5F11 startup - power supply issue/cold 6v6

[6G3Victory]

Hey All,
Been trying to get my new 5F11 build up and running, but had a couple weird things going on..

Was getting 415 volts on the plates of my 12ax7’s. Took them out and put the 6v6’s in. Initially, with just the rectifier in, I was getting 450vdc on the first power supply node (I was expecting a high number unloaded, but not quite that high - 315-0-315 P.T, 5Y3), but with the 6v6’s in, I powered on, saw the 1st node hit 450V quickly, hold for about 5 seconds, and then drop down to 250 in another 4 or 5 seconds, at which point my 200w limiter bulb developed a faint glow, and a hum appeared out of my speaker.

Also, one of my power tubes was cold, even though I confirmed proper voltage on the filament pins.

I should have tried another 6v6 in place of the cold one, but didn’t. I’m wondering if maybe the tube is shorting and that’s why I am seeing a bit of a glow, and perhaps the short is causing the filter caps to struggle to keep up? I’m really not sure.

As for the high preamp tube voltage, I’m not sure if the two are related, or if something entirely else is going funky.

If anyone has any ideas, I’d be very happy to learn what is actually going on!

Thanks,
Kris

[bepone]

remove all the tubes and measure bias voltage ( pin 5 in 6v6 socket) for the start.
and try to rotate bias pot and see if the voltage is changing. if yes , put at the max negative value.

after that we can go on.

[martin manning]

Was getting 415 volts on the plates of my 12ax7’s. Took them out and put the 6v6’s in. Initially, with just the rectifier in, I was getting 450vdc on the first power supply node (I was expecting a high number unloaded, but not quite that high - 315-0-315 P.T, 5Y3), but with the 6v6’s in, I powered on, saw the 1st node hit 450V quickly, hold for about 5 seconds, and then drop down to 250 in another 4 or 5 seconds, at which point my 200w limiter bulb developed a faint glow, and a hum appeared out of my speaker.

For the bulb limiter, 200W is much too high for this amp. I would use 40-60W. Apparently you don't have any shorts, or there would have been smoke.

Get the power stage biased and operating correctly first, with no preamp tubes installed. This will bring the B+ voltage into line by drawing current through the power transformer and rectifier, and avoids hitting the preamp with excessively high voltages. A 5Y3 will typically drop ~50V with normal current draw.

Did you just build the bias supply to the fender schematic, or have you added a trimmer pot to adjust the bias? I would surely do that so you can adjust the bias point to get the best out of the bias-varying tremolo.

Do you have a way to measure idle current on the 6V6's, such as 1 ohm resistors at the cathodes?

Measure the bias voltage at the 6V6 pin 5's, and also at the negative end of the bias supply capacitor. That will tell you if bias voltage is reaching the 6V6's and if you have sufficient negative voltage available.

Also, one of my power tubes was cold, even though I confirmed proper voltage on the filament pins.
I should have tried another 6v6 in place of the cold one, but didn’t. I’m wondering if maybe the tube is shorting and that’s why I am seeing a bit of a glow, and perhaps the short is causing the filter caps to struggle to keep up? I’m really not sure.

Is this cold 6V6 really cold? The filament may not be as visible as the other one.

As for the high preamp tube voltage, I’m not sure if the two are related, or if something entirely else is going funky.

Definitely. See above.

[6G3Victory]

Thanks for the replies, guys,

With only the 5Y3 in, my power tube grid pins (and also the negative lead of the bias cap) are showing me a range of -39 to -49vdc (bias pot). I’m guessing -49vdc is going to stress that 50v bias cap…

I’ve only got the 200w bulb for the limiter so I just went with that. Will look for a 50w when I get the chance.

Martin, my bias configuration is as follows: in addition to the stock 10k bias range resistor, I put a 100k in front of it (in between rectifier pin 4, [no dedicated bias tap, hammond 290cax PT] and the wire leading to the 10k). On to the diode, and then a 10k pot with a 33k, and the 25uf/50v cap, of course.

Figured I’d mention the one 6v6 (V3) was cold to the touch compared to the 6V6 in V4, (which was just as warm as the rectifier), before I go on to install the 6V6s again. I shut the amp off about 30 seconds after the voltage dropped down to 250vdc.

At the same time, I measured the plate pins of the power tube sockets. 454vdc when the bias was set to -39, and 450vdc when the bias was set to -49.

Regarding setting the bias, no, I haven’t installed 1 ohm resistors, but rather I am planning to calculate dissipation using the P to C voltage / OTCT voltage difference method.

Perhaps I’ll try another tube in V3 and measure again?

[bepone]

looks like bias circuit need to be fine tuned.
values needed, in blind shot from here ,lets say -20--35V. you can fine tune and adjust it to cca -30V, after will be reduced to less..

cold tube , it is easy to check heater glow. if there is heater light in the tube all is ok.

need to be installed 1R ohm resistor in series with cathode to the ground, which will be used for current reading.

after that, we can go on.

probably is all ok, need to remove light bulb and start normally, voltages will be normal, but is necesary to have bias circuit ok, and R for current reading installed for fast detection.

[martin manning]

I put a 100k in front of it (in between rectifier pin 4, [no dedicated bias tap, hammond 290cax PT] and the wire leading to the 10k). On to the diode, and then a 10k pot with a 33k, and the 25uf/50v cap, of course.

Add a little resistance to increase the bias voltage (make it less negative) Maybe change the 100k to 120k, or a 33k instead of the 10k. In any case it seems you are biased too cold rather than too hot, which would leave the the voltage high rather than drag it down.

