OK, SY put me up to this, so now its time to abuse the poor old 6AV5 in screen driven P-P. I thought I would mention my initial experiments here, and then I will start a new thread later when I have time to take this further.
I got this wild idea for a new Tubelab prototyping system, so I built it first. This one really does make it possible to prototype a tube amplifier without turning on the soldering iron, and all of the parts come from Home Depot and Radio Shack. But this is another topic that I will cover fully later.
Now that I had a new prototyping lab, I just had to try it out, so I built this amp. The first stage is a LTP phase splitter using a 6BQ7, CCS in the cathode, 10 mA total. Next is the driver. This one took 4 tries to achieve success. 4 tries, 4 different tubes, one hour total time, never touched a soldering iron. The final circuit uses a 5965, 550 volts of B+. It can pump out almost 400 volts P-P into a 100K ohm load (plus the gate capacitance of a fet) at 80 KHz. A mosfet follower drives the screen grids of the 6AV5's. At full power the screen grid swings from -60 volts to over 300 volts. The driver is still the limiting factor. I can't get any more swing because there isn't any more knob left on my power supply. It is maxed at 550 volts. The output stage runs at 550 volts on the plate, and adjustable negative bias on G1. G2 is also adjustable with a second power supply. A third power supply provides -25 volts for the CCS in the first stage. I will draw up a schematic next week.
Does it work? Well, I put in a pair of the wimpy slotted RCA 6AV5's, and tried to make them glow! Operating right on the edge of class B, I can get over 80 WATTS at soft clip! There is NO GLOW visible in the tubes with the room lights off! At 50 watts the distortion is under 3% from 100 Hz to 25 KHz. There is NO FEEDBACK local or global in this amp. Operation below 100Hz is limited by saturation in the OPT. I ran this thing at clipping for an hour, the hottest thing on the bench was the load resistor.
I have not listened to this amp, or made detailed measurements yet. I need to find a better OPT. I am using one from a guitar amp of unknown impedance. I get best results with the 8 ohm load on the 4 ohm tap, indicating the need for a pretty high load impedance.
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I am in the process of documenting the next generation Tubelab prototyping system. I will put it on my web site when I am done.
I bought some (about 200) Schumaker transformers from a guy who was selling them on Ebay about 5 years ago. They were surplus from the closing of the ADA guitar amp company. They are exactly the same physical size as the ones you mention, but they are 6600 ohms. I have been using them for amps ever since.
The transformer that I used here was not one of these. The same guy gave me this one, and said he had more if I wanted them. I have not tested it out. It was the first transformer I found when I was building this amp, so I used it. It is not a Schumaker.
I have found a known transformer, and I am planning to try it later.
Tom, I would like to see the spec for the Harris monster, so I will send you an e-mail. That project (833A-SE) is on hold for lack of 2 equally monster OPT's.
I bought some (about 200) Schumaker transformers from a guy who was selling them on Ebay about 5 years ago. They were surplus from the closing of the ADA guitar amp company. They are exactly the same physical size as the ones you mention, but they are 6600 ohms. I have been using them for amps ever since.
The transformer that I used here was not one of these. The same guy gave me this one, and said he had more if I wanted them. I have not tested it out. It was the first transformer I found when I was building this amp, so I used it. It is not a Schumaker.
I have found a known transformer, and I am planning to try it later.
Tom, I would like to see the spec for the Harris monster, so I will send you an e-mail. That project (833A-SE) is on hold for lack of 2 equally monster OPT's.
Now back to the 6AV5 in SE mode. I decided to perform some basic measurements on the Edcor - 6AV5 combination. I discovered (while playing around with the 6EM7) that the Edcor transformer really likes cathode feedback, so I tested that too. I set an arbitrary 5% test limit for distortion.
In triode strapped mode I got 4.75 watts at 5% with 300 volts and 60 mA. Adding cathode feedback allows 8 watts at 5% using the same voltage and current. Distortion at all power levels is reduced considerably.
In UL mode I got 6.3 watts at 5% with 300 volts and 60 mA. Cathode feedback allows 9.0 watts without adjusting the power supply! This is like adding a second tube.
I have not listened to this yet, but I will soon. It has been my experience that when cathode feedback works, It sounds good. It tightens up the bass without killing the sound.
In triode strapped mode I got 4.75 watts at 5% with 300 volts and 60 mA. Adding cathode feedback allows 8 watts at 5% using the same voltage and current. Distortion at all power levels is reduced considerably.
