Introduction: 6V6 Pushpull Calss AB Tube Mono Amp.

INTRODUCTION

Last time I touched vacuum tubes was around 1967 when I was repairing radios and TV. I remember this old radio HIFI that was stripped from a console back in the early 60s There was a 12 inch speaker and a smaller speaker I think mid range and a small sealed back cone tweeter. There was a huge power transformer on the chases and a bunch of tubes. I do not remember what the output tubes were but I do remember them being in a push pull configuration. The sound was amazing and through out the years that followed I was never able to match the sound quality with all of the solid state amps I have built or heard. Recently with the hype on the vacuum tube single ended amp hoopla I started getting the bug and turned to the Internet for research.

The transistor brought a new theory in the amplifier and speaker industries. The current fad in solid state amps was to through a lot of current at a speaker in a small sealed cabinet. I will not debate with anyone about lots or little bit of power and how they sound.

As an attempt I wanted to build a tube power amp to see if I could find that good sound I heard when I was young. The following document contains the tube amp I chose as my first tube amp. Please note that I have never liked single ended power supplies for audio amps so it was not used.

DISCLAIMER
While this document describes a plan to build an electronic device which has about 360 volts DC under the chases caution must be followed. I will not be held responsible for any injuries incurred from some one following this document.

MSH MODEL-1 POWERAMP (Michael S. Holden) make no representations about the suitability of this information for any purpose. It is provided "as is"
MSH MODEL-1 POWER AMP (Michael S. Holden) disclaims all warranties with regard to this information, including all implied warranties of merchantability and fitness, in no event shall MSH MODEL-1 POWERAMP (Michael S. Holden) be liable or any special, indirect or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortuous action, arising out of or in connection with the use or performance of this information.

This information may include technical inaccuracies or typographical errors.

MSH MODEL-1 POWERAMP (Michael S. Holden) may make improvements and/or changes in the information at any time.

Step 1: Circuit Design History

I did not design the power amp circuit. However I did design the power supply.

The origin of the power amplifier design came from the schematic below, which came from the 1959 RCA RECEIVING TUBE MANUAL Tech Series RC-19

Step 2: Power Supply Schematic

The filter capacitors were sized to keep the amp supplied with power for 10 seconds after power-down while running at full output. This will assure me that there is enough reserve energy to supply any demand the input may supply. While this schematic is separate from the amp schematic both the amp and power supply are in a single chases.

Step 3: Power Amp Schematic

The amplifier schematic has been redrawn to reflect my preferences. The output transformer is a hamondP-T160 at 10W instead of the 25 watt transformer in the ordinal schematic. The components in this circuit will never produce 25 watts and is much more expensive than the 10 watt output transformer. If you decide to replace the 6V6 tubes with 6L6 tubes this transformer as well all other circuit components would need to be reevaluated.

Step 4: Parts List

All components are bran new and purchased from Tubes and More from the internet. https://secure.tubesandmore.com/ The parts listed uses their part number and 2009 prices. You may purchase the parts from anywhere you wish.

Step 5: Drilling Template

This drilling template is in the PDF file and was set up on an 11*17 page size which can be printed and will then give you a full sized template to mark the aluminum before drilling holes.

Step 6: Top Side Component Layout

Step 7: Bottom Side Component Layout

Step 8: Chassis Wood Sides & Paint

The chassis sides are constructed of 7/16 thick solid oak. The over all dimensions are: 12 1/4 by 8 1/4 by 2 1/2. The top of the oak box is rabbit out to fit the 8" by 12" aluminum plate. The corners are 1/4" box joint fit to make a strong and attractive joint. Four coats of hand rubbed Polly Urethane protect the wood sides of the chassis.

The chassis top is a piece of 20 gauge aluminum COVER PLATE, ALUMINUM, 12" x 8", HAMMOND P-H1434-22 To control corrosion the aluminum chassis top is painted with white dry powder paint and is backed at 400 D. F. This method of painting provides a stronger surface which is more scratch resistant than normal paint.

