British PA ID

[fusionbear]

Hello Gents, a client of mine left me this amp to clean up and bring up to specs. It is a rare British PA head labeled "TVM Sound"

It has RS transformers, runs a 240VAC (I have a step up supply) The OT has two 15 Ohm outs on the transformer with separate speaker jacks and a 100 Volt PA line. Can I parallel the OT to get 8 Ohm? Also, it ha a very large potentiometer that seems to be some sort of feedback/ balance control? Need some schematics if possible. The unit works and sounds amazingly great for guitar. Sounds like a JTM45 on steroids. Channel 3 does not work though. It has two 12AX7's on the pre, one EF86 as a Driver, a 12AX7 PI, and two EL34's wired for ultra-linear but are cathode biased.:

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[IMG768]http://i169.photobucket.com/albums/u223/fusionbear/KIMG0030_zpsstwybwjf.jpg[/img]

[IMG768]http://i169.photobucket.com/albums/u223/fusionbear/KIMG0031_zps6jfzvwkq.jpg[/img]

[fusionbear]

The OT is a Radio Spares D1903

It has 3000 primary impedance with 20% UL taps. The Output is labeled 15 Ohms, 15 Ohms and 100V Line

Does anybody have a schematic? Maybe I can parallel the two 15 Ohm outputs? Help?

[Firestorm]

I don't get why there would be two 15ohm outputs. Is it stereo? Probably not, if there's a line output too. I'd hazard a guess that paralleling the outputs makes it 7.5 ohm. But I can't see the photo bucket shots very well on my phone.

That's an interesting piece. Neat (but not Dave Reeves neat). A bit Marshall looking (not theirs but a cop). Early on, there were no reliable PAs in music venues, so there was a rush to fill the void. Didn't last. Hagstrom, in particular, nearly went broke building PA gear.

[Phil_S]

I'm thinking two 15Ω outputs means the OT is bifilar wound (separate secondaries.) If you put them in series, it gets you about 3.8Ω at 3K primary. I'm surmising in parallel you will still have 15Ω at 3K primary because you have not changed the turns ratio and I don't see how Ohm's law applies. The parallel configuration still forces each winding to carry voltage based on the primary:secondary turns ratio. I'm thinking you are simply doubling the output level. It is also problematic if the two 15Ω windings are not identical as there is a balance problem.

Did you try a continuity test to see if it is bifilar wound?

If it turns out the two 15Ω share a common "0" terminal, then I suppose it is impossible to put them in parallel. I'm having difficulty imagining anyone would want to wind a transformer in this way.

[Leo_Gnardo]

I don't get why there would be two 15ohm outputs. Is it stereo? Probably not,

Deafinitely not stereo.

Possible two 15's in series would be the setup for "constant voltage" circuits used typically in distributed-speaker arrays. 70V and 25V are commonly used. I'm no expert on that - anyone who is please chime in. Think, a dozen or more speakers set into office ceilings. Like that.

[Phil_S]

Actually, I really want to know the answer and it is easy enough to figure this out empirically. Assuming it is bifilar wound, bridge the two 15Ω terminals and bridge the two "0" terminals. (Now, hope they are properly in phase and not anti-phased; you'll know soon enough and can correct for the problem.) Apply 5VAC or 6.3VAC from any ordinary tube amp power transformer to the now bridged terminals, the unified secondaries in parallel. See what comes out on the primary. For a 3K primary, the turns ratio should be about 14:1. 5VAC in gives you 70VAC out.

If you get 100VAC out that is the equivalent of 7.5Ω, proving that parallel makes it half. I am thinking you will see 70V.

Please do the test. It will tell us the truth.

[oj]

Does anybody have a schematic?

Interesting Amp !
Did some googling and don't find so much info, more than it was made in Manchester and probably in some connection whit a music store.
Have not find any schematic so u my have to make one.

Best of look
OJ

[fusionbear]

Actually, I really want to know the answer and it is easy enough to figure this out empirically. Assuming it is bifilar wound, bridge the two 15Ω terminals and bridge the two "0" terminals. (Now, hope they are properly in phase and not anti-phased; you'll know soon enough and can correct for the problem.) Apply 5VAC or 6.3VAC from any ordinary tube amp power transformer to the now bridged terminals, the unified secondaries in parallel. See what comes out on the primary. For a 3K primary, the turns ratio should be about 14:1. 5VAC in gives you 70VAC out.