In UL mode I got 6.3 watts at 5% with 300 volts and 60 mA. Cathode feedback allows 9.0 watts without adjusting the power supply! This is like adding a second tube.
I have not listened to this yet, but I will soon. It has been my experience that when cathode feedback works, It sounds good. It tightens up the bass without killing the sound.
Every once in a great while you stumble across a combination that works way too good considering what it is made of. I knew that I was on to something last night when I tried the 6AV5's through the Edcors in UL mode with cathode feedback. The measured performance was good.
Well when I got home from work today Sherri was not home, so I connected up the speakers and cranked it up. This thing positively ROCKS, I played a lot of different music, the CD's that I have previously identified for their abilities in finding amplifier flaws. Complex music pushed well into clipping still sounds good, there is NO intermodulation from heavy bass guitar over a nylon stringed acoustical guitar. Voices and saxes do not get affected at all by drums or bass even with the volume knob all the way up. I can tell by the bouncing current meter that I am not in class A any more, but distortion can only be heard on a few heavy bass notes, and it doesn't sound gross.
There is no bleepin way that I am listening to 3 dollar tubes through 18 dollar transformers. This thing has bass. Crystal clear highs too. I got to run a frequency response test.
Surprise! 3db points are 24Hz to 45KHz AT FULL POWER! 8 WATTS.
Yes I am still using a regulated power supply, and a 5842 driver with PowerDrive. The next step will be to try the real power supply, and try to find a 3 dollar driver tube. When I started using 5842's they were $3, I bought 75 of them. They are not cheap any more.
If I am succesful this could be the ultimate "El Cheapo SE".
Frequency response plot at 8 watts is included. The vertical scale is 1db per division.
Well when I got home from work today Sherri was not home, so I connected up the speakers and cranked it up. This thing positively ROCKS, I played a lot of different music, the CD's that I have previously identified for their abilities in finding amplifier flaws. Complex music pushed well into clipping still sounds good, there is NO intermodulation from heavy bass guitar over a nylon stringed acoustical guitar. Voices and saxes do not get affected at all by drums or bass even with the volume knob all the way up. I can tell by the bouncing current meter that I am not in class A any more, but distortion can only be heard on a few heavy bass notes, and it doesn't sound gross.
There is no bleepin way that I am listening to 3 dollar tubes through 18 dollar transformers. This thing has bass. Crystal clear highs too. I got to run a frequency response test.
Surprise! 3db points are 24Hz to 45KHz AT FULL POWER! 8 WATTS.
Yes I am still using a regulated power supply, and a 5842 driver with PowerDrive. The next step will be to try the real power supply, and try to find a 3 dollar driver tube. When I started using 5842's they were $3, I bought 75 of them. They are not cheap any more.
If I am succesful this could be the ultimate "El Cheapo SE".
Frequency response plot at 8 watts is included. The vertical scale is 1db per division.
Attachments
Wow, and that's with the 5k transformer? What do you think would happen if you used a 2.5k transformer instead? This is very, very exciting... How about a trioded 16gk6 as a driver? They're about a buck a piece off of Ebay. I got a bunch to try some PP stuff but I've seen at least one schematic using it as a driver. Hmm, now that I think about it, the 6bq5 was being used to drive some other 6bq5s. The 6av5 is probably harder tube to drive. Oh well, never mind... 6SJ7? There's got to be a decent, cheap pentode that would fit the budget...
Isaac
Isaac
Damping factor, I can measure that. If I can pry my ears away from it. Next time Sherri is watching TV and I can't make too much noise, I will measure it.
I am still working on this design, but at this point it is a TubelabSE with a 6AV5 wired in place of the 300B using clip leads (don't try this at home, we are trained professionals). Edcor transformers and an external power supply are also wired in.
I am going to see just what parts are needed to make this sound. The first experiment is to lose the regulated power supply.
I have been using PowerDrive which can drive anything, so I don't know how hard the 6AV5 is to drive. More experiments are needed.
I am going to try tubes that I have lots of first
When I have some time I will put together a schematic.
I am still working on this design, but at this point it is a TubelabSE with a 6AV5 wired in place of the 300B using clip leads (don't try this at home, we are trained professionals). Edcor transformers and an external power supply are also wired in.
I am going to see just what parts are needed to make this sound. The first experiment is to lose the regulated power supply.
