Isolation Transformer Upgrade for Old Guitar Amps




Introduction: Isolation Transformer Upgrade for Old Guitar Amps

About: Go sit in the Faraday cage and think about what you've done...

Save your skin! Upgrade that scary old amp with an isolation transformer.

Quite a few old amplifiers (and radios) back in the day drew power by directly rectifying the household "mains" wiring. This is an inherently unsafe practice.

Most guitars connect the bridge and strings to the ground (shield) wire on the guitar cord, essentially using the player as a "noise shield." In transformer-less amps, the Neutral wire of the mains is often used as the "ground." With a two-prong cord, Neutral and Hot can be switched (which could place the amp's ground on the Hot wire!) In other words, playing a guitar amp without an isolating transformer could be like sticking a fork in a wall outlet.

Isolation transformers limit the amount of current that can be supplied to the amp (and consequently to the guitar player) if any shock hazards arise, and eliminate possible "hot" ground issues.

In addition, we'll install a three-prong cord, so the amp has a proper earth ground. And a fuse, too. The earth ground and fuse help to maintain a sane ground reference, and protection from shorts.

And we'll incorporate the changes on a small "module," so as to change the original as little as possible. If someone is crazy enough to revert to the original setup...they can do that.

This mod works with radios, too. In fact, many of these amps were called "radio tube" amps, or "AC/DC amps"--like their radio counterparts, a transformer-less amp could be plugged directly into a DC or battery power supply without modification. A decently-sized bank of batteries were required (over 100V), but that was once commonplace.

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Step 1: ZZZAAAPPPP! It's the Safety Disclaimer!

I'm copying this from my own instructable about tube amp rebuilding :


Seriously. Do this EVERY TIME you work on the amp. If you don't, DO NOT complain if you loose the use of your hand. DO NOT come back and haunt me if you die....

The power 'filter' caps can store fatal amounts of electrical current, and are sometimes termed "reservoir" caps. The caps are connected near the rectifier and are part of the power supply, and aid in converting AC to DC. In fact, they are a standard component in any power supply.

If you're completely lost, and don't understand this , DON'T MODIFY YOUR AMP . You haven't enough knowledge to work on high voltage/current circuits safely...

There are several ways to discharge caps, but here's the easiest:

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)


-- Jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here... If your jumper has a resistor, leave it connected for at least 30 seconds before you touch anything.

-- OR short the caps with a screwdriver. Lay the shaft on the chassis, then bridge to the positive (+) lead of the cap. Be sure the screwdriver handle is insulated (if it's painted, it might not be.)

This may result in a spark... Obviously, your flesh can act as a jumper also (that is NOT a challenge.)

Step 2: So, Does MY Amp Need One?

First, mains-rectified amps were generally small output, 1-5 watts. Manufacturers usually didn't skimp on the larger amps.

If your amp has only one transformer (the output transformer) the answer is YES, you need one. If your amp has two transformers, odds are you don't need an isolation transformer.

Power transformers, the type that's missing from these unfortunate amps, are the largest transformers. They also tend to get warm, so 19 out of 20 times they'll be mounted on the outside of the chassis. The lack of one will be obvious.

Output transformers (and no vintage tube amp can be without one) however are smaller, and might be mounted in various ways, some of which are hard to see. They could be on the outside of the chassis, yes--but also under the chassis, or on the speaker itself. But rest assured--there will be an output transformer somewhere.

But wait--it's not that simple. Some amps isolated the signal path from the mains, but not the filament voltage. If equipped with a three-prong cord, these amps are somewhat safer, as they do offer isolation in most cases.

One sure-fire way to know if your amp lacks isolation is to examine the tubes. American tubes are prefixed with the filament voltage (12ax7 has a 12V filament, 6V6 has a 6V filament, etc.) The AC/DC circuits were designed to run all the filaments in series on a 110V supply. They therefore have high prefixes:

One common set: 50C5, 35W4, 12AU6

...which together equals 97V, so a small resistor was also added in series to drop the 110V voltage an additional 12 to 15V. It should be immediately evident that this was a cheaper way to build an amp. And many were built.

So, from a safely perspective--does your amp need isolation? YES.

Step 3: The Amp

I picked up this funky little Gregory Mark I amp from Craigslist for ~$25. Gregory put date stamps on their cabinets, and this one dates to March 25, 1955. So this little guy is over 50 years old!

