5U4's in parallel

[Cliff Schecht]

So I think I have a transformer that will do the job, with some trickery. The transformer is supposed to be 270-0-270 @ 117V input which will give probably closer to 330 or 340 V per winding with modern voltages. So if the bridge tube rectifier gives me 330*1.414= ~466V under a bit of loading plus 40-50V of drop from the tube rectifiers I should be in the 400-420V ballpark that I am aiming for with this amp methinks. Does this sound reasonable? I've calculated the PT secondary rating at over 150mA per side after a choke input (although the actual amp asks for a cap-inductor-cap before the first B+ so this is actually a capacitor input, not a choke (or should I treat it like choke-input)?

Thanks for all of the help and mind-fodder so far!

[Cliff Schecht]

Aww damn, I had my transformers backwards. The one I want to use with the high current 5V tap is 225-0-225 at 117V and 240-0-240 at 125V. So can I still get in the 410-420V B+ range that I'm aiming for? If not I'll probably repurpose this transformer for the Rocket I owe my buddy.

[Cliff Schecht]

So how about switching to just a choke input and losing the cap? With the power supply designer thingy it's telling me that with a 2H choke I can hit the B+ I'm after pretty easily.. I've never tried this though.. How accurate is this in actually nailing the B+ I'm after?

[jaysg]

a) I'm sure you know that the Victoria solution was to use one GZ-34 instead of plumbing the two sockets

b) Everybody's putting in huge efforts to bring down the loudness. I wouldn't worry about an accurate B+. It'll still be loud.

c) Are you into it because of Eric Clapton...if so, his are a bit different than stock...not the 'signal path' as much as the power supply. You can pm me if you want details.

[Firestorm]

So how about switching to just a choke input and losing the cap? With the power supply designer thingy it's telling me that with a 2H choke I can hit the B+ I'm after pretty easily.. I've never tried this though.. How accurate is this in actually nailing the B+ I'm after?

I don't know how much inductance the original choke had. Is 2H enough to do what it's supposed to do? The point of putting the choke first is that, although it "sags" the voltage considerably upfront, the amp won't sag as much under changing load. It makes the amp less touch sensitive (with regard to compression).

Do not eliminate the cap in front of the choke or the choke will ring like crazy when you switch on.

[tubeswell]

The main thing about a whole-of-supply choke is that it needs to be rated to deliver the maximum (peak) DC current draw of the B+ supply. And what firestorm said about using it in a CLC filter is important too, if you want to get maximum efficiency out of the choke that you end up choosing. Having said that, a whole-of-supply CLC filter has little benefit in a PP amp, as any ripple hum riding on the main signal gets cancelled out in each (opposing) side of the OT primary anyhow. However, a whole-of-supply choke is a good idea in SE amps (because they don't have opposing primary windings), and (therefore) they tend to be hummier. JM2CW

[Cliff Schecht]

So would it be a bad idea to just throw a relatively large resistor right after the rectifier to drop the B+ a bit? I have plenty of current on tap so waste is not a concern and by my calculations/sims I only need maybe 220-470 Ohms to drop the B+ enough. Will this cause an excessive amount of sag? The difference between static and dynamic current draw should only be maybe 20-40mA right? I've done this in SE amps successfully but that's a different beast than a PP 6L6 amp.

The other option I've considered is bridge rectifying only one side of the B+ using the dual 5U4/SS diode setup and running a quad of 6V6's in the output stage. I've been itching to build a quad 6V6 amp forever now, but this may not be the best format (especially for the chassis I have in mind).

I had no idea Clapton used these amps until after I decided to build the 5E8 and did some googling around. Can't be bad though, I love me some Clapton (minus the bad JJ Cale covers!).

[FYL]

I had no idea Clapton used these amps

EC uses/has used a number of very different amps, some are posing as Fender models for endorsement purposes, but *none* are standard 5E8x models.

