DRRI bias problem

[Throstur]

I have this situation where everything is right, but things dont work properly! I put a new set of 6V6´s into a DRRI. When I measure the current with the OT shunt method, (middle of OT to pin 3 on V7 and V8) I get maximum current flow of 7mA through each tube.
I have tested every component in the bias circuit, it seems to be OK, and deliveres -51mA to pins 5 on each socket (with no tubes in). I have 450 Volts on pins 3 and 4 on both sockets.

[sluckey]

I have tested every component in the bias circuit, it seems to be OK, and deliveres -51mA to pins 5 on each socket (with no tubes in).

Surely you mean you have -51VOLTS on pin 5 of each socket? If so, 51 volts is too much for the 6V6s. -35vdc would bring the plate current up to about 20-25ma. The DRRI schematic  shows -37Vdc.

[Throstur]

Thanks Sluckey, yes I meant to write -51Volts.  How can I bring the Voltage down to -37? The resistor, R69, after the diode is 22 Ohm. Should I increase the value of this one?

[sluckey]

The resistor, R69, after the diode is 22 Ohm. Should I increase the value of this one?

Increasing that resistor is one way to bring the bias voltage down. However, that also brings down the voltage fed to the tremolo circuit to turn the trem oscillator on/off. Trem would probably operate just fine with the reduced voltage.

What is the maximum and minimum bias voltage (as measured on pins 5) you get just by cranking the bias pot end to end? Do this with 6V6s pulled. In fact, leave the 6V6s out until you get the bias voltage like you want it.

I'd rather decrease the value of R59 (10K) to bring the bias voltage down. I'd start by temporarily jumpering R59 with another 10K resistor and recheck the bias pot voltage range. A range of -30 to -40 would probably be good. Experiment with different resistors jumpered across R59 until you get the range you want. Then you could replace the two resistors with a single resistor of equivalent value.

[LooseChange]

I've never come across a DRRI that had a problem with the bias voltage range.

[sluckey]

Since his bias is -51v (the max out of the rectifier) I'm kinda suspecting an open circuit between the bottom end of the bias pot and ground. Maybe the pot , or R59, or a wire, poor solder connection, etc... anything that would open the 10K pot and 10K resistor voltage divider. With those  resistance values in the voltage divider, he should be able to go from about -25v up to -50v. We'll know soon enough if he says the bias pot doesn't change the bias voltage.

It's an easy fix on the schematic. Actually finding the problem may be a bit more of a challenge.

[Throstur]

Hey LooseChange, I have retubed a few DRRI myself without any trouble at all with bias voltage range. OK, the maximum and minimum (no tubes in) bias voltage cranking the bias pot end to end is 36,1 to 51,9. I put in a new bias pot I had lying around, and no change. I get -52,9 VDC right after R69 where it should be 49 VDC according to the schematic. I have checked and re-checked all components, wires and values in the circuit and it all seems to be OK according to the schematic.

[sluckey]

OK, the maximum and minimum (no tubes in) bias voltage cranking the bias pot end to end is 36,1 to 51,9.

That's measuring at pins 5 of the 6V6s, right? If so, that sounds OK. That range of adjustment should cause a big change in your plate current. Now I'm questioning something in your OT shunt method for measuring current. If you have a couple 1Ω resistors, tack them in between the 6V6 cathodes and ground. The measured millivolts across those resistors should be pretty close to the measured shunt plate current milliamps reading. Are they the same?

[HotBluePlates]

Now I'm questioning something in your OT shunt method for measuring current.

When you did the OT shunt method, you did set the meter for a current measurement, right? In a lot of better DMM's, this will require you to remove the red lead and plug it into a different socket than you use for a voltage measurement.

I ask because we talk so much about measuring millivolts across a 1Ω or 10Ω cathode resistor that you might forget you need to set up your meter as an ammeter for the OT shunt technique.

[Throstur]

Now I'm questioning something in your OT shunt method for measuring current. If you have a couple 1Ω resistors, tack them in between the 6V6 cathodes and ground. The measured millivolts across those resistors should be pretty close to the measured shunt plate current milliamps reading. Are they the same?

Put 1 Ohm resistors from cathode to ground on V7 and V8. This way I can easily adjust the bias as I like. My shunt method is down the drain then! I trusted it and have used it a lot on Fender black and silverfaces, Marshalls and other amps. I thought it was a good and accurate method!!?? All input on this biasing method is appreciated.

HotBluePlates, I have a very good DMM and when I do this shunt method Black stay´s in Common, move the Red lead to uAmA and switch the meter to mA. I have been using this method for a long time, first time it plays tricks on me as far as I know!

Time is 22.00 here, I,m going to get some sleep now, back in tomorrow. 

[sluckey]

All input on this biasing method is appreciated.

It's potentially dangerous to you and/or your meter.

[Throstur]

Yes I know that these are the hottest points in the amp. I put the amp on standby and use 4 inch long insulated probes to hook it up. One hand in the pocket and the other on the standby switch. I thought this was a very accurate method of measuring how much current a tube draws.

[sluckey]

I thought it was an accurate method also, even though I've never used it. Maybe the current function on your meter is sick. There's an easy way to check it if you want to. First measure the millivolts across one of your newly installed 1Ω resistors. Then disconnect one end of that resistor and connect your meter in series with the resistor. Set the meter to measure current. The meter reading in milliamps should be equal to the millivolt reading across the resistor. This is assuming that your 1Ω resistor is really 1Ω.

[Throstur]

I´ll check my meter Sluckey. Is it all important that the cathode ground resistor is exactly 1 Ohm, not 1.1 Ohm or something like that?

[phsyconoodler]

One thing nobody mentioned,may be silly. He never mentioned actually turning the bias pot.
 

[Throstur]

One thing nobody mentioned,may be silly. He never mentioned actually turning the bias pot.
 

It´s here:

OK, the maximum and minimum (no tubes in) bias voltage cranking the bias pot end to end is 36,1 to 51,9.

I have tested the current function on my meter and it´s alright. The amp is working fine now and I gave it back to the owner who was happy with the sound. This will remain a mistery until I get my hands on a DRRI the next time. 

[phsyconoodler]

If you got -36v at the highest pot setting,then you should have no trouble biasing the amp.
  I bias a different way.I have the amp off and unplugged,caps drained.Then measure from pin 3 of each power tube to the OT center tap.Record the reading.Then turn the amp on and measure the voltage between the same two points(center tap and pin 3)
  Then get out your calculator and divide the ohms into the voltage and you've got your idle bias in milliamps.

e.g. say you get 47 ohms and 1.5v  1.5/47=.03129  or 31ma

[Throstur]

e.g. say you get 47 ohms and 1.5v  1.5/47=.03129  or 31ma

This sounds like a very good and safe method! I will definitely try this out. You would have to put the tubes that you are going to use in the amp before you measure the Voltage, wouldn´t you?

[phsyconoodler]

Yes you need to use the tubes you want to bias but remember you are looking at about 1.5vdc or so  only.