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

[Mike_J]

The power transformer I had made for the Plexi/800 sluckey inspired amp has two sets of taps for the B+ voltage.  The lower voltage is in the neighborhood of 400 volts and the higher tap is around 480 volts.
 
There were two reasons I had the PT made this way.  First, I did some research on some Marshall sites where some people liked lower voltage in the 400 volt range and others liked the voltage around 480 volts and sometimes even higher.  The second reason is they didn't charge me any more money to make it this way so I figured why not.

Now for my questions.  Is it possible to lower the B+ to the power section yet keep the preamp section voltages constant?  If so, how would I go about it?  I guess I should also ask the question does it matter whether or not the preamp voltages are changed along with the HT voltage?

Thanks
Mike

[sluckey]

Is it possible to lower the B+ to the power section yet keep the preamp section voltages constant?  If so, how would I go about it?

Yes. Use larger value dropping resistors on the B+ rail if you use the higher voltage windings. This ain't something I would ever consider making user switchable.

[Mike_J]

Is it possible to lower the B+ to the power section yet keep the preamp section voltages constant?  If so, how would I go about it?

Yes. Use larger value dropping resistors on the B+ rail if you use the higher voltage windings. This ain't something I would ever consider making user switchable.

Sluckey

It does complicate matters when making it user switchable.  Especially with the voltage doubler needed for the PT I have.  I would think if one of the two taps had adequate voltage you might be able to Y off to the preamp with one of the two HT lines and send the other side of the Y to the  switch.  Don't know if it is feasible but it would make for a more versatile amp if it worked. 

Any thoughts on whether the above ramblings would be feasible?  If so I might make a separate amp that is just Marshall with no overdrive but maybe with an active effects unit that can be switched in or out.  All I have to do is have them build the transformer I need for the job.

Thanks
Mike

[sluckey]

Your voltage doubler is really just a full wave bridge. Nothing complicated about that. What you're talking about sounds like two separate rectifier/filter rails, one for the power stages, and another for the preamp stages. I've never seen anyone do that with bridge rectifiers. Not gonna say it can't be done, but if you will draw a schematic (not a layout) of your proposal and study it, then you could say yes this will work, or no, I see some sneak path that will wreck it. But if you still don't know for sure, one way or the other, maybe you would consider dropping the idea?

I think it's just another whistle to throw at your "be everything amp". That amp is already overly complicated.      You need a reality check! And I mean that in a positive way. 

[Mike_J]

Your voltage doubler is really just a full wave bridge. Nothing complicated about that. What you're talking about sounds like two separate rectifier/filter rails, one for the power stages, and another for the preamp stages. I've never seen anyone do that with bridge rectifiers. Not gonna say it can't be done, but if you will draw a schematic (not a layout) of your proposal and study it, then you could say yes this will work, or no, I see some sneak path that will wreck it. But if you still don't know for sure, one way or the other, maybe you would consider dropping the idea?

I think it's just another whistle to throw at your "be everything amp". That amp is already overly complicated.      You need a reality check! And I mean that in a positive way.

Sluckey

Thanks for the good comments.  There is no doubt that this amp is somewhat complex and I am just looking to find a way to further complicate it.  As the layout is currently structured the Plexi normal channel plate voltage would be 150 volts at 480 volts B+.  If I switched the B+ to 400 volts the normal channel plate voltage would drop down to 125 volts.  I don't think that is a practical voltage.  Your suggestion to drop the switch is the right one.

I can't figure out how to show a bridge rectifier on a schematic.  I will have to do some research on splitting the preamp and power amp power strings on my next Plexi/800 build and probably will not use a PT that requires the use of a bridge rectifier because I think I may be able to solve that riddle.  For the meantime I guess I am going to have to change my faceplate and go with the 480 volt tap because I have enough to do already.

Thanks
Mike

[Mike_J]

Your voltage doubler is really just a full wave bridge. Nothing complicated about that. What you're talking about sounds like two separate rectifier/filter rails, one for the power stages, and another for the preamp stages. I've never seen anyone do that with bridge rectifiers. Not gonna say it can't be done, but if you will draw a schematic (not a layout) of your proposal and study it, then you could say yes this will work, or no, I see some sneak path that will wreck it. But if you still don't know for sure, one way or the other, maybe you would consider dropping the idea?

I think it's just another whistle to throw at your "be everything amp". That amp is already overly complicated.      You need a reality check! And I mean that in a positive way.

Sluckey

I prepared a hybrid schematic of the separate power supply rails.  On the preamp side I need 368 volts to operate the effects circuit.  Therefore, I must have the full wave bridge in order to be able to achieve that voltage.  Is the fact that the CT is tied to the power caps okay for the preamp side?  Disregard the 245 volt comment.  Do you think this will work the way I have it drawn?

