JTM 45 negative feedback with Mercury 'Radiospares Deluxe' OT

[Helmholtz]

I think user competence has to play a part in these things, it's surely up to them to plug in the Vdc applicable to the relevant conditions? The vintage tube info doesn't spoon feed so no obvious need for your tool to do so.
Sure it would be nice to have it all singing and dancing, but it may then need to include HT sag for both the plate and g2 nodes (even bias sag too!), along with local ac nfb caused by the individual / shared g2 resistors, and the whole thing would be in danger of becoming unfeasibly complex for both the developer and the noob user.

Completely agree. The tool is a big help as it is.

I just wanted to list some reasons why the real amp may produce an output power lower by 30% or so.

FWIW the Marstran db for early Marshall lists a 64/65 2x12 1962/T with a RS OT Marshall Specs

Thanks for the hint and link. This is a piece of the evidence I am looking for. Could be an exception, though.
Unfortunately there is no information how the RS OT was wired (6.6k/8k, 16/8* ohms).

*actually the RS De Luxe was specified "to suit 3 Ohm, 7 Ohm, 15 Ohm speech coils" and was rated at 30W. The "16" Ohm Celestions of the time were labeled as 15 Ohms.

[pdf64]

All credible reports that I've seen, eg inc Brian Marsland, are that Marshall invariably wired the RS OTs to 6k6:15, UL taps not connected.
And that the 2x12 combos had their 15 ohm speakers wired in parallel, so the loading on those was half that for the heads with 4x12 cabs.
Here's some RS Deluxe OT info https://i.imgur.com/PM942TI.gif
https://i.imgur.com/KCVkehe.gif
https://i.imgur.com/uTNOJ71.jpg
Note that 9k primary taps are also available!
Maybe when they switched to Drake, they just asked Drake for their equivalent of the RS Deluxe, and didn't specify which taps they were using, so Drake just opted for the middle value? It may have been an existing part of Drake's offering, hence the 100V secondary.
I've also read / seen photos, that sometimes an 8k2 resistor was used in place of the 1k shared screen grid resistor, which would have a massive impact on the operating conditions. That may have been a way of limiting the max power output, especially for the 2x12 models, which would help to avoid blowing the greenbacks, 25 watts or even 20 watts back then.

[mikepukmel]

Spectacular! Oh indeed I need bloom and sunshine. And, some sparkle. Don't underrate sparkle. But shazaam, no, don't want that. Heh.

[mikepukmel]

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.
[ATTACH=CONFIG]48956[/ATTACH]

Thanks, Loud.

[mikepukmel]

Ok, I guess you meant your posts in the other thread here: http://music-electronics-forum.com/t46603/
Thanks. I had never considered that the JTM's were using a technically 'wrong' transformer. All the replacements that I've seen for JTM45's using a pair of KT66 have 8K primary and I had never looked at the numbers, I just figured that 8K was 'matched'.
Plugging them into the bmamps calculator gives similar power figures to what you said (higher power for 4K primary).
I guess a practical take away is that anyone who wants more power out of their JTM45 can cut their load impedance in half for power gain. (assuming the OT is rated for the higher power)

Yep, that's the first thread after I really started looking into this thing, and finding out how little reliable info I could find on how the original 'blues breaker era" (I wrote era here since say 3 or 4 years period around when it was released) were wired. I looked at half dozen kits and most were for more standard output transformers, hammonds, etc. Then I started looking for how to calculate what the "ideal" output transformer primary impedance was calculated, and got off onto a very useful, educational track for a while. None of the amp kits have something that says "Ideal output transformer primary impedance for X volts, 2 x KT66 is Y".

This chassis is really small, a lot smaller than I thought before actually trying to do stuff in it. I bought the overpriced rotary impedance selectors switch <frown> and even that with only a few wires is tight. With two switches and all those wires, Im worried about bad oscillation type stuff happening. With my skill set, I don't think I could pull off two switches on the back panel, with all those wires.

For the OT: It sounds like better for a low skill guy like me to twist the sec. bundle, pick an impedance, buy speakers for that, and wire it up hard wired.

[mikepukmel]

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!"

Your comment "struck a note", heh. So, my interest in Mercury, partly, comes from a guy, just some guy, who had a couple of Marshalls, later ones, 1990's, early 2000's?, anyway, he recorded some clips, sounded, meh, then shipped his amps off to his tech, who swapped the transformer set, re-biased, and then he re-recorded similar clips, and I can't TELL you how much better the second clips were. Unbelievable. The harmonics, and dynamics, touch sensitivity, quality of overdrive tone, ... were a heck of a lot closer to what I picture that late 60's holy grail classic Marshall sound to be. But, OK ok, I know, who knows what else the tech did in there. He could have switched out the American Cheese capacitors with the French Brie capacitors, and not told anyone, or washed the screen grid resistors in holy water from north eastern Mongolia.

