5881/JTM45

[BrentD]

Decided to hang onto my JTM45 RI. It's still got the original tubes, the Sovtek 5881/6L6WGCs. I keep finding info online that these aren't really the same as 5881s and they are out of stock. I'm just looking to buy a spare set for the amp - no desire to change tone or output.

Tung-Sol 5881s are in stock as are Sovteks with the larger bases, and JJ 5881s. The JJs are the least expensive and I've had ok experience with JJs in the past.

Aside from a rebias as with any change, will the JJs work?

 

[neikeel]

Why not try some KT66?

 

[Spanngitter]

JJ 5881 do work but I would also vote for KT66 if you have the Version with the Dagnall TX OP 00003 which runs at 8k primary Impedance, something KT66 do love.

 

[Pete Farrington]

Why not try some KT66?

Without modification, do the RIs have sufficient range of bias adjustment to accommodate KT66, especially those of Chinese manufacture?

 

[neikeel]

Yes

 

[C-Martin]

I’d be interested in this info as well. I have a similar situation with my JTM45 RI. Years ago I had a tech look at it because the tubes had failed. Since I was going to re-tube it I asked if I could go KT66 and he told me that he amp needed to be modified, mainly the power supply section or something. I knew nothing so I had him do it. It was quite expensive and it actually failed to work the first time (was recording a project and the amp started farting out) so I had to return it for him to redo/recheck it. After that it worked fine for a couple of years. Then one day, while it was on standby during a recording, my friend asked if he could try it out and when he flicked the standby switch the amp shut off. I haven’t done anything with it since, it’s been stored away and was thinking of selling it but I always get a voice in the back of my head saying I’ll regret that decision.

Anyhow, the tubes used were JJ KT66, and I can’t say I’d recommend them based on my experience. The two pairs I had really didn’t last long, and i never liked how the amp sounded after making the switch. I later heard a JTM45 with some NOS KT66 and thought it sounded way different. I am not saying it’s just the tubes, but that amp sounded and felt completely different to mine. Maybe the tech botched the job and mine needs a to be looked at.

 

[Pete Farrington]

when he flicked the standby switch the amp shut off.

The JTM45 original and RI uses an awful, ‘hot switching’ standby circuit arrangement, a very unfortunate and regretable design choice apparently carried over from the 5F6A tweed Bassman; only use it if you wish to pointlessly cause significant stress to the GZ34.
However, it can allow the use of HT caps that would otherwise require a higher voltage rating.
That's not an issue with the JTM45 or indeed any amp with a GZ34 or other indirectly heated rectifier type.

Check the rectifier in another amp, or just replace it.
Check the HT and mains fuses with a meter (open fuses can look ok).
Don't use standby ever again.
Job done
If it's not been powered up for over a year, initial power up should be slow, eg via a light bulb limiter (100W bulb), variac etc, as the electrolytic caps will tend to deform and when next powered up they pass high leakage current until they reform. So give them chance to do that.

If nothing else, at least follow this procedure https://www.londonsound.org/Vintage valve hi-fi.htm

 

[Bodhi F. Dharma]

The 5881 Power tubes are historically accurate for this amp. The KT 66
tubes came later. In my humble opinion this amp is absolutely glorious right out of the box. Dial it in properly and it will blow your face off with tone. I prefer to jump the channels, open volume 1 up nearly all the way and dial the (tone stack) bass pot way back. You can goose it with a good transparent drive for extra bliss but it’s not really needed.

 

[neikeel]

Depends what you mean by historically accurate.
Sure the first ones with RS Deluxe used USA 5881s but quickly moved to KT66s and all the Drake amps up to early 67 used GEC KT66s, then with another OT change moved to EL34s.
I've tried both in all incarnations and personally prefer the KT66s but of course YMMV.