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

[bob p]

It's funny that you mention using HiFi iron, Leo. YES, you could definitely swap-in Dyna Mk III iron. It would produce good results.

What's funny is that I've seen a lot of people swapping in Hammond UL HiFi iron over the years in Twins, effectively performing the same DL to UL conversion ... The problem is that these people are doing the DL -> UL conversion and changing the operating point of the amp without even knowing it because they think the amps are already operating in UL mode!

I have to wonder if these people mistakenly converting the amps to operate in UL mode might be responsible for some of the talk about why BF amps are superior to UL Fenders. That kind of talk was never accurate in the era of unmodded SF amps, though it's probably becoming true as more people mistakenly swap UL iron into SF amps needing service. Then they end up comparing the tone of their favorite BF amps to SF amps that never were.

[Leo_Gnardo]

I have to wonder if these people mistakenly converting the amps to operate in UL mode might be responsible for some of the talk about why BF amps are superior to UL Fenders. That kind of talk was never accurate in the era of unmodded SF amps, though it's probably becoming true as more people mistakenly swap UL iron into SF amps needing service. Then they end up comparing the tone of their favorite BF amps to SF amps that never were.

Some, maybe. But I remember as soon as the silver-face amps appeared, we didn't like 'em as much. Could be the look as much, or even more than the sound. I was only in my mid teens . . . 50 years ago . . . seems like yesterday. But I've revised my view over the years, if it sounds good it is good. Plus a good player can sound good thru any competent amp. I still prefer the look of Fender's "blackface" amps but advise my customers to not overlook a bargain price on the silvers which by and large still sell at sleeper prices.

I wonder if those who convert to actual UL mode really notice any difference, or is it a matter of "emperor's new clothes" iow "I spent a couple hundred $$$ making the change, obviously it sounds better! You do agree, don't you?" Without being able to do a quick A/B comparison how can one really know? The ear has a short memory. And the degree of difference in distortion between UL and Fender's DL transformers is likely swamped by the distortion created by any speaker pushed to the 100W mark and beyond.

[potatofarmer]

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.

Well... Neither UL or DL will make as much power as "regular" pentode-mode operation. With the possible exception of the KT88 that ostensibly gets an insignificant boost in output. My guess is that they went with the odd choice of DL tap that they did in order to protect the screens while they pushed the plate voltage over the 500V mark. It also keeps the tone pretty close to pentode-mode, though it does increase headroom and improve damping factor.

The Mullard EL34 datasheet does list characteristics at 20% and 43% of the turns if you want to maximize power or minimize distortion. The target plate-to-plate impedance also changes - roughly double what is typically used in guitar amps. I will say that a quad of EL34 through a 3k3 Harman-Kardon Citation 2 output transformer is a wonderful thing.

I have to wonder if these people mistakenly converting the amps to operate in UL mode might be responsible for some of the talk about why BF amps are superior to UL Fenders. That kind of talk was never accurate in the era of unmodded SF amps, though it's probably becoming true as more people mistakenly swap UL iron into SF amps needing service. Then they end up comparing the tone of their favorite BF amps to SF amps that never were.

It's possible. The DL connection does introduce some additional NFB though. CBS also started paying attention to grid leak specs so that meant different plate loads on the LTP to drive the lowered input impedance. On top of that the wiring is just awful, so a greater amount of noise or parasitics might play into it. Also those terrible J-taper 30% audio pots really do make it difficult to dial in. And the Utah speakers aren't very good. But other than that...

A UL output transformer would be fine; not sure if the Dynaco Mk 3 would be the best choice as it's 4.3k. There are some simulated plate characteristics graphs out there: The 6L6 / 5881 Tube

(oh and thanks for posting on my blog; I really should link the articles better and hopefully I gave everyone credit that I should've)

As for the OP's questions:

If it's a 135W Twin, those were only made from the late 70s through early 80s. The Ampwares page says '77-'82 - Fender Silverface Twin Reverb | Ampwares

All the 135W Twins had a master volume with the not-well-liked pull boost.

