Bias problems---

[Valvehead]

working on an orange TH-30 . It has 2 of the el84s red plating , the other 2 do not. Its not the tubes .

Looking at the scem , ive checked all resistors from V5 all the way to pin 2 of the el84s , ground is good , no bad solder joints , and while i had the pcb up i replaced the .1 caps , c45 / 46 coming out of V5 .

with no el84s installed im getting 280vdc at pin 2 of V 6 /7

at V8 / 9 , i get about 0 to 4mv..the meter just kind of jumps around--it wont settle down on a number .

Any thoughts ??

TP13 is 290vdc at "C" so that looks ok according to the scem.

[The Dude]

What resistance do you measure across C45 and C46?

[Randall]

Which two are red plating and which two are not? V6 and V8 share a cathode resistor and bypass cap, as do V7 and V9. That 4 output/2 output switch and it's pal 3w 100K R84 are worth a look also, since it is how two of the tubes get their cathode grounds.

[Valvehead]

v6 / v7 sockets are tubes going red.

i just found something..R84 tests shorted in circuit..but out of circuit tests 100k . both ends of r84 are going to ground no matter where the half pwr is set . R 84 has to be shorted ?? or am i missing something ...

the half pwr switch tests ok

R83 / R85 OK

[The Dude]

If the resistor checks good out of circuit it's good. The half power switch switches out 2 tubes by adding the 100k into the cathode path of those tubes. The switch shorts across the resistor for 4 tube operation. If your problem was there, V6 & V8 would be red plating instead of V6 & V7. I believe the problem is that 280vdc at pin 2 of V6 & V7. You need to find out where that's coming from. That's why I asked the resistance across C45 & C46. Even if it's not the caps themselves (since you changed them), there may be another current path there- a solder bridge, debris, socket issue, etc.

[Valvehead]

ok--sorry i missed that post-i will measure those here in about 30 minutes----in circuit right?

[The Dude]

Yes, in circuit. Basically, we want to know if the plate voltage from V5A is somehow leaking over to the grids of V6 & V7, whether through the cap or some other means. Mostly interested in the resistance reading across C45, but comparing to C46 gives us more data.

[Valvehead]

220k and 240k

[Enzo]

TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

Lift C45 from the circuit and see if the voltage goes away.

[Valvehead]

TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

Lift C45 from the circuit and see if the voltage goes away.

tp10/11 are not going to ground.

ill have to pull the board again to pull c45

[g1]

Have you checked resistance from pin2 of V6 or V7 to ground? Should be approx. 150K.

[Valvehead]

Have you checked resistance from pin2 of V6 or V7 to ground? Should be approx. 150K.

V6/7--the meter slowwwwwly creeps up to around 120- 125k ? this is over 10 or 15 minutes until the meter times out and shuts off

V8/9 instantly measure 150k.

[Valvehead]

TP10, TP11 are the 120 ohm cathode resistors for the power tubes. Either of them shorted to ground?

Your caps from the PI to grids may not measure shorted, but can still leak DC when facing hundreds of volts.

Lift C45 from the circuit and see if the voltage goes away.

c45 removed--still have 230 vdc there at pin2

so its maybe something to do with the v5 area... ??

[Enzo]

Start looking for conductivity or foreign material on the board in that area. Also look for a tip of a component lead nearby touching that trace. If so, trim it back. Follow the trace from pin 2 to anywhere it goes, looking for anything along the sides of that trace somehow touching it.

[SoulFetish]

Lift one end of r76 and r78. Is the voltage still there?
(Assuming you already pull those tubes)