Plexi6v6 voltages wacked

[deiseldave]

Hello. Could someone help me figure out my problem? I took measurements of my new Plexi6v6 build (please see attached), and compared to Mark Huss' measurements, they are wacked! The amp actually works (but doesn't sound very good). But, I I am surprised it works at all, with these measurements.
The overall voltages are a bit lower due to the voltage drop of my tube rectifier (5U4).
However my PI, and V1 pin 8 are not even in the same ballpark. I spent several hours tracing leads, and everything looks correct.
Cold solder joints? Any ideas?
Thanks in advance.

[statorvane]

V1 - Pin 8 160V - that is a typo on Mark's schematic. Should be between 1 and 2 volts, so probably 1.6 volts.

Can you check the value of the tail resistor on V3? Should be 470R. A common mistake is to use a 470K resistor there, which will produce a dirty, anemic sound as opposed to a 470 Ohm resistor there. This is a common mistake. Don't ask me how I know this.

Are your grid resistors wired between pins 5 and 6 (V4 and V5)? It doesn't look like much of a voltage drop there - almost like they are shorted out. You may want to check the value of the grid resistors.

Hope this helps.

[Roe]

check voltages at the electrolytic caps as well

[Stevem]

Why the heck are your heater voltages so low on 2 and 7 of your output tubes?

[deiseldave]

Thanks guys. I'll check all suggestions, re-measure, and get back with you.

[ToneMerc]

Why the heck are your heater voltages so low on 2 and 7 of your output tubes?

Nothing wrong with them, look at them again. There's 6.8VAC there.

TM

[deiseldave]

Statorvane - PI tail is 470R (color code verified and measured), and 6v6 grid resistors measured out as 5.6K. Not sure why I'm not showing more of a drop, there is no obvious short by looking.
Could the low voltages on the PI pins 2, 3, 7, & 8 be caused by all my pots being turned down?
I re-measured everything, with cap voltages included but had to scoot off to work before I could scan and upload. The cap voltages looked ok (to me) I'll let you guys be the judge when I upload them at lunch.
Regarding the heater voltages, I was measuring from ground to each pin, is that incorrect? I shake a bit, and getting in there when its live with both probes worries me. Thanks.

[darefugee]

Have you tried a different tube in V3? Or swap V3 and V2 and see if the voltage issue moves to V2? I ask because when I first powered up my Plexi6v6 there were severe voltage issues at V1 and one of the channels sounded like, well, not very good. Problem turned out to be the tube itself. It was a brand-new tube, too. Swapped in a known good tube, problem was gone and the amp sounds terrific. Too loud, tho.

[ToneMerc]

If you substituted a s/s rectifier with a 5U4 the voltages won't be correct without tweaking the resistor string.

TM

[deiseldave]

I've attached the second reading of voltages, with cap voltages included.
Darefugee - Swapped V2 with V3 - No change
Tonemerc - I had a tube rectifier slot already in my donor chassis, and figured that the lower voltage would act like a little tweak down of a VVR, and wouldn't need to change dropping resistor values from what Mark Huss had. I also saw it as a good thing to get the plate & screen voltages of the 6v6's a little closer to recommended values than what Mark's were. Is my thinking off? If so, what changes from Mark's values do you suggest.
I'm thinking the primary problem here is the low voltages on V3 pins 2, 3, 7 & 8. Anybody got ideas on the next step towards fixing that?
Thanks again.

[xtian]

V3 is your PI, and you should have ZERO volts on the grids, pins 2 and 7.

[deiseldave]

V3 is your PI, and you should have ZERO volts on the grids, pins 2 and 7.

Thanks. I was going by the voltage readings on Mark's original schematic. I looked at Goug Hoffman's 50W Plexi readings, and see you're right.
So, my problem is that I have 18V on V3 pins 2 & 7 that I don't want. I wonder why I have that voltage there.

[xtian]

Well the caps in your tone stack are supposed to block all DC from the previous stage. Check those first.

[martin manning]

V3 is your PI, and you should have ZERO volts on the grids, pins 2 and 7.

Thanks. I was going by the voltage readings on Mark's original schematic. I looked at Goug Hoffman's 50W Plexi readings, and see you're right.
So, my problem is that I have 18V on V3 pins 2 & 7 that I don't want. I wonder why I have that voltage there.

Hold on there hombre! A long-tailed pair should not have zero DC volts on the grids, and anyway you can't measure the grid voltage accurately on pins 2 and 7 because your meter's input impedance will load the circuit. Your PI voltages don't look out of line to me, and based on the cathode voltage you have a reasonable 1.8mA flowing.

[xtian]

Retracted! Martin Knows Best(TM)(R)(SM)