Fender Twin UL > AB763 (re) build.

[pdf64]

Hanging additional circuits off a power tube plate can facilitate oscillation.
Why test the plate's Vdc? Just measure the HT, the difference in Vdc between them at idle is negligible, and as you may be seeing, probing the plate can disrupt idle.
How about setting the bias for a much cooler idle and monitoring cathode currents for 20 mins, to see if they stabilise at a reasonable level?
Perhaps remove the phase splitter tube in V6 to eliminate the possibility of oscillation.
If the cathode currents rise too high, shut it down.

[Sea Chief]

Additional circuits- sorry I don't understand.

Why test vdc? I thought this was inherrantly done in any amp build.. & also re. the dissipation fig needed, so B+ divided by cathode current.. I thought all that was entirely relevant to what I'm doing/ current/ bias issues? I don't understand the Q.

Why wasn't I seeing a steady 495v though? was my fluctuating 680-750v to be expected, considering what I was doing (& not symptomatic neccessarily.. of an imminent series of filter cap explosions?).. if so.. what was I doing wrong??

[pdf64]

Additional circuits- sorry I don't understand.

To be specific, the probes, leads and meter are the additional circuit which you are connecting to the power tube plate when taking a Vdc reading there.
That node is a powerful transmitter; the meter etc that you've connected there can now act as its antennae. The signal from it gets picked up by the sensitive 'receiver' grids of previous stages, creating both negative and positive feedback loops.
The plate Vdc may be reading high because there's a massive amount of Vac, due to oscillation, also there, which may cause your meter to give an erroneous Vdc reading.
Hence I suggest to use the HT voltage, rather than the plate voltage, for the dissipation calculation. Or break the feedback loop by removing V6.
But I suggest that you just bias them each to ~30mA cathode current, dissipation will be fine, the particular value is immaterial for guitar amps as long as it isn't excessive, and 30mA won't be excessive, plus you'll avoid digging another hole of confusion, misunderstanding and general misery here.

[Sea Chief]

Ah I see.. so I got the place to read B+ wrong then?! my oh my.. well its been over a year since I did this in any other amp/ nr a year this one's been at the doctors- Im very rusty then.

Right so please remind me where to read the B+, with all the tubes in place. I understand I can take a reading across the 2 red wire HT where they go into the bias board.. but this isn't reading the B+ at the business place/ the tubes tho is it?

IE don't I need to read each individual tubes' B+ at a socket pin?

[Sea Chief]

To be specific, the probes, leads and meter are the additional circuit which you are connecting to the power tube plate when taking a Vdc reading there.
That node is a powerful transmitter; the meter etc that you've connected there can now act as its antennae. The signal from it gets picked up by the sensitive 'receiver' grids of previous stages, creating both negative and positive feedback loops.
The plate Vdc may be reading high because there's a massive amount of Vac, due to oscillation, also there, which may cause your meter to give an erroneous Vdc reading.
Hence I suggest to use the HT voltage, rather than the plate voltage, for the dissipation calculation. Or break the feedback loop by removing V6.
But I suggest that you just bias them each to ~30mA cathode current, dissipation will be fine, the particular value is immaterial for guitar amps as long as it isn't excessive, and 30mA won't be excessive, plus you'll avoid digging another hole of confusion, misunderstanding and general misery here.

pdf I'm lost. Ive just gone to youtube to refresh myself where to probe for b+, to get my approx 495v figure (I measured this before.. but cannot remember where). The amp tech did -exactly- what I did, black probe to chassis > red probe to the plate/ pin 3 of the power tube (el34 in his case). 495v steady.

Please remind me.. why was mine 680v-750v & fluctuating wildly? (is it bc I had a guitar plugged in??)

[Dave H]

Sometimes (but not always) the amp will oscillate when a meter probe is connected to the plate of a power tube giving wild meter readings. If your amp is as the layout below and you really want to know the B+ voltage you can measure it at the top right of the diode/bias board where the two strings of diodes connect together but I'd do as Pete said just set the bias current to 30mA. Check and reset it back to 30mA after 10-15 min and call it done. It will wander around a little with the mains voltage after that but it shouldn't go up to 45mA unless there's a fault.

[Sea Chief]

pdf help me out here would you. I dont understand a word of your post #18, now that Ive seen eg after eg on youtube & in discussion forums answered by amp techs.. of the -exact- thing I have done to measure plate voltage @ pin 3 of the power tube. Here for eg:

[a Q on a forum like this, he is talking about the power tube]. Q: "I definitely want to avoid getting electrocuted. I am only planning on using the multimeter touching off at pin 3 to ground, with speaker plugged in for sure. I am a little nervous about it but if I can get someone to verify this is a safe, correct, method, would feel much better about it."


A: "I can tell you that I used to run my own amp repair business in Austin, and this is how I did it. Just make sure the chassis is stable and you should be good to go. On Fenders, if I was just doing a quick bias adjustment, I used to put the chassis upside down on the top of the amp. But depending on the size of the transformers, you may have to put something underneath one side to balance it."

