Help with 135w "UL" Twin Reverb

[Bender]

Hello everyone! I acquired this 135watt "UL" loud beast about a couple years ago and I'm finally getting around to giving her the thorough twice over.

It has been serviced and recapped that much I do know - I was told from the seller that it was "moded" to be black faced by the tech on the normal channel... made me chuckle as I know enough about this amp to know it ain't getting blackfaced anytime soon because of its circuit but I digress...the price was right and it didn't seem to bastardized too bad,original transformers etc... and I also checkout out the tech website he's an old dude and reputable works for a studio but herin lies the rub... I can't for the life of me get a hold of the guy so I don't know exactly what's been done to this amp , if anything besides the recapping job and throwing in a crummy Marshall speaker🤢🤢(being replaced with a Weber Chicago as we speak)

In any case I've found the schematic for the 135 but I'm no pro- not by a long shot, and I'm aware of the dangers of servicing & tinkering about such a high power am but unfortunately the only layouts online for the silverface model is the AA270 which doesn't necessarily represent what's going on in a 1979 twin reverb i.e no a bias pot, a master volume,hum balance, and output tube matching pots the reverb taps etc.

So In any case I've taken bunch of pics and have made some notes/ observations- hoping someone more privy could tell me exactly what's up here so I can make decisions on what to do "mod" wise without confillicting witth anythings that's already been done. Long story short all I want to do is the basic tweaks as showcased in PG but again my guys a slightly different beast because of the year and if mods have indeed been made to the overdrive circuit I don't wanna go about the second ladder of these mods if it's gonna conflict with any previous voodoery

https://www.premierguitar.com/articl...n-twin-reverbs
https://www.premierguitar.com/articl...e-twin-reverbs

As seen in those articles it's mostly about phase invertor input & output stage and adding a true bias to the circuit. I'm not even opposed to changing back the "mods" to spec if any, again I just want to know what's up. Basically I want the clean machine that's the point of these amps- but I would like it to be a littler more versatile and play with pedals a little nicer for recording & show purposes.

Now on to the good stuff! Here's a link to the Photobucket with pics & here's my observations/questions.

_Geardude_'s Library | Photobucket

And the documents for those that want them
http://www.thevintagesound.com/ffg/s...270_layout.gif
http://www.thevintagesound.com/ffg/s..._135_schem.jpg

Notes /Questions:

* 68k resistors replaced on Normal Inputs to Pin 1 & 2 of V1
* Hum balance isn't a screwdriver nut like other models I've seen instead it has a pontentiometer. normal?
* no visible suppression capacitors on the 4 6l6gc tubes they'd be there on in the gain tubes correct?
* Power caps recapped (not pictured)
* some capacitors changed to orange drops?
* new chord? pin 7 v2 to vibrato volume pot
* new chord/ potential mod? Middle tone vibrato channel pot to .033 metal cap to 100k resistor
* Input phase invertor resistors replaced with stock 330k on grids of the 12at7 tube pins 2 & 7
- following that the 33k resistor was changed to a 22k resistor as shown PG articles, but the invertor resistors weren't changed to 1m resistors so I guess I can do that!?
* phase resistor output looks different from the article but I think I spotted the 2 47k resistors that go to pins 1 & 6 according to the article I can change pin 1's resistor to 82k & pin 6's to 100k? But they tap off to a wire leading to one of those giant white resistors/ it's not on the board like the article so not sure I can/should change that?
*what are these giant xicon 5w resistors on the power tubes and what about these giant rock wood ones? I remember reading the smaller guys could be clipped off the tubes but don't remember where that article/forum was.
*replaced/new 100k resistor pair next to rockwood resistor/ ground bar ? near the transformer
*replaced/new 68k resistor next to Capacitor going to v9 power tube pin 1
* new 82k & 18k resistors going to pin 1 of v1 possible mods?
* replaced 2 100k resitors going to pin1 v1 & pin 6 v1 &....
- There's a red wire right here two which could be pet or that normal channel mode cascading perhaps?
*new 1.5k resistor to from normal input to pin3 v1 in combo with a new? 100v15uf capacitor.potential Mod?
*new 1.3k resistor with new? 50v cap unknown value as writings on other side to pin 8 v2. Potential Mod?