A good safe rule of thumb is have your speaker wattage at least double what the amp is rated at. My 135W Twin had the original Utah speakers which were Jensen-type 50W. The cones had gotten moldy and torn so I wound up reconing the baskets, and sure enough there were burn marks on the voice coils. On the other hand, they lasted 30+ years.

[bob p]

The Mullard EL34 datasheet does list...

Have you got a nice, clean copy of the Mullard EL34 data sheet? I must have a half-dozen Mullard EL34 data sheets which are all crappy scans of xeroxes.

(oh and thanks for posting on my blog; I really should link the articles better and hopefully I gave everyone credit that I should've)

Maybe I just didn't read enough, but I never realized that that was your blog. I didn't see anything that connected your name on the blog and your screen name here.

I always get a kick out of it when I find a blog post that mentions me. That is, when the blog mentions me in a favorable way...

[pdf64]

This seems reasonable quality to me http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf

[Sea Chief]

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

Ah no.. Ive moved! Im in relative luxury now, detatched house with 2ft thick stone walls.. I can ramp the vol up a fair bit/ but its not pleasant, not for me as a rule.. Im solidly middleaged now (on occasion its fun say with a borrowed 6g3, which alas even as a lovely built Marsh kit with brown cab etc, & offered to me to buy off a mate, was twice what this TR cost me).

My intention isn't neccessarily to put back 100% stock.. just redo the dogs dinner BF mods back to SF stock that's all
(so there's no undue strain on xfmrs or tubes where there shouldn't be). An added MV would be slap bang in the 'twin- reverb, amp' logo bit.. so would ruin the look totally/ no way even if it woud be useful to me. I might well add it in the back s'where tho.

Ive read the UL 135w type isn't a correct platform to revert to BF anyway, whereby the 80w version colud be.. is that correct?

[Chuck H]

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.

I am impressed and in agreement, BUT...

For the purposes of this post and recognizing common nomenclature, screens fed from taps on the OT has, with this amp model anyway, typically been called "the UL Twin". Maybe we don't need to get into a semantic or pedantic debate. And I appreciate you calling attention to the actuality because "I" had never respected the difference before.

[Sea Chief]

Im not really understanding the ul/ dl debate, so I'll not comment..

Whilst waiting for my amp to come back from repair, I tried my DR head into -one- of the (jensen RI C12N) spkrs: sounded good but then after a while some loud odd 'interference' started, strange snowstorm + weird swishing sounds.. so prominent I had to turn off. No silimar noise switching back into my DR cab (2-10 jensen RI's).

Any idea what this could be? Ive never come across this before.

[J M Fahey]

I guess you killed your speaker.
Head was not damaged, since you tested with the 210, but ... did you retest the single 12" after that?
You do not say so.

[pdf64]

In itself, snow tends to fall rather quietly - maybe thinking of accompanying high winds?

Are you certain that the load impedances are the same (usually 8 ohms for a DR) in both cases?
It's always good to verify what any labelling says with a meter.

[Sea Chief]

I was accompanying my snowstorm with additional moderate/ severe winds.. you'd expect to find in a snowstorm. pdf64. yes.

No this is like a swoooishh!!SHHHHHH!!whirrrrrrrrrrywhizzzzssswooooshh.. (you get the idea) sound.

Defo one of the twin's 8ohm spkrs > DR, measures 6.5 etc.

[Chuck H]

And now that you've found the amp still sounds fine with the 2x10 cab, did you ever revisit that single C12n as Juan suggested? It may be damaged.

[Sea Chief]

Yes put it back via one of the c12n's.. & get these weird noises: only after a while tho. Ive never heard anything like it, & I can't match it to any usual dmgd spkr symptoms. Reads 6.5 ohms both do.

[Chuck H]

Well the speaker is the common denominator in problem occurrence right now. Can you test the speaker with another amp at similar output for the same time?

[Sea Chief]

Hi there Chuck- tried again.. no noises. Very strange. It sounded like severe 'interference', pointing twds the amp. But Ive never had any noise at all similar with my 2-10 cabinet (my usual DR cab). I don't have another amp to try alas. Odd.