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

[neikeel]

http://home.comcast.net/~jbjdav26/Neil% ... 202in1.pdf

Use just the preamp bit, ie the split cathode on V1, the push-pull switch is an Alpha I get from TAD in Germany. You need that swithc and an extra couple of caps. With the pre-mv master up full it is stock 1987 (Lead JMP50). Pull the pot out and V1 is then cascaded, balance the gain and MV to taste, it sounds just like a 2204. You can still use the lower input on the lead channel for strats or the lower input below the pot for 2204 lo input if you wish.
It is not fancy or complicated the way I set it out. Steve (SDM) drew the alternative one for people who want to tune it for more gain, but I like stock 2204 as it has plenty of gain for me (I am still a 'Part Time Punk' )

[pearlrider]

http://home.comcast.net/~jbjdav26/Neil% ... 202in1.pdf

Use just the preamp bit, ie the split cathode on V1, the push-pull switch is an Alpha I get from TAD in Germany. You need that swithc and an extra couple of caps. With the pre-mv master up full it is stock 1987 (Lead JMP50). Pull the pot out and V1 is then cascaded, balance the gain and MV to taste, it sounds just like a 2204. You can still use the lower input on the lead channel for strats or the lower input below the pot for 2204 lo input if you wish.
It is not fancy or complicated the way I set it out. Steve (SDM) drew the alternative one for people who want to tune it for more gain, but I like stock 2204 as it has plenty of gain for me (I am still a 'Part Time Punk' )

Nice diagram, copy that for future reference.

Although the added switch thing wouldn’t be for me, the split cathode is of interest. I have thought of adding the preamp from a 70’s JMP lead to a JTM. I find the normal channel a little heavy. I would like to have a usable bass control. Most users turn the bass to zero; I don’t get much above two. I do use it sometimes when I play clean. On my number one I have a 100pf on my bright channel. I use my number one for clean and for the Stratocaster. The extra depth gives the Strat that Hendrix sound, it’s addictive. The fact it has Brimars and GEC’s in it help. I’m looking for a Mullard GZ34 to make the set. The GEC’s are not well balanced though. It still sounds good.

Back to the tube and transformer thing. What other tubes can you use with a 8k or 3.5 transformer. What about KT88, 5881, 6L6 and the 6550. Are these all tubes we can put in the JTM, and which one’s for which transformer. And something I have been wondering for a while. Can I use Yellow Jackets in my JTM, and put EL84’s in.

Once a punk always a punk, Lou Read, Patty Smith, The Clash, and then I moved on to Hawk Wind and Hear & Now. But still a young punk at heart. You know I don’t have one Clash or Hawk Wind record in my collection any more. Time marches on.

[neikeel]

Back to the tube and transformer thing. What other tubes can you use with a 8k or 3.5 transformer. What about KT88, 5881, 6L6 and the 6550. Are these all tubes we can put in the JTM, and which one’s for which transformer. And something I been wondering for a while. Can I use Yellow Jackets in my JTM, and put EL84’s in.

Yes to all of the above.
5881 and 6L6 are better suited to the JTM45 OT but can be used in a 3.4k (I think 6.6k taps on an RS OT are for these)
The KT88 and 6550 can be run with the 3.4k OT but the bias feed grid resistors (usually 220k) should probably be lowered to between 82k and 150k to taste and the bias resistors invariably need changing to get in range.

Once a punk always a punk, Lou Read, Patty Smith, The Clash, and then I moved on to Hawk Wind and Hear & Now. But still a young punk at heart. You now I don’t have one Clash or Hawk Wind record in my collection any more. Time marches on.

Still have all my old stuff, vinyl, upgraded to CD and now on i-pod. Even have all the odd stuff I used to have on C-90 tapes (we used to swap and borrow vinyl form each other to record as the albums were too expensive when I was at school) on my i-pod, the music survives although the attitudes are manifest in cynicism and suppressed nihilism now rather than dress codes etc. Saw Patti Smith again recently and couple of years ago New York Dolls! Still plenty of lively newer bands around too. Odd never really thought of Hawkwind as punk tho'.

[pearlrider]

Wow yellow jackets, I wonder if the JTM will sound like a 1974. I’ll have to take a closer look at them. I haven’t seen a thread on the Metro site for them. I think someone should take one for the team and try them out.

I have read about the 6.6K Radio Spares transformer. I think that that is a one for when I have built a few. But still high on the list of possible builds. It’s not so much the complexity of the build; it’s more the difficulty in getting the parts, rear panel without the impedance selector or a chassis which needs no back panel. There is also the transformer to find. I would love to do an off centre with one of Merrin’s Radio Spares transformers in it, the one with the peg board. This is why I like the JTM & JMP, infinite possibilities.

