New Bass Amp?

[TUBEDUDE]

I spent the last 6 months designing a tube bass combo amp for the living room. I host Guitars, Cigars and Bourbon during Turkey week for my family reunion. I've built 4 different 1 12" guitar combos over the years, and this will allow us all to sit on the amps, leaving the real seats for the "vocalists", ukulele, kalimba, glockenspiel, etc.

The amp uses a two stage pickup tuning circuit, into a James stack, to the effects loop, to a driver, the LTP, 6V6Gs to 6600:8 Ohm OPT into a basslite 10 in a lite horn config. I also made a low FREQ tremolo (stolen from Randall Aiken) with the leftover triode so I could play Starship Trooper. It goes from 1.36Hz to 4.72Hz.

The initial design didn't survive first contact with the enemy, and many changes have occured during the prototyping process, as with all new designs. Many components were clipped out, new added etc. so it is a little ugly and the photos were taken before all work was done. So in the chassis image, the ground bus wasn't installed yet. So the unterminated leads all went to ground.

I'm waiting on carbon fiber tubes to finish up the cab. More pics when finished. I've always used turret board, or FR4 card with eyelet construction. This is my first point to point effort. Mo' later, cvk

[xtian]

Wow! Tell me about bending plywood for the wave guide!

[TUBEDUDE]

I soak the wood with water, then pre-bend the plates with ratcheting cargo straps. After they dry the bend is a little looser radius than the horns curvature, but it decreases the force needed to glue them up, again with more straps.
The smaller plates are another story. With no ability to clamp, or use the straps, I glue the back edge of the plate in, wait 24 hrs., then glue the rest and force the plate along its curved supports and hold it in place for an hour till the glue sets. Yeah, that's a pain in the ass. Twice, as there are 2 smaller horn plates.

[nworbetan]

I really like the hand drawn schematic and layout.

I see one thing that may be a mistake though? After the PPIMV, the second pair of 100nf caps and the 220k resistors are redundant and can be removeand and the mv pot connected to the bias voltage instead of ground.

[nworbetan]

Sorry about the double post.

[TUBEDUDE]

So in effect I would be using the bias circuit as the return instead of ground, by connecting the bias/trem to the MV pot instead of ground? That could put the return 500k Ohms away, as the Trem pot is in line with the bias ground.

[nworbetan]

To be honest I'm not entirely sure what you mean by "return" in this context.

But with a little more thought on it, the change I mentioned earlier would also cause the MV to basically be a balance knob between the trem signal and the audio signal, and I can't imagine that would be very usable. Can you tell this is the first time I've ever looked at an amp with trem and a PPIMV?

Your explanation reminded me that the 4.7k + 220k + (roughly) 250k + (roughly) 100k resistors in between the 6V6 grids and ground are a bit higher than the typical data sheet recommendations of 100k for a fixed bias (or 500k for cathode bias). Probably not a huge deal for your annual living room gigs. It would be more of an issue for someone like a touring musician that's putting a lot of hours on the tubes very regularly and changing them on the road is a bit of a hassle.

[Deric]

Very cool! Thanks for sharing - looking forward to seeing the finished product.

[TUBEDUDE]

To be honest I'm not entirely sure what you mean by "return" in this context.

But with a little more thought on it, the change I mentioned earlier would also cause the MV to basically be a balance knob between the trem signal and the audio signal, and I can't imagine that would be very usable. Can you tell this is the first time I've ever looked at an amp with trem and a PPIMV?

Your explanation reminded me that the 4.7k + 220k + (roughly) 250k + (roughly) 100k resistors in between the 6V6 grids and ground are a bit higher than the typical data sheet recommendations of 100k for a fixed bias (or 500k for cathode bias). Probably not a huge deal for your annual living room gigs. It would be more of an issue for someone like a touring musician that's putting a lot of hours on the tubes very regularly and changing them on the road is a bit of a hassle.

You're right, i normally use cathode bias with 6V6's. Ill reduce the 220k's to around 100k and lower the pot value and see how i can fudge the circuit. Thanks!

Still works without issue at 100k resistors and 100k pot. Will try lower values tomo.

[TUBEDUDE]

About finished. It's not as ugly in person. The dark blue cab reads purple for the camera for some reason. Here's a couple of quick snaps.

20180911_083221.jpg