burned HT fues / Low heater voltage

[sbirkenstock]

Hi Everybody,
a friend brought his broken Marshall 4140 over.
The two HT fuses connecting the transformer with the rectifier diodes were burned. (this amp has a lot of fused by the way....)
One of the KT77 power tubes had a white deposit, so we assumed the tube might be the cause for the burned fuses.
Strangely this is the only tube out of the four that does not have silver deposit on the top, and you can clearly see the getter. Please see attached photos.
So maybe a faulty tube out of production (where the getter did not flash)???


I did replace the KT77 with four new ones and the amp came back to live.
I did check the heater voltage and it was only 5.8V AC.
If I remove two power tubes, it is a basically perfect 6.25V AC.

So it is safe to run the amp with 5.8V on the heaters?
Could it mean that the transformer has an issue and will burn up since it can´t deliver the required voltage?
Also the transformer looks a bit corroded.
I am not sure if that means anything, I heard that rost is no issue but this looks kind of some wax melted out.... Please see attached photos.

Also, would 5.8V be enough for tubes or would they suffer?

Any other guesses why the HT fuses could have burned?

Any hints welcome!

Thank you,
Stephan

[martin manning]

The tube with the white deposit on the inside has lost its vacuum, which causes the silvery getter material to turn white. That one and the one on the far left have been running hot, since the red logo has turned or is turning black. If you haven't done so, check the idle current on all four tubes to make sure that one side or the other of the power amp isn't drawing excessive current, which would indicate another problem such as a leaky coupling cap. The 5.8V heater voltage is within the acceptable +/-10%, but the drop from two to four tubes seems high. If the idle bias is good, I would let the amp idle for a while and monitor the power transformer to see if it overheats.

[sbirkenstock]

Thank you Martin,
I used the TAD Biasmaster and all four new tubes (JJ KT77) are very close.

[pdf64]

Well done checking heater voltage, it often gets overlooked.
However, a corresponding / concurrent mains voltage reading would be a good idea. It may just be that it was running low at the time.

Is there fusing in the heater circuit circuit?

Check for dry joints in the heater circuit.

[R.G.]

Low voltages out of the PT is one possible indicator of too-high current drain pulling the PT down.

(1) is the HT voltage correct when the heaters are low voltage? My first guess would be no.
(2) running it on two output tubes reduces the total HT current; this reinforces the "too much current" guess
(3) rust on the PT is not an issue unless it's really bad
(4) running the power tubes at 5.8V won't hurt them, as the tolerance on the nominal 6.3V heaters is +/-10%; -10% is 5.67V
(5) wax melted out is an indicator that it's been run hot; this is another indicator that there is some current overload; not fatal, but an indicator of earlier abuse
(6) other causes of overloading on the PT include failing rectifiers and/or leaky filter caps

If I were doing this repair for a friend, I would put in new better-than-original rectifier diodes, and ask the friend if he wanted to pay for a full cap job, replacing all the electrolytics in the amplifier. These are things that may not be needed, but could be a problem, and will improve the amp's expected lifetime in any case. I would also clean all the tube sockets.

[Stevem]

That amp originally came with 6550 outputs.

I don’t have time right now to look, but don’t KT77’s need more Heater current ?

Also what is your wall voltage there, under 117?

[didit]

Hello Steve --

That amp originally came with 6550 outputs.

I don’t have time right now to look, but don’t KT77’s need more Heater current ?

Also what is your wall voltage there, under 117?

Heater for KT77 is 1.4A and 6550 (and KT88) is 1.6A.

Encourage working through R.G.'s recommended #1 concern to check and likely cool down power tube bias.

Best .. Ian