Couple of JTM45 questions

thetasigma · Post by **thetasigma** » Fri Feb 24, 2006 3:49 pm

I'm not too up on the whole technical side of things so if anyone can enlighten me I'd appreciate it.

I've been putting EL-34's in my '45 and I've read some things about there being slight impedance mismatches due to the OT's (Heyboer for me) for the JTM being designed more for KT66's. Could someone explain this to me?

My second question is about bias hardware. I was wondering if instead of a resistor and trimmer pot I could just use an 100k linear pot wired in parrallel to the 10uf cap?

Newbie type questions, I know.

Thanks guys!

Flames1950 · Post by **Flames1950** » Fri Feb 24, 2006 4:03 pm

You may not get the optimum life and sound out of the EL34's that way, but you can do it. Besides, the "optimum" sound is really relevant to your personal opinion anyway.
Besides, the 8K primary impedance that JTM45's were built with was hardly optimized for the KT66's either according to our ol' buddy Chris ****** -- his opinion was that the tranny used was a big mistake from an engineering standpoint, that just happened to sound great.
I don't see any reason you can't try a 100K pot for the bias. As long as the adjustment stays stable.

MacGaden · Post by **MacGaden** » Fri Feb 24, 2006 4:07 pm

Flames1950 wrote:..
Besides, the 8K primary impedance that JTM45's were built with was hardly optimized for the KT66's either according to our ol' buddy Chris ****** -- his opinion was that the tranny used was a big mistake from an engineering standpoint, that just happened to sound great..

Hey Flames:

Could you elaborate on that, or point to some info on this ?
And why don´t you see OT´s with dual primaries, so you could go between KT66´s and EL34´s easily with a switch and with the right impedances ?

thetasigma · Post by **thetasigma** » Fri Feb 24, 2006 4:35 pm

So, from an engineering stand point, what would the "optimal" OT primary be for a KT66, and what is the standard primary for EL34s (or does it depend on which OT and or wattage amp you're going for)?

Thanks again in advance, I'm trying to absorb all the useful info I can!

thetasigma · Post by **thetasigma** » Fri Feb 24, 2006 6:11 pm

NVM on my last post, did a search in this forum and found a topic with a real similar header that answered my questions!

Flames1950 · Post by **Flames1950** » Fri Feb 24, 2006 11:41 pm

MacGaden wrote:
Flames1950 wrote:..
Besides, the 8K primary impedance that JTM45's were built with was hardly optimized for the KT66's either according to our ol' buddy Chris ****** -- his opinion was that the tranny used was a big mistake from an engineering standpoint, that just happened to sound great..
Hey Flames:

Could you elaborate on that, or point to some info on this ?
And why don´t you see OT´s with dual primaries, so you could go between KT66´s and EL34´s easily with a switch and with the right impedances ?

I did a quickie search at PP for ******'s posts on the subject, here's some reading material.....as always he can be a bit vague at times, or over-informative at others........

http://www.plexipalace.com/plexiboard/v ... ight=jtm45

http://www.plexipalace.com/plexiboard/v ... ight=jtm45

http://www.plexipalace.com/plexiboard/v ... ight=jtm45