Paul Marossy has a great website dedicated to Gregory amps (in fact, the photos of the Mark I example on his site are mine.)

It's a typical low-wattage practice amp of the time. No tone control, only volume. Probably 1-2 watts of output power. It's great "living room" or recording amp.

Among the mods I've already done was adding a 1/4" jack for the speaker output. I just unplug the small speaker, and run the amp into one of my 4 ohm cabinets. The amp is easily twice as loud through a 2 X 12 cab... (with loads of bass, too.)

But it's also a typical non-isolated amp, and that safety issue need to be addressed...

Step 4: Parts and Tools...


Soldering iron and solder
Drill and bits
Stepped drill bit (for large holes--fuse holder)
Screw drivers, etc.


-- Isolation transformer
-- Fuse holder and fuse
-- Scrap wood
-- Heat-shrink tubing
-- Three-prong cord (scavenged from an old computer)
-- Line wire, misc wire, wood screws, etc.
-- Metal plate for mounting fuse holder
-- Strain-relief for the cord

Step 5: Illustrating the Issues Via Schematics

Here's a schematic for the amp (complements of Paul Marossy's website.)

It's very typical of this amp type. Note the following:

-- the lack of a power transformer.
-- no fuse in the circuit.
-- the 35w4 diode is directly connected to the mains.
-- the GNDs are directly connected to the mains (this one doesn't even have the protection of a "death cap!")
-- the tube filaments are all connected in series, directly to the mains.

How do we fix it?

-- add an isolation transformer
-- add a fuse
-- reroute the ON/OFF switch
-- add a three-prong cord, and a proper earth ground

One issue will be dealt with later: using an iso transformer with a half-wave rectification circuit.

Step 6: Choosing an Isolation Transformer

Unlike many power transformers, isolation transformers have a 1:1 voltage ratio. The output voltage is (for practical purposes) identical to the input voltage. They serve only to "isolate" the device from the high-current potential of the mains. DON'T use an auto-transformer--they don't isolate.

Transformers also have a Volt-Ampere or  VA rating. VA is roughly analogous to wattage (remember, wattage = voltage * amperage, or wattage = V * A.) for resistive circuits, but not for inductive loads. For inductive load, you can "guesstimate" wattage capacity = VA * 0.7, or the wattage of an inductive load is ~70% of the VA.

Wiki page on the Volt-Ampere.

So the first question is:
What is the total power consumption of the amplifier?

I.E., NOT the output wattage, it's only a fraction of the total wattage it takes to run small amps.

Most amplifiers have a power consumption rating on the back. My Gregory doesn't, but it's safe to compare it to other three-tube amps. My little Kay amp consumes 28 watts. My Danelectro DM-10 (4 tubes) is closer to 40 watts.

It's a safe guess that most three-tube amps don't consume anywhere near 40 watts of power, and probably not 30 watts. Since more than half the load of a small amp is resistive (the tube filaments), and 70% of 50VA is 35 watts, then a 50 VA rated transformer should be fine.

So we're going with a Triad N68-X isolation transformer, with a 50 VA rating. Good stuff.

The N-68X is inexpensive, and can be purchased at various online electronics stores. One example:
Allied Electronics (for $11.41 USD.)

Mouser has it, and Digikey probably does, too.

If your amp requires more than 50 VA, Triad also makes larger transformers. Of course, isolation transformers from other manufacturers will work just as well...

Step 7: The Plan

Here's where we decide how to implement the changes.

Wiring the N-68X iso transformer


The N-68X can be used with either 120V or 240V AC systems. 

US 120V

For 120V, place the two primary coils in parallel.

Tie these colors together, and connect to the mains (through the switch, etc.):

-- Black and Red/Black
-- Yellow/Black and Green/Black

Euro 240V

For 220-240V, wire the N-68X primary coils in series:

220V / 240V mains-- Black and Black/Green .
Connect Yellow/Black and Red/Black together.


Use only the two Red secondary wires. The white wire is the shield. Connect it to the chassis (or earth ground) if it's mounted there, or if you experience any noise.

Re-routing the switch

The original ON/OFF switch is mounted on the chassis panel. To keep the switching truly functional, we'll have to route it differently.

We could leave the switch as-is, but then the primary of the isolation transformer would be in a permanently ON condition. Only unplugging the cord would cut the power to the trannie. The switch would still operate the amp, but there would still be some current draw. That's wasteful and "bad form."

