Tube Bias

[dawsonaudio]

Sorry if this is a redundant post, but could I get a little help on adjusting the bias on my new silvertone 1482 diy amp? It currently has a 250 ohm cathode resistor. B+ is 385vdc, pin 3/plates on the 6v6s are at 376vdc and the cathode resistor measures 24.1vdc. 360vdc is what I'm measuring on pin 4 of the 6v6's. It has been a while since my last build using a cathode resistor so I could use a little refresher course on how to adjust the bias. I'm thinking that the tubes are drawing a little too much current.

Thanks for the help here.

Nate

[xtian]

Plug your plate value (376) and tube type (6V6) into this calculator:

http://ax84.com/biascalc.html

That will give you your target dissipation. Aim at 70% for fixed bias, or closer to 100% for cathode bias. So, 37mA is 100% for this plate voltage.

Next, we can ignore screen current for this rough estimate. Just plug in your cathode voltage and cathode resistance value (250 ohms) into Ohm's Law. This gives you 96.4mA, divided by two power tubes, which is 48.2mA, which is too high, as you suspected. Try a 360R cathode resistor, which is what worked nicely in my 1482.

[pdf64]

You will have a 'doh' moment when I point out that you may have forgotten to subtract the cathode voltage from the plate voltage, in order to get the effective plate voltage that's used for calculations.
So the relevant effective plate voltage here is 376-24.1 = 352.
That ax84 calculator assumes 6V6 to be the 14 watt GTA variant, which may not be the case; regular GT ones are usually a 12 watt plate.

[Firestorm]

There's very little point in running a push-pull amp much above 50% dissipation. The 70% figure published by Aiken is a MAX. Adjust until it sounds good.

[pdf64]

To avoid excessive squish, cathode bias tends to sound better when close to class A. Given the high B+ generally used, that often requires idle plate dissipation up near 100%.
From http://www.aikenamps.com/index.php/the- ... on-biasing
'If the cathode-biased amp is... class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.'

[dawsonaudio]

So with a 330ohm the cathode voltage is 26.2 divided by 330 is 79.4/2 is 39.7 mA. Is that the correct method to calculate. Plate voltage is now at 387. This still seems a little too much current. My concern is that the plate voltage will jump too high. I'll try a 360ohm resistor tomorrow. I just found a 390ohm in my stash...not sure what that will do.

[Smokebreak]

What brand of 6V6s are you using?

[dawsonaudio]

These ones are 6v6EH's. I also have a set of Hammond 6v6gty's that have a brown base to them. I was going to try those out as well.

[xtian]

You will have a 'doh' moment

DOH!

Pete wins!

[dawsonaudio]

Installing the 390ohm cathode resistor took the plate voltage to 390vdc and the cathode voltage to 27.7 which is around 35.5mA's per tube. So inputting the plate voltage minus the cathode voltage at 362.3 would give put me in the low 90% range.

Not sure how to get it down much lower than that or if I really need to. Looks like the EH 6v6's can handle up to 450vdc. Not sure about the Hammond 6v6gty's.

Thanks for the help here.

Nate

[pdf64]

Yes, as cathode resistor value is increased, current draw will reduce, and due to B+ sag, plate voltage will rise.
Vintage production 5Y3 might help to bring down the VB+ a little, which would be useful.

[xtian]

Installing the 390ohm cathode resistor took the plate voltage to 390vdc and the cathode voltage to 27.7 which is around 35.5mA's per tube. So inputting the plate voltage minus the cathode voltage at 362.3 would give put me in the low 90% range.

Not sure how to get it down much lower than that or if I really need to. Looks like the EH 6v6's can handle up to 450vdc. Not sure about the Hammond 6v6gty's.

Thanks for the help here.

Nate

I think you're in the "if it sounds good, it is good" neighborhood.

[Smokebreak]

These ones are 6v6EH's. I also have a set of Hammond 6v6gty's that have a brown base to them. I was going to try those out as well.

Sure, check out whatever you have. Keep in mind most 6v6 are 12W, except the JJs which show 14W max.

[dawsonaudio]

Would adding an inductor/coil at the negative side lower the b+? I'm trying to remember how I hooked up my field coil speaker on my supro 1624 build. I'm pretty sure it dropped the voltage quite a bit. I have some small inductors that came from a Hammond Vibrato Line Box that measure 440ohm and look like little hockey pucks. I think they measure 450mh. Not sure if I could use one of these to lower the b+.

[pdf64]

No, my guess would be that such small signal inductors would overheat and short,as they're unlikely to be capable of supporting substantial current, eg full B+. Also they may not be rated for high voltage use.