Deluxe Reverb standby switch

[Lectroid]

Still putting together my AB763 1-channel (reverb, no tremolo) with the Hammond AO-43 iron and a 5AR4/GZ34 rectifier.  The PT has 328-0-328 VAC unloaded, so I'm hoping to get 430-440V at the plates to support 6L6s and approach a Pro Reverb in power.  To wring out every last volt I might have to replace the tube with a SS rectifier.  Everyone I've read here seems to agree (mostly) that a Standby is mandatory when using SS rectification, but (maybe) optional with a rectifier tube.

So I looked at some schematics: The Princeton Reverb AA1164, DR AB763, and Pro Reverb AA165 amps all use a GZ34 rectifier (and different transformers).  The Princeton has 410 plate volts but no Standby switch, while the DR has plate volts of 415 and does have a Standby.  The Pro Reverb has 440 plate volts, also with a Standby switch.

Here's my question:  How can the Princeton Reverb be almost identical to the DR in overall circuit and in the B+ voltages and yet not need the DR's Standby switch?

Is it just because the Princeton was designed to be a light-duty, cheaper alternative to the DR?

If I ran 6L6s and a SS rectifier, with 425 volts or more, would a standby be best practice?

Thanks in advance,

Rich

[sluckey]

Here's my question:  How can the Princeton Reverb be almost identical to the DR in overall circuit and in the B+ voltages and yet not need the DR's Standby switch?

The PR is considerably different from the DR.

Is it just because the Princeton was designed to be a light-duty, cheaper alternative to the DR?

Yes. The PR was Fender's bottom of the line reverb amp in the '60s.

If I ran 6L6s and a SS rectifier, with 425 volts or more, would a standby be best practice?

That's your call. Leo thought a STBY switch was useful.

[pdf64]

… Everyone I've read here seems to agree (mostly) that a Standby is mandatory when using SS rectification …

Dunno where you’ve got that from?
The only scenario for which I consider standby beneficial is in the case of an amp high solid state rectification and an otherwise unprotected DCCF circuit.
If the DCCF’s g-k forward biased voltage was limited by use of a diode etc, then I can’t see any technical merit for the use of standby in that case either.

http://www.valvewizard.co.uk/standby.html

http://www.valvewizard.co.uk/dccf.html

[Lectroid]

@pdf, sluckey,

Okay, this helps.  Sorry for any conceptual errors in stating my case.  I found several threads and long discussions on this site among people with conflicting opinions on the Standby switch.  But not much clarity for a new guy except that smaller, 6V6 P-P amps don't really need a Standby while larger amps seem to be improved by them.

Bottom line, I'm used to using a Standby switch so I'll probably add one for this DR-Pro hybrid and be done with it.

If I ran 6L6s and a SS rectifier, with 425 volts or more, would a standby be best practice?

That's your call. Leo thought a STBY switch was useful.

Yes, it's my call.  I was just hoping to learn how experienced amp builders look at it, some explanation of how to evaluate the situation for different amps.

But hey, "Leo thought a STBY switch was useful" is as good a reason as any.   

[thetragichero]

this is a good read: http://www.valvewizard.co.uk/standby.html

using a resistor across the switch like he does can also sometimes act like a 'low power' mode which is neat. i usually don't use em in tube rectified builds because it saves me the cost of the switch. in higher power amps (which are usually marshall-based and have a cathode follower) i usually do

[dude]

Std/by switch can be used but make sure you have the first filter tied to the switch when on, not the off/on switch. You don't want rushing current to the 5AR4. I didn’t have std/by filtered and had a flash on the rectifier  tube when l switched on stand by with on/off, off.
Also, that PT isn’t going to give you 410/425 volts with 6L6s, 6v6 gives 407 range, you’re lucky to get 390v with 6L6s loaded depending on the bias.
I ran that PT you have with tube and SS, SS might get you 390v at best with 6L6s. If you use SS you don’t need a filter on stand-by, tube rectifier yes.
Don’t know what you’re looking for but if want a DR with 6L6’s, l recommend a higher voltage PT, like 340-0-340 range. That AO-43 PT  is 315-0-315, if you got 425v it was probably unloaded.

[svejkist]

Yes, it's my call.  I was just hoping to learn how experienced amp builders look at it, some explanation of how to evaluate the situation for different amps.

The best explanation you can get is the Merlin link on the subject PDF and thetragichero posted in their answers. In fact, you should read that whole site carefully. And revisit it every few months. You won't get it all at first, but as you grow more experienced, you will find it answers a lot of important questions. Merlin makes a strong argument that standby switches add nothing to most amp designs, regardless of rectification type, and can cause serious problems if poorly designed, and less serious ones if left in standby for long periods. I never use them on any amps. If you want a mute switch, there are better ways of implementing that.

