Biasing Twin Reverb help.

[Sea Chief]

Hi chaps,

been a while. If some may recall I have a (myself rebuilt) '78 Twin Reverb with a bad HUMM issue. Which I couldn't fix.

Y'all consensus after much help trying, was likely a conductive board. Still a possibility: I'll be able to know once back up running.

Since the previous thread, it's just today got back from a UK pro amp builder. It's had a big check-up, renewing a few resistors, etc etc. Has he fixed the HUMM? I bloody hope so! But, it still could be there, meaning the conductive board theory would then be -very- likely. So I'm eager to find out.

Sending it back he says I need to rebias to "40mA" (his suggested figure) with it's 430-440v B+ (I'm not sure why- do they go out of kilter in transit?).

So could anyone help how I do this? Years since I did it. I believe I have to rebias each 'pair' of 6L6.. is that right? It has a bias pot added during the rebuild.


Thanks, Sea Chief

[Chuck H]

Why didn't the "pro amp builder" just do the bias adjustment while he had the amp on the bench?

The bias would be adjusted by changing the value of the 15k resistor on the bias balance pot (schematic value, actual value in amp may be different). We will need to know the idle current as it is and the value of that resistor to advise further.

[Mars Amp Repair]

Hello Sea Chief,
Frist of all, THERE ARE LETHAL VOLTAGES IN THIS AMP. YOU MUST BE CERTAIN THE POWER IS DISCONNECTED & THE FILTER CAPS ARE DISCHARGED BEFORE MESSNG WITH THE AMP.
The easiest and simplest way to set the idle current is to just purchase a simple bias device. However since most folks will only do this once, you can do it without spending that money, but still will need 2 volt ohm meters to do this as a tech would.
I'm listing the procedure here, but please do not attempt this unless you have some experience working on items with high lethal voltages in them.
Now without 1ohm resistors in the cathodes of the 6L6's to measure across (which the tech would have possibly added), the alternate way is to, WITH UNIT UNPLUGGED & FILTER CAPS DISCHARGED;
1) Set the bias control for about midway.
2) Find the center tap of the power transformer, disconnect it, and add an ammeter in series with it and the other point where the wire was initially connected.
3) Set the ammeter for the milli amp scale, fire up the amp, and take it out of standby with volume controls at 0.
4) Now assuming the tubes are matched, the reading will be measuring the idle current for all 4 tubes, so you'll need to divide it by 4. Typically on a Fender w/6L6's, the reading should be about 30ma for each tube (or 120ma for all 4), BUT, it's better to calculate the idle WATTAGE of each tube not the current draw, as even all matched sets of tubes do not dissipate the same wattage.
5) SO, measure the voltage at the center-tap and calculate for wattage by multiplying the current measured times the pcentertap voltage. Note: Technically, you should measure the voltage at the plates of the power tubes, but it is precarious to do that and the reading will be close enough to the center-tap of the output transformer to be viable.
6) Example: Current measured 120ma x Voltage measured 450Volts = 54watts divided by 4 = 13.5watts per tube, which is on the higher end of what's acceptable for a 6L6 tubes in push-pull.
7) Adjust the bias pot for a slightly lower (CCW) current and repeat the calculation.
a. NOTE: As you change the idle current, the plate voltage will also change, so remeasure the plate voltage every time you change the bias current & recalculate.
b. 40mA per tube as suggested by your tech, would set the idle wattage for each tube to about 17watts, which seems a bit high to me.
Hope that helps. Just be REALLY careful to keep in mind that the voltages/currents in that amp can KILL you.

[Helmholtz]

The HT CT current contains 20mA to 25mA of preamp and screen currents.

[Sea Chief]

Why didn't the "pro amp builder" just do the bias adjustment while he had the amp on the bench?

The bias would be adjusted by changing the value of the 15k resistor on the bias balance pot (schematic value, actual value in amp may be different). We will need to know the idle current as it is and the value of that resistor to advise further.

Hi Chuck,

do you mean turning the bias pot? This is all I remember. There's a bias pot (with a slot in the back) I remember it's to adjust slightly, & the figure on the DMM changes. So this part is known.

But where the two DMM probes go, I can't remember. Over a resistor or something?

Why didn't he do the biasing there.. well he did all the testing with a matched quad of (his own) 6L6's. Only 2 of mine were matched you see as it turns out, & 1 wasn't even conducting at all (no idea why- I bought a new matched JJ quad for the rebuild only 5 years ago).

So he's sent it back kindly throwing in 2x Tung Sol 6L6's (presumably matched to each other).. so I can get up & running. I can't afford £110 on (yet another) new quad.. infuriating to even consider it, especially if I have 4 healthy ones, 2 pairs. So as non-perfect it is, I will have to live with it like so.

