Marshall 4140 Amp question

[Arfa Daily]

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 ?

Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/articles/blueglow/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

Arfa

 

[Asimov]

"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" <[email protected]>
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...

 

[Arfa Daily]

"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" <[email protected]>
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

 

[n cook]

Yes, a gentle blue glow is entirely normal ( although a brighter ' hard '
glow can indicate that the vacuum is ' soft ' )

For some interesting stuff on this, see

http://www.jacmusic.com/html/articles/blueglow/blueglow.htm

and

http://members.aol.com/larrysb/blue_glow.html

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 ?

 

[Arfa Daily]

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

 

[Asimov]

"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" <[email protected]>
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

 

[n cook]

--
..

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

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.

 

[Arfa Daily]

--
.


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

 

[n cook]

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

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

 

[Arfa Daily]

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

 

[Asimov]

"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" <[email protected]>
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?!?

 

[n cook]

"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" <[email protected]>
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?!?

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

 

[Asimov]

"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" <[email protected]>
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.