200S Sunn

[Valvehead]

Did idle currents change?

it dropped a little maybe 3 to 5ma

[Valvehead]

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?

those resistors test as they should .
the 12pf plate cap measures 54pf out of circuit
the 390 cap definitely tests 430 PF on my cap meter, i dont get anything in the UF range . its simply labeled " 390"

[Valvehead]

This is the hum it made before it started popping fuses

https://youtube.com/shorts/JsyY2NpMBEk?feature=share

[Helmholtz]

the 390 cap definitely tests 430 PF on my cap meter, i dont get anything in the UF range . its simply labeled " 390"

Oh, I see, schematic says 390µµFwhich is an outdated way of specifying 390pF.

[g1]

Any chance the OT phasing got reversed? Like Helmholtz mentioned, could be oscillation. There is a feedback point at the cathode circuit of the 6AN8, so if it were a phasing issue, removing the 6AN8 or the coupling caps from the PI might kill the oscillation.

[Valvehead]

Any chance the OT phasing got reversed? Like Helmholtz mentioned, could be oscillation. There is a feedback point at the cathode circuit of the 6AN8, so if it were a phasing issue, removing the 6AN8 or the coupling caps from the PI might kill the oscillation.

Looks factory.. would osc cause the rectifier tube to go into a fireworks show?

[g1]

Yes, heavy current at power tubes can make the rectifier do that.
I would guess you have an oscillation either in the 6AN8 stage or earlier in the circuit. Removing either the 6AN8 or the PI coupling caps stops anything (signal or oscillation) from getting through.
I guess with only the first preamp tube removed, you still have the problem?

[Valvehead]

Ok...here we go..
12ax7 out...amp running..does NOT pop fuse !!

tried another 12ax7 - same thing when installed --fuse pops

I have some high readings with v1 out .=

V1 pin 1 408vdc
v1. Pin 6. 408vdc
6.5vac heaters

At the filter caps =

Point A 538vdc (scem says 480)
B. 467vdc (scem says 425 )
C. 417vdc (scem says 320)

the 33k / 1w. At B+C is 37k
the 15k at A + B is. 16k

v1 plate resistors read 100k

[Valvehead]

So the previous guy didnt use a can cap but added the following...Ive drawn out whats going on in the filter section along with the voltages .



heres the sunn scem =

ACDSeeLavoro di stampa (el34world.com)

[Valvehead]

Anyone here ???????

[Valvehead]

Should I try removing the 4 , 100uf and 470ks...and trying 20 uf and a 40uf ? Is there too much filtering in that 1st section ?

[loudthud]

Should I try removing the 4 , 100uf and 470ks...and trying 20 uf and a 40uf ? Is there too much filtering in that 1st section ?

Yes, that amount of filtering is higher than the 5AR4 spec recommends. The 200$ operates the rectifier tubes at the very top of what a NO$ tube can withstand, most if not all of the tubes manufactured today will fail under those conditions.

It is unlikely that this change will cure the Hum. Those big caps may have been installed as an attempt to cure the hum, without finding the actual cause. Verify that the center tap of the output transformer is connected AFTER the choke like the schematic depicts.

[Valvehead]

Thanks---would one of those plug in SS rectifier deals be ok for this application ??

Vacuum Tubes - Type: Solid State Replacements | Antique Electronic Supply (tubesandmore.com)

[loudthud]

Thanks---would one of those plug in SS rectifier deals be ok for this application ??

Yes, that will work and allow you to keep the cap stacks however, you should be aware that you must use the Standby switch to hold off the DC until the tubes warm up. If you fail to do this, the 560V or so B+ will go all the way to the preamp where the caps are only rated at 450V or 500V. Poof, your amp goes dead between sets in the middle of a gig.

This still won't cure the hum.

Addendum: It's possible to add a bleeder resistor across the last B+ filter cap in the preamp to limit the Voltage along the filter string. You'll have to experiment with the value, or do the math.

[Valvehead]

Tried a SS rectifier and amp works , isnt blowing the fuse now. But as mentioned , still loud hum.

Is it possible its the added filter caps/ resistors in my drawing ?