Marshall 4140 Amp question

"Arfa Daily" bravely wrote to "All" (19 Jan 06 10:36:03) --- on the heady topic of "Re: Marshall 4140 Amp question"

AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:355768

AD> In contrast, if the output stage is drawing anything like enough AD> current to damage a transformer - and, IME, these will happily drip AD> wax for at least 5 minutes without failure - then the HT fuse should AD> ideally blow, or the surge limiter / smoothing resistor smoke, glow, AD> and go open. AD> It's probably just a matter of opinion and experience, which you AD> obviously have. I'm just making sure that those who might be reading, AD> and have less experience than you and I, understand the possible AD> dangers and cosequences d:~}

AD> Arfa

A typical primary uses wire about the diameter of a hair. It really can not drip wax for long. In some cases they are damaged merely by high voltage spikes. The insulation breaks down. This is especially true for older transformers because if the enamel got hot over time it tends to oxidize (turns to carbon) and the damage is cumulative.

The thing about audio transformers is that they are to an extent responsible for the tonal character of the amplifier. So if you replace it with something equivalent it might never sound exactly the same. It is a matter of the stray capacitance and feedback in how it was wound. So you understand my being more protective of an audio transformer than of an electro.

Now we know one of the electros is bad. It is possible the voltage stress would have uncovered the weak one right away in a shower of sparks, just as you said. However, an esr meter might be better.

A*s*i*m*o*v

... I remember the 6SN7...

AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:355768

AD> In contrast, if the output stage is drawing anything like enough AD> current to damage a transformer - and, IME, these will happily drip AD> wax for at least 5 minutes without failure - then the HT fuse should AD> ideally blow, or the surge limiter / smoothing resistor smoke, glow, AD> and go open. AD> It's probably just a matter of opinion and experience, which you AD> obviously have. I'm just making sure that those who might be reading, AD> and have less experience than you and I, understand the possible AD> dangers and cosequences d:~}

AD> Arfa

A typical primary uses wire about the diameter of a hair. It really can not drip wax for long. In some cases they are damaged merely by high voltage spikes. The insulation breaks down. This is especially true for older transformers because if the enamel got hot over time it tends to oxidize (turns to carbon) and the damage is cumulative.

The thing about audio transformers is that they are to an extent responsible for the tonal character of the amplifier. So if you replace it with something equivalent it might never sound exactly the same. It is a matter of the stray capacitance and feedback in how it was wound. So you understand my being more protective of an audio transformer than of an electro.

Now we know one of the electros is bad. It is possible the voltage stress would have uncovered the weak one right away in a shower of sparks, just as you said. However, an esr meter might be better.

A*s*i*m*o*v

... I remember the 6SN7...

So do I ...

Are we talking output transformers or power transformers here ? If output transformers, then I would agree that the design of the tranny is critical to the audio performance of the amp. However, I cannot accept that for a high power group amp, the primary is wound with wire as thin as a hair when a) it has a resistance as low as a few ohms per winding half, and b) has to stand peak currents approaching an amp. Even allowing for age, the windings have to get very very hot, and stay so for a very considerable length of time, in order to carbonise the winding insulation coating. When was the last time you managed to burn this coating off successfully with just a soldering iron tip at the better part of 300 degrees C ?

The situation is much the same with power transformers, which do not, however affect the sound providing they can supply the required voltages at the required currents. I can't remember in 30 odd years of repairing these things, ever having to order a replacement power tranny, ( perhaps they're more robust this side of the pond because they have to stand twice the voltage ;-) ) and only one or two output trannies which had failed as a result of clear problems such as a defective output tube which had originally blown the HT fuse, which had then been bridged by silver paper to finish the gig ...

I cannot agree that it would have been better to have found the defective cap by letting it blow up in the OP's face as a result of overvolting. When I was first in this business back in the early seventies, and working for a large TV rental chain, there was a lad at another branch who suffered a serious skin burn and partial loss of sight in one eye as a result of an electrolytic can exploding in his face, so you see my reasons for advising extreme caution with these things, and treating them with considered respect. ESR meters are actually not very good with old can type electrolytics, as their impedance was quite high in the first place, so any readings you get are likely to be misleading if you are used to the readings you get on modern caps, but yes, for all other cases, I would agree that an ESR meter is an essential workshop tool for today's servicing.

Broadly, I am in agreement with you. We've both been around long enough to know how to go about servicing this kit safely, and it sounds as if the OP has too. There are many though, who will be reading this thread, who are not experienced in this type of repair, and might one day decide to pull out that old amp in the garage that uncle Willy used to play when he was in Willy Wang and the CrapKickers, and run it up, and I'd hate for them to get sprayed in exploding cap.

Arfa

Replaced that dual cap and runs cool and no hum after 10 minutes. But after an hour a hum develops, intrusive but not as bad as before and at the same time a serious glow forms on the zinc coloured metalwork of one pair of the EL34s. All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the grids of each EL34, correct values cold. The push and the pull from the preamp are AC coupled, leaking cap here? Swap pairs of EL34s to see if the core glowing ones are in the new position or the original position ?

Yes, most likely problem, if it's not the EL34s themselves ( and if both on one half are doing it, it's probably not ) is the coupling cap between the phase splitter valve and output valves' grids. It is very common, and will often result in hotspots on the anodes and even grid glow. I would just replace both caps anyway. Check also that the bias is not drifting, and if there is a balance pot for the two halves of the output, check that this is set correctly. Check also, any resistors involved in splitting the bias between the two sets of valves. As far as I recall, Marshall usually use a single -50v bias supply, adjustable with a single preset, then distributed to both grid circuits via a couple of fixed resistors. Check that someone hasn't turned the bias down so that the valves are thrashing a bit anyway, and the problem is then just being caused by ' normal ' thermal characteristic drift of the valves, exacerbated by the increased dissipation from low bias.

