JTM 45 clone ghost notes

[casey73]

I built a JTM45 clone on a chassis I purchased from Antique Electronic Supply. They had most of the parts except for the lower voltage British 45 PT, that I purchased from Mojotone.
The only deviation I made from the original was to put 1K screen resistors in place of the original 470 ohm resistors. I've run it with the Valve Art KT66, original Svetlana EL34s and TAD 6L6s. Filter caps are F&T 16/16 dual section. The ghosting is there with all three tube sets.

Could I get rid of the ghosting if I replace them with 32/32 or 50/50 caps?

[martin manning]

There's a very long thread that worked through this. You could start here: https://ampgarage.com/forum/viewtopic.p ... 81#p397481 That one was a Metro, and a solution was found using the existing filters and layout. What schematic did you follow in yours?

[Stevem]

Is the ghosting there at all volumes levels?
If not then at what level of V+ drop from idle on the plates of the output tubes does it start to show up?

[casey73]

There's a very long thread that worked through this. You could start here: https://ampgarage.com/forum/viewtopic.p ... 81#p397481 That one was a Metro, and a solution was found using the existing filters and layout. What schematic did you follow in yours?

I used the original Bluesbreaker version but left out the tremolo circuit. I'll read through the thread. Thanks.

[casey73]

I read through the long thread about the ghosting issue. Since a 5AR4 rectifier can safely handle up to 60uf at the output, is there any reason I can't just replace the 16/16uf caps with 50/50 caps?

[Roe]

50uf is ok, 32 is original. I'd check grounding and the balance of the phase inverter, not just the filtering

[pdf64]

...Filter caps are F&T 16/16 dual section. The ghosting is there with all three tube sets.Could I get rid of the ghosting if I replace them with 32/32 or 50/50 caps?

The caps for the reservoir and screen grid node should be 32uF.

...I used the original Bluesbreaker version but left out the tremolo circuit...

Link to the exact circuit you used is necessary, as the original has a significant error http://raw-sewage.net/images/jtm45orig-schm1.jpg
This is how it should look http://raw-sewage.net/images/jtm45-readable.jpg Spot the difference? The error would result in the choke being overloaded and so losing most of its inductance, resulting in a poorly smoothed HT.

...Since a 5AR4 rectifier can safely handle up to 60uf at the output, is there any reason I can't just replace the 16/16uf caps with 50/50 caps?

Depends on the goal of the project; if it's still to build a reasonably valid clone of the circuit, then no. If it's changed to a 'based on a JTM45', then have at it.
A good approach is get the original circuit working correctly first, before tinkering with it.

[burger]

I've had this issue on a 50w plexi. The mains filter cap was bad - start there.

What's your plate voltage?

This is a handy general trouble shooting resource,
https://robrobinette.com/Tube_Guitar_Am ... ooting.htm

[casey73]

An overdue update on my ghost note issues. I looked at many threads on this and other sites regarding this. I had a few people chime in that the JTM45 is notorious for some ghost notes. I ended up replacing both the 16/16 and 32/32 F&T caps with 50/50uf F&T caps. The ghosting is gone. I still have a minor glitch with high pitched squeal IF you jumper the treble and normal channels, turn both volumes past 12 o'clock, the treble way up and push the presence past 7. After replacing the feedback lead from the turret board to the presence pot, having it go straight down the the chassis and hug the chassis up to the terminal on the pot, and removing the presence pot ground, giving it a separate lead to the ground lug, it still squeals at that setting. My lead dress throughout the amp is very clean and meticulous. I've decided to live with it as I would never normally set the controls like that. And, after quizzing other forums in addition to this I've had other JTM45 owners say this has happened in their amps also.

[Roe]

presence squeal is normally cause by lead dress and lack of faraday cage (add a metal sheet inside the headcab)

[pdf64]

The absence of grid stoppers on the power tubes is an invitation to instability.

[casey73]

presence squeal is normally cause by lead dress and lack of faraday cage (add a metal sheet inside the headcab)

My lead dress is very clean and I do have a sheet of aluminum across the bottom of my head cabinet. I think it is just part of the design. On other forums I've heard similar complaints from other JTM owners. It seems to be a common complaint even in vintage units. Not universal, but not uncommon. Like I said, I'm going to leave it alone since it only occurs on extreme settings most would not use.

[casey73]

The absence of grid stoppers on the power tubes is an invitation to instability.

I have 1.5K grid stoppers mounted to the tube pin, going straight up with lead soldered to the end. I did vary a little from the original design including 1K screen resistors and I bumped the filter caps up to 50/50uf, which stopped the ghosting. I might try higher value grid stoppers next time I have it open. I've read up to 5.6K can work without having a audible effect on the tone. For the time being I'll leave it.

[Roe]

presence squeal is normally cause by lead dress and lack of faraday cage (add a metal sheet inside the headcab)

My lead dress is very clean and I do have a sheet of aluminum across the bottom of my head cabinet. I think it is just part of the design.

I've been able to cure the presence squeal on all plexis and alumium panel amps I've had at my bench (20-30 amps). Just move the purple NFB wire

[Stevem]

Yup, use shielded wire for the feed back fed and go over the orientation of your coupling caps.