JTM45 PT

[Direwolf]

Hey, folks. My next build is going to be a JTM45. From what I can tell, the PT should be ~700v@200ma, 6.3v@5a and 5v@3a and I can get it from Edcor without having to pay the setup fee. Anybody have any input? Should I go with more or less B+ or higher current capacity? I'm going to build it from the original schematic so it won't have any mods initially that would affect power. I'll be using 3-12AX7 and 2-KT66 with a GZ34 rectifier.

Thanks,

Doc

[tubeswell]

I think the original HT winding was more like 325-0-325 (@200mA) FWIW (because its a straight copy of a 5F6A after all - albeit with mustard caps, celestion speakers and Brit made trannys) but 350-0-350 will be okay too 2CW

[Leevi]

The PT should be center tapped 350V-0-350V which means that after rectifier and filtering the voltage is on level 490V which sounds OK.
If you rectify the 700V with a bridge the voltage will be ~980V which is too much. 200mA sounds OK, see the the data sheet of KT66
http://tdsl.duncanamps.com/pdf/kt66.pdf
/Leevi

[Direwolf]

Thanks, guys. I looked at Mercury, Weber and Hammond's specs and figured I was close with the numbers I came up with. What would be the optimal primary OT impedance, 3.4k or 6.6k? I see where Mercury has an OT with multiple primary taps in case I wanted to switch to other tubes but they're pretty expensive.

Thanks,

Doc

[The_Gaz]

Magnetic Components makes a nice PT, and can be picked up here http://shop.amppartsdirect.com/

Lower wattage and higher primary Z will yield less power (455 * 455 / 6600 watts thereabouts), but you don't see *accurate* clones of these using lower primaries like the typical 2xEL34 Marshalls do. Personally, I'd go with the higher voltage and impedance, but it depends on much 'headroom' you want.

[Direwolf]

Thanks for the tip on the transformer, Gaz. I see it's only got 2a on the 5v winding, though. That's cutting it close using the GZ34 as it draws 1.9a. That's one reason I wanted to go with the Edcor.

[plexi50]

Edcor USA makes good transformers. I bought one online for my Ampeg Gemini II last year. BUT! They have to make them per order so expect a 4 week or more delay before recieving it. The told me over the phone that they stock no transformers and are made per order / That was my experience

[Direwolf]

I used them for a 5E3 build that I did a couple of years ago and love their transformers. You're right, it does take a few weeks to get them but it's worth the wait. They charge a setup fee if they haven't wound one to your specs before but after that there's no setup fee. The PT I mentioned earlier is already available so I wouldn't have to pay the setup fee.

[Direwolf]

Again, does anybody have any info on the best primary impedance for the KT66s? I've heard both 3400 and 6600 ohms.

[plexi50]

JTM 45 output transformer 8K primary impedance for KT66 tubes

[Direwolf]

Thanks, plexi50!

[plexi50]

Glad if i can help. I am in fact building the same amp as soon as the chassis gets here. I love EL34's but it's time for some good old KT66's

I have a China cabinet with a light that i am putting my amp chassis in when there done. Have 3 in there so far. This is extreme but i think i might be a reclusive whatcha call it Howard Huges?

Now i just need his money to go with it and im set. Doing a tube section as well as a display

This may prevent me from selling them as well. Dont ask how thats gona stop me

[Direwolf]

Where are you getting your chassis from, plexi50? I'm torn between getting one that is like the original and one larger so I can add some mods later on if I want too.

That sounds like a sweet China cabinet! Much better than, well, china in it!