Jtm 50 / plexi or so

Get support and show off your MetroAmp 50 Watt kit builds.

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fixer2
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Jtm 50 / plexi or so

Post by fixer2 » Mon Mar 07, 2016 8:17 am

What is the difference between a 1968 50and a1971plexi 50 Ibuilt a tube Depot jtm 45 plus and I have el 34 tubes and the both SS rectifier and tube on a switch one or the other so what gives that nice crunch the 71 50watter on metro you channel such a VH bite and Fare warring sound .I would like to build it but I don't see the transformer s on. Valvestorm what ones to get and were.

danman
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Re: Jtm 50 / plexi or so

Post by danman » Mon Mar 07, 2016 11:55 am

Transformers are not that much different between 68 and 71. They may have gone to a standup PT by that year but that won't affect the sound. Look at the two schematics and you will see a couple of differences. 71 likely has the NFB attached to the 4ohm tap with a 1ook resistor and the 68 was on the 8ohm with a lower value resistor. The 71 has less NFB with this setup and gives the amp a rawer sound. The 71 may also have a .68uf bypass cap across the cathode on v2 which will boost the mids a bit. The 71 may also have a larger bright cap on the volume pot of channel one.The circuits are very similar with only a couple of small changes in part values. In the later 70's, Marshall decided to drop the B+ voltage in the 50watters down to around 370-380vdc. The lower voltage caused the amps to break up a bit earlier and sound a little warmer also.

fixer2
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Re: Jtm 50 / plexi or so

Post by fixer2 » Mon Mar 07, 2016 10:15 pm

what type of or model number of transformers and were be some good ones

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neikeel
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Re: Jtm 50 / plexi or so

Post by neikeel » Tue Mar 08, 2016 9:09 am

First JTM45 in 1962 Elstones (5881 tubes)

Offsets and sandwich front production amps from 1963 to early 65 Radispares Deluxe OT and self lead RS Heavy Duty PT with selector on top (450v B+) with 20H RS choke. 5881 tubes later KT66s

Smooth levant and silver maroon logo 1964 only as above (KT66s). Some gold block logos with normal levant and RS trannies?

Gold Block logo first half '65 Drake 784-74 selector on side OT and 1202-55 PT (450v B+ with selector on top) with Drake 354-114 choke (KT66s)

Fisrt script logos Autumn '65 Drake 784-103 OT (selector on chassis )and same PT and choke (KT66s)

Early '67 Drake 784-128 OT, Drake 1202-118PT (420-430v B+) and 354-114 choke (alloy block chassis with JMP logo, steel chassis Black Flag logo).

Some time '67 Drake 784-139 OT and Drake 1202-133PT (High B+ ~ 490-500v)and 354-114 choke. Some amps with SS and some with GZ34 rectifiers. Mainly Black Flag steel chassis.

Late 67 Drake 784-139OT and Drake 1202-118PT with Drake354-114 choke

This continued until 68 when Dagnall C1999 started to appear. In 69 some markets went to Upright 1202-164PT voltages largely the same.

In 1970 voltages on -164s were around 400v and by late 70s as low as 360v on some examples.

This si my personal view on amps I have seen and measured myself or reliable other sources. I have not quite figured ou the precise combinations in mid 67 but that was a period when factories were moving and supplies variable.

There are eriods of overlap (my Autumn 65 JTM45 is script logo and selector on side OT and selector on top PT) If anyone KNOWS FOR SURE please update the above and we can make it a sticky.
Neil

danman
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Re: Jtm 50 / plexi or so

Post by danman » Tue Mar 08, 2016 10:51 am

Great info by Neikeel! That would make for an excellent sticky!

Fixer, if you have decided on a particular model and year that you would like to build, you can contact http://marstran.com/ for any questions on the proper transformers for the build. There are some other options also such as the Metroamp transformers available through Heyboer and Classictone models. Here is a link providing some good info; http://valvestorm.com/Transformers#50watt

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