Need help with a Silverface Vibrosonic short

[martin manning]

You are doing a shunt of the OT primaries, i.e. meter set to measure mA?
What is your pin 3 voltage to ground?
Are the tubes known to be in reasonable condition?

You need to calculate the target current from plate voltage and a chosen percentage of the max plate dissipation for the tube. Say you are at 440V, and you choose 65% Pa max, then you want (30W * 0.65)/440V = 44 mA per tube, 88 mA on each side.

The schematic shows 15k from the wiper to ground; in your pic it is 9k4 (2x4k7). To increase the bias voltage (increase idle current) you'll need to reduce that further.

[Leo_Gnardo]

Great! Do you know what the power tube idle current is now?

My bias probe is on the fritz but assuming I am doing this alternate method correctly (center tap OT primary to Pin3), I am reading average of about 25mA which seems cold. I'm reading about -45V on Pin5 so I'm thinking the funky resistor cluster on the bias pot is perhaps bleeding too much to ground. What target values do you think I need to get to for the bias?

Martin's method is spot on (as usual!)

But I find plenty of Twins (and your amp's a Twin with a 15 inch speaker, no?) running fine with a scant 25 mA per plate. I goose it up to 30, yes it still seems on the cool side but I've never found a tone or any other advantage by running 'em up much higher. YMMV so set a bias, give a listen, no point in running hotter than need be to stay out of crossover distortion territory. I often leave the balance circuit alone except to adjust it, and to set overall bias put a Bourns blue cube trim pot in series with the resistor that sets minimum bias - factory often has 15K here - I've had to on occasion go to 8K2 in series with a 10K pot to find a happy bias point.

[sonicmojo]

It's pretty much a Twin with a 15 inch, yes. Not my amp but I took it on to fix since I do have a similar Twin. I tried stock 15k resistor value and it drops it to 14mA with the existing, used tubes. I swapped in a couple spares tubes and it was about the same. This amp had those replacement spliced together 9.4k resistors on it which puts it to about 25mA. I'm going to try some new tubes in it and probably put back the 9.4k if it is still hanging low but I'll probably not do any other bias mod to it. I thought it sounded pretty good at the 25mA.

[Leo_Gnardo]

It's pretty much a Twin with a 15 inch, yes. Not my amp but I took it on to fix since I do have a similar Twin. I tried stock 15k resistor value and it drops it to 14mA with the existing, used tubes. I swapped in a couple spares tubes and it was about the same. This amp had those replacement spliced together 9.4k resistors on it which puts it to about 25mA. I'm going to try some new tubes in it and probably put back the 9.4k if it is still hanging low but I'll probably not do any other bias mod to it. I thought it sounded pretty good at the 25mA.

I expect those used & spare tubes are pretty well worn out & need to have an unusually low bias voltage to squeeze some bias current thru 'em. Day comes you put a fresh kit of good matched output tubes in, you'll probably need to increase that bias-setting resistor a bit. If they sound good at 25 mA, park it there & smile.

[martin manning]

The voltages on the schematic combined with vintage 6L6GC plate curves do indeed indicate a low current in the range of 25-30 mA, or around 45% Pa max.

Note that the resistor next to the rectifier is a 470 ohm instead of the 1k shown on the schematic, which is also helping to raise the bias voltage.

[matt h]

(deleted)

[sonicmojo]

Thanks for all the feedback. I guess you get what you get on these balance pot fixed bias Fenders unless you go changing out resistors......or otherwise modify the circuit, or just try different tubes. I've learned a lot from everyone's pointers and suggestions. I appreciate it.

And...with new JJ tubes, and the 9.4K on the bias pot, I am getting 30 mA. Close enough, sounds good, call it a day.

[Stevem]

Your still way on the cold side of 7o% at that idle current, you need to be at 40 ma!
If you visit the Jim jones bias table you can see what you need to be at!

[sonicmojo]

Your still way on the cold side of 7o% at that idle current, you need to be at 40 ma!
If you visit the Jim jones bias table you can see what you need to be at!

I hear you, but how do I get there without further modifying this circuit? As noted it already has a couple of mods. If I put it back to spec, it would cool it off even further. Bias voltages now match the schematic. Yes, I could add a trimpot in there and adjust the bias that way but is there something else that I should be looking into as a problem here?

[martin manning]

Don't get hung up on the bias voltage shown on the schematic. What matters is the idle current and plate voltage you are running and the product of the those, which is plate dissipation. Different sets of tubes will have different current at the same voltages, and modern tubes are probably going to show more production variation. Anywhere from 50-80% of the maximum could be acceptable, and as noted above Fender amps are often operated on the cool side.

[Stevem]

And fender amps biased on the cold side with those big .1 uf caps on the out bound side of the PI sound like crap to me when driven to clipping above 6 on the volume knob!
You might want to at least play around with changing these caps to .047, or .022 if you do not want to mess with the bias level any more and see what your ears make of it when played cranked up!

You can forget all this if you only use the amp for its clean tones.