And a Youtube clip, another amp tech @ 1.40 https://www.youtube.com/watch?v=B46w_B_Aoq8
plenty of eg's online concur with what I did.. but you allude to it being wrong for xyz reasons.

I am totally bewildered.

[Sea Chief]

Hi Daveh. I thought it was inherrantly page 1 stuff to need to know the B+. Its the one thing Ive known from any amp built, the fundamental Q, is what this important figure is. So I dont understand why you say 'if you really want to know it..'. You HAVE to know it in order to find out x, y, z (dissipation/ aiming for that 60-70% area is inherrantly important> you need to know the B+ to then divide * etc.. ie whether the amp's running at the correct voltage, for the tubes etc etc/ primary stuff- surely?).

What the heck am I missing here? its like the carpet's been pulled from under me.

[Sea Chief]

I don't have a string of diodes, I have a bridge rectifier so Ive no idea where I masure here.

Let me go back a step. Is it correct that in order to guage if the amp is working as it should.. you should find out what the plate voltage is, at each of the 4 power tubes-? is that correct?

[Dave H]

I don't have a string of diodes, I have a bridge rectifier so Ive no idea where I masure here.

Let me go back a step. Is it correct that in order to guage if the amp is working as it should.. you should find out what the plate voltage is, at each of the 4 power tubes-? is that correct?

You can measure the B+ at the + terminal of the bridge then. The plate voltage of the power tubes will only be a few volts less than that (because of the voltage drop across the OT primary).

We are recommending that the bias current is set at 30mA because at that value the tube dissipation is sure to be OK even if the B+ is over 500V.

[Sea Chief]

Ok Dave & pdf.. Ive just taken your word for it, & I'm forgetting all that voltage faff for now (Ive got alot of + terminals at the bridge.. so bllx to the b+ I say). I turned on & it went up to 45mA, so I set it to 30mA/ it went up to 40mA so I backed the pot back again (prolly at close to min now) to 30 again.. & monitoring it (& playing- proper 1st go woo bloody hoo) for an hour: & it stays there~therabouts! actually 27.9 when I turned off.

So, is that creep 'issue' resolved? & is it deterimental to have it so low at 27.5 mA at all? if it stays there that is I guess.

[Sea Chief]

And bar it being very groundy-noisy when Im not touching strings.. it sounds bloody great too.

Better now than a UL SF twin- no doubt about it.

What can I do re. the groundy noise.. is that the gtr or amp? cant say like so with my 5f1 so gotta be amp?

cheers chaps SC

[g1]

If the noise is only when you are not touching the strings then I think that is fairly normal. At least I get that, others may have different experience but I would think it would depend on shielding in guitar, room environment (emf) etc.

For the B+/plate voltage issue. These guys were not saying you don't need it, they were saying you don't have to check it directly at the plate (pin3). Sometimes this will create squealing/oscillation and weird readings like you experienced. When you have a problem measuring it directly at pin3, you can instead measure the B+ going in to the OT. This will be very close to the same as pin3 voltage, but will get around any problems (oscillation) induced by the test equipment.
In most old style fender amps, you can measure the B+ at the standby switch.

[Sea Chief]

Ok thanks guys. Yes standby switch I seem to recall I measured xyz before.. must have been that.

Well all back together for 1st time, 2 playing sessions now: bar the normal buzz/ bit of motorboating/ noise etc minor stuff I'll sort w'another thread maybe.. amp sounds really great. Like a new amp, less hard than a UL & far more complexity- huge result! the only thing I can check with amp together & in daylight- is the heat from tubes: nothing hot or OTT both sessions/ amp only just warm.

Ok so without being able to measure the bias.. having got success with keeping it steady at 28.5mA say (ie between 27- 30) in my last test.. is this now safe to say it'll not do the creeping up 35>45>.. thing again? or should I be monitoring it on a bench for days to make sure?

[J M Fahey]

Ok Dave & pdf.. Ive just taken your word for it, & I'm forgetting all that voltage faff for now (Ive got alot of + terminals at the bridge.. so bllx to the b+ I say). I turned on & it went up to 45mA, so I set it to 30mA/ it went up to 40mA so I backed the pot back again (prolly at close to min now) to 30 again.. & monitoring it (& playing- proper 1st go woo bloody hoo) for an hour: & it stays there~therabouts! actually 27.9 when I turned off.

So, is that creep 'issue' resolved? & is it deterimental to have it so low at 27.5 mA at all? if it stays there that is I guess.

Congratulations, you are doing fine.

The main point is to be able to set power tube bias :

a) at some reasonable current which does not cause buzzy crossover distortion.
You are there

b) that current must be **stable** ... if it uncontrollably runs up it will soon reach damaging values.
You finally reached that too, so ...

c) Enzoism follows: once you have fixed it, stop "fixing"

3) As of:

Ok so without being able to measure the bias.. having got success with keeping it steady at 28.5mA say (ie between 27- 30) in my last test.. is this now safe to say it'll not do the creeping up 35>45>.. thing again? or should I be monitoring it on a bench for days to make sure?

You *are* measuring it .
You have *already* checked itīs stable.
Again: once you have fixed it, stop "fixing" meaning: close it and go play some Blues (or whatever).