Is this a JTM 50 or something else???

[Laroosco]

I bought an amp recently that a friend built.

He told me that it was basically a JTM50 circuit. Now come the questions

Is the JTM50 different from the 1987 50 watter?

Here's a schematic. It says in the notes that it's based off of a 1959 cicuit, but are the 1959 & 1987 different other than wattage??

Thanks

[Roe]

that no jtm50. it's more like jmp50 lead from 68 or later

[Laroosco]

Good to know.

The amp sounds killer either way. Now to upgrade the trannies

[LordAli]

As I noticed, JTM50 use 100V tap at OT and at least one other tap (usually 16 or 8 ohm - but I'm not exactly sure the impedace). JTM50 used mainly 784-128 OT. Moreover, some JTM50 schematic shows that it use GZ34 tube rectifier.

I built JTM50 amp and after 2 months my tube rectifier went to tube heaven because it is very overloaded by 2xEL34s. GZ34 can supply 250 mA max. by datasheets and if you drive your amp to 50 watts your amp want to drain 310 - 320 mA. This was possibly reason why JMP50 was born with silicon rectifier, which can produce such current to avoid GZ34 paralleling.


In conclusion - main differences between JTM50 - JMP50 :
1) tube rectifier (JTM50) / silicon rectifier (JMP50)
2) 100V tap at OT - 784-128 and some other tap (JTM50) / 16,8,4 ohm taps at OT - 784-139 (JMP50)
3) PT have 5VAC winding for heater for GZ34 (JTM50) / JMP don't have this winding - don't need it and also JMP50 have other voltage levels, because silicon diodes don't load PT so much like GZ34.

[Roe]

unsurprisingly there are lot of exception, but that's the main tendency yes

[JDW3]

Will a JTM50 always burn up rectifier tubes if ran full tilt? I just built a hybrid Metroamp consisting of a JMP50 circuit with tube rectifier and JTM45 PT.

Not fired up yet, but will be within a few days.

As I noticed, JTM50 use 100V tap at OT and at least one other tap (usually 16 or 8 ohm - but I'm not exactly sure the impedace). JTM50 used mainly 784-128 OT. Moreover, some JTM50 schematic shows that it use GZ34 tube rectifier.

I built JTM50 amp and after 2 months my tube rectifier went to tube heaven because it is very overloaded by 2xEL34s. GZ34 can supply 250 mA max. by datasheets and if you drive your amp to 50 watts your amp want to drain 310 - 320 mA. This was possibly reason why JMP50 was born with silicon rectifier, which can produce such current to avoid GZ34 paralleling.


In conclusion - main differences between JTM50 - JMP50 :
1) tube rectifier (JTM50) / silicon rectifier (JMP50)
2) 100V tap at OT - 784-128 and some other tap (JTM50) / 16,8,4 ohm taps at OT - 784-139 (JMP50)
3) PT have 5VAC winding for heater for GZ34 (JTM50) / JMP don't have this winding - don't need it and also JMP50 have other voltage levels, because silicon diodes don't load PT so much like GZ34.

[908ssp]

I know yu guys don't want to hear this but JTM50 is not really a model name. In other words there is no one thing as a JTM50. There are at least three different amps that went under that name and the name had nearly nothing to do with a change in specification. When Jim had a falling out with Terry he took Terry's name off the amp.
Jim, Terry Marshall = JTM
Jim Marshall Products + JMP

Check the spec page on Marstran site you'll see they used that name for a wide variety of specs.

What has wrongly come to be the commonly thought of as JTM50 is in fact a amp with two EL34s and a tube rectifier but this is only one of the many specs that amp had.

[neikeel]

There are a number of odd (or should I say non 50watt Marshall) things about the amp:

-the use of series 100uF 350v mains filters
-the use of single large bias cap
-clipping diodes on the output tubes
-100k resistors to ground on the heater supply
-fender style cap on additional switch/polarity
-switchable pre-PI master volume
-lowered grid resistors on V1

Very much a hybrid design with lots of interesting features, some of which are good (lowered grids, switchable MV) and some a bit strange as I prefer Post PIMV edon't usually like diode clipping and do not see the point of polarity switch and cap (IMO of course!!). I imagine it sounds very 80's in its tone.

[Laroosco]

It doesn't sound 80's at all to me..dialed in with a Gibson it sounds like Billy Gibbons.

It has a Pentode/Triode switch on the back, I always leave it in Pentode mode because he Triode mode sounds too buzzy..could this be using the clipping diodes??

I was also told that this amp is Cathode bias. Don't know how to confirm that without taking it to someone and I've never changed tubes. It run's hot like my Matchless Clubman used to

[Flames1950]

The schematic is not cathode biased. So if that schematic is what you've got, it is a fixed bias amp, and a bias adjustment will be needed every time you switch power tubes.

[PCollen]

Pretty much a modified JMP MV-style amp from V2a onward.
Pre-amp filtering is lowered : 20 vs. 50.
JTM 270K mixer resistors, vs. 470K
The input jack resistor network feeding the V1a and V1b grids is odd
I like the individual bias adj. for each tube.
Switchable MV, and effects loop, other minor mods.

[joegagan]

i have the earlier version of this tim gagan build. tim is my brother. i have not asked him specifcally, but i am pretty sure the diodes to ground off the power tubes are spike protection, not affecting sound. this is a ken fischer ( and others i'm sure ) method of spike protection.

i have mistakenly called mine a jtm45 clone, the same way someone new at starbucks yells out 'redeye' to sound cool.

mine is built in the same type acoustic amp, but the one-12 combo version. it sounds excellent. 16 years and going strong.

[joegagan]

laroosco, the pentode/triode switch is not shown on the schematic you posted. i will ask tim if he has any notes about your specific amp. i also recall he did a few mods to yours that are different from mine- not sure what the exact differences are.

edit-update, below is the actual schem for the amp laroosco owns.
tim built mine first, then 2 years later built the head version in a 2nd acoustic G60. tim later sold it to john fink. i think laroosco got it from john.



more clear schem:
http://www.tigergagan.com/images/schema ... inkall.gif" onclick="window.open(this.href);return false;