SF Twin Reverb (UL).. few Q's

[Sea Chief]

Hi chaps-

picked up a bargain SF Twin Reverb, ultralinear 135w beast recently.. & while its at my amp man for repair (a dogs dinner "BF mod" done to the poor thing, shorted tubes & screen R's went pop: so its being serviced & put back as stock, new grid & screen R's & new matched quad of JJ 6L6's, doghouse recap).. I've a few Q's.

My amp man says doghouse caps orig (silver mallory) & read '78.. but chassis tells me '75. What the dickens?

The spkrs are 50w jensen RI C12N's (BF mod rewired tubes for pentode or s'thing or other, ie 1/2 power 65w anyway, hence these spkrs put in). Any probs having 'only' 100w total? I'll never be diming the thing.

It has a repro BF faceplate (vg quality, I have a DR one with the same pre cbs spiel on).. so I can't tell whether it ever had an MV, as this obvioulsy isn't poss with no hole on the BF faceplate. So, I really need to ID the age of amp > then determine if it falls in the 'defo had an MV' range of dates.

Lastly I see this interesting (similar SF twin) YTube clip, saying 'reverb undone/ rca patchcord from in to out.. gives OD boost' here anyway: safe/ general thoughts? https://www.youtube.com/watch?v=wuoZEPk_f6A

Thx for any help, Sea Chief

[Justin Thomas]

"SF Twin Reverb, ultralinear 135w beast"
I will take this as a "given" for my answers to the rest of your questions.

"My amp man says doghouse caps orig (silver mallory) & read '78.. but chassis tells me '75."
What do the transformers and pot codes say? This will confirm or deny the caps. Also - Fender bought caps by the bazillions. Sure, some could have sat around for 3 years in a bin somewhere before being used. Same for pots and trannies and any other parts.
WHAT on the chassis dates your amp as a 75? Serial numbers cannot be used this late in the game, and any tube charts in the cab were never to be trusted entirely. And even if there is a stamp inside saying "75" or whatever, that's likely to be totally random. As far as I know, Fender never directly dated a chassis.

"The spkrs are 50w jensen RI C12N's (BF mod rewired tubes for pentode or s'thing or other, ie 1/2 power 65w anyway, hence these spkrs put in). Any probs having 'only' 100w total? I'll never be diming the thing."
If you are indeed restoring it to 135W stock, then you may well likely go o w your 100W worth of speakers, domed or not.
A Fender or Marshall 100W can put out 100W long before you can perceive it. And depending on how efficient those speakers are, you may never even HEAR 50W let alone a full 135.

Basically, if I take it on faith that you are truly restoring a 135W Twin Reverb to "stock," then, hell no it's not safe to use with 100W of speakers.

"It has a repro BF faceplate (vg quality, I have a DR one with the same pre cbs spiel on).. so I can't tell whether it ever had an MV, as this obvioulsy isn't poss with no hole on the BF faceplate. So, I really need to ID the age of amp > then determine if it falls in the 'defo had an MV' range of dates."
Fender Master Volumes were issued in 1974. So EVERY Twin Reverb from 1974 til 1981 came with a Master Volume. Look behind the faceplate, bet you'll see unused hole where the Master Volume used to be.

"Lastly I see this interesting (similar SF twin) YTube clip, saying 'reverb undone/ rca patchcord from in to out.. gives OD boost' here anyway: safe/ general thoughts?"
This used to be an "effect" called an "Ice Cube." It was a 470k resistor between two RCA jacks. It's the same thing Fender used to implement their notorious/infamous "Pull Boost," switching the reverb tank out for that resistor. General consensus is that it sounds like butt-ass-crap. I tried it once, good if you're into Ted Nugent and don't mind 10x the noise and hiss.

Justin

[Randall]

"As far as I know, Fender never directly dated a chassis."

Not so. Fender absolutely stamped chassis date codes. A 75 Twin Reverb would range from A81000 to A99999, or B01000 to B15000 for example.

[bob p]

There are no "ultralinear" Fender Twins.

I'm going to chime in with a comment that may not be all that helpful, but I think it's important to say because there's so much misinformation being propagated on the Internet:

There are no "ultralinear" Fender Twins.

Fender actually designed those amps to run in distributed-load mode. Distributed-load mode is used to increase the power output of the tubes, while ultralinear mode is intended to provide optimal power coincident with decreased distortion. These two totally different objectives are achieved by using different turns ratios/tapping points in the transformers.

It's unfortunate, but someone who didn't know what he was talking about gave the 70s Fender amps an "ultralinear" branding out of ignorance; because most people look at the wiring topology while ignoring the transformer specs that incorrect labeling has since become internet-sticky.

What's most sad about this is that the misinformation has become so pervasive that even the transformer companies are now selling UL replacement OTs for Fenders, which were never UL in the first place.

[bob p]

I found this image in the 1951 patent application for Ultralinear operation by Halfler & Keroes. The image represents the difference in operating conditions as a function of the impedance of the screen tap on the transformer primary winding. The dashed bar indicates the operating point chosen by H&K for their UL patent. I added the green bar to represent the lower impedance corresponding to the 12.5% turns ratio that was used by Fender in their distributed load amps.

The diagram illustrates that H&K chose optimal points for low-distortion operation. Their choice of 20% impedance (~43% turns ratio) corresponds to the nadir for IM distortion (C) and (D) while trading off a bit of power (B). In contrast, Fender chose a much lower percentage of primary turns to achieve maximum power (B) and they accepted greater distortion to get it (C)(D).

UL is about minimizing distortion.
DL is about maximizing power.

