Metropoulos Forum

I would like to know something about the use of the 6.6K tap on the Radio Spares OT transformer.
The first thing that I would like to know would be, how long did Marshall use the 6.6K tap and what valves did they use with it. Did they use both the 5881 and the KT66 valves or was it just the 5881.
The second thing I would like to know is about the wiring and impedances. Can I use the Radio Spares OT transformer, wired for 6.6KOhm input and 16Ohm output, with both the KT66 and 5881 valves? Can I do this without changing the values of any of the other components? I understand that the bias dropper resister may need changing. But other than that is there any other values that should be taken into account when using the 6.6K tap? Can I swap the KT66 for 5881 without any other changes?
I am using the 6.6K tap on my Marstran Radio Spares Deluxe transformer clone and I am using it with a pair of KT66 valves and getting some good results. I only used the KT66 valves as they were what I had in at the time. My intention is to put a pair of 5881 valves in as I wish to make the circuit as close as possible to an early 1962 JTM45 as I can. I read somewhere that the 6.6K tap on the Radio Spares transformer was better suited to the 5881 valve. It also may not be necessary for me to use the 5881 depending on what I find out. I am modelling my builds on existing amplifiers. That is I am copying pictures and making guesses. I hope you can help. I have looked at the other threads and not found exactly what I was looking for.

Bump
I saw this while looking in to Taden New Vintage guitar speakers, http://www.gartoneamps.co.uk/regal45.html. This amp uses the 6.6K input on a Mercury Magnetics Radio Spears transformer. There are a lot of sound samples on the page. I have to say they sound pretty much the same as mine dose now. The playing is better though.

marshall appears to have used both the 6k6 and the 8k taps, although I've not been able to verify this myself. the early RS dlx amps had 5881s and the last seem to have had kt66s. both tubes work fine at both 6k6 and 8k if you bias properly. However, 5881 benefit from the 1k flying resistor, especially at higher voltages

This is a good question, because all photos you see of early JTM45s using the RS Deluxe OT have the primary connections obscured by the preamp board. I've also "heard" that they used the 6k6 primary early on with the 5881s, but I don't know if they changed to 8k with the change to KT66. At one point they probably changed to 8k, since the Drake OT they subsequently used (with KT66s) had an 8k primary impedance.

I've tried my own clone at both 6k6 and 8k, but only with KT66s. To my ears, the JTM45 sound *is* KT66s at 8k...something about the clean compression sounds much more right at 8k. As you go down to 6k6, or even lower (like 4k if you mismatch head and cab), it gets progressively harder sounding. A little more headroom and bandwidth, but it doesn't get that quintessential JTM45 sound to my ears.

This isn't really an answer to your question though. It may well be that 5881s sound best at 6k6, and you can definitely swap them in with no other modifications.

BTW, one of the famous JTM45 users was Peter Green, who used a block logo with the large "Clapton" knobs on the Bluesbreakers' "A Hard Road" album. Judging by the looks, that amp was probably a late '64 or early '65, and almost certainly used the RS Deluxe OT with KT66s. It sounds like 8k primary to my ears, but there are too many other factors in play (speaker cab, probably alnicos, recording, reverb etc) to say for sure.

Roe hits the nail on the head with his mention of the 1K flying resister. That would be the one next to the NF resister. I have a lot of pictures saved of JTM45’s with this resister in place. What difference does it make to the tone of the amp? I have seen this resister on amplifiers with both Drake and Radio Spares transformers. Why is it used on some amplifiers and not on others and why is it used at all? Is the 5881 not as robust a valve as the KT66? Or was it just a valve saving device sometimes used and sometimes not, no matter what the circumstance. May be Marshall ran out of 1K resisters one day and decided they could do without it.

I have also seen a 10K resister used on the output of a lot of early JTM45’s. Again this has been used with Radio Spares and Drake transformers. I feel that the use of 5881’s and Radio Spares transformers may have necessitated the use of these resisters. Then as Marshall moved on to Drake transformers they became less necessary and so were no longer used. I would still like to know exactly what difference they made and why they were used.

The impression from what I have read about the 6.6K tap on the Radio Spares transformer is it wasn’t used for very long at all. In fact my impression would be that it was only used on prototypes. This is why I am asking the questions. Was the 6.6K tap used on production amplifiers in 1962? For that matter did Marshall make amplifiers in 1962? I have read some articles that have said that Marshall started production in1963. This would put a bit of a spanner in the works for me as I am trying to build an early 1962 replica. Or I should say a replica of the circuit.

The tone from my JTM45 with 6.6K tapped Radio Spares transformer, as it is now, sounds a bit like a late Tweed Deluxe on steroids. It may be closer to the tone of a Bassman than it is to a Marshall, but not quite as dark. With a 500pf cap on the No 1 pot I could do a great rendition of The House of the Rising Sun. The amp seems brighter than a JTM45 with a Drake transformer in it. I have a 100pf cap on the No 1 pot and it’s still brighter than the other JTM45 I have with the Drake clone in it, and that also has a 100pf cap in it.

As Shakti said Peter Green used a 1964 JTM45. At a guess I would say that they were using the 8K tap by 1964. But does that sound the same as a Radio Spares transformer with a 6.6K tap. How much of a difference is there in tone between the 8K and the 6.6K tap. Thank you Shakti for sharing your experience with the two different taps on the Radio Spares transformer. From what you have said I feel I am going in the right direction with the 1962 build. I am looking for the different tones from the first ten years of Marshall and I think I am close to a1962 JTM45. So far I have a 1962 JTM45, a 1966 JTM45 times two, a 1967 JTM50, a 1972 JMP 1987 and I have stated a 1968 JMP 1986. And they all sound different and all well worth having. For me the grass is always green because I own all the fields. But then there is always the 100 waters’, dam .

Great thread and great info. Perfect place to ask a question about the KT-66's. I've been on the VH thing for so long but lately, the early AC/DC stuff has been interesting to me. Was thinking about building an amp with some KT-66's. It's a long way to the top if you want to Rock N Roll, sounds like a JTM-45 with KT-66's. Is this correct? And if someone could point me in the direction of more info on the amp, that would be awesome.

On a side note, I have a 50 watt plexi build that uses the MM's 045RS-L OPT. Best sounding amp I own, even for VH but I use 6CA7's in it.