Wanted to share a small success story... (Matchless DC30 build)

[turbofeedus]

Most recent project was a DC30 build, I had the TDS transformers from a previous Matchless build that went awry.
I reached out to Nik at Ceriatone to make me the chassis and cabinet.
It's actually been built for a couple months now, and sounding great except for one big issue; high pitched (~12-14kHz) EF86 oscillation.

I tried a couple things to lose the oscillation (moved around wires, swapped tubes) to no avail.
I also modded the EF86 circuit. Matchless went with a pretty large anode load of 330K, which according to Merlin is way too much gain for an input stage.
I changed to the datasheet values (100k plate, 390K screen, 1.2K cathode) and this actually cut down the oscillation in amplitude, and increased the frequency.
But it wasn't gone, and a bit of frustration set in.

I reached out to Nik again, figuring a last effort before posting here.
They immediately recommended an OT shield, which they integrate on all their in house builds.
To be honest, I was skeptical. They're not used very often, and always seemed like a theoretical rather than result-driven endeavor.
Boy was I wrong. Dropped the shield in there and oscillation is nowhere to be found.
I couldn't really believe it, so I pulled the shield back out and boom... oscillation.

I guess that wraps up this build now, just want to share in case anyone was a shield naysayer like myself. Not saying I'm putting them in all my builds going forward, but definitely in the locker.
Also a big +1 to Nik at Ceriatone, was very responsive and helpful, the product is reasonably priced and high quality.
Thank you for coming to my Ted talk.

Photo Jun 04, 9 05 20 PM.jpg

[xtian]

Great TED talk, and thanks for the info about the shield, and the photo. Very cool!

[xtian]

Hey, did you steal those bright orange zip ties from me? That's how I know I've been inside an amp!

[ChopSauce]

Thanks for reporting and congrats for your talk!

I already read about shielding the input jack from another member of another (french) forum, so this is consistent, yes.

Nice gutshot, by the way...

[martin manning]

Looks great. However, I have to say that placing the OT cheek-by-jowl with a high gain input stage doesn't seem like a good idea.

I while back worked on a vintage Supro that had small steel shields attached to the open back chassis, sort of like small local chassis cover plates. This was a Tweed-like arrangement with the chassis mounted on the top of the amp, controls facing up, but the whole power section was in another chassis mounted at the bottom of the cab.

[turbofeedus]

Hey, did you steal those bright orange zip ties from me? That's how I know I've been inside an amp!

Hah actually orange is my least favorite color, I was try to get rid of them. Maybe we should do a color trade XD

Looks great. However, I have to say that placing the OT cheek-by-jowl with a high gain input stage doesn't seem like a good idea.

I while back worked on a vintage Supro that had small steel shields attached to the open back chassis, sort of like small local chassis cover plates. This was a Tweed-like arrangement with the chassis mounted on the top of the amp, controls facing up, but the whole power section was in another chassis mounted at the bottom of the cab.

Yeah, maybe if the board vs tube row swapped position, this issue could be avoided. But hey, the chassis drill template is already made!

[pdf64]

Vox AC50 have a similar issue and solution. Such a screening plate can also be beneficial for (vintage) AC30 that have the top boost circuit.
I repurposed a PA amp where the transformers were at either end of the chassis, I didn’t bother to try it without a screening plate inside the chassis.

[JonSnell]

The screen resistor should be 2M2 ish.
Here is an original schematic of the DC 30.

[turbofeedus]

The screen resistor should be 2M2 ish.
Here is an original schematic of the DC 30.

I modded the whole circuit to reduce the gain; 100k anode, 390k screen, 1.2k cathode. The screen current is a bit high, but I’m happy with it for now.

[Roe]

yes screening between preamp and OT is generally a good idea

[pdf64]

yes screening between preamp and OT is generally a good idea

It’s always seemed a bit wonky to me how many Marshall OTs are oriented such that their primary to anode wiring emerges into the chassis in close proximity to the front panel / tone controls. I wonder why they do that, when it’s just as easy to rotate it and have the secondary that side? The signal voltage is then at most 1/20 of the primary level, and only one polarity to deal with.

On the other hand, if the primary to anode wires are twisted, I guess that should greatly reduce the radiated field and so mitigate the risk of coupling back.
And though the secondary is single ended, its polarity has to be the same as that at the LTP signal input, and hence, generally, the tone controls. So perhaps more risk of a positive feedback loop being significant.

So perhaps overall it may be a ‘swings and roundabouts’ / ‘6 of one and half a dozen of another’ scenario.

[Roe]

yes screening between preamp and OT is generally a good idea

It’s always seemed a bit wonky to me how many Marshall OTs are oriented such that their primary to anode wiring emerges into the chassis in close proximity to the front panel / tone controls. I wonder why they do that, when it’s just as easy to rotate it and have the secondary that side? The signal voltage is then at most 1/20 of the primary level, and only one polarity to deal with.

On the other hand, if the primary to anode wires are twisted, I guess that should greatly reduce the radiated field and so mitigate the risk of coupling back.
And though the secondary is single ended, its polarity has to be the same as that at the LTP signal input, and hence, generally, the tone controls. So perhaps more risk of a positive feedback loop being significant.

So perhaps overall it may be a ‘swings and roundabouts’ / ‘6 of one and half a dozen of another’ scenario.

one of those things that may explain why some amps are noisy. I twist the primaries and secondaries but marshall don't