5E8A build

[paulster]

This amp is on my list of ''next amp to build''. Can someone confirm that the bright and normal channel can not be jump like we can do with a JTM45 or a 5E3 Tweed Deluxe ? And why ?

I think, from memory, that each input hits its own triode so you won't get output from one of the other jacks.

Use a Y-cable and you most certainly can jump the channels, however the input is configured.

Mine is wired so I can effectively do that, although I only use one volume control.

[chopstuck]

I just re-read your first post and I have to ask.

Why would your rectified voltage drop from 450-0-450 ac to 300 dc.
Was the original 5U4 fully functional ?

Sounds like it may be conducting a half wave. 5U4 don't tend to drop that much V+. You said you had big 5v and 6v windings so there was nothing holding the recto back. A half wave might support the 300v you measured.

[Cliff Schecht]

If you were referring to my post then you were misreading. I was using a 225-0-225 transformer with a 4-diode/bridge rectifier. Two of the diodes happened to be 5U4's (two in parallel in the 5E8A) but that was just to stay vintage correct(ish). The 5U4's did drop about 40V but the B+ still sat at ~570V. I used a 4A, 400V (IIRC, used an IRF330) in a zener amplifier to knock off ~150V.

All of this fuss was a workaround to use the iron I had that could support two 5U4's 6A of heater current. In the end I'm glad I went this route instead of compromising and using B+ voltage-correct iron that can only support 3A on the 5V rail. The iron I had cost me next to nothing and it's massive enough to not even warm up much during playing. I also prefer the feel of two rectifiers in parallel, the amp doesn't sag too much but feels better than the SS option IMO.

The 5E8A is a really neat amp and has a unique charm that is very enjoyable. Great lead sounds, tight rhythm and spanky cleans. It's almost as if these are a well kept secret .

[husky]

Clapton's Twins are nothing like standard ones.

They are solid state rectified with 4x output tubes from what I remember.

John Suhr mentioned over at HugeRacksInc broadly how they'd been modified sometime before (since he did them) and the only resemblance to a stock Tweed Twin is the cabinet really!

The original was something Cesar Diaz had done.

[husky]

Clapton's Twins are nothing like standard ones.

They are solid state rectified with 4x output tubes from what I remember.

John Suhr mentioned over at HugeRacksInc broadly how they'd been modified sometime before (since he did them) and the only resemblance to a stock Tweed Twin is the cabinet really!

True they are different but not terribly so.
Primarily the removal of the Tube rectifiers and replaced with 2 more 5881's
Solid state rectifier, bigger filtering and flipped phase on the speakers. He did start playing it with only two power tubes after I convinced him it would be better. It was the stock Triad OT and a replacement power transformer. The voltages were stock but the transformer had more VA.

The other key that was a tough nut to crack on the repro amps I made was the tone of the cabinet. Baffle being VERY important. Oh yeah also Oxford reconed speakers.
The basic circuit of the preamp was stock 5E8A

[husky]

On the stock circuit the High input joins the first two triodes

[paulster]

True they are different but not terribly so.
Primarily the removal of the Tube rectifiers and replaced with 2 more 5881's
Solid state rectifier, bigger filtering and flipped phase on the speakers. He did start playing it with only two power tubes after I convinced him it would be better. It was the stock Triad OT and a replacement power transformer. The voltages were stock but the transformer had more VA.

The other key that was a tough nut to crack on the repro amps I made was the tone of the cabinet. Baffle being VERY important. Oh yeah also Oxford reconed speakers.
The basic circuit of the preamp was stock 5E8A

Thanks for clearing that up, John.

[Zippy]

The other key that was a tough nut to crack on the repro amps I made was the tone of the cabinet. Baffle being VERY important.

Care to share any more regarding the baffle?

Floating? Pine vs ply? Thickness?

Any added stiffener?

Thanks!

[Cliff Schecht]

Clapton's Twins are nothing like standard ones.

They are solid state rectified with 4x output tubes from what I remember.

John Suhr mentioned over at HugeRacksInc broadly how they'd been modified sometime before (since he did them) and the only resemblance to a stock Tweed Twin is the cabinet really!

