Metroamp jtm45 OT dilemma

[dariez75]

Hi guys,
last month I've build a JTM 45 using metroamp jtm 45 BOM and manual. The trafos I've used are Mercury Magnetics PT, OT and choke. When I received the the MM trafos I've read different OT wiring for KT66 despite El34 or other output tubes. I did wiring using the metroamp manual, the amp works fine but I've the dilemma: Could I damge my amp or tubes? The OT is Mercury Magnetics 045jt-16
Thanks for help

[martin manning]

The key parameters are plate voltage, load impedance, max plate dissipation, and bias setting. EL34 and KT66 are interchangeable in all except bias voltage, so as long as you rebias correctly for KT66, you should be fine.

Is the documentation you received with the OT suggesting that it should be wired differently for KT66 vs. EL34?

[dariez75]

Yeah martin, the documentation shows different wiring, if I remember correctly 8ohm in kt66 are 16 ohm el 34 etc. if I found the original file I will post

[maxwedge]

That transformer has a 8k primary which is correct for the KT66 in the JTM45.
Are you trying to run EL34's in your amp?
EL34s require a ~3.5k to 4k load.You can do that by using a cabinet that is half of what you set the selector at. Example: Set the amp to 16 ohms and use a 8 ohm cabinet or select 8 ohm and use a 4 ohm cabinet. You can't do it if you only have a 16 ohm cabinet.
You'll have to re-bias and may need a different voltage dropping resistor for the bias circuit.

[martin manning]

According to internet lore ( http://marshallroadhouse.com/viewtopic.php?f=2&t=12439 ), JTM45's used Radiospares transformers with 6k8 and 8k primaries from '62-'65, and Drake 784-103 with 8k primary in '65-'67.

I believe Mercury Magnetics 045JT-16 is 3k4 primary, with 4-8-16 secondary taps.

I see no problem running a pair of KT66 into a 3k4 load with 400V on the plates. If you want to get close to the original, mismatch the speaker load by a factor of two to get 6k8 (4 ohm tap into 8 ohm cab, or 8 ohm tap into 16 ohm cab). Personally I think the 6k8 and 8k loads are an odd match for the KT66, and the load line really looks much better running into 3k4.

[dariez75]

If I understand I don't need to rehook my rotary impedance switch. With kt66 I just select 4ohm when use 8 ohm cab (1 or 2, I use 2 marshall 1922 in 8ohm) and select 8 ohm when I use 16 ohm cab (for instance marshall 1960).
In the attachment you can see the mercury magnetics wiring layout for tone clone 045-16, OT used in my jtm45. According to metroamp manual and with pdf available on the web there is not difference between el34 and kt 66 hook up. The dilemma rise out when documentation boxed with trafos showed different hook up in el34 and kt 66. I still haven't found the orignal documentation from MM, could be buried in my messy garage but I've try to draw what I remeber.
Thanks guys you are my great help!

[martin manning]

The MM diagram is nice but it does not say what the primary impedance is, and it doesn't give phasing information. I found a reference saying that that part number has a 3k4 primary impedance when the secondaries are used as marked, but you should verify that or measure it yourself.

If it is really 8k, and you mismatch up as I described you will be running 16k, and that could generate very high voltage spikes and ruin the transformer.

Mercury's failure to provide this basic information about their products is just rediculous!

[dariez75]

I agree with you Martin, it is ridiculous! Today I try to sand an email to MM, I hope they answer good.

[martin manning]

Let's hope so. If you want to measure the primary impedance yourself and want help just ask. Again, I see no problem running the KT66 at 3k4 (same as EL34), so you can see which way sounds better to you.

[dariez75]

Thanks for help Martin. I have the same your thought, in fact I just asked to MM why they changed the latest OT wiring layout.
Rock!

[rp]

Personally I think the 6k8 and 8k loads are an odd match for the KT66, and the load line really looks much better running into 3k4.

