JTM45 re-issue Tonestack Mod?

[Gabi]

Mikey!

Thanks a lot!!! it helps!

The trouble is that this JTM45 is a friend's who wanted it wied for a 8 ohm impedance output because he has a 8 ohm impedance cab of some sort.

I wonder if I should just convince him to get a 16 ohm speaker cab and use that instead so that the Presence pot start having effect. Becaus ethe way it is now has no effect at all...

I added the V2a 25uF/50v bypass cap on pin 8 (paralel with the 82R) as Rich suggested and chancged the MID wiper cap from .022 to .01 for a more dramatic range sweep. It worked a little but I still would like to push more.

Some here mentioned the replacement of the 56k (R15 on Markh's schematic) with a 100k resistor. I did not tried that yet though.

thanks for all!

Gabi.

[PCollen]

Hi folks!

Anyone has a recomandation for a mod that makes the tonestack sweep on the re-issue JTM45 (hot-rodded) be more dramatic? I am also thinking of the Presence pot to be more dramatic...

thanks,
Gabi.

I think I read that changing the 250pF treble cap to about 370 pF, leaving the 56K slope R in place, improved the treble control sweep. I haven't tried it myself, but intend to in the near future.

[PCollen]

Thanks folks!

Richie: you mean connecting an electrolitic (25uF/50v) to V2? Can you please mention wich pins or how should I do this?

Larry: thanks! I think I have some polystyrenes somewhere...

Gabi.

Is this Gabi, the amp builder, asking these questions ?

[Gabi]

Is this Gabi, the amp builder, asking these questions ?

Yes

As you see - I know nothing outside my safe and secure 18watt realm
Well let's make that 36 watt

GAb

[PCollen]

Mikey!

Thanks a lot!!! it helps!

The trouble is that this JTM45 is a friend's who wanted it wied for a 8 ohm impedance output because he has a 8 ohm impedance cab of some sort.

Gabi.

Not sure if this will help, or even whether I should 'introduce' this pic here,
but take a look at the OT secondary wiring/notes for KT66 vs. EL34 . For instance, the 8-ohm tap using EL34's 'becomes' the 16-ohm tap when using KT66's. So, if you just yank the El34's out and install KT66's and re-bias, and keeping with an 8-ohm speaker load, you would set the impedance switch to 4-ohms. If this is irrelevant to the problem at hand, please disregard.[/img]

[PCollen]

AAAAAAAAAAAAAAAAAAAAAAAAAAAA


So in order to my Presence to work (which has no effect at the moment),

Gab

On the topic of the Presence control, some amps have the 22K pot and .1uF cap in parallel with a FIXED, and grounded, 4K7 resistor. This eliminates the scratchiness in the Presence control, but I don't know if it provides a more EFFECTIVE Presence control.

[PCollen]

Is this Gabi, the amp builder, asking these questions ?

Yes

As you see - I know nothing outside my safe and secure 18watt realm
Well let's make that 36 watt

GAb

I wasn't insinuating anything, just wondering. No disrespect intended.

[zaphod_phil]

The trouble is that this JTM45 is a friend's who wanted it wied for a 8 ohm impedance output because he has a 8 ohm impedance cab of some sort. I wonder if I should just convince him to get a 16 ohm speaker cab and use that instead so that the Presence pot start having effect. Becaus ethe way it is now has no effect at all...

As long as the NFB is always being derived from the 16 ohm OT tapping, then its shouldn't matter in the least with OT tapping the speaker is connected to.

...and chancged the MID wiper cap from .022 to .01 for a more dramatic range sweep. It worked a little but I still would like to push more. Some here mentioned the replacement of the 56k (R15 on Markh's schematic) with a 100k resistor. I did not tried that yet though.

Both of those were my suggestions. I've spent many happy hours playing with tone stacks - a few too many in fact.

[rjgtr]

That schematic shows the W**** transformer wiring. W**** likes to offer one transformer and "make it work" with a number of amps.

I think I'd just rewire the primary to match the tube impedence if that's possible.

I like using a 47-56k value for the NFB resistor off the 8 ohm tap. It makes the low end a bit more punchy.

The presence control in the original JTM was wired to ground an was used in place of the 4.7k tail resistor. I've also used the alternate method of using a 25k pot in addition to the 4.7k resistor. Both work, but the alternate method isn't scratchy since the presence cap is between the pot and the 10k/4.7k junction.

[Gabi]

Updates...

Larry: I was not offended at all. Hard to do that with me. It actually reflects the truth. I REALLY feel very unsecure \working on anything else but our 18watter
Don't worry about it...

I wired the output tranny as in MarkH's schematic to output 16 ohms impedance. Fired it up, started squealing like CRAZY!!! Swapped the two primary OT leads on the plates, the squeal went away. But still not receiving any effect on the Presence.

Tried the amp with a 16 ohms speaker: The Presence became juuuust a little more obvious. In fact what does the Presence do anyway? I wonder if lowering R21 (10k) would be of any use?
Also I placed a trimmer in the place of R26 (27k) and trimmed it around 15k.

At this point I am not sure what else can be done...

Thanks foar all your help!!!!

GAbi.

[jaysg]

The Presence became juuuust a little more obvious. In fact what does the Presence do anyway? I wonder if lowering R21 (10k) would be of any use?
Also I placed a trimmer in the place of R26 (27k) and trimmed it around 15k.

15k with the 16ohm wiring or 8 ohm wiring? I'd check/replace the presence cap. It sounds like you're working with a bad value part.

fwiw, I never realized how much feedback these are using.

[Gabi]

Hi Jay...

15k with the 16 ohms OT wiring.

I have a .01uF cap on the Presence wiper... Looks the same on the schemo...

thanks,
Gabi.

[Gabi]

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[Gabi]

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[zaphod_phil]

So what did you end up with in your tone stack?