big volume drop with interactive volumes

[koen]

After a thorough check and lots of resoldering of my Mission Amps 5E3, I noticed that when I turn the unused volume past 10, there now is a lot of noise, and the volume almost drops to zero. I know that the volumes are interactive, but seems seems very drastic, and much more drop than before I 'serviced' my amp.

What's the most likely cause of this? Bad soldering at the volumepots, the inputjacks, V1, or maybe something else?

I've also noticed that with low tones (bass), I can hear crackling, but it sounds more electric than mechanic.

Other than that, the amp sounds just great, and is even quieter than before.

[pdf64]

Do both channels work ok otherwise?
Is it the same on both channels?
Have you tried a different tube (V1 12AY7)?
Confirm the Vdc on all the tubepins is ok.

[d95err]

Bad tube? Bad pot? Bad coupling cap?

[koen]

Thanks for your input and additional questions.

Both channels work ok otherwise.

I'll check the voltages this weekend, and will also try another tube in V1.

How do I know if a coupling cap is bad?

[Jazz P Bass]

Leaking Coupling Cap

With the amp on, at idle, measure the Vdc on both sides of the two coupling caps.
One side will be plate voltage.
The other side should be zero volts.

[koen]

Should I measure the Vdc on all pins with the tubes in their socket?

[pdf64]

For the purposes of this, yes.
I was thinking that maybe your resoldering exercise had overheated and damaged a resistor or cap.
If you could measure the Vdc with the controls in a 'problem' and 'no problem' settings, it may be useful.
Will only need the pre-amp tube voltages for this.
Some measurements may cause a loud noise from the speaker, so use a dummy load, or point it away from you and be prepared.

[koen]

Yeah, I'm familiar with the clicks from the speaker when using a DMM
Just as long as it doesn't damage the speaker I can live with it. I'll post back this weekend with the values.

Oh, should a guitar be plugged in for this?

[pdf64]

'Oh, should a guitar be plugged in for this?'

Best not to, unless it's volume is turned down. With the volumes that high it's likely to feedback, and the extra current demand on the amp's power supply (from putting out that signal) will lower the Vdc readings.

[koen]

Allright, had some time to look more at the amp. Here are some observations:

• It happens with both channels

• I tried several tubes in V1, it doesn't go away

• When I measure the value of the 1 MOhm resistor in the input jacks, I measure 0 (zero) Ohms for the Bright Channel, and 1 MOhm for the normal channel. When I plug in a cable, both measure 1 MOhm.

• The grid wire to pin 2 (normal channel) of V1 is very microphonic. The bright grid wire (pin 7) almost not. V1 is also microphonic (all tubes I tried are).

And the voltages I measure are:

V1
pin 1: 120V
pin 2: 0V, and can hear a radiostation
pin 3: 1.8V
pin 4,5 : filament
pin 6: 117V
pin 7: 0V, radio
pin 8: 1.8V
pin 9: filament


V2
pin 1: 146V
pin 2: 0V
pin 3: 1.2V
pin 4,5 : filament
pin 6: 179V
pin 7: 2.9V
pin 8: 44V
pin 9: filament


This is with all potmeters at zero, when I open them there's a very loud plop from the speakers.

[pdf64]

'It happens with both channels
I tried several tubes in V1, it doesn't go away '

Are you refering to the noise, volume drop, or both?
Does the tone control setting affect the volume drop?

Everything there seems normal, except for

'When I measure the value of the 1 MOhm resistor in the input jacks, I measure 0 (zero) Ohms for the Bright Channel, and 1 MOhm for the normal channel. When I plug in a cable, both measure 1 MOhm.'

The input sockets have shorting switches built in, so that the input gets shorted if nothing is plugged in.
It looks like the shorting contact on the normal channel either isn't properly wired up, or there's a problem with the actual socket/switch.
Have a close look into this difference between the bright and normal channel sockets.

We really need the Vdc measurements in the fault condition.
If the 'very loud plop' is a concern, connect the meter leads with the volumes low, then increase them to the settings where the problem would normally occur.
Now you see why a dummy load is a good idea for fault finding!

[koen]

What I meant was that the volume drop and the increasing noise occurs on both channels when the unused channel is turned up to 12.

I already stared at the sockets for several times, and didn't see any differences, but will do so again.

The coupling caps are fine.

I'll also measure the Vdc again in the fault position. BTW, how do I make a dummy load?

[koen]

The voltages on pin 2 and 7 on V1 are always 0, with any settings on the volume pots.

And did find an error on the input jacks, the switch part on Normal 1 was bad: without a plug the switch wasn't closed. Could this cause the issues I'm having? I don't have a replacement jack now, so cannot check if that is the case.

I was looking at the connections the whole time, not at the switch part, thanks for forcing me to look at it again.

To be continued...

[koen]

Quick update: I ordered a Switchcraft 12A jack and some 1MOhm resistors just in case their legs break off, should be here this weekend. In the meantime, I'll just use the bright channel.

[pdf64]

To make a dummy load, get a chunky resistor of the same resistance as the correct output impedance load for your amp, power rating about 2 x your highest power amp, for example see

https://taweber.powweb.com/store/resord.htm
RES8-50 $1.55 8 Ohms, 50 watt power resistor used for speaker loads, attenuators, etc

Either mount it to a heatsink or stand it off to allow air circulation (depending on the type of resistor).
A well vented enclosure is a good idea as it will get hot, may damage leads which accidentally touch it.
Connect it to amp either with a flying lead, or mount a jack socket on the box and use a speaker cable to amp.
See
https://amptechtools.powweb.com/truload.htm
for 1 way of doing it.

'The voltages on pin 2 and 7 on V1 are always 0, with any settings on the volume pots.'
How about V2?