Low Voltages And A Couple Of High Ones?

[Statick]

I've got my 50W Plexi build fired up. It has a slight hum that I haven't started diagnosing because I wanted to check voltages first.
Almost across the board, I've got voltages that are lower than what's in the Metro instructions. I've found a few posts where people had higher than expected voltages, but I haven't seen low voltages as a problem. See the attached table.

I've also got around 20V on the grids of V3 where I think I should have zero.

Almost all the parts came from Valvestorm. The iron is Classic Tone. All readings were taken with tubes in and both switches on. This is not my first amp build, but I'm by no means an expert, so any help is very appreciated.

- Should I be concerned about the low voltages or are they close enough?

- Where would you look first when hunting down the voltage on the grids of the PI?

[guitarist1977]

What ClassicTone transformers have you used?

Those voltages on V3 pins 2 & 7 are normal

Please add photos of your build.

Alex.

[Statick]

What ClassicTone transformers have you used?

PT: 40-18089
OT: 40-18025
Choke: 40-18058

Those voltages on V3 pins 2 & 7 are normal

Great! Thanks.

Please add photos of your build.

Working on photos now.

It seems like there's something going on with the bias. With the bias pot in the min and max positions, I get these readings:

MIN / MAX
Rectifier + : 360V / 400V
Cathode Current V4 : 4.8mA / 0.0mA
Cathode Current V5 : 5.7mA / 0.0mA

I might try lowering the value of the 47k bias resistor.

[danman]

I am using the same PT in one of my builds and I believe my plate voltage is right around 412vdc. Yours is measuring slightly lower but that could be because of a slightly lower ac wall voltage. What are you getting at the outlet in your house? I notice that you are reading -45 on pin 5 of each power tube socket so it appears that your bias voltage is there. Take a reading at pin 5 while rotating the bias pot to make sure that you get a nice even bias voltage sweep. It could be that you just got a set of tubes that bias up at the cold end of the normal range. You can try another set of power tubes or you can lower the value of the resistor that is inline with the bias pot. If you currently have a 47k there, try dropping down to 33k to see if this will bring the tubes current draw up to a more acceptable level.

[Statick]

I finally got another pair of power tubes to try and they do bias differently (better?). Now, at the middle of travel of the bias pot, I've got -44V on the grids. B+ is 391V.

A newly discovered issue that is probably related to everything else:

I get almost no cathode current at idle. I'm reading about 0.8mA. If I crank the bias pot temporarily, I can get it up to about 3mA.

I have triple-checked that the resistors from cathode to ground are 1 Ohm. I have verified that the ground is a good ground. It doesn't matter which set of power tubes I use.

What would cause relatively good (although slightly low) voltage readings and no cathode current?

[guitarist1977]

Funny thing. I got the same issue this morning. In my case it turned out that screen resistor (1K 5watt) was bad and the screen voltage was 0.

Try to check this one.

[danman]

You have listed your cathode current draw in ma but do you have your meter set for mv when taking the reading? I'm sure that you do but I just wanted to check first to make sure. Can you post some pics of the power tube socket wiring?

[danman]

Also check socket pin tension to be sure that you are getting solid contact. It would be unusual for two new sockets to have issues with tension but anything can happen.

[Statick]

Funny thing. I got the same issue this morning. In my case it turned out that screen resistor (1K 5watt) was bad and the screen voltage was 0.

Thanks for the suggestion. I just double-checked both of the screen resistors. They're good and I'm getting good voltages at the screen.

You have listed your cathode current draw in ma but do you have your meter set for mv when taking the reading? I'm sure that you do but I just wanted to check first to make sure.

Yep. I'm using the mVdc setting. I've also tried two different multimeters to make sure it wasn't that.

Also check socket pin tension to be sure that you are getting solid contact. It would be unusual for two new sockets to have issues with tension but anything can happen.

How would I check this? Looking into the tube sockets, the clips in the individual pin holes all look identical. Short of sacrificing an 8-pin tube and building some kind of dummy-tube for testing, how can I make sure that all pins are making contact?

Can you post some pics of the power tube socket wiring?

Three pics attached.

Thanks so much to anyone who is taking the time to read this and possibly respond. I'm kind of at a loss with this one.

One more thing, because I went to the show last night and may never have as good a chance to use this.... !!

[gloeidraad]

I think the cathode (pin 8 ) is not grounded. Its difficult to see in the pics.

[Statick]

I think the cathode (pin 8 ) is not grounded. Its difficult to see in the pics.

You're right, it is a little hard to see in the pics.
On both sockets, pin 8 is connected to pin 1 which then grounds through the 1 Ohm resistor. I've got good continuity from pin 8 to ground on both.

[guitarist1977]

Can you post pictures that will show everything inside the chassis?

[danman]

I feel bad because I missed the voltage readings on pins 2 and 7 of v3 that you posted. Are you showing a DC voltage on these two pins? Should be AC signal voltage. I'm wondering if you have a bad coupling cap. Also check that the presence pot is properly grounded.

[danman]

I don't think we have discussed how the amp sounds either. Are you getting any output at the speaker? Is it producing the expected volume or is it weak?

[Statick]

Alright! Progress!

I was getting some hiss and hum at the speakers, but had not yet plugged in a guitar. Once I did that, I got a shorting sound when playing anything but the softest notes. A quick once over (for at least the fifth time) and I found the culprit. A not-so-solid joint on the ground of the first cap can. I fixed it and fired it up. My initial reaction was that I had made it worse because I couldn't hear any hiss or hum when I switched it on. Amazingly, it is now dead quiet and seems to be working.

I can't get more than about 28mA of cathode current with the bias pot max'ed out, so it looks like a resistor change is in order. That's easy enough.

Thank you to all who took the time to read this thread and respond.

Here's my current voltages, in case they help anyone in the future who is looking to solve a similar mystery. Now all of my "low" voltages are closer to expected values: