Fender "The Twin" (red knob) bias question

[Chevy]

Hi guys,

Long time no see, no talk.... been offline for well over a year; nice to see some familiar "faces" still here, and that this forum is still around! Good show!

Anyhow, just doing a repair on a Fender The Twin (red knob), and am curious about the markings on the schematic and the chassis that say to adjust the bias to 40mV, then set the balance.

With 2 tubes on each side putting out 40mA each, you'd get 80mA thru 1 ohm R, you'd get 80 mV..... no? What am I missing here? D'oh!?

Best,
Chevy

[Chevy]

bump

Anyone know anything about this.....?

Thanks kindly!

[pdf64]

The bias probe sockets measure the voltage across the 1R cathode ground return resistor on one side. The balance sockets measure the voltage between the 1R resistors on each side, ie adjust so that both sides measure the same 40mV and the voltage between the balance sockets is 0V. So it's recommending an average of 20mA / 6L6, a bit cold at about 10watts. Peter.

[Chevy]

Cold bias

Thanks, Peter,

Thought I was missing something that I just couldn't see...
20mA per tube is something I've never done.... seems way too cold. Do techs actually adjust these amps that way?

What do you mean by "10 watts" ?

So it would be correct/safe for me to adjust the bias to, say, 70mV, and then do the balance, and everything should be about "normal", right? The amp's power supply should be able to handle the extra current....?

Just can't wrap my wits around the 20mA thing. Seems the amp was designed for this.....?

Thanks very much!

[Amp Kat]

The 20 ma's is very cold as was determined by pdf64 as 470 VDC X .02 amps= 9.4 watts which is very cold for a 6L6. I'm not a big fan of the bias/balance arrangement in these amps and if it were mine I'd revert it to blackface standards. You could still bias it to at least 70% which would be 17.5 watts instead of 10 watts and your voltage across the 1 ohm resistor would be 74 at 37 ma's per tube for 17.5 watts. Of course in low power mode this would almost cut in half from the lower plate voltage.

[Chevy]

Thanks for the input, guys, much appreciated! Been out of the loop for too long!!

Since this is just a repair, I'm not really inclined to modify anything.

But I still don't know if the amp will actually survive a "normal" bias of about 35mA per tube..... as in will the power transformer handle it or not? Off the cuff, that would add about 25W+ of load to the xformer. Seems like a small amount.... Anyone done this successfully?

[Pedro Vecino]

Sorry, but I d´ont understand... Twin amp red knob has two "independient" bias circuits (one per side). Not general bias and balance between sides.
The margins of adjustment they have are very low and you may not get a good setting with a standard medium grade 6L6.
The solution to obtain standard margins of adjustment (for example 34mA per tube) is to reduce the value of the resistor that every bias potentiometer has connected to ground.
The value of these resistors is 100K (they are located in the same pc board) and you can reduce them to 50K installing in parallel another of the same value.
No problems with this adjustment. I have prepared several but I use always external sockets to measure the current across the plates.
Also you need to verify the bias adjustment when the reduction of voltage is applied (lo power).
Regards

[Enzo]

Chevy - ignore the numbers for a moment. How does the amp sound when set at the specified currents? OK, it is colder than most, but does it sound like a Fender? Does it sound OK? Remember, this amp has a low power setting too, and that switches voltages in the bias circuit as well as elsewhere. If we heat it up, how will that track on th low power setting?

There is no threat to the transformer.

AMp Kat, this is not Fender bias balance. This is the same system as recent MArshalls or even the old SVT use. There are two ppins, one for each side. Each independently measures current through its side tubes. The only "balance" it does it in readings. What they are doing is setting current to 40ma on one side, then instead of measuring 40ma on the other side, they get you to connect a meter between sides and set side 2 so the reading is zero. All that means is side 2 is at the same level as side 1 - our 40ma. Using this method means if the side one reading interacts a little, you don;t have to go back and tweak - both sides will be the same. You could just as easily set side 1 to 40ma, then set side 2 to 40ma, then go back to side one to see if it moved.



Pedro, it is not a "Twin Amp" amplifier. It is a Fender "The Twin" amplifier.

The circuitry (as I explained above) itself is not a balance circuit, but the procedure they use is. The only difference is where you connect the meter leads.

I don;t think he wants to modify the customer's amp - we don;t know how hot the existing circuit will get his tubes.