Thanks
Mike

[Mike_J]

Your voltage doubler is really just a full wave bridge. Nothing complicated about that. What you're talking about sounds like two separate rectifier/filter rails, one for the power stages, and another for the preamp stages. I've never seen anyone do that with bridge rectifiers. Not gonna say it can't be done, but if you will draw a schematic (not a layout) of your proposal and study it, then you could say yes this will work, or no, I see some sneak path that will wreck it. But if you still don't know for sure, one way or the other, maybe you would consider dropping the idea?

I think it's just another whistle to throw at your "be everything amp". That amp is already overly complicated.      You need a reality check! And I mean that in a positive way.

Sluckey

Here is my latest attempt at a schematic for a Dual HT Power Supply.  It should be the same as the last one I sent you but I found some schematic elements for the switches.  Hopefully it will make sense to you.

Thanks
Mike

[sluckey]

Look at the complexity of the switching. Do you really want to do that? And why???

[Mike_J]

Look at the complexity of the switching. Do you really want to do that? And why???

Sluckey

The only reason I wanted to do it is because I read somewhere that some people prefer the amps at 400 volts versus the higher voltages.  Most of them are EVH fanatics.  They go to great lengths with Variacs and other means that have the potential of starving the heaters and lowering the voltages to the preamp tubes well below their circuit specifications.  I believe my method is a more correct way of accomplishing the objective of lowering the B+.

I have very little experience playing Marshall amps.  I know a lot of the music I enjoy was recorded and performed using the amplifiers.  I am just trying to get as much versatility out of the amp as possible.  I have no idea if the difference in tone makes the additional effort worthwhile but I just see it as an additional 4pdt toggle switch and a 1000 volt bridge rectifier which is not that big of deal.  About $5 worth of parts.

If your objection to this is that the difference in tone would not be enough to warrant any additional effort being made then your point is very valid because why should I do anything that would make little or no tonal difference.  However, if the only concern is a little added complexity then I don't mind the added complexity as long as the way I drew the circuit will work.

I guess to answer your question I like some of EVHs work too.  I just want to know whether the circuit I drew will work properly to accomplish the objective of lowering the voltage to the power tubes while leaving the preamp tube voltages intact.

Thanks
Mike

[HotBluePlates]

Look at the complexity of the switching. Do you really want to do that? And why???

If your objection to this is that the difference in tone would not be enough

His objection is about safety: Most of those multi-pole switches are small and have closely-spaced lugs. If your wiring is less than perfect, you will create short-circuits and probably a burned transformer. Good wiring would involve careful soldering and probably heatshrink covering every exposed lug (which then requires a decent heat gun or some careful use of a very hot hair dryer).

How about bringing the PT secondary wires out to a terminal strip (be sure it's a VERY good one)? You could bolt down the wires running from PT to terminal strip, and have a 2nd set of wires to be bolted at the terminal strip running to the rectifier. This would not be "user changeable" but it could be changed if you wanted to test each mode, and would be safer wiring than using a switch.

[SILVERGUN]

Hey Mike,
I've been reading some of your posts and just figured I'd throw my 2 cents out there...

If it's the EVH trick you're after,, then why separate the power tube voltages from the preamp tube voltages?

Using a variac to lower voltages at the input would cause all of those B+ voltages to be affected proportionately, and that would have something to do with the magic....right?...

Respectfully,
SG

[Mike_J]

Look at the complexity of the switching. Do you really want to do that? And why???

If your objection to this is that the difference in tone would not be enough

His objection is about safety: Most of those multi-pole switches are small and have closely-spaced lugs. If your wiring is less than perfect, you will create short-circuits and probably a burned transformer. Good wiring would involve careful soldering and probably heatshrink covering every exposed lug (which then requires a decent heat gun or some careful use of a very hot hair dryer).

How about bringing the PT secondary wires out to a terminal strip (be sure it's a VERY good one)? You could bolt down the wires running from PT to terminal strip, and have a 2nd set of wires to be bolted at the terminal strip running to the rectifier. This would not be "user changeable" but it could be changed if you wanted to test each mode, and would be safer wiring than using a switch.

VAC,
HotBluePlates

Thank you for your comments.  The 4PDT switch I am considering is rated at 16A 125, 10A 250 VAC, DC 500 100M ohm min, AC 1500 V 1 minute.  The only negative is that it has screw terminals instead of solder terminals.  If this is not safe then I agree with you 100% that I need to pick one of the voltages or the other and stick with it.  Do you think that switch would be adequate for the job I need it to do?  The switch body is about 1-1/2" X 1-1/2" so it is pretty hefty.