I could tell you I think "this amp sound better than that one" but could not explain in specific technical terms.

[mikepukmel]

All credible reports that I've seen, eg inc Brian Marsland, are that Marshall invariably wired the RS OTs to 6k6:15, UL taps not connected.
And that the 2x12 combos had their 15 ohm speakers wired in parallel, so the loading on those was half that for the heads with 4x12 cabs.
Here's some RS Deluxe OT info https://i.imgur.com/PM942TI.gif
https://i.imgur.com/KCVkehe.gif
https://i.imgur.com/uTNOJ71.jpg
Note that 9k primary taps are also available!
Maybe when they switched to Drake, they just asked Drake for their equivalent of the RS Deluxe, and didn't specify which taps they were using, so Drake just opted for the middle value? It may have been an existing part of Drake's offering, hence the 100V secondary.
I've also read / seen photos, that sometimes an 8k2 resistor was used in place of the 1k shared screen grid resistor, which would have a massive impact on the operating conditions. That may have been a way of limiting the max power output, especially for the 2x12 models, which would help to avoid blowing the greenbacks, 25 watts or even 20 watts back then.

I lost a post someplace, sorry if this is sorta/kinda dupe: Trying to narrow down and get the amp built, great after all your fantastic suggestions and info.

So, with the 320 - 0 - 320 PT + GZ34, I will wire 2 x KT66's for 6.6K. Thanks Helmholtz
For the OT Secondaries, I don't have the skill set to pack 2 switches into that tight area on the back panel, in this little chassis, and definitely don't have the skill set to route wires around to a place with more room.

Most of this discussion was to figure out what speakers to buy ( 2 x 8's or ???) So, if I get 2 x 8 ohm speakers, I can route the 6 wires to the output jack (the other output jack hole has a post phase inverter master vol, thanks also for all the info there).

If the first shot is wired for 16ohms, then no mods to the feedback ckt is necessary. I will put an extra turret or two onto the board, so that I can put a couple of feedback resistors and just move a solder in jumper between feedback resistors, when switching between 16 and 4.

Does anyone think that the probably mismatched original amps had some kind of 'mojo' due to: OT primary impedance not optimal for the KT66's, OT secondary impedance not wired for correct speaker load? I sorta tend to think this is a "no" but might have had something to do with somewhat lower longevity of some of the original amps.

The last oddity is the reports that amps had various screen grid resistors. There's a schematic of the earlier re-release of the JTM 45 done in the 80s, it shows a I think extra 1.5K screen grid that feeds the other two 470 screen grids, so around 2k. What effect does dropping to the screen grids have on tone?

[mikepukmel]

From PDF:
"...I've also read / seen photos, that sometimes an 8k2 resistor was used in place of the 1k shared screen grid resistor, which would have a massive impact on the operating conditions. That may have been a way of limiting the max power output, especially for the 2x12 models, which would help to avoid blowing the greenbacks, 25 watts or even 20 watts back then."

Ive seen lots of comments on this as well (as mentioned above).


470 5w: http://raw-sewage.net/images/bb62ri-schm1.jpg
470 + 1k: http://raw-sewage.net/images/jtm45-readable.jpg

one of the kit guys: http://www.ceriatone.com/ceriatone/w...1-Dec-2016.jpg

[mikepukmel]

Until solid evidence I tend to doubt that Marshall used the RS type OT in original Bluesbreaker combos. The reasons are 1)The KT66 equipped combos were produced only in the end of the JTM45 era ('65/'66), when most JTM45 tops already used the 8k Drakes. 2) The original schematic of models 1961/1962 shows an OT with a standard multi-tapped secondary. 3) The OT in the schematic has a 100V secondary winding, which only the Drakes had.

Thanks Helmholtz, so far I haven't been able to find a scan of a Marshall schematic with RS OT.

[Leo_Gnardo]

Your comment "struck a note", heh. So, my interest in Mercury, partly, comes from a guy, just some guy, who had a couple of Marshalls, later ones, 1990's, early 2000's?, anyway, he recorded some clips, sounded, meh, then shipped his amps off to his tech, who swapped the transformer set, re-biased, and then he re-recorded similar clips, and I can't TELL you how much better the second clips were. Unbelievable. The harmonics, and dynamics, touch sensitivity, quality of overdrive tone, ... were a heck of a lot closer to what I picture that late 60's holy grail classic Marshall sound to be. But, OK ok, I know, who knows what else the tech did in there. He could have switched out the American Cheese capacitors with the French Brie capacitors, and not told anyone, or washed the screen grid resistors in holy water from north eastern Mongolia.

I could tell you I think "this amp sound better than that one" but could not explain in specific technical terms.

