JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

[Leo_Gnardo]

so, then, more wires coming out of a transformer isn't better?

Whatever the boss tells 'em to write. Maybe they get a free doughnut or sumpin'. One thing, and one thing only has ever impressed me about Mercury: their marketing department. They really could sell ice cubes to Eskimos. Ummm,,,,, Inuit, sorry, I gotta get modern.

Time was, in the 1950's, early 60's Radio Spares had their generic OT's made this way. Nobody expected to have a choice of multiple output taps ready at the flip of a switch. Hi fi hobbyists bought one or two and wired 'em appropriately for their speaker(s). Putting one in a guitar amp was a sort of off-label use, a "punt" no doubt by Ken Bran who was called on to whack together a couple of amps to satisfy Jim Marshall's customers clamoring for more-or-less copies of Fender's 5F6A Bassman. That's where it started and look where it went, good for them! For those modern day hobbyists (you) who want to build near-as-dammit copies of Marshall's earliest efforts, I guess you gotta invest in the Mercury OT, as inconvenient as it is to use. Whether it really sounds like the real thing, well you're going to have to come up with an authentic Blues Breaker or JTM45 to compare. In any case, it looks like a wonderful project. You've already managed to build your Deluxe Reverb style amp, so I reckon this is within your abilities. Good luck!

[Malcolm Irving]

…
Time was, in the 1950's, early 60's Radio Spares had their generic OT's made this way. Nobody expected to have a choice of multiple output taps ready at the flip of a switch. Hi fi hobbyists bought one or two and wired 'em appropriately for their speaker(s). Putting one in a guitar amp was a sort of off-label use, a "punt" no doubt by Ken Bran who was called on to whack together a couple of amps to satisfy Jim Marshall's customers clamoring for more-or-less copies of Fender's 5F6A Bassman. …

Yes. In those days, in the UK, we had hobbyist magazines called 'Practical Wireless' and 'Practical Electronics'. They advertised those RS transformers (and capacitors etc.) every month and articles in the magazines showed you how to use them for home-brewed 'hi-fi' amps and even the odd guitar amp (and I do mean odd). I expect Ken even bought the first few transformers by retail mail order! Anyway, it turned out well for them, as you said.

I still like the sound of those early Marshall amps better than the modern versions.

I think the key thing in the 'design' of those amps was interaction with the musicians, particularly Pete Townsend of The Who. What, you want even more treble boost? You like it when it feeds-back? You want more distortion?

[jmaf]

You can add a small 1k resistor in series with a 100k linear potentiometer instead of the -FB resistor and vary the pot from zero up until it pleases you. The 1k resistor will save the pot from smoking. Then measure the pot value and add a same value fixed resistor in its place.

Try the amp open loop too, who knows you may not even want a neg FB anymore. I have several open loop amps I built, they sound wild. JTM45 is Bassman, totally moddable amp in so many aspects.

[Chuck H]

Likely so. Wouldn't doubt Mercury has a claque audience right under their own roof to write "reviews."

All you have to do is read their reviews to know it's planted BS. They read like a Mercury ad. No one talks (or writes reviews) like that. "My amp lacked a certain bloom and compression that the originals have. I put in the MM Super Don't Suck transformer and now my tone is amazing! And my teeth get whiter every time I play since I installed it! I've had to beat the girls off with a bat!"

[The Dude]

[jmaf]

[Chuck H]

Ha ha! That reminds me of a customer I had when I was a "tinter" for a paint store. A "designer" had brought a color in for a match and complained that the color match "didn't have enough LOVE in it". I had to consider how to explain to such a person that I didn't have "LOVE" on my tint wheel colorants.

[g1]

If having the exact OT is critical, it seems kind of blasphemous to use a different impedance tap.

[loudthud]

I tried drawing out the connections from the attachment in post #6 from Chuck H. The polarity of the 8 Ohm example is reverse compared to the other two. The series arrangement for 16 Ohm is not ideal.

Try this: Always ground F and take the feedback from A and use a feedback resistor for a 4 Ohm tap.

For 4 Ohm: Connect A to C (Output). Connect B to E. Connect D to F (ground).
For 8 Ohm: Connect A to B to D. Connect C to E (Output). F is ground.
For 16 Ohm: Connect A to B. Connect C to D. E is output, F is ground.

Below is a schematic for using two DPDT switches. Both switches need to be in the correct positions for it to work. Warning: If one of the switches is in the wrong position, you could damage the OT.

[Helmholtz]

Just being curious, the OP says he wants/needs to use a 4 Ohm speaker load. Is this made up by two 8 Ohm speakers wired in parallel?
If so, why not wire the speakers in series for 16 Ohms?

[Chuck H]

Just being curious, the OP says he wants/needs to use a 4 Ohm speaker load. Is this made up by two 8 Ohm speakers wired in parallel?
If so, why not wire the speakers in series for 16 Ohms?

I was thinking the exact same thing. The original amp using a transformer that this MM is supposed to be a "clone" of was 16 ohms with the tap from 16 ohms. And, according to loudthud, the 16 ohm arrangement "is not ideal". But is THAT the way the original was wired? If so, and since Mike is chasing original mojo with so much of this effort, just build the thing as a frikin' jtm45!?!

As you mention, two 8 ohm speakers can be wired for 4 ohms or 16 ohms. Likewise with four 16 ohm speakers. I think it would be less trouble to rewire a cabinet, or even put the 16 ohm option on a switch, than it would be to take a clone effort this far only to limit the "clone" to a single and incorrect output impedance.

JM2C

[pdf64]

Yes, best to keep things the same where feasible; changing the feedback resistor from 27k to 12k will alter the corner freqs of the presence control a bit, then pushing you to tweak the 0.1uF cap up a bit to compensate, and before you know a few more changes some to mind and it ends up a different amp.

[Helmholtz]

I always wondered how original model 1962 Marshall Bluesbreaker combos were wired. On one hand Marshall is reported to have always used the 16 Ohm versions of the G12 (alnico or ceramic) and this would result in an 8 Ohm speaker load. On the other hand the original circuit diagram indicates that the OT was wired for 16 Ohm. With the RadioSpares OT you have to decide for a single output impedance. And if they used the 8 k primary and the 16 Ohm secondary, the speaker load would have reflected to 4 k for the KT 66 tubes. This would in fact have increased available clean output power from around 27 watts to over 40 watts.

But maybe Marshall only used Drake OTs in the Bluesbreakers with the speakers connected to the 8 Ohm tap and the feedback loop to the 16 Ohm tap?

[g1]

On one hand Marshall is reported to have always used the 16 Ohm versions of the G12 (alnico or ceramic) and this would result in an 8 Ohm speaker load. On the other hand the original circuit diagram indicates that the OT was wired for 16 Ohm. With the RadioSpares OT you have to decide for a single output impedance. And if they used the 8 k primary and the 16 Ohm secondary, the speaker load would have reflected to 4 k for the KT 66 tubes. This would in fact have increased available clean output power from around 27 watts to over 40 watts.

I'm a little unclear, but it seems you are saying they would get more power mismatched than with proper impedance match? Could you explain? Shouldn't 8K primary for pair KT66 give max. power?

[Helmholtz]

I'm a little unclear, but it seems you are saying they would get more power mismatched than with proper impedance match? Could you explain? Shouldn't 8K primary for pair KT66 give max. power?

No, 8k is strongly mismatched. See my previous posts. Max. power is achieved with Raa between 4k and 5k. Please check data sheets (loadline construction). Actually the KT66 is quite similar to the 6L6.