New plexi 6V6 lives ! Good sound- out of spec PI voltages.

[chopstuck]

I have finished up a nice sounding Mark Huss Plexi 6V6 but my PI voltages are way off his published voltages. Everything else in very near the mark except the V1 p8 which can't be 160v on the first Rk. My screen voltage is higher than the plate so should I increase my 1k resistors ?

I'm at a loss to explain these

Expected vs. measured as per Mhuss's first schematic at mhuss.com

v3 12ax7

exp actual

p1 246 170
p2 114 20
p3 146 31
p6 241 171
p7 110 20
p8 146 26


I'm at a loss to explain the differences. The amp doesn't have much clean headroom and the amp goes into distortion quite quickly.
any hint's or ideas ?

[Colossal]

I have finished up a nice sounding Mark Huss Plexi 6V6 but my PI voltages are way off his published voltages. Everything else in very near the mark except the V1 p8 which can't be 160v on the first Rk.

Yes, it is a great sounding amp. No, you are right, 160V is way off. Mark's pin 8 is 0.76 and I got 0.84 for mine. Other builders got 0.90, 0.91, 0.74 so some variance is expected.

My screen voltage is higher than the plate so should I increase my 1k resistors ?

Now that seems a bit odd. Have you checked to be absolutely sure you've got B+1 tied into your plates (coming from the the OT) and not your screens? FWIW, I've got 447V on my plates and 412V on the screens (using a 1k 5W sag resistor and a 10H choke in series).

I'm at a loss to explain these:

Expected vs. measured as per Mhuss's first schematic at mhuss.com

v3 12ax7

exp actual

p1 246 170
p2 114 20
p3 146 31
p6 241 171
p7 110 20
p8 146 26


I'm at a loss to explain the differences. The amp doesn't have much clean headroom and the amp goes into distortion quite quickly.
any hint's or ideas ?

No, clean headroom is not that amp's forte'! It cleans up a bit if you back off on the guitar volume but there's always a bit of grit on tap if you dig in.

BTW, your V3 pin 8 is a little lower but it should not be 146V nor should pin 2 be 114. I'm pretty sure those are typos. Other builders got 38, 37, 56V and I got 63 for pin 8. Pin 2 ranges from 22 to 37V in others' builds.

You may need to bump up your supply voltage just a bit to V3.

[jbrew73]

check the actual values of the pi cathode and tail resistors. looks like it could be a 1k instead of 10k.

[chopstuck]

I found that Colossal was right, Thank's. I had taken the screen supply from the first node after all. Correcting this allowed me to decrease my 6v6 current draw down from 20ma to 14ma each which gave me more headroom. My 6V6 screens are now 369v with my plates at 412v. Power tubes are happier. My only question now is the low (unchanged) PI voltages from the first post.
The low end sounds a little sputtery on breakup and I tend to keep the bass below 1-2 or it falls apart under distortion. I don't know if it's blocking distortion or it my hifi OT (from a Grommes EL84 mono tube amp)is conveying more bass than I want it to. Any thoughts?

[Structo]

Yeah, I built a simple version of that amp with just one preamp tube, the PI and two 6V6 power tubes.
I put a lot of switches and stuff on it, like different cathode caps, 1/4, 1/2 and full power on the 6V6.
I think I changed the MV to a boot strapped MV.

I'll bet you read a resistor color code wrong or got the B+ out of order.

I recently changed my amp to a 6V6 Rocket but that Plexi circuit really sounded good, some of the best crunch I have gotten from a small amp.
And, my BYOC Tubescreamer really made that amp come alive, since I was using it for a gain stage basically.

Probably a simple fix, just walk away for a while and come back with some fresh eyes.

Or post some large pictures so we can help you sort it out.

You posted right when I was typing.
Check the 470R cathode resistor on the PI to make sure it is not a 4K7, 470K or other similar.

For some reason a lot of people get that 4 and 7 and the multiplier wrong a lot, I know I have.

[chopstuck]

I finally traced it to the the presence pot and cap. When disconnected, the voltages read close to the published ones . This is with NFB still hooked up. maybe my cap is faulty. anyway too late tonight to check.

Thanks for your help so far !

[Gaz]

The PI is grounded through the presence pot, so if there's a problem there - with the ground - then I'm sure your voltages would be wacky. I bet it's that if all the other component values check out. Good luck.

[Structo]

Do you have a speaker connected or a dummy load?

Are the OT primaries phased correctly?

[chopstuck]

The first time I powered it up I had the primaries reversed and got all the nasty positive feedback. I fixed that. I have the amp hooked up to a 4 ohm 2x12 cab as a load.