[Justin Thomas]

UL, Ultra-Linear, Underwriters' Labaratories, Uber-Load... :O

[Sea Chief]

Thx for help chaps- so I'm still not sure whether I can date the amp by the chassis (75) or caps, or xfmrs.. I guess there's a difference of opinion here on this. I dont have amp to hand, its at prob one of if not the very best amp-builder in UK.. so its in good hands/ will be well serviced. He didn't make any major concern re. the spkrs 'if you don't whack it up' (which I won't by a very long chalk) that is, so again a difference of opinion to Justin.

Yes I saw the MV hole, but chassis likely made on mass with all holes/ then this one covered by faceplate -if- different versions with/ w'out the MV.. was what I was thinking. Ok so it had an MV. The one thing I can't redo as it'll totally ruin the BF faceplate.. shame.

So didn't the clip sound any good? odd I thought it sounded pretty interesting & very far from 'butt-ass-crap' so I'm not sure why you say this. Is this safe to do if I want tho? I couldn't care much about added hum or noise unless totally ott.. clip doesn't point to anything nasty added afaict.

UL.. I'll leave this for debate, way over my head apart from its a bigger cleaner TR beastie.. hence being affordable s/h.

Thanks alot- SC

[pdf64]

...UL is about minimizing distortion.
DL is about maximizing power.

Not disagreeing with how you've characterised the implementations, but my understanding was that UL was a specific case of DL, eg any OT with a primary tap for g2 was DL, with tap at a specific point for that tube being UL; the UL tap point being different for different tube types, I think the 6V6 is only 20%.

The overdrive in the clip sounded ok, though I'd be surprised if some of the plethora of pedals wouldn't serve better.
However, I suspect that the degree overdrive in the clip relies on there being a master volume available, and its tone requires the pull boost circuit (which includes some treble reduction before the master volume, in boost mode).

[Sea Chief]

Well, I recently bought a costly £160 'OKKO diablo' as distortion was an itch I do fee the need to scratch.. & altho I have to use an attenuator to get ~fairly good results with my DR alone, I was told a good OD pedal would be much better, this was meant to cover all manner of tones. Not even remotely close: no touch sensitivity, the bass seemed to evaporate once on compared to off: it was chalk & cheese to my DR. One 'organic', the pedal just 'fake distortion', just about unuseable/ very uninspiring. The reason I dont just use the attenuator is A) its a quickly made diy attenuator which does sap the treble, & B) the wear on the tubes. Sold pedal for £90 a few weeks ater.. but I could do w'out pouring £70 down the drain on awful pedals.

[pdf64]

Be warned that the taper on the original pots of these was quite steep; so without the MV, setting a reasonable (eg domestic/small venue) volume level may be something of a hair trigger.

[Justin Thomas]

Seachief,

I tried the reverb bypass in a Dual Showman Reverb, a pretty different amp. The sound in the clip was passable, but I can chalk my impressions up to a YouTube video. If I ever get a 135W TR to try myself, I will be happy to.

I guess my question is, is it truly being restored to stock, or only partially? The MV may be necessary to get the sound in the video - in the video, he set the Master low AND had to use the Pull Boost. That said, FWIW, in 1980, most Fender amps were available in a choice of BF or SF cosmetics. The logos on the BF were horizontal instead of slanted. Also, it's not a crime to put a hole in a repro faceplate. And if your amp guy is as good as you say, it should be no problem at all.

Justin

[Justin Thomas]

Be warned that the taper on the original pots of these was quite steep; so without the MV, setting a reasonable (eg domestic/small venue) volume level may be something of a hair trigger.

Not to mention, the definition of "bedroom volume" varies by individual. I consider a Bassman head on 7 through a 2x15" to be acceptable bedroom volume.

I seem to remember you having a pesky neighbor, chief...

Justin

[bob p]

Yes I saw the MV hole, but chassis likely made on mass with all holes/ then this one covered by faceplate -if- different versions with/ w'out the MV.. was what I was thinking.

Fender did that in the 70s -- they'd manufacture one chassis and use it on several amp models. Those models that didn't use all of the holes simply covered them by the face plate. This tells us two things:

1. You can't necessarily ID the original amp model by it's chassis, because one chassis was used for several models. For example, my Bassman 10 was built into a Bandmaster chassis. All of the holes for the bandmaster are there: pots, transformer mounts, etc.

2. You can date the amp by the appearance of the MV hole on the far right by the pilot lamp. That MV hole didn't exist on any model prior to the introduction of the MV amps. (Yeah, I know that was obvious...)

[bob p]

Not disagreeing with how you've characterised the implementations, but my understanding was that UL was a specific case of DL, eg any OT with a primary tap for g2 was DL, with tap at a specific point for that tube being UL; the UL tap point being different for different tube types, I think the 6V6 is only 20%.

Agreed! Thanks for clarifying that -- I could have worded it better and I appreciate you doing that for everyone.

Technically the Fender implementation would be a specific subset of DL, and UL would also be a specific subset of DL (without overlap in the Venn diagram). Because the two implementations were designed for different objectives, IMO it's wrong to say that Fender ever made any UL amps. According to the H&K definition of UL in their patent and their HiFi embodiment, UL is a specific case where that tap location is chosen such that distortion is minimized. My point was to say that Fender never built UL guitar amps. Fender wasn't after lowest distortion, they were after highest power output.

[Leo_Gnardo]

My point was to say that Fender never built UL guitar amps. Fender wasn't after lowest distortion, they were after highest power output.

But ... if you DID want a low distortion amp you could swap in say a Dynaco Mk III output transformer, yes? I've seen it done. Also consider the Sunn amps that did use Dyna OT iron.

Good discussion & clarification thanks to bob p & others.

And chief, I remember that sensitive-eared neighbor of yours too. How's he getting along with your 135W Twin? Has he gone completely deef, or moved on to West Undershirt?