JTM1 Impedance Issue

[LOUDthud]

I think the output impedance is too low in the 1W mode. In the low power mode there is significant resistance in series with the speaker. Try disconnecting the negative feedback, C11 or R15. It still might be too low because of the triode output stage.

[Littlewyan]

Sorry guys been real busy with another amp rebuild this past week. Will pick this up again next week. I'm getting to the bottom of this!

[sturgeo]

Did you manage to fix the issues?
I've just finished my JTM45 build and want to build it a baby brother

The Andertons JTM145 is what I'm aiming for, like you i'm in the UK so would like to use the ampmaker transformers i believe you may have used.

[Littlewyan]

I actually have a few ideas on how to improve the amp which I'm hoping to try out soon. I'll let you know!

Edit: The ideas include installing a static presence control (no pot just a resistor) and changing the pre amp to be more like the JTM45.

[Littlewyan]

Picking up on this again. I wonder if its the conjunctive filter at play here? as I remember it was quite distorted before I put the filter in. Perhaps the filter isn't working well with my TX? Also I'm skeptical about doubling the output impedance as its already 22.5K so if I double it it will be 45K!

Just played through the amp again now and I think it can definitely do with a lower B+ and lower OT Impedance.

https://www.youtube.com/watch?v=g8q2jJXvNLg

Listen to how sharp his notes sound. Mine is far too soft and has no definition.

[sturgeo]

Pity the ampmaker transformers aren't quite right for these, £10 for the OT is a bargain

I might just try the Hammond 125a, I think smokebreak used one of these, not sure on the impedance though.

[billyoung]

removed

[Littlewyan]

I measured this amp and at 280V on the plates and with a 22.5K impedance it was pushing out 1.5W at full whack with distortion. Clean was something like .7W.

Anyway I have an idea! If Matec is right and the original JTM1s had a 10K Impedance then the conjunctive filter will be for that impedance, hence the 10K resistor (actually this is a bit low as its usually 1.3 times the impedance). Anyway my OT is 22.5K and as a result I think the conjunctive filter is affecting the frequency response a lot more than it should. So I'll try and find a 22K resistor to stick in there and will see what it sounds like .

[jazbo8]

If Matec is right and the original JTM1s had a 10K Impedance...

I seriously doubt that would be the case, just look at the load line... but you can easily test it, by putting a 4R speaker on the 8R output to hear what it sounds like.

[Littlewyan]

Dam, you're right, I did try that. Maybe my output transformer just doesnt need a conjunctive filter.

I tried doubling the output impedance but it pretty much sounded the same. I'll try disconnecting the filter to see how it sounds and ill try removing the low power switch temporarily in case its still partly in the circuit with the amp in high power mode.

[cassoulet]

Hi,

This is my first post here, though I've been following and reading a lot about JTM1 and other builds for some time.

Littlewyan, why not have a look at this OT from OEP:

http://www.oep.co.uk/pdf/N35A002F%20issue%205.pdf

It is sold for about 15 quid VAT and shipping included in UK. You can find it here at rs-online:

http://uk.rs-online.com/web/p/audio-tra ... s/2106475/

You can get a 15k primary impedance, which I think should be ok with the JTM1 clone and the like.

I'd like to give a try to this OT, but the french rs online shop want to charge me more than double to cross the channel. I figure that's Europe.
I'll have to find another solution. I am sure this OT is better than Hammond 125A or B; plus, full specs are available in the vendor datasheet, with curves and everything.

Also, I have completely redrawn the JTM1-HTM1 and JCM1 schematics from Matec in order to clear things. I can send it if you like.

Cheers from France.

[sturgeo]

I was about to pull the plug on the 125a but I'll have a look at the one from RS.

The JCM1 schematic would be helpful, I'm going to remove the Jubilee clipping and look at adding the #34 or #36 mod without the extra gain stage, test it out on the bench and then possibly have a dpdt switch between those and stock.

I better start learning how to do turret layouts!