What should B+ on 300B push-pull be?

[andy2]

I have a 300B tube amp in push pull config. It uses 2 300B tube per channel and has 22W output per.

The B+ voltage is measured at around 520V. Is this too high or is it just right?

[tubelab.com]

Without knowing anything about your circuit, it is imposible to tell. The maximum plate to cathode voltage for some 300B's is 400 volts, some 300B variants can go higher. If your amp uses direct coupling, and, or cathode bias the total supply voltage will be higher.

My 300B push - pull runs at 380 volts. I can get 25 watts out of it before distortion sets in, but it is no longer in class A at this point. Class AB begins at about 14 watts.

[alexmoose]

How does a 300b pp amplifier sound? I have never heard push-pull triodes before

[tubelab.com]

I got the idea after reading an article written in a magazine (VTV, I believe) by Kevin Kennedy (kevinkr). I simplifed his circuit and then modified it to work with tubes and parts that I had on hand. From the moment that I plugged the amp in I knew that I had something. This amp has the ability to ROCK loudly, yet retains most of the detail associated with the 300B. I have built dozens of amps since then, both SE and P-P. Many of them were sold, modified, or dissasembled, but this one remains as it was when I built it. It has a sound that I have yet duplicated. I tried to improve it by swapping OPT's and coupling caps, but it didn't help, and often hurt the sound. So I put it back to the way it was when I built it, including the $20 OPT's. It is rare when you just put something together, and it is perfect, but this was one of those amps.

I believe that the low plate resistance and low distortion of the 300B allow a zero feedback push pull amp that has enough power and great dynamics to handle any style of music with ease. I have spent the last two years trying to get the same sound with an SE amp, without total success. I have come close, but not close enough to allow the replacement of this amp.

I will try an update of this design in the future using many of the concepts that I have learned in the 5+ years since I built this amp, but I will not touch this one until I have duplicated ( and hopefully improved) the sound.

[andy2]

300B in PP configure has most the SET characteristics but better bass performance.

[andy2]

Originally posted by tubelab.com
I got the idea after reading an article written in a magazine (VTV, I believe) by Kevin Kennedy (kevinkr). I simplifed his circuit and then modified it to work with tubes and parts that I had on hand. From the moment that I plugged the amp in I knew that I had something. This amp has the ability to ROCK loudly, yet retains most of the detail associated with the 300B. I have built dozens of amps since then, both SE and P-P. Many of them were sold, modified, or dissasembled, but this one remains as it was when I built it. It has a sound that I have yet duplicated. I tried to improve it by swapping OPT's and coupling caps, but it didn't help, and often hurt the sound. So I put it back to the way it was when I built it, including the $20 OPT's. It is rare when you just put something together, and it is perfect, but this was one of those amps.

I believe that the low plate resistance and low distortion of the 300B allow a zero feedback push pull amp that has enough power and great dynamics to handle any style of music with ease. I have spent the last two years trying to get the same sound with an SE amp, without total success. I have come close, but not close enough to allow the replacement of this amp.

I will try an update of this design in the future using many of the concepts that I have learned in the 5+ years since I built this amp, but I will not touch this one until I have duplicated ( and hopefully improved) the sound.

Would you share the schematics of your 300B push-pull amp? Here is the one I got:
http://www.geocities.com/hai_vu1/schematic.JPG

Well if you prefer not to, would you see any thing on my schematics that need to be improved?

[Giaime]

Well, you could start by removing the negative feedback. Look under the first stage: at 6SL7 cathodes there is that network, you simply remove the 100k resistor, disconnect the cap from the two resistors and connect it to ground. This could give some improvement or worse sound, it depends: however I don't think it will change much since from a rapid look the negative feedback seem extremely low, practically non-existant.

If you prefer the sound with negative feedback, you could also try to increase it: replace the 100k resistor with a 75k, 56k, 47k, 33k whatever you like best.

Otherwise the amp seems well built: other mods could be done, but only by redesigning entirely the input and driver stages.

[tubelab.com]

Would you share the schematics of your 300B push-pull amp?

I would, but I don't have one! I didn't document my experiments very well when I built this one, and I really didn't expect it to turn out as well as it did. I have the schematic of the driver board and PC board layout in an old version of Eagle. Due to many requests I am planning to try to document the amp, but I haven't had the time to do it.

The design came from the VTV article that I referred to earlier. It is on Kevin's web site here:

http://www.kta-hifi.net/projects/amp...mp/300bpp.html

I started with that design, used a simple unregulated supply (against Kevin's recommendations), substituted 9 pin tubes (6SL7 became a 5751, and 6SN7 became a 6CG7) and used an SRPP stage for the driver (it was in vogue at the time). I would consider his amp a more robust design, although it is complicated.

I used cheap components, including some guitar amp output transformers, and I consider the amps success more luck than skill. I tried substituting some serious iron including some vintage UTC LS-57's but the sound became harsh. I think that the gentle rolloff in the OPT's above 15KHz compensates for the cheap caps, or something, but any attempts to tweak this amp have resulted in me putting it back to the way that it was. I finally found a set of Sovtek 300B's that are reliable, and this amp has been playing, untouched for over 4 years.

[lt cdr data]

550 volts with 100volts across the cathode resistor makes 450 volts, use a 5k6 output transformer.

Driver, use 5687 interstage coupled, and 6072 anode loaded, 100k resistor, no feedback.

best circuit there is.