Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: musicfixerupper on March 02, 2011, 04:16:33 pm

Title: hum gets louder when PI tube is removed
Post by: musicfixerupper on March 02, 2011, 04:16:33 pm

Hi, i am working on a super reverb clone and am having problem with a hum that gets louder when the PI tube is removed. Seems weird that it get loud when the tube is removed.

Title: Re: hum gets louder when PI tube is removed
Post by: LooseChange on March 02, 2011, 04:34:26 pm

At least it does not hum when its off!  :smiley:
Not sure what is ther problem but I can understand the hum being louder with the tube pulled.
Something is obviously wrong with the PI wiring or the voltage supply.

Title: Re: hum gets louder when PI tube is removed
Post by: octal on March 02, 2011, 06:12:51 pm

I would start by checking the bias supply for ripple and correct DC voltages on both control grids of the output tubes. (I think LC was getting at this.) When the PI tube is plugged in, its plate resistance can serve like a voltage divider to reduce hum that is back-fed in from the bias feed resistors going to the output tubes.

Has this build ever worked properly?

Title: Re: hum gets louder when PI tube is removed
Post by: sluckey on March 02, 2011, 06:31:45 pm

Quote

Hi, i am working on a super reverb clone and am having problem with a hum that gets louder when the PI tube is removed. Seems weird that it get loud when the tube is removed.

Here's an idea to consider...

The hum may be caused by a big imbalance in current flowing in the OT primary. May be poorly matched tubes, bad tube, or a faulty OT, or biasing that causes one tube to conduct more than the other. When you pull the PI tube, you break the NFB loop causing the gain of the output stage to increase, which makes the hum louder.