Metroamp jtm45 OT dilemma

[M Fowler]

You can use 3k4 to 8k on KT66 tubes done that many times.

For a true JTM45 I would use 6k to 8k I usually use 8k.

Mojotone's new 50w bass uses Heyboer JTM45 transformers 3k4 OT with KT66's.

[dariez75]

Ok guys, Patrick of Mercury Magnetics answer me about the OT 045JT-16, the primary impedance for KT66 is "any time 3.4K for EL34's and 5.5 for KT66's.
So what can I do?

[dariez75]

Sorry 6.6k for kt66's

[martin manning]

They are saying mismatch it up just as we have been discussing. You can just wire it as for EL34 and change the impedance switch setting so it is at half of the speaker ohms when you are using KT66.

[dariez75]

Thanks Martin and thanks guys, so if I play kt66 using 8 ohm in 8ohm cab could i damage the OT?

[ToneMerc]

Thanks Martin and thanks guys, so if I play kt66 using 8 ohm in 8ohm cab could i damage the OT?

It's 4-8-16 @ 3.4K, which will become 8-16-32 ohm @ 6.8K. Thus, connect your 8 ohm cab to the original 4 ohm tap.

The tag board style Radiospares OT is impossible to implement with the normal single pole impedence switch. Here's why MM makes it easier by offering an easy wire version, although you lose some of the original configurations with your model. Here's the Marstran version, MM offers this as does Chris Merren.

FWIW, I heard that a few left Marshall actually wired for 9K.

TM

[martin manning]

Thanks Martin and thanks guys, so if I play kt66 using 8 ohm in 8ohm cab could i damage the OT?

No, no problem doing that. I'd try it that way just to see how it sounds.

[dariez75]

Interesting thing Fowler about mojotone new 50W 3.4K wiring...

[M Fowler]

I've run octal power section Trainwreck Rockets at 4k3 OT with KT66's no problem they sound great.

Also Dr Z Route 66 build 6k6 OT UL taps with KT66's.

I put KT66 in same family as 6L6GC as far as what primary to use.

Others will come up with all the technical reasons why not to do that but I use my ears, well what's left of my hearing and I don't care about all that technical stuff.

Mark

[dariez75]

Now maybe understand (but if I mistake tell me soon...)what happens last week end during sound check before gig: I was running my jtm45 kt66 at half-3/4 PPIMV power for 20 minutes when quickly (during a solo lead part) the amp turn to very very low volume (headphone level). The amp was (now I know) in 3.4K primary with impedance selector on 8 ohm plugged in one Marshall 1922 8ohm cab. I was try to change the selector to other impedance but not sound at all, only little audible note or chord. I turned off the amp and I putted in my car. When I was at home I turned on the amp and it was fine and loud as before the previously accident. So, could be the mismatching impedance determinded the OT's iron core saturation and the followed lower output volume?

[martin manning]

So, could be the mismatching impedance determinded the OT's iron core saturation and the followed lower output volume?

Saturation is something that occurs transiently during periods of high current flow through the OT (peak current flow during loud playing). I am only guessing but I would suspect some problem like a bad solder joint or a bad component (like a pot say), where the signal path is interrupted. Unfortunately you will probably have to wait until the problem occurs again before you can go looking for it. One clue you have is that it may be triggered by high temperature since it fixed itself after cooling down.

[dariez75]

Oh, what a hassle...

[dariez75]

Now I run in that configuration, very nice soud with two 8 ohm cab connected!

[rp]

Unfortunately you will probably have to wait until the problem occurs again before you can go looking for it. One clue you have is that it may be triggered by high temperature since it fixed itself after cooling down.

Welcome to the wonderful world of intermittents. Ever see the Gene hackman movie "The Conversation"? The final scene where he rips his loft apart to the rafters looking for a bug?

Actually don't worry be happy, half the time little oddities like that go away, just something settling in.

[dariez75]

Great RP! Fine citation!