Fender Twin Reverb 135w Ultralinear full overhaul and rebuild. Design discussion

[Rolteus]

I'd like to share a design I've been working on for some comments/feedback.
I'm no expert as you can see.
Here is a summary of what I'm proposing.

The design uses Hoffmans AB763 Two Channel 17" board with bias on the board.
It's perfect for my modified circuit.

I'm attempting to correct some issues with the Ultralinear and maybe add some features that I feel it needs.
Got rid of Reverb and Vibrato and added a Fenderish style Presence.
Put a Tube Buffered FX loop send and return on the front of the amp with jacks on back. The return level acts as a proper master volume.

Reverb and Vibrato were so poorly implemented in this amp they steal all the fire. Deleting them and installing a proper FX loop
to allow the use of proper reverb and delay makes this amp useable again and much more flexible.

The loop is really transparent and solves some issues with master volume amps with an FX loop.
Since the amp is clean I opted out of the traditional MV and the PPIMV entirely. The Fender MV was junk and nobody wants that thing anyway.   

The loop return works as a final program volume.
I may add the PPIMV if the amp is too much.
Having channel volume, loop send level, and loop return level makes the amp much more flexible
and you're able to dial in a sweet sound at a lower volume and drive your fx in the loop properly.
You can even use the loop levels as an added gain stage and it's pretty hot.
I added a gain stage to the normal channel to fix the phase issues and give that channel a bit more push and
maybe allow the channels to be used together with a jumper.   
After that I added a cathode follower to sum the two channels into the FX loop.

Any feedback, comments, or criticisms are greatly appreciated.

I'm kind of riffing here just wondering if this can work.   

[AlNewman]

Does the amp not work or something?  You can do whatever you want to it, but the amp won't be worth anything when you're done, if that matters.
Besides the output transformer, it isn't difficult to modify the ultralinear to an ab763 circuit, and you have the added master volume pot/switch, as well as the ground switch on the back to add some mods.
Anyways, if you would like comments on your ideas, it's best to make up a schematic.  Pretty hard to "read" a layout.

[Rolteus]

A customer brought me this amp we we've talked extensively about what he wants. This amp is a favorite of the Pedal Steel players which is why he wants this particular amp.
He's not worried about what it's worth or reselling it. This is more of an experiment utilizing the Ultralinear transformers with some added design features to the original design.
Backstory:
The amp fell off the shipping truck upside down and dented the chassis and broke the cabinet.
We figured rather than try to 'fix' the amp we will make a head shell and do a refurb/update/restore/custom job and go all the way.

I'm not working with a schematic for this amp perse, I'm using the Fender Twin Reverb 135w Ultralinear schematic
and modifying the layout. I do intend to create a new schematic but I'm designing the amp in DIY layout creator.

He's not worried about money. We're getting the chassis powder coated titanium white and doing all CDE Orange Drop caps, metal film resistors
and a new turret board and adding a choke. Basically new everything with a bunch of changes and new features.

I'd rather work in layout than design by schematic. This amp was once stock Fender but it had been recapped and was pretty tired.

It's been gutted for powder coating. We're going to give it the classic car restoration treatment with some updates and upgrades.

I'm enjoying this experiment and the opportunity to create something totally unique and bring this old amp to life.
The original amp was pretty boring and noisy. The reverb and vibrato are marginal at best. That old master volume had to go!

For pedal steel the amp doesn't have the punch and harmonic richness they need for that style of playing.

These are never used stock they're always modded in certain ways for that instrument and style.

So this is just for fun because we can we do it.
Thanks for the comment.

[AlNewman]

Cool.  Yes, if the amp is damaged, I can see rebuilding it for sure.  On that note, I'm sure it took a lot of time to figure the layout, and you're basically asking people to reverse engineer your drawings to offer opinions on viability.  If you're serious about asking opinions, a schematic will offer a lot more results. 

[Willabe]

If you're serious about asking opinions, a schematic will offer a lot more results.

That's a big circuit, need a schematic to see what's going on.

[Rolteus]

I get it.
I'm working from the original Fender Schematic that's crudely drawn by hand and copied on a rotten potato. 
I have all the values and most of the amp is basically stock except for the things I removed and a couple things I added.
I'll be creating a schematic once I'm finished with the design.
I can read and design in schematic but I find it easier to create this design in layout because of all the work it would take to transpose the hand drawn schematic in a schematic editor. My goal is to create a new schematic of my new design once it's finished.
Also I purchased a Hoffman Turret board and used his turret layout for this amp because it's basically the same amp so his layout and the values from the original schematic work work perfectly together. Plus removing reverb and vibrato frees up a couple of tubes for other stuff.