I failed to mentioned I will make a separate circuit board for the rectifiers with turrets or eyelets to tie into.  I think it will also help in placement to reduce the chances of AC noise entering the inputs.

Thanks
Mike

[Mike_J]

Hey Mike,
I've been reading some of your posts and just figured I'd throw my 2 cents out there...

If it's the EVH trick you're after,, then why separate the power tube voltages from the preamp tube voltages?

Using a variac to lower voltages at the input would cause all of those B+ voltages to be affected proportionately, and that would have something to do with the magic....right?...

Respectfully,
SG

SG

I have a very nice Variac that would do the trick but I am concerned about starving the heaters.  HBPs opinion regarding the safety of the available 4pdt switch will be the determining factor in which direction I will go.  If he doesn't think it is safe then I will definitely choose the higher voltage tap and move on from there.

Thanks
Mike

[SILVERGUN]

Hey Mike,
I've been reading some of your posts and just figured I'd throw my 2 cents out there...

If it's the EVH trick you're after,, then why separate the power tube voltages from the preamp tube voltages?

Using a variac to lower voltages at the input would cause all of those B+ voltages to be affected proportionately, and that would have something to do with the magic....right?...

Respectfully,
SG

SG

I have a very nice Variac that would do the trick but I am concerned about starving the heaters.  HBPs opinion regarding the safety of the available 4pdt switch will be the determining factor in which direction I will go.  If he doesn't think it is safe then I will definitely choose the higher voltage tap and move on from there.

Thanks
Mike

Since you're only switching the HT winding(s), your design wouldn't affect the heater winding

[SILVERGUN]

Your dual HT tap PT is a great platform for doing just what you are thinking of.....without the side affect of starving the heaters

If I was doing what you're doing and going after the VH thing,,,,I wouldn't hesitate to do just that...a basic low/high B+ switch that only switches between the 2 HT taps is ideal.

 
 

[Mike_J]

Your dual HT tap PT is a great platform for doing just what you are thinking of.....without the side affect of starving the heaters

If I was doing what you're doing and going after the VH thing,,,,I wouldn't hesitate to do just that...a basic low/high B+ switch that only switches between the 2 HT taps is ideal.

 

SG

Thank you for your support concerning this matter.  The key is as to whether it can be done safely.  That is a determination someone like HBP is qualified to make.  I think the switch is more than adequate for the job but I would like confirmation from HBP because I would trust his judgment concerning the safety of the switch.

Thanks
Mike

[SILVERGUN]

Just for the record, here's what I'm suggesting for you...

It does not take the bias winding into consideration, and that will be unaffected as well, so it will result in a "colder" bias condition (when switched to the 'low voltage' setting), because the bias voltage will stay the same as the B+ is lowered........

That will contribute to an overall more overdriven output stage......could be cool.

***I'm going off of memory that you do have a separate bias winding***

[HotBluePlates]

The 4PDT switch I am considering is rated at 16A 125, 10A 250 VAC, DC 500 100M ohm min, AC 1500 V 1 minute.  The only negative is that it has screw terminals instead of solder terminals.  If this is not safe then I agree with you 100% that I need to pick one of the voltages or the other and stick with it.  Do you think that switch would be adequate for the job I need it to do?  The switch body is about 1-1/2" X 1-1/2" so it is pretty hefty.

Well, you seem determined to give it a go. Your switch also sounds bigger than I envisioned. It is likely workable, then.

Since you have screw terminals (a blessing in this case), you won't need to worry about solder blobs on the switch (solder bridges and stray wire strands were the big issues I was concerned about). Do yourself a huge favor and find ring terminals which fit the screws on the switch. Strip the PT wires, crimp & solder the ring terminals, with heatshrink over the "non-ring" part of the terminals. Now they're ready to be bolted in place on your switch. It probably doesn't include a locking mechanism for the screws, so add your own lock washers (you don't want these to loosen, ever, without you putting some effort into removing them).

I'd encourage you to have the amp at least in standby (preferably off) when switching the high voltage levels. Also ensure you have enough rectifier and filter cap voltage rating reserve for unexpected high wall voltage.

[TIMBO]

Would a VVR on the power tubes achieve what you are wanting. Replace the 1M with fixed resistors on a switch.

[Mike_J]

The 4PDT switch I am considering is rated at 16A 125, 10A 250 VAC, DC 500 100M ohm min, AC 1500 V 1 minute.  The only negative is that it has screw terminals instead of solder terminals.  If this is not safe then I agree with you 100% that I need to pick one of the voltages or the other and stick with it.  Do you think that switch would be adequate for the job I need it to do?  The switch body is about 1-1/2" X 1-1/2" so it is pretty hefty.

