Strange voltages in new 6V6 plexi

[pjd3]

Hi everyone, I'm hoping one or two of you might offer some guidance to one very baffled little boy here.

First time just now, I powered up the amp with all the tubes in, made sure I had maximum negative bias V to the power tube grids,
And had my meter already set to look at the plate of V1. I wanted to see right off that voltages were being divided down reasonably.
I expected to see around 160 vdc, as illustated in sluckeys schematic. Seemed reasonable for this circuit. But, I watched the voltage climb up near 400 volts. That worried the hell out of me so, I turned the amp off, discharged the caps and did some continuity checks.
The power node resistors are adding up in series as they should, filter caps are arriving at their proper nodes as they should and no where else.
I haven't measured any other voltaged exept pin 6 on V1 (plate) as I was extemely concerned about all these tubes seeing hugely high voltages. I"m referrencing off of chassis/HV centertap ground....

I'm really at a loss to understand where and why this could be happening. The voltage from the 5V4 rectifier was up near 495 vdc everywhere when I initially powered up with Only the rectifier tube which I though was very enexpectedly high, but was ensured that I would see that voltage sag once all the tubes were in - which made sense.

Now, I'm scared to death to turn the amp on as I don't wan't tubes to go on me. Some of them are moderately pricey NOS tubes.
Any advice on this would be very appreciated.

Thank you everyone,
best,
Phil

[sluckey]

made sure I had maximum negative bias V to the power tube grids,

So, what was the actual maximum voltage on pin 5 of each 6V6?

Set the bias for about -40v on pin 5 then recheck the other voltages.

[pjd3]

Hey Sluckey,

The range was -31.9 to -58.5 so I just kept the pot counter clockwise for -58.5 volts at the grids of the 6V6's. The bias always seemed to perform well. I spent sometime looking at that before I put in any tubes at all.
It was just a little bit of a shock to see nearly 400vdc on the plate of V1. At this moment I have a 5751 in V1 but I don't believe that has anything to do with it.

As the amp was warming up I heard a pop from the speaker. Hope that wasn't something burning out. I know amps can make sounds as they are warming up. But, this is what it is. Me turning things on for the first time and not knowing quite what to expect, and finding surprises.

Thanks Sluckey, if you think of anything to look for, send it my way,
Thank you,
Phil

[pjd3]

Oh I'm sorry, I just re-read your suggesting to set the bias at -40 volts and recheck voltages. I'll go do that now.

What I'll do is pull the rectifier tube, set the bias for -40 volts and put the rectifier tube back in.

thank you,

Phil

[pjd3]

I set the bias to -40 on pin 5 of the 6V6's and watched the voltage at the plate of V1 - pin 6. I shut down the amp as it began to rise above 350 volts.
I then pulled out all three preamp tubes and checked the plate of both power tubes which was about 430 vdc, which is somewhat higher than what I had expected - I thought I'd see around 400 plate volts with 330-0-330 and a 5V4 which is supposed to give 10-15 volts lower maybe than a GZ34.

So, I'm getting the clue that there could be some mis-wiring in my build that is bringing very high voltages to the plates of the preamp tubes, well, I only know about V1 at this point. I havent measured any other preamp tube voltages yet, since I get to concerned about leaving the amp on for anymore time than I have to. I really don't personally know what voltages that high will do to 12ax7's and 5751's. They are NOS RCA's and Sylvania's but, that doesn't really tell me a whole lot at this point.

Thank you for checking in.

Phil

[sluckey]

Do you have the 6V6s plugged in? If not, put them in. If they are in what is the voltage across the 1Ω cathode resistors?

[pjd3]

Yup, all the tubes were in including the 6V6GC;s. Their plates were around 430vdc.
I'll look at the voltage across the 10 ohm resistor that are in series with the cathodes.
Also, I read somewhere that rectifier tubes are known to occasionally put out more voltage that they are intended to. Perhaps I should try the GZ34 from my Fender amp, see it there are any differences.

Best,
Phil

[pjd3]

I just took a moment to put in a JJ GZ34 to measure its ouput - 450 vdc. Thats with the power tubes in but no preamp tubes (and bias at -40 vdc).

I'm not yet versed on the behaviors of various rectifier tubes but I wouldn't have expected to see nearly 50 more volts from a 5V4G than a GZ34.

The 5V4 had been outputting 495 vdc and the GZ34 is outputting 452 vdc.

I'll go check the cathode resistor voltage.

Thanks,
Phil

[pjd3]

I will get back. I found what looks to be a miss-wiring to the phase inverter. The diagram has a yellow wire that looks like it stops at pin 8 but, upon closer inspection it appears to go to pin 3. My eyesight sucks, and I really need to get better lighting and magnification. this can really bite my ass.

