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Old 16th November 2010, 10:24 AM   #1
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Default Marshall Amp strange wiring

Hi
I just bought a very old used Marshall type (knock-off) amp, which I'm about to modify. It uses 38 VDC for heating the 3 preamp tubes (in series) and the same -38VDC for bias for the output tubes (4xEL34), which are heated separately by 6.3 VAC. I googled tons of Marshall and other schematics without any luck in finding a commercial amp with such arrangement.

My question is: are there any drawbacks to this type of wiring - noise,, sound detoriation, etc... In case "yes" - one option would be to use the 6.3 VAC supply for the both the preamp and output tubes, which however may require to leave only two EL34 tubes (50 watts) - but is such modification worth?

please help. 10x
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Old 16th November 2010, 11:00 AM   #2
DF96 is offline DF96  England
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Using DC for the input valves will reduce hum, and allow sloppy heater wiring to be used. This means the manufacturer can use cheap valves with poor heater-cathode leakage, and he doesn't have to learn how to do AC heater wiring.

The main snag with this arrangement is that if one valve heater goes open-circuit, they all go cold. You then have to swap each in turn to find the one which has failed. A conventional parallel heater circuit would mean that only the failing one goes cold. I some cases this might mean that only one channel fails.

On balance I would leave it as it is, unless you want to learn how to do AC heaters.
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Old 16th November 2010, 11:02 AM   #3
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Well, when you play the amp, is the noise level ok? If yes, there's really no point in messing with the wiring.

DC heating on the preamp tubes should actually provide a really great noise situation in the context of guitar amps - very rarely people bother with DC heating in guitar amps.
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Old 16th November 2010, 11:33 AM   #4
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Thanks for the replies.

The amp has an audible buzz which does not come from the preamp section (it is still there even when a turn the master and volume knobs all the way down, and it does not increase as I turn them up). I suspect the buzz may be due to old filtering caps, low quality resistors, insufficient B+ filtering, lack of a choke for the B+ supply to the output transformer, etc.. I'm planning to work on all that and maybe add star grounding too.

So based on your comments I'm leaning towards leaving the preamp heating as it is since it is actually better in terms of noise.

Have you any thoughts on the quality of sound of DC vs. AC heating in preamp tubes. I remember reading on the net that DC heating takes away warmth from tubes?
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Old 16th November 2010, 01:02 PM   #5
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In indirectly heated tubes, my experience is that there is absolutely no difference in sound quality.
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Old 16th November 2010, 08:53 PM   #6
taj is offline taj
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The buzz is probably old filter caps. Time to replace them. No big deal.

If it was my amp and I was using it a lot (depending on it), I'd add another power supply (transformer, diodes & filter caps) for the preamp heaters and lose the mickey-mouse heater wiring. Those parts are really cheap and the installation is easy. You don't need a pi supply for guitar amp heaters.

I'm just thinking of the hassle and downtime if something screwed up with those series heaters while you were in the middle of a paying gig. If that's not your situation then it probably doesn't matter.

I would also (personally) have no problem with losing a pair of output tubes. Same sound, but 50W Marshalls are a much more reasonable volume for stage use. I can't stand 100W guitar amps for anything less than 200 ft wide stages. But that's just me. (speaking as a sound guy.)

..Todd
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Old 16th November 2010, 10:23 PM   #7
taj is offline taj
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The buzz is probably old filter caps. Time to replace them. No big deal.

If it was my amp and I was using it a lot (depending on it), I'd add another power supply (transformer, diodes & filter caps) for the preamp heaters and lose the mickey-mouse heater wiring. Those parts are really cheap and the installation is easy. You don't need a pi supply for guitar amp heaters.

I'm just thinking of the hassle and downtime if something screwed up with those series heaters while you were in the middle of a paying gig. If that's not your situation then it probably doesn't matter.

I would also (personally) have no problem with losing a pair of output tubes. Same sound, but 50W Marshalls are a much more reasonable volume for stage use. I can't stand 100W guitar amps for anything less than 200 ft wide stages. But that's just me. (speaking as a sound guy.)

..Todd
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Old 17th November 2010, 01:27 AM   #8
Enzo is offline Enzo  United States
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My word, all this commotion over heaters.

Series DC heaters is not a rare approach in guitar amps. With the same 150ma of heater current running through them all, you don;t have to crank several amps of 6.3VAC down the board. One very commmon problem - usually alleviated by hard-wiring - is the high current 6VAC burning up connector pins that carry it between boards.


yes, if one of a series string opens its heater, they all go out. bip, bip, bip down the sockets with a voltmeter tells you in seconds which one it is. And really, how many open heaters does any one of us encounter? In the last 25 years of running a pro audio repair shop, I can count the open heaters I have seen on my fingers. That really is not much of a consideration.


It is DC, which means it won;t add any hum. Yes, one can design noise free AC heated circuits, but the series string of DC won;t require any extra measures, the transformer winding can be lighter, and the same winding can do double duty running relays or creating the bias supply.

recap your B+, and don;t forget to recap the bias supply.


A couple examples of series heaters: The very popular Peavey Classic 30 has three 12AX7s in series heaters on DC, and their gain monster 5150 amp also wires the heaters in series string in the preamp.
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