Nuad - Jtm-45 Ri (my First Marshall)

[JohnnyN]

A week ago I spotted a used JTM-45 RI from early 1990s at a decent price, on our counterpart to CL here.
Yesterday I went to have a look and listen. Some hum and hiss as expected, but not annoyingly.
I took it home for about 40% of the price for a new one - not bad I guess.

After playing it at home for a while I noticed some odd sounds and cutout. Although it was mostly when using the low input of ch1 it sounded like a tube gone bad.
So I preplaced the tubes (Shuguan ECC83 and Sovtek 5881WXT), with some newish Harma Retro ECC83 (Tung-Sol RI tested and rebranded by Watford Valves in London) and a matched pair of Tung-Sol 5881 tubes I had. Unfortunately I did not have a rectifier tube.

But first I checked the BIAS on the Sovteks and was surprised that they where as low as 23mv measured at my probe. I checked with another amp-meter and got the same result before believing it
I set the Tung-Sols at 32mv @400V plate voltage which should be 55% of max plate dissipation.

While that dealt with the odd sounds and cutout it enhanced the hum and hiss. The hum is here even on standby, so I'll get a rectifier tube and maybe some filter caps. Perhaps that'll bring the hum to a minimum?
Maybe I should try some other output tubes too - I guess the Drake PT won't like KT66. I really liked the TAD short bottle 6L6 in my Blues Deluxe, but I'm open for suggestions

Anyway a great sounding amp with a view to some fun hours playing and probably with the soldering iron too

Thanks for reading and don't be shy to pass on knowledge and advice on this beast



A few more shots of the innards:

These are the tubes that was in it:

 

[Gunner64]

From what I've seen on the subject the jtm45 ri is just one of the lucky heads that have a reputation as a bit of a hummer..even in standby. And a bit of a hum is normal for these. It Seems to be associated with the p.t.'s magnetic field and the o.p.t. picking it up. Ive heard of people rotating the power trans 90 degrees to try to minimize it. Just what ive heard on the subject..maybe a techie will join in. The jtm45 is one of the few marshalls i havent owned...I should look into that..

 

[neikeel]

Gunner84 is correct re the PT orientation and the coupling effect, don't know why they oriented it that way the vintage ones do not. Brian Wallace had a batch of PTs made up to negate this issue, not sure if he still does them.
Things to check would be equal bias. This version does not have a heater hum-balance pot (might be worth seeing what a.c. volts you have either side of the heater centre tap (should be 3.15vac on each half).
You could try redo ing the heater wires with tighter twist, make sure the grounds are all tight and maybe a zoned grounding scheme (as per Larry Grounding on Metroamp - if it reopens).
The JTM45 RI is a nice amp and of course KT66s work in one of these with the Drakes! The only issue s the top lip on the head cab which needs trimming to clear the tubes.
Just for giggles try seeing how the amp sounds biased a bit hotter too. No higher than 70% tho'. If it does not sound any better then of course dial it back a bit. Bias is not a magic thing - tweak it until it sounds best and THEN check you are in the safe range.

 

[Ghostman]

Congrats on the new Marshall.

What kind of music will you play on it?

doesn't matter...

 

[JohnnyN]

Thanks for the useful advice

Meanwhile it appears the JTM have been approved by an expert at our home

 

[jack daniels]

Congrats Johnny! You mentioned filter caps. If this amp is an average of 26 yrs. old and never had the electrolytics changed, then swapping out ALL of the electrolytics would not hurt one bit.

From 'The Tube Store' re: Shuguang KT66

The Shuguang is functional art. A great job has been done in nailing the look of the original KT66. This is a huge tube when compared to others used in guitar amps. You are advised to have a careful look at space availability before installing these guys. The specs on this tube are very similar to the 25-watt originals. They draw more heater current than a standard 6L6: 1300mA instead of the usual 900mA. You should be sure that your amp's power transformer has the capacity to handle the extra current draw. Bias these tubes hot for best results. While most tubes we tested were biased at around 60% of their rated plate dissipation, the Shuguangs sound very bright and crystalline in this range. Some may find it too bright. At 75% of rated dissipation, these tubes really provide that feeling of rolling thunder. If you want Country clean, bias them for 60-70%. If you need earlier breakup and a beefier sound then turn up the heat, but keep an eye on your power tranny.

