Selmer Truvoice 18 Watt Project

[Shawnobi]

Hi guys,
Got myself a new project. This one a little more challenging. It's a 1950's Selmer TV18. I'm jumping the gun a little in so far as it is still in the post, but have already started research. My first main issue is that this amp ran KT55 Beam tetrodes in the power section (fed via drop resistors from the mains I believe - though rest of amp uses Iso Transformer). KT55's are like hens teeth and there seems to be no direct swap that I can find. Heater voltage is 52V @ 0.3A! Way more than a later KT88. So aside from waiting around for a set of 55's to come up. I was wondering if people think it feasible to convert to use different tubes. I don't yet have a schematic so just looking to get a general opinion on best approach.

KT55 Info here: http://www.r-type.org/exhib/aaj0167.htm
Selmer TV18 infor here: http://www.vintagehofner.co.uk/galle...ry5/cat52.html


Cheers,

[Ted]

Has this amp got a mains transformer? KT55s were designed for AC/DC sets which typically used rectified mains for B+ and series connected heaters (often with huge wirewound droppers) across the mains. The chassis was often connected to one side of the mains through a resistor or capacitor or both. When you say it has isolating transformers, do you mean it has one to generate the B+, or that the input is isolated?

By modern safety standards it isn't. Safe.

If it really has no mains transformer I'd seriously consider rebuilding the amp with one (if there's space). This would allow you to use any tube you like in the output stage.

[Shawnobi]

As I understand it it does have a mains transformer but it was bypassed for the KT55 heaters via dropper resistors. I think those can be seen in the image link above.

[Ted]

What that advertisement refers to is an isolating transformer on the audio input. Not an isolating mains transformer. This amp is almost certainly an AC/DC amp with no mains transformer.
In this case, the isolating transformers make it reasonably safe.

I couldn't find a schematic for the TV18, but here's one for the TV15 which uses PL82s for the output stage. Note the absence of a mains transformer, and the three input isolating transformers on the left hand of the drawing:

http://www.vintagehofner.co.uk/brita...atic/tv15.html

[Shawnobi]

Thanks Ted. Hopefully when it arrives in a few days it will come with the scheme as the seller claimed to have one (as he has another TV18 he has restored). Will most likely looked to add a mains transformer I think. I'll post back with some photos when I have the thing.

[Ted]

A possible replacement for the KT55s would be to use 50L6s (which are still just about available) - you'd need to rearrange the heater chain and probably change the dropper values, but the base is the same as a KT55. You might well need to change the cathode resistor too. But really. Put an isolating mains transformer on it.

[olddawg]

What makes this amp desirable and even worth the trouble? I know that tone and aesthetics are personal things, but have yet to play a line string filament amp that I liked, especially with weird tubes, and all the other issues to make them even usable and safe seem to be over the top to me. Just curious. Is it a rare presentation piece?

[Ted]

They are pretty rare (probably justifiably) here in the UK. However they can be picked up for a sensible amount of money, unlike a Fender or Vox of that vintage.

It has a cabinet, a chassis, pots, knobs, output transformer and valve sockets which could be used to build something nice. The 12AX7s might even be reusable.

And most British amps of this vintage had terrible speakers which haven't been improved by 55 years of existence.

[Shawnobi]

What makes this amp desirable and even worth the trouble? I know that tone and aesthetics are personal things, but have yet to play a line string filament amp that I liked, especially with weird tubes, and all the other issues to make them even usable and safe seem to be over the top to me. Just curious. Is it a rare presentation piece?

It's a valid point. I've repaired a few valve amps now. Fender silver faces, Peaveys and newer PCB stuff. It's all part of a learning curve where I hope to design and build my own amp eventually. So it's not a display peice or something to make money on but a learning tool. Plus I like stuff that is a bit quirky.

Regarding desirability, strangely not too sought after for the early ones. Despite being used originally by the Beatles and the Animals (House of the rising Sun guitar was a Selmer). But after they were recorded Vox stepped in and had them parading their amps so those are the ones that everybody seems to want.

[Enzo]

And before we redesign the thing to use different tubes, why not wait to see if the tubes it already has need replacing? They may be fine, rendering all the effort moot.

[Shawnobi]

And before we redesign the thing to use different tubes, why not wait to see if the tubes it already has need replacing? They may be fine, rendering all the effort moot.

No speaker, or tubes with it sadly.

[Shawnobi]

An interesting twist on this. The owner contacted Me to say that this is one of the later models running 6V6 power tubes. So we may be ok. Anyway, I'll keep you up to speed!

[Ted]

In which case what you have is almost certainly a TV20 (or something close - the correlation between schematic and actual build is often pretty loose on Selmers of this vintage).

It's unlikely (but not impossible) that anything with 6V6s in it would have series connected heaters.

Selmer TV20 Schematic

[Shawnobi]

In which case what you have is almost certainly a TV20 (or something close - the correlation between schematic and actual build is often pretty loose on Selmers of this vintage).

It's unlikely (but not impossible) that anything with 6V6s in it would have series connected heaters.

Selmer TV20 Schematic

The plot thickens!
Really hoping it arrives soon as I'm itching to see the circuit first hand. Thanks for the continued input, Ted.

[Shawnobi]

So, who wants a looky!



Will start to trace where those trannies are going first. Lots of capacitors that will most likely need binning. Hard to tell if the work to run with 6V6's is from the factory or a later mod. Wiring seems quite messy over to the right but I have no point of reference with a mint example.
Need to follow the wiring from the big blue accordian thing to suss what job it performs (Load resistor with cooling fins?).
I'm pretty excited. Lots of work ahead.