6V6 plexi build, signal cutting out

[flintrock]

Power section side

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[ampfab]

Have you installed your phase invertor tube into v1 or v2 I see if your voltages change?
Maybe install tubes from v1 or v2 ino the phase invertor spot and rerun your test of the power section.

[Tillydog]

I see the ground bus isn't soldered to the pots (no bad thing) and that you've used an isolated input jack - do you have any connection between the ground buss and the chassis?

Not that, then...

[morcey2]

I was on the high scale so that would be why heater voltages look low. But definitely some high voltages going on here.

In another post, you said that you're dropping about 30V across the plate resistors. Is this on all gain stages (V1a, V1b, and V2a)? If it is consistent, you would be pulling 0.3mA through each triode.

Can you measure the cathode voltages with the meter on a lower voltage scale on your meter? The cathodes, other than the CF, would be in the sub-1V range. Also, make sure that it's measuring DC voltage, not AC. The fact that you're measuring 3V on each of the heater pins is telling that you're measuring with the meter set for AC. That's what I would expect to see on each heater lug relative to ground.

Matt

[flintrock]

No I was on the DC 1000v scale. Cheap Ideal dmm. Swapped out V1 and V2 for two other known good preamp tubes and walla! I' m rockin' now and all voltages are in line. I think one of the sidea on V2 might have been bad. I tested them on my BK700 and they were good but barely. Hard to test those ax7's. The only thing not functioning is the fat switch and one of the nfb settings (I did the 3 way switch) is howling. Stay off those and it soundz great! Thanks everyone so far!

[morcey2]

No I was on the DC 1000v scale. Cheap Ideal dmm. Swapped out V1 and V2 for two other known good preamp tubes and walla! I' m rockin' now and all voltages are in line. I think one of the sidea on V2 might have been bad. I tested them on my BK700 and they were good but barely. Hard to test those ax7's. The only thing not functioning is the fat switch and one of the nfb settings (I did the 3 way switch) is howling. Stay off those and it soundz great! Thanks everyone so far!

Great! I've built a bunch of Plexi6V6s and there one of the most versatile amps out there. You'll enjoy it. For power tubes, I like the JJ 6V6s. They sound sterile and a little harsh in tweed-ish stuff, but are awesome in the Plexi's.

For the howling, swap the OT primary leads. I've yet to get the leads correct on any of my builds with NFB. I'm 0 for 12-ish in that department.

Matt

[flintrock]

I have it sounding pretty good. Ordered up some JJ 6V6's to try in comparison to the National Union's I have in there now. Ya think swapping those OT leads around will make a difference? It doesn't howl on the 0 nfb or 47K nfb, just when its set on 100k nfb. Curious about that but one if the OT leads is too short, I'd have to extend it (thats what she said).

[flintrock]

Well it sounded good at first, its sounds compressed the louder you get. Not sure if its the power tubes (which test good) or the output transformer maybe? I used a Hammond 1760H. Everything else looks to be wired correctly at least to me.

[Stevem]

If your measured audio signal across the outputs tubes pin 5 is more than about 36 vac, than you are drivin the crap out of the 6v6`s and the compression will be quite large and the distortion could be unpleasent depending on the brand of tube.

[hans-jörg]

If your measured audio signal across the outputs tubes pin 5 is more than about 36 vac, than you are drivin the crap out of the 6v6`s and the compression will be quite large and the distortion could be unpleasent depending on the brand of tube.

Hi,

even the half is way too high.

Vg1 max 8,5 Vac (Data sheet)

Hans-Jörg

[flintrock]

Is AC signal measured pin 5 to ground or another pin?

[flintrock]

My JJ 6V6's came in today. As with any tube I get, I put it on my tester first (BK 700) to make sure I get good tubes. These however test at a 60 which is replace on my meter, and 70% life. They test exactly the same, plus they have Tube Depot labels on them with a 22 bias point. This is my first time with 6V6's, they can't possibly test that low can they? They look brand new. Seller claims to have bought and never used..hmmm. Ordering another pair.

[martin manning]

I have a (supposedly) matched pair of JJ 6V6's here that tested 97% and 100% of reference current, and gm's of 3.80 and 3.75 on my uTracer. The spec gm (GE 1955) is 4.1. Sounds like the two you have are definitely on the low side.

[Firestorm]

I've read that if the 83 rectifier tube is on its way out, you can get low readings. That said, I've tested heavily used Philips 6L6s that read much stronger than new Russian or Slovak ones.

[flintrock]

My meter has had the solid state conversion in place of the 83 tube.