Standby switch location
Question:
OK, I know this subject has been beaten into the ground, but the fact remains that I’ve yet to see a clear-cut consensus on the best place in circuit for a standby switch. I’m building a 5E3 deluxe clone (cathode bias, B+ = c.350VDC, PT = 307-0-307). This is my 6th homebrew and 1st 5E3, but this time I’m vowing to "do it right"…commercial chassis w/eyeletboard from Weber, new top quality trannies from Hoffman and overall, a real clean job and NO SCROUNGING!! 
I’ve seen several schemes for standbys but they all have their own disadvantages. I’m certainly open to suggestions and am curious as to what folks may think would be the "lesser of 4 evils". Here’s what I’ve been considering. Remember, this amp has cathode-bias. 1) Standard Fender BF. After the 1st filter and before the OT-CT and choke. Advantage: there is absolutely no plate voltage on the output tubes and preamp tubes are also saved. Disadvantages: switch designed for 250VAC is expected to control 400+VDC…arc-city. Also, without the bleeder resistors you see on larger Fenders, there will be quite a surge when you flip the switch. Besides, the eyelet board provided with the Weber chassis needs one more terminal to do it this way. I guess I could always add a terminal strip somewhere. <bg 2) Lift the PT-CT. This only works with cathode bias. Fender did it in some of their tweeds. Advantage: minimal layout changes from stock tweed fiberboard layout. Switch handles minimal voltage swing (as long as it doesn’t float too high). I’m not sure, but does this allow the plate voltage to float? Also, I assume there will be quite a surge when the switch is engaged. 3) Lift the output tube cathodes: Also minimal layout changes and switch is only exposed to 20VDC. Disadvantages: (speculating here) will the H-K voltage swing wildly and/or draw current through the filament when switch is disengaged? Preamp tubes remain fully energized. Will there a huge surge through the OT when the switch is flipped (possibly spiking directly into the speaks?) 4) I’ve seen this one used in the AX84 project: Use a DPST standby and switch both of the PT-HT leads. Seems to circumvent the bugaboos associated with 2 and 3, and since the first stage filter is uncharged when on stdby, there won’t be a huge spike when the switch is thrown. Disadvantage: you need a DP switch, you’re still switching 307VAC (or more, depending on the project) on a switch designed for 250VAC, and the voltage between the poles can exceed 850V during the AC maxima. I’m not sure how these switches are rated *between* poles. Of course, there are other schemes which probably don’t merit mentioning. My "favorite" is on Silvertones where all they did was to tie the output tube grids together. Doesn’t save the tube one iota, but it does effectively mute the amp. Well, that’s been the source of my insomnia lately. Anybody have any constructive ideas? BTW, I’m leaning towards the BF scheme with a 500K resistor added across the 1st filter. TIA –Mike Schway Mike Schway | [Picture your favorite quote here]
Response:
If you opt for std. BF, why not go with a higher rated switch (save the old one). – Hide quoted text — Show quoted text – OK, I know this subject has been beaten into the ground, but the fact remains that I’ve yet to see a clear-cut consensus on the best place in circuit for a standby switch. I’m building a 5E3 deluxe clone (cathode bias, B+ = c.350VDC, PT = 307-0-307). This is my 6th homebrew and 1st 5E3, but this time I’m vowing to "do it right"…commercial chassis w/eyeletboard from Weber, new top quality trannies from Hoffman and overall, a real clean job and NO SCROUNGING!! I’ve seen several schemes for standbys but they all have their own disadvantages. I’m certainly open to suggestions and am curious as to what folks may think would be the "lesser of 4 evils". Here’s what I’ve been considering. Remember, this amp has cathode-bias. 1) Standard Fender BF. After the 1st filter and before the OT-CT and choke. Advantage: there is absolutely no plate voltage on the output tubes and preamp tubes are also saved. Disadvantages: switch designed for 250VAC is expected to control 400+VDC…arc-city. Also, without the bleeder resistors you see on larger Fenders, there will be quite a surge when you flip the switch. Besides, the eyelet board provided with the Weber chassis needs one more terminal to do it this way. I guess I could always add a terminal strip somewhere. <bg 2) Lift the PT-CT. This only works with cathode bias. Fender did it in some of their tweeds. Advantage: minimal layout changes from stock tweed fiberboard layout. Switch handles minimal voltage swing (as long as it doesn’t float too high). I’m not sure, but does this allow the plate voltage to float? Also, I assume there will be quite a surge when the switch is engaged. 