Well, you seem determined to give it a go. Your switch also sounds bigger than I envisioned. It is likely workable, then.

Since you have screw terminals (a blessing in this case), you won't need to worry about solder blobs on the switch (solder bridges and stray wire strands were the big issues I was concerned about). Do yourself a huge favor and find ring terminals which fit the screws on the switch. Strip the PT wires, crimp & solder the ring terminals, with heatshrink over the "non-ring" part of the terminals. Now they're ready to be bolted in place on your switch. It probably doesn't include a locking mechanism for the screws, so add your own lock washers (you don't want these to loosen, ever, without you putting some effort into removing them).

I'd encourage you to have the amp at least in standby (preferably off) when switching the high voltage levels. Also ensure you have enough rectifier and filter cap voltage rating reserve for unexpected high wall voltage.

HBP

Great advice.  I am going to make a check list of your recommendations and incorporate every one of them into my build.  Thanks again for your input.

Mike

[Mike_J]

Would a VVR on the power tubes achieve what you are wanting. Replace the 1M with fixed resistors on a switch.

Timbo

Thank you for your reply.  The amp already has three master volumes.  If I put another one on it I believe I would be in agreement with everyone that is telling me the amp is over complicated.  Maybe on another amp.  I have never installed a VVR in any of my amps.  Given most of them are 100 watt amps I should probably study it.  Just not right now.

Thanks
Mike

[Mike_J]

Just for the record, here's what I'm suggesting for you...

It does not take the bias winding into consideration, and that will be unaffected as well, so it will result in a "colder" bias condition (when switched to the 'low voltage' setting), because the bias voltage will stay the same as the B+ is lowered........

That will contribute to an overall more overdriven output stage......could be cool.

***I'm going off of memory that you do have a separate bias winding***

SG

Yes, I have a dedicated bias winding.  When I was working on the power supply schematic I went back and forth as to whether I should treat the PI as part of the power section and therefore have it increase and decrease with the HT voltage.  Decided having it on the preamp side would be easier.  I hope the lower voltage setting will give the amp a different and pleasing tone.

Thank you for your input.
Mike

[SILVERGUN]

Yes, I have a dedicated bias winding.  When I was working on the power supply schematic I went back and forth as to whether I should treat the PI as part of the power section and therefore have it increase and decrease with the HT voltage.  Decided having it on the preamp side would be easier.  I hope the lower voltage setting will give the amp a different and pleasing tone.

The lowered voltage to the PI will result in a reduction in headroom in that stage, and will contribute to the PI clipping...with the amount of gain you have in your preamp plan, you should wind up with one seriously square wave.

Funny you mention it because I just suggested using a 12AT7 in the PI on your other thread right before I read this post. I believe it lends itself well to the circuit when you've got that much preamp gain on hand.

I'm totally benind you and look forward to the end result....my only fear for you is the fear of the unknown.

[sluckey]

Here is my latest attempt at a schematic for a Dual HT Power Supply.

I believe that will work. You'll probably need more filtering in the upper B+ rail.

[Mike_J]

Here is my latest attempt at a schematic for a Dual HT Power Supply.

I believe that will work. You'll probably need more filtering in the upper B+ rail.

Sluckey

Thanks for your reply.  Do you have a suggestion as to values?  Maybe 220uF @ 450 or 500 volts?  Do I also need to boost the caps for the screen supply?  Do I need the 1A fuse on the preamp side?

Looking back at my ODS - HRM build I used two 200uF @450V in series on the HT and two 47 uF @ 350V in series for the screen supply.  Probably good choices for this amp too don't you think?

Thanks
Mike

[Mike_J]

Yes, I have a dedicated bias winding.  When I was working on the power supply schematic I went back and forth as to whether I should treat the PI as part of the power section and therefore have it increase and decrease with the HT voltage.  Decided having it on the preamp side would be easier.  I hope the lower voltage setting will give the amp a different and pleasing tone.

The lowered voltage to the PI will result in a reduction in headroom in that stage, and will contribute to the PI clipping...with the amount of gain you have in your preamp plan, you should wind up with one seriously square wave.

Funny you mention it because I just suggested using a 12AT7 in the PI on your other thread right before I read this post. I believe it lends itself well to the circuit when you've got that much preamp gain on hand.

I'm totally benind you and look forward to the end result....my only fear for you is the fear of the unknown.

