Low Voltage JTM45 - Maybe?

[dbishopbliss]

Two years ago I built a Supro 1690T style amp but it was never the amplifier I hoped for. I could never figure out why the Tremolo would not work and I could not use it in band rehearsal. For some reason when the drummer would really get going the amp would start making squealing noises. Not feedback. Maybe microphonic tubes, but I swapped all them with tubes that did not have problems in other amps and the same thing happened. But mostly, it was not really the sound I thought I wanted so it's time to rebuild.


Because the chassis and the PT are for a JTM45, I figure it makes the most sense to build something along that style of amp. However, the PT and OT are not quite your typical JTM45 iron.


The power transformer is a Mojotone British 45 Low Voltage Transfomer. The Plate Voltage is Lowered from standard 350-0-350 to 300-0-300 AC. I have tried searching the internet but I cannot find anyone that has documented why you would use a Low Voltage PT for a JTM45 and what (if any) adjustments to the circuit would be required. I would appreciate any information or thought you might have about this.


The Output Transformer was intended for a Tweed Super/Pro/Bandmaster. It's not quite the right spec for a JTM45. It has a 6K primary whereas the Mojotone British 45 OT has a 6.8K primary (for 6L6/KT66 tubes). What difference do you think this would make to the sound?

I'm open to other suggestions of an amp that might make better use of this iron?


As always, thanks!!!

[pdf64]

… why you would use a Low Voltage PT for a JTM45 and what (if any) adjustments to the circuit would be required. I would appreciate any information or thought you might have about this.

A lower HT supply will reduce the amp’s power output. eg perhaps about 20-25W here.
I would just build the amp using the standard component values, then on testing, tweak as required. It’ll probably be fine as is.

… It has a 6K primary whereas the Mojotone British 45 OT has a 6.8K primary (for 6L6/KT66 tubes). What difference do you think this would make to the sound?

Nothing not noticeable / worth worrying about.
In regards of power output, it’ll match the lower HT a bit better, ie increase power output a tad.

The biggest concern is whether the OT will accommodate the high degree of NFB of a JTM45. The circuit might not cope with much phase shift (usually up in the ultrasonics) before stability problems occur.

[dbishopbliss]

The biggest concern is whether the OT will accommodate the high degree of NFB of a JTM45. The circuit might not cope with much phase shift (usually up in the ultrasonics) before stability problems occur.

How do those problems exhibit themselves sound-wise? Could it be the issues I was experiencing with the Supro?

[pdf64]

The biggest concern is whether the OT will accommodate the high degree of NFB of a JTM45. The circuit might not cope with much phase shift (usually up in the ultrasonics) before stability problems occur.

How do those problems exhibit themselves sound-wise? Could it be the issues I was experiencing with the Supro?

Typically, an amp with such instability will sound bad at certain settings, eg presence at minimum.

With your Supro build, my best guess is that there’s a bad connection somewhere.

[bmccowan]

Two years ago I built a Supro 1690T style amp but it was never the amplifier I hoped for. I could never figure out why the Tremolo would not work and I could not use it in band rehearsal. For some reason when the drummer would really get going the amp would start making squealing noises. Not feedback. Maybe microphonic tubes, but I swapped all them with tubes that did not have problems in other amps and the same thing happened. But mostly, it was not really the sound I thought I wanted so it's time to rebuild.

You built a "Supro 1690T style" amp. It's hard to know what that means. If you did not follow a known schematic, the problem could be where you diverted, or as pdf suggest, a bad connection. Maybe I am just stubborn, but I would want to get that amp working right before accepting defeat and rewiring into something else. The squealing issue may be a problem that has other more subtle issues that are resulting in an amp you do not like. If fixed and you still don't like it, then by all means use the bones to wire up something else.
I have two amps with the preamp that is in the 1690T - neither amp squeals and the trem works in both. So if you follow the schematic - it will work.
If you care to post the schematic of your amp, folks here will try to help.

[dbishopbliss]

I have two amps with the preamp that is in the 1690T - neither amp squeals and the trem works in both. So if you follow the schematic - it will work. If you care to post the schematic of your amp, folks here will try to help.

Here is a link to the original build thread. I used this schematic.



I don't every really use a tremolo so I finally gave up on it. Perhaps a fresh set of eyes after a year (or two) might help.


Initially, I liked the sound but as I mentioned, I was having trouble with it in a live situation with my drummer. He is VERY loud. Its a strange phenomenon because the squeal is sort of a cross between a ping and a squeal. It doesn't seem to start unless I am playing with the drummer. But after it starts he doesn't have to be playing for it to continue.

[bmccowan]

I used that schematic in repairing/rebuilding mine - they are original Valcos. Eric from Dirty Girl knew/knows his stuff. Last I knew he was out of amps and building beautiful muzzleloaders. Note that V2b has some variations. My amps had the variation that Eric termed "VERY aggressive" - I concur. So aggressive that I do not think I could play mine near max volume without it falling into some kind of feedback nightmare. Just a thought. There are other variations on that schematic, and I imagine some work together better than others. A squeal that is triggered and then sustains, kind of seems like feedback/oscillations.

[tubeswell]

Here is a link to the original build thread. I used this schematic.

Never tried a 1690T, but that PI with a common unbypassed cathode resistor looks like a recipe for instability. I have built a few 6420s and they are great. No instability problems. Maybe try the PI from that?

https://el34world.com/charts/Schematics/files/Supro/Supro_6420_thunderbolt.pdf

[pdf64]

  … that PI with a common unbypassed cathode resistor looks like a recipe for instability. …

Yes, it can increase linearity in the linear range of operation, but pushed into overdrive can cause weird stuff to happening.
Whilst Merlin doesn’t advise against it, my experience leads me to suggest splitting the cathodes.
http://www.valvewizard.co.uk/paraphase.html