3) Lift the output tube cathodes: Also minimal layout changes and switch is only exposed to 20VDC. Disadvantages: (speculating here) will the H-K voltage swing wildly and/or draw current through the filament when switch is disengaged? Preamp tubes remain fully energized. Will there a huge surge through the OT when the switch is flipped (possibly spiking directly into the speaks?) 4) I’ve seen this one used in the AX84 project: Use a DPST standby and switch both of the PT-HT leads. Seems to circumvent the bugaboos associated with 2 and 3, and since the first stage filter is uncharged when on stdby, there won’t be a huge spike when the switch is thrown. Disadvantage: you need a DP switch, you’re still switching 307VAC (or more, depending on the project) on a switch designed for 250VAC, and the voltage between the poles can exceed 850V during the AC maxima. I’m not sure how these switches are rated *between* poles. Of course, there are other schemes which probably don’t merit mentioning. My "favorite" is on Silvertones where all they did was to tie the output tube grids together. Doesn’t save the tube one iota, but it does effectively mute the amp. Well, that’s been the source of my insomnia lately. Anybody have any constructive ideas? BTW, I’m leaning towards the BF scheme with a 500K resistor added across the 1st filter. TIA –Mike Schway Mike Schway | [Picture your favorite quote here]
Response:
– Go to http://www.ampage.org/chat_amp/ for more answers. GaryC
– Hide quoted text — Show quoted text – OK, I know this subject has been beaten into the ground, but the fact remains that I’ve yet to see a clear-cut consensus on the best place in circuit for a standby switch. I’m building a 5E3 deluxe clone (cathode bias, B+ = c.350VDC, PT = 307-0-307). This is my 6th homebrew and 1st 5E3, but this time I’m vowing to "do it right"…commercial chassis w/eyeletboard from Weber, new top quality trannies from Hoffman and overall, a real clean job and NO SCROUNGING!! I’ve seen several schemes for standbys but they all have their own disadvantages. I’m certainly open to suggestions and am curious as to what folks may think would be the "lesser of 4 evils". Here’s what I’ve been considering. Remember, this amp has cathode-bias. 1) Standard Fender BF. After the 1st filter and before the OT-CT and choke. Advantage: there is absolutely no plate voltage on the output tubes and preamp tubes are also saved. Disadvantages: switch designed for 250VAC is expected to control 400+VDC…arc-city. Also, without the bleeder resistors you see on larger Fenders, there will be quite a surge when you flip the switch. Besides, the eyelet board provided with the Weber chassis needs one more terminal to do it this way. I guess I could always add a terminal strip somewhere. <bg 2) Lift the PT-CT. This only works with cathode bias. Fender did it in some of their tweeds. Advantage: minimal layout changes from stock tweed fiberboard layout. Switch handles minimal voltage swing (as long as it doesn’t float too high). I’m not sure, but does this allow the plate voltage to float? Also, I assume there will be quite a surge when the switch is engaged. 3) Lift the output tube cathodes: Also minimal layout changes and switch is only exposed to 20VDC. Disadvantages: (speculating here) will the H-K voltage swing wildly and/or draw current through the filament when switch is disengaged? Preamp tubes remain fully energized. Will there a huge surge through the OT when the switch is flipped (possibly spiking directly into the speaks?) 4) I’ve seen this one used in the AX84 project: Use a DPST standby and switch both of the PT-HT leads. Seems to circumvent the bugaboos associated with 2 and 3, and since the first stage filter is uncharged when on stdby, there won’t be a huge spike when the switch is thrown. Disadvantage: you need a DP switch, you’re still switching 307VAC (or more, depending on the project) on a switch designed for 250VAC, and the voltage between the poles can exceed 850V during the AC maxima. I’m not sure how these switches are rated *between* poles. Of course, there are other schemes which probably don’t merit mentioning. My "favorite" is on Silvertones where all they did was to tie the output tube grids together. Doesn’t save the tube one iota, but it does effectively mute the amp. Well, that’s been the source of my insomnia lately. Anybody have any constructive ideas? BTW, I’m leaning towards the BF scheme with a 500K resistor added across the 1st filter. TIA –Mike Schway Mike Schway | [Picture your favorite quote here]