Thank you for hanging in here with me, I will get this right.

Best,
Phil

[Paul G.]

Yes, the two cathodes on the Phase Inverter are tied together. Not sure that's your only problem, but it needs to be addressed anyway if you want a properly performing amp.

[Paul G.]

Also voltage will not drop across a resistor if there is no current being drawn. If the socket is wired correctly, swap another tube in there just to see. A bad tube may not draw current. I once spent a week chasing down hum in an amp that turned out was coming from a bad 12AX7.

[pjd3]

Guys, I figured it out. Kick my ass. I didnt' ground the preamp. (I also left out a 100K plate resistor on the stage going into the cathode follower). What made me look for this was something my amp tech mentioned months ago when was asking him about safety and dangers. He talked about occasionally finding high voltages showing up in all kinds of places where they shouldn't be and that this is pretty much always a grounding issue, which makes sense. It you dont give the voltage somewhere to go it has no way to disperse itself.

so I made the ground connection and all the proper voltages showed up where they should. Now that was nice to see.

However, there is 452 volts on the plates of the 6V6GT's. That kind of baffles me a bit - 330 vac x 1.3 (GZ34) = 429 volts maybe minus a few more from the OT coil and whatnot. They are new NOS Sylvania's so, I suppose they are OK if I don't push the bias current. One tube reads 19mA and the other reads 24mA - bias voltage is -40 vdc.

The amp sounds fricken "GREAT" !!! Very happy. 6 months of planning, collection parts and building made for a nice, crisp, rich and quiet 20 watt or so Plexi. all the controls had good usable range, PPIMV worked great, the adjustable NFB was subtle but usable and there wasn't any funny stuff to be found. Well, lets see what happens when I get to crank it up a bit. Its late so I had to keep it tame.

I want to thank you all for your ideas and direction, and espectially to Sluckey for dishing up a great little amp and hanging in with me on the build. I can't wait to get this thing on stage with my pedal board. I think the bands going to be happy. Its a toneful little guy!

Hey, good night everyone and thank you again. a night of trials and success all squished into one.
Best,
Phil

[Colossal]

.However, there is 452 volts on the plates of the 6V6GT's. That kind of baffles me a bit - 330 vac x 1.3 (GZ34) = 429 volts maybe minus a few more from the OT coil and whatnot.

Your mains input voltage is higher than the power transformer primary, e.g. your transformer has a primary wound for 117VAC input, but you have 122VAC coming out of the wall.

Congratulations on your amp. 6V6 Plexis are fun.

[pjd3]

Thanks Colossal, and you really have me thinking about primary PT voltage. When I first got the classictone 40-18016 I rigged it to the wall voltage to check all the secondary voltages and found very close to the 330-0-330 it is rated at. Then after I put it in the amp I found the same, but, last night I measured it again and it had increased from 330-0-330 to 342-0-342 which did surprise me. I shouldn't have been surprised but, if you measure something a bunch of times over a course of weeks, its odd when its suddenly that different.

Now, tomorrow, I'm going to put in the 5V4A and see what its giving me for voltage now that I grounded the preamp, installed the missing resistor and have been running the GZ34 with good signs. Previous posts would show how the 5V4 was giving me 40-50 volts higher than the GZ34 which seems crazy. Its known for always outputting somewhat lower voltage than a GZ34.

I'll run the results by tomorrow. another thing though, the insides of the 5V4 were tilted like the leaning tower of piazza! I asked Mike at KCA about it but he said it tested fine before it was shipped. I'll report findings tomorrow.

Thanks man,
Phil

[pjd3]

Just as a final report,

all is great using a 5V4 with a couple of 6V6GT's in an 18 watt plexi. It brought the B+ down slightly, maybe from 452 down to about 444, not a whole lot but, thats OK. The 6V6's are seeing around 442 on the plates which is still a bit higher than initially anticipated but I'll live with it. Perhaps when I get the whole rig set up with the 18 watt plexi being a dedicated British sound, I'll try a 5R4, or one of the ones that don't use 3 amps of heater current, and see and hear how a lower B+ works with it.

Amp sounds great but, not yet totally great heavily overdriven. Its got some fizz to it but I blame that on the speaker I"m using (Weber C12B) and maybe due to using a 5751 in V1, just right now. And I wouldn't be surprised if a lower B+ could help that out, but thats OK for now. I'm going to use it as a clean pedal platform just for a little while. All in all, very happy with the result,

Thanks again for all your comments and guidance.

Best,
Phil D