No bias current reading ?

[gtrplayr1976]

Hey folks. I have a bias probe connected to the two middle tubes of this Twin, and I'm getting 0 current. The plate voltage is normal ,and the tubes are lit. I will check more voltages this evening I was just concerned about running it with a zero current reading. By the way it's been completely refurbed. Nearly all components ,and it's been blackfaced. Any ideas on this?

THANKS !

[nsubulysses]

Check the voltages on pin 4 and 5 of the tube socket. Pin 4 should be nearly as high as the plate voltage. Pin 5 should be your negative bias voltage, just guessing but probably somewhere between -30V and -60V.

If the screen grid resistor on pin 4 is open the tube will not conduct. If pin 5 bias voltage is too negative the tube will be biased into cutoff.

[Enzo]

The two middle tubes? OK do the two outer tubes work the same way? Or do they provide some expected current reading?

Does the amp work and amplify?

[g1]

Have you used this bias probe with good results in other amps or on previous occasions?
Some use volt range, some use current range.

[gtrplayr1976]

Thank you gentlemen !

Pins 3,4,and 6 are all around 460VDC. Pins 1,and 5 are near -85 VDC. I think I may have found AN issue with the bias pot. The side with the resistor reads open between it ,and the wiper.
I haven't tried anything through the amp yet Enzo ,but I do hear a slight thud when I turn it on ,so something is coming through the speakers. The other tubes are the same Enzo. I just use the inside two when I check bias.
g-one, I have been using this one for a few years ,and it seems to have done fine. Is there another way to read current with my Fluke ?

[Mick Bailey]

-85v will cut off the tube, so no current flow.

[g1]

No problem with the probes then if you have used them before. The fact that you have -85V bias is why your tubes are not conducting.
Try another pot or tack in resistors for temporary testing. Make sure the bias filter caps are high enough voltage rating that the -85V is not damaging them!

[gtrplayr1976]

No problem with the probes then if you have used them before. The fact that you have -85V bias is why your tubes are not conducting.
Try another pot or tack in resistors for temporary testing. Make sure the bias filter caps are high enough voltage rating that the -85V is not damaging them!

I think they're 100volt. I may have another pot but not sure. Where would I tack the resistor(s) ? Directly to the wiper ?

[J M Fahey]

You should put a new bias pot there, of course, but just as a temporary measure, replace it by a fixed resistor, say between full trimmer resistance and 3/4 or 1/2 of that, meaning some value which it *might have been set to, and remeasure bias.

It will be the correct one only by sheer chance, but at least it wonīt be *that* far away and will let you check that the rest of the bias circuit is still working (hopefully )

And if you are itchy to play and a replacement is a week away, use a standard pot of nearby value, on flying leads, to find proper bias, and then replace it with 1 or 2 fixed resistors as needed to get that value.

[gtrplayr1976]

I have the bias working for now. I will replace the pot with a new one later. I have checked all voltages and see a high voltage on V5 pin 1 plate. I also have -56 vdc on the grids. Some hum on most grids when I touch the pins with the meter. Some very loud. Is this normal ?

[J M Fahey]

Thatīs what grids do

[gtrplayr1976]

Thatīs what grids do

Thank you So the hum is normal ? My concern is the higher than normal plate voltage on V5.

[g1]

If the hum is only when touching the grid with the probe, then yes that is normal.
Not sure what Twin this is? V5 is usually the trem tube and high plate voltage at pin1 may be normal, do you have the trem off or on? Does it sound normal?

[gtrplayr1976]

Thatīs what grids do

If the hum is only when touching the grid with the probe, then yes that is normal.
Not sure what Twin this is? V5 is usually the trem tube and high plate voltage at pin1 may be normal, do you have the trem off or on? Does it sound normal?

Thanks g-one. Sorry it`s the SF Twin reverb I converted to BF that I`ve been messing with for a couple of years. Yes I only have hum when I touch the grid with a probe. I just decided to plug a guitar in and see what happens. Worked ok ,but had no controls except the guitar. Found I had the input grid ,and volume grids backwards. I`ll get that straight ,and go again.

[gtrplayr1976]

I got the grids straight ,and all seems to be working now. I just have to test the tremolo ,and reverb ,but I have to put it back it the cabinet to do that ,or at least get it close enough to connect. Someone had mentioned somewhere on this forum about using a shorted plug to turn on the tremolo ? If so does that mean you just twist the hot ,and shield together on the plug ?