Switchable Tube Rectifier (50watters)

[TweakNcuss]

Even though I've found other diagrams, it seems I always have to use my brain to figure out how to do it the right way. Was blowing B+ fuses when I followed another schematic. Rectifier uses a 5AR4. Any thoughts, comments, questions?

[soundguruman]

A well, actually, if the DC output of the rectifier is disconnected from the amplifier circuit (load)...
as it is in your modification...
the rectifier may short circuit, because it is not loaded.
B But actually, there is a solid state rectifier tube, that plugs into the tube socket, it costs about $10. You can switch from tube rectifier to solid state rectifier when you want, without shorting the amp out.
C And also, the switch that you are using must be rated 500 volts or higher, not a normal toggle switch.
And also, when you switch from solid state to tube, the B+ voltage will go lower, and your bias will need re-adjusting when you switch.

[joey]

I would do it how mesa does it, and put the switching on the AC side, this takes care of a couple of problems. if you don't want to parallel the rectifier valves, and want to keep the lower secondary HT voltage when using it, you can try to source a TPDT to take care of the bias issue.

[shakti]

Bumping this, as I haven't found a definitive answer to this question: which is the best way to do a tube/solid state rectifier switch? I am going to do this on an upcoming JMP50 build. I'm building it as more or less a straight-up clone of a friend's late '67 tube-rectified JMP50, but I want to add solid state rectification for the more typical 1986 sounds.

There are two basic ways to do it; to do the switching either on the AC side (before the rectifier) or the DC side. I know Germino does it on the DC side on his Club 40 amp, but apparently Mesa do it on the AC side (according to Joey). So what are the pros and cons of each method?

AC switching cons:
- hard to find properly rated switch? However, Metroamp's dual voltage 12-series uses a 3PDT switch to switch between two sets of HV AC secondaries on the standby switch. Couldn't the same switch be used to switch one set of HV AC secondaries to two different rectifiers?
- if I use the standby switch to do the switching as described, the amp will have to "ramp up" when using the tube rectifier, as it doesn't get hit with AC before I switch it off standby.


DC switching cons:
- possible short circuit as described by soundguruman above?

Any other opinions on this issue?

[shakti]

Another possible problem: if I use the 3PDT swithc that Valvestorm sells, and that George uses for the dual B+ switching in his 12-series, the middle position is off. That's all fine and dandy if I set it up for switching on the AC side, where it would serve its function as a standby switch also.
However, if I use this type of switch on the DC side, could I be in trouble if I have the switch accidentally in the middle (off) position? Then I would have both a diode and tube rectifier hooked to the switch, but with no output. I'm concerned with Joey's statement above about short circuiting the rectifier when it has no load.

OTOH, wouldn't that be essentially the same as what the standby switch does - breaking the B+ supply?

[neikeel]

OTOH, wouldn't that be essentially the same as what the standby switch does - breaking the B+ supply?

Exactly the same

If you use that switch with common ac feed in centre and up to rectifier valve and down to diodes (or visa versa) you are replicating the stock standby set up (I am going to do this on my next build, only potential issue I see is variabilty in voltages when setting the bias but the 50watter bias feed is from half the ac it is just the plate voltage difference!)

[shakti]

Thanks, Neil! I'll see which way I'll end up doing it. I'll probably put the switch in the back after all, as I want to use the Castelco switches on the front.

Difference between tube and solid state rectified plate voltage shouldn't be more than 15-20V maximum. That's not enough of a difference to make it necessary to adjust the bias.

However, I've read that many late 60s and early 70s 50-watters have replaced PTs due to the way the mains and standby switches were wired. The details escape me, but might it be that having the AC on the standby switch could be a problem? Or was it that the bias supply was taken from the far end of the standby switch...so the tubes were without bias in standby, then a sudden surge when flipping the standby?

[ivan H]

Hi, I was thinking of adding switchable rectifiers but have decided its easier 2 use a plug in SS rec so I can swap them when I want. I'll bias it up with the SS in. Regarding standby switches, when I asked about them on DIYaudio forum Kevinn O'connor answered saying he doesnt know why they're used & some countries require them 2 b removed before the amps can b sold there, advising they b used as a power tube cathode lift if u really want 1. Just thought I'd pass it on. Cheers

[shakti]

If you check in the 50W forum, I made a thread for one of my builds where I explain how I did it. Works really well.