Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on September 15, 2021, 03:36:08 pm
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Looking at the tube depot jtm45. They run the standby between the PT HV and the rectifier. Any issues with doing it this way? This would seem to solve valve wizards issue with blasting the filter cap with a fully charged rectifier tube?
(https://i.ibb.co/whhBNDG/stnd.png)
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Schematic:
https://s3.amazonaws.com/tubedepot-com-production/spree/attached_files/jtm45plus_schematic_v4.pdf?1391458123
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Thank you acheld. :icon_biggrin:
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Honestly, I can't make head or tails out of layout drawings.
With regard to this design: I'm no wizard of any kind, but:
1. One of the advantages of the GZ34/5AR4 is it's slow warmup and thus it's controlled startup current flow. This design strips that advantage away, by allowing the filaments to remain energized, without HT. So, when the standby switch is flicked closed, the tube gets slammed. I really don't know if this is important, or not, but it doesn't seem right.
2. I do believe in diode protection for your tube rectifier. Cheap insurance! So it is interesting to see that they've used (what are normally protection diodes) as an alternative rectification source. OK, nothing inherently wrong with that I suppose. But I still want my circuit protected.
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valve wizard's reasoning dont fly with me. Plenty of DPST switches out there and they are not a pain to wire. .
The switch will be subject to less arcing if is placed in the AC part of the circuit, e.g. prior to the rectifier. However, with a two-phase rectifier this would mean using a double-pole switch, which is a pain. A simple trick is to put the switch after the rectifier but add another rectifier diode after the switch, plus a resistor to reference the switch to ground.
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valve wizard's reasoning dont fly with me. Plenty of DPST switches out there and they are not a pain to wire.
DPST switches that can handle the acv/current are pretty big, you need room for them. A larger body switch will normally be better for this than a smaller body switch.
If you simply put the stand by switch after the 1st B+ filter cap(s) it will last a long time. Very easy to do. Use a good switch that can handle the dcv/current like the standard Carling.
Fender did this on the BF AB763 amps, thousands of them.
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Looking at the tube depot jtm45. They run the standby between the PT HV and the rectifier. Any issues with doing it this way?
The schematic circuit is hot switching, ie when switched out of standby, the hot rectifier has to charge the reservoir cap.
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Looking at the tube depot jtm45. They run the standby between the PT HV and the rectifier. Any issues with doing it this way?
The schematic circuit is hot switching, ie when switched out of standby, the hot rectifier has to charge the reservoir cap.
The schematic shows the switch between the HT and the rectifier. What issues does that present? When switched, the rectifier juices up the same as if there were no standby and you turned on the power
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When switched, the rectifier juices up the same as if there were no standby and you turned on the power
No, the rectifier’s cathode will be hot, and the uncharged reservoir cap will, momentarily, be an effective short, drawing very high current through the rectifier.
That’s hot switching and it isn’t good for rectifier operational life.
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I had an amp set up with hot switching with a GZ34, every time l switched the S/B to play the rectifier tube “flashed”, not good. I changed it, no flash.
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....That’s hot switching and it isn’t good...
Uh, the plan is from TubeDepot. Good for them. Sell those bottles!
(Actually probably an oversight, but a very prettily-drawn oversight.)
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so i assume everyone here puts the standby switch on the DC side of things?
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Here's how Marshall did it. https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_pa_45w.pdf (https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_pa_45w.pdf)
better schematic https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf (https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf)
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It only matters if there’s a rectifier valve to consider.
Generally, given the choice, it’s better to switch AC rather than DC, as any resulting arcs should tend to self extinguish. So the switch contacts should last longer.
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Here's how Marshall did it. ….
That’s hot switching.
Marshall sell valves too :icon_biggrin:
Marshall have a long history of ‘design by Bubba’, especially in regard of standby switching arrangements.
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would really appreciate some guidance on this standby switch placement. Valve wizard says put it after the reservoir cap but Rob has a 2W 100K across the switch and the switch is between the Cathode on the rectifier and reservoir cap. Which way ? I havent seen any "official" schematics or layouts showing this. THis is a JTM build.
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Here's how Marshall did the STBY switch...
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf
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If you were to implement sag resistors in lieu of the GZ34. They could be switched in and out of the power supply circuit without harm. The resistance value could be selected to approximate the voltage drop of the GZ34.
JTM JMP Marshalls did not use tube rectifiers. MOJO simply stuck a GZ34 into the JTM JMP circuit without regard to the effect of hot switching tube rectifiers.
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If you must have a GZ34/5AR4 and a Standby switch, follow Valve Wizard's advice and place the switch after the first capacitor.