SG

Thanks for the 12AT7 suggestion it is a good one.  Also, thanks for your support.  I am fairly confident in the Marshall side of this build as I can ground the potential problem area, OD & FX out of the signal path.  However, the excitement is in adding the OD & FX to the amp.  It could be one high gain monster or it could be unusable only time will tell. 

What I am confident in is there are people on this forum that can help increase the odds that the overall build will be a success.  This amp may stretch the boundaries a little bit but I think that is a good thing.

Thanks
Mike

[terminalgs]

I would ditch the idea of a toggle switch in favor of using an octal tube socket and two different octal plugs (with covers, of course).  One is labeled 480V, the other is labeled 400V.  With an octal plug, you can connect 1,2 to be the 480V PT leads, 3,4 to be the 400V PT leads, 5,6 to be the circuit to your rectifier,  and 7,8 would be a series interconnect of one of the PT's primary (i.e. MAINS,  in series between the power switch and the PT ).

On the 480V plug: connections are 1-5, 2-6, 7-8, (3,4 NC)
On the 400V plug: connections are 3-5, 4-6, 7-8, (1,2, NC)

The advantage of doing it this way: (1) when going between the two voltage selections, the amp is effectively OFF because of the mains lead in series and (2) the plug and sockets are rated for 500+VDC, (3) you get a 1-2 second delay in the time it would physically take you to swap plugs.

If you want to be super snazzy, use 5,6 instead of 7,8 for the mains interconnect.  Make pin 6 be the connection from the power switch so if someone doesn't read the socket label and inserts an EL34, then the main voltage is on pin-6, a pin not used by the tube.


also, you may already be planning it this way...: I would design the entire power supply (all electrolytics for everything from the power tubes to the preamps) to operate at acceptable voltages with your 400V power tube voltage.  Do so such that the preamps are at some acceptable voltage.  When you switch to 480V,  those preamp voltages will increase, but they should still be acceptable.

[Mike_J]

I would ditch the idea of a toggle switch in favor of using an octal tube socket and two different octal plugs (with covers, of course).  One is labeled 480V, the other is labeled 400V.  With an octal plug, you can connect 1,2 to be the 480V PT leads, 3,4 to be the 400V PT leads, 5,6 to be the circuit to your rectifier,  and 7,8 would be a series interconnect of one of the PT's primary (i.e. MAINS,  in series between the power switch and the PT ).

On the 480V plug: connections are 1-5, 2-6, 7-8, (3,4 NC)
On the 400V plug: connections are 3-5, 4-6, 7-8, (1,2, NC)

The advantage of doing it this way: (1) when going between the two voltage selections, the amp is effectively OFF because of the mains lead in series and (2) the plug and sockets are rated for 500+VDC, (3) you get a 1-2 second delay in the time it would physically take you to swap plugs.

If you want to be super snazzy, use 5,6 instead of 7,8 for the mains interconnect.  Make pin 6 be the connection from the power switch so if someone doesn't read the socket label and inserts an EL34, then the main voltage is on pin-6, a pin not used by the tube.


also, you may already be planning it this way...: I would design the entire power supply (all electrolytics for everything from the power tubes to the preamps) to operate at acceptable voltages with your 400V power tube voltage.  Do so such that the preamps are at some acceptable voltage.  When you switch to 480V,  those preamp voltages will increase, but they should still be acceptable.

Terminalgs

Unfortunately I am not familiar with the use of plugs.  I would need a schematic and part numbers to follow what you are saying.  It is way over my head.

The advantage to the way I have wired it with the switch is the preamp voltages will be consistent whether the B+ is 400 volts or 480 volts.  Only the HT and the screen voltages will change.  There is a 17% difference between the two voltages.  To me that seemed like too much of a swing in the preamp section which is what led to my decision wire the supply the way I did.

I am always open to new ideas but it seems like turning the standby switch to off and changing the HT voltage using two switches is as easy as this can be done.

Thanks
Mike

[terminalgs]

Hi Mike,


this is what I'm talking about.





If the procedure to switch between voltage involves two different independent switches (voltage switch, and  a stand-by switch),  someday, someone (you, or someone else) will throw the switches the wrong way, at the wrong time.  If the amp is likely to blow up in that event, it is a very bad design decision. 


I drew the plugs as a straight line, but you can use an octal tube socket.  the plugs are $2ea (you need two for this), the socket is $2-$20ea, depending on your quality tastes.


See "9 Pin Tube Base" on Doug's "tube Sockets' page in the store. 

[Mike_J]

Here is my latest attempt at a schematic for a Dual HT Power Supply.

I believe that will work. You'll probably need more filtering in the upper B+ rail.

