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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.
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.
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Signing UpStep 1Circuit 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
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
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(no phun intended)
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.
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.
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!)
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
Great catch!
Happy listening!
Qa
Haha, just caught that one. Did you "Google" my nick? If not, check it out. You'll get a laugh out of it. :)
Qa
No I am not that clever. I had no idea yor nick is the name of a tree.
Very good job.
Thank you for your kind observations. "Re-design" interesting name.
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.
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.
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.
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.)