Troulbleshooting my 5e3: Bias reading 65mA

[Jonny toetags]

I just completed my 5e3 (after some great grounding info from this forum) and fired it up with the usual steps.
The amp sounds real good, dead quiet (or as quiet as my other 2 5e3's). After about 20 min of playing (volumes on about 3, volume controlled by guitar vol knob), i let it cool down a bit and moved it to my workbench for some checking and noticed that even after about 30 min of cooldown, the PT was extremely hot to the touch.
I decided to fire it up yesterday and check the bias with my meter and I'm reading 65mA. I know deluxes run hot, but not that hot!
I've never had to troubleshoot a 5e3 build before and was wondering if anyone had some insight as to how/where to start?
This is my first time using Classic tone PT and OT. Other than that, everything is pretty much the same as my other builds.

[Tom Phillips]

Please post the 6V6 plate and cathode voltages so we can understand the complete situation. Also verify, by measurement, the resistance of the power amp cathode resistor you have installed.

[Jonny toetags]

Here are some readings i took:

5Y3GT (NOS RCA): pin 2 and 8= 329 vdc
pin 6= 350.7 vdc
6V6: pin 3= 311 vdc on one and 309 vdc on the other
pin 4(both)= 237.4 vdc
pin 8 (both)= 5.97

Preamps have 111vac and 110vac on pin 1

[Tom Phillips]

OK.
Please measure and post the voltage on pin(s) 8 of the 6V6s and the resistance of the cathode resistor. (The resistor that connects pin(s) 8 of the 6V6s to chassis ground.

[Jonny toetags]

OK.
Please measure and post the voltage on pin(s) 8 of the 6V6s and the resistance of the cathode resistor. (The resistor that connects pin(s) 8 of the 6V6s to chassis ground.

Resistor measure 41.8 ohm
I made a typo before...pin 8 is 5.97, NOT 7 on the 6V6's
I made the correction

[Justin Thomas]

Schemo calls for 250ohm cathode resistor... that's what, about 400% low? I'd start there!
Edit: is that with the resistor still in the amp? Is the bypass cap shorted? Or resistor shorted? Or just wrong part or value printed on it?

Justin

[Tom Phillips]

Resistor measure 41.8 ohm...

The stock value for that resistor is 250 Ohms. If we are both talking about the same resistor then I think that low value is causing your problem.

[Jonny toetags]

The stock value for that resistor is 250 Ohms. If we are both talking about the same resistor then I think that low value is causing your problem.

It is a 250 ohm/5W resistor


I should menton i'm using Mission's schematic with the classictone OT and PT.
For the PT, they told me the following:
For our 40-18021 power, use the two BLKs for the BLK and WHT leads. Connect GRN/YEL lead to ground (using Missions schematic). There is no BLU lead - it's not used on this amp. All of the other colors are the same.
I'm worried something might have fried the resistor? if so...what?

[Jonny toetags]

Schemo calls for 250ohm cathode resistor... that's what, about 400% low? I'd start there!
Edit: is that with the resistor still in the amp? Is the bypass cap shorted? Or resistor shorted? Or just wrong part or value printed on it?

Justin

Resistor still in the amp (just measured my Mission, still in the amp and its 266ohms)
The resistor def. says 250ohms on it

[Tom Phillips]

If I understand you correctly the resistor is marked 250 Ohm / 5W but it now measures 41.8 Ohms.
Disconnect the capacitor that's in parallel with the resistor and measure the resistor again to be sure. You can also measure the resistance of the cap and it should be a very high (out of range) value after the meter reading stabilizes.
One or the other or both parts (if they measure bad) need to be replaced. Then try the amp again with another set of 6V6s. Parts do experience early failure. It could have been the resistor, the cap or one of the power tubes.

[Jonny toetags]

If I understand you correctly the resistor is marked 250 Ohm / 5W but it now measures 41.8 Ohms.
Disconnect the capacitor that's in parallel with the resistor and measure the resistor again to be sure. You can also measure the resistance of the cap and it should be a very high (out of range) value after the meter reading stabilizes.
One or the other or both parts (if they measure bad) need to be replaced. Then try the amp again with another set of 6V6s. Parts do experience early failure. It could have been the resistor, the cap or one of the power tubes.

Pulled the resistor...it measures 246 ohms outside the circuit.
when i measure the cap...it shoots up to M-ohm and stays at 0

[Tom Phillips]

Pulled the resistor...it measures 246 ohms outside the circuit.

Then the resistor is probably OK. They usually don't recover or flip flop between a very low value and the normal value.

when i measure the cap...it shoots up to M-ohm and stays at 0

Is that a "zero" or and "Over" range indication? If "Over" range then the cap is acting normally.

We need to figure out why you got that 41.8 Ohm reading. Maybe something else is inadvertently in the path from the cathodes (Pin 8) of the 6V6s and chassis ground. Give it a good look over. Also measure the resistance from pin 8 to ground now that the parts are removed or lifted. Do you still read a low resistance? You should read infinity / Over Range or whatever your meter shows for a resistance higher than it can read.

[Jonny toetags]

Then the resistor is probably OK. They usually don't recover or flip flop between a very low value and the normal value.


Is that a "zero" or and "Over" range indication? If "Over" range then the cap is acting normally.

We need to figure out why you got that 41.8 Ohm reading. Maybe something else is inadvertently in the path from the cathodes (Pin 8) of the 6V6s and chassis ground. Give it a good look over. Also measure the resistance from pin 8 to ground now that the parts are removed or lifted. Do you still read a low resistance? You should read infinity / Over Range or whatever your meter shows for a resistance higher than it can read.

With the capacitor and resistor pulled, resistance at pin 8 to chassis is 0.

[Tom Phillips]

With the capacitor and resistor pulled, resistance at pin 8 to chassis is 0.

Well...I think your meter interpretation is different than intended.
We are back to the question "When you write "0" do you mean "zero" or the letter O of the alphabet which would be your meter's way of displaying "over" or "out" of range.
Try this:
Touch the two probes together when the meter is set to measure resistance. The display should indicate something that means "zero" resistance or close to it. That is a dead short.
Next separate the probes with just air in between them. Whatever the meter displays is infinite (or at least very very very high) resistance. That is known as an "open" / "infinity" / "Out of Range". Use one of those words rather than "0" to be clear. I suspect that you are reading an open not "zero." Do you agree?

[Jonny toetags]

Well...I think your meter interpretation is different than intended.
We are back to the question "When you write "0" do you mean "zero" or the letter O of the alphabet which would be your meter's way of displaying "over" or "out" of range.
Try this:
Touch the two probes together when the meter is set to measure resistance. The display should indicate something that means "zero" resistance or close to it. That is a dead short.
Next separate the probes with just air in between them. Whatever the meter displays is infinite (or at least very very very high) resistance. That is known as an "open" / "infinity" / "Out of Range". Use one of those words rather than "0" to be clear. I suspect that you are reading an open not "zero." Do you agree?

Ok, took the board out and checked for shorts and checked all the connection (resoldered a few).
The meter with air between shows 0.L (zero.L) infinite.
With those two parts still out of the circuit (completely), pin 8 to the chassis is reading just under 50 ohm now.
There is definitively no short on or around the board.