Figured I’d mention the one 6v6 (V3) was cold to the touch compared to the 6V6 in V4, (which was just as warm as the rectifier), before I go on to install the 6V6s again....
Perhaps I’ll try another tube in V3 and measure again?

Swap the 6V6's and see if the issue stays with the socket or the tube.

[6G3Victory]

Alright, so I have determined that the issue of one 6v6 not heating up is socket specific, not tube specific. Also, I see both filaments glowing, regardless of the socket it’s in.

I replaced the 10k in the bias circuit with a 33k and my plate voltages went up a bit - to 285v plate V3(the cold tube socket) and 262v plate V4.
Still powers on quickly to 450, this time it drops down to about 235v then climbs up to the above numbers.

I’m not getting any fluctuation of plate volts when I twist the bias pot, even though I got a range of bias voltage through the pot, before I swapped resistors…I forgot to check the bias voltage again. Will do first thing tomorrow.

Ok, and now a new thing. I noticed that the 56k NFB resistor was reading 1K5. I figured it was quite unlikely the resistor would be so far off, but I pulled it out, measured and got 56k, and again measured a resistance of 1K5 on each side of that empty 56k spot. I have no idea what is going on there, or whether or not it’s causing the issue with the power tubes. I know the NFB involves the OT, which is sending my plate voltages to the 6V6s, so I’m guessing that might be what’s up.

This is my 3rd build. My first two went off without a hitch. Which was great, but it also kind of means I didn’t learn much. So even though I am lost here, I’m happy that I’m actually going to learn some new things, from experience.

[martin manning]

Alright, so I have determined that the issue of one 6v6 not heating up is socket specific, not tube specific. Also, I see both filaments glowing, regardless of the socket it’s in.

This is confusing. Both tubes show the filament glowing when plugged into in one socket, but neither one glows if you plug them into the the other socket, correct? If so there is a problem with the socket, a bad solder connection, or a broken lead.

I replaced the 10k in the bias circuit with a 33k and my plate voltages went up a bit - to 285v plate V3(the cold tube socket) and 262v plate V4.
Still powers on quickly to 450, this time it drops down to about 235v then climbs up to the above numbers.
I’m not getting any fluctuation of plate volts when I twist the bias pot, even though I got a range of bias voltage through the pot, before I swapped resistors…I forgot to check the bias voltage again. Will do first thing tomorrow.

Check the B+ and bias voltages with only the rectifier tube installed. From 315-0-315, I think you should see ~400 unloaded (no power or preamp tubes installed), which should drop to ~330 loaded.

Ok, and now a new thing. I noticed that the 56k NFB resistor was reading 1K5. I figured it was quite unlikely the resistor would be so far off, but I pulled it out, measured and got 56k, and again measured a resistance of 1K5 on each side of that empty 56k spot. I have no idea what is going on there, or whether or not it’s causing the issue with the power tubes. I know the NFB involves the OT, which is sending my plate voltages to the 6V6s, so I’m guessing that might be what’s up.

The OT secondary is a virtual DC short to ground, so you have the 1k5 V2a cathode resistor in parallel with what you are trying to measure.

So even though I am lost here, I’m happy that I’m actually going to learn some new things, from experience

Exactly right.

[bepone]

Still powers on quickly to 450, this time it drops down to about 235v then climbs up to the above numbers.

you have voltage drop because of the bulb in series with the amp, remove the bulb, important remove the feedback resistor,
start normally, and you will be ok. very short powering on, 30sec, just to confirm that all is more or less ok.

[6G3Victory]

Hey guys, thanks again for the replies,

Martin, sorry for the confusion. I meant to say, each tube’s filament glows, whether in socket V3 or V4.

But I found out what was happening - I wasn’t getting a bias voltage on V4’s grid pin, nor a range of plate voltages with the adjustment of the bias knob. So I looked for continuity in the bias voltage path, from the 220K resistor over to the grid pin of V4, and there was none. To me this explains why tube V4 was getting really hot, while tube V3 was cool to the touch - V4 was probably getting cooked due to no voltage on its grid!

I reflowed and got the bias voltage to the grid. Now, the bias voltage level reacts to the pot, as do my plate voltages, which range from 360/361v to 397/400v. And, both tubes warm up together. So, great! *Note* these voltages were measured with the preamp tubes installed as well.

Preamp tube plate voltages are a bit high though (ranges due to bias adjustment):

V1A: Plate 213-233. Cathode +1.57 to +1.72

V1B: Plate fluctuates (tremolo) Cathode 0v

V2A: Plate 314-345v Cathode 0v

V2B: Plate 315-345v Cathode 0v (should be around 50v!)

So, those preamp voltages are my newest mystery..

[sluckey]

V2A: Plate 314-345v Cathode 0v

V2B: Plate 315-345v Cathode 0v (should be around 50v!)

Sounds like neither cathode resistors are connected to ground. Measure resistance between V2 pin 3 and ground. Measure resistance between V2 pin 8 and ground.

If you followed the original Fender board layout the most likely culprit is a short underboard jumper that connects both resistors to ground.

[sluckey]

Any joy?

[6G3Victory]

Sluckey,
No, not yet. I went back and checked for continuity/resistance values along every point of the 12ax7 cathode to ground paths and everything seems to check out.

So, I’m not sure what’s up!

[sluckey]

Sluckey,
No, not yet. I went back and checked for continuity/resistance values along every point of the 12ax7 cathode to ground paths and everything seems to check out.

So, I’m not sure what’s up!

Set meter to read resistance. Connect one meter probe to chassis. Connect the other probe to pin 3 of V2. What resistance do you read?

Repeat for V2 pin 8.

[6G3Victory]

Sluckey,
V2 pin 3: 1.43k
V2 pin 8: 59.4k