Biasing a Twin

[csjoyner]

The Joyner Boys are back again with another problem. We have a twin build that has some issues. It is half from Mojotone and half from Weber. It started as an all Mojotone but we had issues with it and decided to redo the board with turrets. Anyway, the problem followed throughout. First, the amp is not as loud as we expected. We did a Super Reverb build at 40 watts and it is much louder than the twin. The second problem, which I assume is the real problem causing the first is that the bias reading when the bias pot is full open is only 7.5mv. I have used multiple bias probes to make sure there is no problem there. Any Ideas where to start? Here are some voltage readings. Let me know what else you need. Thanks

The power tubes were the same:
3. 445
4. 444
5. -59

The bias circuit:
From the PT 66v
Between the cap and diode -82

Here are some pics for your eyeholes.

[sluckey]

-59V on pin 5 is causing the bias reading to be low. Need to drop that to about -50V to run the tubes hotter.

I would pull the four 6L6s. Connect the meter to pin 5 of an output socket. Set the bias pot to center of rotation. Then experiment with increasing the value of that 470Ω bias range resistor that feeds the diode on the board. The target is to get as close to -50V on the meter as you can with the bias pot set to center. The resistor value will likely be greater than 470Ω but less than 1000Ω. When you get the right resistor then turn the bias pot from end to end. It should swing below -50v and above -50v. If satisfactory leave the bias pot set for max negative voltage. Now put the tubes back in and readjust the bias pot using a bias probe.

[csjoyner]

Sounds doable. I’ll give that a try and let you know.

[Stevem]

Both the blackface super reverb schematic and the black face twin reverb schematic call for -52 volts at the junction of the two 220K grid load resistors, and both amps have within 10 volts of the same V+ in play mode, so that -52 volts it what I would start off with .

Once all is well I bias/ idle amps like these Fenders at .035 to .038 amps which works out to about 65% dissappation .

If you have well built output tubes and need the output stage to clip sooner then bias the tubes up to .042 amps, but note that you will chew thru output tubes faster!

I also noticed that your speaker jacks are not the shorting type and that’s not good for the life of the output transformer should the amp be played without a speaker load like if the speaker cable gets ripped out, or if the speakers blow.

If you do not want to swap over to shorting type jacks ( like your input jacks are) then I would at least add a 250 ohm 7 watt resistor across the 4 ohm output jack to provide the needed load in case there is a mishap!

[Ten Over]

Move the yellow wire to the wiper of the bias pot.

Change the dropping resistor from 470 to 3300.

Change the tail resistor from 27K to 15K.

[csjoyner]

Which should I try first? I’m not ashamed to say a lot of the electrical stuff is a bit over my head. Also, the main output is a switch output. The secondary is not. I am planning on hooking up the other 2 OT wires to create an output for 8ohms and 16ohms for versatility. Any opinions on that? I’ve heard twins can get squirrely at 16ohms.

[sluckey]

Did you try my suggestion?

[Ten Over]

Which should I try first?

Do what sluckey said. It will probably take a 3.3k bias range resistor in place of the 470R resistor to get what you want.

With the yellow wire connected as shown, the bias voltage will default to its hottest setting should the wiper fail to maintain contact with the track. With the yellow wire connected to the wiper which is connected to the other end of the track, the bias will default to the coldest setting upon wiper failure. If this is of concern to you, then do what I said.

[csjoyner]

Work is a beast. It will probably be this weekend before I get to it. Plus I’m waiting on more resistors to come in.

[csjoyner]

Do what sluckey said. It will probably take a 3.3k bias range resistor in place of the 470R resistor to get what you want.

With the yellow wire connected as shown, the bias voltage will default to its hottest setting should the wiper fail to maintain contact with the track. With the yellow wire connected to the wiper which is connected to the other end of the track, the bias will default to the coldest setting upon wiper failure. If this is of concern to you, then do what I said.

So like this? (See pic) What you said makes sense.

[Ten Over]

So like this? (See pic)

No. Leave the jumper in that connects the wiper to the lug on the pot that goes to the diode. The method in the pic is even worse because the power tubes will have no bias voltage at all if the wiper fails.

[csjoyner]

So like this? (See pic)

No. Leave the jumper in that connects the wiper to the lug on the pot that goes to the diode. The method in the pic is even worse because the power tubes will have no bias voltage at all if the wiper fails.

So just like the pic but jumper the two lugs with the wires on them? I just want to be sure I understand what you’re saying.

[sluckey]

Don't make this complicated. He said "Move the yellow wire to the wiper of the bias pot." Just do what he said. Nothing more.

The wiper is the center lug.

[Ten Over]

So just like the pic but jumper the two lugs with the wires on them? I just want to be sure I understand what you’re saying.

You already have the two lugs jumpered, so just move the yellow wire like this:

[csjoyner]

Alright, here is what we did and what got. We moved the yellow wire to the wiper. We had to switch the dropping resistor to 3300 and the tail resistor to 15k to get to the point where the 5 pin was reading roughly between -45 to -55.

With a plate to cathode voltage of 426v and using a Bias calculator I determined I wanted to run the bias at 41mv, just below “average”. However, the bias circuit fully open would only yield 36mv. So we replaced the tail resistor with a 10k. This gave us a value on pin 5 of -38 to -50.

This gave me the the 41mv bias number I was looking for. Pin 5 on the power tubes is now -44.1. The amp is now loud. It sounds great.

Any suggestions on anything? Will this be ok to keep it like this? Is there a better number than the 41mv to bias the amp to. I want to keep it clean. Also, I am using 7581A tubes. Thanks.