SF Twin reverb Ultra linear transformer Blackfacing

[diydidi]

Hi
Have 135W SF twin. Can the models be blackfaced without changing the output transformer??
If so, HOW?

[J M Fahey]

Why would you change the output transformer?

Just ignore the UL taps and wire screens the classic way.

Only problem is that V+ may be too high, but that's the way Fender made them, and no fault of the OT .

What are you trying to achieve anyway?

Besides losing a couple Watts or having a somewhat different Lab measured distortion, the nature of the beast won't change that much.

Silverface high power Twins sound much better than Internet lore claims.

[potatofarmer]

Just ignore the UL taps and wire screens the classic way.

Only problem is that V+ may be too high, but that's the way Fender made them, and no fault of the OT .

Agreed. You'll need to add a B+ node for the screens so you might be able to drop a couple volts that way, but expect the screens to still be hanging out above 500V.

The OT taps are so close to ordinary pentode-mode connection that I would just leave it alone. If you want to change the tone of the output section, there are a couple mods you can do, but it depends on your goals.

[Jazz P Bass]

[QUOTE=diydidi;373737]Hi
Have 135W SF twin. Can the models be blackfaced QUOTE]

One question: Why?

#1: B+ is nowhere close to a BF
#2: The wiring on the SF amps is a nightmare, so unless you totally gut the amp.....
#3: the SF amps sound fine.

Just my opinion mind you.

[Leo_Gnardo]

Hi
Have 135W SF twin. Can the models be blackfaced without changing the output transformer??
If so, HOW?

I don't sweat the ultralinear output section, nobody wants to pay for a new OT, so that all stays as-is. I find a big step can be made towards blackie tone by altering the PI components - first and foremost those 330K resistors get changed to 1M. If there's a honkin' big 0.01 uF cap at the PI input, bust that back down to 0.001 uF, if you must maybe 0.0022 but no bigger. Change component values in the PI as necessary until it matches the BF and that's all that's necessary. 47K plate R's become 100K and 82K. Cathode R's too. Have a listen after swapping out these few parts & you'll likely find that's enough tinkering, stop there, play & smile.

[diydidi]

Thanx for all the replies guys, BUT, I still want to be able to mod it close to BF bias circuitry too.

[Chuck H]

IMHE...

Modding to BF type circuitry with higher voltages STILL SOUNDS GOOD. Maybe a bit better than the SF circuitry, YMMV.

As mentioned, you can bypass the UL taps. You would need to add an appropriate screen grid node by adding a choke to the amp (expensive).

Will it sound like a BF twin (Let's say, a '64 twin?)? Nope. But it will be a different tone than the one you have now.

If you don't like the amp you have nothing to lose but the cost of the components and the time for modification (which I assume you would invest for free anyway ).

To see how it will sound you'll need to build it. Also IMHE you'll want to include some sort of power supply sag via a rectifier tube (I didn't, and wouldn't do that) or a resistor to simulate one. I actually used a Pi filter to both add resistance to the power supply and reduce ripple. It worked out very well in my case.

The amp I built uses EL34's though!?! It switches between being a BFish amp and an AC50ish thing with a top boost preamp. No high gain tones, but aside from that it's undoubtedly the best tone machine I've ever built. It runs about 470Vp. That should be a bit lower than your Twin. Adding a carefully considered Pi filter or even a straight up "sag" resistor would probably get you in a similar range though.

FWIW, what you propose should work to get a more "classic Fender" tone. There may be a little more to it than you planned though.

Incidentally... I like the Voxy channel better

[Leo_Gnardo]

Thanx for all the replies guys, BUT, I still want to be able to mod it close to BF bias circuitry

One stage bias filtering instead of two? Okay..... I don't see any advantage, the output tubes don't much care what the bias source is as long as it doesn't sag or fail. So you can mod the bias supply to match or at least closely resemble BF style, no problem there.

[potatofarmer]

One stage bias filtering instead of two? Okay..... I don't see any advantage, the output tubes don't much care what the bias source is as long as it doesn't sag or fail. So you can mod the bias supply to match or at least closely resemble BF style, no problem there.

No problem there as long as you add a new power supply node for the screen grids. The bias balance control is how these null out all the ripple that shows up on the screens. Or a CLC / CRC filter could be used before the plate supply; that's how Sunn/Dynaco did it.

My 2 cents, leave the bias balance control and just add a bias level pot.