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i am so disgusted i'm going to leave out the standby switch :l2:
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would really appreciate some guidance on this standby switch placement. Valve wizard says put it after the reservoir cap but Rob has a 2W 100K across the switch and the switch is between the Cathode on the rectifier and reservoir cap. Which way ? I havent seen any "official" schematics or layouts showing this. THis is a JTM build.
If you simply put the stand by switch after the 1st B+ filter cap(s) it will last a long time. Very easy to do. Use a good switch that can handle the dcv/current like the standard Carling.
Fender did this on the BF AB763 amps, thousands of them.
Like this;
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_ab763_schematic.pdf
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would really appreciate some guidance on this standby switch placement. Valve wizard says put it after the reservoir cap but Rob has a 2W 100K across the switch and the switch is between the Cathode on the rectifier and reservoir cap. Which way ? I havent seen any "official" schematics or layouts showing this. THis is a JTM build.
If you simply put the stand by switch after the 1st B+ filter cap(s) it will last a long time. Very easy to do. Use a good switch that can handle the dcv/current like the standard Carling.
Fender did this on the BF AB763 amps, thousands of them.
Like this;
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_ab763_schematic.pdf
No caps to ground or resistors running through the switch at this point?
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No caps to ground or resistors running through the switch at this point?
What do you see looking at the schematic?
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No caps to ground or resistors running through the switch at this point?
What do you see looking at the schematic?
see nothing but doesnt means it shouldnt be there
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What shouldn't be there? :dontknow:
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I like having and using a standby switch.
But why do you want/think you need a standby switch? :dontknow:
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Couple of observations:
1. While standby switches are "expected" on valve amps, their utility is limited and there are always (in my view) minor design tradeoffs when they are placed.
2. Over the past year, I've paid attention to when I actually use a standby switch, and when I do, why. I found that I use it very infrequently. But I get it, most folks want one, even me (my inner voice is saying "but what if you need one down the line").
3. I personally prefer the Fender implementation of the standby switch. It works, has been around for my entire lifespan, and does not seem to cause trouble.
4. There are a few other tried and true standby switch schemes, all of which seem to work fine. The Mojo idea is not one of them.
5. If you read Merlin's page on this he argues against using standby switches. He then goes on to enumerate various design choices for specific problems. Most of these choices are not needed in our amps, but a lot of folks seem to interpret these choices as best practice -- and they may be IF you have a specific problem to solve, but not helpful if you don't. And remember, he really does not think much of standby switches to begin with.
Last bit of advice: It's ok to try out what you think will work best for you! It may or may not work, but that is some of the fun of this journey.
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..........the Fender implementation of the standby switch. It works, has been around for my entire lifespan, and does not seem to cause trouble.....
I suspect it is because they use those vacuum-cleaner switches. The linked plan will have a big voltage kick when the switch is opened. So will a vacuum cleaner motor. There's a little lightning bolt inside my Electrolux rocker when I turn it off. Wee little compact switches will just arc-over internally. The big Carling switches have been around for nearly eighty years. (My 'lux has a large custom switch under the rocker.)
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Yoiu know JoeS you can install a standby switch but you can just leave it in "off" position (up on a Fender amp, not in standby) IOW you don't have to use it. Yet, if it's there and down the road you decide to sell it the geetar player who wants it would probably want to have it. I have a GZ34 in my super reverb and I hardly every use standby because of the time delayed ramp up of the rectifer, but then again I'm not actively playing gigs - I'm just a home noodler. After looking at Marshall JTM45 schematics and reading what's been said I might be convinced to put the standby switch after the first power supply filter cap, like the big Fender amps do.
Merlin also says if you must use one you can strap a current leak resistor on the switch I guess to keep the caps charged up. Have I done it,.... no.
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Yoiu know JoeS you can install a standby switch but you can just leave it in "off" position (up on a Fender amp, not in standby) IOW you don't have to use it. Yet, if it's there and down the road you decide to sell it the geetar player who wants it would probably want to have it. I have a GZ34 in my super reverb and I hardly every use standby because of the time delayed ramp up of the rectifer, but then again I'm not actively playing gigs - I'm just a home noodler. After looking at Marshall JTM45 schematics and reading what's been said I might be convinced to put the standby switch after the first power supply filter cap, like the big Fender amps do.
Merlin also says if you must use one you can strap a current leak resistor on the switch I guess to keep the caps charged up. Have I done it,.... no.
I would assume putting after the first filter cap eliminates the need for a resistor. I would think to still put a .04uf 1kv cap to ground to reduce pop
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Have you given any thought to just a MUTE switch?
There are different ways to implement a switch that will just kill the output of the amp.
Standby switches are just soooo 1974
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I like having and using a standby switch.
But why do you want/think you need a standby switch? :dontknow: