Marshall 4140 Amp question

I rarely deal with valve/tube amps and am more familiar with radio output-matching transformers primary-windings having DC resistance of windings of hundreds of ohms. Been in a shed for years and no known history so treading cautiously. Measuring the DC resistance of each side to centre tap of this Marshall amp, shows only about 15 ohm each way. Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each side of this impedance matcher. Does 15 ohm look right for each of these primary coils ? Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems fine.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
n cook
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I will check a couple of my amps tonight and see what the primary _resistance_ is on the output transformers. One uses a pair of EL-34s each channel, the other a pair of 6550s each channel. They are audio-amps, not guitar amps, but the readings should be similar....

Peter Wieck Wyncote, PA

Reply to
Peter Wieck

Yes, thanks. The only ones I've got lying around are small low wattage radio receiver type ones with range of 600 to 800 ohms on high voltage side. I could not find anything googling. It had obviously been overdriven as charred but functional gate resistors.

Reply to
n cook

Probably doesn't have any shorted turns and is OK. I put a 1KHz AC LCR meter on the coils and it comes out to 32 Henries for each half. I wonder what guage of wire to give only 15 ohms for all those turns.

Reply to
n cook

My thoughts are that a guitar amp which spends most of its time driven to distortion will have transformers able to take considerable abuse without failure. So, I expect it to be much lower than an audio amp (which I will check anyway).

Peter Wieck Wyncote, PA

Reply to
Peter Wieck

I found this in the archives, putting matching transformer henries ohms as keywords in google usenet, on a guitar group

"Primary winding, by direct meter measurement with a Fluke 8600-A: DCR Blue side to center tap = 44.91 ohms DCR Brown side to center tap = 51.03 ohms "

I assume DCR is DC resistance of that matching transformer. Mine was measuring 14 and 15.5 ohms which seemed a bit unbalanced but no worse than this example

Reply to
n cook

One side of center will have more DC resistance because more wire is required for the same number of windings.

GL, Bill

Reply to
- exray -

This may sound like a silly suggestion but .....

Why not just ring Jim Marshall, and ask ? They're only up in Milton Keynes, and are a pretty friendly company.

FWIW, for a paralleled pair of EL34s in this application, I'd say that the readings you have obtained are fair. If it has been standing for a long time, I would strongly advise bringing the power up slowly on a variac. The main resevoir and smoothing caps, may not be in a state to accept HT right away, and might need reforming.

If you are a reader of " Television " magazine, look out for the new one that will be coming out at the end of March. It's going to be called " Technology @ Home ", and when I get round to it, you can read my ramblings on repairs to valved group and PA amps.

Arfa

Reply to
Arfa Daily

"n cook" bravely wrote to "All" (18 Jan 06 12:52:44) --- on the heady topic of "Marshall 4140 Amp question"

Hi,

That 15 ohms is about right for a high power output transformer.

The first thing is to check the input grid coupling caps for leakage. Then the screen grid resistors for opens. Then remove the finals and apply power briefly to measure that the grid negative bias is healthy. Check the tubes. Mount them back in. Then on to actual running tests. Beware of the B+, high DC with ++joules is quite painful and lethal. Do discharge all supply electros first thing before working inside!

A*s*i*m*o*v

nc> From: "n cook" nc> Xref: core-easynews rec.antiques.radio+phono:359961 nc> sci.electronics.repair:355619

nc> I rarely deal with valve/tube amps and am more familiar with radio nc> output-matching transformers primary-windings having DC resistance of nc> windings of hundreds of ohms. nc> Been in a shed for years and no known history so treading cautiously. nc> Measuring the DC resistance of each side to centre tap of this nc> Marshall amp, shows only about 15 ohm each way. nc> Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each nc> side of this impedance matcher. nc> Does 15 ohm look right for each of these primary coils ? nc> Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems nc> fine. nc> -- nc> Diverse Devices, Southampton, England nc> electronic hints and repair briefs , schematics/manuals list on nc>

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... "'Blues,' for all you people who paid to get in." - Pete Townshend

Reply to
Asimov

I agree with Asimov regarding leaky grid coupling caps, but I am not a fan of running tubed / valved group amps without the output bottles in place, no matter how briefly ( but if you must, take note that Asimov does say briefly - IMO 40 seconds max ). Without the output stage drawing current from the HT line, this can rise easily by 20% or more, from its nominal running value, which can stress the power supply caps, and any other decouplers along the way on the HT line.

This is not quite such a bad thing if the amp employs semiconductor HT rectifiers, as the HT rail will be high of its nominal running value until the outputs warm up anyway, and this factor is designed in when the voltage ratings for the caps are chosen by the designer.

However, if the amp employs a thermionic rectifier, its output will come up slowly, as will the load imposed by the outputs as they warm up. This results in an HT rail that comes up to a value, and stays there. If you now run the amp up with the outputs removed, the output of the reccy will come up with no load on it, which is an unexpected condition for the designer, and the resulting unloaded voltage may well come very close to, or even exceed the voltage ratings of some caps on the HT line.

You should be able to check the bias voltage with the outputs in place. Marshall generally tend to use a negative supply of around 50v for the bias source, potted down and fed to the outputs' control grids via a few resistors.

If a seperate bias supply were missing or low, this would more than likely result in excess output stage current demand, with consequent lowering of the HT rail. Glowing anodes are the order of the day here, but I would rather have that, as the outputs will stand this for a considerable time without damage, rather than have exploding or shorting electrolytics, brought on by voltage stress.

Arfa

Reply to
Arfa Daily

What is the problem with the amp?

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Reply to
Shoreline Electronics

"Arfa Daily" bravely wrote to "All" (19 Jan 06 01:49:16) --- on the heady topic of "Re: Marshall 4140 Amp question"

AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:355730

AD> lowering of the HT rail. Glowing anodes are the order of the day here, AD> but I would rather have that, as the outputs will stand this for a AD> considerable time without damage, rather than have exploding or AD> shorting electrolytics, brought on by voltage stress.

Arfa,

High voltage electros have a surge rating about 20% higher than the wvdc specified on the can. A brief surge less than the few seconds it takes to get a reading will not usually be a problem. OTOH leaving the tubes in runs the real risk of having a large current burn out a transformer winding. Which risk would you prefer to take; ending up with on your workbench: a smoking rectifier with a zapped electro, or an open winding in a cherished rare vintage audio output transformer?

A*s*i*m*o*v

... Transformer designers take turns doing it.

Reply to
Asimov

no

voltage

up

now

bias

Amp is 1975 , from electrolytics, and uses Si rectifiers. Amp was in a damp shed for maybe 15 years. Doesn't look too bad for that , after blowing out the cocoony bug nests etc (1KW Martingdale ). Charred/burnt 1.5K grid resistors and blown HT fuses. All valves ( all marked Marshall) checked out good on Avo CT160 - I'd forgotten how problematic , with high current valves, to get the initial zero on the meter before rotating the SET mA/V. I always power up kit left idle for a long term with a variac + current meter + thermal trip. Is it still perceived wisdom in such circumstances to power up valve amps with full speaker load on output with all valves in place and to power up transistor amps intitially without load ?

Reply to
n cook

That's my usual recommendation.

Arfa

Reply to
Arfa Daily

Townshend

place,

current

come

designer,

of

time

,

amps

up

On the slow variac power up, I usually power up to about 80 per cent mains with no valves in and then add the valves and then go low to 100 percent.

Reply to
n cook

Hi Asimov

Please don't get me wrong - I'm not challenging what you say. Merely pointing out that there's two schools of thought on this. I agree that high voltage electros should stand +20% on their indicated voltage rating, but that is when they are youthful and in good condition ( unlike me these days ... !! ). The point is that any measurements done with the tubes removed, MUST be done quickly - which is why I said 40 seconds maximum in my post.

As far as the smoking reccy and zapped electro versus the open transformer go, personally, having been in the vicinity of many exploding electrolytics in the old days, I would rather risk the transformer damage, as I can't remember ever having seen a transformer - power or output - in a high power group amplifier, damaged from an output tube or bias problem. Remember that old electro cans, at best had a ' pip ' in the rubber seal at the bottom, and at worst, nothing to assist with venting, unlike the proper bungs or rupture lines designed into modern ones. This is why they had a tendency to go off like hand grenades, spewing boric acid soaked paper everywhere.

In contrast, if the output stage is drawing anything like enough current to damage a transformer - and, IME, these will happily drip wax for at least 5 minutes without failure - then the HT fuse should ideally blow, or the surge limiter / smoothing resistor smoke, glow, and go open.

It's probably just a matter of opinion and experience, which you obviously have. I'm just making sure that those who might be reading, and have less experience than you and I, understand the possible dangers and cosequences d:~}

Arfa

Reply to
Arfa Daily

After a check yesterday evening, I got ~100 ohms on the primary side across the entire winding (ST-70), I did not disconnect any wires to measure. So, based on a guitar-amp, I would expect your measurement to be just fine. I did not get a chance to measure the Scott as I had less time than I thought.

However, this evening I will have more time, and I will use the Fluke meter rather than the R/S glove-box unit.

Peter Wieck Wyncote, PA

Reply to
Peter Wieck

That's a good way of doing it, and avoiding the potential pitfalls that I have pointed out regarding over-volting the smoothers. About the only slight difference that I would normally do with an amp of this type that had been standing unused for a long time, would be to first come up to about 30% on the variac, and leave it running like this for 10 minutes, then come up to

50% for 20 minutes to half an hour, before going up to 80% to do the checks. This will ensure that there are no issues with the caps reforming.

Arfa

Reply to
Arfa Daily

"n cook" bravely wrote to "All" (19 Jan 06 09:11:40) --- on the heady topic of "Re: Marshall 4140 Amp question"

nc> From: "n cook" nc> Xref: core-easynews sci.electronics.repair:355765

nc> Amp is 1975 , from electrolytics, and uses Si rectifiers. nc> Amp was in a damp shed for maybe 15 years. Doesn't look too bad for nc> that , after blowing out the cocoony bug nests etc (1KW Martingdale ). nc> Charred/burnt 1.5K grid resistors and blown HT fuses. nc> All valves ( all marked Marshall) checked out good on Avo CT160 - I'd nc> forgotten how problematic , with high current valves, to get the nc> initial zero on the meter before rotating the SET mA/V. nc> I always power up kit left idle for a long term with a variac + nc> current meter + thermal trip. nc> Is it still perceived wisdom in such circumstances to power up valve nc> amps with full speaker load on output with all valves in place and to nc> power up transistor amps intitially without load ?

The burnt 1.5K grid resistors might indicate the last user tried to operate the amp without a load and after the smoke out set it aside. You are a wise man. Why did you need our help again?

A*s*i*m*o*v

... The current limits placed are based on resistance

Reply to
Asimov

a

HT

This

will

the

lowering

would

electrolytics,

current

power

mains

percent.

slight

checks.

I've replaced the burnt stuff and its back working. Unfortunately after about 10 minutes hum makes an appearance and after half an hour becomes excessive. I stuck a piece of 20 to 35 degree C thermochromic paper to each of the 4 can electrolytics and one is heating up. I assume its normal for each of the 4 EL34 to have a blue glow observable through some of the holes in the internal metal-work as well as normal orange heater glow ?

Reply to
n cook

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