The Clash to Hawk Wind thing was my way of saying that my tastes haven’t stayed the same. I think I was into Joy Division somewhere between those two. In fact I think the first band I was ever in to was The Bay City Rollers, but we can just keep that one between me and you ‘ay *. I’ve still got some Joy Division, no Bay City Rollers though. If I did make a 6.6 model I would like to hear it with the 6550 valves.

[pearlrider]

I didn’t think I would get to start a new build for at least another few months. But then hey presto, the tax man gives me a big load of my own money back and I’m off again. I think I’ll call this one tax man.

I have just ordered a fine set of transformers from Marstam. I have ordered a 1202-118, a 784-128 and a 1999. So now I think I may get my answer. I can hear for myself the difference between the transformers and two tube sets. I chose this set up because Marshall used them and I can get a matching set from Marstran. The two amps, side by side, same caps, same resistors, same everything, except transformers and power valves. Even the same ham fisted builder. It will be interesting to hear how big a difference there is.

I’ll let you know how it turns out.

[pearlrider]

I got my answer, and wow what an answer.

[neikeel]

Ok....spill!

Was I right or was I right

[pearlrider]

Hi Neikeel

Well what can I say, it’s sweet, woody, and well worth the work I put in. It’s a very different sound to the other JTM’s, not as much sparkle and depth of bass. This JTM50 is loud and aggressive when pushed, not quite as creamy as the KT66 powered amp. An acoustic element when played clean and a powerful bass when pushed hard, a very characteristic Marshall sound. Neikeel, you were right about the biasing, it’s biased at 35, this made it that little bit softer and sweeter, and this JTM50 is a sweet sounding amp.

Have a look at the wiring. The capacitor next to the rectifier tube is a 16-32uf. The 16uf post is the post closest to the rectifier tube. The wiring between the board, the power tubes, and the 16-32 cap gave me some cause for thought. I went for the same wiring as is in my JTM45. Other than that the rest is as you would expect. The wiring is JTM45 except for the 32-32uf on the board and the 16-32uf next to the rectifier tube. There is also a 560pf on the bright channel mix resister, a 100pf on the number one volume and the V1a cathode capacitor is a 220uf. There is also two pair of 68k input channel resisters. Take a look at the wiring and tell me if it could have been dun differently, or better.

One thing, the ac on my heaters is 2.8, is this within tolerance. My voltages are always a little out. Ill have to make up a voltage chart on this build and post it here. The Allan Bradley’s are all OK, I tested them. Maybe my meter is out.

I should thank the usual suspects Metro, Valve Storm, Marstran, and all the posters on this forum who helped me. Another tone tapped, another mystery solved. And now on to my next build. The next build will be a stock pre 1970 lead JMP50. And then after that, a 1962 radio spares loaded JTM45 with 5881’s. My goal, other than world domination, is to build the first decade of sub 50 watt Marshall amplifiers, or even include the 1970 and make it the first 10 years. Well what do you think? May be world domination would be less mad.

[neikeel]

Hi Neikeel

Well what can I say, it’s sweet, woody, and well worth the work I put in. It’s a very different sound to the other JTM’s, not as much sparkle and depth of bass. This JTM50 is loud and aggressive when pushed, not quite as creamy as the KT66 powered amp. An acoustic element when played clean and a powerful bass when pushed hard, a very characteristic Marshall sound.

.
That is the important bit!

Have a look at the wiring. The capacitor next to the rectifier tube is a 16-32uf. The 16uf post is the post closest to the rectifier tube.

I played back and forth and found 16uF for screens and 32uF of that cap for the PI worked best for me - more tactile and pick response that way.

There is also a 560pf on the bright channel mix resister, a 100pf on the number two volume and the V1a cathode capacitor is a 220uf. There is also two pair of 68k input channel resisters. Take a look at the wiring and tell me if it could have been dun differently, or better.

I copied my original and had a 100pF tubular ceramic on vol 1 pot and like you 560pF (closesst to 556 on original) over 270k mixer and 250pF in tone stack.
I found 270pF on tonestack works wel, and if you have used the 32/32 with extra 10k in the preamp try 470k mixers instead.

One thing, the ac on my heaters is 2.8, is this within tolerance. My voltages are always a little out. Ill have to make up a voltage chart on this build and post it here. The Allan Bradley’s are all OK, I tested them. Maybe my meter is out.

No the only thing that influences your heater voltage is the ratio between the mains input and your chosen mains tap. What mains tap are you using and what is your wall voltage.

The next build will be a stock pre 1970 lead JMP50. And then after that, a 1962 radio spares loaded JTM45 with 5881’s. My goal, other than world domination, is to build the first decade of sub 50 watt Marshall amplifiers, or even include the 1970 and make it the first 10 years. Well what do you think? May be world domination would be less mad.