Sluckey

The F&T electrolytics that are available are only 220uF at 300 volts.  That means running two of them in series would give me 110uF at 600 volts.  Do you think that is enough voltage for a 480 volt B+ amp?  There are no name caps that are 220uF at 350 volts.  They claim to be high quality and low leakage.  Any suggestions?

Thanks
Mike

[sluckey]

I wasn't referring to increase the size of the caps. I meant you need to put a CLC pi filter following the fuse. You lost all that filtering when you split the B+ rail. Your original CLC filter is still on the power amp rail but it does not benefit the preamp rail.

[SILVERGUN]

Mike J.,,,,just for the record.

IMHO, just dropping the B+ on the power tubes is going to give you the audible equivalent of turning the amp's PPIMV down from 8 to 6.5

Lowering all of the voltages however, will produce a more noticeable effect and probably get you closer to your tone goal......if the VH1 tone is truly what you're after and not just a more complicated or unique B+ rail. 

I'll shut up now. 

[Mike_J]

Mike J.,,,,just for the record.

IMHO, just dropping the B+ on the power tubes is going to give you the audible equivalent of turning the amp's PPIMV down from 8 to 6.5

Lowering all of the voltages however, will produce a more noticeable effect and probably get you closer to your tone goal......if the VH1 tone is truly what you're after and not just a more complicated or unique B+ rail. 

I'll shut up now. 

If I can accomplish the same thing by just reducing the PPIMV then it would not make much sense to go to the effort of doing the dual HT taps with split rectifiers.  I have very little experience with these things but as sluckey just mentioned in his last post the circuitry to split the rectifiers is getting more complicated by the moment.  With more caps required I start to get into space issues.

By lowering all of the voltages I just need a second 2pdt switch the way I wired it in the original layout, correct?  In other words, a switch that will send one of the HT secondary voltages (i.e. 400 volts) or the other.

Thanks
Mike

[Mike_J]

I wasn't referring to increase the size of the caps. I meant you need to put a CLC pi filter following the fuse. You lost all that filtering when you split the B+ rail. Your original CLC filter is still on the power amp rail but it does not benefit the preamp rail.

Sluckey

I am coming around to your original way of thinking.  This gets into a space issue with more power caps at which point I suppose I need to do the center tap thing again.  It is becoming a mess that is not worth the effort.  I thank you for bringing this to my attention as it adds a considerable amount of additional complexity to the design.  With the additional caps I have to start thinking about where to put them and it does not seem as feasible.

I am also wondering whether the existing power supply will provide adequate filtering for the overdrive channel.  If I increase the uF rating on the B+ tap then I lose the originality of the Marshall amp.  This is something that I won't know for sure until the amp is finished.  My chassis man thinks he will have my chassis ready during the next few days.  In the meantime I will start the circuit boards.

Thanks
Mike

[Mike_J]

Hi Mike,


this is what I'm talking about.





If the procedure to switch between voltage involves two different independent switches (voltage switch, and  a stand-by switch),  someday, someone (you, or someone else) will throw the switches the wrong way, at the wrong time.  If the amp is likely to blow up in that event, it is a very bad design decision. 


I drew the plugs as a straight line, but you can use an octal tube socket.  the plugs are $2ea (you need two for this), the socket is $2-$20ea, depending on your quality tastes.


See "9 Pin Tube Base" on Doug's "tube Sockets' page in the store.

Thank you for the explanation.  I think it is getting too complex for my first build that is not completely paint by numbers.  I think I am going to back away from the dual HT idea.

Thanks
Mike

[SILVERGUN]

Mike J.,,,,just for the record.

IMHO, just dropping the B+ on the power tubes is going to give you the audible equivalent of turning the amp's PPIMV down from 8 to 6.5

Lowering all of the voltages however, will produce a more noticeable effect and probably get you closer to your tone goal......if the VH1 tone is truly what you're after and not just a more complicated or unique B+ rail. 

I'll shut up now. 

If I can accomplish the same thing by just reducing the PPIMV then it would not make much sense to go to the effort of doing the dual HT taps with split rectifiers.  I have very little experience with these things but as sluckey just mentioned in his last post the circuitry to split the rectifiers is getting more complicated by the moment.  With more caps required I start to get into space issues.

By lowering all of the voltages I just need a second 2pdt switch the way I wired it in the original layout, correct?  In other words, a switch that will send one of the HT secondary voltages (i.e. 400 volts) or the other.

Thanks
Mike

One switch will do it....
Yup Mike...it's just that simple...see my schematic in reply #16
I have tried both methods on my breadboard and sincerely believe that you will be going in the right direction if you drop the entire rail.

