JTM45 RI, tube questions

[SG John]

I have a 1989 JTM45 reissue that I'm thinking about putting new tubes in. It currently has Sovtek 5881WXT power tubes, and Sovtek 12AX7WXT+ preamp tubes. It sounds pretty good, but I would like to get it sounding stellar.


1. What would be the best upgrade for a 12AX7 tube?

2. What are my best possibilties for power tubes? I've been going through archive here on TGP and it seems that I can use EL34 tubes. Can I also use KT66 tubes?
Two tubes that I am thinking about trying in this amp are:
http://store.metroamp.com/product_info.php?cPath=21_57&products_id=319
and
http://www.kcanostubes.com/products/342/NOS-Tesla-EL34-Matched-Pair-NOT-JJ.htm

I play hard rock with a mix of blues, psych, and punk. The amp is powering a Silver Jubilee 4x12 with Celestion Greenbacks. Any information would be greatly appreciated. Thanks.

[phsyconoodler]

JTM 45's and KT66's are a match made in heaven!The Chinese Shugang
tubes are good sounding tubes if you bump up the bias a little from advertized specs.They like at least 45ma.Of course people will tell you otherwise but those tubes like it hot.
Can't speak for ValveArt tubes.EL34's sound good too.

[hamfist]

The Chinese Shugang
tubes are good sounding tubes if you bump up the bias a little from advertized specs.They like at least 45ma.

Anyone else find this ?

[hamfist]

The Chinese Shugang
tubes are good sounding tubes if you bump up the bias a little from advertized specs.They like at least 45ma.Of course people will tell you otherwise but those tubes like it hot.


As I understand it, the Plate voltage makes a lot of difference. With a Plate voltage of 400V (as in, for example, the Metroamp Jtm45), 45mA would give you a power dissipation of 18W (0.045 x 400), which is actually right at the top of the generally accepted range for a KT66 tube. My Jtm45 runs at 450V, therefore 45mA would fry the tubes in short order, giving a power dissipation of 20.25W (0.045 x 450) !

[phsyconoodler]

Not true for the Shugang.They can and do take high bias.

[hamfist]

Not true for the Shugang.They can and do take high bias.

Interesting. What sort of power dissipation do you successfully run them at ? And also, what sort of tube lifespan are you getting ?

[SG John]

Thanks for the replies. It sounds like I should have Bob D. check the plate voltage before we start trying different tube combinations. I'm sure he'll have some input as well.

Any other input regarding power and pre amp tubes will be greatly appreciated.

Thanks again, and have a great Christmas.

[hamfist]

Any other input regarding power and pre amp tubes will be greatly appreciated.


I've tried a lot of different pre-tubes in my Ceriatone Jtm45. V1 (nearest the input jack) makes the most difference. My favourite is a Mullard short-plate 12ax7 in V1. RFT 12ax7 was also very good. Least favourite were Dutch Amperex 12ax7 - some nasty, nasty mids going on with those ! I've got slightly less expensive old stock tubes in V2 and V3, but only for longevity really. My ears couldn't hear a difference between Old stock and JJ's in those positions.

[jezzzz2003]

My vote definitely goes toward the Shuguang's, excellent sounding tubes in older amps as theyre higher output than most others and sound glorious inside these amps, I think they come stock now, do the preamp valves say Marshall in gold and 00047?

[SG John]

My vote definitely goes toward the Shuguang's, excellent sounding tubes in older amps as theyre higher output than most others and sound glorious inside these amps, I think they come stock now, do the preamp valves say Marshall in gold and 00047?


How do I look into Shugang's? I've looked a couple of the links here on TGP, and haven't seen them.

The tubes that were stock were Sovteks.

I believe that KCA had the short plate Mullard 12ax7 preamp tubes, and will be getting some of those.

[jezzzz2003]

How do I look into Shugang's? I've looked a couple of the links here on TGP, and haven't seen them.

The tubes that were stock were Sovteks.

I believe that KCA had the short plate Mullard 12ax7 preamp tubes, and will be getting some of those.

Order some from a reputable tube vendor, Ive also tried NOS Mullards throughout and was unphased by what I heard compared to the Shuguang's, their a very strong sounding tube in these older simple cct amps, hell, even Aspen Pittman recommends them for these applications, just done expect them to sound good through anything modern is all.

[freaksho]

Thanks for the replies. It sounds like I should have Bob D. check the plate voltage before we start trying different tube combinations. I'm sure he'll have some input as well.

Any other input regarding power and pre amp tubes will be greatly appreciated.

Thanks again, and have a great Christmas.

definitely look into it more before just dropping in a set of KT66's.

for more evidence, here (http://raw-sewage.net/jtm45ri_mods.html) is something i ran across today (from the likes of Lord Valve and such). i just typed "jtm45" into google and this is the first thing that popped up.

sounds like it would be worth the effort though.
good luck.

[twangbanger]

Those must be the stock tubes that came w/ the amp! Ive got a JTM 45 reissue that I installed the Valve Arts KT66's and some of the new tung sol 12 ax7s . Sure helped! you should see a dramatic difference w/ any of the above recommendations as the tubes in there now have to be worn out.

[SG John]

Thanks for everyones input. I was at Bob D's when he went through the amp and gave me a bit of an education in Marshall amps, tubes, and electronics in general.

We went with JJ EL34 Power Tubes. There was no way a KT66 would not vibrate against the transformer. He also told me that there is only one real KT66 available, and unless I was willing to cough up $600.00 for a pair I should move on.

We also replaced the #1 preamp tube with a JJ 12AX7. He said I should start with #1, and if I really wanted to I could change the others later.

He also looked at the values of many of the resistors and capacitors and commented on Marshall correcting many of the problems of the older amps with my reissue. Because it was a first year reissue('89), it still had the eighties style pc board and seperate control knobs. I can change it to turret board costruction if I so desire in the future.

Bob set the tubes a little on the hot side, replaced a couple a couple of resistors, and sent me on my way. I A/B'ed the JTM along with my DST30. Bob's improvements helped out the amp quite a bit and I was very happy with the results. My drummer on the other hand... well, let's say I couldn't hear much of him.