JTM1 Impedance Issue

[Littlewyan]

Yes lets try and get to the bottom of this.

[Matec]

Yes lets try and get to the bottom of this.

Yes this is the way!

[martin manning]

An 18-22k (a-a) PP transformer mis-matched up by one tap would result in a 9-11k Class B load line, which looks pretty damn good for a 12AU7.

Re the poor sound quality, is it possible that there is some other problem? A wiring error? Or, maybe the non-standard half power resistor nw?

[Littlewyan]

It sounds like a wiring error but I've checked the wiring again and again, can't find an issue. The half power switch is setup the same way as the schematic attached except instead of the 82R Resistor I'm using a 150R Resistor and I can't remember what I'm using for the 8Ohm Tap.

I'll try and take a photo tomorrow so you guys can see.

[tictac]

Is this the actual schematic?

The Neg Feedback injection point looks wrong, usually goes into the cathode of the gain stage prior to the splitter....

That tone circuit looks a bit odd as well; seems like it would strip away a bit of gain....

Also 100k grid resistors on the 12AU7?

Hmmmm.....

TT

[Littlewyan]

Yes thats the actual schematic.

http://ampgarage.com/forum/viewtopic.php?t=24671

There are 24 pages there of info regarding the JTM1 if you want to know more.

[Matec]

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.

[img:548:549]https://dl.dropboxusercontent.com/u/132 ... 5V_10k.png[/img]

The JTM1 normally works with a 230V supply voltage.

You must then make another chart with a different quiescent point for this amp.

Cheers.

[Littlewyan]

230V I thought it was 250V? Mines at 280V at the moment.

[martin manning]

Wow! It's supposed to be 230 +/-10% as a standard, but in reality what that means is the UK can stay where it is at 240 and the EU can stay where it is at 220 with some margin.

[Matec]

250v is the maximum voltage supplied by the source, before the heat up valves, then falls to around 230V.

This to the power transformer with secondary HT 180V.

[epis]

Wow! It's supposed to be 230 +/-10% as a standard, but in reality what that means is the UK can stay where it is at 240 and the EU can stay where it is at 220 with some margin.

A little bit of misunderstanding, Matec meant B+ 230V DC

[jazbo8]

Guys,

My apologies, in my previous post, I clicked on the wrong link for the graph, duh! Here is the correct one with B+ at 250V (middle of the range based on the mains voltages), anyway, it still looks a bit steep for me, and the estimated output is now only 0.85W.

[img:552:550]https://dl.dropboxusercontent.com/u/132 ... 0V_10k.png[/img]

[Littlewyan]

Well like I said my B+ is at 280V at the moment, wonder if lowering it to 250V would do the trick. I've got a 2.2K Resistor and a 33uF before the OT Centre Tap to bring the B+ down. If I find a 3.3K that would give me 250V.

One thing I will say is I found the amp to be sharper sounding without the 10K Resistor and .0022uf cap across the plates of the 12AU7 but then it sounds really buzzy. What also wont help is these values were picked to work with the 18K OT and obviously mine is 22.5K.

[jazbo8]

Well like I said my B+ is at 280V at the moment, wonder if lowering it to 250V would do the trick. I've got a 2.2K Resistor and a 33uF before the OT Centre Tap to bring the B+ down. If I find a 3.3K that would give me 250V.

One thing I will say is I found the amp to be sharper sounding without the 10K Resistor and .0022uf cap across the plates of the 12AU7 but then it sounds really buzzy. What also wont help is these values were picked to work with the 18K OT and obviously mine is 22.5K.

I doubt it will sound much different by lowering the B+, also bear in mind that the 250V is the net Vak voltage, so the the B+ on the CT is 260V in the example. Your 280V isn't really that far off. Just to make sure, what is the cathode voltage of the 12AU7?

Removing the Zobel network on the primary gives you more ringing so more HF distortion could be heard, but again, it is not the primary tone shaper.

The fact that you like how the amp sounds at 0.25W suggests that the same tone should be reproducible at 1W, are you using the same settings and the exact same speaker when auditioning the two modes?

[Matec]

Littlewyan

I do not know exactly what might be wrong with the tone of your amp.

But let me ask you: Have you tried to reduce the NFB resistor (R15) to half of its value? (And while increasing C11 to double the amount).

Let me explain my idea. IF by chance the actual impedance of the OT is 10K, the NFB network is correct and will help in the sound of power. Already with an OT 22k The NFB will not be fully active which can cause a more strident sound.

This is just a suggestion.

(The value of R9 is 56K?)

Cheers