Ampeg B25 hum issue

[Axtman]

I cannot solve the 60 cycle hum issue in my Ampeg B25 amp. The hum is always present and is affected by the control knobs. The hum gets louder as the volume knob is turned up on each individual channel.

I replaced filter capacitors, swapped tubes, tightened grounding nuts on controls, etc. all to no avail.

What do you recommend?

[Axtman]

Here is the schematic for those of you playing along at home. ;-)

[trobbins]

Did it always hum?
Do you have no input jacks connected?
(a) Does the hum level change when the hum pot is adjusted?
Does it hum with a shorting plug used for the external amp input? Do (a) again.

[nickb]

What happens to the hum if both volume controls are set to zero?

Does the hum balance control have any effect?

If you measure the hum ACV across the speaker how big is it?

[Axtman]

trobbins,

I have had this amp kicking around in my basement for a while. I believe I bought it cheap because it was broken (hums).

The amp hums when nothing is connected to the input jacks. I cleaned and tightened the jacks and even tested them with a shorting plug.

I checked the resistance on the hum balance control and it is fine. The hum balance control does not alter the hum.

When the external amp jack is shorted the hum is reduced but not eliminated.

[Axtman]

Nickb,

If both volume controls are set to zero, the amp makes a small hum. If either (or both) volume control(s) is turned up the hum increases.

The hum balance control has no effect on the volume of the hum. this control does work. I check it and does lower/raise the voltage on the heaters.

The hum starts off at about 0.05vac and goes to about 1.5vac with all the controls up.

[trobbins]

One area to focus on is the output section, from the external amp jack onwards, as that shows up a hum.

Have you checked the 7027 output stage idle current balance using an add-in probe, or using added cathode sense resistors, or using the anode to B+ voltage drop and output transformer winding resistance method ?

Have you confirmed the feedback driver and PI stage operating dc voltage levels, versus the schematic indicated levels?

With respect to the input stages, the test with the volume control indicates noticeable hum is generated from the input stage. Are you able to do a temporary link from the input grids (K,L) to their individual cathode resistor ground ends ? That should isolate any issues that may occur back to the input jacks.

[Axtman]

You are not going to believe how I fixed the hum!

When I first started working on the amp I replaced the power cord to a 3 prong grounded cord and had removed the "death" capacitor. The way the earlier model B25 amps were laid out the power and polarity switches were located on the front panel and the standby switch was located on the rear panel. Since I had removed all the line switching wires, I decided to remove the switch entirely and move the standby switch to the front panel. I got tired of reaching back and forth to turn the amp on and take it off standby. It makes way more sense to have them on the front panel anyway.

Well guess what happened to the hum when I moved the switch?!

The standby switch is a really strange design. It grounds or lifts the center tap of the B+. On Fender amps the standby switch makes or breaks the B+.

I will be honest. I have no idea how the standby switch location would cause the hum. Any ideas?

[nickb]

The issue will have been the length and proximity of the wires to the standby switch to other sensitive areas. Those wires carry big pulses of current at 120Hz which will tend to interfere with whatever they are near.

Aside from moving away, you can help to mitigate the noise by twisting together the all three B+ winding wires and routing to the standby switch and then to the destination. The object it to keep the area of the current loop for that power circuit as small as possible.