Did that amp have the same tubes in it before & after transformer changes? I bet no. Furthermore such a difference - improvement - could also be attributed to different microphone, its placement, speakers, room ambience, recording equipment between first and second samples. Who knows if the first was recorded on a potato, with a buck and a quarter mic pointed the wrong way as it seems many are when I hear the soundtracks on "comparison" videos. Seems nobody ever takes into account the crusher compressor/limiter built into the phone/camera used for such recordings. The "after" recording might have a good mic - heck even an SM57 makes great guitar recordings - placed in the sweet spot and jam up to the speaker grill. Unless all the factors in recording this supposed A/B test are identical, the results are meaningless. No need to swap in special caps/resistors. Just moving the mic a couple inches can make a world of difference. Given the opportunity, I could "prove" what a disaster it is to Mercury-mod an amp with a defective A/B test.

In the test you mention, at least the person doing it admitted he re set the amp's bias. THAT right there could make all the difference in the world.

[Helmholtz]

I've also read / seen photos, that sometimes an 8k2 resistor was used in place of the 1k shared screen grid resistor, which would have a massive impact on the operating conditions. That may have been a way of limiting the max power output, especially for the 2x12 models, which would help to avoid blowing the greenbacks, 25 watts or even 20 watts back then.

Well, my JTM 50 (PA version, ca.1966) came with a shared screen resistor of 11k (two 22k in parallel). It was equipped with KT66s and has the original 1963 circuit, but when I measured the Drake OT, I found it was the 3.2k EL34 version. Made me use the 8 Ohm output for 16 Ohm speakers.
I think the main idea behind the high screen resistance was to protect the KT66 screens in an attempt to bring screen voltages down the max. rated voltage of 400V, especially in amps with exceptionally high B+. Of course it also reduces max. output power, but this seems only desirable for the combos.

"Mismatching" changes the loadline and thus changes clipping characteristics, higher load impedance resulting in softer and earlier compression.

The Drakes finally allowed to use an impedance selector.

BTW, the first 2x12 combos had the Alnico G12s (as pictured in Doyle's book), rated at 12W to 15W.

[Helmholtz]

Thanks Helmholtz, so far I haven't been able to find a scan of a Marshall schematic with RS OT.

Neither have I. I was speaking of this, which is the oldest original I could find. Also shown in Doyle's book. It seems to have the choke wired incorrectly.

http://raw-sewage.net/images/jtm45orig-schm1.jpg

[Malcolm Irving]

The choke does seem to be wired incorrectly - it takes the plate currents through the choke (which would need a high current rating on the choke). All the other schematics take the plate currents before the choke.

I built a JTM45 replica and was not sure what to do about screen stoppers. I decided on 1k shared plus 470 each side, but from the power resistors readily available used 1.5k shared plus 470 each side (to be on the safe side).

The transformer is 8k primary, when you put the speaker on the expected output tap.

With current production KT66 (Genalex made in Russia) the amp produces 32 watts clean to a dummy load resistor and has been very reliable (touch wood!). I have been gigging it regularly (about once a week) for more than 2 years without any problems.


This particular amp seems to have more bass than I expected - I always end up with the bass and mid at zero and the treble at about 5, using a Les Paul. I have a sneaky suspicion that I may have used a wrong value component somewhere (in the tone stack?) but I haven't taken the time to investigate. The amp sounds great and I don't want to change that.

[Helmholtz]

The choke does seem to be wired incorrectly - it takes the plate currents through the choke (which would need a high current rating on the choke). All the other schematics take the plate currents before the choke.

There is another original schematic (model 1963) in Doyle's book which shows the same odd choke wiring. I was trying to be cautious when saying "seems to have the choke wired incorrectly", because the JTM45 was a copy of the Fender Bassman and the 5E6-A schematic also shows this choke arrangement. Of course it would have required a BIG choke, especially at the specified inductance of 20H. Maybe they found out later that PP stages don't require much plate supply filtering.

[pdf64]

...This particular amp seems to have more bass than I expected - I always end up with the bass and mid at zero and the treble at about 5, using a Les Paul. I have a sneaky suspicion that I may have used a wrong value component somewhere (in the tone stack?) but I haven't taken the time to investigate. The amp sounds great and I don't want to change that.

Yes, I found my JTM45 'based amp's' response was overly bassy too; I guess it was a copy of a Bassman
The tonal balance seems better with ~220pF bright cap on the normal channel, and ~470pF on the bright channel (no peaking cap on the 270k mixer resistors); and the presence up full.
The tone takes on a weird hollow sound with the treble turned much above halfway (linear taper), hence the added bright caps.
It's not oscillating, honest
The shared-individual 1k-470 screen grid resistor arrangement seems to smooth out the tone, even when overdriven, compared to 1k individual resistors.
May be due to a local feedback mechanism, eg as with a p-p shared unbypassed cathode resistor.