Convert JTM45 to JTM50

Get support and show off your MetroAmp JTM 45 kit builds.

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chopperferrari
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Convert JTM45 to JTM50

Post by chopperferrari » Sun Aug 21, 2016 1:19 pm

Hi Guys,

I want to convert my JTM45 to a JTM50. I want to keep KT66s and the valve rectifier but pop in a alternate 50watt power transformer (to improve inductance away from flubby bass). I've found a few options of alternate transformers. Has anyone knowledge of which model is best to go for with this configuration?

thanks,
Ben

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Re: Convert JTM45 to JTM50

Post by ivan H » Sat Aug 27, 2016 10:27 pm

Hi, the 1202-55 transformer that you have in your JTM45 isn't what causes the flubby bass & will do fine if you want to convert the circuitry to 50 watt spec. I take it you are talking about a JTM45 clone so depending on who made the PT it may have slightly different B+ voltage. I have a Marstran (Heyboer) 1202-55 in a JMP50 build % its fine, delivering around 460VDC. The Metroamp (also Heyboer) 1202-55 will deliver around 410VDC IIRC. Either way, a 1202-55 has a 150mA secondry winding which is the same as the 50 watt PT's, & heater windings are the same, 5VAC @ 3A & 6.3VAC @ 5A. The 50 watt transformers are the 1202-118, which is the most common & will deliver around 420VDC, or some early JTM50's used a 1202-133 which will deliver around 460VDC with a GZ34/5AR4 rectifier. Hope this helps. Cheers

Edit; I think Classic Tone sell a 133 PT & list it for use in either a JTM45 or 50 watter. Cheers

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Re: Convert JTM45 to JTM50

Post by neikeel » Sun Aug 28, 2016 7:39 am

chopperferrari wrote:Hi Guys,
I want to convert my JTM45 to a JTM50. I want to keep KT66s and the valve rectifier but pop in a alternate 50watt power transformer (to improve inductance away from flubby bass). I've found a few options of alternate transformers. Has anyone knowledge of which model is best to go for with this configuration?
thanks,
Ben
Hi Ben, that's daft!

If you have a JTM45 with KT66s running a JTM45 OT with valve rectifier, changing the PT does not make a JTM50!!!

A JTM50 runs EL34s with suitable OT and filtering (higher than a JTM45 but lower than a JMP50.

Flubby bass comes from the filtering and OT type and to an extent the output tubes.

Sure say you want to use a big PT with 500v on the plates and high preamp voltages, that does not make it a JTM50.

There are lots of threads on how to trim the flabby bass from a JTM45 (25uF V1 cathode cap, 500k bass pot, finely adjust your preamp droppers to get 190v on V1, etc)
Neil

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Re: Convert JTM45 to JTM50

Post by Littlewyan » Sun Aug 28, 2016 5:48 pm

What are you trying to achieve tonewise? What do you want to play through this amp? As I have a JTM50 and the bass is pretty good, but I do have to keep the bass control on 0 and have the treble and presence pretty high. It'll never be a metal amp if thats what you want :lol:

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Re: Convert JTM45 to JTM50

Post by chopperferrari » Sat Sep 10, 2016 6:03 am

neikeel wrote:
chopperferrari wrote:Hi Guys,
I want to convert my JTM45 to a JTM50. I want to keep KT66s and the valve rectifier but pop in a alternate 50watt power transformer (to improve inductance away from flubby bass). I've found a few options of alternate transformers. Has anyone knowledge of which model is best to go for with this configuration?
thanks,
Ben
Hi Ben, that's daft!

If you have a JTM45 with KT66s running a JTM45 OT with valve rectifier, changing the PT does not make a JTM50!!!

A JTM50 runs EL34s with suitable OT and filtering (higher than a JTM45 but lower than a JMP50.

Flubby bass comes from the filtering and OT type and to an extent the output tubes.

Sure say you want to use a big PT with 500v on the plates and high preamp voltages, that does not make it a JTM50.

There are lots of threads on how to trim the flabby bass from a JTM45 (25uF V1 cathode cap, 500k bass pot, finely adjust your preamp droppers to get 190v on V1, etc)

Ha! Now I read it back it does seem daft.

I'm really want to end up with tighter bass, less flub and the amp less floppy feeling.

I'm good with cathode bypass caps, pi cap mods and the Help reduce the amount out low end, but not the kind of low end.

I'm guessing increasing the pre amp filtering to 33uf would help?

Also I'm not familiar with preamp droppers - can you elaborate?

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Re: Convert JTM45 to JTM50

Post by Littlewyan » Sat Sep 10, 2016 8:52 am

Its quite a saggy amp mate. You would need to lose the valve rectifier, up the filtering across the board and then look at perhaps converting it to lead spec or just putting a smaller cathode bypass cap on V1. However it will still be a bit saggy as the Power Transformer sags a lot.

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Re: Convert JTM45 to JTM50

Post by Roe » Sat Sep 10, 2016 12:31 pm

try running a 8ohms load from the 16 ohms secondary (or 4 from the 8). If you like it, try a 784-128 or -139
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

chopperferrari
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Re: Convert JTM45 to JTM50

Post by chopperferrari » Mon Sep 12, 2016 6:13 am

I have tried running 8ohms load from the 16 ohms at it's pretty good.

I've got a drop in SSR and that helps too.

I'd like to try upping the filtering as this might be the magic bullet for me. What would you suggest? at the moment i'm just running the metro stock filtering F&T Dual 32uf can, F&T Dual 16uf Can and a Tad 16uf.

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Re: Convert JTM45 to JTM50

Post by Littlewyan » Mon Sep 12, 2016 1:09 pm

So at the moment am I right in saying V1 and V2 share the same filtering node?

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Re: Convert JTM45 to JTM50

Post by chopperferrari » Mon Sep 12, 2016 3:24 pm

You got it - V1 & V2 are sharing the 16uf on the board

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Re: Convert JTM45 to JTM50

Post by Littlewyan » Mon Sep 12, 2016 5:21 pm

OK split those with a 10K dropper and give them both 32uf each and bring the PI up to 32uF to begin with. Thats the level of filtering I have in my JTM50 and I quite like it. Its tight enough for me.

If thats not enough and you're not going to use a valve rectifier then I would take the mains up to at least 64uF (2 x 32uF). You might still be able to use a GZ34 with that level of mains filtering but you might find you'll go through them quite quickly!

Or if you just want some easy modifications for now then bring the PI up to 32uF and bring the shared V1/V2 node up to 32uF.

How far do you want to go with this? What kind of tightness do you want?

chopperferrari
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Re: Convert JTM45 to JTM50

Post by chopperferrari » Mon Sep 12, 2016 6:41 pm

Brilliant, thanks. I'll order a duel 32uf for the pre amp and split it as suggested and try that. I'll post back when I've got them installed.

I don't want the amp to be super tight, it's just so squishy. Always loved the sound of the Majors & Super Bass.

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Re: Convert JTM45 to JTM50

Post by Littlewyan » Tue Sep 13, 2016 1:51 am

Late Super Bass Amps had 50uf caps across the board. I personally think that its a bit extreme to have 50uf in the pre amp though.

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