Thanks for the comment.

[pdf64]

Here's the Fender schematic https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

Both the premise behind the need for the changes, and the changes themselves, seem wrong headed to me. Changes have been made seemingly without an appreciation of the technical rationale of why the original design choices might have been made 

Regarding the premise -
What's wrong with Fender black / silver panel reverb and trem? After 6 decades they're still a popular industry standard. But I accept that not everyone finds them useful.

I acknowledge that a well designed and implemented fx loop can be beneficial after an overdrive preamp, especially a channel switcher. But with a clean preamp, I don't perceive the benefit?

What's wrong with the stock master volume? I'm not defending the pull boost distortion feature, although I know one player that likes and uses it

Regarding the changes -

Why are the fx loop send and return controls considered to be better, an improvement, on the sock master volume?

What's the thinking behind the use of a 1M send level control between the fx send buffer output, and the fx send socket?

Why change the power amp operating mode? Pentode operation at the high stiff HT produced by the power supply would seem to require additional mitigation for screen grid overdissipation. That doesn't seem to have been a consideration (apologies if I've missed something).

The changes have removed the HT potential divider, which allowed 500V caps at downstream HT nodes to be used without risk of their rating being exceeded from the higher than typical HT voltage of these amps. Maybe it's intended that 600V rated HT caps are used?

The layout at the FWB HT rectifier has a significant issue, in that both the HT winding CT and rectifier negative output are connected to the same node, chassis common. That will draw fault current and blow the mains fuse.

[Rontone]

I'd like to see a schematic of just the loop if possible, just at the loop and just wondering are the resistors marked "S" for send and "L" for the return?

I assume so as the normal channel has "N" and the Vibrato channel "V"


I'd recommend changing the colour for the HT line and nodes on your layout, the ground rail and HT rail are both blue and quite close to each other and crossing over

[Rolteus]

I've corrected the HT issue you're seeing. I added the 30k 20w resistor to correct downstream voltage for the preamp.

I appreciate the input. I'm learning. This is an experiment. In the original post I stated this amp was dropped off a truck and the chassis was bent pretty badly. It's been recapped and sounds pretty tired. The amp isn't worth much any more as a collector so we decided to experiment and have some fun and not take it too seriously.
I am working directly from the schematic you shared. This is a pretty bold project were just trying to get something different out of this amp. Not sure exactly what to expect as we plan on modding the preamp once it's finished and we can hear how it sounds to start with. Who knows it might sound stellar?

Thanks again!
.

Here's the Fender schematic https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

Both the premise behind the need for the changes, and the changes themselves, seem wrong headed to me. Changes have been made seemingly without an appreciation of the technical rationale of why the original design choices might have been made 

Regarding the premise -
What's wrong with Fender black / silver panel reverb and trem? After 6 decades they're still a popular industry standard. But I accept that not everyone finds them useful.

I acknowledge that a well designed and implemented fx loop can be beneficial after an overdrive preamp, especially a channel switcher. But with a clean preamp, I don't perceive the benefit?

What's wrong with the stock master volume? I'm not defending the pull boost distortion feature, although I know one player that likes and uses it

Regarding the changes -

Why are the fx loop send and return controls considered to be better, an improvement, on the sock master volume?

What's the thinking behind the use of a 1M send level control between the fx send buffer output, and the fx send socket?

Why change the power amp operating mode? Pentode operation at the high stiff HT produced by the power supply would seem to require additional mitigation for screen grid overdissipation. That doesn't seem to have been a consideration (apologies if I've missed something).

The changes have removed the HT potential divider, which allowed 500V caps at downstream HT nodes to be used without risk of their rating being exceeded from the higher than typical HT voltage of these amps. Maybe it's intended that 600V rated HT caps are used?

The layout at the FWB HT rectifier has a significant issue, in that both the HT winding CT and rectifier negative output are connected to the same node, chassis common. That will draw fault current and blow the mains fuse.

[pdf64]

... It's been recapped and sounds pretty tired. ...

Maybe there's something wrong with it?
These amps are brutal clean amps, if it's not then something is probably wonky.
Especially if the reverb and trem sound as bad as you report.

When repurposing any amp, the key thing is that the stock OT and PT were designed and specified to work together using the original output stage.
Anything else is up for grabs, but the best chance of a successful outcome is when the stock power supply and output stage design is retained.

Operating without the screen grid taps off the OT primary will almost certainly put excessive stress on the output valves at high power output.

Another thing is that the speaker sockets  / wiring to them doesn't look quite right on the layout.
The stock ext socket uses changeover contacts to switch impedance, with the socket sleeve isolated from the chassis. 8ohm arranges the main and ext loads in series.