Vintage JTM-50 - S/11295 - 1968 - what it it worth? How to rennovate (if at all)

[StuC]

Looking at the ones I've recorded, I think it's more likely they had a box full of caps with 1962 date codes than various later years all coincidentally made in week 20-something. I'm keeping an open mind though.

 

[TAZIN]

Hunts. It's spaced like that: H TU, so tempting to see the H as the year, which would make it week 46 of 1962. It could be week 24 of 1966... I haven't yet found one with a middle digit that would confirm which way around to read these. I've only come across H and Y as the first letter in Marshalls.

I believe the date code format for these capacitors is: week, week, year.
For example:
Y ID --> 3rd week of 1968
H YD --> 20th week of 1968
H YN --> 20th week of 1967
H TU --> 24th week of 1966

 

[StuC]

I believe the date code format for these capacitors is: week, week, year.
For example:
Y ID --> 3rd week of 1968
H YD --> 20th week of 1968
H YN --> 20th week of 1967
H TU --> 24th week of 1966

That's certainly true for some, which is why I'm keeping an open mind, but there are other Hunts caps where it's definitely YWW... if it is WWY, why break up the week number with a space (surely more logical to separate the year from the week number?), and why do we only see caps with week numbers beginning 0 or 2 across several years?

 

[TAZIN]

That's certainly true for some, which is why I'm keeping an open mind, but there are other Hunts caps where it's definitely YWW... if it is WWY, why break up the week number with a space (surely more logical to separate the year from the week number?), and why do we only see caps with week numbers beginning 0 or 2 across several years?

Like I mentioned earlier Hunts used the year-week-week format for most of their electrolytic caps starting in 1966 after changing from the week-week-year format. Their film-foil caps seem to have stayed with the week-week-year format for the most part. I think the small range of date codes for the 0.05uF/1000v caps stems from being somewhat limited in application usage. Even Marshall didn't use this cap that much. The 50 watt amps manufactured from roughly late 1967 thru 1968 tend to use this cap but once you get into 1969 the usage is more sporadic. The only 100 watt amps (late 1967 thru 1970) that use this cap were the USA export versions.
I have seen some marked; W YA (10th week of 1969), in Marshall amps produced in 1969.

 

[Seanxk]

Like I mentioned earlier Hunts used the year-week-week format for most of their electrolytic caps starting in 1966 after changing from the week-week-year format. Their film-foil caps seem to have stayed with the week-week-year format for the most part. I think the small range of date codes for the 0.05uF/1000v caps stems from being somewhat limited in application usage. Even Marshall didn't use this cap that much. The 50 watt amps manufactured from roughly late 1967 thru 1968 tend to use this cap but once you get into 1969 the usage is more sporadic. The only 100 watt amps (late 1967 thru 1970) that use this cap were the USA export versions.
I have seen some marked; W YA (10th week of 1969), in Marshall amps produced in 1969.

The 50w Pa on post 39 is actually an Erie W YA cap, so with Feb 69 it does fit ( brain fade on my part though it's not a Feb 69 assembly as I state in that post, but April 69 on the plain label )

 

[StuC]

Found a W NU - 176, so clearly week first.

 

[Brek]

Every time I read one of these threads I am mind blown at the depth of knowledge you guys have, amazing ‘barn’ find OP, lol. So what’s it worth in present condition?

 

[Mick10]

Every time I read one of these threads I am mind blown at the depth of knowledge you guys have, amazing ‘barn’ find OP, lol. So what’s it worth in present condition?

I was thinking the same thing. Please keep us updated on what happens with this amp.

 

[neikeel]

Progress
Afraid filter caps all dead:

Guts to follow

 

[neikeel]

Ok so new filter caps, bias caps and presence pot.
I left the snubbers on the diodes and 220R in HT line.
Nice as it has Mullard preamp tubes and Mullard EL34s so sounds spot on!
I'll try some phone clips (see how they come out first - not the amp - my playing!)

 

[redfish]

Yove done an amazing job Neil!

 

[AndyD]

Fantastic job Neil. I bet you enjoyed working on it! Lovely amp.

 

[neikeel]

Fantastic job Neil. I bet you enjoyed working on it! Lovely amp.

I did not have to do much at all really. There are a couple of grounding tweaks that I did to yours to lower the noise floor that I would do for myself but as it is I think Redfish is cool to leave it as stock as possible and let a new owner decide.

 

[Torren61]

Ok so new filter caps, bias caps and presence pot.
I left the snubbers on the diodes and 220R in HT line.
Nice as it has Mullard preamp tubes and Mullard EL34s so sounds spot on!
I'll try some phone clips (see how they come out first - not the amp - my playing!)

Is there a source for that PT mounting plate?

 

[Ivan H]

Is there a source for that PT mounting plate?

Valvestorm. Cheers

 

[Torren61]

Valvestorm. Cheers

Not available

 

[neikeel]

Brian Wallace would normally sell you one with one of his laydown PTs!

 

[AndyD]

I did not have to do much at all really. There are a couple of grounding tweaks that I did to yours to lower the noise floor that I would do for myself but as it is I think Redfish is cool to leave it as stock as possible and let a new owner decide.

Whatever you did, it works! Its lovely and quiet.

 

[redfish]

Whatever you did, it works! Its lovely and quiet.

Is the slight hum you find when cranked up on this era of JTM-50's:

1) a generic thing? (suboptimal grounding design), or
2) just found on a few amps? (nonuniform contruction and testing?),
3) is it just something that comes with age?
4) or something else?

 

[neikeel]

There seems to be a variability in background hum. There are undoubtedly design faults in the grounding scheme - earth bus on pots for example. Sometimes it works sometimes not. But there are tricks !