Burning Screen Resistor

[pdf64]

Even a half watt one ohm resistor will handle 700 mA...

It may be that 700mA cathode current is exceeded in the zero bias state.
Don't ask me how I know, but 1ohm 0.6W MRS25 MF resistors used in this application will quickly fuse if bias fails

[pompeiisneaks]

I like the question about 'moving tubes' does the problem follow the tube or the socket? You can narrow it down to a specific socket if it doesn't move, and then solve either the problem in that socket's connections OR maybe you've got a bad socket.

~Phil

[bal704]

Sorry for the delay....It follows the socket. I'm using the bias jacks connected via a 1R resistor method, so I'll swap those out and see if that fixes anything.

[pompeiisneaks]

Sorry for the delay....It follows the socket. I'm using the bias jacks connected via a 1R resistor method, so I'll swap those out and see if that fixes anything.

good find. It could also be the socket, inspect it for arc marks or cracks etc. Depends on what type it is, if its ceramic it could have a crack, if it's some form of plastic it may have melted somewhere, or just a pin isn't making great contact, so try re-tensioning the pins etc.

~Phil

[sluckey]

There is also a good chance that your OT is now damaged as a result of having zero volts negative bias on pin 5 of the output tubes.

Easy way to check... Disconnect the NFB wire from the speaker jack and swap the OT primary plate leads between sockets. Did the high current move to the other socket?

[bal704]

Using a adapter cable, I'm putting youtube music through the amp. I'm getting a nasty buzzy sound out of the speakers. While you can still hear the music, there's a horrible buzz on top of it. Using an audio probe, I'm getting very clear, nice sounding music to pin 5 of each output socket.

I'll check the socket and swap wires as suggested when I get a chance. Maybe try new tubes. Hate to think I smoked the OT.....

[pdf64]

Using a adapter cable, I'm putting youtube music through the amp. I'm getting a nasty buzzy sound out of the speakers. While you can still hear the music, there's a horrible buzz on top of it....

Using a desktop PC as the signal source? If so, try a battery powered device such as a smartphone or mp3 player.

[bal704]

Lifted the negative FB, swapped the OT leads between sockets. The difference in bias current follows the OT leads. For example, my white lead had 40mA on the right socket, now has 40mA on the left socket after I moved the white lead there.

[sluckey]

Sorry to hear that.

[bal704]

Does that mean my OT is toast?

Sorry to hear that.

[martin manning]

It would seem so. Some other measurements you could make to confirm that would be measuring the DC resistance of each side of the primary (the resistances should be close, within 10% or so, and around 75Ω), or putting some AC on the secondary and looking for matching AC voltages on each side of the primary.

[pdf64]

I'm a bit sceptical; what is the Vdc from the OT CT to each plate?
I think that to affect plate current so much, the 'cold' leg would have to be dropping a lot of Vdc.

[pompeiisneaks]

I'm a bit sceptical; what is the Vdc from the OT CT to each plate?
I think that to affect plate current so much, the 'cold' leg would have to be dropping a lot of Vdc.

You may be skeptical, but considering that it's been proven by deduction that :

a. it's not the tubes (problem stayed with socket with tube moves)
b. it's not the sockets (problem moved with transformer wires)

What other options do you consider next?

Additionally it does make sense to me if for some reason there's a shorted winding, say one winding shows 75 ohm like sluckey says, what if the other shows 40 due to a shorted winding? That means current flows through that half much more readily due to less resistance. If you set the bias to some reasonable value for one side, the other's going to not play as nicely. No?

~Phil

[pdf64]

It does not seem feasible that even a 'worst case' OT winding, shorted completely from plate to CT, could cause such a discrepancy in plate current.
Consider that if one OT leg was 75 ohm and the other 0 ohms, given the same g1 and g2 voltages, if the plate current of the 75 ohm leg was 20mA, the plate of the 'shorted' leg would be 1.5V higher than that of the 75 ohm side.
That difference is negligible, it would make almost no difference to plate current; what controls plate current are the g1 and g2 voltages. The function of g2 is to 'decouple' plate current from plate voltage.

See the tube charts, eg the closest comparable being at the top of p7 of http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf
With g2 at 400V and g1 -40V, plate current is flat over a wide range of plate voltage.
It should be apparent that a volt or 2 change in plate voltage will have a negligible effect on plate current.
The plate voltage would have to drop from the usual ~450V to ~100V for the plate current to fall from ~50mA to ~25mA.

For such a change in plate current to have been caused by a discrepancy in the winding resistance of the OT primary legs, with the 'hot'side resistance less than 100 ohms, the 'cold' side resistance would have to be ~15k.
Hence my enquiry about the Vdc across either leg of the OT primary.

Hope that all makes sense.

[pompeiisneaks]

It does not seem feasible that even a 'worst case' OT winding, shorted completely from plate to CT, could cause such a discrepancy in plate current.
Consider that if one OT leg was 75 ohm and the other 0 ohms, given the same g1 and g2 voltages, if the plate current of the 75 ohm leg was 20mA, the plate of the 'shorted' leg would be 1.5V higher than that of the 75 ohm side.
That difference is negligible, it would make almost no difference to plate current; what controls plate current are the g1 and g2 voltages. The function of g2 is to 'decouple' plate current from plate voltage.

See the tube charts, eg the closest comparable being at the top of p7 of http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf
With g2 at 400V and g1 -40V, plate current is flat over a wide range of plate voltage.
It should be apparent that a volt or 2 change in plate voltage will have a negligible effect on plate current.
The plate voltage would have to drop from the usual ~450V to ~100V for the plate current to fall from ~50mA to ~25mA.

For such a change in plate current to have been caused by a discrepancy in the winding resistance of the OT primary legs, with the 'hot'side resistance less than 100 ohms, the 'cold' side resistance would have to be ~15k.
Hence my enquiry about the Vdc across either leg of the OT primary.

Hope that all makes sense.

That seems to make sense all things considered, but I still venture my question, what is the cause of the behavior then if you've ruled out everything?

It must be the only thing left, but there's nothing I can see as 'left' It's not tubes, sockets, connections, or OT. What else would cause a tube to have significantly different current in this situation?

~Phil