To use the original switch, a simple two-conductor wire can be attached, and run down to make/break the incoming AC connection to the isolation transformer.

Connect the earth ground

With the three-prong cord addition, a true earth ground is available.

Attach a wire from the center prong (should be Green, but verify) of the plug and connect it to the chassis.

Optionally, the transformer casing can also be grounded.

Power -- connecting the isolated AC

OK, here's where things get a little "iffy."

The Simple Way:

The transformer's secondary can be connected directly where the old power connections attach. In this case

Wire 1 ) to the rectifier plate, and the series filaments
Wire 2 ) to the chassis ground

The order of the secondary wires doesn't matter--the AC from the transformer is isolated, so there's no Hot or Neutral side. They are both Red for a reason...

The Correct way:

Read the next Step--it deals in depth with half-wave rectification...

Step 8: Fixing the Half-wave Rectifier Problem

But wait--the 35W4 tube is only a single diode , so the rectification is half-wave , rather than full-wave.

Is that bad?

Well, yes. As the name implies, half-wave rectification only uses one half of the AC waveform, and blocks the other half. Power transformers are really designed to be symmetrically loaded. The flux field collapses as one peak falls, and the transformer expects an equal load--and an equal amount of magnetic force from the complementary peak.  Without a load on half the cycle, the collapse of the field causes the transformer core to become saturated much more quickly than normal. That puts a "standing" DC voltage on the transformer. The N-68X, being a small transformer, isn't designed to handle this.

Half-wave rectification isn't quite as much a big deal on your household "mains." The current draw is small compared to the available current. The resulting asymmetry only changes the total waveform fractionally. But even that could be enough to create noise in other devices...

When I first installed it, I tried to use the N-68X with the circuit, as-is. But it immediately became obvious that the transformer became too hot, considering a current draw less than 30 watts.

Solving the problem

A larger isolation transformer might nullify the problem, but when using the N68X the best solution is to rectify twice -- once with a solid-state bridge rectifier to shift the negative voltage over to positive; then rectify again with the 35W4 tube. That will eliminate our asymmetry, since there will no longer be any negative voltages for the tube rectifier to block.

See the fifth illustration for this "combination" technique... Note that the output of the combination is full-wave , despite passing through a single diode rectifier after the bridge. So there's more current potential for the amp circuitry than before. Plus it's probably quieter, too.

And note that the peak voltages of the tube rectifier (diode) are lower than the solid-state bridge. Note also that half-wave rectification need not be done with a tube diode--a solid-state diode functions just as "well" for this application.

Where to insert the SS bridge

There are two good options:

Option A ) between the isolation transformer and the entire amp circuit. Since rectified AC (pulse DC) holds the same potential as regular RMS AC, the total voltage doesn't change.

If the filaments were fed solid-state rectified and filtered DC the voltage would be too high, because the total voltage would approach the peak voltage, rather than being an average. And the filaments would fail. However, the filtering caps come after the tube rectifier, so that's not a problem.

In addition, the SS rectifier could be mounted back on the iso module. Since I didn't do that initially, I placed it on the chassis.

Option B ) after the filaments, and feed the tube rectifier only (only the DC parts of the amp cause asymmetry.) This would work fine. But it also requires a bit more rewiring.

I chose the first option...

Why include the tube rectifier at all?

The bridge produces all the rectified current the amp needs...why keep the 35W4?

-- Leaving the 35W4 will keep the peak DC voltages at a lower level than the more efficient SS bridge by itself. The 50C5 power tube wasn't designed for plate voltages much higher 120V. Since AC peak voltage is higher than it's RMS value, rectification circuits tend to output a higher DC voltage (theoretically 1.414 times higher than the RMS.) But as stated previously, tube diodes are less efficient.

-- All the tube filaments are still connected in series, so removing the 35W4 would have created a new problem--how to drop the voltage on the series string of filaments (the remaining two tubes) by an additional 35V.

Leaving the 35W4 tube in place solves both these issues.


Is all this absolutely necessary? Well, with a large enough Isolation transformer, maybe not.

A 100 or 150VA rated transformer could safely deal with halfwave issues for a <50 watt amp, I'd say.

Step 9: Option C (busting the Hum)

OK, it's a year later, and then some...

These changes do seem to introduce hum to some AC/DC tube circuits. For a few reasons: SS rectifiers are more efficient, the filtration is a little lacking, and fullwave rectification shifts the PS wave peaks from 60Hz to 120Hz.

