JTM1 Impedance Issue

[Littlewyan]

I built myself a JTM1 not too long ago which has a low power switch taking the amp from 1W down to 25mW I believe. Now on the low power setting the amp sounds great, nice Marshall grind with a sharp sound but as soon as I switch to the high power setting the notes go out of focus and sound so soft that there is no grind or sharpness and it just sounds pants like it has a bad valve or something.

I did some testing and I think the low power switch is mismatching the load which is why it sounds good. The OT has a 22.5K Primary (Amp is supposed to have 18K) and I found that in low power mode if I mismatch the primary by doubling it the amp starts to get that out of focus sound. But I've tried halving the load when the amp is in high power mode and it gets a bit better but no where near as good as the low power mode.

I should add that when the amp is in high power mode the low power circuit is completely out of the circuit. Any ideas? At the moment all I can think to do is replace the Output Transformer.

[jazbo8]

So if you double the load for the low power mode, it sounded good, but neither straight or halved load for the high power mode, sounded good, is that right? What happens if you double the load for the high power mode?

I am using a small 10k OPT with 16R speaker connected to the 8R tap, so Ra-a=20k, it sounds ok to me, I don't have the low power resistor divider installed... Since I am not a good player so I may not be in the position to judge whether if it really sounds good or not...

[Littlewyan]

I'll have to try and do audio clips. Basically low power mode sounds great, high power mode naff. Double the impedance on low power mode and it still sounds ok but sounds closer to high power mode. Halve the impedance on high power mode and it still sounds naff. I should add I am using an external attenuator on high power mode but I did try it without and still the same.

I mean the difference is night and day between the two modes.

Edit: Maybe the amp is supposed to sound the way it does in high power mode, as if you watch the Marshall demo it is a very soft sounding amp. Think thats because of the use of Triodes in the output stage.

https://www.youtube.com/watch?v=8P2OmLNUAvE

Its only with chords it sounds bad. The JTM1 in the demo sounds a bit sharper than my amp, probably due to the slightly lower impedance.

[jazbo8]

I see, it is a bit odd, since the problem is the reverse of what typically happens, i.e., when the output is lowered, the sound gets worse. In this case, the low power output actually sounds better than the high power output. Interesting, will have to look into it a bit...

[Littlewyan]

I think the low power mode is mismatching the sound so much that its making it sound better. Could be OT is bad and isn't actually 22.5K as advertised. Maybe I'll replace it, would be a pain though as I'd have to take the board out or at least lift it a bit to unscrew the OT Bolts

[jazbo8]

There is no mis-match if you are using the resistor divider as shown in the schematic. But you are saying even without the mis-match, the low power output still sounds better. I doubt the OPT could be defective, it may not be "22.5k" though, did you measure the impedance ratio?

[Littlewyan]

My resistor divider is different to the schematic. Can't remember what values I used but its on the original JTM1 thread. Havent measured the OT, might be worth doing tho. I'll look later at how to measure the ratio.

[jazbo8]

It really doesn't matter what values you used for the resistor divider, they have no impact on the impedance seen by the output tubes.

[Matec]

Hello Guys!

By the time we were trying to figure out the layout of marshall 1W, unfortunately, we could not have the impedance measurement of OT.

Later other people have written to me, and some of them got the impedance values of the "TX OT00039". The obtained values were 10k => 8/16.

On another occasion was ordered an OT to Dagnall, with the same specifications with which they manufacture the TX. It was also given the component with 10K PP.

By the words of the designer and the specifications of the components, I believe this is the real value of the transformer.

Already for my part I "recommend" it was 18k, in order to not overload the 12AU7. Just for that.

Cheers

[Matec]

Hello Guys!

By the time we were trying to figure out the layout of marshall 1W, unfortunately, we could not have the impedance measurement of OT.

Later other people have written to me, and some of them got the impedance values of the "TX OT00039". The obtained values were 10k => 8/16.

On another occasion was ordered an OT to Dagnall, with the same specifications with which they manufacture the TX. It was also given the component with 10K PP.

By the words of the designer and the specifications of the components, I believe this is the real value of the transformer.

Already for my part I "recommend" it was 18k, in order to not overload the 12AU7. Just for that.

Cheers

[jazbo8]

Hello Guys!
Later other people have written to me, and some of them got the impedance values of the "TX OT00039". The obtained values were 10k => 8/16.

Already for my part I "recommend" it was 18k, in order to not overload the 12AU7. Just for that.

Cheers

10K, nah that just seems too low, has anyone actually build the amp and try it, does it still sound like the Marshall? I think your 18k is a much more suitable value.

[Matec]

10K, nah que just Seems too low, has anyone Actually build the amp and try it, does it still sound like the Marshall? I think your 18k is a much more Suitable value.

To me it also seems very Low. But who rode reported that the sound was like that. There was still some adjustments to the circuit, so it's early for conclusions.

[Littlewyan]

I am certain I read a post from the designer stating that it was 18K

Here is the replacement OT

http://www.tube-town.net/ttstore/Transf ... :6799.html

Edit: Can't find it now but I'm sure I read a post from a designer stating that the JTM1 and JCM1 had 18K Transformers but the DSL1 and JVM1 had 22K or something. All I've found was a post from James Merchant who designed the JTM1 and JCM1 saying that he preferred a much lower impedance.

[Matec]

It is peculiar.

I searched a lot for OTs to 1W. And had not seen that ad.

Must be recent.

Perhaps the manufacturer has taken this information on some forum.

About the 10k, I have numbers that seemed pretty convincing.

[jazbo8]

About the 10k, I have numbers that seemed pretty convincing.

I will try and plot the load line when I get a chance, so we can nail down this important parameter.

[Edit] ok, here is the 10k load line, looks rather reckless to me... the estimated output is 1.65W which is also significantly higher than the 1W rating for the amplifier. I'm still not convinced that 10k is the right spec for the OPT.

[incorrect graph removed]