If you get 100VAC out that is the equivalent of 7.5Ω, proving that parallel makes it half. I am thinking you will see 70V.

Please do the test. It will tell us the truth.

I will do the testing tonight and post the results...

Thank you very much for this info!!!

[fusionbear]

So, it was bifiliar. whoever ever worked on it previously has the output jacks wired out of phase. Once I corrected the phase, I was able to put in parallel. It needs a cap job. All the HT filter caps are leaky. I installed some temporary ones for testing. I am going to blue print the circuit. It is an interesting design in that the stages are: In>Triode>TS>Recovery Triode>EF86 Driver>PI>Power Tubes. I think the EF86 is overdriving the PI and giving it the nice OD tones when cranked. I don't think the power tubes are overdriving much. Plate voltage on the power tubes is 300-402 B+

[pdf64]

It is an interesting design in that the stages are: In>Triode>TS>Recovery Triode>EF86 Driver>PI>Power Tubes

That reminds me of the Selmer PA100; it may have used as 'inspiration' http://bmamps.com/Schematics/Selmer/selmer_pa100.pdf

I've got a TVM PA200 2 x KT88, probably made a few years later.
It uses what looks to be the same metal cab and chassis as yours, and has a UL output, but HT is nearly 600V and the pre-amp is transistor.
I've stripped it out, fitted a more useful OT, and built a Marshall type bass amp from it.

[SilverFox]

Fusionbear- EF86 Driver>PI>Power Tubes I think the EF86 is overdriving the PI and giving it the nice OD tones when cranked.

I'm looking into the same thing with the Sunn-Dynaco power amp section designs. As I recall you have a modded Dynaco in the form of the, Weber 6s100? Your customers amp might resemble a souped up version of the Mullard 5-20.
silverfox.

[JMFahey]

Won't comment on other problems, but as a side note, paralleling 2 "15 ohms" windings will not turn them into a 7.5 ohms one but still 15 ohms (windings are not resistive and all what matters is voltage, which does not change) ,at most it's still a 15 ohms winding but has 1/2 the DC resistance, so it's slightly more efficient ... but nothing else.

Why did they wind it that way?
Dunno, but sometimes I see that trick (I use it myself if needed) to be able to use thinner enamelled wire secondaries.

Fender 2 ohms output transformers are sometimes wound paralleling , say, 4 x 0.80 mm dia wire (piece of cake to wind) instead of a single 1.60 mm diameter one, which is a PITA (way too rigid and secondary tends to be "round" instead of "square" if you get what I mean, because corners do not bend to 90 degrees and to boot are rounded.

Or sometimes (I also do that if needed), sometimes Factory has unused surplus of some odd wire diameter, so it may pay to design some way to use it.

I see no other practical explanation.

2 x "4 ohms" secondaries, as a difference, makes a lot of sense, because paralleled they are 4 ohms, in series 16 ohms, in fact some Hammond transformers used to come that way.

Now the 16 ohms/64 ohm option does not make much sense ... unless it happens to match some popular constant voltage line standard, say 25/50/70/100.

*Some* installations , those who share a couple hundred feet or yards with 120/240V mains along same conduit, , demand balanced , say, 50/70/100V lines, and this dual winding might fit some of those.

Just thinking aloud on the meager data available

[pdf64]

Might there also be potential for better primary-secondary coupling if the secondary is wound in sections?

Sowter's preferred arrangement for secondaries is 4 x 1ohm section outputs, which can then be arranged for 1, 4, 8 (well, 9) & 16 ohm loads, see bottom of page http://www.sowter.co.uk/single-ended-ou ... ormers.php

Rather more relevant to hifi than MI amps though.

[trobbins]

Definitely wire the two 15 ohm windings in parallel to provide any subtle improvement in improved coupling and reduced leakage inductance and lower operating temp. Don't forget to ground one end.

Fusing the secondary, and some subtle grounding tweaks would also seem in order. The chassis seems to be used for some convenient 0V connections - which may be better managed if the 0V returned to a more appropriate spot on 0V bar.

[trobbins]

It may actually be worth checking the OT turns ratio to confirm the 15 ohm marked windings are for 15 ohm loading.

As there are two separate windings, one could ambiguously presume that loading each winding with 15 ohm was the design intent, but that would then present an 8 ohm loading to the primary. So the windings could actually be 8 ohm windings, and meant to be used with 15 ohm loading on each winding.