• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

2 Volts AC on my bias supply

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#2
Probably not! LOL I assume you're using a CRC or CLC filter at least? What are the values?

FWIW in my designs, I use bridge rectification, and a CRC filter (like 100uF/2k/100uF) and then a zener shunt. No noise. I also used a half wave like you describe using a 6CA4 paralleled, and a 47u/4.5H/220uF filter feeding a 75V VR tube (SR75 I think). Also no noise 🙂
 
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#3
Dummy me. CR, forgot the second C. Point taken about the bridge rect. I'll do that next time or maybe this time if I cant get rid or the AC. I'm thinking one of the small high inductance low current Hammond chokes wouldn't hurt either.

Thanks Koda.

Steve
 
#4
Anytime, Steve!
Realistically, bridge rectification isn't necessary, but the diodes are smaller and cheaper than the larger filter components 🙂

If you're ever down in Toronto, A1 Parts has 4.5H chokes for $5 (Also, to whoever might read this: don't be a douche and go buy all the damn coils, please?) They seem to be good for about 50mA. Perfect for filtering bias and class A gain stages 🙂

58.jpg

Coil-58 on this page: Chokes & Coils
 
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#6
Koda, thanks for the tip on the small chokes, I've just ordered two. I've been to A-1 a couple of years ago. Got a few Hammond trannies on order from A-1.

No wonder I was getting hum, that 2 volts was going straight to the control grids of the OP tubes. Added caps after the R and it's much better but not perfect, a work in progress.

It never occurred until now that you need a really clean bias supply. Always learning.

Cheers, Steve
 
#9
Steve Morley said:
Koda, thanks for the tip on the small chokes, I've just ordered two. I've been to A-1 a couple of years ago. Got a few Hammond trannies on order from A-1.

No wonder I was getting hum, that 2 volts was going straight to the control grids of the OP tubes. Added caps after the R and it's much better but not perfect, a work in progress.

It never occurred until now that you need a really clean bias supply. Always learning.

Cheers, Steve
Click to expand...

A1 has the best prices for all things Hammond if you're in Canada at least.

Yup the bias supply needs to be clean, but because it's such a low power, it's easy to filter 🙂

Another idea is to make a delon doubler to get extra voltage, and then make R larger, but it's probably cheaper and easier to make C2 bigger or use a VR tube or zener diode in a shunt regulator 🙂
 
#13
Bias V at the diode and first C is -65. At the second C is -50. I need about -35 to -40 at the grids of the tubes. By the way, the amp is acceptably quiet now. I've been experimenting with grounding and found wiring error as well.

So this is the setup:
10 or so years ago I was given a Heathkit AA-100 (P-P 7591 pentode mode integrated with GZ34 rect.) I harvested the trannies and junked the rest though the original 7591s were still OK so I gave them to a friend for his Finale amp.

The amp I've built is still in a quasi breadboard state. I copied the Dynaco Doctor's driver circuit (6DJ8s) and these feed NOS CV391 fixed bias, pentode outputs. I suspect the CV's are going to be flaky and they don't really like the almost 400 volts I'm putting on their screen grids, they're rated for 300 max on G2.

The next step is to remove the loctal sockets for the CV's and replace them with octals so I can use some NOS GE 7581s I've had for a while. (This was my backup plan all along.) The GE's should take the high voltage without any problems. The CV's will live to fight another day in an amp running about 300 volts B+.

When the current amp is complete it will power my wife's system in our living room, displacing a Dynaco EL84 clone.

Cheers and thanks for the help, Steve
 
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