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6l6 mystery amp

Carbsandcaps said:
I have the pots centered. I removed the caps from pin 5 and it still has low voltage. With full b+ to the screens and lower voltage on the plates isn’t that going to make the tube flow like crazy?
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This is not correct.

Oh, to have the schematic... ;)
 
I put a 1k resistor and 47uf cap between the b+ and screens to lower it a bit. Now I have to figure out the cathode resistor. I’m just worried the power supply might be not be big enough
 
I did find another tap out of the transformer that was taped up it’s got 360vac it shared the center tap with the other tap that’s being used. I wonder if I should use that for the screen voltage instead
 
Carbsandcaps said:
I did find another tap out of the transformer that was taped up it’s got 360vac it shared the center tap with the other tap that’s being used. I wonder if I should use that for the screen voltage instead
Click to expand...
Usually, the screen voltage is part of the main power supply. One tap off the main transformer.

This schematic is for example's sake. :)

super_reverb_plate_and_screen_taps.jpg
 
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its honestly not uncommon to feed the screens from the same place the transformer comes from. Maybe not ideal, but it can be done, and having the screens 5v or so higher than the plate thanks to drop within the transformer isn't unheard of. UL designs also typically have screen voltage slightly higher than plate voltage for the same reason. Triode-wired also runs screen equal to plate voltage for obvious reasons. Thats not why this is being dumb, though its possibly something that can be tweaked later and might want to be depending on what the output transformers actually are. We're just dealing with idle conditions here which is way less complex.


54v / 310R = 175 ma per pair or 87.5ma per-tube. Somehow making the resistor larger has increased the current by 50ma per pair which just isn't possible. Since the plate voltage is up it seems like the current has dropped though. Since the pot is 150 ohms, thats about 75 ohms per side at 87.5ma so there is an extra 6.5 volts across that, so basically 60 volts cathode to ground.

just plugging some numbers into a tube curve calc, assuming 360 cathode to screen and plate, and aiming for 87.5ma it should be about 25 volts from cathode to ground, and you're clearly well above that. Given 350v at the plate and an expected ~60 at the cathode, thats only 290 volts across the tube which would need even less bias voltage.

Something is wrong. I don't know whether its measurements or what but this just does not make sense.

Can you confirm the grids are at zero volts and that its not oscillating? Maybe yank whatever tube is immediately upstream of all this, or unhook the coupling caps from the upstream stage to make sure we're not chasing the wrong problem here.
 
gadget73 said:
its honestly not uncommon to feed the screens from the same place the transformer comes from. Maybe not ideal, but it can be done, and having the screens 5v or so higher than the plate thanks to drop within the transformer isn't unheard of. UL designs also typically have screen voltage slightly higher than plate voltage for the same reason. Triode-wired also runs screen equal to plate voltage for obvious reasons. Thats not why this is being dumb, though its possibly something that can be tweaked later and might want to be depending on what the output transformers actually are. We're just dealing with idle conditions here which is way less complex.


54v / 310R = 175 ma per pair or 87.5ma per-tube. Somehow making the resistor larger has increased the current by 50ma per pair which just isn't possible. Since the plate voltage is up it seems like the current has dropped though. Since the pot is 150 ohms, thats about 75 ohms per side at 87.5ma so there is an extra 6.5 volts across that, so basically 60 volts cathode to ground.

just plugging some numbers into a tube curve calc, assuming 360 cathode to screen and plate, and aiming for 87.5ma it should be about 25 volts from cathode to ground, and you're clearly well above that. Given 350v at the plate and an expected ~60 at the cathode, thats only 290 volts across the tube which would need even less bias voltage.

Something is wrong. I don't know whether its measurements or what but this just does not make sense.

Can you confirm the grids are at zero volts and that its not oscillating? Maybe yank whatever tube is immediately upstream of all this, or unhook the coupling caps from the upstream stage to make sure we're not chasing the wrong problem here.
Click to expand...

The coupling caps are all out, there’s .1vdc on the grids. Is it possible that this is a 6v6 amp and not a 6l6? Here’s the specs on the power transformer. It’s rated at 250ma. It’s the r-35bc IMG_3758.png
 
Mr Boochie said:
I'm sorry, but I'm still not following you. A 250mA PT would work in a 6L6GC push-pull pair amp --- IF there weren't too many other sub-circuits in the unit drawing current.

But this doesn't address your voltage reading issues. (I don't think). :idea:
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I’m just trying to figure out if the tubes are drawing too much or if the pt can’t keep up
 
voltage seems too high to be for a 6v6. 380 volts with cap-input 5U4 rectifier like the blurb says seems about right. 6v6 you'd want closer to 300. 380 volts less the bias voltage is pretty near perfect for a 6L6G/GA/GB/5881 type though.

looks like its got taps, which would give a lower voltage supply. Shame its a little on the low side, a second pair of taps with the second 5v winding is a nice way to make for a solid screen supply. Could still be done, using something like a 5ar4 or 5V4 plus the relatively low current draw of the screens would give a little more voltage. Direct heat on the plate side, indirect on the screen source to bring it up after the plates go hot. It would make a little less power than datasheet spec but it would still do acceptably. Thought for another time though.


something just seems to be drawing too much current, and I don't know why. The bias resistor should be more than large enough. No coupling caps eliminates leakage and oscillation. Only other thing that seems to make sense to me is the tube(s) are bad but all 4?
 
Ok so with no tubes in it except the rectifier I get 450v.
With 1 output tube I get 310v
With 2 output tubes I get 305v
With 3 output tubes I get 290v
With 4 output tubes I get 287v
 
I'm unsure if it matters with your amp, but do you have a proper resistor or speaker load on the output transformer?
 
Mr Boochie said:
I'm unsure if it matters with your amp, but do you have a proper resistor or speaker load on the output transformer?
Click to expand...
I have nothing on the output transformers. They look like a multi impedance design. I can’t tell which wire is which and some of them are tied together but I thought that dc couldn’t be affected by a transformer. They’re all tied to this organ style speaker plug IMG_3764.jpeg
 
Carbsandcaps said:
I have nothing on the output transformers. They look like a multi impedance design. I can’t tell which wire is which and some of them are tied together but I thought that dc couldn’t be affected by a transformer. They’re all tied to this organ style speaker plug View attachment 3224469
Click to expand...
I think that you should put a load onto the secondary. Either a dummy load (resistor) or speakers. :idea:
 
Ok I put a speaker on the output and didn’t change anything. I been put a 1k ohm resistor on the cathode just to test it and it only brought the voltage up to 330v.
 
Im so confused. I’ve tried multiple combinations of resistors and according to the online bias calculator, this should be barely drawing any amperage yet it’s dragging the voltage down. I even put diodes in place of the rectifier and it’s still draws the voltage down by 100v once the tube start flowing. I’m really wondering if this transformer is bad. Do power transformers get weak? I thought it was all or nothing with them.
 
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