How can I keep high and low B+ taps from changing preamp voltages

[Mike_J]

If we put the bias on an on-off-on switch and we flip the switch to the middle position which is off or standby are we getting any bias to our power tubes?

Yes. The bias voltage will actually increase to some larger negative value when the switch is in the middle position. So it's a safe condition for the tubes. But so what. Even if you did kill the bias voltage, you also killed the B+, so the tubes are double safe.

Would bias problems go away if you abandoned the dedicate bias tap, and used the 400v or 480v tap instead?

That's very possible.

Sluckey

If we abandoned the dedicated bias tap then we lose the ability to adjust the bias for each voltage level.  I like that feature, don't you?

Thanks
Mike

[Mike_J]

There will be no more "funny spikes" than you would normally have by switching any standby switch.

Here's my idea of simultaneously switching the bias. You could make it more elaborate by switching another pot so you would have an adjustment for either voltage setting.

Sluckey

Attached is the layout for the dual bias control schematic you drew.  This subject has been solved to my satisfaction unless someone has any comments they would like to make.

Thanks
Mike

[terminalgs]

If we abandoned the dedicated bias tap then we lose the ability to adjust the bias for each voltage level.  I like that feature, don't you?

The idea is that the bias circuit, being a constant function of the input voltage, would produce a bias voltage proportional to the input voltage (400V or 480V).


If your target bias is -43V for the 480V circuit,  the same bias circuit, if provided 400V instead, would (I think) produce -36V.


You could put a trim pot in that bias circuit.  If you have 480V selected, and you trim your bias to -38V,  when you switched to 400V, the bias would be -32.




Did you find a 4 pole ON-OFF-ON switch?

[Mike_J]

If we abandoned the dedicated bias tap then we lose the ability to adjust the bias for each voltage level.  I like that feature, don't you?

The idea is that the bias circuit, being a constant function of the input voltage, would produce a bias voltage proportional to the input voltage (400V or 480V).


If your target bias is -43V for the 480V circuit,  the same bias circuit, if provided 400V instead, would (I think) produce -36V.


You could put a trim pot in that bias circuit.  If you have 480V selected, and you trim your bias to -38V,  when you switched to 400V, the bias would be -32.




Did you find a 4 pole ON-OFF-ON switch?

Terminalgs

Thank you for your response.  It seems to me it would be easier to set the bias for each setting to my liking and then it is done.  It is probably because I am familiar with adjusting bias voltage but not familiar with what you are saying.  It is not that I don't believe your way will work, it is just that I am familiar with the method Sluckey recommended and still don't quite understand your method.

I bought the 4 pole ON-OFF-ON switch at Del City.  It is part no. 7920008 and is quite beefy.  Lists a dielectric strength of 1500V AC for one minute.  I bought five of them and five of the ON-NONE-ON switches which is part no. 7920007.  Should last me a while and have more than  enough beef for any guitar amp I will ever make.

Thanks
Mike

[Mike_J]

I know I said I was done with this subject but the thought of the preamp plate voltages dropping 17% when the switch was changed from 480 volts to 400 volts bothers me.

The new dual adjustable bias adjustments for the amp that Sluckey proposed gives tonal flexibility to the amp through the ability to adjust the 400 and 480 volt bias supplies individually.  I really am enthusiastic about this feature.  However, the preamp plate voltage drop of 17% leaves the adjustable B+ as less than perfect.

We still have one switch left on our 4pdt switch.  Could we use that for a resistor in the power string to make the plate voltages the same, or different if that is what was wanted, when the B+ was switched from 400 to 480 volts or vice versa.  As it is now the plate voltages will drop 17%  when the 400 volt setting is selected. 

If we could set the preamp voltages so that they are correct to Marshall or Dumble specifications at 400 volts then it would only be necessary to add another dropping resistor to the 480 volt side to get the voltage the same or different if that is what you were looking for.  The only issue I see is how to run the wire from the switch to avoid adding noise to the amp.  Does anyone know if that will work or have any suggestions?

I have attached the power string for the amp.  I think the dropping resistor would need to be between node B and anything downstream of it.

Thanks
Mike

[sluckey]

ARRRGGGH!

[terminalgs]

I know I said I was done with this subject but the thought of the preamp plate voltages dropping 17% when the switch was changed from 480 volts to 400 volts bothers me.

Be aware that in the original EVH variac backed Super Lead Plexi scenario,  when Edward cranked the 480V down to 400V, the preamp voltages dropped as well..




so....  don't over think the mojo.




If you are looking for alternate uses of the extra switch pole...  consider some LED or incandescent bulbs to indicate "EVM mode" or "Angus mode"...

[Mike_J]

ARRRGGGH!

That's funny.

Thanks
Mike

[Mike_J]

I know I said I was done with this subject but the thought of the preamp plate voltages dropping 17% when the switch was changed from 480 volts to 400 volts bothers me.

Be aware that in the original EVH variac backed Super Lead Plexi scenario,  when Edward cranked the 480V down to 400V, the preamp voltages dropped as well..




so....  don't over think the mojo.




If you are looking for alternate uses of the extra switch pole...  consider some LED or incandescent bulbs to indicate "EVM mode" or "Angus mode"...

Terminalgs

Compromise position would be to place an extra turret between the B mode on the power string and the downstream supply.  For the time being I could just put a solid wire there.  If I decided to experiment then I could use the extra spot on the switch and a three conductor shielded wire to make the changes I was considering above.

Thanks for your input, it will probably turn out for the best.
Mike

[tubenit]

My experience with stuff like this is that I very quickly figure out a preferred position for the tone I want and then never really use that feature anymore.  Once that happens,  I often remove the switching feature in favor of a fixed position.  Simple example of that for me was pentode/triode switches.  It got left in pentode 98% of the time & I removed the pentode/triode switch.

Maybe your experience will be different and the feature will be worth while to you?  Your amp already has so many tonal options to it, I am skeptical whether you will use this very much?  My guess is that there are quite a few other features that you will switch or adjust before getting around to using this feature?

With respect, Tubenit

[Mike_J]

My experience with stuff like this is that I very quickly figure out a preferred position for the tone I want and then never really use that feature anymore.  Once that happens,  I often remove the switching feature in favor of a fixed position.  Simple example of that for me was pentode/triode switches.  It got left in pentode 98% of the time & I removed the pentode/triode switch.

Maybe your experience will be different and the feature will be worth while to you?  Your amp already has so many tonal options to it, I am skeptical whether you will use this very much?  My guess is that there are quite a few other features that you will switch or adjust before getting around to using this feature?

With respect, Tubenit

Tubenit

Thank you for the reply and your support.

That is why I told my metal man to make me two chassis.  I will see what works and doesn't and remove what doesn't from the first amp.  I think the chassis that is left over will be a good trial chassis because it will certainly have enough holes for tubes and controls to breadboard almost any amp.  If I am happy with everything the way it is, or will be, then I have a chassis for my next project on the shelf.

I am going to start making the boards today.  My metal man says my chassis will be completed this week.  Of course he also said that last week so we will see.

Thanks
Mike

[sluckey]

My "ARRRGGGH!" was not out of disgust. It was because I knew you could not stand the idea of having an unused pole on that switch. 

What you proposed will work. The resistor needs to be shorted by the switch when in the lower B+ position. (See attached.) You can likely use a 3 watt MF resistor but you'll have to determine the value experimentally. You don't need shielded wire. B+ wires  are not noisy, nor are they subject to picking up noise from other wires. Filter caps will take care of that.  And surely this switch will be located in the power end of the chassis.

Now that you have it all figured out I want you to at least think about it from a safety standpoint before you pull the trigger. Do you really want your very critical negative bias voltage on the same switch with 4 PT HT wires and 2 B+ wires? Is your switch up to the task, and I don't just mean current rating? Will the switch provide satisfactory isolation between the poles? If I was considering doing this, I would be using a high quality mil spec switch. Otherwise I'd probably want to use my safety gloves to operate the switch.

[Willabe]

Now that you have it all figured out I want you to at least think about it from a safety standpoint before you pull the trigger. Do you really want your very critical negative bias voltage on the same switch with 4 PT HT wires and 2 B+ wires? Is your switch up to the task, and I don't just mean current rating? Will the switch provide satisfactory isolation between the poles? If I was considering doing this, I would be using a high quality mil spec switch. Otherwise I'd probably want to use my safety gloves to operate the switch.

This is a very important point! (I would not do this in an amp.)

But if I did I would also get a switch like Sluckey said with a plastic handle bat.


                       Brad   

[Mike_J]

My "ARRRGGGH!" was not out of disgust. It was because I knew you could not stand the idea of having an unused pole on that switch. 

What you proposed will work. The resistor needs to be shorted by the switch when in the lower B+ position. (See attached.) You can likely use a 3 watt MF resistor but you'll have to determine the value experimentally. You don't need shielded wire. B+ wires  are not noisy, nor are they subject to picking up noise from other wires. Filter caps will take care of that.  And surely this switch will be located in the power end of the chassis.

