Tube Amp Rebuild (and Mod)

Striving for that old school sound, you purchase a 'vintage' guitar amp. But it doesn't sound quite right. Well, any amp more than 20 years old is gonna need work...

What's the deal with tube amps? Why all the fuss?

Yes, they do have a special sound, one that digital 'modeling' can't quite get right... It's actually the limitations of tubes that impart that special tone--that natural compression and smooth breakup. Sure, they keep improving solid-state amplifiers--but a look at the majority of mid-range to high-quality amps, the current models (mostly all tube, tube/ss hybrids, etc.) should convince you that it's very difficult to capture 'that sound' without tubes.

Does a 'rebuild' ruin the value of my vintage equipment?

No. Maybe. I don't know. Does changing the tubes mean the amp is not longer 'vintage?' Every old amp will need a rebuild at one point or another. If this negates it's 'vintage' status, then there is no such thing as a vintage amp! Functional amps, anyway--o.k. for nutso collectors who don't actually use 'em.... Who cares about them!

This little jem is a 1961 Kay 503A instrument (guitar, harp) amplifier. Output is in the 3-4 watt range. A nice studio, or "living room amp."

Here's a taste, and there's more video on the last step:
#1 -- Mod A (my first attempt--If you only have patience to listen to one clip, play #2):

(recognize the Trash-o-caster?)

#2 -- This is Mod B, IMHO, a much better sound:
(pickups switched from middle to middle/neck phased mix, about half way)

#3 -- Quick one added, with a Gibson Les Paul, just to show the amp has some bluesy distortion..
(pretty 'raggedy' playing on this one, but it's enough to catch the sound...)

All the videos are 'clean'--guitar and amp only, no FX.

---

Danger! Danger! No, really, DANGER!!!!!

A tube amp, even an unplugged tube amp, stores enough electrical energy in it's capacitors to kill you! YES, KILL YOU. Don't touch it unless you take precautions. See the section on DISCHARGE THOSE CAPS!!!!!

I'm not a tube repair technician, or an expert electrician. I'm just a hobbyist. Don't take my word, do your own research, and please be careful !

Step 1 Symptoms: How do I know my amp needs a rebuild?

--Hum: Usually a sign of bad filter caps. Poor routing is often a cause. Adding a three-prong cord will help.

--Weak signal: Bad tubes, failing output transformer.

--No highs/loss of tone: Bad tubes, coupling or bypass caps.

--Unpleasant distortion (not the kind you want): Often, this is bad tubes. Also could be the coupling caps or cathode bypass caps. Or could be filter caps, if the signal is modulated with 60 hz noise.

--Weird noises (squeals, crackles, etc.): Probably bad tubes. Sometimes failing coupling caps.

--In each case, look to the speaker, also. A torn or worn speaker cone could be the culprit. In rare cases the output transformer may be failing.

---

This amp had the following problems:

--good tone for 5 minutes, then a nasty, harsh clipping distortion on louder notes (yep, bad tubes were the cause.)

--Nasty hum (filter caps and routing problems.)

--Dull, muddy tone (needed new caps and tubes.)

--The original two-prong power cord needed replacing.

Comments

[Ceiling cat]

Hi.  I was wondering if you could help me.  I have an old Lindell three tube amp and it's not working properly.  When I turn it on, as soon as the tubes warm up there is a VERY LOUD hum , and it's unaffected by the volume dial.  Could this be a bad filtering cap?

[cgillyard]

Yes bad filtering caps are usually the cause.

[gmoon]

Hi. Older amps almost always need new filter caps at some point. Loud hum regardless of volume does seem to point in that direction.

It could be other things, too, though. Sometime the tubes themselves develop hum and have to be replaced.

You could try pulling the preamp tube and then turning on the amp--unless it's a "radio tube" type amp that has all the filaments wired in series (like the amp in this instructable.) In that case the amp won't work with one tube pulled.

Always nice to have a few backup tubes anyway, so new tubes are good investment...

[Ceiling cat]

Thanks. I'm actually on my way out to Radio Shack for new caps.  As for the tube  configuration, I think it is how you said.  It's a very simple circuit.  And don't worry, I have plenty of tubes around.  I bought a bin of four tube radios and a box full of tubes at a swappers meet for $10.  Thats where I got the 12AU7 for my tube boost pedal.  I have like seven.

[gmoon]

Ok, cool, you've eliminated tubes as a problem.

Good luck finding the caps at RadioShack, though. The store near me doesn't carry electrolytics at the capacitance & voltage needed for a tube amp...(even for the relatively low voltage of "radio tube" amps-- 120V-150V.)

[Ceiling cat]

Unfortunately I found out it's the same story here.  I biked a few miles to RadioShack only to find they didn't have the caps I needed.  Not even the right capacitance.  It wasn't a total loss, though.  I got some electrical tape and solder that I needed badly.  And I got a ride home with a friend I ran into.

