Convert 68 Superbass Kit to JTM45/100 specs ?

[Polaris26]

Hi all,

I was wondering if it would be practical or even possible to convert a 68 superbass kit into a 45/100 style amp? I heard some
demos online of a 45/100 and I think it would be more my taste at this point. Since I built the plexi kit years ago when I was in a band, I hardly ever use it now. Could I use the existing iron and boards? I want that KT66 sound and the smoother tone of the 45 - possibly even just use it in '45 watt' mode or have a switch between 2 and 4 KT66's?
Can I get some of that JTM 45 sound by modding the existing amp for KT66's? Something like between these two amps:

http://www.youtube.com/watch?v=smz0J_mmNYI
http://www.youtube.com/watch?v=WKGYd-TnyAo


thanks!

Dave

[neikeel]

The iron is different (OT is 4k primary impedence vs 1.7k and the PT has higher voltage, 530v compared with 480-490 of the SB) the main circuit board is nearly identical and the filtering can be re-jigged to give 45/100 values.

You can use KT66s with a mismatch correction using the selctors depending on your cab but it would not be the same.

I would suggest a replacement OT (1202-84) and 32uF filter cans, see if that works for you without a new PT.

A JTM45/100 is not a double JTM45 as it has ss rectifier and IMO a nicer amp overall but it would benefit from a decent attenuator (the Alex is probably best value for performance, IMO).

[Polaris26]

Hello Neil -

Thanks for your comments. I was searching online today for more information about the 45/100 and I came across this site: http://folk.ntnu.no/roef/JTM100.html

In it, the author claims that the earliest versions of the 45/100 had an effective 8K primary impedance, whereas later versions were brought down to the 'correct' 4k impedance. I am wondering which sound I am hearing on recorded examples (I am assuming the 4k is more likely).

Also, what might be the importance, if any, of trying to emulate the effect of using two output transformers instead of one larger one, as was done on the earliest examples?

I guess what I am trying to figure out is which exact version of the circuit I want to emulate, or does it even matter that much?

What I liked about the 45/100 as apposed to the straight '45 was that it seemed to have something a bit more edgy or gainy
happening in the harmonics, without losing that smooth overdrive tone of the straight '45.

I guess I could start by switching my Superbass to KT66's, rewiring the circuitry on the board, and using the Superbass OPT set to present a 4K load, and see what it sounds like.

As for plate voltages, I think I like the idea of lower voltages because I like a lot of sag and early/smooth breakup rather than the Superbass as it is now, which seems to sound a bit harder and less easy to overdrive (I don't use any pedals).

I am assuming I could even just start by subbing in a quartet of KT66 to the stock circuit, set it up to be 4K, and re-bias to see how it sounds, and then maybe mod away from there, no?

thanks!
Dave

[neikeel]

Yes you can, just need to alter the impedance selector setting and possibly the bias resistor, that is all

If you have the later SB filtering (50/50 caps all around it might be a bit stiff, 68 spec is 50uF, 16uF, 48uF and 32/32 so not far from the 32, 16, 32, 16 (or 32/32 on sl later ones like the Hendrix SN 7026 amp).

Personally I would change the OT for the correct one, but see how you like it.