AX84 Variable Bias

[DeepBlue]

OK I will post my specs in a few. I just wanted to say I did put a 370k resistor before the grid stopper to ground, but I will remove this and place it on the grid directly to ground. I will get back to you. Someone has commented it sounds like my pot has DC current on it as it scratchy…my MV pot I mean…

[DeepBlue]

Ok so the grid is at 30V. While the Pin 8 (Cathode is at 28V. I am thinking perhaps I should just try to put the grid leak right off pin 5? Right now I have a 370K Resistor running from the MV side of the grid stopper to ground. I would have thought this would have been my ground reference for the grid, but perhaps something is wrong? I will try perhaps a bit lower say 220K and directly to ground from the pin on the OT side of the grid stopper this time.

[DeepBlue]

So just one more update…after measuring the DC across all cap’s in my tone stack, I see that both the Mid and the Bass are passing almost 130V of DC across them! I believe that this should not be so am I correct? I am planning to switch out the cap’s as well as ad a grid leak from pin 5 to grnd using a 220K resistor. If anyone could confirm that in fact the DC across my cap’s in the tone stack would be incorrect. It seems that this would then run through my volume pot and cause the scratchy sound I am hearing? So I am about to swap the caps out as well as ad the grid leak.

[beelzebum]

The mid, bass, and treble caps should all be blocking about 130v. if you are reading 130v across them, then they are blocking the dc from pin 3 of your cathode follower. This is what you would want to see. If you read 0v across any of those then that is a problem. That would mean that it was shorted. Take a voltage reading from your caps on the tone stack side to ground to see if there is any voltage leaking into your tone stack. also if you are reading 30v at you grid and only 28v at your cathode, then your bias voltage is positive 2v and you are melting your tube. If your cap is leaking voltage into your tone stack then it would also be dumping a positive voltage onto your grid. check your caps you should read a voltage across them, that means they are blocking your dc.

[DeepBlue]

Thanks Beez. You are so helpful man. I swear. Let me get back to you. I am working on things right now.

[DeepBlue]

Oh also I am so sorry, I mistyped. My Cathode is 27V and my Grid was 29V. Sorry for the mistype. I just rechecked quickly. That seems a very small bias though is it not? -2V?

[DeepBlue]

OK So I think I might have the problem. So the Treble Cap is showing only 8V DC across it, but then has almost 182V DC from the Tone stack side to ground. The mid and bass show the opposite. The show around 130V DC across them, as you mentioned was normal, and then about 5V DC to ground. So suspecting I should checking wiring and replace the capacitor on the treble cap.

[beelzebum]

If your grid is +29vdc and your cathode is positive +27vdc then your grid bias is still +2vdc. Normaly on cathode bias, you would have about +27vdc on your cathode and 0vdc on your grid. this would give you a grid bias of -27vdc, which would be fine. The goal for cathode bias is to keep the grid referenced to ground. Have as close to 0vdc as possible on your grid. You say you are at +29vdc.

[beelzebum]

Yes, it sounds like your treble cap is bad.

[DeepBlue]

Oh yes I did know that Being silly. Start getting things backwards after hours of numbers back and forth. Ok so let me check my treble capacitor and make sure its working. Ill get back to you on the voltages. Yes I thought the grid should be 0V.

[DeepBlue]

So now with bias pot at 0R. Readings such:

Va 372 V
Grid 0V
Cathode 26V

No DC on tone stack side at all, each tone stack cap reading Dc voltage across also.

After adjusting the bias to about 45mA the Cathode reads 37V. So just to be sure that things seem ok now…my bias pot is now at 500ohms and the voltage is 37V across it with 45mA of current. So its power across the pot is (37V * 45e-3 Amps) = 1.67watts correct? Do my numbers now seem better?

[beelzebum]

Everything looks good with those voltages. Actually there is even less wattage flowing through you bias pot because it is in series with a 220 ohm resistor, according to the schematic for the variable bias pot, which is carrying about a third of the load. To read the actual wattage on the pot you just read the voltage drop across the pot itself, from pin 1 to 3 and times it by .045 amps. The pot is probably dropping about 25v and the 220R is probably droping about 12v. That would mean your pot is only carrying about 1.2W. I hope everything works out well from here. If you have any more problems just post them up.

[DeepBlue]

Awesome! I think things are working, the amp sounds great, no scratchy pot , no change voltages or current with higher volumes. I cannot thank you enough Beez! So now after I enjoy this for a bit Im moving on to perhaps the 4-4-0 push pull amp! Thanks man

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