1986, JTM50, Early Spec 1959 or JTM45/100..........

VICOwner

Gold Supporting Member
Messages
1,847
Well, I got the power transformer and choke for the 45/100 today. I’m shipping 56Tweed his JTM45 on Monday. I worked today until 4pm and got home to the Marstran shipment. The 1204-43 is huge. Weighs 13lbs. Checked voltages and I’m getting unloaded voltage of 388-391 on the low taps and 448-451 on the high taps. We’re using the low taps for tube reliability but I’d sure love to get another build and put KT88’s in it with the high taps. I’m disappointed because I had really nice vintage boards made for both these amps and found out that the material became conductive. I’ve ditched all the stock of that material because I don’t trust it. Sucks because I had 3 other boards fully perfed and Harwin turrets installed. I’m back to FR4 and G10. I’ll report as I make progress now on the 45/100. I have the transformers installed and wired. I’m finishing up the flying leads on the power board and getting ready to put both boards in. More tomorrow.
 

VICOwner

Gold Supporting Member
Messages
1,847
Under what circumstances does a board become conductive? Thanks!
I can’t tell you why exactly that it happens other than the dielectric strength goes away. The material I had is called a few different names. It could be called micarta kite (epoxy paper phenolic), paxolin, Tufnol. I’ve had some epoxy linen. I guess some of the fibers get conductive.
The G10 and FR4 is epoxy fiberglass based and has superior dielectric strength. I just like the color of the paxolin. Wish someone would mix up a color batch of FR4 that was just like the old Marshall boards.
 

VICOwner

Gold Supporting Member
Messages
1,847
Well here it is. Since I got the transformers yesterday and I was eager to get working on this amp, I finished putting it together. I’m tired and my ears are ringing. The only mishap was that I installed the bias supply diode backwards and during pre checks I had positive voltage in the bias supply. Loud!
My JTM50 makes 60 watts clean. This 45/100 is 72 watts clean and 121 full output but it’s knocking boxes off my shelves. Haven’t played it long but it’s impressive. Have to put the aluminum shielding plate down inside the cabinet. Just ready for a nap. Here’s a few pictures.


 

glogulus

Silver Supporting Member
Messages
906
Well here it is. Since I got the transformers yesterday and I was eager to get working on this amp, I finished putting it together. I’m tired and my ears are ringing. The only mishap was that I installed the bias supply diode backwards and during pre checks I had positive voltage in the bias supply. Loud!
My JTM50 makes 60 watts clean. This 45/100 is 72 watts clean and 121 full output but it’s knocking boxes off my shelves. Haven’t played it long but it’s impressive. Have to put the aluminum shielding plate down inside the cabinet. Just ready for a nap. Here’s a few pictures.


Oh man! That looks great! Can’t wait to try it out!
 

glogulus

Silver Supporting Member
Messages
906
W
I had @VICOwner build a regular JTM45. Love the big iron, but just not sure I can handle this in all its glory. @Bluesful is the other one with an amp in progress, but his trannies are currently on a world tour.
With the advancements in reactive loads, big iron doesn’t bother me as much. I’ll see how it sounds with the attenuators I’ve got. I’m sure the jtm45 you’ve got coming is going to sound great too!
 

Bluesful

Silver Supporting Member
Messages
38,955
Well here it is. Since I got the transformers yesterday and I was eager to get working on this amp, I finished putting it together. I’m tired and my ears are ringing. The only mishap was that I installed the bias supply diode backwards and during pre checks I had positive voltage in the bias supply. Loud!
My JTM50 makes 60 watts clean. This 45/100 is 72 watts clean and 121 full output but it’s knocking boxes off my shelves. Haven’t played it long but it’s impressive. Have to put the aluminum shielding plate down inside the cabinet. Just ready for a nap. Here’s a few pictures.


Awesome.

With the advancements in reactive loads, big iron doesn’t bother me as much.
Yep, when others keep asking about 20W amps, I keep thinking about 100W amps.
 

VICOwner

Gold Supporting Member
Messages
1,847
Awesome.



Yep, when others keep asking about 20W amps, I keep thinking about 100W amps.
Not only 20 watt amps but how to attenuate it for “bedroom “ volume levels. Let’s put a PPIMV on it too, and start a discussion on why it is so lifeless. Get a 100 watt Plexi, at least a 4 x 12 and crank it. Play it for at least an hour. Now that’s an experience.
 

EXP

Supporting Member
Messages
3,157
Well here it is. Since I got the transformers yesterday and I was eager to get working on this amp, I finished putting it together. I’m tired and my ears are ringing. The only mishap was that I installed the bias supply diode backwards and during pre checks I had positive voltage in the bias supply. Loud!
My JTM50 makes 60 watts clean. This 45/100 is 72 watts clean and 121 full output but it’s knocking boxes off my shelves. Haven’t played it long but it’s impressive. Have to put the aluminum shielding plate down inside the cabinet. Just ready for a nap. Here’s a few pictures.


That looks really clean!

How much do you charge for a 45/100?
 

Bluesful

Silver Supporting Member
Messages
38,955
Well here it is. Since I got the transformers yesterday and I was eager to get working on this amp, I finished putting it together. I’m tired and my ears are ringing. The only mishap was that I installed the bias supply diode backwards and during pre checks I had positive voltage in the bias supply. Loud!
My JTM50 makes 60 watts clean. This 45/100 is 72 watts clean and 121 full output but it’s knocking boxes off my shelves. Haven’t played it long but it’s impressive. Have to put the aluminum shielding plate down inside the cabinet. Just ready for a nap. Here’s a few pictures.


Where are you getting all those old caps from?
 

VICOwner

Gold Supporting Member
Messages
1,847
The China hat retainers are on eBay and they are listed as asbestos free. I don’t know what they are made from but they sure look good. The mustard caps are sold as sets also, but they are for early 1987 spec amps and come with 1 2200pF, 6 .022uF and 2 .1uF. I don’t use the 2200uF and valvestorm had the extra .1uF caps. They are expensive but that’s what we have to have. I purchased several sets because they are getting rare. The Lemco mica caps are the same way. I’ve only found the Lemco caps for this amp but have a few RS and SRC 250pF and nos 560pF ceramic disc caps for future builds.
 

RAG7890

Gold Supporting Member
Messages
1,298
Transformers have finally arrived in Melbourne.

Home stretch.......
Great news on the transformers. It shouldn't be long now. I'm really looking forward to seeing and hearing this thing is all its glory!
I spoke to Barry about a few other things we've been working on & he mentioned the Iron was almost at his place.

Your Amp, sans Iron, is pretty much finished. Will be an early Christmas present. :)

:beer
 




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