VHT Pitbull 45 Cuts Out

[nosaj]

I would firstly check the bias as they do seem a little on the warm side. 230+°C is hotter than I would like. It could be swamping the output transformer or draining too much power from the main HT supply.

Once you get a trustworthy meter follow up with Jon's advice.
nosaj

[ca7922303]

I would firstly check the bias as they do seem a little on the warm side. 230+°C is hotter than I would like. It could be swamping the output transformer or draining too much power from the main HT supply.

First checked bias on this one, was getting (396v) which calculated into (35ma) and (115%) disapation. Raised voltage to (417v) calculated into (22ma) and (75%) disapation. Temperature on power tubes came down to about 350°F on three(3) power tubes, but one went to 550°F at the base of tube using IR temp gun. Then began having red plating on power tubes. Checking now in groups of only two (2) tubes at a time to find bad sockets or bad tubes.

[ca7922303]

From left to right looking at back of amp, V4(EL84)just red plated and went to 600°F before I shut it down. Installed tubes in V2/V3 only and no red plating, but V2(-12vdc pin#2) and(500°F). V3(-6vdc pin #2) and (160°F). V1(-7vdc) and (135°F). V4(-12vdc) and (500°F) but not red plating. Voltage at pin#7s is(424vdc). Currently running it with V1/V3 only.

[ca7922303]

The two(2) 5w 100ohm resistors read(100ohms)
5w 390R resistors with one leg up reads(32m ohms).

[ca7922303]

I have some 470 ohm 5w resistors. Any harm in using one in place of the 390r 5w?

[The Dude]

Trace out the circuit and see what the resistor is for/where it is in the circuit. Resistors burn open for a reason and you're going to need to figure out why it burnt. There must be a shorted component, arcing tube, etc. somewhere. Also, that 390 ohm 5W resistor has been replaced before. It's an NTE/substitute part. Hopefully the last tech at least put in the correct value.

[nosaj]

Are you sure the PCB isn't carbonized? It looks pretty bad. you can check to see if its conductive sticking the probes into 2 different burnt sections and check ohms.
nosaj

[ca7922303]

Trace out the circuit and see what the resistor is for/where it is in the circuit. Resistors burn open for a reason and you're going to need to figure out why it burnt. There must be a shorted component, arcing tube, etc. somewhere. Also, that 390 ohm 5W resistor has been replaced before. It's an NTE/substitute part. Hopefully the last tech at least put in the correct value.

The leg of 5w 390r cement resistor, nearest the 500v filter cap, is connected to all four(4) EL84 power tubes pin #7. Still looking for where other leg of that resistor connects.

[ca7922303]

Are you sure the PCB isn't carbonized? It looks pretty bad. you can check to see if its conductive sticking the probes into 2 different burnt sections and check ohms.
nosaj

Not getting anything checking for conductivity of board. The red paint that you see inside of power tube pins is where copper was exposed on pcb. Nothing reading on those spots either.

[ca7922303]

The leg of 5w 390r cement resistor, nearest the 500v filter cap, is connected to all four(4) EL84 power tubes pin #7. Still looking for where other leg of that resistor connects.

Other leg of 5W 390r cement resistor connects to V1 & V4 pin #9s. Does not connect to V2/V3. That leg also connects to the two(2) 5w 100 ohm cement resistors. Connects to the legs farthest from 500v filter caps. And that cap is reading .39k ohms. Had a bad connection to it. Using alligator clips it reads correct.

[g1]

Do the other ends of the 100R's connect to pin9 of V2 and V3 ?

[ca7922303]

Do the other ends of the 100R's connect to pin9 of V2 and V3 ?

Get to that shortly, had to step out.

[ca7922303]

[QUOTE=g1;n975529]Do the other ends of the 100R's connect to pin9 of V2 and V3 ?[/QUOTE
V2 pin #9 connects to other end of 100R next to V2. V3 pin #9 connects to other end of 100R next to V3.
V1 & V4 pin #9 are connected.
V2 & V3 pin #9 are not connected.
V1 & V2 pin #7 are connected
V3 & V4 pin # 7 are connected
V1/V2/V3/V4 pin#3, #4/#5 are connected

[g1]

So the set-up for those resistors is similar to the Peavey classic 50 (schematic attached). The 390R is between the plate and screen supply nodes, like R58 in the peavey supply.
Two of the power tubes get their screen pins directly connected to the screen supply, like V6 and V7 in the peavey. The other power tubes have 100R series resistors between their screen pins and the screen supply, like V4 and V5 in the peavey.

Any time the 390R overheats or burns up, a bad power tube(s) is a likely suspect.

[ca7922303]

So the set-up for those resistors is similar to the Peavey classic 50 (schematic attached). The 390R is between the plate and screen supply nodes, like R58 in the peavey supply.
Two of the power tubes get their screen pins directly connected to the screen supply, like V6 and V7 in the peavey. The other power tubes have 100R series resistors between their screen pins and the screen supply, like V4 and V5 in the peavey.

Any time the 390R overheats or burns up, a bad power tube(s) is a likely suspect.

In post #40, I corrected statement about 390R being bad. It is reading(.39k ohms). Both 5w 100ohm resistors reading (100 ohms) in circuit).Yet just about every EL84 I put in it is red plating at this point?