Putting EL34’s in a JTM45, or making a JTM50

[pearlrider]

Putting EL34’s in a JTM45, or making a JTM50. Which is it? Is it still a 45 with the 34’s in it or is it a 50. Or is it the transformer that makes the diferance.

I had been led to believe that I would have to change my output transformer if I were to put in EL34’s in my JTM45 build. Then it would be a JTM50.
Then I read things like this from, 45 Degrees of Tone.

To get our ears accustomed to the JTM45 sound, we began by firing up our ’65 head with a Les Paul. Normally, this head has EL34s in it, but we borrowed the Genelex KT66s from the Mojave and biased the amp to accommodate them. It made sense to us to use KT66s, because they were what the amp was designed for. With everything looking good, we flipped it from standby and beheld the beauty of this vintage masterpiece.

I have read other articles stating similar things. What is the truth? Can I put a pair EL34’s in my build and re bias, as stated in the article above.

I am planning a JTM50 build. I was thinking of building a JTM45 and then changing the output transformer for the 784-139. I thought that that would be all there was to it. Or is there more to it than that, or is there less. Is there something I am missing? Or do I need to change the transformer at all. Just put in the EL34’s and re-bias.

Comments would be welcome.

[neikeel]

I have done all three!

Go for the Marstran -128OT with EL34s and turn the bass down! Having said that a Metro -139 worked very well but ran a bit cleaner, made with NOS Pihers and Mustards and Mullard El34s it now lives in Canada with a very happy owner.

[pearlrider]

Hi Neikeel.

Thanks’ for the reply.

Can you use KT66 and EL34’s with the -128 and if so, what do you do about the difference in Ohm’s. 3.5 needed for the EL34’s and 8 for the KT66’s. Is there something more than a change in biasing needed?

Also is it the -128 OT transformer that has the 100volt tap. There are Drakes on e bay with a 100v tap. They’d look good if I knew what to do with the tap.

May be someone could tell me what the 100 volt tap is for. I know it has something to do with the PA, but what, I don’t know.

The -139 is the more likely. I have heard that about them being cleaner, and also louder. I think I could use that. What did you think of yours? Sounds like you miss it.

[neikeel]

You can use either tube with either OT just use a mismatch between selector and load to make sure reflected impedence matches. Personally I would decide what I want and go with the correct OT. ie using KT66s with an EL34 OT if you have the load of 16ohms set your amp selector at 8. Visa versa for EL34s in a 8k OT go one higher so 16ohm selector and 8ohm cab. You would need to rebias, of course, if swapping back and forth.

I have a 67 JTM50 which sounds amazing for what it is, lots of gain on tap so I cope with losing the other to Canada. I also have a replacement clone built too so do not feel sorry for me. Funny because that amp with that particular Metro -139OT never quite cut is as a 1987 with split cathode and diode rectification, but as a JTM50 with a GZ34 and resistors changed to Pihers sounded pretty special (new owner had paid up front so no backing out )

The 100v tap is for the PA feed. Both my 67s have them wired up to the selector (16, 8 and 100v). The ones Brian Wallace sells are 16, 8 and 4 (I have one in my current clone). Where on the internet did you see the ones with the 100v tap?
If you do get one of those just shrink wrap it thoroughly and carefully and hide it in the OT end bell.

[pearlrider]

Well I’ll be, that’s good information. I can use that. I’m a paint by numbers sort of builder. Learning as I go.

I know someone with a pair of Mullard EL34’s. And now I know I don’t have to build another amplifier, I may buy them. Saying that though I don’t need much of an excuse to build another amp. In fact I think I’ll build one anyway. But in the mean time I can try out a pair of Mullards in one I’ve already built. This is my latest build.
Have a look at some of the other pictures on the Shack. The pictures with the red boards are my number one and the pic's with the brown are my number two. The boxs all look the same. The speaker cabs are, one with 55Hrz G12H's and Celestion blue in the other.

The decision on what OT transformer to put in my JTM50 is going to be more difficult than I thought. I don’t want the 100 volt tap. The 4Ohm would be more useful. The information you’ve given me on the -139 pushes me more in that direction. It may have been the sag from the rectifier tube that made the difference in yours. There is plenty of time to make that decision. The split cathode thing is also a good idea, I was thinking about that. I wasn’t sure how well it would go in a JTM50

Brian Wallis’s -128’s look tempting. I need more information on what things sound like, -128 or -139. This is the link to the -128’s on e-bay. http://www.ebay.com/itm/Drake-Output-Tr ... 19d8e421ad" onclick="window.open(this.href);return false;


Thank's

[OnTheFritz]

The 100 volt tap is buried on the Marstran 128. It apparently affects the tone, but it's not used in any way.

[pearlrider]

The 100 volt tap is buried on the Marstran 128. It apparently affects the tone, but it's not used in any way.

I have heard about the Marstra 128 OT’s having a buried 100 volt tap. They look good and have a good rep. I don’t see them on the Marstan web site though. I’ll have to send Marsran an e mail.

[Brian Wallace]

Then I read things like this from, 45 Degrees of Tone.

To get our ears accustomed to the JTM45 sound, we began by firing up our ’65 head with a Les Paul. Normally, this head has EL34s in it, but we borrowed the Genelex KT66s from the Mojave and biased the amp to accommodate them. It made sense to us to use KT66s, because they were what the amp was designed for. With everything looking good, we flipped it from standby and beheld the beauty of this vintage masterpiece.

Pearlrider,

The truth about the 1965 Marshall used in the 45 degree's of tone is that the amp did not have the proper output transformer in it. It had a 784-139 OT and this explains why it had the EL34 tubes in it. So in the history of Marshall tube use, the group was taking the correct tubes out and replacing them with the wrong tubes.

[pearlrider]

Then I read things like this from, 45 Degrees of Tone.

To get our ears accustomed to the JTM45 sound, we began by firing up our ’65 head with a Les Paul. Normally, this head has EL34s in it, but we borrowed the Genelex KT66s from the Mojave and biased the amp to accommodate them. It made sense to us to use KT66s, because they were what the amp was designed for. With everything looking good, we flipped it from standby and beheld the beauty of this vintage masterpiece.

Pearlrider,

The truth about the 1965 Marshall used in the 45 degree's of tone is that the amp did not have the proper output transformer in it. It had a 784-139 OT and this explains why it had the EL34 tubes in it. So in the history of Marshall tube use, the group was taking the correct tubes out and replacing them with the wrong tubes.

The authors of the article, 45 degrees of sound, did like the sound of that particular amplifier. Although a JTM45 with a -139 and a pair of KT66’s dose make it a rear beast. Not the traditional set up.

I am looking to make a JTM50. I am trying to work out what would be the best combination of tube and transformer. Of course EL34’s, but what transformer, or even transformers. Was there a difference in the power transformer? If so I would like to talk to someone who could set me up with a good matching set. A power, a -128 and a 1999 choke. I would be happy to make an accurate build using a matching set. A matching set from someone who knows transformers. If not, I may go with the Metro power -55, a -139 from whomever and a 5 Hy choke from MM. These are my transformer chooses for my JTM50.

This build will be a departure from the Metro instructions, and my strict following of those instructions. So I am giving this one a little bit more thought.

[Brian Wallace]

It wasn't a rare beast, it was a JTM45 with a replaced OT. That OT being the 784-139 which wasn't made until 1967.

The correct transformers for a tube rectified JTM50 are the 784-128, 1202-118 and the c1999.

[pearlrider]

Is there that much of a difference between the 1202-118 and the 1202-55?

Is the B+ higher in one or the other?

As far as I can see, Magnetic Components only provide one power transformer for both the JTM50 & the JTM45. Is the 55 a good sub for the 118. Given the difference in output power you can get in the 55.

I think It may be fun to just role the dice and let my ears decide on this one. I'll let you know how it go's.

[Brian Wallace]

The 1202-118 B+ will be around 420VDC while the 1202-55 B+ will be around 455VDC. The 1202-55 also has a lower current capability so it will sag more than the 1202-118. It's up to you as to what one you want to use.

