Selmer Truvoice 18 Watt Project

[Ted]

This has no mains transformer and no sign of any input isolation (maybe the transformers are attached to the input jacks?).
It also has rather a lot of valve sockets compared to the TV15 and TV20 schematics. It has exposed 240V wiring on both sides of the chassis.

The output valve heaters are connected in series (pins 2 & 8, brown wiring). I wouldn't be too certain that this thing had been modified to run 6V6s.

The big blue accordian thing is a selenium rectifier.

As you say, a lot of those caps are ready for the bin. There's no guarantee that any of the other bits are any good either - people don't usually strip all of the valves out of an amp for no reason.

It's a mess (although I've seen much worse), and it isn't safe to connect to 240V.

If it was my amp, I'd strip the whole thing, buy an appropriate mains transformer and rebuild it as a TV20, maybe with silicon rectification.
Like I said earlier, you have a cabinet, a chassis, valve sockets, an output transformer and a choke. Build your 6V6 based choice of amp onto the chassis.

What I wouldn't do is try to use it as.

[Shawnobi]

If it was my amp, I'd strip the whole thing, buy an appropriate mains transformer and rebuild it as a TV20, maybe with silicon rectification.
Like I said earlier, you have a cabinet, a chassis, valve sockets, an output transformer and a choke. Build your 6V6 based choice of amp onto the chassis.

What I wouldn't do is try to use it as.

Wouldn't dream of plugging it in in this state. Will most likely rewire the whole thing. Want to get my head around it first though. Write down all the component values and put together a scheme. A fair chunk of work to do but I'm looking forward to whatever it ends up being.

[Shawnobi]

Here is a pic of the input panel. It shows the isolation trans' on the inputs. Strongly coming around to your idea of rebuilding as a TV20. A scheme for the 18 seems impossible to find, otherwise I would rebuild as that with a mains transformer. But I thing a TV20 based build seems a sensible way to go. I have the scheme for that sat here.

[Shawnobi]

I've decided on building a simplified version of the Selmer TV20 design. Can anyone suggest the secondary winding values this circuit would have used? It shows 6.3V for the heaters, I'm guessing 5V for the tube rectifier. Would the other winding be 330v to keep the 6V6's happy? Need to select a suitable mains Tranny. Thanks.

[Ted]

That looks like a pretty sensible cut-down of the TV20 circuit.

Yes. 5V for rectifier heater.

As for transformers. You might want to read up on some basic theory on how B+ relates to transformer voltages.

Duncan Amps has a PSU simulator that is quite useful:
Software

For transformers try Bluebell Audio (They do the Hammond range which are pretty good quality, and Phil is very helpful)
Bluebell Audio

Or for something a litlle cheaper try Living In The Past:
Valve Mains Transformers

[Shawnobi]

Thanks, I'll check it all out

[Shawnobi]

Could anybody assist in identifying the wiring for the Selmer output transformer?
Wires are twisted as they came out of the thing. (Picture attached).

[Shawnobi]

Found this which may be of use to people. Not quite sure if it is helping me yet!

Transformer and Wire Color Standards for Tube amps


From the ARRL handbook:

BLA - grounds, grounded elements, returns
BRO - heaters/filaments, off ground
RED - power supply B+
ORA - screen grids (and base 2 of transistors)
YEL - cathodes (and transistor emitters)
GRN - control grids, diode plates (and base 1 of transistors)
BLU - plates (and transistor collectors)
VIO - power supply, minus leads
GRA - AC power line leads
WHI - bias supply, B or C minus, AGC


Is there a standard scheme for power transformer wire colors?
From Orr's Radio Handbook:

Primary leads -------------- black
(if tapped)
Common ----------------- black
Tap -------------------- black/yellow
End -------------------- black/red

High voltage secondary ----- red
Center tap ------------- red/yellow

Rectifier filament winding - yellow
Center tap ------------- yellow/blue

Filament winding No. 1 ----- green
Center tap ------------- green/yellow

Filament winding No. 2 ----- brown
Center tap ------------- brown/yellow

Filament winding No. 3 ----- slate
Center tap ------------- slate/yellow

I have seen primaries with other color schemes; if it includes black it should be a primary lead of some sort.
I've seen transformers with only one filament winding that was brown. I've seen high voltage secondaries with
red/white centers. And on old transformers, the colors may have faded to the point brown looks black or
green looks black or brown, and so forth. I'm not aware of any standard for filament winding numbering,
so if there is more than one, verify the voltages. The rectifier winding will normally be 5 volts,
but again, verify it. If you have an old tranny with odd or indistinguishable colors on the leads,
verify them. In fact, it's a good idea to verify any PT before using, just to be safe. Miswirings are,
AFAIK, rare, but do occur, as do shorts.

Is there a standard scheme for output transformer wire colors?
Derived from the ARRL Radio Amateur's Handbook:

Single-ended transformers:
Plate lead (pri.) -------------- blue (or brown)
B+ (power supply) lead --------- red
speaker (typ. +) (sec.) -------- green (or yellow)
speaker return (sec.) ---------- black

Push-pull transformers:
Plate lead (pri.) -------------- blue (start)
B+ (power supply) lead --------- red (center tap)
Plate lead (pri.) -------------- brown (finish)
speaker (typ. +) (sec.) -------- green (or yellow)
speaker return (sec.) ---------- black

Ordinarily the black side should also be grounded, if the speaker is grounded. Replacement transformers are
not always identical, so if you get squeals or other odd sounds when hooking up a new output transformer,
try reversing the output leads. "Start" and "finish" are arbitrary terms with respect to this configuration.

Is there a standard scheme for interstage coupling (such as phase inverter) transformer wire colors?
From the ARRL Radio Amateur's Handbook:

Plate finish lead (pri.) ------- blue
B+ (power supply) lead --------- red (whether center tap or not)
Plate start lead (pri.) -------- brown
Grid finish (sec.) ------------- green
speaker return (sec.) ---------- black (whether center tap or not)
Grid start (sec.) -------------- yellow

[Ted]

Go back to your original pics.

You can see that pins 3 and 4 of each valve (anode and screen grid) are connected to the red/orange and blue/green pairs. Screen grid connected to a transformer tap suggests that it's an ultra-linear circuit. I'd guess that the yellow wire goes to B+. If there's continuity between yellow and red/orange, and yellow and blue/green that would confirm it. The red and black are speaker wires. The other two wires look as though they go to the speaker output and are possibly for negative feedback.

[Ted]

Incidentally. If you look on page 35 of this:

http://www.tubebooks.org/Books/GEC_approach.pdf

There's a design for an ultra-linear AC/DC KT55 amplifier with which should give you some idea of how your amps transformer is wired, and is probably pretty close to the circuit of your TV18.

[Shawnobi]

Thanks for the book link. Trying to read as much as I can currently.
Just measure the tranny.
Yellow > blue/green - 8.5 Ohm
Yellow > Red/orange = 8.2 ohm.
So I figure that rings true. I also guess that I can keep this wiring the same for my TV20 design and ignore the negative feedback.

[Shawnobi]

Finally started building this thing. Only original Selmer parts are the O/P tranny and Choke (Not fitted in pic) and the bulb! Mains tranny courtesy of BLS electronics.
I'm still a little shaky about the O/P tran wiring but what's the worst that could happen eh?!!
Big knobs!

[Shawnobi]

That moment when you realise you forgot to drill the hole for the guitar jack!