135w Ultralinear "1/2 Blackface" Mod

[22uf]

Hi there.

I have just picked up a 1977 TR that I would like to "soften" up without replacing the PT or OT or adding a choke. I don't want to substantially modify the amp in that way. I'd like to just maybe change some caps and resistors to see if the amp can give up a bit more. Its a bit stiff and sterile sounding as it is, even with new Eminence Legend speakers.

The amp has been fully serviced: all new filter caps, replaced all electrolytics. 470ohm resistors have been changed for cement and the rest of the caps are the original blue mylar jobbies.

I'm new to all this but I'm ok with soldering and following a circuit etc.

I keep reading about the 'half-blackface' mod for this model but I can't find any definitive info on what that atually is.
Google suggests its changing some values in the PI circuit to blackface spec and possibly also some mod to the rectifier (full wave in this model)? Signal cap changes may also affect the perceived 'warmth' and 'complexity' factor?

Would some kind soul be willing to suggest certain non-epic-scale mods that I could make to get a softer/less hi-fi tone out of this amp? I know it will never overdrive, but I use a klon for that! :)

Any/all suggestions would be kindly appreciated. Even if its just where to go! :)

[Latole]

I mod a ulta linear Vibrolux Reverb ( AA270 ) to a Blackface AA964. I keep original transformers. It sound great.
I work on amps ( and more )  since 20 years

New to amps mod, be careful before doing anything, you may damaged the amp.
It is not a amp to learn how to.
Tubes amps have lethal voltage

A better lead dress is the first ( and only ) thing I'll do.

1- Is the amp working fine now  ?
2- Check amp after each small mod.

[22uf]

1- Is the amp working fine now  ?
2- Check amp after each small mod.

Hi. Thanks for the reply!

1. Yes, the amp is working fine now. I have some 60 cycle hum which is not mechanical but the tubes are tired.
2. I plan to! :) No big changes, incremental adjustments.. :)

** I have a resistor on a clip which I have used to safely discharge the filter caps.
I measured them with the meter and watched them empty out.

The amp is fully serviced and I can't see anything obviously wrong or in need of work. Its relatively unmolested for its age.

I have cleaned the chassis and checked the heater wires are clear of everything. I have cleaned the pots and the input jacks.
I have also re-tensioned the socket pins.

I believe the Bias Balance is still just a balance and has not been converted to Bias Adjust etc.
I think that will be one of the first mods I make: add a 25k trim pot on there so I have Balance & Adjust. I do not intend to pay for matched output tubes if I don't have to. :)

I think (I am not a expert obviously) that this is a good platform to make some tasteful/subtle improvements/safety upgrades and tone mods to?

That is why I purchased it anyway. I wanted one that was in good shape and that I could easily figure out without having to unwravel decades of hacks! :)

[pdf64]

For any Fender, beefing up the screen grid resistors above 1W is a retrograde change IMO, unless HT fusing is also fitted. Just replace them with 1W flame retardant MO types.
I strongly suggest to fit 1 ohm cathode resistors (eg between terminals 1 and 8, move the chassis braid to 8), then you can easily assess each tube's idle current and its response to signal.
Try it with one leg of the bright cap on the master volume lifted.
Try it through some decent speakers - the stock ones were poor, coping with the high power at a low cost may have been the purchasing goal.
Later SFs seems bassy, a big part of that is due to the steeper taper of the bass pot used (about 30%, same as the treble); it needs to be set lower and can be a hair trigger between not enough and too much. If it's bugging you, try a regular CTS audio taper 220k in there (10% taper, same as BF).
Take note of the OT secondary / speaker output arrangements, they're unlike any other Fender, before or since, eg speaker jacks in series, impedance switching built into the jack. Don't remove the insulation washers off the jack unless you've thought it through.
Also the humdinger heater balance pot is usually damaged, due to power tube shorts, so check its resistance and operation is good.

I suggest not to undertake any typical 'blackfacing' type mods until you've got it working right and have heard it with decent speakers. The circuit design changes were sound, user driven (eg get rid of bass flub at high power) and done by highly competent engineers.