So in the quest for a hum-free amp, I've modified the circuit somewhat. This has made the little Gregory amp almost totally free of nasty hum. Your mileage may vary--each amp is a little different.

NOTE about this section:
There's cost for converting to higher voltage DC filaments--increased power consumption. The power draw for the 120V AC filaments is 18 watts; 25.2 watts for 168V DC filaments. Keep that in mind. Note also that this mod may raise the plate voltage for the 50C5 output pentode somewhat higher than the recommended voltage...this has worked fine for me, but YMMV.

Option C

This option Inserts another filter cap after the SS rectifier. It's a little odd, as the additional filter cap is placed between the two rectifiers. Nothing technically wrong here, just unusual...(as are two rectifiers, but we know that works.) We're just feeding the second rectifier a current source that's less...wavy .

However, Option C introduces a complication: With even a moderate filter cap, the filament voltage is much closer to DC than the original AC.

That's good, right? DC is quieter. Yeah, but the DC voltage resulting from rectifying and filtering AC is closer to the peak AC voltage, and can't be treated as an "average"... So the new DC voltage is higher--TOO high, in fact. The old AC-to-DC formula is in play...the DC voltage is approx 1.4 times the AC RMS, approx 168V. This would surely burn out the filaments.

Taming the Higher Filament Voltage

But there's already a series resistor inserted with the three filaments to drop the voltage--for line AC (115-120V). We just need to increase that resistance so it can handle the higher voltage.

So how do we figure the new resistance value for Rv? A few facts...

-- the three tubes (12AU6, 35W4, 50C5) drop a total of 97 volts (12 + 35 + 50 = 97).
-- each tube draws 150 mA (0.150 Amps ). That's important.
-- the stock Rv value is 160 ohms (for 120V).
-- the new filament supply voltage is 168V .

Hmmm, each tube draws 150 mA. AaaHa! Current is equal for all components in a series circuit. So the current draw of Rv must match.

Time for good ol' Ohm's Law (R = E / I, or resistance = voltage / current). Let's check the original value:

120 - 97 = 23 extra volts to drop.

To achieve the same current draw for Rv: 23 / .150 = 153 ohms. Good! That's almost spot-on to the 160 ohm spec'ed value.

The New Rv Value

Estimated DC voltage for the filaments: 120 * 1.4 = 168V
168 - 97 = 71 volts to drop.

71 / .150 = 473 ohms. That's SO close to a standard value...

470 ohms is the new Rv resistor value. Rv is dissipating 10.5 watts, 15 watter is required.

This has been tested, and worked perfectly--the very first time (yeah!)

Yes, this ups the current draw (total wattage) of the amp, without increasing the output power. OK, not quite true--the output pentode now has a higher plate voltage, so the output is slightly increased. The higher filament voltage is drawing about 7 additional watts. The iso transformer does get a little hotter.

The New Filter Cap

Pick a reasonable value here. I used 22uF / 250V, but upped that to 100uF / 250V. It works just fine, and obviously the 100 uF cap is a bit quieter.

Other Anti-Hum Mods

I've moved the initial SS rectifier ground directly to the bolt that holds the rectifier to the chassis. Probably helps a bit. The first (filament) filter cap is also grounded here.

Also moved the isolation transformer a little farther away from the speaker voice coil. It's easy to experiment with this...just clamp the transformer "module" in different spots and test. Didn't have much effect, but it can't hurt.

Don't forget to clean and reseat input jacks, especially if they are grounded directly to the chassis. That's a common source of hum.

Step 10: Building an "Isolation Module"

I built it as a small self-contained module, mounted on a block of wood.

There are other ways, of course. All the components can be mounted directly on the cabinet itself. The cab plywood is rather thin for this amp, so best to use the wooden block for a base.

Make the module base

A scrap piece of poplar 1x2 was used, cut to a length that easily fit all the components.

Add a fuse holder

The fuse holder is a pretty standard type. It's mounted in a small piece of galvanized metal plate (originally a truss plate.) Metal plate is definitely the best choice for securing this kind of fuse holder device. Thin plywood wouldn't be secure.

A stepped drill bit was used to drill the hole for the fuse holder. Wood screws were used to attach the plate to the base.

Mount the transformer

This is straight-forward. The N68-X transformer is attached with a pair of wood screws.

Make internal connections

Wire the module using the schematic / wiring diagram on Step 7. You can find it below.