To give the project little flair all three transformers were dissembled and red power paint was placed on the outer skins.

This dry powder paint process is quick and more durable than normal spray paint. I researched it on the web and gave it a try. I will use it again.

Step 9: Chassis Wiring

The first thing to do is remove the aluminum chassis top from the chassis sides. This makes it much easier to wire. There is a ground loop which runs all around the chassis and is grounded to the chassis. Lay the components out while keeping in mind the wiring path and mechanical requirements. You will probably want to layout the components in some other format other than mine. I have shown my layout to give some ideas. I tried to balance the weight of the components and still have reasonable short wire length. The wire used was 22, 20, gage solid copper hookup wire with insulation of 600vdc.

Solder lugs were placed at different locations on the wire side of the chassis. A drill was used to start the tube sockets holes and finished of with a step drill. The hole sizes are listed on the layout sheet in the Chassis Construction chapter.

Rubber grummets �e uses any time a wire pass through the top side to the bottom side of aluminum top.

Step 10: Testing & Evaluating

The frequency response chart was created using a small VB program I created. If any one wants a copy please email me at MS.Holden@comcast.net and I will email you a copy of the program. The amp seams to suffer performance below 30hz. This depends on the level of the input voltage so I picked an arbitrary level.

Step 11: Lower End Frequency Response

The top graph below shown at 20hZ shows a great deal of distortion and crossover issues. By bumping it up to 30hZ most all of the problems went away. I would want to start the lower end frequency response at 30hZ. Wonder what is wrong?

Step 12: Tooles Used

In the picture below are some of the main tools used in this project. Also these tools were used: Soldering iron, Multi meter, power drill, wire strippers, diagonal cutter, assorted screwdrivers, nut drivers, and others.

Step 13: In Conclusion

I am sure that there are better tube power amps out there. This one works fine for me. A 10W tube amp with a high SPL speaker of 90 dB or better is a good combination and delivers the sound that I heard when I was a boy.

Below is the mono HIFI preamplifier I built to go with the Model-1 Power Amp.

Step 14: Project in PDF

The PDF file has higer resolution and moer information in it.

Comments

author
robot797 (author)2012-02-11

that is a nice amp you have there

(no phun intended)

author
msholden (author)robot7972012-02-12

Thank you for the complement. Still using it daily.

author
tiogaamplification (author)2016-05-20

Very nice amp! I have no logical explanation as to why the ancient design 6V6 sounds magical but it really seems to be a forgotten resource. Have had fun paralleling them for increased power but the singles still impress IMHO.

author
LowerT (author)2014-12-11

my eyes are still there, but failing fast, so for the life of me i cannot see the pots for the tone controls? could you direct me? very nice job sir.

aidan

author
msholden (author)LowerT2014-12-11

6V6 Pushpull Calss AB Tube Mono Amp. has no tone controls MSH Tube Preamp dose have tone controls. I hope this helps if not pleas reply

author
LowerT (author)2014-12-11

my eyes are still there, but are failing fast, so for the life of me i cannot see the tone control pots. could you direct me?

very nice job sir.

aidan

sorry if this post comes through multiple times. stumbling around here due to poor eyes

author
rexmaxus (author)2014-08-13

Do you happen to have the original schematic with the 25 watt transformer?

author
msholden (author)rexmaxus2014-08-15

author
msholden (author)2014-08-15

author
johnnyjcap (author)2012-12-05

I "must " have these specific pieces of equipment in order to build this amp?
not the hand tools, the audio gen. &PC OSCILLOSCOPE?
i'VE GOT EVERYTHING ELSE. Please Advise?

author
msholden (author)johnnyjcap2012-12-06

Hi

Pleased to here from you.
If you build as shown the amp will work fine without an "O" scope and without a audio generator.

author
johnnyjcap (author)2012-12-05

Nice Handsome Amp.

author
johnnyjcap (author)2012-12-05

"thinks to himself" as realization sets in. 'how am I going to be able to solder this?" I will need 3 hands, thats it! 3 hands. Problem solved!
Or my son, with the promise of beer when finished. When finished! steadier hand before rather than after! If & when I get off my ass & build something like this.
You have, dear Sir, created what I will now dream about for many nights until I get off my butt & do something.
Thanks, Thanks so much! J.

