Weber copper cap SAG in solid state rectified amp

[proaudioguy]

I have a Deluxe Reverb II.  I would like to use a 5U4GB in the amp but it has a solid state rectifier.  Anyone have a schematic of what Weber is doing with those copper caps to get the 50v drop?  I understand they get quite hot.  I assume some kind of power resisters.  Do they need heat sinks?


Btw, I have already reduced the capacitance on the power supply.  I have actually made a lot of small mods to get it as close to a deluxe reverb as possible.  The only thing I haven’t done yet is split the V1 in half.  If I had an old BF DR, I would be more likely to use a 5U4 than a GZ34 as I am looking for the compression and grit, rather than the clean.  The DRII is STIFFER than my Twin.

[kagliostro]

Franco

[d95err]

I’d use a 25W or 50W power resistor with an integrated heatsink. They are fairly small and cheap.

Ohms law to calculate the resistance you need for the desired voltage drop. Or just experiment. I’d try something like 200-300 ohms.

Note that resistor power ratings are valid only at reasonable temperatures. You may need additional heatsinking or ventilation.

[kagliostro]

https://www.ebay.it/itm/401514561098?hash=item5d7c21fe4a:g:0FUAAOSwOtVavMgn&amdata=enc%3AAQAIAAAAwN2hGVqJYuPC4JerF309IgxiJyPBkJLkhyo0%2FFKhKahMqQL0%2Fi2kn4yZ1Ra5g49D1f%2BYanO1ZL1PAgl2BNYK5zQ2Club8I9CQBDNE6rsXAlAd%2Bnr0UT3d8X28oP6QRP5DuMzcHfbJBCm%2BvWBTASq9FYAi8n7x35jmt%2Bjh%2FVjT6O4XdaZOwBgIjncJwAC4z6M0UuG1Gy6EColD4JHeqQo6mM%2FDvRNfHbfdic%2FNDP%2FGgDgcrB43znne%2BLm%2FekvRtY0jw%3D%3D%7Ctkp%3ABk9SR_CX2oSfYw

Franco

[WimWalther]

I’d use a 25W or 50W power resistor with an integrated heatsink. They are fairly small and cheap.

Maybe I'm confused, here.. but doesn't the OP want to convert the amp FROM a SS rectifier TO a 5U4 (tube) rectifier?

No resistor needed for that, the vacuum diode's purveyance characteristic provides the voltage drop.

[proaudioguy]

The DRII has a bridge rectifier.  Would you still wire them in series like this or would you put one on the grounded side of the bridge and one on the B+ side?


I have been reading since I posted.  Seems Weber uses a thermistor for in rush, but since this amp never had a tube rectifier, is that necessary?  Perhaps it already has something in line for that?

Franco

[proaudioguy]

If I can get the same result without cutting a hole in the chassis and changing the power transformer for one with a 5v winding, I’d love to do that.

I’d use a 25W or 50W power resistor with an integrated heatsink. They are fairly small and cheap.

Maybe I'm confused, here.. but doesn't the OP want to convert the amp FROM a SS rectifier TO a 5U4 (tube) rectifier?

No resistor needed for that, the vacuum diode's purveyance characteristic provides the voltage drop.

[kagliostro]

You told you want to use a 5U4GB on your amp

If you don't want (socket) holes you can arrange the circuit without it, simply add the SAG resistor to your SS rectify

---

Note that in the Weber solution the resistors are indicated to be 2 (in series) and not one, but it is for space and dissipation reasons, obviously you can use only one SAG resistor if you want

Franco

[pdf64]

Indeed, simply insert a sag resistor between the existing FWB rectifier output and the reservoir cap. eg if you want a noticeably saggy response then try 100R 15W.
An aluminium clad chassis mounting type may be a good idea.

[jjasilli]

Ditto to pdf64.  FWIW I use an inductive power resistor  rather than non-inductive - on the theory that some type of reactance will better emulate a tube rectifier.

[stratomaster]

Here's what they did to emulate a tube rectifier on Brian May's custom AC30 builds.

I'd personally use a single UF5408 in place of the 3x 1N4007s, but that's just to save some eyelets and soldering.

[proaudioguy]

That fuse on the B+ is interesting.  How do you figure out the size of the fuse?  I supposed you could solder in a resister and put signal through full tilt, and measure the resister to see what the current draw is.  I don’t see fuses for 4oo odd volts, x amps.  Do you need to adjust the fuse based on the total POWER when the voltage and current are different?  Why aren’t fuses rated in WATTS instead, to make it easier to figure out the sizing?

[mresistor]

They probably started with 100ma and increased the fuse value until it didn't blow, then added some safety margin amount to that. 

[sluckey]

That fuse on the B+ is interesting.  How do you figure out the size of the fuse?

