Cathode Capacitor Smoking

[harry]

Hello,

I am building an amp based on this power amp and PI:

Linear Conchord 30 & L50 Schematic Diagram


and this preamp:

http://www.blueguitar.org/new/schem/vox/ac30_pre.gif


Sounds good but output is quite low.. voltage at EL34 plates is 320 ish... this seems low, but I am using a choke, whilst the original amp did not.

Also, something smokes when I turn it up. I thought it was the cathode capacitor, but I'm not sure now. It may be the cathode resistors. I used the resistors from the original amp until new ones arrive. Could they go old and smoke? Would an under-rated resistor cause smoke?

Any suggestions would be much appreciated!

Harry

[tubeswell]

Yep underrated resistors will burn. (Amps following thru the resistor x Volts dropped by the resistor = Watts dissipated by the resistor as heat)

BTW a choke filter will make the B+ higher than a resistor filter (because a choke has lower DC resistance). What are the voltages (incl the VAC from your PT secondaries)?

[harry]

Thanks for the reply tubeswell.

okay, VAC from the PT secondary is 720V (so.. 360-0-360?), after rectification it is 390Vdc, after the choke 330Vdc.

The plate voltage on the EL34 starts at 320V and then drops slowly but steadily as I play. I think the 5AR4 is failing. If I increase the voltage going to the plates of the preamp tubes, it really struggles when I play loud.

Might have to delay this until I can get a new rectifier.


(at least it stopped smoking now!)

[tubeswell]

A choke shouldn't be dropping 60V - Check the DC resistance it should be about 100 -150R across the choke. Off the top of my head maybe one (or both) of the filter caps on the CLC filter is starting to fail? (long shot guess). How old are the caps? You can put another cap in parallel with it and see if the voltage comes back up. Also have you tried other tubes (as well as the rectifier?)?

[Chuck H]

Second that. What's old and whats new???

What are the volts on top of the cathode resistor? what is that resistors measured value and labled watt rating?

If the filter caps are old, how old?

Are you feeding the plates through the choke? Reading your post, it sounds like you are.

Where did you get the choke? What are it's ratings?

Are you certain that all electrolytic caps are installed in correct polarity?

+ 20 more Q's, but I'll start there.

Chuck

[stokes]

Sounds like the cathode resistor is either under-rated or is biasing the tubes way too hot,which would cause the volts to drop at the plate.In any event find out why the resistor is smoking before going any further.

[harry]

Thank you very much for the replies, tubeswell, Chuck, stokes..

I replaced the cathode resistor, and I don't think it was that which was smoking. In fact, whatever it was blew up. Sounded and smelled like a capacitor but I cannot see what it was... has this ever happened for you? I have good eyesight but I am baffled.

Now there is hum, so I'm guessing either power supply filters were bad, or one of the power tubes is now not operating. I'm going to replace the filter caps and see what happens. I had tried putting extra caps in parallel where I thought the voltage might be approaching the working voltage of the filter caps... I think I will just start from scratch with fresh caps.

I could not work out what was going on with the choke. It measures about 250 ohms resistance. I took it from a carlsbro 100-watt EL34 amp. The application in its previous circuit can be seen here: Carlsbro CS100 T.C. Amp Schematic

I was feeding the plates through it; in the end I swapped it round but it caused hum, suggesting poor filtering?

Back to the bench!

[tubeswell]

In that CS100TC schematic, the choke is only supplying the pre-amp (including the PI). It was probably the choke that was smokin'. Get a new choke, but don't hook up the plates through it.

[harry]

OOPS.. well, I mis-read that schematic..

[harry]

Also, in the linear conchord schematic, in the middle of the first stage of filter caps, there is a point labelled "C". This isn't referenced anywhere else. Does anybody have any idea what this might be?

[harry]

Got rid of the choke, installed new filter caps, still hums a lot, fades in and out as you turn it on, then it settles down to a steady (fairly loud) level. Guitar output is quiet still. So there's something amiss in the power/PI stage.

[harry]

Tried it with one power tube, amp worked, albeit with hum. Put tube in other socket and just got crackling. Changed cathode capacitor and noticed that I hadn't soldered the ground (shared with cathode resistor). Works fine now, albeit a little quiet for a 30 watt amp.

Added some extra filtering, turned it on and 5AR4 went in an arcing mess. Oh well. At least the amp works.

Thank you everyone for your suggestions and help.

Here is a photo of the little beast. It needs a case.

[harry]

(Filtering was still quite low - 49uF in total - so should have been acceptable for a 5AR4)

[Howlin' Mad Mac]

Hope you get all the bugs out soon. Looks like a sharp little beast.