New JMP 50 style build in JTM45 chassis

[gktamps]

Hi,

First, thanks to Miles and Richie for giving me some initial ideas and guidance in starting this build, and MattH has pointed me towards DC elevated heaters and Matt and Miles towards CF protection. I'm now putting this out to you all to ask for your input.

I want to build a JMP 50 style amp in a JTM45 chassis, using 50 watt transformers and EL34s. I had been considering dumping the tube recto but Miles has suggested leaving it there, and having the SS bridge rectifier switchable to tube. I've done that on a couple of my other builds, using diodes on a switch near the tube recto, so though dumping it would save $ and free up chassis space, not sure what to do there.

Looks like the Tubedepot chassis puts both can caps inside and spaces the output tubes and recto tube farther apart, so considering that chassis - any thoughts?

Considering Marstran 784-139 OT and Marstran 1202-118 PT. This sound OK?

Despite the high-end choice of transformers, I don't know if I want to go through the trouble and cost of using NOS mustard caps, etc. I want it to sound great, but am not building a clone, so I suppose the choice is down to Mallorys or Sozos, eh?

Would like to add a MV in one hole, channel two input, but not sure if VVR is also the way to go with this design. I've read here that some of you feel the MV is well suited to this amp.

Don't need to do anything fancy with bias circuit, because I don't plan on making this versatile with power tubes - just EL34s.

Thinking I would pretty much follow the Tubedepot jtm45+ layout, and tweak as needed. If anyone knows of inherent problems with this, can you share?

Thanks very much!

Greg

[matt h]

(deleted)

[M Fowler]

The mojotone JTM45 kit is really a JTM50 it uses EL34's with 3k4 OT and has Tube recto it's a great sounding amp.

Also, WWW. triodestore.com has a JTM50 kit and layout to look at.

[NickC]

Here's a build log for the JTM45 I made; might be of some help:

https://tubeamparchive.com/viewtopic.ph ... c&start=15

[Reeltarded]

When I build a JTM that normally has a tube rec I don't make it switchable, I just build the bridge on the tube socket so all I would have to do is clip the one end of the bridge and insert a tube, but it's not switchable.

I think my BB amp runs like 390v with a GZ and 418 or so with a diode bridge. Brian's transformers are more typical of vintage spec, and higher B+.

You have to ask Brian what the expected voltage is in either configuration there. I have no idea.

[gktamps]

Matt H - Oops, I didn't think that through. I had originally thought of discrete diodes, not a bridge rectifier, but I've looked at so many different schemos I started thinking along those lines. I'm really thinking about diodes on a switch in addition to a tube, but will give serious thought to your suggestion for sag resistor/switching instead of tube - thanks!

Miles - I misquoted you - you did say clip the diodes to insert a tube, not switchable. Thanks for headsup about Brian's tranfo voltages - I was intending to use his JMP50 transformer which is 285-0-285. I'll get in touch with him. For comparison, Classic Tone 40-18095 has taps for 345-0-345, 312.5-0-312.5, and 280-0-280. I used Classic Tones in my 6V6 KF50 build and am pleased with them, but if there is a step up without squandering cash on MM...

Mark - thanks for the pointers to Mojo and Triode. I've had ordering and shipping issues with Mojo in the past, and am somewhat inclined to get parts elsewhere, but I haven't broken out the chisel and block of stone yet...

Thank you NickC for the link to your build - very nice.

Thank you, all,

Greg

[martin manning]

Re the Marstran PT, my 2204-ish amp uses JMP164, listed at 281-0-281, and has ~375VDC on the EL34 plates with SS rectification.

[gktamps]

Hi Martin,
Hmm. I don't see that particular model on the Marstran in-stock page, but did find Brian's chart of AC Primary-Secondary relationships.
http://www.marstran.com/JTM45%20PT%20Information.htm
Thank you,
Greg

[matt h]

(deleted)

[martin manning]

I see 1202-164 PT is now listed as a special order part. These are all FW rectifier circuits.

[gktamps]

My head hurts. And my toe from stubbing it as I stumble my way through the terminology.

Yes, Matt - right you are, full wave rectifier, not full wave bridge, but for my build, I was planning on placing two diodes on a switch, as I have done this on two other builds.

It appears there are full wave bridge applications on some Marshall's, such as 100 watt 2204, correct? Interestingly, I read on one forum that a tube rectifier can still be used as a bridge rectifier by using two silicon diodes as the other half of the bridge.

But me, just thinking of the two diodes. Don't have much use for over 800V

[matt h]

(deleted)

[Reeltarded]

/continue rant

Count me in that number too

/rant maybe over

[gktamps]

I'm detecting a trend here, though the data signal is weak.

You guys are the Yodas of rant, and I'm still trying to learn how to lift my craft out of the swamp.

[Reeltarded]

Haha

Tube rectifier arguments are all but over. It's like global warming. Ask both sides. The science speaks for itself.



And then use silicon.