[Ceiling cat]

Oh, and earlier today I found a fuse holder with a mount for free.  I'll have to do the fuse/three-prong-plug mod sometime.

[Ceiling cat]

Good news!   Last week I bought some new caps at a local amp repair shop called "Mike Tech".  I soldered them in and took it back there and he formed them with a variac for free.  I brought it home, hooked it up, and the results were great.  It sounds beautiful!  It's got a great tube sound when I crank up the volume.  Very happy with it, and glad my dad's old amp is still in the family and finally working and being played again.

[hg341]

my grandpa was move out to a smaller house and geting rip of most of his stuff a few years back and i took some thing that look like a ham raido

i hadnt realy look at it since then in till last weekend and found out in was a 300w tube amp by eico

im trying to get speakers for it now

[gmoon]

Pretty cool. Should be worth fixing up.

(don't run it without speakers...)

[hg341]

im sry i get all execited and grously over-estamate what some things can do....
its only 14 watts

buts its the famed hf-81

every review i saw was like this

http://www.stereophile.com/integratedamps/606eico/

[gmoon]

Definitely worth fixing up.

12-14 watts of tube power is LOUD. Loudness is logarithmic--subjectively, twice as loud as 14 watts is 140 watts.

[hg341]

on the back of it it has screws for the speakers marked g 4 8 16 and 32
what dose this mean(i think it the ohms but i dont know...) i do know what g is...
1st is what im talking about 2nd is a bouns

[cgillyard]

That is your speaker impedance terminal strip. Use them to connect your speakers to the corresponding impedance screws. For example if you have a set of 8 OHM speakers. Connect the positive (+) terminal of your speaker to the 8 OHM screw and the negative (-) terminal of your speaker to the 0 screw to the amp. Sometimes that 0 terminal screw on your amp is also known as C or common. Hope this helps. Let me know if anything. Thanks.

[gmoon]

Yeah, that's ohms. For an 8 ohm speaker, connect one wire to "8" and the other wire to "g" (ground).

Must be stereo, so two speakers (at least) are required--one per channel.

1st is what im talking about 2nd is a bouns

Sorry, I didn't get that...

[hg341]

ok im looking to buy some speakers for this what would be best 2 8 16 or 32?

[gmoon]

None is inherently better; all the better speakers come in different ohms loads. Just work with what's available.

You can also wire speakers together. Two (identical) speakers in series doubles the resistance; Two in parallel halves the resistance...

[hg341]

cool thanks for that

"1st is what im talking about 2nd is a bouns" i was talking about the pics
the 2nd one is the uncoverd bottom of my amp no real need to show but its still fun to look at.

[carpe_noctem]

i was wondering, what kind of output you could get from just one tube? I've been thinking about making a tube headphone amp but haven't seen any schematics anywhere (maybe i haven't looked hard enough), and i wasn't sure whether it would be plausible...maybe just a tube preamp and if the signal is still too weak a little bit of solid state amplification (i heard FET's sound more "tubie" then most opamps)...i should probably just look at some schematics and see how they manage the headphone jack

[gmoon]

A single tube is a little problematic, especially if you're looking for a classic tube sound.. A hybrid approach has been done--tube preamp with an FET or a LM386 to drive the headphones would work.

The other possible approach would be to use triode/pentode paired in a single tube. There are quite a few out there. You might even try one of the low-voltage tubes intended for cars and portable radios. This page has some possible combo tubes... These are safer to use than normal high-voltage tubes.

[carpe_noctem]

thanks

[s3mamill]

I just got this exact same amp. I thought it would be a good starter project since its a simple circuit, but it seems over my head, since I have very little experience with electronics, wiring diagrams, etc. Now I'm considering some options: 1. Have it serviced by a pro. It would be safer and the work would be done right. Low risk, but higher cost. 2. tearing out what's there and installing a kit in the body. Higher cost, but doing the work myself. 3. Trying your mods and risk messing it up irrevocably. Low cost, but high risk.

When you said "I'd tear the amp down to the bare chassis and rebuild from there." did you mean rebuild from a kit or design a circuit yourself? Thanks for a great instructable!

[gmoon]

Hey, that looks good.

By "tear it down and rebuild," I meant that a less "hacked" approach would probably lead to a cleaner, more organized, less cluttered circuit. As-is, it's gonna be more difficult to replace the caps in 30 years. The tangled wiring is more fragile than a well-planned strip-down / rebuild, too.

However, it's extremely quiet and sounds great, so I'm not too disappointed.

I'm not sure what approach is right for you. If you try rebuilding it yourself (and fail), you can always take it in to a profession to get it sorted. Just be REALLY CAREFUL if you do.

