135W twin question

[goldstache]

Thinking of taking a 135W twin and converting it to 2-6l6GC and going Full Wave recto for around 275vdc operation.
Will include adjustable fixed bias and balance.
Owner wants single 12”


So……
2-6l6 4K primary
1-12” 8ohm speaker (loaded via 4 ohm speaker out)


With the current free up from only 2 power valves operating and full wave recto (non bridge)
Should be able to get it 70% diss or better.


Also, keeping UL taps intact.


Not above cathode biasing either.


Owner wants a smaller amp, but wants it to be in that rectangle, rather than selling and buying a different critter.


I’ve heard ab763 preamps at those voltages, and they still work pretty ok.
Some HT node shuffling and oscillator tweaks usually.
Also lifting bottom R of bulk V divider for PI/preamp. Likely landing similar to 763 circuit.
Any thoughts, pitfalls spring forth for anybody?
Thanks.


https://schematicheaven.net/fenderamps/twin_reverb_sf_135_schem.pdf

[pdf64]

That's going to be a dreadully heavy 25W combo!

[tdvt]

That's going to be a dreadully heavy 25W combo!

I would have to agree.

That is a lot of cabinet & a pretty beefy transformer set for a 2-6L6/1-12" amp.

Though I am more of a small amp guy & without really trying, over the last year I ended up with 3 big Fenders. 2- '79 UL Pro Reverbs (both nearly just salvage condition & cheap) & more recently a BF Twin Reissue on a trade.

 None are light or fun to lug around, in fact the Twin had been converted to a head to make it more manageable.

The UL Pro circuit is almost identical to the UL Twin you are working on, just 2 less power tubes. One Pro will be rebuilt stock, the other with a 15" (Vibroverb inspired probably) but I basically wish they were smaller/lighter/not as loud. But as I said, they were cheap.


If at all possible, I would try to go smaller if you are contemplating that extensive of a rebuild.

[Willabe]

Sell it, get something that works for what he wants.

The UL taps aren't really UL taps, their not at the correct tap %. Their really for limiting the dcv on the screens.
 

[Dave]

For around a hundred bucks (I haven't checked prices in a while) he could get a cheap and reasonable facsimile of a 100/85 watt Twin PT and OT that would fit right in there. You could still remove a pair of power tubes. If he wants 25 watts you could do the old tie the screens to the plates "half power" trick.


Dave

[goldstache]

Yeah, my pal and I have been over the efficacy Vs sensibility deal.


It’s gonna sit in his studio. But doesn’t want the headroom.


The screen taps seem to be about 15%, so I do agree the UL is a bit of a misnomer.


To try out full wave non bridge, can’t I just lift the bottom of PI/preamp resistive divider. And ground PT CT, and lift (-) of bridge module from ground? To audition.

[sluckey]

To try out full wave non bridge, can’t I just lift the bottom of PI/preamp resistive divider. And ground PT CT, and lift (-) of bridge module from ground? To audition.

yes

[passaloutre]

I had a 70s non-UL Twin that I modded for 4 JJ 6V6S, cathode biased, with a switch to lift two of the cathodes and disable one speaker, for half power. The tone out of that thing was unreal, but as someone mentioned, it was an awfully heavy 20w amp.

[pdf64]

There's a ready made 1/2 HT voltage supply at the HT winding CT (red/yellow). Just take the 250V DC feed from there.
That's what the 25W option on the later iterations was done.

[goldstache]

Thanks very much. I have derived screen/preamp supplies on other projects, from totem junction.
Did cross my mind.


Pull some moves later tonight, and update.


Appreciate the input, gets the gears going.

[Willabe]

I had a 70s non-UL Twin that I modded for 4 JJ 6V6S, cathode biased, with a switch to lift two of the cathodes and disable one speaker, for half power. The tone out of that thing was unreal, but as someone mentioned, it was an awfully heavy 20w amp.

Hmmmm, that sound like a great option for an amp that stays home/studio. 

And with that iron set, should be rock solid bottom end, as long as you don't strip out the PSU B+ cap values.

[goldstache]

Okay lifted pi/preamp divider on HT. Took standby switch feed from middle of reservoir totem (PT CT node). Also reduced reservoir totem to 50uF total.
Got 240v on plate and rebuilt bias rail for lower V-, and added adjustable along with balance.


Operating around 80% diss.
90ma or so.


Still sounds pretty large. But in conjunction with the stock master vol..
Many a sound to be had, at less girth.
Verb/trem are working fine.


I may tweak the pi/preamp dropper (2k7 stock) (190v feed to PI)
But seems to be closer to what the owner was after.


Since we are putting only one cone in a twin.
Was looking for 12” 8R 50w or so speaker. Any full sounding, early breakup offerings you’d like to recommend??
Just gotta order the cone.
Thanks
-joe