And this amp range are not ultralinear, the OT's primary taps for the screen grids are at about 12.5%, so the amp operates basically in regular pentode mode. My guess is that the intent and effect of the use of the tapped primary was to reduce screen grid dissipation, rather than to create a more linear audio response. Screen grid dissipation being an issue due to the higher HT voltage and increased OT primary impedance, compared to previous models.

[sluckey]

I strongly suggest to fit 1 ohm cathode resistors (eg between terminals 1 and 8, move the chassis braid to 8), then you can easily assess each tube's idle current and its response to signal.

You want pin 1 to be grounded, not pin 8.

[22uf]

For any Fender, beefing up the screen grid resistors above 1W is a retrograde change IMO, unless HT fusing is also fitted. Just replace them with 1W flame retardant MO types.
I strongly suggest to fit 1 ohm cathode resistors (eg between terminals 1 and 8, move the chassis braid to 8), then you can easily assess each tube's idle current and its response to signal.
Try it with one leg of the bright cap on the master volume lifted.
Try it through some decent speakers - the stock ones were poor, coping with the high power at a low cost may have been the purchasing goal.
Later SFs seems bassy, a big part of that is due to the steeper taper of the bass pot used (about 30%, same as the treble); it needs to be set lower and can be a hair trigger between not enough and too much. If it's bugging you, try a regular CTS audio taper 220k in there (10% taper, same as BF).
Take note of the OT secondary / speaker output arrangements, they're unlike any other Fender, before or since, eg speaker jacks in series, impedance switching built into the jack. Don't remove the insulation washers off the jack unless you've thought it through.
Also the humdinger heater balance pot is usually damaged, due to power tube shorts, so check its resistance and operation is good.

I suggest not to undertake any typical 'blackfacing' type mods until you've got it working right and have heard it with decent speakers. The circuit design changes were sound, user driven (eg get rid of bass flub at high power) and done by highly competent engineers.

And this amp range are not ultralinear, the OT's primary taps for the screen grids are at about 12.5%, so the amp operates basically in regular pentode mode. My guess is that the intent and effect of the use of the tapped primary was to reduce screen grid dissipation, rather than to create a more linear audio response. Screen grid dissipation being an issue due to the higher HT voltage and increased OT primary impedance, compared to previous models.

Wow! Thanks for taking the time to type all that out! Much appreciated. 

I hear you both re: getting the amp to be the best it can be before doing any fundamental mods.

Here's my to-do list so far then:

1. Upgrade bias balance bot to bias and bias balance pot(s).
2. Assess the hum pot for function and replace if neccessary
3. Beef up the screen grid resistors above 1W - these have already been replaced with 5w 470ohm ones. (Done)
4. Fit 1 ohm cathode resistors.
5.
6...

- The taper on all the EQ's is basically just on/off. This seems to be consistent with those I have seen on similar vintage Fenders. The advice I was given was to 'set to 'on' and leave it there'. I'm happy with it tbh (I'm not the guy who's chasing micro adjustments in his tone stack hehe) but duly noted on the CTS upgrade option.
- The bright cap on the MV is a mod I will add to my "to try after basic preventative maintenance" list. :)
- I have fitted decent speakers. Its got 2x Eminence Legend 1258s in it which I like. I've just picked up a Celestion Creamback 65 which I intend to swap in to give it a bit more girth overdriven.
- Duly noted on the output arrangements. I read this elsewhere recently when researching if I could use 16ohm speakers together @8ohm and on the 135w models you can if you plug them in to the extension speaker jack etc. Thanks though. Thats really important to know!

I have also read that these amps weren't really 'ultra linear'. Its interesting how loathed they seem to be among the internet headcount. Its not a blackface but then I don't want a blackface Fender really. I have Vox and Marshall for that stuff. My only problem with it is that it sounds a bit 'thin' and lacks that almost-stereo vibe you get from a nice 2x12.

I have attached some close ups of the current socket arrangement. The 470ohms bricks measure dead on.
The original grid control resistors are there still. Should they be replaced as a precaution? They seem to test OK.

[22uf]

I strongly suggest to fit 1 ohm cathode resistors (eg between terminals 1 and 8, move the chassis braid to 8), then you can easily assess each tube's idle current and its response to signal.

You want pin 1 to be grounded, not pin 8.

Noted!