It's not exactly like lowering the PPIMV, but I gave you that example to try to demonstrate the little 'audible' difference that it would make to just do the power tubes by 17%...

In my mind, the extra circuitry needed doesn't justify the outcome.......i.e. a lot of work for a little change in tone
But on the other hand, the small amount of work needed to drop the entire rail will yield a greater change in tone, and more closely mimic the use of a variac.
I wouldn't steer you wrong. 

[Mike_J]

Mike J.,,,,just for the record.

IMHO, just dropping the B+ on the power tubes is going to give you the audible equivalent of turning the amp's PPIMV down from 8 to 6.5

Lowering all of the voltages however, will produce a more noticeable effect and probably get you closer to your tone goal......if the VH1 tone is truly what you're after and not just a more complicated or unique B+ rail. 

I'll shut up now. 

If I can accomplish the same thing by just reducing the PPIMV then it would not make much sense to go to the effort of doing the dual HT taps with split rectifiers.  I have very little experience with these things but as sluckey just mentioned in his last post the circuitry to split the rectifiers is getting more complicated by the moment.  With more caps required I start to get into space issues.

By lowering all of the voltages I just need a second 2pdt switch the way I wired it in the original layout, correct?  In other words, a switch that will send one of the HT secondary voltages (i.e. 400 volts) or the other.

Thanks
Mike

One switch will do it....
Yup Mike...it's just that simple...see my schematic in reply #16
I have tried both methods on my breadboard and sincerely believe that you will be going in the right direction if you drop the entire rail.

It's not exactly like lowering the PPIMV, but I gave you that example to try to demonstrate the little 'audible' difference that it would make to just do the power tubes by 17%...

In my mind, the extra circuitry needed doesn't justify the outcome.......i.e. a lot of work for a little change in tone
But on the other hand, the small amount of work needed to drop the entire rail will yield a greater change in tone, and more closely mimic the use of a variac.
I wouldn't steer you wrong. 

SG

I came across an on-off-on switch with the same high voltage capabilities of the other on-none-on switch.  Seems like that would be safer than two Carling 2p2t switches.  Would you agree?  I could place the HT taps on the ends and run the middle of the switch to the rectifier.  When the switch is placed in the middle it is in standby.  Go one direction it is 400 volts the other is 480.

Hopefully HBP is looking in and can comment on the safety of doing it this way.

Thanks
Mike

[Mike_J]

Hi Mike,


this is what I'm talking about.





If the procedure to switch between voltage involves two different independent switches (voltage switch, and  a stand-by switch),  someday, someone (you, or someone else) will throw the switches the wrong way, at the wrong time.  If the amp is likely to blow up in that event, it is a very bad design decision. 


I drew the plugs as a straight line, but you can use an octal tube socket.  the plugs are $2ea (you need two for this), the socket is $2-$20ea, depending on your quality tastes.


See "9 Pin Tube Base" on Doug's "tube Sockets' page in the store.

What is your opinion concerning the use of one beefy on-off-on switch with the 400 volt tap tied to one side and the 480 to the other and the middle lugs go to the rectifier?  That way the standby switch (middle position) is in the same switch.  I would only use one rectifier in the power supply string.

How can you throw the switch the wrong way?  One switch has both HT voltages on it and the other is a fairly common standby switch.  I don't see how I could get in trouble as long as the switches are adequate to handle the voltages.

I hate to admit it but I am still stuck in the weeds with my 1948 Western Electric circuitry.  The drawing is still over my head.  I appreciate your efforts in that regard but I think it will take me some time to graduate to your level as far as the wiring you refer to is concerned.

Thanks
Mike

[SILVERGUN]

SG,

I came across an on-off-on switch with the same high voltage capabilities of the other on-none-on switch.  Seems like that would be safer than two Carling 2p2t switches.  Would you agree?  I could place the HT taps on the ends and run the middle of the switch to the rectifier.  When the switch is placed in the middle it is in standby.  Go one direction it is 400 volts the other is 480.

Hopefully HBP is looking in and can comment on the safety of doing it this way.

Thanks
Mike

Mike,
That would work, but I personally would be hesitant to combine the voltage selector switch with the standby switch (even though it does seem convenient)

I just don't want to be cycling that switch that often, and the standby is the most often cycled power switch on the amp.

Just my opinion,,,but your idea is fine and would work as you intended

[sluckey]

I could place the HT taps on the ends and run the middle of the switch to the rectifier.  When the switch is placed in the middle it is in standby.  Go one direction it is 400 volts the other is 480.

That's a nice simple solution for switching the B+. And the standby switch in many Marshall amps actually switches the PT HT leads on/off.