Some pointers:

-- The switch and fuse should be on the Hot "mains" wire.
-- When routing the switch wire, avoid the signal path where ever possible.
-- Connect the transformer primary wires as noted. This is US, 120V wiring. Euro wiring will be different (and is explained on Step 7.)
-- I used "wire nuts" to connect the wires, but soldering is more secure. Once I'm satisfied with the setup, I'll replace the nuts with solder, and cover with heat-shrink tubing.

Add some strain-relief for the cord

I used plastic wire channels to fix the cord in place. Electrical cords must have some strain-relief, or flexing will quickly lead to disconnections or shorts.

Step 11: Installation

Ok, now to hook everything up...

Fix the module in place

Yep. That means attaching the module somewhere inside the cabinet.  I used wood screws; whatever is adequate will work. Mounting it some distance from the chassis is fine, and might be advantageous in some circumstances.

Attaching the earth ground (from the three-prong plug & cord)

An important safety feature in any amp is a valid external earth ground. This helps protect the amp (and the player) in a very simple way:

Should any parts fail, or any connections loosen and cause a short circuit, the ground wire provides a "safe" current path, while ensuring that the current flow from a short will also blow the fuse. If the fuse blows, you know there's a problem to fix. And you won't be using potentially dangerous equipment.

The center prong wire from the three-prong cord is the earth ground. In the US, this should be the green wire. Test it anyway, to be sure.

Connect it directly to the chassis. It does not go through the isolation transformer.

Connect the Power switch

Route a two-conductor wire from the switch on the front panel, down to the incoming AC line. Line cord, like the type used in lamps or extension cords works fine. Buy it by the foot at hardware and home improvement stores (Home Depot, Lowes, etc.)

Drill a hole through the chassis if necessary (I did.) Install a rubber grommet in hole, to prevent the wire from rubbing across the chassis, a creating a short circuit.

Route the wire away from the signal path, if possible.

Connect the transformer secondary to the amp

As discussed in the "half-wave" step, there are several way to do so.

But in any case, a double-conductor wire should be connected to the RED secondary wires on the isolation transformer. The wire can then be fed through the chassis using the original power cord entry hole.

Add the solid-state bridge rectifier

This is discussed in depth in Step 8, and schematics are included. Check the photo below for a wiring example.

A bolt-on type of rectifier was used. A new hole was drilled in chassis to accept the mounting bolt.

Once soldered in place, heat-shrink tubing was added.

1 Person Made This Project!


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93 Discussions


1 year ago

Hi, like the info! working on a sears 1448 amp in case type from the 60's. Same tubes but it used a transformer for the heater of the 12au6 and a 100ohm 5w resistor instead of the 150 ohm. So I removed the iso xfmr for the 12au6 and wired all three heaters in series. Now thinking of option B which puts the rectified dc to just the rectifier and leaving the heaters AC. So I did this and the heater volts at the 12au6 are 13.5 with the 100 ohm 5w(may need to up it to 150) and the plate voltage at the 50c5 was at 149.3 too high, so I put the 470 ohm in before the ss bridge on the AC side to reduce volts going into the ss bridge and this brings the 50c5 plate down to 100v and the heater of the 12au6 down to 9v. Now this does sound good and no hum and I used a 25w 470 ohm. So this reduces the voltage for both the dc and ac circuit as it is in place before the ss bridge and the ac heaters (also removed the 100ohm 5w from the heaters as the 470ohm is in place before anything) So this brings voltage down for both the AC and DC part of the amp. I was thinking of going with a 400 ohm resistor to allow a little more voltage to the amp and bring up the plate and heater volts just a bit like 10 to 20 volts on the plate of the 50c5 and might bring up the 12au6 from 9v to a bit higher as well. Your thoughts? I will include original schematic and the idea schematic I came up with. The heater for the 50c5 is at about 44v with the 470ohm in place so upping that will up the 9v to the 12au6 a bit I would think. I plan on using a 50w 400ohm after the iso secondary before the ss bridge and heaters with the heaters tapped after the resistor but before the ss bridge. The amp sounds great with the 470 ohm 25w resistor but like I mentioned the voltage is just a bit low and the 470ohm resistor gets pretty hot (not too worried) but feel the lower 400 ohm won't need to work as hard and allow a bit more volts to the amp plus it will be a 50 watt, should be less work for the iso xfmr also. With the 470 ohm in place the iso xfmr gets warm but not hot after 30 minutes of play. I also added a 100uf 250v cap between the ss bridge + and 35w4. I also replaced all caps including signal and coupling and replaced all resistors including the 1ma pot so again the amp sounds great but hoping the 400 ohm works a bit better. Update: The 400ohm works but still voltage is a bit low and after some figuring I will go with a 330ohm. Included a couple more pics, I put the guts in a homemade chassis and made a combo out of it. Final schematic with 2 power resistors a 150ohm and a 330ohm at 25w each not the 50w as in the schematic (next to last pic) but this gets my voltages right and it sounds great. Thanks for the useful info here. Last pic is the gut shot of the final build.

