New to me 1967 Marshall JTM50

[Trouserpress]

Hello everybody,
when I picked up above mentioned amp I also found out about my long forgotten membership here (see welcome post in welcome section) and thought the purchase of that JTM was a good opportunity to finally post something.

When I told the canny folks at TDPRI (that’s where I usually hang out when I hang out) about it they recommended to recap to play or preferrably leave it as is to sell.

As far as I can tell the only things that changed: mains socket and maybe feet (2 grey ones and 2 black ones). The Serial is S/10598.

I myself am probably much more curious than canny - as well as a sucker for tone and everything vintage. Having heavily struggled with myself for awhile I finally ordered new filter caps and put them in. Decisive factor was an email answer from Marshall confirming the original looks and advising "not to power it up for any amount of time with the original electrolytes in it".
What do you think? Am I being sacrilegious or human or worse? (pics of recap to follow)

 

[StingRay85]

Amazing looking black flag!

I guess it's running on EL34? Probably not a bad idea to fit in screen grid resistors. No biasing resistor. Tube rectifier. Is it hot switching?

I've never seen ceramic style octal sockets, maybe they are also replacements?

Just wait for the experts to arrive, they'll you exactly what is original and what isn't.

 

[Trouserpress]

When I picked it up the previous owner had already powered it up via variac and it appeared in beautiful working order. I had not switched it on since I took it home on Easter. Yes it has EL34, GZ34 and no bias pot but it should have a bias resistor.

Yesterday after I installed the new filter caps and plugged the voltage selector into 240-250V it passed the light bulp limiter test but the pilot light would not come on. Voltages seemed alright (plate voltage about 420; negative bias voltages -30 and -37) and the guitar signal was amplified very well. Then all of a sudden the signal crumbled to low noise distortion and I got a dreadful fright - anticipating the worst. But I found out that the pilot light was suffering from loose connection and most likely did compromise the heater circuit. I fixed the light and everything came up fine again.
I'm still shaking a bit and waiting for the delivery of the new 8uF-180V bias capacitors.

The 1st filter (on the outside) is a 32uf, the 2nd (blue) is 16uF + 32uF and the 3rd (black) is a 32uF + 32uF. They're all F&Ts, rated 500Vdc and have the exact values of the old ones.

 

[StingRay85]

With a single pole switch, if it sits in between the rectifier and the reservoir cap, no current limiting resistors on the secondaries, it's so called "hot switching" and might take out your GZ34 or even worse.

 

[Nick1234]

Amazing amp. Congrats.

 

[Trouserpress]

This is the first time sticking my nose into a Marshall. Are you talking about the stand-by switch? There‘s a lot of volts waiting to get a pathway to the filter cans.
I can see or rather feel that there is a problem. What if I never use that switch - if I always leave it ‚on‘?

 

[TAZIN]

Looks original with the exception of the Indicator light. The front panel switches are not the usual type so they may have been replaced. The white octal sockets are original, and I've seen them on other builds such as #10554. What's odd is the riveted preamp sockets. That's something you see in 1968 builds.

 

[Trouserpress]

Looks original with the exception of the Indicator light. The front panel switches are not the usual type so they may have been replaced. The white octal sockets are original, and I've seen them on other builds such as #10554. What's odd is the riveted preamp sockets. That's something you see in 1968 builds.

I was wondering too about the indicator light and browsed the internet, finding only one other JTM45 with a similar one in the USA. To me it looks nicer than the square ones.
What do I do about that stand-by switch - would it help if I bridged it?

 

[chocol8]

This is the first time sticking my nose into a Marshall. Are you talking about the stand-by switch? There‘s a lot of volts waiting to get a pathway to the filter cans.
I can see or rather feel that there is a problem. What if I never use that switch - if I always leave it ‚on‘?

My advice would be to never use the standby with a tube rectifier. The rectifier will come up slowly and give the amp a "soft start" vs the surge using a standby switch.

 

[neikeel]

Nice amp.
Shame about the lamp hole and IEC being hogged out (you can buy original size lamps and IECs that don’t need the metal work affected but most don’t bother to look). But I guess those things make it more affordable for players, as it stops the collectors with silly deep pockets being interested.
I’d have tested the caps first but the F&Ts are a good choice. I use 250v 10uF Phillips BC caps as much for aesthetics as function (I know I’m sad). Most 8uF caps are too small to bridge the gap.
Lot said about switching of the GZ34 amps. I don’t use standby personally so not an issue.
Enjoy.

 

[Trouserpress]

Nice amp.
Shame about the lamp hole and IEC being hogged out (you can buy original size lamps and IECs that don’t need the metal work affected but most don’t bother to look). But I guess those things make it more affordable for players, as it stops the collectors with silly deep pockets being interested.
I’d have tested the caps first but the F&Ts are a good choice. I use 250v 10uF Phillips BC caps as much for aesthetics as function (I know I’m sad). Most 8uF caps are too small to bridge the gap.
Lot said about switching of the GZ34 amps. I don’t use standby personally so not an issue.
Enjoy.

I bought two 10uF F&T caps that are exactly the size of the originals. Unfortunately these are 350V and I‘ve read somewhere that overpowered electrolytic bias caps induce hum into the circuit. So I put them aside and wait for those cheaper 8uF 180V MOD caps (silver) to arrive.

Is a bleed resistor across the standby switch as a safety measure on top of not using it a good idea?
Edit:

 

[TAZIN]

What do I do about that stand-by switch - would it help if I bridged it?

I would not bridge the Standby switch for safety reasons (e.g. needed for troubleshooting).

 

[neikeel]

I’d just not use the switch, leave it on
The issue with bias caps is not the voltage. There is a theoretical issue with too big (uF) value due to slow charge up time.
Please share the source of the hum rumour so I can understand their logic.

 

[Trouserpress]

I’d just not use the switch, leave it on
The issue with bias caps is not the voltage. There is a theoretical issue with too big (uF) value due to slow charge up time.
Please share the source of the hum rumour so I can understand their logic.

I wanted to get a grip on that Marshall universe so I sifted heavily through the internet during the last few weeks thus took a while to find that quote. And while I'm reading it again I might have taken that post too seriously. It's the 4th one:

Reforming the electrolytic Bias Capacitors and cathode cap? - Metropoulos Forum

Let me thank you all for dealing with my issue and for your advice!

 

[neikeel]

I wanted to get a grip on that Marshall universe so I sifted heavily through the internet during the last few weeks thus took a while to find that quote. And while I'm reading it again I might have taken that post too seriously. It's the 4th one:

Reforming the electrolytic Bias Capacitors and cathode cap? - Metropoulos Forum

Let me thank you all for dealing with my issue and for your advice!

Ah, Damien and Larry - well familiar with those gentlemen and good opinions. If Larry says the electros don't work well that low on voltage I will have to agree, partcularly as I have found low rated cathode bypass caps work better than big ones too.

 

[Trouserpress]

That‘s typical: sordid details in all ‚things audio‘

 

[StingRay85]

Quite amazing that these gems can still be found in Germany.

I would say replace the EH power tubes with some genuine XF2 Mullard tubes.

 

[Trouserpress]

Quite amazing that these gems can still be found in Germany.

I would say replace the EH power tubes with some genuine XF2 Mullard tubes.

Very cool that you could identify these. I had no clue. Where would you look for XF2 - Ebay and ..CL ..?

 

[StingRay85]

I would simply take them of my own stash of 200+ Mullard tubes

 

[neikeel]

I would simply take them of my own stash of 200+ Mullard tubes

Sounds like you need to donate to a good cause.