You still need to consider what you will do with the bias voltage. Maybe use a 3PDT ON-OFF-ON switch to change B+ and bias with one toggle.

That said, I'd still just pick one voltage winding and keep the whole thing simple.

[terminalgs]

What is your opinion concerning the use of one beefy on-off-on switch with the 400 volt tap tied to one side and the 480 to the other and the middle lugs go to the rectifier?  That way the standby switch (middle position) is in the same switch.  I would only use one rectifier in the power supply string.

That would work.

How can you throw the switch the wrong way?

I mistakingly got the impression that two independent switches were in the works.  a DPDT for the voltage, and separate stand-by switch.


The plug type of voltage selector was very common on european tube gear in the 50's/60's, only on the primary side of the PT for 230,220,115,110 voltages etc..

[Mike_J]

What is your opinion concerning the use of one beefy on-off-on switch with the 400 volt tap tied to one side and the 480 to the other and the middle lugs go to the rectifier?  That way the standby switch (middle position) is in the same switch.  I would only use one rectifier in the power supply string.

That would work.

How can you throw the switch the wrong way?

I mistakingly got the impression that two independent switches were in the works.  a DPDT for the voltage, and separate stand-by switch.


The plug type of voltage selector was very common on european tube gear in the 50's/60's, only on the primary side of the PT for 230,220,115,110 voltages etc..

Terminalgs

You weren't mistaken at all.  One of the choices is using two on-none-on DPDT switches.  The first switch would have the 400 volt tap attached to one end of the switch and the 480 volt tap attached to the other end.  The common connections would go to one of the ends of the second switch which is the standby switch.  The common side of the standby switch would feed the full wave bridge rectifier. 

The switches are Carling's with 125 volt at 6A and 250 volt at 3A ratings.  Whether this is adequate for the job I do not know but it is the same rating Fender used in their SPST switches I have used for years without any problems whatsoever and they were carrying almost double the AC voltage.

I have attached a layout of what I was discussing above.

[Mike_J]

I could place the HT taps on the ends and run the middle of the switch to the rectifier.  When the switch is placed in the middle it is in standby.  Go one direction it is 400 volts the other is 480.

That's a nice simple solution for switching the B+. And the standby switch in many Marshall amps actually switches the PT HT leads on/off.

You still need to consider what you will do with the bias voltage. Maybe use a 3PDT ON-OFF-ON switch to change B+ and bias with one toggle.

That said, I'd still just pick one voltage winding and keep the whole thing simple.

Sluckey

The on-off-on switch I would use is a 4pdt so there is room for what you are talking about although I am not sure what you are talking about where the bias voltage is concerned.  Could you please explain what I would need to do in more detail? 

Another option is to set the bias on whichever voltage gives me the most conservative bias setting when the other voltage is selected.

I may end up taking the simple method after I have tried the more difficult method.  Just want to see if I can get any sounds I like out of having the both choices.

Thanks
Mikes

[Mike_J]

I am not a proponent of  400-off-480 volt switch, where the off is standby. 

Remember you have capacitors charged, and a quick switch from 480-0-400 can introduce some funny spikes.

Drgonzonm

Are you, like HBP, in favor of turning off the amp to change voltages instead of just switching the standby switch off?  Is there a way to curb the funny spikes?

Thanks
Mike

[sluckey]

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.

[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

Thanks.  I think I can make room for another pot and 47K resistor on the board.  Good thing I am asking these questions before I finish my boards.  I am going to make a layout and post it to make sure I have it right.

Thought of another question.  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?  If not, are they going to redplate?  May require a separate switch for the standby and a 4pdt on-none-on for the 400V - 480V - 400 volt bias - 480 volt bias.  But in standby we are removing the voltage from the power supply so why would there be redplating?  Just thinking out loud.

Thanks
Mike

[terminalgs]

Yes, I have a dedicated bias winding.

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


Would the bias change be proportional in such a way that it results in the correct bias (or close enough?)

[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

I am getting enthused by this.  Certainly with voltages of 400 and 480 and separate bias adjustments for each you could completely adjust each voltage to taste and get two unique sounds.  Much better than average in my opinion.

Thanks
Mike

[Mike_J]

Yes, I have a dedicated bias winding.

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


Would the bias change be proportional in such a way that it results in the correct bias (or close enough?)

Terminalgs

Thank you for your reply.  I think what is so good about Sluckey's approach is the ability to adjust the bias for each voltage.  You wouldn't then just be stuck with what was left over but you could adjust bias for each voltage to taste.  I like that idea a lot.

Thanks
Mike

[sluckey]

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.