Trouble shooting weird bias voltage on a 102 build

[mgotoo03]

Hey all,

I've been trying to chase down an issue with my 102 build that happens after the amp is played through for 5-10 minutes. All readings (initially) seem reasonable (6L6 344V plates, Bias at 43mA). After playing a bit the sound cuts out and I'm measuring -120ish(!) volts on pin 1 of the 6L6's and the plates drop to about 285V. I can post some pics a little bit later, but has anyone experienced something like this? Has been a repeatable event after every attempted troubleshooting so far. I used the normal ampgarage 102 layout (from Tony). Just checked after cool down and the bias voltage is -35V before standby, then starts creeping up after flipping the standby switch.

Thanks for any suggestions.

[alnight]

That's interesting. Bias pot tail resistor and ground all good?

[mgotoo03]

I don’t think so. Here are my readings on the bias pot lugs:

No power, bias pot
1. 32.5k
2. 30k
3. 27k

With power on (with and without standby)
1. -2 Mohm
2. -2 Mohm
3. -1.9Mohm

I added a pic because I didn’t believe the reading.

Also, I tried to adjust the bias pot when the amp went into the weird voltages. It’s on an amp cradle so I tried to reach under from the front. Got a bit of a shock when my fingers touched the bias pot and my wrist touched the outside of the choke at the same time.

[pdf64]

With power on (with and without standby)
1. -2 Mohm
2. -2 Mohm
3. -1.9Mohm

A circuit needs to be totally deenergised before valid resistance measurements can be made.
The meter applies a known constant current and measures the resulting voltage. The voltage is divided by current and the resulting resistance reading is displayed.

Any existing current flow in the resistance being measured will cause error.
And a high voltage will wreck the meter.

[Guy77]

Hey all,

I've been trying to chase down an issue with my 102 build that happens after the amp is played through for 5-10 minutes. All readings (initially) seem reasonable (6L6 344V plates, Bias at 43mA). After playing a bit the sound cuts out and I'm measuring -120ish(!) volts on pin 1 of the 6L6's and the plates drop to about 285V. I can post some pics a little bit later, but has anyone experienced something like this? Has been a repeatable event after every attempted troubleshooting so far. I used the normal ampgarage 102 layout (from Tony). Just checked after cool down and the bias voltage is -35V before standby, then starts creeping up after flipping the standby switch.

Thanks for any suggestions.

344v on the plates sounds too low. Are you sure its not 444v?
What model power transformer are you using?

Cheers
Guy

[mgotoo03]

It’s a Classictone 40-18004, and yes, sorry, it is 440V at the plates when behaving

40-18004.pdf

[pdf64]

Got a bit of a shock when my fingers touched the bias pot and my wrist touched the outside of the choke at the same time.

That indicates there's voltage between the pot metalwork and the choke shell. One of those parts isn't at the same potential as the chassis.

After playing a bit the sound cuts out and I'm measuring -120ish(!) volts on pin 1 of the 6L6's and the plates drop to about 285V

A problem with using numerous electromechanical connections between circuit OV common returns and the 0V common conductor, ie the chassis in this case, is that operation is dependent on every one of those connections being good.

My guess is that there isn't a reliable connection between the pot metalwork and the chassis.

Soldered connections are best between things that you really want to stay connected.
If not soldered then a high torque fastener with a sharp toothed star washer between the chassis and the soldered lug. But that's not feasible with a pot, because the soldered connection is to the outer shell, which is crudely crimped to the body. Yes a star washer can be fitted between body and chassis, but the threads won't support high torques.

Your loss of a reference connection between bias supply 0V common and the HT 0V common is leaving the output valves without bias.
Hence they're conducting heavily and loading the HT down.

[mgotoo03]

Thank you. I will consolidate the bias pot and cathode grounds and revisit the pot/chassis ground contact.

[mgotoo03]

Seems fixed! I put the bias pot and power tube cathodes on the same ground, and also removed paint from the chassis around some of the transformer/choke bolts to get better grounding. Also put a good lock washer on the bias pot and really tightened all ground points. The amp is singing again and sounding great! Thanks all!