Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: aro on March 30, 2015, 09:03:57 pm

Title: Different voltages in the same filter cap \ Bias
Post by: aro on March 30, 2015, 09:03:57 pm

With one of my Marshall 2204 clones on the operation table, waiting for a filter cap replacement, I borrowed another 2204 clone to play.

As I was checking it, I saw that one of the filter cap lugs shows 370 V and the other lug 440 V. The other filter cap shows 310/320.The voltage at the HT fuse is 440 V.

Is this difference between filter cap lugs normal?

Also, the bias pot doesn't have enough range to get to 40mA. What can I do? Do I have to adjust the bias resistor? Thank you.

Title: Re: Different voltages in the same filter cap \ Bias
Post by: PRR on March 30, 2015, 09:33:58 pm

It is 3 caps in one can doing 3 jobs.

It is normal to find different voltages. The 440V may be the main raw power. 370V may be the driver/PI. 310V would be preamps.

It is like how I filter my dirty well-water. I have a tap for flushing right out of the well. Then I have a screen-filter and a tap for the garden-hose which can be a little brown. Then I have a string-mesh filter which gives the "clean" water for the house. And likewise the flowing pressure drops from raw/straight to garden-hose to house water.

Title: Re: Different voltages in the same filter cap \ Bias
Post by: aro on March 30, 2015, 09:46:10 pm

Thank you.

Now I'm just wondering what to do about the bias. I remember years ago I had this issue in another Marshall clone - not enough range - and I solved it with, I think, a resistor somewhere in the bias circuit. I sold that amp though so I can't check what I did... and for the life of me I can't remember.

Title: Re: Different voltages in the same filter cap \ Bias
Post by: aro on March 30, 2015, 10:59:42 pm

When I look at this schematic, which resistor would I have to change (to a higher value)?

http://mhuss.com/MyJCM/JCM800_2204.gif (http://mhuss.com/MyJCM/JCM800_2204.gif)

I think it should be RB1 (47K), RB2 (220K), or the 15K between them. RB1 looks like a good candidate, but not 100% sure...

Title: Re: Different voltages in the same filter cap \ Bias
Post by: aro on March 30, 2015, 11:20:20 pm

Well, from what I read online, RB2 sets the range of available bias voltage, so I'll lower that one and see how it goes.

Title: Re: Different voltages in the same filter cap \ Bias
Post by: aro on March 31, 2015, 09:55:57 am

RB2 was the one. I put a 27K in series with it and it's all good.

Title: Re: Different voltages in the same filter cap \ Bias
Post by: EL34 on March 31, 2015, 02:50:23 pm

Make sure your bias caps are up to snuff also
If they are failing, they can change your bias voltage