Kustom 72 Coupe cathode resistors

[dmartn149]

I got this Kustom 72 coupe at a local auction. It's power tubes are running really hot, one of them was starting to turn red when I shut it off. I checked the cathode resistors, in circuit, one measured 84 ohms the other was "OL" I've seen advice on the internet that these resistors should be changed to 100 or 110 ohms for cooler operation. Does this seem like a good idea? Is there anything else I should check?
72Coupe-Schematic.pdf

Thanks

[Enzo]

WHy not fix the OL one and see what happens. On most meters I know that means open or at least over scale. 84 ohms of ran 82 ohm resistor is certainly close enough.

Also, this amp uses a dual bias system, you have the cathode bias by resistor but also the -28v fixed bias on the grids. Verify you have the -28v on every pin 5 of the power tube sockets.

Verify B+ on pins 3 and 4 of every power tube socket as well.

Also it is entirely possible one or more of your power tubes is bad. That could explain red plating, but also might have caused the resistor to burn out.

[dmartn149]

Thanks Enzo. I don't have any experience with these amps, but it seems that a lot of the people who use them think that they run "too hot" and that the fix is either to install fans or change the cathode resistors. I did't want to do that without checking here first.
I'll check the pin 5 voltage and the B+ tomorrow morning.

[Enzo]

Maybe they do, but a good rule of thumb in any shop is to fix the problems first, THEN go about modifying, upgrading, changing, etc.

[dmartn149]

Ok, that's what I'll do.

Thanks

[pdf64]

When 2 or more tubes share a cathode bias resistor as here, both tubes must be good, and preferably have broadly similar cathode currents at idle, for sufficient bias voltage to appear.
Often, it seems that vintage amps were intended for lower mains voltages than are prevalent today in the USA. A good indicator of that is to measure the heater voltage at idle; if higher than 6.6Vac when fed with your regular mains voltage, then it may be beneficial to consider further action.
But yes, identify and fix the actual fault first.

[dmartn149]

Also, this amp uses a dual bias system, you have the cathode bias by resistor but also the -28v fixed bias on the grids. Verify you have the -28v on every pin 5 of the power tube sockets.

Verify B+ on pins 3 and 4 of every power tube socket as well.

I have -29.4v on pins 5 and 446 and 447v on pins 3 and 6. Measured with no tubes in amp.

[Enzo]

Then my prime suspects are the tubes.

Take out the power tubes. Pick one of the sockets that were not red plating before, and install ONE of the tubes. Does it seem to function OK? Now pull it and test the next tube in the same socket. Do all four tubes one at a time in that socket. DO any of them act up or red plate? They would be bad. You can also then take ONE tube known to be OK, and check each of the remaining three sockets one at a time with it.

Now you have verified all four tubes and all four sockets. Or you have discovered a problem with one or more of either.

[pdf64]

...Take out the power tubes. Pick one of the sockets that were not red plating before, and install ONE of the tubes...

I'm not keen on that because the cathode bias part of the mixed bias system requires 2 functional tubes per cathode resistor in order for the intended cathode voltage to develop.
With only 1 tube in place, the cathode voltage will lower than normal, and it will likely run hot.
Maybe it will be hot enough to red plate, maybe not, but it just doesn't seem a good test to me, especially as the screen grid HT node will be a lot higher than normal with only 1 screen grid drawing current / dropping voltage, across R74, and a much lower load on the HT supply generally; hence the operating conditions will be rather hotter than the designer intended.

It would be a good idea to verify that the power tube pin 5 voltages are 0Vdc.

[dmartn149]

I have other 6l6 amps. Would it be any risk to them (the amps) to try the suspect tubes?

I have one that's cathode biased, and another that's fixed biased.

[g1]

A bad tube will sometimes take out screen resistors, so that is a possibility if trying these tubes in another amp.
Did you replace that burnt cathode resistor? If you put something around 150 to 200 ohms there, you could use that pair of sockets for testing. That would address pdf64's concerns about higher idle without all tubes installed.

[dmartn149]

I just finished replacing that resistor. Put the tubes back in the sockets that each came from. After leaving it on for about 20min. then playing through it about 15 it seems a little hotter than most, but no red plates. I think I'll button it up, and enjoy it as is. Lots of cool features on this one.

Thank you-all

[Enzo]

SOMETHING burnt that resistor out. COuld have been a previous tube, who knows. Before you button it up, I recommend you check voltage across both resistors to see if they are similar. Also verify screen voltage on each socket.

[dmartn149]

SOMETHING burnt that resistor out. COuld have been a previous tube, who knows. Before you button it up, I recommend you check voltage across both resistors to see if they are similar. Also verify screen voltage on each socket.

OH MAN! I just finished putting it all back together including the cage over the power tubes. It sounds good and running cooler. maybe I'll open it up again later