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Marshall 1987xl Bias

Started by Littlewyan, December 21, 2013, 01:50:41 PM

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Littlewyan

Hi guys

I had to buy new EL34s for my 1987xl recently as one went bad and have had a few issues with the new valves. The valves are Svetlana Winged C EL34s from Karltone and get burned in for 30-40 hours. Now when i first received them they werent matched as they were about 4ma out. I didn't think that was toooo bad so biased up to 70% which was 38ma if i remember rightly and then played through it. After I played through it I found one valve had drifted and was now 10ma out, also noticed that the HT voltage was going up and down by 4 volts by itself, with no drive. So sent the valves back, got new ones and these biased up perfectly. However after a bit of playing same thing happened! Altho this time it was only 4ma and the HT Voltage didn't appear to go up and down at first but originally it was 440volts and now its 449volts. Also I found that the plate current had drifted up by 5ma. So rebiased, Swapped the valves and the readings follow the valves. When I first turn the amp on I do hear a very slight hint of motorboating, however it doesnt increase with volume and the valves arent redplating. Played with the amp for another hour, rechecked the bias and its now gone down a bit and both valves are matched again!!!! Any ideas?

Roly

Quote from: LittlewyanAny ideas?

Yeah; you've been reading too many Hi-Fi influenced forums?

"Back in the day" the only people who gave a stuff about matching were Hiper-Finatics.  Valve guitar amps?  Just bang in whatever you could get at the local radio repair shop, make sure the bias was high enough to stop them red-plating, and away you go.

My personal view is that matching is a racket to part guitarists from $$$'s.  Matched?  When?  Under what conditions?  At how many points?  How long ago?

Valves drift, it's a fact of life.  I'd also observe that the low distortion that Hiper-Fi-ists are looking for isn't what guitarists want, that a bit of imbalance in the output stage may actually sound better than perfect balance.

{but then I'm a cynical old coot}

Quote from: LittlewyanWhen I first turn the amp on I do hear a very slight hint of motorboating

If this really is low frequency instability you need to look at the power supply filter electros, particularly those for the preamps.

BUT, this could be initial feedthrough of the tremolo oscillator, if fitted, or the valve structures expanding during warm up, which can sound a bit like motorboating.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

#2
Yer thinking about it the PI isn't balanced itself which in most cases sounds better (for the guitarist). The source of interference actually turned out to be one of those LAN Adapters that goes through the power cable. I had the amp plugged into the same four way, just by chance i plugged it into it's own wall socket and realised the amp was silent when I switched on.

Now I think there may still be an issue with a cap somewhere as when I turn the amp off it drains the power pretty quick, yet there is nothing that I can see that would do this? No resistor to ground from the HT line or anything.

Roly

Quote from: Littlewyanwhen I turn the amp off it drains the power pretty quick

This may be quite normal (particularly if you don't have any signs of distress such as excessive heating anywhere).

If you have the supply voltage, an idea of the current being drawn, and the total value of the caps on the HT rail you can time how long it takes for the supply to drop to 1/3rd of initial value.  From the supply voltage and current you can calculate the effective load resistance, then apply;

t= CR
seconds, farads, ohms

... to see if the voltage decay falls in the general ballpark, say within 2:1 of your CR value.  If it's roughly right then you don't have a problem, but if the decay is much faster than the CR value (and you don't have an excessive current draw) then one of the HT caps may be on the way out.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

Hmm I could remove the HT Fuse and put my meter in it's place, suppose it wouldn't hurt as I would only be doing it for 30 seconds. Thanks Roly

Littlewyan

Right, Current Draw is 88ma, Voltage is 445V, so that gives us 5562ohms of resistance. 5562 x .0003F = 1.66875seconds so yer thats about right. When I turn the amp off its probably 2/3 seconds by the time I get my meter on it and the highest I've managed to read is 228V. So all good :)

Roly

Your meter is likely to have an internal fuse on the current range anyway, and if you are not blowing fuses and there are no signs of distress it is generally safe to substitute a meter for the fuse.

Really this calculation of time constant is quite rough, but it's only to work out were the decimal point is, 0.1sec, 1sec, 10sec, etc, and yes this looks quite reasonable.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

Well in that case the Amp is all good. Thanks for your help

Enzo

First, matched tubes.  There is no standard as to what is matched.  You might think they had to be dead on identical, but not likely.  So 3%, 5%, 10%?  4ma is not a large difference.

Your B+ or HT voltage is not regulated, it is simply a rectified and filtered power supply.  Your mains voltage is not stable.  PLug your AC voltmeter into a wall outlet and watch it change.  If you are on 120v mains, then with a 480v (close to yours) high voltage, it will change 4 volts for every one volt of mains voltage change.   My 120v wanders as high as 125v and as low as 117v here.  That is an 8v spread which represents on this amp a 32v spread of B+.

And if your B+ varies, so will the current through the tubes.  This is normal.  If you set a bias for 38ma today, don't expect it to be 38ma every day.

Littlewyan

Good point, didn't think of that. That explains why the voltage is changing overall but often i'll find one valve drifts much higher than the other after i've played through thr amp. Then if I turn it off, let it cool, turn it back on and they may then have a similar current draw. Once one of the valves went from 35ma to 40ma whereas the other stayed at 32ma. I'm not too worried as the amp works fine and the bias is still below 70%, just think its odd for only one of them to drift.

bobhill

Simplest way to figure out what you have is to swap the tube positions. See if the changes follow the tube or the socket. (Ignore NA bias for those of you who have valves in your amps ::) )

Littlewyan

The current draw does follow the valves so its not that particular socket. I've tested all of the capacitors and they're fine so i'm just going to leave it. The valves aren't red plating and the amp sounds great so may as well leave it.