JTM45 Power Transformer

[Littlewyan]

Hi Guys

I'm getting parts together for a JTM45 build and am struggling to decide on the power transformer. I've already got a 6.6K OT and will be getting a 10H Choke but I can't decide on the HT Voltage.

I have found a transformer which gives a 345VAC Secondary but I think this will be a bit high. I imagine with that I'll probably get 460/470VDC after the valve rectifier? The other transformer I've found for the job in the UK is the Hammond 290LX which will give a 300VAC Secondary which means I'll end up with 390/400VDC after the rectifier. Only issue with that is I think it looks pants http://www.hammondmfg.com/jpeg/290LX_B.jpg

I mean its got to look good as well right . I should also mention that I live in the UK and wish to only buy parts from Europe so I don't get stung on import VAT.

There is a third option and thats buying the transformer Marshall use in their JTM45 reissues, which I THINK is 300VAC secondary but I haven't got a clue what the current ratings will be. Plus I'll need to find a wiring diagram for it as it is supplied as a replacement part.

What do you guys think? Is there another supplier in Europe that I'm missing out?

[Stevem]

I just used a Mojotone jtm 45 low voltage type (300/ 300 SS recto) in a blackface bassman and it had enough current to make 42 watts rms and 87 watts peak if that helps you decide on B+ voltage?

[Littlewyan]

Its more the tone than the wattage I'm concerned about. I've listened to a JTM45 that had 400VDC with 6.6K OT and it sounded fantastic. I might just go for the Marshall PT and try and find a wiring diagram for it. The Hammond PT would be perfect except the inside of it is just white and looks crap.

[jelle]

Have you seen this?

http://www.classictone.net/40-18054.html

International shipping is possible too. Or use shipito.com.

[Littlewyan]

International shipping is possible but expensive. I would LOVE that classic tone transformer, believe me I would , but shipping is $65 and then I would have to pay another $40 to get it into the country. So the transformer would end up costing me $200 which is £135. If I bought the Hammond Or Marshall Transformer they are about £79 including shipping.

You know I think its quite funny how Marshall amps are British yet we have hardly any aftermarket parts for them over here.

If I bought the transformer with the 345V Secondary what do you think my B+ would be if I used a GZ34? I've been reading that it could be about 470VDC or is that way too high? I don't think I want to have a B+ any higher than 420VDC.

[joeboo88]

Hey Littlewayn. I wonder how much shipping would be from Winnipeg Canada over the pond to you. I get Hammond transformers shipped to an electronics store here where I live, shipped from Ontario and my cost is zero. I have a 290bx PT and a 1750q OT in my jtm45, they sound fine, but I don't have anything to compare with.....I'm like you,shipping from the US costs a lot. I really want to buy a OT from RJ for my express build, but the Canadian dollar isn't worth much right now (or ever)
Let me know if I can help?

[Littlewyan]

Getting Hammond Transformers isn't an issue believe it or not, I can order them from Mouser and as the Power Transformer will be over £50 its free delivery with all import VAT paid! Or I can order them from a German shop. Thanks though.

Doesn't that transformer run a bit warm? As the 6.3VAC Secondary is only rated at 3A and with 3 x 12AX7s and 2 x KT66s you'll need at least 3.5A?

Edit: 291CEX looks like it'll do nicely. Heater secondary is rated at 4A which is on the limit a bit for me I like to over spec my transformers if I can. If I can't find any others then that will be what I'll use. Its a better option than the 290LX which Hammond state is for the JTM45!

[joeboo88]

Littlewyan
I thought this PT I used wouldn't be up to the task with the low ma. I had used that on a tweed deluxe I built but scrapped it.
I played the amp for an hour, and it only gets a little warm.
I love the bass on this amp......and the sustain I get.
Oh sorry, I am running 5881's not kt66's
good luck with the build,cheers

[Littlewyan]

I think I'll just go for the 345VAC Transformer. At the end of the day I've listened to a clip of George M showing off his 3 way power switch on his 68 Marshall that puts the amp in either 470V or 400V mode and the sound difference is very slight. Its probably more the feel of the amp. Plus I've read that most people tend to get 420VDC after the GZ34 in these amps when using KT66 valves.

If anyone has any experience with this please chime in.

[pdf64]

£72 + vat etc, looks nice http://livinginthepast-audioweb.co.uk/i ... mrvt1566dt
If not in a hurry, these look the bees knees http://www.ig-transformatoren.com/catal ... /50rl?c=48

[Littlewyan]

The Danbury PT is the one I've had my eye on (OT that I've got is Danbury). Although as I probably wont be buying it for at least a month I think I'll wait for the other one as it does look nicer and has the option for a 325VAC Secondary. Thanks . I actually forgot about the IG Transformatoren PT and you just reminded me!

[jelle]

Would it be possible to add a bucking transformer for the HV secondaries to knock a few volts off if needed? I would think this would work, but I have never tried it.

[lovetone]

The original 60's amp used 350-0-350 if you look for the model 1963 the schematic has a valve chart with voltages, in the early 1987 amp they used the same transformer with diodes and fitted EL34's.

Hope this helps.

[lovetone]

Heres the schematic I was thinking of

[MHProd]

It depends what kind of sound you're after... If you want something close to an original you need 450vdc on the tubes and 8k primary OT. Merren transformers are the best for authenticity.