That about wraps up the whole enchilada. Thanks and appreciate any feedback/guidance ya'll could give. In spotting these mods!

[Leo_Gnardo]

Wow what a project! I'm not gonna answer all at once, but first let's look at these:

*what are these giant xicon 5w resistors on the power tubes and what about these giant rock wood ones? I remember reading the smaller guys could be clipped off the tubes but don't remember where that article/forum was.

The Xicon 5W on the tube sockets are screen grid resistors. Sure I read somewhere some guy said you could clip 'em off, IF you want your amp to go silent. So . . . let's leave 'em be. They replaced some 1 or 2 watt carbon comps which may have been roasted. As for Rockwood ww's, the smaller one is the first dropping resistor in the high voltage power supply, and the big one crammed in the corner is a "bleeder" working as a sort of rough regulator for the hi voltage supply. They ain't broke? don't fix 'em.

I'm sure our team of experts will sound off on all your other points, stay tuned. And welcome aboard!

[Richard]

I think this is the correct schematic.
The Fender Amp Field Guide

[pdf64]

Obviously Jeff is a very successful and popular tech but I think all the changes suggested in the articles linked are retrograde.
The amp will tend towards flab, flub and fart, due to the low frequency response being extended / phase splitter gain and output potential increased. The AB763 TR kinda mitigated these issues with the 1nF coupling cap at the phase splitter input; unfortunately this seems to have been overlooked in the articles.

My perception is that the 135 was carefully designed to mitigate blocking distortion, the cause of these issues.
If you want the amp to be loud, clean and responsive then leave it as it is; adding an overall bias voltage trimmer is a helpful service feature of course.
If you want to 'open the amp up' some then try reducing/removing the NFB.

The biggest weakness with these amps were the stock speakers, which hamstrung the poor thing; flat tone, inefficient and their voice coils popped anyway. Pretty much anything else is an upgrade.

From the photos, it seems that the amp may have been modded to some degree. Unfortunately the convoluted wiring on these makes trying to work out the full extent of these difficult. Sorry but the photos aren't great.
Something looks to have changed in the phase splitter circuit.

[Bender]

Wow what a project! I'm not gonna answer all at once, but first let's look at these:



The Xicon 5W on the tube sockets are screen grid resistors. Sure I read somewhere some guy said you could clip 'em off, IF you want your amp to go silent. So . . . let's leave 'em be. They replaced some 1 or 2 watt carbon comps which may have been roasted. As for Rockwood ww's, the smaller one is the first dropping resistor in the high voltage power supply, and the big one crammed in the corner is a "bleeder" working as a sort of rough regulator for the hi voltage supply. They ain't broke? don't fix 'em.

I'm sure our team of experts will sound off on all your other points, stay tuned. And welcome aboard!

Thanks for the welcome! Active member a handful of other forums but popping my cherry here! Good to know about the 5w xicon a and rockwoods thanks for clearing that question mark up really appreciate it!

[Bender]

Obviously Jeff is a very successful and popular tech but I think all the changes suggested in the articles linked are retrograde.

If you want to 'open the amp up' some then try reducing/removing the NFB.

The biggest weakness with these amps were the stock speakers, which hamstrung the poor thing; flat tone, inefficient and their voice coils popped anyway. Pretty much anything else is an upgrade.

From the photos, it seems that the amp may have been modded to some degree. Unfortunately the convoluted wiring on these makes trying to work out the full extent of these difficult. Sorry but the photos aren't great.
Something looks to have changed in the phase splitter circuit.

