5E3 Build (SS Recto) From "Scratch"

[LedZepp007]

OK, thanks everyone for tolerating my ignorance in my other threads. I have worked my way up from a simple pedal to the Airbrake (which went surprisingly well) and then overestimated my ability by trying to fix a Silverface TR.

At any rate, I've read and read and READ. I'd like to try a 5E3 head from "scratch." I am planning on using a Hammond 1444 16x8x2 chassis (with plate and cage), with Hammond 290CAX and 1760H transformers, and the Mojo 5E3 small parts kit. The trick is I am also wanting to use 4007 SS diodes for rectification rather than the 5Y3. By my calculations, this means that (in order to get approximately 350 VDC B+), I would need approximately a 500-525 VDC secondary on my power transformer (hence, the 290CAX). Other than that, the circuit would remain unchanged. I plan on generally following the standard 5E3 layout/schem. I do have a few questions first:

-Are my transformer selections OK?
-Right now, I'm not planning on using a faceplate. Will I need to paint the 1444 chassis? Is the chassis pre-prepped for finish?
-For rectification, I can use a full-wave design with two diodes if I utilize the center tap, right?

Thanks!

[Phil_S]

I think I'd look for plate voltage a bit higher. In any case, simply take your target and divide by sqrt(2). That will get you within about 5%. Different PT's are harder or softer, leading to some variance in the end result. You'll need about 100mA capacity, but might get by with a bit less on this particular amp.

In other words, if your target is 400VDC, then 400/1.4 = 285. So you need 285-0-285. I think you've done the math correctly, but you asked for confirmation. So, for 350VDC, you need 250-0-250.

Check the Hammond site for rectifier wiring and expected results. There is a good one page chart there.

[rock_mumbles]

I used a 300V 150mA non-center tapped PT for the ss diode rectified (with 100 ohm sag resistor) 5E3 I built for a friend.
It ended up at around 360-370V actual plate voltage (plate minus cathode)

The 1760H is more like a Deluxe Reverb OT (6.6k) than a 5E3 OT (8k)

[ToneMerc]

I'll throw this out there; since you are using a non-standard chassis make your layout and board is orientated to way the chassis will be mounted, whether on the cabinet floor or hanging.

TM

[LedZepp007]

Thanks again for the help, all. I look for an 8K primary OT.

The board should be mounted from the "ceiling" with the bottom plate being toward the floor and the cage on "top" of the chassis. That actually is a really good point, though, that I might not have thought of.

How well does a sag resistor work? Sounds cool. Maybe I'll make a "sag" switch.

In terms of rectifier design, Hammond seems to suggest full-wave bridge rectification. If I do that with four 4007's, will I need to adjust my filter caps at all from the stock values?

[LedZepp007]

If I use insulated standoffs to mount the circuit board, do I need an insulating board?

[martin manning]

If you use a stiff fiberglass (G10) board, then you can just use any kind of standoff that is 3/8" or so, and no insulator board. I would not recommend using a fishpaper board anyway, unless you want to build a clone. If you're not up to making one, Watts Tube Audio will sell you a ready-to-use G10 5E3 eyelet board for $25. http://www.turretboards.com/circuit_boa ... _index.htm

[rock_mumbles]

Thanks again for the help, all. I look for an 8K primary OT.

The board should be mounted from the "ceiling" with the bottom plate being toward the floor and the cage on "top" of the chassis. That actually is a really good point, though, that I might not have thought of.

How well does a sag resistor work? Sounds cool. Maybe I'll make a "sag" switch.

In terms of rectifier design, Hammond seems to suggest full-wave bridge rectification. If I do that with four 4007's, will I need to adjust my filter caps at all from the stock values?

The type of rectifier used depends on the power transformer you have ... if you have a non-center tapped PT then you will use a four diode bridge rectifier ... if you have a center tapped PT then you will use a two diode full wave rectifier.
For the amp I built for my friend, I used a 300V non-center tapped PT so I used a four diode (full wave) bridge rectifier ... if I would have had a 300V-0V-300V center tapped PT I would have used a two diode full wave rectifier.

In a 5E3 circuit a 120 ohm sag resistor drops around 10V and is put in to give a small amount of sag like a tube rectifier has ... the sag resistor doesn't really give the same response as a tube rectifier though ... it's not worth switching it in and out ... the effect is really small

To build a good modern take on a 5E3, I'd use a NOS 5Y3 and a Deluxe Reverb PT ... the tube rectifier makes a difference in the amp's feel