Fender 5F4 Super-Amp Glowing Plates

[Gabriel E.]

My '57 Super-Amp started sputtering recently. The sound got thin, quiet and distorted. Looked in the back and V4 (a 6L6) was glowing cherry red. I shut down immediately. After it cooled down I switched the tubes and the same position (not the same tube) started glowing. Checked plate voltage and V4 reads 401 while V5 reads 419. Resistance in the OT is also uneven - 132.3ohms on the V4 secondary and 95.8ohms on the V5 secondary.

I couldn't find any dead components - this amp has upgraded filter caps and power supply resistors.

Any ideas? Is the OT shot?

[clintronics]

Do the grid resistors read 470 ohm?

[Gabriel E.]

It doesn't have grid resistors.

[clintronics]

oops my bad. should have dug out the schematic. OP xfrm gets my vote.

[R.G.]

Come on, guys - think!

A pentode is a current source. It conducts only as much current as the grid-source voltage tells it to. You could SHORT the OT from CT to the plates and not have the tubes overheat as long as you keep control of the grid-source bias.

Overheating tubes is caused by too much heat - which comes from too much power dissipation, which comes from voltage times current. The voltage is fixed or even a little low on the hot tube, so - the current there is too high - .

Find out what is causing the signal grid or screen grid or both to make too much current flow. Start by measuring the grid voltage on the suspect tube. Is it as negative as the grid on the other tube? Why isn't it?

[clintronics]

This may be very unscientific but Id swap the OT wires to the ouput tube sockets to see if the problem shifts to the other tube socket or stays with the original.

[Gabriel E.]

Ok. I swapped the secondaries and it's V4 that's still glowing. So I should rule out the OT right? I'll measure the grid/screen voltage now.

[Enzo]

When a tube is red hot, the first thing to check is the bias voltage at the grid. If the bias is not there, you either have an open resistor or the coupling cap from the phase splitter is shorted/leaky.

[R.G.]

Hey, Enzo, I guess it would be better if I guessed what the folks want to hear and tell them to do that, eh?

[Mark Black]

It ALWAYS turns out to be a bad transformer, doesn't it?

Seriously though - this could be as simple as a poor connection between the tube socket and the tube pins, most likely at pin-5. A thorough re-tension would at least help rule that out and improve things in general.

[Enzo]

It does amaze me in the shop how large a percentage of people with ANYthing wrong are convinced it must be the transformer.

My mother, rest her soul, all her life, when ANYTHING went wrong with the TV, always said the same thing, "It must be the picture tube, they haven't perfected them yet." One time I recall I could not convince her that the loss of sound was not a picture tube gone bad. Mom, the picture is still there, see?

[clintronics]

I joined this forum to learn, because I, unlike you guys don't know everything. If an output tranformer with a center tap has one side reading 50% more res/ind than the other half wouldn't it be safe to assume this tranformer is not in perfect shape?

[Enzo]

No.

There are two ways to wind the coil. The easy way is to start winding. When you get to half the turns number, connect the center tap, then continue winding until all the turns are on the bobbin. Now the inner turns are all tighter around the core, so they are shorter lengths of wire, but the later half turns are wrapped around the earlier ones, so they are longer wires. The longer the wire, the higher the resistance. SO it is quite common to have lopsided resistance readings. Transformer work on turns ratios, not resistance, so they get away with this.

The "better way" is to wind with two pieces of wire at the same time, so both halves of the winding are about the same length. Then the resistances will be about the same.

Now it is certainly possible a transformer has some shorted turns on one side of the CT, and that would cause a lower resistance. But usually shorted turns show up in various ways. On RG's site, I think there is a transformer tester. That would be something to have around and would answer any question as to the tranny being shorted.

Perhaps I sounded a trifle arrogant, sorry. I don't claim to know everything, but I am pretty good at troubleshooting. The resistance mismatch at the same time as the excess current in the one side would be an example of correlation not being causality.

[Mark Black]

Great description of why you find DC resistance mismatches on OT primaries Enzo. I never really thought about the "nuts & bolts" reason before.

In any case it was reported by Gabriel E. that he had swapped transformer leads without improvement (he said "secondaries" but by the description & previous posts I think it was probably primaries). So that pretty much rules out the OT.

So, Gabriel E., have you tried re-tensioning the tube sockets? If you measure -52V (for example) at the tube socket pin-5 terminal solder-side it would not necessarily mean that bias voltage was getting to the tube itself if the contacts were relaxed. I have even seen tube sockets where the terminal itself was broken in half and there was no real connection between the wiring & the tube. Some tugging on the wires to the socket might reveal such a thing.

The fact the problem has remained with that particular socket through the previous troubleshooting tells some kind of a tale...

Best of luck!

[Enzo]

I don't think we ever got a grid voltage report.