I have a similiar task!
So far have a couple of JTM50s (one original and one the clone in DIY section) an orignal '68 JMP50 (lead), a 2204 clone with original trannies, 1987/2204 switchable with Marstran trannies, 66 JTM45 clone, offset clone (with 5881s and orignal RS OT). Also have a 69 master PA that is going to go into a standard headbox and special chassis as a hotrod in sheeps clothing (mixing my metaphors!!). I have the 100watters and th edifferent 4x12s covered pretty much too, but that is another thread

[RussB]

I just want to say that is a beautiful build pearlrider! Nice work and attention to detail

[pearlrider]

Thanks’ RussB

I have to do it that neat so that I can stay on top of what it is that I am doing. If I didn’t I would loose track and make mistakes. It makes learning as I go easier, I can see where the wires are going.

I’m also interested in documenting my builds. Keeping it neat and tidy and only changing the necessary parts gives me the answers I need.

It’s still good to know people are looking and appreciating my work.

At this moment in time I’m making a voltage survey of my JTM50. Ill post it when I’m dun.

[pearlrider]

Voltages testing for JTM50 build.

All the voltage samples were taken with the valves in, the volumes set to zero and a 60 watt cab connected. The bias was set at 35 mille volts.
Incoming power at the on-off switch was 246vac. The output from the output transformer was, at the hot tap 332vac - 332vac and at the cool 294vac - 294vac. The heater tap for the rectifier valve, taken at the transformer was 4.65vac and the other valve heater tap was 2.84vac. With no valves in, that rose to 2.86vac.
The valve pin voltages are;
V1 pin 1 200vdc, pin 2 signal, pin 3 1.785vdc, pin 4 2.72vac, pin 5 2.72vac, pin 6 200vdc, pin 7 signal, pin 8 1.796vdc, pin 9 2.71vac.
V2 pin 1 159vdc, pin 2 signal, pin 3 1.464vdc, pin 4 2.73vac, pin 5 2.73vac, pin 6 277vdc, pin 7 159.7vdc, pin 8 167vdc, pin 9 2.72vac.
V3 pin 1 228vdc, pin 2 signal, pin 3 85vdc, pin 4 2.73vac, pin 5 2.73vac, pin 6 223vdc, pin 7 signal, pin8 39vdc, pin 9 2.72vac.
V4 pin 1 ground, pin 2 2.74vac, pin 3 424vdc, pin 4 419vdc, pin 5 -32vdc, pin 6 424vdc, pin 7 2.73vac. pin 8 ground.
V5 pin 1 ground, pin 2 2.75vac, pin 3 424vdc, pin 4 419vdc, pin 5 -32vdc, pin 6 424vdc, pin 7 2.74vac. pin 8 ground.
V6 pin 2 425vdc and 4.65vac, pin 4 332vac, pin 6 332vac, pin 8 425vdc and 4.65vac.
All measuring equipment was tested and found to be within tolerance.

One other point, V1 pin 1, with no valves other than the rectifier valve, showed a voltage of 411vdc. And also a band new JJ GZ34 put the B+ up to 460vdc. These JJ’s are all over the place. It would be nice if they settled around the 440 mark.

Well there you go folks; I hope you don’t find anything too bad in there. If there is any thing wrong, all as I can say is, it sounds good to me. But then maybe I should get out more.

[neikeel]

Which PT primary are you hooking up to with your mains?
The Marstran PT has 100, 110, 120, 220,230 and 240v options.
If you have 246vac in and using the 240vac tap with a B+ of 460v (ie quite high) I am surprised your heaters are so low.
Both mine have approx. 6.3vac on the heaters but 420v on the output plates (using brand new Mullard GZ3s using the same connections.

[pearlrider]

Which PT primary are you hooking up to with your mains?
The Marstran PT has 100, 110, 120, 220,230 and 240v options.
If you have 246vac in and using the 240vac tap with a B+ of 460v (ie quite high) I am surprised your heaters are so low.
Both mine have approx. 6.3vac on the heaters but 420v on the output plates (using brand new Mullard GZ3s using the same connections.

I am using the yellow input on the transformer. The Marstran website has that as the 240vac primary.

The 460vdc output was from a brand new JJ GZ34, put in after the test was dun, and was not the JJ GZ34 used in the test. The valve in the test, as can be seen in the test, is giving 425vdc. This valve when first put in gave a reading of 447vdc. May be they just need time to settle in.

I’m not unduly alarmed by the heater voltages. I was thinking something could be draining the power, a short or something. But then the transformer runs cool and silent and the amp sounds good. The only thing I can think of is, to pull the transformer and bench test it. Or I could get another set, build that JMP and see what voltages I get out of that. Now that sounds like a plan. Ill let you now how it go’s. In the mean time I will keep poking at this one. That’s all for now folks.

[rgorke]

So, which do you like better, the lower or the higher voltage?