Super Champ XD

[lowell]

Hey folks,

I'm a little out of practice these days. Got this amp in the shop, with no sound, dead. There is no bias current. Literally 0mv.

Vplates - good 410v
Vscreens - good 410v
Bias voltage is about -33v
Cathodes are good with about 500-700mv, and they measure pretty much zero ohms to ground/common (there is a diode and 1ohm in parallel in these amps)

I've brought the bias voltage all the way down to -5v... zero change in the bias current.

I've checked that the rectifier ground has continuity to the cathode grounds as well to be sure, and it's all good.

I'm getting 150ohm across both primaries, and about double, 315ohm, from end to end.

My thoughts - Bad output transformer. Would you all agree?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

I'm confused by this. You say you have "no bias current". Then you say you have 500-700mV at the cathodes. That's boat loads of voltage across a 1 ohm cathode resistor translating to very hot bias- well into red plate range. When you say you have "no bias current", where and how are you measuring it?

Edit: Have you checked the speaker? Do you have a scope?

[Enzo]

Are the tube heaters glowing?

[xtian]

I'm confused by your post. On the schematic, at TP22 you should expect 40mV (0.040V) DC. That's where you measure the current flowing thru the pair of power tubes. 500-700mV would be burning up.

Are your power tubes shot?

[The Dude]

......Are your power tubes shot?

If not, they soon will be!

[g1]

It sounds to me like the cathode resistors are open and you are getting .7V across the diodes. But you said you measured close to zero ohms across those 1 ohm resistors. Maybe check them again.

[lowell]

Duh, sorry, like I said - out of practice.

Vk is .6mv - so basically 0mv.

I have 0mv across the primaries, and with shunt method - 0ma.

Yes tubes are glowing.

There is clearly no current going thru the tubes

[lowell]

Well, I'll have to say this is a FIRST for me. It was the tubes. I didn't try a new pair of tubes cause I've never seen BOTH power tubes in a push pull amp not working (not drawing current). Strange... all fixed! Got 40mv Vk dialed in, amp works great.

Sorry to waste everyone's time.

[vintagekiki]

I'm a little out of practice these days. Got this amp in the shop, with no sound, dead. There is no bias current. Literally 0mv.

Vplates - good 410v
Vscreens - good 410v
Bias voltage is about -33v
Cathodes are good with about 500-700mv, and they measure pretty much zero ohms to ground/common (there is a diode and 1ohm in parallel in these amps)

I've brought the bias voltage all the way down to -5v... zero change in the bias current.

I've checked that the rectifier ground has continuity to the cathode grounds as well to be sure, and it's all good.

I'm getting 150ohm across both primaries, and about double, 315ohm, from end to end.

My thoughts - Bad output transformer. Would you all agree?

+ HV and -Bias voltages are good
OT resistances (2 x 150 = 300 Ohm) are good. Output transformer is good.
Plates voltage (TP37 / TP38) and screen voltage (pin 4 V2 / V3) are good but no bias current (TP22).
Check that D3 is not shorted.
Replace V2 / V3

1)
Sorry for the late reply.
I didn't see you find the fault and fix the amplifier.
Congratulations.