1965 JTM45 with EL34s: Dropping Resistors?

[Trouserpress]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?. Between V4 and the board you can see 3 grey components (most likely resistors?) that might probably have been put in to take some high voltage stress off of the power tubes.

I'm no engineer and not quite sure if that would be a proper arrangement. Please take a look and tell me what you think. Thanks,
Frank

 

[Creach]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?. Between V4 and the board you can see 3 grey components (most likely resistors?) that might probably have been put in to take some high voltage stress off of the power tubes.
View attachment 158666
I'm no engineer and not quite sure if that would be a proper arrangement. Please take a look and tell me what you think. Thanks,
Frank

Hello Frank,
It should be a 1K resistor going to V4, and from this 1K a 470R on V4 and a 470R on V5.
On your picture the 1K resistor was replaced by the grey resistors and the 2 x 470R was replaced by a wire.
Here is a schematic and a picture of a JTM45 (with kt66) from 1965 that i saw a few years ago.

 

[Trouserpress]

Thanks Creach, so the original 1k(2w) in series with 2x 470R(1w) have been replaced by 3 assumably sturdier grey resistors and 2 leads. I wonder what value these might be and if that’s really all you have to do to complete a KT66 to EL34 conversion….???
I would have to buy it to get some more information (I‘m interested in doing so).

 

[Creach]

Thanks Creach, so the original 1k(2w) in series with 2x 470R(1w) have been replaced by 3 assumably sturdier grey resistors and 2 leads. I wonder what value these might be and if that’s really all you have to do to complete a KT66 to EL34 conversion….???
I would have to buy it to get some more information (I‘m interested in doing so).

Right after the JTM45 is the JTM50 with el34.
The difference is JTM 50 has no resistor at all.
But for el34 a 1K can be placed instead of the 470R (and no need to have the first commun 1K).

 

[Helmholtz]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?.
View attachment 158666

Judging from the OT wires this amp has a Drake OT.
Either a 784-103 (8k) for KT66s or a 784-128 (3.5k) for EL34s.
If intended for EL34s the bias supply should have a 220k series resistor.

 

[Trouserpress]

Here's a foto from above. Does the shape tell what kind it is?

 

[Helmholtz]

Sorry, no.
Drake 103 and 128 have the same shape.
But it's obviously a Drake.
What are the secondary wire colors?

 

[2L man]

If options are those two the OT Winding Ratio/impedance is very easy to measure in few minutes. Remove power tubes and rectifier tube and there does not come HV to secondary circuit***. Then bend two wires to U and push between 2 and 3 on other power tube socket and between 7 and 3 on another. Then turn mains switch On and there come 50Hz or 60Hz sound from loudspeaker when filament AC go to OT primary thru Anode sockets. Disconnect loudspeaker and measure output AC voltage and AC between U-wires.

***Keeping HV/stby switch Off make that double safe and removing HV fuse make it triple safe.

 

[ThreeChordWonder]

I hope this helps rather than clouds the picture, but...

On my Mojotone kit, there are 470k 1-watt resistors anchored in unused (with EL34s anyway) pins #6 and terminated at pins #4. Both are fed off a 1K 5-watt ceramic / cement resistor on the turret board to pins #6 and thence through the resistors to pins #4.

Some suggest increasing the 470s to 1K, which drops the screen grid voltages further, and which therefore, presumably, drops the output of the tubes slightly, making them more reliable or last longer, I don't know, not an expert by any means.

I've also seen a comment somewhere that the early designs didn't use 470s or 1Ks, but instead put the full voltage on pins #4.

The comment said, however, this was with "robust, early" (I presume BBC or military spec) EL34s.

I wouldn't risk that with contemporary ones.

I would also suggest using at least 2-watt, probably 5-watt items, as the feeding resistor is rated for 5. The current is supposed to split evenly, but any imbalance, or if a tube blows, could see more (or all) of the current going to one tube.

 

[NickKUK]

I remember on a schematic on the bluesbreaker that:
* tube meant kt66
* solid state meant el34s

No idea idea if this correct.

 

[ThreeChordWonder]

[Edited - mainly for typos]

Marshall started developing the JTM45 in 1962, using 5881 tubes and 6L6 tubes if they couldnt get 5881s (American made, 1960s UK import restrictions, etc., etc.). In 1964 they switched to KT66s. Those are classic "Mk II" models, I believe, but don't quote me on that. In 1966 they switched to EL34s. Also in 1966, they started calling the amp the JTM50 and or JTM50 Mk III. The JMP50, however, is a completely different circuit.

The first amp to use solid state (diode) rectifiers were the 100 watt versions which started appearing in mid 1965, and first used by some bloke called Pete Townshend. EDIT - The JMP50 (as opposed to the JTM50) also switched to diode rectification.

More here:

History of the JTM50

The Black Flag JTM50 is a rather rare and coveted after amplifier made by Marshal between 1966 and 1967. It was one of the transitional steps between the JTM45 amps and the plexi panel JMPs and later metal faces. During the short period of time it was produced, the JTM50 saw a few...

solodallas.com

And here:

The first 100 watt Marshall

These first 100 watt amps are known today as JTM45/100 because they had the same Plexiglas control panel as the JTM45 (also bearing the label “JTM45”) on a slightly larger chassis which was able to accommodate four KT66, as well as a rear panel on which t

www.fuzzfaced.net

 

[Trouserpress]

Sorry, no.
Drake 103 and 128 have the same shape.
But it's obviously a Drake.
What are the secondary wire colors?

Could you tell from the picture in post #1? It’s alI have.

 

[M0usey]

Hi. This is a photo of my block end '66 JTM. It's had EL34s since I bought it in about 1979. It has 1k resistors across the power tubes and a Drake 784-103 8k op transformer. The long green resistor is 150r. It's still in regular use, getting gigged 3 or 4 times a month. Hope this helps.

 

[ThreeChordWonder]

^ Very nice. Some more gut shots would be great if you have the chance.

Also, my circuit shows 5.1K resistors on the conttol grids (pins #5). Does yours have these?