200S Sunn

[Valvehead]

Im stumped on this one...here we go..

69' 200s. Blowing the main fuse. Also loud hum.

Amp came in and it worked but had an old pair of Ruby 6550. 1 was bad. I had the owner get some new Sovtek kt88.
new tubes in ..bias is good , voltage is good...now we have a loud 120hz hum. i have replaced the 12ax7 and the 6AN8 .

If the amp is powered on with a bulb limiter, 60w or 200w bulb....No short...bulb doesnt get any brighter...no matter what.

If i power it on with the 6AN8 in--no limiter---rectifier tube puts on a BIG light show and fuse blows. Power on without 6AN8---fuse is ok. Its not the 6AN8..i have a replacement here. Have several rectifier tubes---they all do the same.

Voltages at the 6an8 are a bit high at pin6 = 102vdc , and pin 3 = 115vdc = scem says 75v/ 90v .

Someone replaced all the filter caps 1 year ago . I doubt its that ? Amp worked fine for 1 year . None of the filter caps show a short .

I cant find the fault. Pulling my hair out....can you guys walk me through this one ?

ALSO--- ive cleaned and scrubbed the tube sockets with alcohol and contact cleaner thinking it was an arc there---cant see any issues




ACDSeeLavoro di stampa (el34world.com)

[Delta362]

Is the 390uuf cap on node E at the power tube shorted or leaky?

[Helmholtz]

Check the 0.25µ coupling caps for leakage/short.
To test you can just lift one end of each and see if the amp powers up without issues (with all tubes in).

[Helmholtz]

new tubes in ..bias is good , voltage is good...

How did you check bias and "voltage" when the amp blows the fuse?

[g1]

What happens when power tubes are removed?

[Valvehead]

How did you check bias and "voltage" when the amp blows the fuse?

it wasnt blowing it for a while...just kind of came out of nowhere

[Valvehead]

What happens when power tubes are removed?

power tubes out , but other tubes in = fuse does not blow

[Valvehead]

Is the 390uuf cap on node E at the power tube shorted or leaky?

not shorted , tests 430pf out of circuit

[Helmholtz]

it wasnt blowing it for a while...just kind of came out of nowhere

Ok, but how did you check bias: Measuring bias supply, grid voltages or idle currents?
And what did you mean with "voltage is good", what voltage?

[Valvehead]

Check the 0.25µ coupling caps for leakage/short.
To test you can just lift one end of each and see if the amp powers up without issues (with all tubes in).

They are .1 in this amp . They test .1 out of circuit / not shorted.

With power tubes out , the voltages across these are
-73v / 352v
-74v / 136v

with both caps lifted out , all tubes in = fuse does not blow

[Valvehead]

Ok, but how did you check bias: Measuring bias supply, grid voltages or idle currents?
And what did you mean with "voltage is good", what voltage?

shunt method at OT , idle current

voltages test good at pins 3/4/5/ of power tubes and as listed above at 6an8

[Helmholtz]

not shorted , tests 430pf out of circuit

I guess you mean 430µF instead.
Anyway, capacitance doesn't tell about leakage.
And DMMs read high capascitance when a cap is leaky..

[Delta362]

Since pin 6 is more positive than it should be, with the tubes out, did you confirm the resistances for both sets of plates and cathodes of V2? What about the plate cap on V2?

[Helmholtz]

with both caps lifted out , all tubes in = fuse does not blow

Did idle currents change?

[Helmholtz]

shunt method at OT , idle current

What current values?
Curious, because a grid bias of -74V looks extreme. Should result in low idle currents.

Is your meter battery fresh?
What are actual B+ and heater voltages?

Do you have a scope?
There might be oscillation involved.

This amp uses hot standby switching. Modern GZ34s don't like that and tend to arc.