Thanks for the reply, I know the pics aren't the best but I was hoping there were enough for those "in the know" to be able to spot some mods off the bat such as the phase splitter circuit & the normal channel. In anycase, after browsing for the NFB resistor and I can't spot it in the usual area's so I'm thinking that might have already been clipped as well... it's my guess that maybe something was done on the normal channel */or the tone stack to give some more flexibility & difference in tone but that's just me trying to remember what was said during the sale.

As for the speakers, it came with 2 Utah/Pyles,one broke and was replaced with a junk Marshall before my purchase....the Pyle is apparently the worst of the bunch, but honestly it doesn't sound bad to me, but maybe down the line I'll flirt with either a c12n or weber california after seeing how the pyle plays with a Weber chicago

[Tom Phillips]

Thanks for the reply, I know the pics aren't the best but I was hoping there were enough for those "in the know" to be able to spot some mods...

It would help if there was a photo shot of the complete guts. I could only see various isolated sections.

[Bender]

It would help if there was a photo shot of the complete guts. I could only see various isolated sections.

Just added that and some brighter photos. In the previous photo set I tried going from right to left ( normal channel inputs to master fader) with close ups on suspected mod/replacement part sections

[Bender]

BUMP! Just scrapped all the old photo's and added new & brighter ones. Any help from those that would like to chime in would be appreciated! Would also like to know if anything looks like it might need replacing or what to test for at an amp this age that's been already been recapped.

_Geardude_'s Library | Photobucket

[Bender]

Alright so the plot thickens an obvious thing to also mention is that v6 appears to be a El84 tube and not a 12at7 as well... the label has since faded but its larger than the standard preamp tube sizes

[Enzo]

Larger doesn't make it an EL84, does it SAY EL84 on it? Look at the structure inside, is it one big thing like an EL84? or does it have two side by side things? the original 12AT7 is a dual triode, but so are taller tubes like 12BH7 or 6CG7. Look at the wiring. Are pins 4 and 5 tied together and heater 6v applied to 4/5 and to 9? Are plates 1 and 6? etc. 12BH7 will drop in place of a 12AT7 electrically, though it is not the same specs. 6CG7 has the same active element pinout, but 6v must go to pins 4 and 5, so would require rewire of the socket for heater.

On the other hand, someone could have stuck a EL84 in it and wondered why the amp didn't work.

The trouble with identifying "mods" is that if you give the same circuit modifications to three of us here, and we all do them, they won't look the same when we are done. I will route my wires different from Tom, and mount my extra resistors and what not in different spots than Richard. My larger blue caps will look different from someone else's smaller yellow caps. and so on. SO what I have to do, is look at your pictures and trace out the circuit I see, then compare that to a schematic of the stock item. LAborious, and no offense, but more than I will do. You can trace out the existing circuit for your amp, it isn;t hard. Use the stock drawing as a starting guide.

Notes:
A pot is a pot, and the circuit has no idea if it has a shaft or a screwdriver slot to turn it.
Non-original caps tells us nothing. They could be same value or changed value, you'd have to check. I am not a cap snob, so Orange drops are fine, so are Mallorys. Frankly I prefer to use axial mallory caps over radial fat orange drops.
You mention swapping the two 47k plate resistors on the phase inverter for 82k and 100k. But those values are ones used with a 12AX7 phase inverter instead of the 12AT7 stock. It is all about the circuit, not the parts. You cannot look at any part out of its context. So if someone did that, then one hopes they put a different tube in there as well.

[Bender]

Thanks for the input, I'll check out what you mentioned and report back. Learning more and more which is great! yeah I'm not sure about the tube As it doesn't have a label here's a pic

http://i1067.photobucket.com/albums/...psdxca83al.jpg

I was just going based off of photos but if it's such a drastic difference/mod for it to be the el then it's probably not that obviously looking more like a 12bh7 but let me check the pins for what you said.

As for the recapping I was talking specifically about the power caps

[Enzo]

Clearly that tube is a dual something, you can see the two sections with a space between them.