[Pedro Vecino]

Sorry, Enzo. Really it´s The twin, not Twin Amp.
In the last two that I have had (practically simultaneously) the average per tube if not remember bad was of about 10mA with the adjustment to the maximum (!). Definitively a poor sound.
To manipulate the margins in a bias circuit to obtain the opportune performance cannot be considered a modification but simply an slight adaptation (a conceptual question...)
With 32/34 mA for tube, bias adjustment in lo power mode was moderated to low. It´s also operable altering the value of the 6K8 resistor but I did not consider it necessary.
Regards

[Amp Kat]

Chevy - ignore the numbers for a moment. How does the amp sound when set at the specified currents? OK, it is colder than most, but does it sound like a Fender? Does it sound OK? Remember, this amp has a low power setting too, and that switches voltages in the bias circuit as well as elsewhere. If we heat it up, how will that track on th low power setting?

There is no threat to the transformer.

AMp Kat, this is not Fender bias balance. This is the same system as recent MArshalls or even the old SVT use. There are two ppins, one for each side. Each independently measures current through its side tubes. The only "balance" it does it in readings. What they are doing is setting current to 40ma on one side, then instead of measuring 40ma on the other side, they get you to connect a meter between sides and set side 2 so the reading is zero. All that means is side 2 is at the same level as side 1 - our 40ma. Using this method means if the side one reading interacts a little, you don;t have to go back and tweak - both sides will be the same. You could just as easily set side 1 to 40ma, then set side 2 to 40ma, then go back to side one to see if it moved.



Pedro, it is not a "Twin Amp" amplifier. It is a Fender "The Twin" amplifier.

The circuitry (as I explained above) itself is not a balance circuit, but the procedure they use is. The only difference is where you connect the meter leads.

I don;t think he wants to modify the customer's amp - we don;t know how hot the existing circuit will get his tubes.

Yes I realize that and I just worded it wrong but I do understand the arrangement but either way you look at it 20ma's is 20ma's and that is very cold and hard to believe it's not into crossover distortion in either mode. I do agree however that if it sounds good leave it alone as it could be the nature of the amp.

[drewl]

seeing how most people with this amp use the preamp distortion I bias it as it says and run the tubes a bit cold. They last longer, amp sounds fine and it doesn't come back with any problems!

[Chevy]

Thanks for info.... one more power question!

Guys,

Thanks so much for the input!!! Very much appreciated. This beast is just different...

OK, I got the bias pots to fix the amp, got 'em in there, and the amp seems to be fine now, but I can only get about 62 watts out of it into a resistive load at either 4 or 8 ohm loads (Z switch set in correct position in both cases) at 1 KHZ.

Here's the background: Amp came in with the bias pots busted in half due to amp falling on it's back. One or more tubes were apparently glowing cherry and there apparently was some smoke along with that. I could not find evidence of anything being burnt..... but 2 of the screen R's were about 50% too high off spec.

Customer is flat broke, but a steady gigger, and I'm having to use his old mis-matched spare tubes.... (I don't have a full new set on hand, either, for a test) 2 newish Sovtek 5881's on one side, and two very old Chinese 6L6's on the other. Bias is set at 40mA/side. B+ is at around +445 with the amp just about clipping at output. Power supplies seem to be healthy. The screen 470R's were all replaced due to being far off spec.

I'm thinking the tubes are the problem, but not enough experience to know for sure. The tubes test OK (all around 80 - 85%) on my old tube tester. The output of the amp clips evenly on top and bottom of waveform; so this tells me both side are working about evenly. I ran the amp for over half an hour at around 25 watts, 1 KHZ, and no evidence of any problem. Resistance of both OT primaries is even, at around 45 ohms???? if I remember the number correctly; but they are even for sure.

But why so little power out? Just the tubes? Could the OT have been damaged somehow too?

[Enzo]

40ma a side? That is 20ma per tube then. Try adjusting it for 40ma per tube.

[Chevy]

40ma a side? That is 20ma per tube then. Try adjusting it for 40ma per tube.

Enzo, By what you and others have said, I kinda decided to run the amp as per Fender design (stencil on the amp says to adjust to 40mV) and run it at 40mA per side. This is 20mA per tube. The waveform on the scope looks ok to me, symmetrical and clipping evenly.
I temporarily upped the current to almost 50mA/side, and that did nothing for the power output.... don't know if that was enough to make a difference.... I can always up the current, but if the amp is designed to run at 20mA/tube and produce 80 watts or so, I shouldn't have to increase the current to get the right power out..... no?

I assume this amp should put out 75-80 watts or so at it's best. Is that a correct assumption?

[Enzo]

You are right, sorry. I got distracted.

Can you drive it to full output through the power amp in jack?


I'd expect a 100 watts out of this thing.

How are you determining this 62 watts?

It is a guitar amp, not a hifi, so if you are balking at the onset of distortion, that could be the problem.

There are test conditions specified on th schematic, and there are test points throughout with signal levels. How does all that stack up?