SF Twin - bad 6L6s?

[MaxHdRm]

Hi, I’m working on a 135W SF Twin. http://www.schematicheaven.com/fende..._135_schem.pdf I installed 1 ohm 1% resistors between pin 8 and the braided ground on the 6L6s. I’m measuring around 450VDC on pins 3 and 4, and -53VDC on pin 5 of the 6L6s. When I measure the voltage drop on the 1 ohm connected to 6L6s V10 and V9 I measure 36 and 32 mV. On V8 and V7 I measure 15 and 0 mV. I swapped the tubes as pairs and the measurements follow the tubes. Bad tubes? Thanks for your help!

[Enzo]

Pick a socket. Stick one tube in it. Take a measurement.

Now remove that tube and stick another tube in the same socket. take a measurement.

One at a time do the other two tubes.

Since they were all tested in the same socket, any difference betwen tubes is the fault of the tubes themselves.

If your tubes are all over the map, then replace them.

[MaxHdRm]

Thank you Enzo! That worked out.
I replaced them with a matched quad and I'm measuring on all four 6L6s;
Pins 3 & 4: 460
Pin 5: -53
1 ohm: 21 to 19 mV

Thanks Again

[jbltwin1]

135 watt twin

That's pretty cold. You should try for somewhere in the vicinity of 30mA per tube on that amp

[MaxHdRm]

Yeah, I thought it was cold. There is an "Output Tubes Matching" pot, not a true bias pot. I believe the bias is fixed for two tubes and the circuit will only allow the other two to match them. I'm not sure what to do. I assume I would need to change the resistors for the two that are fixed so that they are in the 30mA range then use the matching pot to bring in the other two tubes. I'm only shooting from the hip here, I need to do some more research. If someone can point me in the right direction, that would be great. Thanks!

[jbltwin1]

135 watt bias

If I remember correctly, there is a resistor slapped on the back of the bias balance pot that you can alter that is the bias limiting resistor. I'm not SURE that's where it is but if you alter that one, it will adjust your bias limit. OR, you can get a multi turn 15K pot and put in it's place and ADJUST the bias. THEN you have the best of both worlds. Mike.

[Dave Curtis, dB AudioTech]

If I remember correctly, there is a resistor slapped on the back of the bias balance pot that you can alter that is the bias limiting resistor. I'm not SURE that's where it is but if you alter that one, it will adjust your bias limit.

I believe there's a 33K resistor from the wiper of that pot to ground in this amp. It's 15K in my '76 Pro Rev.

OR, you can get a multi turn 15K pot and put in it's place and ADJUST the bias. THEN you have the best of both worlds. Mike.

Two things:

1) 15K isn't large enough (by itself; in this amp).

2) replacing the existing resistor with only a pot will allow the bias V to be adjusted to 0v, ensuring a rapid demise of the output tubes.

Put the pot in series with the 33K (since 33K is fairly cold with these tubes), then it will only reach that point. Also set up the pot as a variable resistor (one leg tied to the wiper), so the voltage goes through the element, the resistor to gnd. If you leave one leg open and wire the voltage through the wiper to the other leg, you risk losing bias voltage if/when the wiper fails. Instant cookage.

[jbltwin1]

135 watt bias

Good advice for him. I was shooting from the hip on the values and if he IS cold, that would be good advice. Just remember, with control grid voltage, more negative voltage means colder tubes. Less negative lets the tube conduct more, therefore more current.

[MaxHdRm]

Bias mod

Is this the circuit being suggested? Please see attachment. Thanks

[jbltwin1]

135 bias

Yup-that should work just fine. I should add that I PERSONALLY don't like pots in the bias circuit but they ARE a lot easier. Scratchy pots in th ebias circuit lead to trouble but hey, what the hell do I know! Fender did it for years so who am I to complain!

[52 Bill]

If the original resistor is 33K, adding a 15K pot in series with it will only allow the bias to be set colder than it already is. Try using the 15K pot with a 22K series resistor.

[MaxHdRm]

My thanks to all for the help! Any suggestions for vendors that carry quality pots?

[jbltwin1]

135 watt bias

THAT'S what he needs. Although, you call it colder, it will actually reduce the amount of control grid voltage. THAT will make his tubes run HOTTER. THAT is what he needs according to his post.