author
MolecularSandwich (author)2011-05-13

What was the total cost of the build?

author

As I remember ti was around $500

author
Xellers (author)2010-02-12

Excellent instructable! I am building something similar at the moment and am very impressed by your work!

author
msholden (author)Xellers2010-02-14

Hi Xellers,
Thanks for your observation.
Good to here from other tubies.  One thing you may or may not know is in the placement to the power transformer and the output transformer is you may reduce hum if you place one of the two bodies 90deg in respect to the other.  I think I will test this on my next project.  I think this was talked about here on this web site.

author
Xellers (author)msholden2010-02-17

I just finished designing and building a single-ended amplifier using a 8FQ7 medium-mu dual triode and a 5AQ5 power pentode (Almost identical to the 6V6GTA) found inside of one very old TV. Surprisingly enough, even though the tubes are very worn, its works amazingly well and there is no hum whatsoever!

TV Tube Amplifier.jpg
author
msholden (author)Xellers2010-02-17
Hi Xellers,
Interesting!  First I like to see an isolated power supply, good job.  You can also use the same idea less the rectifier circuit to make a bench isolated power supply.   This is real handy when working on tube circuits such as an American All 5, which has one side of the 120-vac line power to the chassis.  It seams that somewhere I saw a web sight, which premiered on using TV tubes for audio.
 
This is nice. Will you make this an instructable I am sure there would be interest in it.
author
Xellers (author)msholden2010-02-17

Thank you for your kind comment! First of all, I would like to admit that the isolation transformer was actually not included in the original design; when I was testing this little guy, I used an enormous 1kW isolation transformer for safety, but when I tried eliminating it in the final version, the voltage multiplier circuit repeatedly gave me trouble, so I settled for the current version. Also, right now, the entire thing is wired up on my desk and performs beautifully, but I can't find a chassis for it! What sort of enclosures would you recommend (keeping in mind that I can't really afford to buy anything big at the moment)? Also, can you please explain (or link me to) how you did the metalworking in your design? I have never used an aluminum chassis and would like to learn how.

Once again, thank you!
-Xellers

(PS: an instructable will come as soon as I order new tubes and sockets to build a better stereo version - I might even try replacing the 5AQ5 with a 6V6 if I have the money!)

author
msholden (author)Xellers2010-02-17

I have prepaired a small document which may help you in prepairing a chassis for your project.

Construction of an aluminum chassis for electronic project���.pdf
author
Xellers (author)msholden2010-02-17

Thank you so much! I never realized that this could be so easy - I always assumed that a big budget and a workshop full of professional tools was required!

author
Quercus austrina (author)2010-02-11

Have you tried a larger value input capacitor? I have seen the old schematics where the ("your") 0.1 uF is the standard for audio input, but this is usually very low level audio - between a detector stage and a preamp. Try a 1 uF (preferably a 10 uF) capacitor for the higher input levels you are dealing with. In my own designs, I have used 10 uF caps for inputs on both Mic preamps and power amps. This should help the situation.

Qa

author
Quercus Austrina,

Great catch!
Some times you cannot see the forest for the trees. The network of C1 and R1 form a High pass RC filter that has a cutoff frequency of 3 Hz. You are 100 percent correct the value of this component should be changed. I would like to keep the 470K resistor at its current value. By changing C1 to a 4.7MFD will improve the lower end frequency response considerably.
 
I will change the capacitor and then plot the frequency response on this page. I will also update the schematic
 
Thanks so much for your help
author

You are very welcome. Always willing to help a fellow enthusiast. Can't wait to see the new plots.

Happy listening!

Qa

author

"Some times you cannot see the forest for the trees."

Haha, just caught that one. Did you "Google" my nick? If not, check it out. You'll get a laugh out of it. :)

Qa

author

Quercus austrina,

No I am not that clever.  I had no idea yor nick is the name of a tree.

author
Re-design (author)2010-01-03

Excellent intructable.  Pro. quality build also.  Photos good and the freq. resp. graphs are a nice touch.  Shows us something we can't hear.

