Dr Z Mini hum?

[chuckb]

hi guys, i have a Dr Z mini with the attenuator that's giving me lots of loud hum. Not to sure if it's 120 or 60 cycle. I checked all the grounds and tightened all chassis grounds. I put new tubes in it and still the same. did a visual check of the board and all solder joints look good. no obvious signs of burnt or damaged caps or resistors. There is no change in the hum on the volume pot, although when the attenuation pot is moved through its settings, the hum lessens but i guess it would when attenuated. any help would be appreciated.

Chuck.

[DrGonz78]

So was this hum always present or something that has recently developed? Not sure how old the amp is but even then who knows when a filter cap is going to act up. Have you tested for excessive B+ ripple? Also, check the two 100 ohm resistors for the virtual center tap on the heater circuit. If one of them drifts in value that may effect hum cancelling ability and you get increased 60hz hum. Lastly you know the hum is not from the first preamp stage and the volume has no effect. So you would need to signal trace or scope from that point of the circuit to see if you can locate any problems in the signal path. Here are two schematics and please confirm for us how accurate these drawings are to your amp. For example, one drawing shows two 100 ohm resistors for the heater center tap and the other one has two 50 ohm resistors.

[chuckb]

thanks for the reply. no the amp wasn't always like this. the schematic with the 50 ohm resisters on the heater center tap is what is on this amp but there are as follows 16 ohm/475v- 22 ohm/450v- 100 ohm/350v filter caps. here is a pic of the inside of the amp.

[chuckb]

iv'e checked the voltages on the filter caps and are as follows, (A)-281v, (B)-278v,and (C)-261v. R15 resistor is 2.2k where the schematic has 5.6k. Except for the filter caps,(mine has 3 and the scematic has 4) and R15 is different. ,everything else on the schematic is the same.

[Dave H]

There will always be hum in a single ended amp with a power supply like this one. As a check try moving the OT wire from Point A to Point B to see if that reduces the hum.

[chuckb]

so you want me to switch A to B and B to A at the filter caps?

[Dave H]

so you want me to switch A to B and B to A at the filter caps?

No, just move the top red wire (not the one running through a hole in the board) from the right side of the 5W 500R Xicon over to the left side of the resistor where the yellow wire is connected.

[chuckb]

so your thinking it might be the 5w/500R resistor?

[Dave H]

so your thinking it might be the 5w/500R resistor?

No, I was thinking that with the OT connected directly to the first filter cap there wouldn't be enough filtering for an SE output stage. SE usually needs a pi filter. For a quick test you could move the wire to make it a pi filter and if that works perhaps add another RC (or LC) node.

[chuckb]

i did some voltage checks and here are what i got, (V1a)-p1=174v, P3=1.3v?(schematic shows 6.3v),V1b-P6=198v.
V2-p7= 268v, p3=8.1v. Looks like voltage on pin 3 on V1a is low according to the schematic.

[Dave H]

Looks like voltage on pin 3 on V1a is low according to the schematic.

1.3V looks right for V1a pin 3. 6.3V must be the heater voltage. The hum doesn't change with vol pot setting so it's not from V1a.

[chuckb]

I checked the bias using a Quadstage BiasPro MQ13 which is built using a 1ohm resistor where you measure the voltage drop across the resistor and it reads 45mv. which divided by 1 ohm is 45ma.

[mhuss]

Ouch. 261 volts time 45mA is 11.7 watts, right at the limit for an EL84 -- and today's EL84s do not like being pushed to their limits...

I'd try another EL84 and recheck the readings. If still similar, add another 20-30 ohms to the EL84 cathode and see where things stand.

[g1]

If it wasn't always like this, then first try other tubes as mhuss mentioned.
And for the power supply voltages you measured, also check the AC ripple.

[chuckb]

yes, i'm going to replace the filter caps and see if that is the problem. not to sure how to measure the ripple.