Vibrolux Reverb (AA270?) - pics & questions

Hi all - I recently picked up this silverface VR and am in the process of making a few changes. I'm no expert but do have a technician degree with a fair amount of bench time (a bit rusty but yes, I can safely discharge caps), but have never done this kind of work to a vintage amp so I have a few questions.

First, the bias balance circuit:




I have started to convert this to the adjustable circuit by removing the 3.3k resistor from the bias supply to the pot and the 50mfd 70vdc cap that was connected to ground. I moved the 15k resistor to the top leg of the 10k linear pot, connected a wire from the bottom leg to the bias supply and connected a wire from the center pin to the 100k resistor on the board (bias splitter? not sure what this is called). My question here is regarding removal of that 100k and installation of the 220k resistors as per the earlier design. How will this affect the sound? Should I be using CC here?

Filter caps:



Next, I installed a 3 prong cord (removed AC receptacle and ground switch) and replaced the filter caps with F&T 16uF 475v versions. The resistors installed in the doghouse with these are 10k and 2.2k, which I am planning to replace with equivalent 3w metal film versions. I didn't think there was a reason to stick with CC here, anyone feel differently?

Next, the bias supply.



So, I have an F&T 100uF 100v cap that I'm installing in place of that Mallory, and was thinking of using another metal film resistor to replace that 1k CC resistor. I believe the original is only 1/2w - what is recommended for a replacement?

Next, plate load resistors:



I was thinking of trying a 12ax7 in the PI for some extra gain - are the 47k plate resistors still appropriate in this config?

General question about plate resistors - I was thinking of leaving them alone and seeing how the amp performs with the other changes, but I'm also tempted to just change them while I'm in there now. Sorry to rehash this again, but there seems to be different views of what type of resistor is best here. I do have some 3w MF in the appropriate values (47k, 100k), but I'm wondering if I should just stick with CC? The Ohmite Little Demon series are available in 0.5w-1w and they would certainly look the part but performance is #1 for me. I guess I don't fully understand where it makes sense to stick with CC over MF.

Of course, pretty much any of this is reversible but I'd like to get as much done in this first pass as I can. Next on the list is to replace the 7 electrolytics on the board with 25uF 50v Sprague Atoms.

OH I almost forgot - my power tubes have 470ohm 1w CC screen resistors and the 2000pf caps. I was thinking of clipping the caps and replacing the CC resistors with MF - is this the standard, and what power rating do most people go with, just 1w?

I'm awfully tempted to go through the entire amp and clean up the wiring but I'm thinking this is bad form. I understand that wires are run a certain way to reduce noise, so I think I'll just get it back together and see how she sounds.

Thanks for reading and for any suggestions/comments!

 

By the way, this amp has the 5U4 rectifier.

Something else I'm planning is some 1ohm 3w resistors on the power tubes for biasing.

EDIT: bonus, this amp came with all it's original glass - RCA blackplates & RCA/GE for the little guys. I only played it very briefly as I wanted to get the caps done ASAP, but everything was working well. Not much breakup at 6-7, so I'm interested to get it biased and try the 12ax7 PI swap and the V1 omission. Speakers are the original Oxfords, which will be swapped for Weber DT-10s if all goes to plan.

 

I'm at the bench now - love the smell of solder in the evening

I was planning to keep the 5U4 for sure. I finished the filter caps tonight and did replace the 2.2k and 10k doghouse resistors with MF 3w versions. I'm going through the board electrolytics now. I'll add the 1ohm bias resistors and will leave everything else alone, but I'm still wondering about adding the 2 x 220k resistors in place of the single 100k where the bias pot connects. Do I NEED to make this change since I've modified the balance circuit?

 

Yeah, for sure! Just wondering if it's "improper" in regards to functionality to not add the 220k resistors after making all the other changes. I want to make sure I have the correct range of adjustability.

This is definitely a great model - had one just like it about 10 years ago which is why I jumped on this one (the great price didn't hurt!). I'm very excited to fire it up by the weekend!! Just waiting on one more part...

 

Awesome, thanks everyone for your opinions!

So far, all I have done is the electrolytics, the doghouse resistors, and will be finishing the bias supply and adding the 1ohm 3w bias resistors to the power tubes once the parts arrive. I will leave everything else alone and see how it sounds. I tend to agree it doesn't make sense to blindly blackface an amp that already sounds good - I just wanted to make sure I was getting all the preventative maintenance kind of stuff on the first run. And yes, I DO want more breakup but this amp wasn't biased properly in the first place so I need a better baseline. Of course things can always be changed later if need be, but I suspect that now that the caps are changed and I can bias it I will just spend some time getting to know it as RussB suggested.

I was surprised at the drift on the 25uF board electrolytics - they were all in the range of 32-39uF!

Something else interesting that I did not see on the VR AA270 schematics but did find on this AA270 Dual Showman Reverb schematic - a 5uF 50v electrolytic connected to pin 8 of V5:

http://www.el34world.com/charts/Schematics/files/fender/DUAL_SHOWMAN_REVERB_AA270.pdf

I see now that the AA270 Twin Reverb layout has the same thing I only noticed the value after I pulled it off, and just went ahead and put a 25uF 50v in there same as the others. Maybe I should order another 5uF for this spot?

 

This is a good point, Mike! I was just going by the instructions on the Hoffman site.

Does anyone have a comment/suggestion on the 5uF 50v electrolytic connected to pin 8 of V5? I guess I'll order a new 5uF for that spot, but what would be the affect of running it with a 25uF 50v?

 

Thank you, good sir. I will have her up and running again by tonight or tomorrow at the latest and will report back with my thoughts for anyone who might be interested. Thanks again for the help

 

What power rating would you suggest, Mike? I read another thread here where someone was in the habit of using a 0.6w 1ohm that worked fine under normal conditions but would fail and protect the trannies if one of the power tubes went kaput. Thoughts?

 

Excellent, I will follow this advice. Thanks guys!