Now that you have it all figured out I want you to at least think about it from a safety standpoint before you pull the trigger. Do you really want your very critical negative bias voltage on the same switch with 4 PT HT wires and 2 B+ wires? Is your switch up to the task, and I don't just mean current rating? Will the switch provide satisfactory isolation between the poles? If I was considering doing this, I would be using a high quality mil spec switch. Otherwise I'd probably want to use my safety gloves to operate the switch.

Sluckey

You are correct.  An unused pole on a switch or an unused triode will drive me up a wall.  It feels like I didn't get my $3 worth out of the switch. 

As for the safety factor, I have attached the data sheet for the switches.  The ON-OFF-ON and the ON-NONE-ON are rated the same.  Seems pretty hefty to me.  Willabe sent a post suggesting a plastic handle bat.  Something like a rubber boot to go over it would be a good idea.  Getting electrocuted by my amp sounds like a bad day to me.  Need to make sure I avoid that.

Good news from my chassis guy.  My chassis are ready and I am headed over there to get them.  I will be getting this project off the ground today.

Thank you for the schematic.  Presuming the switch with a rubber boot on it is deemed safe enough, I may be adding it.  Going to try the amp with the lower voltage on the preamp plates first to see if I like it.  If not I will experiment to see if I can find some setting that makes having a high and low voltage capability worthwhile.

Thanks
Mike

[sluckey]

If the switch is up to the task I would not be concerned about an insulated toggle. Those Chinese symbols on the drawing do raise my eyebrows though.

[Mike_J]

If the switch is up to the task I would not be concerned about an insulated toggle. Those Chinese symbols on the drawing do raise my eyebrows though.

Sluckey

Do you think it would be a good idea for me to send you one in the mail.  You may be able to tell if it is any good or not.  As far as the Chinese characters, while I don't disagree with you that they are not ideal but what isn't made in that part of the world today.  Don't know where Carling is making their switches now but I would be surprised if it wasn't somewhere in Asia.  I know that the CTS pots are being made over there and based on my recent experience with the 12A jack, I would expect they are now being made over there as well.

I do know that everything that is made in China is not junk.  However, if the Chinese are told to make something cheaply they will do that and it will be junk.  If they are told to make quality components they seem to be able to do that too.  I am buying them from Del City Electronics out of Wisconsin.  I will call them and see if they have a tech that can enlighten me further regarding the switches.  Will let you know what I find out.  In the meantime if you want one of the switches let me know and we can make arrangements to send one to you.

Thanks
Mike

[Mike_J]

If the switch is up to the task I would not be concerned about an insulated toggle. Those Chinese symbols on the drawing do raise my eyebrows though.

The tech that I spoke with at Del City Electronics says he has been with the company for two years.  They have been selling the switches for the entire time he has been there and he hasn't heard one complaint or had any returns where the switches are concerned.  I think they are probably as safe as anything available today.  Certainly in our litigious society if someone was electrocuted by one of their switches he would know.

Thanks
Mike

[sluckey]

I'll bet you're the first person to use one of those switches in a configuration to control high voltage AC and high voltage DC.

[Mike_J]

I'll bet you're the first person to use one of those switches in a configuration to control high voltage AC and high voltage DC.

Is there a way to throw 1500 VAC and 500 DC at the switch and test it?  Maybe even higher DC to be safe.

What would we test for.  I guess you wouldn't want the power in one switch to transfer into another switch.  They would need to be isolated from each other.  Tops on my list is you wouldn't want the power to jump to the toggle switch.  That would not be good.

Question:  Is it the high volt AC and high volt DC combined in the same switch what you are most concerned about?

Thanks
Mike

[Mike_J]

I'll bet you're the first person to use one of those switches in a configuration to control high voltage AC and high voltage DC.

Sluckey

I had another thought.  If I am adjusting the voltages from the screen node I will be at around 480 VDC and probably more at start up.  If it is only rated for 500 VDC then it is not beefy enough I wouldn't think.  Might be a good idea to keep DC off the switch if it is at or near the stated limit of the switch for safety sake.

Thanks
Mike

[Willabe]

I would be using a high quality mil spec switch. Otherwise I'd probably want to use my safety gloves to operate the switch.

It feels like I didn't get my $3 worth out of the switch.

A new $3 switch (as apposed to NOS surplus switch) is not a high quality mil spec switch with 500dcv+ specs. It's not so much about where it's made it's the spec it's made to (and IF what they claim is real world reliable in at those claims/spec in use) and I really doubt that a new $3 with that many contacts, what 4P2T(?) is built to high voltage mil. spec.

