Just Finished a Monotone 18watt TMB

[ejendres]

Just finished my build and I'm having a couple issues, I'm hoping you guys can help me out.

Overall my voltages look OK, just on the low side, particularly for the preamp tubes. I can post them if you think that would be relevant.

I cranked the amp up to 10 (with an attenuator) and I'm not having any oscillation or weird behavior so that's nice. I am getting some radio interference but my input wires are unshielded so I think that's the cause of that.

The biggest issue is that I'm getting a fizz/buzz on the low-end. It's particular noticeable during palm mutes. I did a quick clip of it below. It's more noticeable on the TMB channel but I hear it a little bit on the V-T channel as well. Any ideas what this might be?

I did a double check on both the attenuator and the cab by running my VHT Pitbull through the setup. There's no fizz or buzz with that amp.

https://on.soundcloud.com/RpiAm

[xtian]

Fizz can often be parasitic oscillation. Can sometimes be fixed by improving lead dress (please post photos). Sometimes needs a snubber cap to fix the fizz.

[ejendres]

Sure thing. I have shielded wire coming in to do the input properly and I plan to switch to a star ground for the pots when I make that change.

[xtian]

Many of your turrets look like they could use more solder. But more importantly, consider your preamp tubes. Each one has two triodes, using pins 1,2,3 for one triode, and pins 6,7,8 for another. The wires connected to pins 1,2,3 can be grouped tightly together, or pins 6,7,8 together, but you want to avoid one set getting too near another set. For example, the two blue wires on V1 get really cozy close to their plate resistors--separate them as much as possible. Same for the yellow cathode wires. Same issues for the other two preamp tubes.

[ejendres]

Many of your turrets look like they could use more solder. But more importantly, consider your preamp tubes. Each one has two triodes, using pins 1,2,3 for one triode, and pins 6,7,8 for another. The wires connected to pins 1,2,3 can be grouped tightly together, or pins 6,7,8 together, but you want to avoid one set getting too near another set. For example, the two blue wires on V1 get really cozy close to their plate resistors--separate them as much as possible. Same for the yellow cathode wires. Same issues for the other two preamp tubes.

Because of the way my tube sockets are clocked in the amp I would have a hard time separating all them as they cross over each other.

The tube is the same regardless of the triode used correct? I could switch pins 1,2,3 and 6,7,8 without issue? Doing that would help to separate all those.

[The Dude]

Got a schematic so we can see what we're working with?

[Helmholtz]

Do you have a scope?

[ejendres]

Got a schematic so we can see what we're working with?

Schematic and layout are here:
https://www.mojotone.com/British-Sty...-Amplifier-Kit

[ejendres]

Do you have a scope?

I have access to a really nice one at work.

[xtian]

Because of the way my tube sockets are clocked in the amp I would have a hard time separating all them as they cross over each other.

The tube is the same regardless of the triode used correct? I could switch pins 1,2,3 and 6,7,8 without issue? Doing that would help to separate all those.

No worries. Just, where the wires must cross, make sure they either cross at 90 degrees, or separate them as much as possible--remember this is a 3D creation, so you have more room to work with than the layout suggests. Basically, never run them close and in parallel, to avoid parasitic oscillation.

[mozz]

18watt TMB

EL84, fizz fuzz buzz, conjunctive filter may be needed.

EDIT: or bias them real hot and get rid of the crossover notch.

[Helmholtz]

Scoping will show if there's oscillation or crossover distortion.

[ejendres]

After chatting with an amp building buddy I added a 'zener clamp mod' to the he power amp to prevent bias excursion and a grid stopper on v2b. I've attached the modified schematic.

These mods tightened up the amp and I like how the lew end sounds better however all it didn't fix the noise I'm hearing, it actually made it more obvious. Check out the clip below.

https://soundcloud.com/ejendres/loose-farts

Any thoughts on what else to try?

What should I look at with the scope? And what am I looking for? Any guides I can read? I haven't ran a scope since an Electrical Engineering course in college.

[mozz]

18 watt website has your answers. What are the tubes running at now? Any tech should have a scope to check for crossover.