Tube Swapping Question (6V6, 6L6, KT66, EL34)?

[doveman]

Considering building a Marshall JTM45 from kit and found a guy selling a used "Union Jack 45" loaded with 6V6 for less than the kit cost. He claims that you can swap the 6V6 (with 6L6, Kt66 or El34) and just rebias. I know my Brown Note needs the dual output transformer to be "switched" to make a switch between 6V6 and 6L6 ... not just rebias.

When I look at the Union Jack website it is unclear if it's just an option at build or truly switchable by the owner with rebiasing. They have another model called "the vintage" that seems to make the claim of 20 watts to 80 watts by swapping tubes.

Does this make sense? Could you switch tubes and simply rebias between all of these tubes on a JTM45 (roughly a bassman circuit) clone? I posed the question on the Jackson website and asked the seller a few questions but thought I might get a response here quicker.

I would not want to have the amp be limited to 6V6 and 20 watts.

[sliberty]

The governing factors will be the transformers. Assuming that the Union Jack 45 was originally meant as a true JTM45 copy, there should be no problem running 6L6's, 5881's, KT66's and probably even EL34's. But if the transformers are meant for 6V6's, then those larger tubes can be problematic.

[edit]
Their website says that the amp comes standard with EL34's, and can run 6L6's or 6V6's. 5881's and KT66's are essentially like 6L6's, so they will also be viable.

You should be good to go.

[doveman]

He says it's loaded with 6V6s now ... so I'm thinking it's got the 6V6 transformer in it ... so I'd be stuck. Or ... he's running 6V6s in it when it should be 6L6s ... but that would probably be burning up the tubes.

A reply from the amp builder about his product indicates he makes the transformer selection based on the customer's tube selection.

So I'm going to steer clear ... I'm not sure what I'd be getting and I don't think the seller can tell me.

I'm going to go back to site though and look at the 45 again and see ... but I'm still worried enough to pass I think.

[ToneMerc]

He says it's loaded with 6V6s now ... so I'm thinking it's got the 6V6 transformer in it ... so I'd be stuck

A JTM45 if setup properly will most likely have a 6.6k or 8k output transformer, which by the way is perfect for a pair of 6V6's. Thus, it's not unreasonable for the amp to be setup this way. In fact my JTM45 is setup with an 8k OT and a modfied bias circuit with enough range to bias 6V6 to KT66's. In my case it's as simple as pulling the KT66's, installing the 6V6's and then rebias.

TM

[doveman]

He says it's loaded with 6V6s now ... so I'm thinking it's got the 6V6 transformer in it ... so I'd be stuck

A JTM45 if setup properly will most likely have a 6.6k or 8k output transformer, which by the way is perfect for a pair of 6V6's. Thus, it's not unreasonable for the amp to be setup this way. In fact my JTM45 is setup with an 8k OT and a modfied bias circuit with enough range to bias 6V6 to KT66's. In my case it's as simple as pulling the KT66's, installing 6V6 and then rebias.

TM

He said he's going to ask the builder directly ... we'll see what he says. To many possibilities for my money just yet ... but all this helps. Thanks guys.

[doveman]

Here are the two transformers in the amp ... what do you think ... safe for all the tubes? Currently running 6V6 ... I want to go to the higher output tubes.

40-18025 and 40-18033 (see attached specs)

Thanks in advance for the help ...

[sliberty]

Looks to me like the amp was built with the transformers that are appropriate for the larger tubes, even though it has the smaller tubes installed. Should be fine I would think.

[doveman]

I'm pretty sure I can run the EL34s with it. I also figured out from the pictures that the choke is a W022699 which appears to be what a Bassman would use.

I think I'm OK on this using the EL34s ... might be able to try some others too. I'm not sure the 6V6 should be used with these transformers.

[Firestorm]

I'm not sure the 6V6 should be used with these transformers.

Put an 8-ohm load on the 4-ohm tap, or a 16-ohm load on the 8-ohm tap and the 6V6s will see 7K2, which they will like fine.

[doveman]

I'm not sure the 6V6 should be used with these transformers.

Put an 8-ohm load on the 4-ohm tap, or a 16-ohm load on the 8-ohm tap and the 6V6s will see 7K2, which they will like fine.

Ah .... yes that would work. My intention would be to run this with at least the EL34s. But this would work if I did want to use the 6V6s. I could do some further investigation of the 6L6 and KT66 but the original builder confirmed today to the seller that you could run 6V6, EL34 and 6L6. There was no mention of the KT66 or the load that you suggest.

So I think I would be ok.

[sliberty]

Plate voltage may be pretty high for 6V6's though.

HT = 345V

345*1.4 = 483V

350V-420V should be fine, but 483V is pretty extreme for those poor little 6V6's.

If you plan to use a tube rectifier, that will drop some voltage, but you still might be on the high side.

[doveman]

Plate voltage may be pretty high for 6V6's though.

HT = 345V

345*1.4 = 483V

350V-420V should be fine, but 483V is pretty extreme for those poor little 6V6's.

He says the 6V6s are NOS and special ... I might rescue them for use in another amp I have ... I sometimes run it with 6V6s. I want to run El34s in this one.

About to pull the trigger I think. Thanks for the help everybody.

[sliberty]

Older 6V6's may handle more voltage, but maybe not. New JJ's can take some extra "heat" so to speak. Hey, it's only glass, right?

[NickC]

KT66 tubes are great in a JTM45. Highly recommended.

[sliberty]

KT66 tubes are great in a JTM45. Highly recommended.

+100