[sluckey]

Actually, Fender mounts the 1.5K grid stopper between pin 1 and pin 5 so you can't use pin 1 for the 1Ω current sense resistor. Probably just need to disconnect the ground braid from pin 8 and connect the 1Ω between pin 8 and the braid.

[shooter]

I have fitted decent speakers. Its got 2x Eminence Legend 1258s in it which I like. I've just picked up a Celestion Creamback 65 which I intend to swap in to give it a bit more girth overdriven.

I paired an Eminence re-issue Legend with the Creamback and I haven't found a better match for any amp I've built.  so you're on track for speakers 
didn't look a schematic yet;
a no-solder change, try an AY, AT, or AU in the "V1" spot.

[DummyLoad]

add a choke for the preamp ckts. - use small(er) 10-15H 25-40mA part. ditch the MV and silly drive ckts. blackface the LTPI and you'll have an awesome sounding, very quiet, very powerful amp.

JBL D120 or EV-12L sound spectacular in these.

--pete

[pdf64]

What benefit does adding a choke to the preamp HT supply bring 

...I could use 16ohm speakers together @8ohm and on the 135w models you can if you plug them in to the extension speaker jack etc. Thanks though. Thats really important to know!
...
The original grid control resistors are there still. Should they be replaced as a precaution? They seem to test OK.

Yes, as a quirk on this model, plugging into the ext speaker jack should provide an 8ohm compatible output. However, I wouldn't rely on the shorting contact of the main speaker jack to complete such a crucial circuit, considering the 4 decades of crud that's accumulated on to those unenclosed, unexercised contacts, which the speaker plug has been holding open all that time. Rather, insert a shorted plug into the main jack.
The power tube sockets have been replaced, they wouldn't have been ceramic type originally, so the associated resistors may have been replaced when that was done. If original, considering their 'mission critical' role of keeping the tube biased, and the thermal / mechanical stress they're subject to, I think replacing them is good practice.
Apologies for the bad guidance re socket terminal #1; the last few amps I've been working on have had EL34 compatibility, where mounting the control grid stopper across the socket hasn't been an option, so I was somewhat complacent / negligent earlier; thankfully sluckey spotted my error 

[2deaf]

Here is a schematic.

[2deaf]

Here are a couple of bias mods.  I have a lot of documentation on this amp if you want to know how it is wired.

[Latole]

Original Fender /CBS schematic

http://ampwares.com/schematics/twin_reverb_sf_135.pdf

[2deaf]

Here is a schematic that came with a Twin I had decades ago.

[Latole]

Here is a schematic that came with a Twin I had decades ago.

Same as mine few minutes ago 

[sluckey]

Same as the schematic in Hoffman's library.

[2deaf]

Same as mine few minutes ago

Mine's better.

[Latole]

Same as the schematic in Hoffman's library.

That is why IS better 

[2deaf]

Now that I have looked at the pictures, I see you have the export version.  It appears to be hard-wired for 240V.

[pdf64]

Now that I have looked at the pictures, I see you have the export version.  It appears to be hard-wired for 240V.

It's a good idea to prevent wandering fingers from tinkering with that setting, accidentally or otherwise; pretty common to find them turned a notch higher or lower than where it should be.

[shooter]

get a softer/less hi-fi tone

maybe go the other way with speakers   
sell/trade for a Vox   
pull 1/2 the power tubes, make it cathode biased 

[sluckey]

...

[66Strat]

I would lose the master volume and maybe wire in a Presence control in its place. Consider re-voicing the pre-amps, maybe the Normal channel along the lines of a 6G14 Showman and the Vibrato along the lines of an AB 763 Super Reverb.

[22uf]

Actually, Fender mounts the 1.5K grid stopper between pin 1 and pin 5 so you can't use pin 1 for the 1Ω current sense resistor. Probably just need to disconnect the ground braid from pin 8 and connect the 1Ω between pin 8 and the braid.

Noted, noted. Thanks. Ordered the bits. :)

add a choke for the preamp ckts. - use small(er) 10-15H 25-40mA part. ditch the MV and silly drive ckts. blackface the LTPI and you'll have an awesome sounding, very quiet, very powerful amp.

JBL D120 or EV-12L sound spectacular in these.