I have been using PowerDrive which can drive anything, so I don't know how hard the 6AV5 is to drive. More experiments are needed.
I am going to try tubes that I have lots of first
When I have some time I will put together a schematic.
You might pick a driver from the data here: http://www.pmillett.com/pentodes.htm - 12HL7 looks particularly good, but most any of them would do, and they're (mostly) cheap TV tubes.
I have read his work, and collected a large pile of video output tubes to test. Many of them are on his list. I have also called the tube vendor that I have known for years, to find out which ones are commonly available. Some of them like the 12HL7 are cheap, but not always available. Yes Antique Electronic Supply had them on sale last year for $0.70, but I called them the day that I got the flyer only to learn that one person had bought them all! I have a large box (thousands) of 6AK5's, a little VHF pentode. Time to run some tests of my own.
I was thinking along the lines of a dual triode, one half for each channel. This would be closest to what I already use. There are lots of TV tuner triodes that are cheap and readily available. Unfortunately the price goes up if they get talked up by the audiophile community. Case in point, the 6AQ8.
I have had good luck with the 5670 (got boxes full of those too), but the gain might be a little bit short for this application.
I need to find a good single tube driver, since I have been using the 5842 for everything, and these have gone from $3 to $30 in 5 years. Time to find a new tube. I plan to do some serious testing after I get back from a 2 week trip in April. Until then I am at work late most nights, last night was an exception, I left early at 6:30.
I was thinking along the lines of a dual triode, one half for each channel. This would be closest to what I already use. There are lots of TV tuner triodes that are cheap and readily available. Unfortunately the price goes up if they get talked up by the audiophile community. Case in point, the 6AQ8.
I have had good luck with the 5670 (got boxes full of those too), but the gain might be a little bit short for this application.
I need to find a good single tube driver, since I have been using the 5842 for everything, and these have gone from $3 to $30 in 5 years. Time to find a new tube. I plan to do some serious testing after I get back from a 2 week trip in April. Until then I am at work late most nights, last night was an exception, I left early at 6:30.
tubelab.com said:
I have hoarded about 20 NIB 6CL6's and half as many new 6AG7's to try as plate coupled drivers. These tubes will swing a lot of volts into quite a low plate load impedance. Have you any experience with these two types yet?
I have a SET amp in development that uses eight 6080 triode sections (4 bottles) in parallel in A1. Miller is pretty high. I was thinking of trying the 6AG7 since it is an octal and I would prefer that in this particular amp design for mainly cosmetic reasons. No miniature tubes, too new looking.
I have plenty of both types, but I haven't tested them yet.
I have a warehouse with about 100,000 tubes in it. Mostly odd voltage TV tubes, and WWII era military surplus, none of them individually boxed. I have been slowly trying out the types that I have lots of for audio projects. At my current rate it will take more than a lifetime just to sort them all. The previous owner spent a large portion of his life organizing them. When he passed on, the company that cleaned out the warehouse just dumped everything into 35 large boxes. Many were broken. They were given to me in exchange for a few days of work unloading the trucks (lots of WWII era surplus).
Before you all ask, NO there were no audiophile tubes in the lot. The original owner sold surplus (including tubes) so any tube that was worth money was sold long ago. I have at least 1000 (probably more) 6AL5's and almost as many 2D21's. A few boxes are full of old thyratrons and HV rectifiers. What can you make with these?
Got lots of 6AS7GA's too, these were not yet popular with the OTL crowd, I guess. They are dirty, and many have loose bases, but I will get around to playing with these sooner or later.
I have a warehouse with about 100,000 tubes in it. Mostly odd voltage TV tubes, and WWII era military surplus, none of them individually boxed. I have been slowly trying out the types that I have lots of for audio projects. At my current rate it will take more than a lifetime just to sort them all. The previous owner spent a large portion of his life organizing them. When he passed on, the company that cleaned out the warehouse just dumped everything into 35 large boxes. Many were broken. They were given to me in exchange for a few days of work unloading the trucks (lots of WWII era surplus).
Before you all ask, NO there were no audiophile tubes in the lot. The original owner sold surplus (including tubes) so any tube that was worth money was sold long ago. I have at least 1000 (probably more) 6AL5's and almost as many 2D21's. A few boxes are full of old thyratrons and HV rectifiers. What can you make with these?
Got lots of 6AS7GA's too, these were not yet popular with the OTL crowd, I guess. They are dirty, and many have loose bases, but I will get around to playing with these sooner or later.
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