Most will be auctioned very soon, including:

The '57 5E8A modified by Cesar Diaz (larger PT, 4x power tubes, SS recto, ceramic speakers), used from '86 to '97 or so
http://www.bonhams.com/usa/auction/19226/lot/92/

John Suhr's later ECT repros from the Fender Custom Shop, used from '97 to '01 or so
http://www.bonhams.com/usa/auction/19226/lot/93/

Oh, and you can also bid on a Clapton-endorsed "Fender / T-Mobile A Limited Edition Fender T-Mobile myTouch 3G Mobile Phone". With Layla ring tones as standard?

[Firestorm]

So would it be a bad idea to just throw a relatively large resistor right after the rectifier to drop the B+ a bit?

This would have almost the exact opposite effect of the choke. The resistor voltage drop will vary with the current. The resistance of the choke's winding does this too, but its inductance counteracts the effect, so above a certain inductance the voltage drop is very nearly flat. For example, with the 5U4 feeding a choke filter, the voltage difference between full load and half load is only 15 volts, while with the choke moved from the front of the PS (or removed altogether) the voltage drop difference from full load to half is around 50V.

The point on the tube's curve where the voltage drop flattens out is a function of the choke's inductance. With a 300-0-300 PT, a 2H choke doesn't "flatten" until 100mA; a 10H choke goes flat at around 30mA.

[Firestorm]

The '57 5E8A modified by Cesar Diaz (larger PT, 4x power tubes, SS recto, ceramic speakers), used from '86 to '97 or so
http://www.bonhams.com/usa/auction/19226/lot/92/

I'd love to see exactly what Diaz did to it. I think his recollection of the mods is somewhat incomplete. And what in the world is overheating so much that it melts insulation, but doesn't take the amp out?

[FYL]

I'd love to see exactly what Diaz did to it. I think his recollection of the mods is somewhat incomplete.

Iron from a '78 Super Twin/Showman, solid state recto, 4 x 6L6GCs, '73 Fender-branded CTS ceramic speakers lifted from a SF Bassman 100. Rewiring job, some voicing. That's all.

And what in the world is overheating so much that it melts insulation, but doesn't take the amp out?

Some PVC-coated wires don't like the vicinity of big bad power tubes in a cramped chassis...

[Firestorm]

I'd love to see exactly what Diaz did to it. I think his recollection of the mods is somewhat incomplete.

Iron from a '78 Super Twin/Showman, solid state recto, 4 x 6L6GCs, '73 Fender-branded CTS ceramic speakers lifted from a SF Bassman 100. Rewiring job, some voicing. That's all.

He went to 4x6L6 but left the stock OT? Interesting.

[jaysg]

He went to 4x6L6 but left the stock OT? Interesting.

According to Suhr, EC didn't like the 4x configuration and ran 2x mostly....and yes, oddball decisions on Diaz's part. Suhr was supposed to clone that amp, not get all creative.

[FYL]

According to Suhr, EC didn't like the 4x configuration and ran 2x mostly....and yes, oddball decisions on Diaz's part. Suhr was supposed to clone that amp, not get all creative.

John Suhr did a great job at making replicas - 4 have been manufactured in the first batch (well, if you can call garage hand built ops a batch process), two for EC, one gifted to BB King - who later gave it to one of his techs, one or Mark Knopfler.

JS mostly used custom parts, heavier wiring and everything but stuck to the original Diaz approach and voicing for the ECTs.

EC used the amps on stage for 4 years or so, lately using the wrong small signal tubes - 12AX7s add some nasty fizz and ugly disto, 12AY7s sing, then switched to the Cornell DC aka the Custom 80, then more or less came back to the ECTs fitted with the right tubes...

BTW, one of EC's Cornell amps is also for sale:
http://www.bonhams.com/usa/auction/19226/lot/97/

[Structo]

Sometimes I think these tech guys like Diaz dispel misinformation to confuse the masses.

The "bigger cable" was probably Teflon insulated wire, that could take the heat.