--pete

I've seen some posts where a fellow on TDPRI did something similar. Not for me though - too advanced. I'm a complete novice at this stuff. Also I want to preserve the inherent quality of the amp without making major circuit alternations. Mainly because I'll probably want to sell it on to fund the next project. Thanks for the suggestion though. :)

What benefit does adding a choke to the preamp HT supply bring 

...I could use 16ohm speakers together @8ohm and on the 135w models you can if you plug them in to the extension speaker jack etc. Thanks though. Thats really important to know!
...
The original grid control resistors are there still. Should they be replaced as a precaution? They seem to test OK.

Yes, as a quirk on this model, plugging into the ext speaker jack should provide an 8ohm compatible output. However, I wouldn't rely on the shorting contact of the main speaker jack to complete such a crucial circuit, considering the 4 decades of crud that's accumulated on to those unenclosed, unexercised contacts, which the speaker plug has been holding open all that time. Rather, insert a shorted plug into the main jack.
The power tube sockets have been replaced, they wouldn't have been ceramic type originally, so the associated resistors may have been replaced when that was done. If original, considering their 'mission critical' role of keeping the tube biased, and the thermal / mechanical stress they're subject to, I think replacing them is good practice.
Apologies for the bad guidance re socket terminal #1; the last few amps I've been working on have had EL34 compatibility, where mounting the control grid stopper across the socket hasn't been an option, so I was somewhat complacent / negligent earlier; thankfully sluckey spotted my error 

Will do. The schematic says 1.5k 1/2 watt (500mV) resistor. It doesn't specify what voltage rating they need to be though? I presume these don't carry any/much current and the lowest (240v iirc) I could find for sale will suffice?

Thanks for the heads-up re: shorting the 4ohm speaker input. I have some 1/4" to 3.5mm adaptors I can modify for the task. The replacement sockets are actually plastic! Very cheap and nasty. I have ordered some actual ceramic replacements (they were cheap so why not).

Here is a schematic.

Thanks for the schematics all! I did find the original Fender one a bit hard to follow the redrawn one is much clearer!

[22uf]

Here are a couple of bias mods.  I have a lot of documentation on this amp if you want to know how it is wired.

Yes, please, I'll take everything you have! I'm keen to learn as much as possible and especially if someone has already done all the hard work.. :)

The bias mod diagram is ace, I'll definitely be following it!

Now that I have looked at the pictures, I see you have the export version.  It appears to be hard-wired for 240V.

Good spot! It was hardwired by the previous owner who said the selector knob kept getting knocked to the wrong setting by roadies.

I have read that if you intentionally set these amps to 260v (where in the world has 260v?!) but feed it 240v it can actually improve tone. Other people said that was a terrible idea and would cause issues long term.

Needless to say, no plans to change it!

get a softer/less hi-fi tone

maybe go the other way with speakers   
sell/trade for a Vox   

I have an AC30CC2X which I use for everything else. You are dead right re: the speakers though. Celestion is the wrong way .. I've been testing the Creamback 65 + Legend 125 combo and while it sounds much better for crunch, the clean tone is basically nuked. The speakers don't seem well matched in sensitivty (100.1db vs 97) and I kept trying to EQ out the midrange, but the EQ on the amp basically stopped being effective at all. Back to 2x Legend 1258s. Maybe the 75w Creamback might be a better match but I'm not in a position to pay £125 to find out. 

I would lose the master volume and maybe wire in a Presence control in its place. Consider re-voicing the pre-amps, maybe the Normal channel along the lines of a 6G14 Showman and the Vibrato along the lines of an AB 763 Super Reverb.

I actually find the mv useful? I can dime channel volume and set mv to 5-6 and get a light (very light) drive. I can then use a Klon to get overdrive and if I use the pull out boost (which as far as I can tell looking at the schematic just mainlines the signal from the 12ax7 direct to the output) it sounds basically the same as putting a distortion on top of the overdrive and sounds pretty lary (in a good way). I don't want to remove the MV anyway: mainly I think because I am playing it at less than concert pitch and the MV really does help get a usable practice/bedroom tone out the amp. I honestly don't notice much degredation in the tone either - certainly nothing I can't boost back in with the EQ on the amp.
The Eminence speakers are very tight though. It would probably not be so great with V30's or w/e. It works for me anyway, so I'm sticking with it. :)

I do like the idea of re-voicing the normal channel as that is essentially useless/redundant. Might as well! Do you have any advice on how to go about that? :) Point me in the direction of some written material?

