Help troubleshooting 5F1 issue

[jlatrace]

I have a 5f1 that I put together several years ago and have been playing it off and on. A couple of days ago I was playing it (on 10) and I lost a power tube. I retubed and when I play at high output I see a flashing inside the new power tube and I lose some sound.

Any suggestions on what might have blown along with the power tube? All the resistors test ok on my voltage meter.

My voltages at idle are as follows:

Recto p8= 389 vdc

6v6 p3= 378 vdc
p4= 320 vdc
p5= 0 vdc
p8= 19 vdc

12ax7 p1= 171 vdc
p6= 172 vdc
Thanks,
Les

[tubeswell]

Need more information - 378 does sound a teency bit on the high side for the plate - but whether it would be harmful or not depends on your tube dissipation. If its arcing, it might be biased too hot. Need to know what is your cathode resistor resistance? (Then we will be able to determine tube dissipation and work out if its within the ballpark).

On the other hand, it could be your old tube worked better at a higher voltage, or it could be that when it went down, it did something funny to the PT.

BTW what kind of rectifier tube have you got in there? And what voltage is your HT winding? 389 sounds high for a 5Y3GT off a 'standard' 5F1 PT HT. (By standard HT winding, I mean something around 320-0-320).

[MWJB]

378v at the plate isn't too high in a real world situation, yes high compared to the Fender schem, but wall voltages have risen since then and whether the amps ever ran the voltages quoted is debatable. Champ PTs (125P1B) are more likely to have a B+ secondary nearer the 350-0-350VAC mark.

With a stock 470ohm cathode resistor, even a no name 6V6 should last for years at over400v on the plate (measured to ground).

Check for arcs at the tube socket & any stray filaments of wire that may cause arcs, check speaker & connections/integrity of the speaker wire, especially if you are using an RCA jack (replace with 1/4").

Check the output transformer, you can apply a 0.5VAC signal to the OT secondary and, with the power tube pulled, measure voltage developed accross the primary & work out turns ratio...on the other hand, if you don't have a signal source & 2 meters, simply subbing the OT might be quicker & cheaper?

[jlatrace]

Thanks for the help. I will run these to ground later today.

I have a quick follow-up question. You suggested I check the OT and compute the turns ratio which I understand will comfirm that my OT is properly matched with my speaker.

Will this also tell me if I have fried my OT?

(My cathode resistor is stock at 470ohms and my line voltages run high at around 122vac. I am running a 5Y3GT recto. )

[MWJB]

"Will this also tell me if I have fried my OT?" Yes, if you have a 4ohm speaker & matching secondary on your OT, you should see a voltage of between say 18.5VAC (TR 37:1) & 21.6VAC (TR 43:1) accross the OT primaries with 0.5 VAC applied at the secondary.

You MUST use 2 meters, one to ensure that exactly 0.5VAC is going into the secondary, if you try & use the dial on your signal generator to set the voltage you'll get a spurious reading.

You could also measure the DCR of the OT primary, but you really need a reference from an identical working OT.

[MWJB]

Should clarify that this voltage test is done without speaker attached.

[jlatrace]

MWJB
Well I think I found the issue (or maybe two issues).

I checked the tube sockets more closely as you suggested and found a glob of solder on one of the PT filament pins that likely was causing the arching.

However, I also I noticed that the pin 3 wire was cracked, but still touching pin 3 on the socket pin. I guess this also could be aggrevating the issue if not causing it.

Anyway, I think I've found the issue and avoided more problems later.

Thanks for the help,
Les