True they are different but not terribly so.
Primarily the removal of the Tube rectifiers and replaced with 2 more 5881's
Solid state rectifier, bigger filtering and flipped phase on the speakers. He did start playing it with only two power tubes after I convinced him it would be better. It was the stock Triad OT and a replacement power transformer. The voltages were stock but the transformer had more VA.

The other key that was a tough nut to crack on the repro amps I made was the tone of the cabinet. Baffle being VERY important. Oh yeah also Oxford reconed speakers.
The basic circuit of the preamp was stock 5E8A

This is the impression that I was under. Thanks for the great info. I still hold that I built the 5E8A out of curiosity about the dual rectifier setup more than anything. I didn't know about the Clapton connection until somebody else pointed it out to me on here.

I can't see the amp needing more power TBH, part of why I really dig this amp is how well it sings when cranked without killing me volume-wise and how amazing the cleans are when dialed back. I use all NOS 6072's in the first few tube sockets as I find that swapping in 12AX7's make this amp unbearably gainy. Any comment on the tube choice that Clapton uses? (tube type, brands, etc..).

It's a fun circuit with lots of versatility on those few knobs. I also prefer the dual rectifier setup, it gets way too spongy and flabby with a single rectifier. The trick IMO is finding a PT that can easily handle the dual rectifier setup. I think the Triad PT I ended up using was close to 12 lbs with solid copper leads coming out of the transformer. A nice little note: rating the current capacity of each winding on a transformer with solid copper leads is a lot easier than that of a stranded lead PT. You can get out your calipers, measure the diameter, convert this to circular mils (google can do this for you!) and use the 700 circular mils/Amp rule. Mine ended up being rated for about 5.8 Amps (ish, don't remember the exact number) but this is more than close enough in my book.

I'm interested as well on the baffle issue as well as what are "appropriate" speaker choices for the amp. I've heard it sound amazing on an old Fender open back 2x10 with whatever stock speakers were in it (might have been Oxfarts) but it also sounds very good on my 1x12 closed-back/front-port alnico SRO loaded cab. Then again, that 1x12 sounds good with anything .

[bluesfendermanblues]

Oxfarts

[Herec]

What am I missing about not being able to use the 807's in a head? I think I don't understand where that 10z resistor goes. I'm thinking you can solve the oscillation problem if you really want the 807's.

Curious about this as well.

[Cliff Schecht]

There is no reason other than that the adapters I made are built from old relays and are I think about 2" tall. Tack this onto the length of an ST-shaped 807 bottle and I'm at I think about 10" tall worth of tube. If I want to mount this amp into it's own head cab with the controls facing up and tubes facing down (i.e. traditional tweed Fender style) then I need at least 10" of clearance on top of the length of the chassis. In my case it makes the amp unbearably tall for a head but if I put this amp in a combo then I could use 807's no problem. I'd probably use some sort of homemade tube retainers if I build a combo though.

If I don't use the 2 Ohm Super transformer I just bought in this tweed Twin then I'm probably using it on my upcoming '63 Vibroverb build. One of the two (probably the Vibroverb) is going to become a combo or I'm going to build a cab that switches between 2, 4 and 8 Ohms (16 as well if I can squeeze that in there).

[Structo]

Is that correct that the power tubes plates and screens are run from the same power supply node?

I wonder why they did it like that rather than the typical way of having just the screens after the choke.

[Cliff Schecht]

Yup. I didn't bother with screen resistors or anything because the little 6L6GB's I'm using (JAN-Sylvania IIRC) are some tough little fuckers. No signs of screen glow as far as I can tell.

It was an experimental time for Fender so it's not surprising. Since it's part of a push-pull circuit I don't see anything like power supply ripple being an issue.

[bluesfendermanblues]

Is that correct that the power tubes plates and screens are run from the same power supply node?

I wonder why they did it like that rather than the typical way of having just the screens after the choke.

Same with the Tweed Super (5F4) - The whole amp is run through the choke.
Adds a little more sag. (Don't do this on the smaller chokes)

However, its easy to move the OT wire from B2 to B1, which is the usual spot in all other amps.