Arfa

"n cook" bravely wrote to "All" (20 Jan 06 16:37:24) --- on the heady topic of "Re: Marshall 4140 Amp question"

nc> From: "n cook" nc> Xref: core-easynews sci.electronics.repair:355957

nc> Replaced that dual cap and runs cool and no hum after 10 minutes. nc> But after an hour a hum develops, intrusive but not as bad as before nc> and at the same time a serious glow forms on the zinc coloured nc> metalwork of one pair of the EL34s. nc> All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the nc> grids of each EL34, correct values cold. nc> The push and the pull from the preamp are AC coupled, leaking cap nc> here? Swap pairs of EL34s to see if the core glowing ones are in the nc> new position or the original position ?

You should have checked the coupling caps for leakage first. If then it still glows it could be a bias problem on that pair. Seems like you are well on the way to restoring the amp.

A*s*i*m*o*v

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Not a problem with the DC blocking caps at final push-pull separator stage. Replaced and still hum after 3/4 hour. No DC on the ganged volume pots. Monitoring the negative bias for the output EL34s.

10 minutes in, the -ps voltage at smoothing cap is -52V. The voltage to the (schematic marked) B pair is -39.2V before going to 1.5K then g1 of each EL34 For C pair -39.2V also 25 minutes in B = -37.8, C=-34 45 minutes in B= -36.9 , C=-32.5 hum is getting quite noticable and -ps rail is still -52 Increasing bias pot from its original of 8K to max of 22K brings the -ps down to -53.5 now B=-39 and C=-33 less hum but still going more positive over time and hum increasing. I stopped before the B pair started glowing like before. I was expecting from these voltage readings that it would be the C pair that would start complaining. Switching off the amp, not just to standby, for a couple of minutes , brought things back to original cold situation and another 3/4 hour presumably before hum gets too much.

One thing that concerns me is each of these 1.5K g1 resistors has one end to the valve base pin and the other floating in space , not soldered to an insulated pin just the wire connecting through - would that be as original? The schematic shows 1.5Ks but could someone have replaced 4 of a different value? or is it EL34s leaking too much and need changing.

I think that it's getting to the point now where you've got to start trying valve substitution. However, before rushing out and buying new ones, you can start by removing both the C valves ( these are the ones that glow ultimately - yes ? ), then removing one of the ( likely ) OK B valves and putting it in the C side. The amp will run quite happily with just one valve in each side ( it's a trick that I teach owners to allow them to finish a gig if they have a serious valve failure ).

You will then be running it with two valves that have performed OK when they were both in the B side, so if it now works ok, next put them into the two unoccupied sockets instead. If it still continues to work ok, then the chances are that it is a valve problem. If it doesn't, then it must be a bias issue. This assumes of course, that it's nothing to do with the output tranny, which could suffer a partial insulation breakdown, resulting in shorted turns, when it has been running a while. Again, this could be checked by swapping the winding ends betwen anode pairs, and seeing if the bad behaviour swaps sides.

Arfa

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Swapping each pair over, improved matters a lot. The bias stayed about 6 volts more negative and much less noise for the same time intervals from switch on. So to save buying new bottles , a matter of switching off for 2 minutes every hour or so. I can only assume it is due to over driven valves - well something had to burn up those resistors and knock out the HT fuses. Is there a name for this leakage? effect

Not that I'm aware of. I suppose that there probably is if you're a valve manufacturer, though. Might be worth looking into the cost of some new valves. Prices have come down a lot since the chinese have got involved. Apparently, much of the old valve manufacturing equipment from UK plants, was sold to Russian and far east countries. There's often some good deals to be had on e-Bay. Also worth checking with your local music shop, if you've got one. Valve PA amps are still as popular as ever with ' real ' guitarists, and my local shop buys in valves in quite large quantities, so prices are not too bad.

Arfa

"n cook" bravely wrote to "All" (23 Jan 06 17:22:52) --- on the heady topic of "Re: Marshall 4140 Amp question" nc> From: "n cook" nc> Xref: core-easynews sci.electronics.repair:356259 nc> Swapping each pair over, improved matters a lot. nc> The bias stayed about 6 volts more negative and much less noise for nc> the same time intervals from switch on. So to save buying new nc> bottles , a matter of switching off for 2 minutes every hour or so. nc> I can only assume it is due to over driven valves - well something had nc> to burn up those resistors and knock out the HT fuses. nc> Is there a name for this leakage? effect The input grid on a tube can become "poisoned" by deposited material coming from the plate and this diverts some cathode current. As it starts to get hot it starts to emit its own electrons and loses some control over the plate current and it increases. It is especially noticeable in tubes that are driven very hard. I saw an extreme of this effect in a 12AX7 phase splitter where 3K3 cathode and plate resistors were mistakenly installed at the factory, instead of 33K. Must have been a colour blind worker. Anyway, the

12AX7 was practically a short. Just for fun I arced the input grid with some very high voltage (5KV) and it cleared the junk on it enough that after this it worked as a tube again. A*s*i*m*o*v ... You mean 15" sparks are SUPPOSED to come out of this thing?!?

had

Sounds plausible, well at least there is not the complication of minority carrier recombination in thermionic devices.

"n cook" bravely wrote to "All" (24 Jan 06 08:40:28) --- on the heady topic of "Re: Marshall 4140 Amp question"

nc> From: "n cook" nc> Xref: core-easynews sci.electronics.repair:356334

nc> Sounds plausible, nc> well at least there is not the complication of minority carrier nc> recombination in thermionic devices.

There is still heavy ions to think about in heavy iron.

A*s*i*m*o*v

... "Bother!" said Pooh, as he saw the sparks and smelled the smoke.