For an example, Doug sells a carling SPST (big body) switch for $4.50 and I don't know if that's mil. spec, I don't think so. We know that that switch works for a standby switch for many years at 450+dcv but it's only make/break 1 wire connection in the B+ rail, there's nothing right next to the contacts to arc-over too.

The carling short bat SPDT, 3 position, center off (big body) switch he sells is $5.30. (Don't forget there's not more only external contacts, there's more internal contacts too, so more $$.)


                         Brad   

[sluckey]

Question:  Is it the high volt AC and high volt DC combined in the same switch what you are most concerned about?

Bingo!

But also the bias voltage is in very close proximity. You got a lot of expensive stuff riding on the bias voltage pony. Maybe I'm being over cautious. But if you only paid $3 for that switch I would not call it a quality switch.

[Willabe]

But also the bias voltage is in very close proximity. You got a lot of expensive stuff riding on the bias voltage pony. Maybe I'm being over cautious. But if you only paid $3 for that switch I would not call it a quality switch.

No your not, for the reason's you have stated.

Bias voltage goes so do the output tubes and maybe the OT. Or the B+ could arc over to the bias voltage and you now have put 450+dcv's on the power tube grids? That's not good.

I've been here now ~4 years? I don't remember anyone trying this, I'm not saying it can't be done but your dealing with the highest voltages and the most current (plus the -bias) in the amp. (Maybe someone did and they will post their results with the switch brand they used?)


                Brad   

[Mike_J]

But also the bias voltage is in very close proximity. You got a lot of expensive stuff riding on the bias voltage pony. Maybe I'm being over cautious. But if you only paid $3 for that switch I would not call it a quality switch.

No your not, for the reason's you have stated.

Bias voltage goes so do the output tubes and maybe the OT. Or the B+ could arc over to the bias voltage and you now have put 450+dcv's on the power tube grids? That's not good.

I've been here now ~4 years? I don't remember anyone trying this, I'm not saying it can't be done but your dealing with the highest voltages and the most current (plus the -bias) in the amp. (Maybe someone did and they will post their results with the switch brand they used?)


                Brad   

If anyone can or knows someone who can test one of these switches I would be glad to send one or more to them to test.  It is probably no secret by now that I like to poke at the edges of what can be done.  However, I will not do it at the expense of safety.  I am somewhat stubborn but I strive to not be stupid.  My goal is to make amps that will last in excess of fifty years and operate safely.

The switches supposedly were $16 switches marked down to $3.  However, if you can buy them for $3 they are $3 switches.  I mentioned how much I paid for the switches because I thought it was relevant and it most likely is.  Generally, you get what you pay for but 20 years ago you could get a nice leather bombers jacket in China for $18.  Not all of the product pricing coming out of Asia makes economic sense.  Much of it is government supported to promote job growth. 

The reason I am babbling on about this is the only true way to know whether the switches are any good would be to test them under stress loads.  If nobody on the forum considers it a hobby to test switches I guess I am out of luck.  To do it properly I suppose it would have to be cycled thousands of times while under stress load which I am thinking might not make for a very enjoyable Friday night.  I don't think I could get my wife to spend Friday night flipping a switch back and forth over and over again when the purpose of the test is to make sure it won't electrocute anyone.  I know for sure I am not volunteering for the job.

I suppose I should take the conservative approach and pull out a Carling DPDT switch and test each voltage individually to see if it even makes enough of a difference to justify the fuss. 

Thanks
Mike

[tubenit]

Mike,

We all want you to be successful and enjoy the amp (safely).  Please note the "caution" that has been conveyed by  some very experienced builders regarding this idea.

Anything that Sluckey was hesitant about from a safety standpoint, I would not even remotely consider. 

Respectfully and with best regards,  Tubenit

[Mike_J]

Mike,

We all want you to be successful and enjoy the amp (safely).  Please note the "caution" that has been conveyed by  some very experienced builders regarding this idea.

Anything that Sluckey was hesitant about from a safety standpoint, I would not even remotely consider. 

Respectfully and with best regards,  Tubenit

Tubenit

I agree 100%.  If a component is unsafe or may result in premature failure of the amplifier then the appropriate thing to do is to not use that component.  I know you have been encouraging me to take a more conservative approach because of the experience you have.  Part of what I like to do is explore possibilities.  It is how I learn. 