The fuse needs to be rated for more current than the amp will ever draw during normal operation. Then usually double the current rating to prevent nuisance blowing. Notice there is also a fuse in the CT ground connection. One of these fuses is redundant, kinda like putting a fuse in the line and neutral sides of the power cord. Silly.

Why aren’t fuses rated in WATTS instead, to make it easier to figure out the sizing?

A fuse is NOT a power consuming device. A good fuse will have no voltage dropped across it. No voltage means no power.

[PRR]

.... measure the resister to see what the current draw is.

This rarely works well because they put the fuse in the PULSATING current part of the circuit. Few meters, even 'TrueRMS', will read this correctly. If you just want a resistor here for voltage-drop it wants to be much smaller than you figure from DC approximations.

> A good fuse will have no voltage dropped across it.

And a bad (blown) fuse will have Zero current through it.

You can't touch both ratings at the same time.

"250VAC" fuses do tend to serve acceptably after a reasonable size transformer in 440V DC supplies in non-life-critical applications. The glass may shatter, which is why we like well enclosed or internal fuseholders. The usual tubeamp transformer won't do much over 1000 Watts in any kind of fault, and won't make a flame to cause instant incineration. It may set the drapes on fire but a proper venue has exits to clear the room quickly. (Yes, the Station fire was fireworks not an amplifier.)

[proaudioguy]

I have been considering adding this type of fuse to all my tube amps to keep the transformers from destruction, but this si the first time I have seen it in a schematic.  Any others you all suggest I look at?  I’m OK with just copying other’s work on my personal amps, to increase reliability.

[pdf64]

… I'd personally use a single UF5408 in place of the 3x 1N4007s …

What’s the benefit?
Compared to a series triplet of 1N4007, a single UF5408 has only 1kV voltage rating and 4x the reverse current.
Granted, for an AC30 mains transformer, 1kV diode rating per phase is probably adequate.
https://www.vishay.com/docs/88503/1n4001.pdf
https://www.vishay.com/docs/88756/uf5400.pdf

[stratomaster]

… I'd personally use a single UF5408 in place of the 3x 1N4007s …

What’s the benefit?
Compared to a series triplet of 1N4007, a single UF5408 has only 1kV voltage rating and 4x the reverse current.
Granted, for an AC30 mains transformer, 1kV diode rating per phase is probably adequate.
https://www.vishay.com/docs/88503/1n4001.pdf
https://www.vishay.com/docs/88756/uf5400.pdf

Relatively low power amp, less so than even the AC30, so you can get away with it.  The only benefit is fewer parts, less to solder, fewer turrets.

[PRR]

> 4x the reverse current

Hard to think of a case where 0.2milliAmps matters.

Yes, I did work with Selenium, Germanium, and even CopperOxide rectifiers so I know the issue.

[WimWalther]

Yes, I did work with Selenium, Germanium, and even CopperOxide rectifiers so I know the issue.

Ten years ago I helped a friend replace the original motor controller on an electric forklift from the 1960s.

I'd never seen selenium rectifiers so huge in my life. There were a pair of them, each with a dozen 5-6" square plates.. and this doesn't include the rest of the electronics.

The new controller assembly was about the size of a 1lb. block of cheese. Must have used mosfets or IGBTs.

[AlNewman]

The deluxe reverb 2 is a completely different amp than an ab763.
A few minor mods probably won't scratch the surface.  If you were to try, you could start at the phase inverter, power section, nfb.
It's it's own monster.  Probably a perfect pedal platform.  Sell it and buy something you like.

[proaudioguy]

The deluxe reverb 2 is a completely different amp than an ab763.
A few minor mods probably won't scratch the surface.  If you were to try, you could start at the phase inverter, power section, nfb.
It's it's own monster.  Probably a perfect pedal platform.  Sell it and buy something you like.

It is modded to the point that the entire clean channel is AB763 all the way through the phase invertor except for the Presence control, and the first stage tube that is 2 triodes.  I am about to split that when I get this other amp back togehter.  The only other difference is the tube rectifier.  If I can get the clean channel sounding like the DR (and its just a bit fatter in the low mids), then I will work more on the dirt channel.  These 2 channels are completely separate from that first tube to the phase invertor.

[pdf64]

… and its just a bit fatter in the low mids) …

That may be partly due to user perception, ie the J taper bass pot used with the II makes lowish bass settings (that are easily achievable with the 10% AB763) tricky to achieve, something of a hair trigger between say 1 and 2 on the knob.

Also affecting the low end there’s the 22nF (compared to 1nF) at the power amp input, the 22nF coupling caps to the output valves (compared to 100nF), and the slightly lower degree of NFB, 1/22 compared to 1/18. 

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxereverb_ii.pdf