Very good job.

author
msholden (author)Re-design2010-01-03

Re-design,
Thank you for your kind observations. "Re-design" interesting name.

author
cerwinthedoc (author)2009-10-27

Wow, great tutorial.

Just one question though, would this amp be suitable for guitars? I have been interested in building a tube amp for quite a while, but I would be using it mainly for guitar.

author
msholden (author)cerwinthedoc2009-10-28

Hi! cerwinthedoc,
I do not play a guitar but I did apply one to this amp.  This power amp has more gain than a power amp normally has.  It sounded OK to me but you would need a pre amp to put between the guitar and the power amp.   This amp is more targeted toward HiFi applications.  When this amp starts distorting or overdriving the sound does change.  I cannot tell you if this is what you arte looking for.
Thanks for your intrest.

author
Normjr (author)2009-08-29

This has to be, so far, bar none, absolutely, positively, One of THE BEST Instructables that I have EVER been to. It is also the most professionally documented. The photographs are crisp and clear and are provided not only at the most important steps, but also many of the intermediate steps. The instructions are easily understood, easy to read, with proper grammar and correct spelling. It, in nautical terminology, “Blows all of those mint box instructables clear out of the water and sends them all to Davy Jones' Locker”. msholden you certainly have done one “bang-up” job, you are the only second instructor that I have subscribe to. This Instructable, in my not so humble opinion also deserves the “Grand prize of Grand Prizes award” if there is one.

author
msholden (author)Normjr2009-08-31

Normjr, Thanks for the vote of confidence. This is my first attempt with an Instructables. I have several other projects which will be posted in the future. Hope to improve as I go. Thanks!!

author
student.mckinney (author)2009-07-21

I would imagine that the last time you touched vacuum tubes, you got a nasty burn on your finger....

author

It does not take me long to look at a hot tube.

author
msholden (author)2009-06-30

gmoon, All amp components were chosen around the 6v6 tubes. The power supply will supply all that the 6v6 tubes can handle. Supplying 590 mv to the input of the amp will provide the maximum load the 6v6 tubes can handle. The 6au6 sharp cutoff tubes perform well in this amp. I have had no problems with microphonics. I am not an expert in vacuum tube output topology. I thought of this amps output topology as having two complementary darlinton out put.

author
gmoon (author)msholden2009-07-01

The typical output for a cathode-biased, push-pull pair of 6V6's is 12 watts. A fixed-bias 6V6 PP amp could probably output 14 to 18 watts. The max output of the tubes has more to do with the B+ voltage and the bias voltage, than the input signal. Although choking off the input signal will reduce the output, but that doesn't seem to be the case here.

Remember, push-pull Class AB amps can output more current since one tube is usually "resting" sometime during the cycle (just check a 6V6 datasheet for PP output power.)

The output is certainly complementary (push-pull), but tubes are voltage-driven, not current-driven like a bipolar transistor. So a tube has high input impedance by nature--it's input impedance isn't dependent on the load. The gain stages in tube amps are voltage amplifiers, rather than a current amplifier like a darlington.

10 watts is a reasonable figure if the tubes are run at a lower voltage, of course.

Small-signal pentodes have a reputation for microphonics, (which is why they are generally not used anymore in guitar preamp stages.) I have a couple old "radio tube" guitar amps that use a 12AU6, and they sound great. But at higher volumes it might be a problem. I'm glad you're not experiencing any problems.

Your amp uses the second stage of the 12ax7 as a cathodyne phase inverter. Since the bottom half of the phase-inverter is is a cathode-follower, a cathodyne PI is a "unity gain" stage (no amplification.) Each phase of the PI is fed to a different 6AU6 for more amplfication before the power tubes...

This topology is very unusual. Generally the last stage before the power amp is the phase inverter. Your amp has a gain stage, the phase inverter, then a gain stage for each phase (the pentodes), after the phase inverter. Then the power stage, of course.