1448.jpgschem 1.jpgguts.jpgback.jpgfront.jpg2 resistor schematic.jpglatest guts.jpg

Reply 1 year ago

Hi Mike. Sorry about the late reply. I checked Ibles only a week or so ago, and didn't see your post (? who knows).

Cool, I see you built from scratch using the Silvertone schem. I don't doubt the tubes can take a bit higher voltage than spec'ed. I think they were all +- 10% back then, and many could be abused further. I don't think you'll have any problems at all with the resistors.

Working on replacing the baffle board of my Silvertone 1482 today...

Good luck and rock on.


2 years ago

Here is picture of the schematic and the outside of the kodak kodascope


2 years ago

Hi. I have a kodak kodascope pageant with a symilar schematic. I want to add the isolation trans former and fuse as you did. I plan to use a 100va transformer because i prefer not to add an aditional rectifier. I plan also to replace the 3 selenium rectifiers with a 1n4007 diode followed by a 300 ohms 2w resistor(the value is a guest)


Reply 2 years ago

I want to know if there will be an ac short with the actual ground to the chassis when i install the green ground wire from the transformer to the chassis.


I've read your brilliant article about the iso transformer upgrade but I'm a bit confused about applying the concepts to the new Kay 703C amp I bought off of ebay recently... Attached is the schemo... it appears that only a part of the filaments are not isolated...?!?! Am I getting this right? Any particular guidelines to apply the concepts and not end up frying some component or myself in the process? I'm not sure how to go about this especially relative to rectifying this circuit adequately... I know a fair bit about electronics but not as much as you that's clear...Thanks.

Ian W.


Reply 5 years ago on Introduction

That is one really strange schematic... There isn't any isolation for most of that amp.

The only thing that's isolated on that amp is the 18V filament for the preamp pentode. The only purpose I can think of for that transformer is that the preamp tube filaments draw a different amount of current than the other tubes, so they couldn't be used in series. The preamp voltage is still AC, even with the iso trans, so it doesn't have a DC supply for the preamp filament (which would be one reason to separate that heater from the rest). The tube filament voltages look perfect to run in series, so it's puzzling unless there are differing current requirements.

The wall AC runs in series through the iso transformer primary winding, the power tube, a voltage dropping resistor (R9) and half of the rectifier filament--where it splits off through the other half of the rectifier filament, and then becomes the B+ for both the signal tubes. Meaning the B+ voltage is wired in series with half of the rect heater, so they figured the current draw of the plates and sundry would balance with the draw of the half filament...

Very weird. Maybe the current draws for all the tubes are different, and they creatively figured a way with the transformer and half of one filament to make it work. Sometimes products are designed around an existing stock of tubes (like they got a truckload cheap).

I'd add an isolation transformer for the whole thing, right at the cord. It's too complicated and weird to change the rectification. Get a bigger iso transformer if you're worried about the half wave issue...


Reply 2 years ago

Damn. I wish I scoured these comments a little closer back when I was ordering parts. The Kay 703c is the same amp that I have been using this instructable for. And I've spent the last couple days scratching my head trying to figure out that rectifier issue.

I have one quick question first. The 703 is a 33W (consumed) amp. Would a 100VA iso be enough to handle the half wave issue?

This my first real tube project (not counting a kit), so I am a little green. The data sheet for the 36AM3 says that the heater tap (pin 6 where AC is connected) is meant to be used as a current limiting resistor and that it has a resistance of 45 ohms when the output current is at it’s max. That could be what you were saying already (3 years ago), but I just wanted to pass that along in case it makes that circuit make a little more sense.

