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I am new to the forum and am based in Tete, Mozambique where finding electrical components can be a real challenge. I have a JTM30 that has finally, with my assistance as well as through a manufacturing issue, given up the ghost. I would like to use the components, as many as possible anyway, to build an amp using the 18W layout/schematic. I would be using the 5881/6L6 valves.

In all honesty, I do not know how valve amplifiers work (Internet to the rescue) but I have a fair amount of electrical experience in the motor trade and repairs of band related equipment. Having survived for 15 minutes hanging on to some 380V high tension cables and having inserted a spanner into a generator/mains change over switch whilst it was connected, I am fairly aware of the dangers involved.

Could someone please advise me if this is doable and, if I use the 18W layout, what I may have to change.

Many thanks in advance!
Chabenda

Oh well...........it seems that either I have asked a totally idiotic question or this forum is dead.

The first thing that strikes me about the JTM components is the small size of the transformers. They seem tiny.

I'm sure someone will help soon.

So, for one, you cannot use the sIII schematic as is. The power supply rail will need to change in order to get the correct voltages to all the valves. There are 2W resistors between the filter capacitors that are used to drop the voltage along to power rail to the needed values for each valve. Also, the 6L6 valves will need to be biased differently from EL84s. I'm not sure what will work well but you will almost certainly need to use separate cathode resistors with a much higher power rating else they'll fry. Maybe Zaphod_Phil et al can chip in here.

Many thanks indeed ezietsman. First prize for me would be to get as much headroom as possible for a clean sound but, at the end of the day, what counts is doing something with all of these lovely bits, other than throwing them away. I would like to squeeze a little more power out of this - 18 to 25 watts perhaps?

Cathode biased 6L6 should get you there easily enough. A stock 18 watter doesn't have enormous amounts of clean headroom (with 6V6 or EL84) but you'll get a bit more with the bigger tubes. One could tweak the TMB to have a bit less preamp gain though.

Think it might be plausable but with xformer from, that you'll be in the 440 v range B+ so like some one said dropping resistors gonna be different. I guessing but you'll have 30+watts on that and i don't think you'll be able to you a vvr vrm as I don't think it will work with a non centre tap transformer

My best guess would use a JTM 45 schematic loose the feed back circuit and sub in a 1k 10 watt resistor thereabout for the choke
Loose the bias circuit. Shared bias resistor 270 R 10 watts thereabouts (thats the part not sure of what resistor size)
As for the rest you could use the 18 watt values in preamp

Ah.. here my absolute ignorance comes into play. Looking at the JTM30 transformer schematic that I have, it states 325v or 361v. I am assuming, perhaps like an idiot, that this is the B+...

Many thanks for your interest and much needed assistance!

That's not the B+ yet. Those are the HV secondary taps. Maybe yours has 2 and one can decide which to use. So those are connected to the rectifier, which will likely give you close to double that voltage rectified, and then, once everything in the amp is connected it will drop by a good amount. I think a ballpark figure is around 70% of that value. So for the 325V taps you get B+ of around 450 which I think is good for 6L6. The other tap will give about 500.

These are the transformer schematics that I found on the net -

OK it looks like your PT is not centre-tapped. I think you'll need to use a full-wave bridge rectifier, you'll get a B+ of somewhere between 325V and 360V. I may be wrong. Your output transformer does seem really small but the old amps had pretty much oversized ones in general. Dunno what the small one will sound like. You'll need to check what that kind of voltage will be like with 6L6. Looks like at those voltages you can manage around 25 Watt with a Push Pull amp (this is one).

Your PT is not really the best for an 18 watt style a p. It will have to be a Solid state bridge rectifier since there is no centre tap. That could really mess with your voltages and feel of the amp.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks very much for your reply. I'm quite eager to do something (for a guitar application) with the major components of this amplifier and would really appreciate any advice you could give. I have a host of other amplifiers but would like to use this as an opportunity to learn something as well as have a bit of fun.

Well, first you need to determine what the PT is capable of in term of B+ and what the OT is designed for. You should be able to go from there. An 18 watt amp need about 350V B+ with tubes in which for a 200 ma PT is about 400-420 V no load.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks Stephen - time to get measuring