[pearlrider]

The 1202-118 B+ will be around 420VDC while the 1202-55 B+ will be around 455VDC. The 1202-55 also has a lower current capability so it will sag more than the 1202-118. It's up to you as to what one you want to use.

The point you raise about the power transformer is interesting.

As it is I am getting 400 B+ from my Heyboer, Metro 1202-55. I live in the UK and get 245vac at the wall. I have used both JJ and TAD GZ34’s, and I get this on both my JTM45 builds. Is it true that the more B+ the more head room, the less B+ the less head room.

I am also interested in the current capability point you raise. You say that the -55 has a little more sag. Could you expand on this? This could be useful to know. Adding a little more sag to a JTM50, low B+, a pair of EL34’s and a -139 OP, that mite not be a bad thing.

Having the knowledge to change the subtleties of an amplifier must make for a better build. Also the availability of good parts, transformers and tubes especially. I often wonder how far we can go with the tube situation the way it is. The tube situation is a discussion for another time.

Also knowing how an amp will sound before I start could save me some time and money. I can see in time I’m going to be knee deep in home made amplifiers. I’ll be like that grand mother who knits woolly jumpers and gives them out at Christmas, except I’ll be handing out amplifiers. Well practise makes perfect.

[neikeel]

Most modern GZ34 are nowhere near as good as a new Mullard GZ34. It will give you 20 more volts of B+ than the Sovteks etc. I have a cheapo one that I use to set up and test. Once all is running I always use a Mullard GZ34.

The sag referred to is what happens when the amp is running hard, the sound softens slightly and you get a richer sound and I personally think more harmonics too.

I have a Metro 45 that I built, as well as a JMP50 lead (with Marstrans) made with a 2 in 1 preamp. If you have a JTM45 why not build a JMP50 with a -139 OT, -118 PT (get one from Marstran with 5v taps so you can use a GZ34 if you want. If you are really clever add a 2 in 1 pre amp and use Thorlief's switching so you can use diode or valve rectification, use EL34s. Then you will have all bases covered!

[pearlrider]

Most modern GZ34 are nowhere near as good as a new Mullard GZ34. It will give you 20 more volts of B+ than the Sovteks etc. I have a cheapo one that I use to set up and test. Once all is running I always use a Mullard GZ34.

The sag referred to is what happens when the amp is running hard, the sound softens slightly and you get a richer sound and I personally think more harmonics too.

I have a Metro 45 that I built, as well as a JMP50 lead (with Marstrans) made with a 2 in 1 preamp. If you have a JTM45 why not build a JMP50 with a -139 OT, -118 PT (get one from Marstran with 5v taps so you can use a GZ34 if you want. If you are really clever add a 2 in 1 pre amp and use Thorlief's switching so you can use diode or valve rectification, use EL34s. Then you will have all bases covered!

The GZ34 situation is dire. I’m looking for a good Mullard or two. I had a TAD in my number one. It lasted less than 30 hours. By the time I took it out, it was down to less than half of its power. I’m using two JJ’s as of about 5 hours use ago, one of each in my two builds. I have read that they are the best of the Chinese GZ34’s. Saying that though, I can’t be to down on the new valves, the TAD KT66’s are not too bad.

I have read that sag refers to the amplifiers lack of ability to react to power demand. It looks to me Marshall was trying to reduce sag and compassion as they went from JTM to JMP. This is something I would like to explore. I would like build these three amps and find out how it all worked.Two amps I should say, I've built one of the three. I do like sag and compression, but then again I was a punk, so I also like it a bit knarly.

I think rather than put a switchable rectifier in one of my amps, I would just build two amps. My clean and dirty channel is two different amplifiers as it is. Two builds twice the fun. I am more of the Ken Fisher school of amp building, less is more. I would be more likely to take things out than put them in. Could you tell me what a 2 in 1 pre amp is, I don’t know what that is.

I think most of all I would like to build simple amps of each type, JTM50 and JMP50, transformers from Marstan or Merrin, Sozo’s, Allan Bradleys the usual good quality parts. A pair of projects for the next two years.