And one more question (sorry if it’s answered somewhere else), that standing DC voltage that occurs, is it causing the iso to use more power? Or are we just overrating the iso to deal with extra heat? It seems pretty straightforward in step 8, but since I am tube green I just wanted to be crystal clear exactly how you meant that.

Thanks for the instructable. This has been fun, if a little frustrating. And time consuming….



3 years ago

If full wave rectification increases the available voltage, can you insert a second 35w4 instead of the diode bridge? This way you can have all tube rectifier, smoother voltage and the tube will eat up the extra voltage produced.

I'm asking because I am doing a similar project right now. I managed to get my amp pretty quiet but had to up my filter caps to where the tone became too hard for me. I'd like to be able to go as low as 20uf without the 60hz hum. Right now 50uf is fairly quiet. I used this article to get my power supply set up in the first place. (thanks!)

my tubes are 12ba6 12ba6 35c5 35w4. fujiya ex-1 1950's tape recorder,originally.


Reply 3 years ago

My bad for not answering sooner -- busy!

Not really with one additional 35w4 -- those are single diodes (fine for half-wave). A true bridge is four diodes.

Tube amps with full rectification do so with an expensive trick: they use dual rectifier tubes (5y3, 5u4G) together with a center-tapped HV transformer (like 300V,0,-300V). The voltage swings positive & negative around that center tap, and half the available voltage is dropped -- i.e., end-to-end on a 300,0,-300 PT, the full voltage swing is 600V! (and the output would be 600V with a standard SS four-diode bridge).

They did this because :

A) they absolutely knew full wave was better.

B) Iron and copper were once much cheaper than tube rectifiers. Now the opposite is true -- metal is expensive (and heavy) but sand (silicon) semiconductors are (literally) dirt cheap! Plus there was always a trade-off regarding current draw of the tube filaments, and double the rectifier filament current wasn't deemed acceptable.

Full rectification end-to-end is very do-able with two dual-rectifier tubes. I'm sure it's been done.


3 years ago

Hey this is a great article and I will need to do this on one of my amp. It is a Canadian made amp labelled Regal. It's probably from the mid 1960s.

I do have one question, it uses a 12ax7, 50L6 and 35Z5 rectifier. I looked at the data sheet for all parts and they can all accept plate voltage of 200V. Is it really necessary to drop the voltage (from your measured 168V) in that case?

Thanks for this very informational and documented piece.


Reply 3 years ago

Thanks! You'll need to trace the voltage for the tube heaters (filaments), not the plate voltage, to set your limit. The heaters have at most 10% over-voltage tolerance.

That's all this voltage modification does, it doesn't drop the voltage on the signal path part.


3 years ago

Great article man! Very Helpful!

Forgive me if I missed it in the article, but what size fuse should I use, or how do I go about calculating? Does it need to be slow-blo?


Reply 3 years ago

Sorry missed your comment earlier.

What's the wattage for your amp? Wattage is voltage X amperes, so it's pretty easy to figure the fuse from that...


3 years ago

Thanks for all your help, my amp works great, no hum, tremolo works great.


Reply 3 years ago

Cool! Thx.


4 years ago

Thanks , I've got a amp basically like yours, made by Guyatone in Japan in the early 60s. The badge says Marathon, but the same amp has badges of Kent, Tremo-tone and a few others. It's an accident waiting to happen the way it is. Thanks again Your the man.


4 years ago

Did you use a half wave bridge rectifier ?


Reply 4 years ago

Changed the original tube rectifier (halfwave) to a fullwave SS. It's discussed in the Ible; see steps 8 and 9...


4 years ago on Step 7

Yeah , good project !

I would like to comment on a couple of things . Even the amps with a power transformer aren't totally safe if they came with a 2-prong plug . I have a 1968 Fender Dual Showman tube amp that had a 2-prong plug and a " GROUND " switch which would reverse the mains connections . you could flip the switch once you started to get ZAPPED or the amp got noisy ! That one was an easy fix , I replaced the power cord with a 3-prong type , and grounded/earthed the chassis . ( I like old " tube " stuff ) one of my old radios , a Hallicrafter s-38 eb communications receiver has a " hot " chassis , I will probably get one of those transformers for it , or get a bigger transformer and have an isolated power strip to plug things like that into , haven't decided yet . Oh yeah , the idea of keeping the 35w4 tube is good , in addition to keeping the filament voltages correct , it would not be good for the other tubes to have full plate voltage while they were warming up .

Cheers , take care , and have a good day!!...73