If you're not confident you can do this safely, take it to an amp tech...

[Packerswin14]

I'm having a hard time identifying the power transformer in this amp, and it's getting REALLY hot.  The numbers are 521P1-98 839013.  The schematics don't give any voltages or info.  Any ideas?

[Pentagrid]

Hello Packerswin14, play safe & disconnect immediately. You don't say if the Transformer in your amp has been rewired as part of restoration work... if yes, check to see all the windings are connected correctly. It's possible to do this with a multimeter & a little intelligent guessing.

If it hasn't been touched, then check for a short-circuit from B+ to earth. If this proves false, there may be an internal short in the Transformer:(

But, please do take care as allowing a Transformer to run hot for any reason is dangerous, not only to you but the surrounding electronics.
John Roberts.
Wellington.
New Zealand.

[FireandIcebike]

Man, it's crazy that you posted this because I was just wondering why my old Gibson Falcon amp shocked me when I was barefoot! How long do the filter caps hold a charge?

[FireandIcebike]

I'm pretty sure if you're not a licensed electrician, you shouldn't be farting around in the back of an amplifier. At least that's the impression I'm getting. I finally found a guy that specializes in this stuff, so hopefully he can fix me up and I'll be jamming out instead of worrying about dying working on my amp.

[gmoon]

The last person you'd want inside your amp would be a "licensed electrician." They deal with a very specific set of predefined circumstances--NOT what you find in tube amps.

If you're not willing to learn (or unwilling to take the risk), seek out an experienced amplifier technician.

[FireandIcebike]

I ended up doing just that. Thanks.

[gmoon]

Caps can hold a charge for a loooooong time (although the older ones usually don't, 'cause they're failing.) Were you pokin' around inside or just playing? Isolation can be a problem with these oldies, too.

I've been dumb enough to play my old Ampeg in the basement with barefeet....wasn't a painful shock, I could just feel the current flowing from my left hand to the floor. One lesson learned.

[titsanonymous]

i have an ampeg, its the reverb rocket. its busted tho. my dads friend said that a guy gave it 2 him and then it sat in his basement and he gave it to me. when i plug a guitar into it the light turns on but there is no sound. could it be the tubes?

[73lonestar]

a short in a output tube can cause the fuse to blow replace the fuse 250ma pull out the output tubes the larger ones and and look for voltage form chassis ground for g1 g2 and plate b+ should be about 350 to 450 volts g1 look for bias and make sure amp has speaker load bias differs on the type of tube el34 is about -42 6l6 -52 el84 -15 6550 -62

[titsanonymous]

do i need to discharge the amp to do that?

[73lonestar]

i find it so satisfying to build an amp from tubes the art work of human is awesome and money is the other drag parts are kinda pricey i can feel sick or stressed out but being in my work bench figuring out am i going to use 6aq5s el38s or how high of voltages will be best at this currant and making someones jaw drop after they see your work is cool after a week drilling the holes bolting up transformers and sockets at the end everyone is jamming out loud and sweet.

[73lonestar]

well i will say the voltage is high but you need to make these test with the amp on it is critical to measure the bias if not you can burn a set of good tubes the pin out on a 6l6gc or 5881 is 1 neutral 2 filament 3 plate or b+ 4 g2 screen b+ slightly lower than 3 5 control grid g1 bias is preasant 6 neutral 7 filament 8 ground notch clock wise upside down chassis between 1 and 8 i have 7591 tubes but they have the tall bases cant look to see how it works but you can get the pin out on google please be aware of the high voltage preasant and unplug the amp before you discharge the caps replace any resistors that are out of tolorance if it is self bias look at the 400 ohm resistor 7 watt or maybe 10 on a 6l6 it would be 220 ohms g1 grids are about 300k ohm to chassis 1/2watt resistors also look for proportion in the center tap primarys to the plates maybe around 30 to 60 ohms from center tap on output transformer and look at the preamp tubes and look at the voltages there to pins 1 and 6 are b+ on a 12ax t or u7

[gmoon]

It could be the tubes...but it could be other things too, like bad connections. Is there any sound at all? (humming etc.)

[titsanonymous]

no sound at all

[gmoon]

Yeah, that could be a lot of things (including bad tubes). Could also be a bad output transformer, disconnected speaker wires, etc.

[FireandIcebike]

I was just playing. Man, I didn't think an amp could be all lethal and junk! Thanks for the info.

[gmoon]

Yeah, actually quite a few people have died from this stuff. Very likely your guitar is wired to use you as a ground (through the strings.) On some old amps, the chassis ground might instead be a lethal voltage in respect to another 'ground' (you in your bare feet, or a mic stand, for instance.) There's a little info on Step 3 about that, if it's an amp without an isolation transformer. But even the amps with isolation can be dangerous--that's why I added the three-prong cord here....