Again, read this over and over until you understand it,

http://www.valvewizard.co.uk/standby.html

[pdf64]

… not much clarity for a new guy except that smaller, 6V6 P-P amps don't really need a Standby while larger amps seem to be improved by them.
…

No properly designed amp NEEDS standby.
Often its implementation is an awful codge that causes problems.
Any perceived improvement it provides is in regard of user convenience, rather than technical benefit / advantage.

[Lectroid]

tragic here, dude, svejkist, pdf64

This is all great stuff.  Studying theory can make strong suggestions one way or another, but the best education for me comes from the experience of other builders.

No properly designed amp NEEDS standby.
Often its implementation is an awful codge that causes problems.

... standby switches add nothing to most amp designs ... and can cause serious problems. I never use them on any amps.

i usually don't use em in tube rectified builds

Now that's three-part harmony.    

Also, that PT isn’t going to give you 410/425 volts with 6L6s, 6v6 gives 407 range, you’re lucky to get 390v with 6L6s loaded depending on the bias.  I ran that PT you have with tube and SS, SS might get you 390v at best with 6L6s. If you use SS you don’t need a filter on stand-by, tube rectifier yes.

@dude,
Extremely helpful to learn that about the AO-43.  Thank you for jumping in with real numbers.  I will now focus on making a strong DR instead of a weak Pro Reverb.

A question if I may:  My PT tested out at 328-0-328, from 120 volts.  My design, adapted from sluckey's AB763 Lite, leaves out two 12AX7  tubes, (for triodes): the tremolo tube and the Normal channel 1st stage.  Do you have any sense of whether, and by how much, that might increase the plate voltage on 6V6s?  My guess is: yes, by 8 - 10 volts.  I'd like to know what you think.


Everybody, thanks again for all this help.  As someone who's come to this addiction late in life, I couldn't have made it this far without this community.   

[dude]

How did you test that AO43 PT to get 328 volts?

[sluckey]

How did you test that AO43 PT to get 328 volts?

The voltage depends on the load of the amp. I have two amps that use that AO-43 PT. Secondary voltage was checked at the same time all other voltages were checked with the fully operational amp idling and line voltage set to exactly 120VAC with a variac.

My Plexi6V6 measured 330VAC.

My Supro S-6424 measured 320VAC.

[pdf64]

The voltage depends on the load of the amp

And also (just to note for @Lectroid) on the mains voltage that it’s being fed from the wall outlet.

[Lectroid]

Ok, my ignorance is showing off again...

I just bench-tested the amp on a variac set to exactly 120VAC output.  That's how I measured the output voltage at 328V, just like Uncle Doug taught me.  In all discussions of PT output I've read here, I never knew until now that a bench test was just a first approximation, that PT output should only be measured in an operational amp while idling.  Now I know.  Sorry for the confusion.  Makes sense, though.  Thanks to sluckey and pdf64 for setting me straight on that.

@dude,
In a parallel thread of mine about the AO-43 choke, you wrote:

I used that A0-43, PT and 14H choke with 6V6’s, 6L6s and EL-34. with GZ34 and SS on all of these tubes: B+ with GZ-34 was about 400/405 (all tubes), SS about 15v more, all tubes

So I'm moderately confused.  It looks like there's more to this than I understand right now.   Can anyone explain the discrepancy?   

I know, once I get the amp built, it will probably become more clear. 

[sluckey]

Can anyone explain the discrepancy?   

What discrepancy?

[dude]

Steve, l was referring to DC volts in an operational amp, loaded with tubes,  B+ voltage not A/C. I thought Lectroid was referring to B+. Perhaps l didn’t read the tread correctly.


Sorry Lectroid, all the voltages l gave you for various tubes was D/C or what the B+ would be in a fully wired and running amp. Hope that clears things up for you. I thought you were talking B+, so yes 120v primary for that PT would give about 328-0-328 A/C.

B+ about 400 D/C. Sorry for the confusion.


I ran that PT with 6L6’s and was completely happy with the tone, in a 6V6 plexi amp, just the B+ was around 385v. Depending on what you’re looking for the B+ voltage with that PT in a Pro with would be lower then a Fender Pro, or that’s all l was saying.
Hope things cleared up, you’re doing a good job.

[Lectroid]

Thanks to all for getting me back on track.  For whatever confusion arose out of my ignorance, I apologize.  If I can't ask the right question, how can you give me the right answer?

My 328VAC *1.414 gave me an estimated 466VDC.  After dropping an estimated 40 volts through the 5AR4 to be conservative, I came up with ~420VDC at the plates. 

Or, looking at it another way using the Tube Data Cheat Sheet factor of 1.3 for the 5AR4, (328 * 1.3), I get 426 VDC.  420 vs. 426?  Close enough for imaginary numbers.     

I'm still moderately on track and I've learned a whole lot about power supplies that I didn't know before.

Thank you, everyone.  I am grateful.

[thetragichero]

have a play with this. you can add current taps for the various stages to get a better approximation of what the loaded power supply will provide. power tube datasheet is your friend for plate and screen currents and whatever operating point you're going for