Need Schem for SS & Tube Rect. Combo

[flythair1]

Hello MEF Community,

It's been awhile since I posted here, I guess it was a couple of years ago when I built a 5E3 circuit in a donor. I do lurk here often and pick up many useful tips.

I just finished a AX84 SEL build and I included a option for SS rect. or tube. I have the SS rect. working well and I wonder if anyone might have a link to a schem for a switchable SS/Tube rectifier system.

Does there need to be a ss diode between the output of the tube rect. and the common point? Any other tips pertaining to doing this type of switchable rectification would be welcome.

Scott

[Bruce / Mission Amps]

Here's a quick sketch of the way I do it....

[flythair1]

Hey Bruce, thanks for your reply!

I thought that was the way to do it, I was not sure if you needed a diode in the line from the tube cathode(K) up to the common point of the two sides to prevent some kind of back voltage into the tube when in SS mode.

Again, thanks for your help. I'll try it out today and report back.

Scott

[Bruce / Mission Amps]

Originally Posted by flythair1

Hey Bruce, thanks for your reply!

I thought that was the way to do it, I was not sure if you needed a diode in the line from the tube cathode(K) up to the common point of the two sides to prevent some kind of back voltage into the tube when in SS mode.

Again, thanks for your help. I'll try it out today and report back.

Scott

No other diode is needed because the rectifiers already are diodes... including the tube rectifier's elements.

[bnwitt]

Why not just use an SPDT switch and switch the outputs of the rectifiers instead of the inputs? Fender's SRV Vibroverb doesn't even go that far. They just switch on and off the output of the silicone rectifier on top of the output of the tube rectifier. That along with switching in and out a bias dropping resistor for the B+ change.

[flythair1]

Hi bnwitt, thanks for your interest.

I did install the switch system Bruce, and it works perfectly. Thanks again.

The Fender schem shows a very neat method for compensating for the B+ change in a fixed-bias amp, I suppose you could use the same method for cathode-bias? It looks like the amp is double rectified when in tube mode, first through the SS diodes and then to the tube.

This AX84 SEL I'm working on is built with the variable cathode-bias. I wanted to include the ability to switch from SS rect to tube just to see if the tone changed much in the single ended design. Last night when I got it working, I decided to try as many different tube rectifiers as I had. I made a chart for the nine different tubes that include the voltages for Plate, B+, B+1, B+2, & B+3. I also checked the mA draw for each tube without making any changes in the settings.

The tubes are all either American NOS or UOS, no Russian, Chinese, Slav etc.
(2)5AR4, 5Y3GT, (2)5U4, 5Y4G, 5V4, 5V3, 5R4, were tested as well as SS mode.

The only tube that made a real difference was the 5Y3GT, a NOS Sylvania. The voltages with SS & all of the tubes except the 5Y3 were from 401-412 Plate, and 421-428 B+. The mA readings at the bias I had it set at were 41-42 mA. The 5Y3 produced a plate of 360 and a B+ of 376. The mA draw was 37. The SS mode read 405 plate & 426 B+ with 42mA. Four of the tubes had higher plate & B+ than the SS rect.

My first impression of switching between the SS & 5Y3 is tightness vs. looseness. I need more time to really hear the difference in the two, I couldn't crank it up much last night.

I guess my biggest suprise is the small difference in most of the rectifiers. My sample is small and perhaps other brands would produce different results. For now, I'll run the 5Y3 to see the most variation in the voltages possible. I will have to try re-biasing a little as well when changing back & forth.


Scott

[Bruce / Mission Amps]

Originally Posted by flythair1

...
The only tube that made a real difference was the 5Y3GT, a NOS Sylvania. The voltages with SS & all of the tubes except the 5Y3 were from 401-412 Plate, and 421-428 B+. The mA readings at the bias I had it set at were 41-42 mA. The 5Y3 produced a plate of 360 and a B+ of 376. The mA draw was 37. The SS mode read 405 plate & 426 B+ with 42mA. Four of the tubes had higher plate & B+ than the SS rect....

Scott

Boy some of that doesn't seem right to me... especially the part about some of the tube rectifiers have less drop (higher B+) then the solid state diodes, which because of design parameters, will only drop a volt or two max.
Other then the 5Y3GT, yes, the tube rectifiers will not have a huge drop across them if the current through the tube rectifier is low.
As an example the 5AR4 has close to a 250ma rating and at 250ma might only drop 15-20 vdc, while the 5Y3GT is rated around 125ma but will drop as much as 35-60vdc with a large load, such as 125ma, across it.

And as far as the Fender schematic... the tube rectifier is only used to drop B+ voltage (as a series impedance) ... the SS diodes are engaged all the time, doing all the AC to DC rectification and the two vacuum tube plates are in parallel (and less impedance)...
the tube rectifier is not acting as a full wave rectifier in that application the SS diodes are.
There is nothing wrong with that idea but the tube is not the actual rectifier in that amp... I'm pretty sure it is only used to soften the B+ as more current is drawn through the tube, which will not happen with the SS diodes.
You could do a similar thing (albeit the anodes will not be in parallel) by putting the SS diodes in series with the hi-v power tranny leads before the vacuum tube anodes... and then just short out the SS diodes with a dpdt switch.
There are a few ways to do this and I did not mean to suggest "my way" was the only way or best way.
It is just the way I've always done it so the SS diodes and the tube rectifier allow two separate ways to apply raw B+ to the first filter cap and or the rest of the B+ rail.

[flythair1]

That was the odd thing to me also Bruce. I was under the impression that the minimum voltage drop with a tube rect was about 15 volts and varied somewhat between tube types up to the 5Y3 that had the most drop at 50v+-.

The amp & guitar were plugged in normally and played a little with each different tube. I've never had a chance to do a direct comparison of SS vs. tube rectifiers like this and maybe it means nothing. My amp may be to blame, my tube sample may be too small, who knows? I am using an odd-ball power tube, a 7868.

Anyhow, your dual rectification method is what I had in mind but being a noob, I needed some confirmation before I went ahead. Thanks again for your interest & help.

Scott