[22uf]

Change your battery!

Why do I get the impression that you already noted my blood type, cloned my license plate and have iniated construction of a "nest" under my house? 

[shooter]

attention to detail, it's what makes a good Tech really good 

[22uf]

attention to detail, it's what makes a good Tech really good 

I'm sure all the more successful serial killers (the ones nobody has ever heard of) were amp techs. 

So I was testing those speakers today and I noticed that this fella was getting *really* hot and smelly with the chassis on the desk.

Is that normal? What does that big resistor do?

[shooter]

Is that normal? What does that big resistor do?

if it's the 30k R; (hard to read in image)
20W gets hot, look at the schematic, power supply section, just right of the SB switch, It's there to bleed off the caps

[22uf]

Is that normal? What does that big resistor do?

if it's the 30k R; (hard to read in image)
20W gets hot, look at the schematic, power supply section, just right of the SB switch, It's there to bleed off the caps

Ahh OK. I guess the smell is just it being on the desk for a few days and its accumulating 'particulates'. :)

I really could not get that creamback 65 to work with the Legend 1258. Did you use the 75w version not the 65w?

[shooter]

came from the '80s 65w
never put it in anything over 40watts since I only have 4 acres 

[2deaf]

Kind of rough, but they come in handy sometimes.

[2deaf]

...

[2deaf]

Your AC power is different, but the rest of the amp should be the same.

[2deaf]

...

[2deaf]

...

[shooter]

Kind of rough

They are nice examples of what life was before we let computers obfuscate   

[Willabe]

Same as mine few minutes ago

Mine's better.

 

[Willabe]

Here's the link for a SF 135w UL Bassman I rebuilt for a friend;

https://el34world.com/Forum/index.php?topic=23749.msg255442#msg255442

I did a LOT in that amp. He LOVES it now!

[22uf]

...

Wooah! Thank you very much!! 

Here's the link for a SF 135w UL Bassman I rebuilt for a friend;

https://el34world.com/Forum/index.php?topic=23749.msg255442#msg255442

I did a LOT in that amp. He LOVES it now!

Now that's a good read!

1. Wired 2 of the 4 - 6L6's as triodes, took them out of UL. This added even order harmonics.
2. Biased the 6L6's up to 60% dis.
3. Disconnected the -FB loop, with 2 - 6L6's still in UL, that still gives the amps power section some FB.
4. Changed the screen grid R's to 1K.
5. Rebuilt the PI to BF values. Most importantly, 1M grid returns and 82K/100k plate R's.
6. Installed 3 x 10ohm sag R's in the B+.   

I think I'll try these mods when I'm up to scratch. I see a lot of posts advocating the PI changes as a bare minimum for improved tone.
I don't dislike the tone as-is, its just a bit bright and lacks that "woody" character I associate with the earlier amps. Its also super focussed in the highs and has no 'sag' whatsoever. Mitigated somewhat by the midrange-y speakers but it could do with a little garam masala..

[Willabe]

There's more than that list that I changed, read reply #66, I changed the preamp tubes bias/set up;

https://el34world.com/Forum/index.php?topic=23749.msg255912#msg255912

[22uf]

There's more than that list that I changed, read reply #66, I changed the preamp tubes bias/set up;

https://el34world.com/Forum/index.php?topic=23749.msg255912#msg255912

So to clarify the title of this thread was the '135w Ultralinear "1/2 Blackface" Mod' and I consider that to be the PI circuit component changes as detailed here: https://www.premierguitar.com/articles/19916-ask-amp-man-twinterventionrescuing-rotten-twin-reverbs

^ In the above article the boost switch is disconnected from the circuit. That leaves me a push pull free to re-allocate to some other purpose. Since its not possible to wire it up to deliver bacon on demand, maybe something less extravagant?

But thanks to all the awesome info you guys have provided I'm feeling confident enough to have a stab at a few more tweaks. 