So, I have 1&4 positions with my JJ's. And 2&3 positions with Tung Sol's. As suggested by him. I aim to bias the two sets that being 1&2, & 3&4 (I think this is the idea) to a similar figure, of 40mA.

Thanks, SC

[Sea Chief]

Mars Amp Repair Thanks alot for the reply.

What I need to do, is the same procedure as before. Because I need familiarity, A) because I'm not too knowledgeable about all this, & B ) because I need to do it as safely as possible: undoing PT wires & I'm then in unfamiliar territory, & this means I'm unsure. Not good.

Now, I know that I -didn't- undo any centre tap wire before. So with respect, I'm a bit confused as to why this is suggested, even if it might be an alternate way. A better way even.

So there must be a procedure (this is really what I'm aiming for advice on) whereby the two probes are placed, in the amp somewhere; perhaps over a resistor (?? I seem to recall this is the way I've done this before, in other amps too.. but where, I cannot remember).

If I google UL135w (which this amp was, before being rebuilt with Blackface additions) I get info on a dial at the back, which is now redundant. No use.

Thanks, Capt

[Helmholtz]

Please post a pic of the power tube sockets. Maybe you already have 1R cathode resistors.
As you obviously know how to solder, it shouldn't be a problem to add them.otherwise.
Using 1R cathode resistors is the best and cheapest solution.
With them installed you just measure the mV drop across each resistor.

[Sea Chief]

Mars Amp Repair

Sorry I missed this part of your reply..

Now without 1ohm resistors in the cathodes of the 6L6's to measure across (which the tech would have possibly added).

THIS is what I believe he -has- added (or rather, I added when I rebuilt it 5 years ago). But I see only a big green 470r & small brown 1.5k at each socket.. not 1r. Anyway hopefully this photo might clarify things..



Thanks, SC

[pdf64]

The camera angle couldn't be much worse, but I can see a 1R resistor on at least 1 socket.

[Sea Chief]

The camera angle couldn't be much worse, but I can see a 1R resistor on at least 1 socket.

I've taken many photos, clear as day unlike this ^ one, apologies. But they'll simply not load onto a post. Spent ages trying, doing my *******g head in.

What can I do? Tbh I can't see a 1r resistor! Only a big green 470r & a smaller brown 1.5k across the socket.

SC

[Sea Chief]

JesusH. Finally got another one up.

Hopefully this helps, clearer. You may need to zoom in.

[Helmholtz]

Seems all four 1R resistors are there. The are the small blueish/grey resistors connected between socket pin 8 and the chassis ground lug.

[Sea Chief]

Ooh I can see the 1r resistor! Little blue sod hiding away at the side.

Right, so if anyone can remind me what to do with it? Then I'm back on the same track as I did previously.

Apologies Mars Amp Repair & kind of you, but it's too complicated for me to do your more intensive method. I can only cope with putting a DMM across this resistor, tweak the bias pot aiming for an mA figure! Or mV as I do know I read it as.

And to be honest, I cannot understand for the life of me, why this "1r" method isn't as good as another, if the aim is to set a figure of say 30mA by tweaking the bias pot: I mean if you go a huge longer route.. what is the logic to taking this longer route-? I can't see any reasoning to it being a 'better' way if the result, is the same, that being to set a bias figure of 30mA for eg. Idle wattage, dissipation (is it? A percentage like 70% or something?) it's all an unknown physics language to me in which I can't see a trace of logic, to follow, to be able to start to understand any of it: I try but it remains unknown, like advanced quantum mechanics!

Thanks, SC

[Helmholtz]

As said, the 1R method is best.
Adjust for around 35mV (corresponding to a current of 35mA) across each 1R resistor.
With unmatched tubes it will not be possible to get exact same readings at all tubes.

[Sea Chief]

Seems all 4 four 1R resistor are there. The are the small blueish/grey resistors connected between socket pin 8 and the chassis ground lug.

Hi HH, yup great so my pics have been of use. I remember soldering these in 5 years ago now too.

So I'm all set, everything plugged in: speakers/ footswitch/ reverb tank, & my DMM at the ready. A 2 year odyssey of pain trying to fix this amp, is tantalisingly close to a possible conclusion:

I'm inches away from being able to conclude that if (god forbid) the HUMM is still there- then the theory that a conductive board holds merit (& I throw the fkn thing in the river tbh) or..

that if the HUMM has gone (or at the least seriously diminished)- then my theory that a bad component was the likely culprit, holds merit.

Other things done by my pro builder like a better ground path for a wire here & there, wrapping a shield around the Vibrato, were the same before the HUMM started.. so it cannot be attributed to these logic surely suggests.

Thanks, SC