Advice on a 6550 pp amp

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Advice on a 6550 pp amp

Postby jbesound » Sat Mar 26, 2016 3:21 pm

I bought a used 6550 amp and wanted to get opinions on ways to improve it. I reverse engineered it and have already changed a few things such as lowering the supply to the phase inverter from 406 to a safer 373. The 12At7 is only rated for a plate voltage of 300v and the 285v cathode to anode was a little close for me. The 3 10k resistors around the balance pot also seemed odd. I was hoping somebody could explain the reason for doing so. It is a stereo amp and I have included schematic for 1 channel. I am currently considering changing the biasing circuit 1 of 2 ways.

1. Removing the 10k resistors and adding 10k pots (1 per tube) to allow individual tube bias (I know I can do that now with the balance pot).
2. Switching to cathode bias by grounding the 220k resistors and using LM317HV's 1 per tube as current sources which would allow switching to KT88's without re-biasing.

Thanks for taking the time and providing your input.
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Re: Advice on a 6550 pp amp

Postby DeathRex » Sun Mar 27, 2016 10:41 am

I've been going through a pp6550 with purejazz, and was puzzled to find out that 6550's maximum control grid resistor should be 50K for fixed bias. Seems low, but that is what the datasheet calls for. Yours has a 220K control grid resistor, that would be too high and should cause problems with control grid voltage stability.

The 3 10K resistors around the bias pot is a ground return for adjustment, without them the bias balance pot wouldn't do anything. You should have a bias voltage pot, and along with the bias balance pot, it's almost like having individual pots. It had better be a good pot, if the wiper doesn't make a connection, your bias goes from -55 to ground. :(

The 12AT7 looks fine. I forgot what kind of phase splitter that is called. I like Shannon's LM334Z. You could put one in there, since it's already capped off.

Purejazz is working on a RCA SA1004. http://www.diyaudio.com/forums/tubes-va ... build.html
Look at post #4 for a schematic.
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Re: Advice on a 6550 pp amp

Postby jbesound » Tue Mar 29, 2016 12:30 am

Current plan is to implement a Poseidon driver as suggested. MKIII amps didn't have screen grid resistors whereas mine has 1k. Thought of dropping them to 100 ohm. Any thoughts?
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Re: Advice on a 6550 pp amp

Postby DeathRex » Sat Apr 02, 2016 9:00 am

That schematic hopefully isn't drawn correctly. You might have the 2 screens reversed, and the cathode is connected to the suppressor grid.

If you change the amp to self bias, the tubes would usually want to see a higher resistance load, than with fixed bias, and power will decrease.

You can also run 6550s in triode mode, but you'll have to decrease B+ to 440.
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Re: Advice on a 6550 pp amp

Postby jbesound » Thu Apr 14, 2016 8:51 pm

Rebuilt the amp with a Poseidon front end with the feedback and voltage amp resistor and capacitor changes for Hammond 1650N OPT's. Sounds great and the driver is simpler than before. Thanks Shannon Parks for a great design.
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