So now my To-do list looks like this:

1. Upgrade bias balance bot to bias and bias balance pot(s) as per 2deaf's awesome layouts. Bias amplifier to 60% dis (after completing task 4).
2. Assess the hum pot for function and replace if neccessary. It works fine. My amp doesn't have any discernable hum once the bias balance adjustment pot is twiddled to the right spot (about 2 o'clock). Its a very noticeable change from noisey to silent on the trim. The amp sounds crisper and more 'Fendery' and less midrange/flub when properly adjusted. (Done)
3. Beef up the screen grid resistors above 1W. These have already been replaced with 5w 470ohm ones. (Done)
4. Fit 1 ohm cathode resistors as per Sluckey's suggestion: "Fender mounts the 1.5K grid stopper between pin 1 and pin 5 so you can't use pin 1 for the 1Ω current sense resistor. Probably just need to disconnect the ground braid from pin 8 and connect the 1Ω between pin 8 and the braid."
5. Replace battery in MM. 
6...

Question(s) regarding the mods you mention as being the most impactful tonally Willabe:

3. Disconnected the -FB loop.
4. Changed the screen grid R's to 1K.
6. Installed 3 x 10ohm sag R's in the B+.   

3. Is it worth messing with the -FB if continuing to run the amp in UL?
4. Is a -500ohm reduction in screen grid R value going to make any signifcant impact on tone in UL?
6. Sag R's would basically slow down the filter caps and give the amp some 'sag', but presumably that's not much use in a 135w clean amp, as it never gets loud enough to enjoy the benefits?

General question: is there any need/experienced recommendation to disconnect the hum balance pot and go with two 100 ohm resistors across the back of the lamp assembly instead? Purely as long term preventative maintenance? Or is it a case of it ain't broke? I suppose if I did that I could repurpose the hum balance pot for something else (that isn't going to harm anything if it fails at a later date)?

It has also been suggested by someone elsewhere to: "It's also good to blackface the reverb circuit.  Do this by changing the cathode resistor on V3 (reverb driver) and adding a 22uf/25v bypass cap.  Pins 3 & 8 will be joined on V3, with a 470R or 680R cathode resistor.  Change this resistor to 2K2 and add the 22uf/25v bypass cap!  This runs the tube much cooler!"

I think I saw Uncle Doug do this in one of his repair videos. 

[Willabe]

Question(s) regarding the mods you mention as being the most impactful tonally Willabe:
3. Disconnected the -FB loop.
4. Changed the screen grid R's to 1K.
6. Installed 3 x 10ohm sag R's in the B+. 

3. Is it worth messing with the -FB if continuing to run the amp in UL?
4. Is a -500ohm reduction in screen grid R value going to make any signifcant impact on tone in UL?
6. Sag R's would basically slow down the filter caps and give the amp some 'sag', but presumably that's not much use in a 135w clean amp, as it never gets loud enough to enjoy the benefits?

#3. Try it and listen for your self. It's 1 wire to disconnect.
#4. Same as above, try it and listen.
#6. It doesn't slow down the filter caps, it sags the B+ dcv. If you play full blast or close to it, the amp stays sagged. If you play 4 to 6 maybe 7, then it sags the peaks in the B+ which is noticeable, compression. Same with the larger screen grid R, sags the dcv peaks, compression. 

General question: is there any need/experienced recommendation to disconnect the hum balance pot and go with two 100 ohm resistors across the back of the lamp assembly instead? Purely as long term preventative maintenance? Or is it a case of it ain't broke? I suppose if I did that I could repurpose the hum balance pot for something else (that isn't going to harm anything if it fails at a later date)?

You just wrote;
My amp doesn't have any discernable hum once the bias balance adjustment pot is twiddled to the right spot (about 2 o'clock). Its a very noticeable change from noisey to silent on the trim. The amp sounds crisper and more 'Fendery' and less midrange/flub when properly adjusted.

[sluckey]

I would leave the hum balance pot as is. IMO it's superior to the resistors or a real center tap.

[pdf64]

...
2. Assess the hum pot for function and replace if neccessary. It works fine. My amp doesn't have any discernable hum once the bias balance adjustment pot is twiddled to the right spot (about 2 o'clock). Its a very noticeable change from noisey to silent on the trim. The amp sounds crisper and more 'Fendery' and less midrange/flub when properly adjusted. (Done)...

