JTM45/Bluesbreaker Output Impedance selector mismatch?

[slegros]

One thing I have heard of some doing with JTM45s/BB is to set the output impedance selector to 16ohms for an 8ohm speaker load. Apparently with these amps the OT has a high impedance of about 6.6k, and as a result its fine to do on these amps. Apparently running with the amp set at 16ohms for an 8ohm load is supposed to give the amp a punchier more aggressive sound.

Anyone try this? Any opinions?

[stratman_el84]

Generally speaking, most tube amps will tolerate up to a 2:1 mismatch in load impedance with no harm. Give it a try and see what you think of the sound.

I would, however, urge using a bit of caution & discretion if we're talking about a vintage amp. Stressing original/expensive/rare components a few decades old isn't generally wise.


Strat

[SatelliteAmps]

Marshalls, in general, really do not like 2:1 mismatches. They tend to melt output transformers.

[slegros]

Marshalls, in general, really do not like 2:1 mismatches. They tend to melt output transformers.

Im no expert, but from what I know of the JTMs they have an unusually high OT primary impedance of around 6.6k for 8ohm load, compared to Fenders which are in the 4.0k range, and Marshall 1959slp which are about 3.4k. Apparently this is why the 2:1 mismatch is ok on JTMs (dropping the primary to 3.3k, still reasonable)or so the theory goes.... Apparently the result is a more plexi-like sound. Does any of this sound right?

Thanks!!!

[jackaroo]

I've heard and read this from many reliable sources...just the JTM 45.

[RussB]

What particular BB/JTM 45 are you talinkg about? What OT does it have?


what do they say at the metro amp forum

[slegros]

What particular BB/JTM 45 are you talinkg about? What OT does it have?


what do they say at the metro amp forum

Marshall reissues, but AFAIK it applies to all JTM45/BB using JTM45/BB correct OTs. Thx Ill have a look at the metro forum!

[slegros]

Heres what I came across on one site that got me thinking about this:

6.Output Impedance. The JTM reissue output transformer has a primary of about 6.5k, which is about 65% higher than your typical two-tube Marshall iron. This might be because some of the very earliest JTMs had a similar unit, or because they simply wanted to further differentiate the sound from the 50W reissue. Regardless, if you run the impedance selector set as marked, you'll not get more than 25W out of one, and the waveform will be squashed---typical of upward mismatch. If the plate voltage was higher than it is, it's likely that flyback arcing would be a problem. If you like the tone as it is, you won't hurt anything, most likely. But if you set the selector one notch higher than the load, i.e. set it at 16-ohms for an 8-ohm cabinet, your amp will be happier, you'll see 40W or so, and the tone will be much more Marshally. It has been said that using the impedance selector in this manner will make the amp louder and it will sound less "constricted."

Any amp gurus out there that can confirm or deny this? Id love to try this in my BBRI, but don't want to fry my amp! Thanks!

[GearHeadFred]

This might be due to the KT66 output tubes. I run EL34's in my JTM45 clone.. and use the 16 ohm taps for an 8 ohm cab due to the lower plate resistance of EL34s (relative to KT66s).

I wired it to the 8 ohm taps for an experiment and it was noticeably "tamer" (not in a good way) with that mismatch.

[AbbeSauniere]

I did it for years with my BB RI and never had problems with the transformer. More open and more dynamic is how I would describe it. A bit more roar. Don't think it has anything to do with the power tubes. Just the way the transformer is built. I've done it with 6L6s as well and had the same result. More open roar.

[zombiwoof]

Supposedly, this had to do with the transformers on the originals, and was carried over to the reissues. My brother's BB RI came from the factory with the impedance set to 16 ohms, in my mind that verified what I'd heard.

Al

[drbob1]

So, what if you're running KT66s. Better to keep the selector and the impedance matching?

[zombiwoof]

So, what if you're running KT66s. Better to keep the selector and the impedance matching?

I don't think the tube type makes any difference here, it's supposedly an issue with the transformer they used.

Al

[zombiwoof]

By the way, if you do an internet search on "Bluesbreaker Mods", I think you'll find a lot of info on easy mods that get the reissue more in the ballpark of the originals, the mods usually mention this impedance issue. Also, changing the speaker, tubes, and putting in the vintage value filter caps, the reissue has higher values there that harden the response of the amp.

Al

[zombiwoof]

This page mentions the impedance thing:

http://www.raw-sewage.net/jtm45ri_mods.html

there's more info about the original issue on other pages.

Al