That's interesting, As one day I want to do a very early JTM45 I'll keep this in mind. I just know of KT66s reading about Williansons and I think that called for 6-10K but for a pp triode connection with nfb @15W, IIRC. I have a feeling Marshall and Bran used whatever they had at hand and didn't even know what a load line was. I bet you could've run rings around them Martin, at least back when they had the little music shop. Very likely they chose 8K looking at RS trany catalog page, based on the popular hifi literature regarding KT66s at the time, which recommended that OT for KT66s and left it at that. I think the OP should therefore at least try 4K on the KT66s and not just follow rote, as should we all.

I've played w/ halving doubling the load on my amps and it's really hard to notice a big difference, I'll hear a bit more bright and sizzle one way little fatter the other, not a big difference really, I never tried it with a 4x12 sealed cab however and that's where you can really hear stuff. I really think w/o real KT66s, and some mullard pre's while we're at it, it'd be impossible to tell if 8K was magic serendipity or just no big deal.

Mercury's failure to provide this basic information about their products is just rediculous!

I've said it before, if they were baking boutique cookies I could see keeping a secret, but it's engineering so it's worse than ridiculous it's unacceptable. Plus this is the data sheet to a paying customer, not just not putting info on the web for the supposed competition to steal. Doubly unacceptable.

[maxwedge]

According to internet lore ( http://marshallroadhouse.com/viewtopic.php?f=2&t=12439 ), JTM45's used Radiospares transformers with 6k8 and 8k primaries from '62-'65, and Drake 784-103 with 8k primary in '65-'67.

I believe Mercury Magnetics 045JT-16 is 3k4 primary, with 4-8-16 secondary taps.

I see no problem running a pair of KT66 into a 3k4 load with 400V on the plates. If you want to get close to the original, mismatch the speaker load by a factor of two to get 6k8 (4 ohm tap into 8 ohm cab, or 8 ohm tap into 16 ohm cab). Personally I think the 6k8 and 8k loads are an odd match for the KT66, and the load line really looks much better running into 3k4.

It should be 8k if it's made for a JTM45. The thing is Marshall made a mistake when they used the transformer that they did and that is the reason for the inefficient classic sound of the JTM45. I agree, the KT66 is more suited for a 4k load and it cleans the amp up a lot and that's okay if you want it to sound more like a Fender.

[ToneMerc]

According to internet lore ( http://marshallroadhouse.com/viewtopic.php?f=2&t=12439 ), JTM45's used Radiospares transformers with 6k8 and 8k primaries from '62-'65, and Drake 784-103 with 8k primary in '65-'67.

I believe Mercury Magnetics 045JT-16 is 3k4 primary, with 4-8-16 secondary taps.

Personally I think the 6k8 and 8k loads are an odd match for the KT66, and the load line really looks much better running into 3k4.

No exactly internet lore; I worked on an 1965 JTM45 that had the tag board RS OT that was wired as 8K with about 460V on plates. Yeah the load line might look better, but the mismatch contributes to the tone for what real JTM45 sounds like. If you want something that sounds a bit more focused, build a JTM50( El34/3.4K)

When I built my JTM45 clone, I followed the KT66/8K route and never looked back.

TM

[ToneMerc]

According to internet lore ( http://marshallroadhouse.com/viewtopic.php?f=2&t=12439 ), JTM45's used Radiospares transformers with 6k8 and 8k primaries from '62-'65, and Drake 784-103 with 8k primary in '65-'67.

I believe Mercury Magnetics 045JT-16 is 3k4 primary, with 4-8-16 secondary taps.

I see no problem running a pair of KT66 into a 3k4 load with 400V on the plates. If you want to get close to the original, mismatch the speaker load by a factor of two to get 6k8 (4 ohm tap into 8 ohm cab, or 8 ohm tap into 16 ohm cab). Personally I think the 6k8 and 8k loads are an odd match for the KT66, and the load line really looks much better running into 3k4.

It should be 8k if it's made for a JTM45. The thing is Marshall made a mistake when they used the transformer that they did and that is the reason for the inefficient classic sound of the JTM45. I agree, the KT66 is more suited for a 4k load and it cleans the amp up a lot and that's okay if you want it to sound more like a Fender.

Maxwedge, you are right, the greatest mistake ever made!

TM

[rp]

OK, I'm convinced, 8k it will and must be.