In this instance after exploring the options it is only appropriate to accept the recommendations of wise counsel.  I appreciate everyone's participation in trying to find a solution to what I was trying to accomplish.  I think it comes down to the fact that we do not have sufficient confidence in any toggle switches larger than the Carling DPDT switch. 

The Carling switches have been used successfully for many years.  It would be very nice if a 4PDT toggle switch existed that was known to be reliably able to do what I wanted to be able to accomplish as has been discussed in this post.  Unfortunately, I don't know of any that fit that category.  Our other option was to split the rectifiers but that is impractical because too many electrolytics would be required and  limited space precludes the use of same.

Thanks to all that have participated in this post.  I have learned a lot from this exercise.

On a side note, I got my chassis today and they look great.  Will get to work building the amp and figure out what works and doesn't work.  What is worthwhile and what isn't worthwhile.  I think having the separate tone stacks will in the long run be a good thing.  The active effects loop that can be switched out of the signal path should be worthwhile.  The overdrive circuitry is up in the air and is a we will have to see type of thing.  The different voltages and appropriate bias and plate voltage settings is an experiment I am looking forward to.  Only time will tell whether that experiment is fruitful.

In the end if I end up with a Plexi/800 amp with individual tone stacks that can each be used with an A/B/Y switch the project will be a success.  I look forward to your future help and wise counsel and again much appreciation for the help and support you have given me.

Thanks
Mike

[Willabe]

Mike, We all want you to be successful and enjoy the amp (safely). 

Yes, absolutely, what Tubenit said.   

It is probably no secret by now that I like to poke at the edges of what can be done.

That's fine, it can be very good to poke/push things and experiment to get new results, new boundary's. The guys here and on other fourms do that all the time. 

But........    some things are much easer to 'poke' than others. Like high dcv and current.   

Again I'm not saying and I don't see anyone else saying it can't be done, it's just this 1 needs to be thought out from a stress and safety point with the right parts to get'er done.


                          Brad     

[tubenit]

Mike,

I love to see innovative ideas and amp designs!  Your amp easily qualifies as a unique "one of a kind" approach.  I am anticipating your amp sounding great!  I think once it's built and you have tweaked it, it will a remarkable and superb creation. 

 

I look forward to your project coming together. I think you have taken a very thoughtful and reasonable approach to this & all your efforts will pay off for you, IMO.

A bunch of us are rooting for your success and will cheer you on (and help when and if you need some input).

With respect, Tubenit

[sluckey]

Yeah, what he said! Now quit talking 'bout it and get busy!   

We're all eager to see this amp fired up.

[Mike_J]

Mike, We all want you to be successful and enjoy the amp (safely). 

Yes, absolutely, what Tubenit said.   

It is probably no secret by now that I like to poke at the edges of what can be done.

That's fine, it can be very good to poke/push things and experiment to get new results, new boundary's. The guys here and on other fourms do that all the time. 

But........    some things are much easer to 'poke' than others. Like high dcv and current.   

Again I'm not saying and I don't see anyone else saying it can't be done, it's just this 1 needs to be thought out from a stress and safety point with the right parts to get'er done.


                          Brad     

Brad

You and others provided very sound advice.  I appreciate your input.

Thanks
Mike

[Mike_J]

Mike,

I love to see innovative ideas and amp designs!  Your amp easily qualifies as a unique "one of a kind" approach.  I am anticipating your amp sounding great!  I think once it's built and you have tweaked it, it will a remarkable and superb creation. 

 

I look forward to your project coming together. I think you have taken a very thoughtful and reasonable approach to this & all your efforts will pay off for you, IMO.

A bunch of us are rooting for your success and will cheer you on (and help when and if you need some input).

With respect, Tubenit

Tubenit

Thanks as always for your kind words and inspiration.

Mike

[Mike_J]

Yeah, what he said! Now quit talking 'bout it and get busy!   

We're all eager to see this amp fired up.

Sluckey

Got both of my chassis and the faceplate and back plate came in today.  I drilled the holes for the tubes today and drilled and tapped the holes for the bias circuit that will be place one the inside of the front of the amp.  Tomorrow I will put the faceplate and back plates on and get started on the boards.  I think I have all my parts here or they are on there way over the next few days.  After the faceplate and back plates have had a day to set I will drill the holes for the jacks, pots, switches, etc.  Then it is a matter of installing the transformers and hope for the best.

I will try to get my daughters to take some pictures for me.  My camera proudly proclaims "Sponsor of the 1980 Olympics" on it so needless to say it won't be of much help but my daughters are able to take pictures so I will try to post some tomorrow.

Thanks for all your help.

Mike