Screen voltage higher than plate voltage?

[dude]

Fender Vibro Champ BF.  I have an old SE OT that has a 34:1 ratio, want to replace the undersized 5 watt OT with this old paper wound  that is about double the size of original, guessing about 8/10 watts.I want to use a 6L6B tube, a Sovtek 23 watts dissipation. Kept the schematic original, except use a 5K dropping R instead of the 1K first node, wanted to lower the screens a bit. Bias cathode R is 247 ohm and measures 20.30v across it.  The B+ is 356v, screen 324v, plate to cathode is 290v ?, plate to ground 310v.
Why is my 6L6B screen higher than my plate voltage? does this indicate that the OT is bad, or the 6L6B? (using a 10" 8 ohm speaker, PT has a CT)

https://el34world.com/charts/Schematics/files/Fender/Fender_vibro_champ_aa764.pdf .

[pdf64]

So what’s the resistance of the OT primary?
I guess 600-700ohms.

[Esquirefreak]

Maybe bias a little colder to get the plate current lower and thus rising the plate voltage? If I did my math correctly your plate dissipation is roughly 25w. Maybe try a 330Ω resistor? I'm just thinking out loud.

One question I have, though, is if the PT will handle the filament draw of the 6L6 in the long run?

/Max

[dude]

So what’s the resistance of the OT primary?
I guess 600-700ohms.

Yes, 626 ohms

[dude]

Maybe bias a little colder to get the plate current lower and thus rising the plate voltage? If I did my math correctly your plate dissipation is roughly 25w. Maybe try a 330Ω resistor? I'm just thinking out loud.

One question I have, though, is if the PT will handle the filament draw of the 6L6 in the long run?

/Max

Yes, l can try that, running a little hot at about 25watts but l can’t see the plate voltage rising mote then 10v’s, probably less.
I know the 6L6 would like to see 5k and with the 34:1 OT l get 9k with the 8 ohm speaker but that wouldn’t lower the plate voltage

[sluckey]

Bias cathode R is 247 ohm and measures 20.30v across it.

Why so hot??? Put a 470Ω on the cathode. Any better?

[HotBluePlates]

... screen 324v, plate to cathode is 290v ...
Why is my 6L6B screen higher than my plate voltage? ...

You've already found out "why."

But know that unless the plate voltage is <70v then having a screen voltage higher than the plate doesn't matter.  In fact, when you play the screen voltage is higher than plate voltage about half the time anyway.

[dude]

Bias cathode R is 247 ohm and measures 20.30v across it.

Why so hot??? Put a 470Ω on the cathode. Any better?

Don’t have a 10 watt, can l use a 5 watt 500 ohm or have to use (2) 250 ohms in series. SE 6L6, rather use one 5 watt, 500 ohm

[sluckey]

Don’t have a 10 watt, can l use a 5 watt 500 ohm or have to use (2) 250 ohms in series. SE 6L6, rather use one 5 watt, 500 ohm

5 watt may be OK. Put it in. Then measure the voltage drop across the resistor. Do the math. P=E²/R. Double that number. If less than 5, you're OK.

[dude]

5 watt may be OK. Put it in. Then measure the voltage drop across the resistor. Do the math. P=E²/R. Double that number. If less than 5, you're OK.

With the 5 watt, 470 ohm l get about 18 watts dissipation, l think the Sovtek is 19 watts, says 6L6B so, good enough.Math: 26.42 sq’d = 698/470= 1.48 x 2 = 2.9 wattsHope l got that right, so my 5 watt R is fine. After 10 minutes of heavy play, turned off amp, 470 feels very hot to touch, wouldn’t want to leave my finger there long. Am l correct?Voltages: B+ 378, plate to cathode 314v, 26.42v across KR, 470 ohm R.From ground plate 342, screen 344, as HBP say not an issue.Just want to sure l got the math right on the 5 watt cathode bias R..?Amp sounds good but higher voltage then specs on 1st 12ax7, 209/190, l’d like to brown the preamp out a bit, l think raising the B node R from 10k to maybe 18k might do it..? I also took the 22uf first gain stage cathode to 4.7, might need to go back to 22uf. Sound right.Thanks again

[sluckey]

Math is right. Once you have the power amp right, the preamp voltages are just a matter of changing dropping resistors. Resistor should be hot. Mount it with plenty of air space around it.

[dude]

Now, l need to keep my eye on the PT, enough to handle the 6L6..?
I assume, and the PT looks original, after the 10 minutes digging in, PT not hot, but a little warm. I think others have used a 6L6 with no issues in older sixties BF amps…?

[HotBluePlates]

Fender Vibro Champ BF.  ...

Now, l need to keep my eye on the PT, enough to handle the 6L6..?
I assume, and the PT looks original ...

Look at the transformer part numbers listed on the schematic you linked.
Now look at the transformer part numbers listed on the Princeton Reverb schematic and compare to the numbers on the transformer in your amp.


    "Forum Lore" is that the "blackface & silverface Princeton and Princeton Reverb amps used the puny Champ/Vibro Champ power transformer."

    Reality IMO is that the "Champ/Vibro Champ used the oversized Princeton and Princeton Reverb power transformer."  It can support 2x 6V6 and 4x 12A_7s but is loafing along with half that much in the little amps.

Measure the heater voltage to confirm the transformer is not over-taxed.  I bet it comes in even a little above 6.3vac.

... Amp sounds good but higher voltage then specs on 1st 12ax7, 209/190 ...

The schematic is not Gospel, and they have errors here & there (even though Fender is generally pretty good).

Look at that Vibro Champ schematic & the Princeton Reverb schematic again.  Same power transformer part number, right?  And that PT is under-loaded in the Vibro Champ, which normally leads to higher voltages, right?  So what do you see for AC Volts output of the high-voltage winding in each schematic?  315v on the Vibro Champ schematic, but 340v on the Princeton Reverb schematic.

Neither schematic figure is correct for what you'd measure in an a physical amp.  I used to own a 1965 Vibro Champ with the 125P1B power transformer.  As-is, I had to variac the outlet voltage down to 106.9vac to get 6.3v on the heaters (because the PT was very under-loaded).  When I did that, I had 323vac output on the high-voltage winding and 366vdc at the 6V6 plate.

[dude]

I used to own a 1965 Vibro Champ with the 125P1B power transformer.  As-is, I had to variac the outlet voltage down to 106.9vac to get 6.3v on the heaters (because the PT was very under-loaded).  When I did that, I had 323vac output on the high-voltage winding and 366vdc at the 6V6 plate.

Ok, got ya. I have a bucking transformer to keep voltages down. But in a previous post you mentioned to keep the 470 biasing R and use the rail R’s to get the voltages you want to screens and preamp. Ok, but l can’t lower the plates except with the bucking transformer, (l have a few setting on it), l can lower the screens with dropping R’s and preamp. I needed a 10k on Node A (was 1k) to get screens in 325 range, and needed a 33k to get preamp voltage in the 180v range, which l perceived the tone was best.  IMO, if the preamp is too high over 200/220, the amp looses  compression and tone to me, sounds more like fixed bias, too sterile for me. Upping those nodes valves , even if extreme against the original schematic, is not an issue, l assume?
I appreciate your knowledge.

[HotBluePlates]

... I have a bucking transformer to keep voltages down. ... l can’t lower the plates except with the bucking transformer ...

Low plate voltage on the 6V6 doesn't really matter.  If you like what you get using the bucking transformer, so be it.

... Upping those nodes valves , even if extreme against the original schematic, is not an issue, l assume?

Nope, you're good as long as you like the results!