 

[jack daniels]

http://www.legendarytones.com/metropoulos-jtm45-kit/

http://www.legendarytones.com/marshall-jtm45-reissue-modification-part-2/

http://www.premierguitar.com/articles/45_Degrees_of_Tone_The_JTM45_Roundup

 

[neikeel]

http://www.legendarytones.com/metropoulos-jtm45-kit/

http://www.legendarytones.com/marshall-jtm45-reissue-modification-part-2/

http://www.premierguitar.com/articles/45_Degrees_of_Tone_The_JTM45_Roundup

Been there done the mods and back again, I would not mod it if I had my time again, if you want to tinker build a clone.
IMO leave it stock with the best tubes you can stretch to, don't go MM OT and TBH the turret board was a fun exercise but added v little.
JD well may be right about the filter caps although the RI has filtering to spare (50/50/50/50) whereas the originals have 32/32/16/16 unless the caps are dried out and faulty (v. rare for LCRs in my experience - best modern caps that you can't get any more!!).

 

[thegaindeli]

If you remove those LCR filter caps, PLEASE contact me! I will buy them from you.

 

[jack daniels]

Been there done the mods and back again, I would not mod it if I had my time again, if you want to tinker build a clone.
IMO leave it stock with the best tubes you can stretch to, don't go MM OT and TBH the turret board was a fun exercise but added v little.
JD well may be right about the filter caps although the RI has filtering to spare (50/50/50/50) whereas the originals have 32/32/16/16 unless the caps are dried out and faulty (v. rare for LCRs in my experience - best modern caps that you can't get any more!!).

Do the LCR's have that great of a reputation for minimal leakage after 26 yrs. of usage?

 

[JohnnyN]

Thank you all for your advice

The previous owner had it serviced when he bought it from a friend 10 years ago, and it seems like it haven't been touched except for the tubes.
Since it actually sounds pretty good as is, I'll try to keep it that way, except for some tube rolling

Oh and I'll check all the electrolytic caps before replacing them. But I'm amazed that they actually work to some degree after all these years.
I was also very pleased to hear that the Drake transformers are the good ones

Thanks again for your replies!

 

[jack daniels]

If it was serviced 10 or > yrs. ago then the tech should have looked/checked for cold solder joints. This should be SOP when going over any new amp. Not saying this particular amp needs it but, it never hurts to take note.

 

[marshallmellowed]

After playing it at home for a while I noticed some odd sounds and cutout. Although it was mostly when using the low input of ch1 it sounded like a tube gone bad.

The cutting out when plugging into the low input jack sounds like a dirty high input jack. On older amps, sometimes the contacts in the jacks not being used do not always "make" when unplugging a cable from them. You can try plugging/unplugging several times to improve the connection between the contacts. If that doesn't work, they will need to be cleaned. This can also occur with FX loop jacks.

 

[JohnnyN]

The cutting out when plugging into the low input jack sounds like a dirty high input jack. On older amps, sometimes the contacts in the jacks not being used do not always "make" when unplugging a cable from them. You can try plugging/unplugging several times to improve the connection between the contacts. If that doesn't work, they will need to be cleaned. This can also occur with FX loop jacks.

Thanks! I think you are right about that. It's gone and it is possibly just from the input jack being "massaged" by a bit of use. However the new tubes didn't hurt at all

 

[deee]

the way the heater filament wires are run on these amps can be a hum problem. especially around preamp tubes 1 and 2. unless you have experience with high voltage equipment it's too dangerous to mess around inside the unit with the power on though, so discus the issue with a tech.

 

[dro]

Once I started running clean, regulated power. My JTM45 RI became so quiet. At first I thought I had forgotten to plug in the speaker.

Furman regulator P-1800 AR