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Old 15th July 2011, 01:36 AM   #1
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Default Amp PSU standby and PA switching

Hello there me again, not too long ago i posted about a tube rectifier, well i decided on a GZ34 and apparently i can get one of those easiest.
Anyway, the plan is to be able to switch between rectifiers (SS diode and the GZ34) and have a standby switch. the other idea was to have a push-pull parrallel 12au7 output and a parrallel el84 triode SE output. I'l post a schematic with my ideas on doing it but theres a few things im not sure on. (* indicates a ceter off position for a DPDT) (note: a 33 ohm resistor may be put straight after the SS rectifier so there isnt a huge voltage difference)

1. i once heard about flash over or something if a standby switch is before a tube rectifier, so i dont think id be able to use a DPDT C/O on the rectifier select switch right?

2. the DPDT on the PA switching could be C/O instead so standby could be here, but the other half would be to control speaker taps to speaker jacks, (speaker would switch betweek OPT's) would this be ok to use while the amp is turned on and as a centre standby? or would i have to power everything down?

all help and contributions will be greatly appreciated.
Ricky
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Old 15th July 2011, 02:02 AM   #2
tomchr is offline tomchr  United States
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You probably won't be switching the rectifiers on the fly. They'll need time to warm up and such. On the tube amps I've built, I've measured the output THD go down versus time following the initial power-on. I normally wait a good 15 minutes before doing any measurement. So I think your rectifier switch idea may have a flaw in it...

Another thing is that it'll be difficult to find a switch that can handle the high voltage at the currents you'll be running.

Many rectifier tubes drop significant voltage across them (50 V or so) whereas diodes drop about 1 V. So your B+ will be higher with solid state rectification, which may require the amp to be re-biased.

Switching between SE and PP will require switching the OPT. Good luck with that one... You will need to have a load on the secondaries at all times (including during the switch) to prevent the flyback from potentially arcing over the secondary of the OPT.

Those are all challenges that can be solved, but is it worth the hassle?

What's C/O by the way? I'm familiar with NO and NC but have never seen C/O in conjunction with a switch spec.

~Tom

Last edited by tomchr; 15th July 2011 at 02:05 AM.
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Old 15th July 2011, 10:13 AM   #3
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im sure the rectifier switching will work, as it is done with a fair few other amps, with the GZ34 the drop isnt quite 50v closer to 20. which can easily be dropped with the SS diodes. as for the switch i have a switch, im sure its not going to be more than 10A :P and it can definately handle the voltage as ive used them in much bigger builds (520v) and this one is looking at around 318v max (then take ripple the choke filter etc.) as for C/O its centre off, so that in the middle position the switch wont be touching any contacts (on/off/on).

switching the output transformer, i was thinking just switching which OPT B+ went to, then switching which one the speaker went to, would you still need a load if you switched both OPTs off? (speaker would also be completely detatched).

Schematic of idea posted here
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Old 15th July 2011, 02:20 PM   #4
TheGimp is offline TheGimp  United States
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You could stick a resistor inseries with the diodes to drop the same voltage as the tube rectifier.

If you disable the grid drive (but leave it biased as is) to one tube in a PP amp it becomes SE, but the PP has to run class A to do that.
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Old 15th July 2011, 03:53 PM   #5
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Quote:
Originally Posted by tomchr View Post
Those are all challenges that can be solved, but is it worth the hassle?

~Tom
It probably is considering im building this for recording use, i want to get as much tonal possibility at low power in a single amp, mainly for space, im starting to run out of room with what ive got at the moment...

and i think i mentioned the series resitor with the diodes. the main thing i want to be sure of is the flash over thing with the rectifier and is switching the HT and load simultaneously on the OPT be ok? and will i need 2 switches or three to be able to do the switching and standby?
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Old 15th July 2011, 04:16 PM   #6
TheGimp is offline TheGimp  United States
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Run a small cap across the switch contacts to reduce the chance of flash over.

Always switch your OPT in stdby without B+ applied.
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Old 15th July 2011, 04:24 PM   #7
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Quote:
Originally Posted by TheGimp View Post
Run a small cap across the switch contacts to reduce the chance of flash over.

Always switch your OPT in stdby without B+ applied.
just for the tube rectifier contact? and will this still be ok in a centre off position? or would i be able to use a centre off to switch out the OPT?
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Old 15th July 2011, 04:52 PM   #8
Gilgy is offline Gilgy  United Kingdom
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I'm guessing this is for a guitar amp? Even allowing for the sometimes eccentric nature of guitar amp designers I'm a little confused bit your design goals here. All the rectifier and OPT and speaker switching just sounds ridiculously complicated to me - really what you're doing here is designing two entirely different amps. Why not just build two separate amps?
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Old 16th July 2011, 12:11 PM   #9
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well the rectifier switching isnt that uncommon especially in MESA/boogie amps, and as for building 2 amps separately i dont quite have the necessary funds... and its just easier having everything in a single small box... all i wanted to know is if my ideas had any chance of working, and i think ive got the power section sorted, only thing im not sure of is the centre off standby with the rectifier, ive got a couple so i may fry one and see how it goes.s
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