red knob twin in flames

[rfhess]

Hey can someone help me with this one. Opened up the amp and found r303,r302 burned away. Also found broken solder joint on tube socket pin number 3 . I replaced replaced r302 and r303 with 47k I hope this is the right size. Now amp has no punch to it and sounds muddy. I checked voltage on tube socket and they are 496vdc,57vdc and 4vac. The choke looks like its burnt.what should DC on choke be mine is 247vdc Please help , thanks

[soundguruman]

It looks as if the power supply section, or the power transformer secondary may be cooked.

[Gtr_tech]

Don't guess....consult the schematic and replace any failed components with the same value/wattage/voltage.

http://www.webphix.com/schematic%20h...s/the_twin.pdf


If you are talking about the balancing resistors on the filament winding, those are marked as 47 OHM, not K-ohm. 100 ohm is the std value here....its not critical, but I wouldn't go above 200 ohm. These are commonly taken out as a result of output tube shorts. Replace the bad tube(s), then replace the burned heater balancing resistors.

The choke is in the screen circuit, so check the filter caps after it for extreme leakage or shorts. A shorted choke won't cause a voltage drop (unless its shorted to the core), but it won't act as a choke either....

[Jazz P Bass]

I do believe R302 & R303 should be 47 ohms (not K/ 47000). 1/2 watt resistors.
These are on the tube heater power supply.

[Enzo]

I agree with the above posts, the two resistors are 47 ohms each, make them 100 if you want to, doesn;t matter. And they burn up usually when a power tube shorts to heater.

You report three voltages on a tube socket. OK, which tube and what pins? On the power tubes, pin 3 and 4 of each tube should have full B+ voltage, and the pin 5 of each the bias voltage, about -60v. Assuming high power mode.

Note this amp has a high/low power switch, and that changes B+ to the power tubes, while leaving the preamp alone. If your B+ is real low, make sure that switch is set for high power.

[rfhess]

The pins readings are 1.-62vdc. 2. 3.3vac. 3. 496vdc 4. 496vdc 5. -65vdc 6. 496vdc 7. 3.3vac 8. 0

[Enzo]

Those sound fine. Something is off here though. If you have 490v on the screens, how can you have 247v on your choke? CHeck that again.

You need to see those same voltages at every power tube socket. Don;t skip any. A failed tube can burn out a screen resistor on its own socket without affecting the others. so we always must check all of them.

ANd I don;t see, did we install new power tubes, or are these still the old ones? Worn tubes could be part of your problem, especially with those resistors as evidence.

[rfhess]

OK 'I will check that again. There's 2 wires on the choke, how do you test vdc ? Ground and one leg or a cross the 2 legs? OK on the tubes I tested with my peavey classic ... I have two sets also thanks for your help

[Jazz P Bass]

OK 'I will check that again. There's 2 wires on the choke, how do you test vdc ? Ground and one leg or a cross the 2 legs? OK on the tubes I tested with my peavey classic ... I have two sets also thanks for your help

If you want to measure the voltage drop, you can measure across the component.
That measurement will help when determining something like the wattage of the device.
What Enzo is calling for, I believe, is a Vdc measurement at the coil.
The front of it, the back of it. (it's a piece of wire, I don't see it changing "much")
Do both.
Red lead, measurement.
Black lead, ground (zero volts)

[rfhess]

chokes ok i must have been on low output when i checked it . now i getting low(12to14) dc on pin 8 of two tubes ?i also found two green resistors on the bias and balance board burnt out . do you know what these should be?

[g1]

They are probably the 1ohm 1watt cathode resistors (R215 & R216) on the schematic. There should be 2 diodes in parallel with them as seen on the schematic (D202 & D203). If it doesn't have the diodes install them. The resistors probably burnt due to bad power tubes (maybe not the ones you are using now).

[rfhess]

hey g-one I can't get the bias on back of amp set to specs, I'm getting DC on the test ports. Low output sounds great high output I'm getting intonation wave . I put in 4 new ruby 5881/6l6wgc tubes . Thanks for all the help

[rfhess]

OK I got balance and bias set but I still have a waving sound on high output can someone point me in the right direction

[g1]

Can you be more specific what you mean by "intonation wave" or "waving sound"? It's possible you have excess ripple in the B+ due to faulty filter caps. Take an AC voltage reading where you were measuring the 496vdc on the power tubes.

[rfhess]

I found a loose wire near r214 and r217 , I repaired that and now the amps back to its old self. I would have never fixed it without your (everybody) help thanks a million!!!!