BTW--nice job, I like the build.

author
msholden (author)gmoon2009-07-01

gmoon, Please bare with me. Step 14 has an attached PDF which I got the Graph and placed below. The pdf is quite large but all has much more resolution and all is clear. This amp will put out 13.7 RMS watts at max. The data sheet for the 6v6 for a class AB1 push pull says max out put 14w. The power supply is rated 150ma at 339v which exceeds the max plate voltage of 315vdc. Could you be thinking 6L6 instead of 6V6. I have reviewed other class AB1 push pulls and as you said the output topology is different in this amp. You have a great knowledge of vacuum tubes which I admire. I have a guestion for you. In this amp the plate voltage is suposto be the same on both 6AU6 plates and it is not. One is 231v and the other is 182v. I switched both 6au6 tubes and the problem follower the tubes. I bought two more 6au6 tubes and it appears that depending on the performance of each tube will affect the two plate voltages. All most like some thing is floating. Thanks your comments. Please keep in mind that this amp was designed for HIFI and when I rated the output I always select the point of the start of ringing as the out put. I do list max output (over-drive) for those who like to work with the distortion.

capture30.jpg
author
gmoon (author)msholden2009-07-01

Please don't take my comments as criticism; I'm merely musing at an unusual design.

Although the datasheets spec 315V max, it's not too unusual to run NOS 6V6's at a higher voltage (and power output.) Back "in the day," if you had to replace the tubes every 4-5 months, it was no biggie (at least when searching for "that sound" with guitar amps.)

Would I recommend running so far out of spec? Nope, I'd stick to 12 watts max. (a cathode-biased 6L6 pair might output ~16-20 watts, depending on the B+ V.)

Re: the 6au6's--I'm sure there are differences, tube-to-tube (transconductance, etc.) Translating that to the PP power tubes, it's variations like that that call for "matched pairs" and POT adjustments of the bias voltage (if it's fixed.) A cathodyne phase-inverter isn't very symmetrical to begin with--so you're not necessarily feeding it a perfect signal to start. I suppose it's possible to carefully balance the 6AU6's (by adjusting the cathode-bias resistors, R16 and R17), but that might be fighting the very nature of the cathodyne PI...(and be defeated every time the 6au6's are replaced.)

That's what makes me wonder about the overall design--following a cathodyne PI with pentode gain stages. Small-signal pentodes are known for having even more gain than triodes, so a single pentode gain stage might be equal in gain to two triode stages. Since the phase inverter is so far away from the power stage, that could magnify any asymmetry. None of that would matter much for a guitar amp, but for hi-fi??

Since cathode-biased amps generally aren't as powerful as fixed-biased maybe they just figure anyone using it would just ease back off the input if it starts to distort, and that would enforce the 10 watt limit (?)

Here's a link to the Kalamazoo Bass 30, which has the same topology, although the PI halves are followed by a triode, not a pentode stage before the power tubes... (it's got a pot to balance the power tube bias, too. But that's probably more critical with fixed bias than cathode bias like yours.)

author
gmoon (author)2009-06-30

Interesting design. I have a Kalamazoo bass amp that also has two amplification stages after the phase inverter (which is not the usual way to do it)--although they are triodes, not pentodes like yours. Do you find that the 6AU6's are microphonic? (Based on my experiences, I don't think I'd have gone with a smaller OT. The 6V6s must be running cold with those cathode-bias resistors, and maybe that's keeping the power down.)

author
lemonie (author)2009-06-28

I love the "mess" in steps 7 & 9, reminds me of my old Cossor radio. Lovely. L

author
msholden (author)lemonie2009-06-29

lemonie, I always say to my self that I will keep the wiring neat. As I start wiring neatness becomes less important as I continue wiring. By the end of wiring my mind has shifted to the testing phase and sometimes I look and wonder where my promised standard went to.

author
lemonie (author)msholden2009-06-29

It's natural and it looks good. There maybe something nice about regimenting things into grids, but I like the functional arrangement of this old technology. L

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