SS/Tube Rectifier Switch Question

[Explorerman]

Hi, I’m looking for a design for installing a switch to change between Tube and SS rectifier. This is for a 18w TMB build, after a lot of searching through old forum posts I’ve come up with the attached schematic (sorry it’s a bit messy). Is someone able to verify this for me?

Diodes 1 and 2 are just protection diodes, diodes 3 and 4 are the SS rectifier. These utilise the unused pins on the EZ81 for a connection point.

The switch replaces the standby switch with the middle position OFF. I’m hoping a 240VAC rated toggle switch will suffice?

The zener diodes (the number of zeners may vary depending on the SS voltage) are to drop the voltage back to where the tube rectifier value/sweet spot of 340-345v since there will a rise in voltage when using SS rectifier. Does this look right?

Cheers!

[JMPGuitars]

Regarding the rectifier switch, that's the right idea, but they should be setup with a DPDT switch. I would personally use UF4007 over 1N4007 diodes. You'd probably also want to use 2 in series for each side before considering the remaining diodes for voltage dropping, which may or may not be necessary.

You may also want to consider adding a sag resistor. That will help with the voltage, but also add some level of sag to simulate a tube rectifier. A higher value = more sag, so you can tune that how you like. Most often I see something around 100R 5W to 10W resistor.

You can also reference the Superlite TMB schematic for how the SS rectifier and sag resistor are setup: files/schematic_superlite_tmb_w_ss_rectifier.gif

Thanks,
Josh

PS. I didn't review anything else on the schematic besides that one thing.

[Explorerman]

Thanks Josh, I didn't really want the sag resistor as I want to tighten the overall response, if I do want the sag I would just switch to the tube rectifier. I'm just confused on whether the zener diodes as I've drawn above would work because after reading Robrob's method he mentions for cathode biased (I'm pretty sure my 18W TMB is cathode biased?) placing the zener stack on the PT center tap to ground!

https://robrobinette.com/5e3_Modificati ... ier_Switch

[JMPGuitars]

Thanks Josh, I didn't really want the sag resistor as I want to tighten the overall response, if I do want the sag I would just switch to the tube rectifier. I'm just confused on whether the zener diodes as I've drawn above would work because after reading Robrob's method he mentions for cathode biased (I'm pretty sure my 18W TMB is cathode biased?) placing the zener stack on the PT center tap to ground!

https://robrobinette.com/5e3_Modificati ... ier_Switch

The amp is cathode biased (assuming you didn't change that). I fake bet $1 that you'll probably prefer the sound with some form of a sag resistor. A low value will give you just enough that it doesn't sound sterile, but not slow down fast leads. Then again, EZ81 sag isn't super significant.

If you search this site, I think you'll see that the preferred method of zener use is in series on the B+ line. For example: viewtopic.php?f=4&t=24656&p=237214#p237214 - there are obviously other methods as well.

I don't think you'll need to drop much voltage, but we'll see what your voltage comes up with when you get there. If your EZ81 B+ is around 340 and the SS B+ is 350, that's all good unless you want to modify the B+ for either more headroom or more gain.

Thanks,
Josh

[geoff 1965]

what is your HT, 290-0-290?
mine is 275 diode rectified which gives me 360VDC then i have a 100 ohm sag which drops it to 350 so 290 will be quite higher.

[Explorerman]

It’s a 290-0-290 HT, so yeah could be high.

Cool, so what I have drawn with the layout of the zeners should work ok (using a SPDT)? Yes, the number of zeners will vary depending on how high the SS rectified volts are. Will a SS rectifier with no resistor sound sterile?

Also, wanted to confirm the B+ line is referring to the voltage to ground measurement between at the standby switch (section of the output of rectifier and first filter cap)?

[geoff 1965]

before i added the sag resistor i ran my B+ straight from the bridge rectifier at 360VDC and it definately was'nt sterile! the headroom,response & tone were superb, the sag resistor added "touch sensitivity" which you'll get with the ez81.
have you found any feedback on the effect the post diode zeners have on the B+?

[JMPGuitars]

It’s a 290-0-290 HT, so yeah could be high.

Cool, so what I have drawn with the layout of the zeners should work ok (using a SPDT)? Yes, the number of zeners will vary depending on how high the SS rectified volts are. Will a SS rectifier with no resistor sound sterile?

Also, wanted to confirm the B+ line is referring to the voltage to ground measurement between at the standby switch (section of the output of rectifier and first filter cap)?

Yes, you can use a standby switch in that configuration for the rectifier switching. Some people/companies use a DPDT ahead of that to keep the voltage off the tube when it isn't in use. Either way is fine.

Yes, that is the B+, or sometimes referred to as HT. 345VDC +/- 5V is golden. More than that is more headroom and less gain. Less than that is lower headroom, more gain, and muddy sound.

Sterile may not be the right word, but it won't sound or respond the same without at least some sag. Maybe that's what you want, you won't know until you try. Try it both ways and see what you like better.

Thanks,
Josh

[geoff 1965]

yeah good idea to try both ways, just a point of interest have a look at the 20 watters in the downloads,they have the "tighter response" you are looking for,diode rectified with a 22 or 30 ohm sag.

[Explorerman]

Thanks for your help, i’m surprised i couldn’t find too much info or schematics with switchable capabilities between the two. Like I said, one source i found had the zener diodes on the centre tap position to ground. Just wanted to confirm what i’ve drawn will be ok using multiple (as required) zeners in line between the SS rectifier output and the load.