There are 2 screwdriver adjustable pots on the back panel.
I think you may be getting the 'output tube matching' pot (aka bias balance) mixed up with the 'hum balance' pot.
For their adjustment instructions, see http://www.thevintagesound.com/ffg/manuals/vibrosonic_twin_dualshowman_79_pg3.jpg

Regarding the Jeff Bober article, I’m somewhat sceptical that there’s a sound technical rationale behind the suggestions.
The ‘BFing’ of the LTP seems kinda random and knee jerk.
That the 1M resistor in the pull boost circuit (ie when it’s not engaged), could load down the signal (to any more than a barely measurable degree) seems nonsensical. That plate’s output has to overcome the heavy loading of the 50k trem intensity pot, whether it also has to drive the 1M012 + 10nF of the pull boost isn’t going to matter a jot.

[22uf]

Well I just spent a week practicing with the amp and gigged it yesterday evening.
It took a while to dial in the EQ - you really do need a dramatically different EQ for single coils vs humbuckers. Unlike most amps where you can get away with just adjusting the bass/mids a bit.

I also swapped out V2 with a new Shuguang A12AX7 that came out of a Marshall Origin head (branded Marshall). That tightened things up a LOT.
Honestly I don't understand the hate for these amps. Its loud, tight, plenty warm and the bass and mids really project. With the addition of some light overdrive from the Soul Food (klon clone), it sounds wonderful and delivers all the crunch I'd want at more or less any volume.

In short, I will not in fact be making any changes to the tone-effecting elements of the amp. I will just add that bias trim pot, check the bias isn't insane (if its cold, I'm still leaving it as it sounds great) and swap those carbon composite screen resistors for some flameproof metal film ones. I DO use the master volume but not the pull out boost, but since it has no impact on the tone, I can't see any reason to mess with that either.

In summary, this is a great amplifier that does everything I need (classic rock, blues and jazz) and sounds great. It does benefit from a bit of compression but the reverb and tremolo is glorious.  I'm totally at a loss to understand why people say they suck. 
The modern speakers make a huge difference I guess ..

[pdf64]

Good call, well done for taking a considered, deliberate approach and getting the amp working right before buying into received wisdom regarding BF mods.
And though it may seem somewhat naff, a couple of players I fixed these for find an occasional use for that pull boost; after all, it’s a pretty unique  sound that can’t easily be replicated 
Pleased it’s worked out for you

[22uf]

Good call, well done for taking a considered, deliberate approach and getting the amp working right before buying into received wisdom regarding BF mods.
And though it may seem somewhat naff, a couple of players I fixed these for find an occasional use for that pull boost; after all, it’s a pretty unique  sound that can’t easily be replicated 
Pleased it’s worked out for you

It really is that. Its a very unique sound through a drive pedal! 
I don't forsee myself ever using it tbh. From what I read (random internet knowledge) it was not intended as a filth knob by Fender in the first place? Wasn't it designed as a 'clean' boost?

The amp "pops" in to life around 5. Below that it sounds terrible, completely throttled. I wonder if the haters hate it for not being 20 watts?

[22uf]

I've been playing this amp as my #1 for a good long while now. I can even lift it in and out of the car after a course of steroids and a high protein diet.

Only mods made were to pull V5 and put NOS RFT's in V2 and V4. I left the ancient JJ ECC83S in V1 as I think the amp sounds less vintage/Fender'y with the hotter bias when V1 is removed.

I make good use of the master volume and the preamp starts to distort at 5 on the vibrato channel volume. At 10 its full on overdrive. I have an OCD 1.7 for when I need more (not very often) and I find all it needs for solos is a bit of compression. I have had good results with a TS808 (original 1984) but the only pedals apart from a tuner I regularly use with this is a Flashback X4 and a Bonamassa Dunlop Wah. 

I have bumped this one to ask a follow up question: the power tubes are about dead. They weren't new when I got the amp, I'm certain, but I was wondering if, since I never play this past 5-6 on the master volume, would it extend the life of the 6l6GC's any if I uprated the screen resistors to (say) 2k/5w? I really don't need 120dbs of clean headroom - and never will with all venues now being either small club or mic'd. $100 for 4 "cheap" tubes every 6-8 months is a bit of a kick in the nuts is all.