AC Elevated Heaters . . . .

RickyLee

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I knew there was a subconscious reason why I reached for that 10,000uF filter cap when I was not getting my results with the lower capacitance values (besides not having many cap values on hand).

This Granger board is using two 6,800uF caps for its DC filtering. But then, they are using a voltage regulator as well. You can power the three relays or use only one or two relay sections with their power supply, so they advise.

https://grangeramp.com/wp-content/uploads/2020/01/3_relay_board_full_wiring_diagram.pdf

Would you say that is dangerously high filtering for that circuit? I have only a 470uF, and then some 3,300uF. I had a 1,000uF the other night. Have to find what I did with it lol.
 

thetragichero

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merlin suggests 4700-10000uf. not enough filtering can result in more noise from dc heaters than from ac heaters
 

myersbw

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Wow...that's an adventurer!!! :) Ok...I know you're trying to use a relay...have you thought about something like the Mojotone JFET switch...using mixing resistors for the channel junction and just using jFET's to short to ground the channel you don't want? Might save some current demands a relay coil consumes.
 

RickyLee

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Well, best I can get is 5.4VDC with one 12AX7 loaded up. My head is spinning as I tried so many combos of single cap values, two cap values in parallel with one direct on rectifier positive pin and 2nd cap jumped to 12AX7 pins 4/5.

I started with 220uF and worked my way up to two 6,800uF caps parallel.

The sweet spot was two 3,300uF caps parallel. When I increased from there I went from aprox 5.41VDC down to about 5.36VDC.

This has me realizing I will be joining my two xfmr secondary taps together either in series 12V or parallel 6V on that Jubilee Clone transformer swap down to 18W iron. That transformer has less heater amps than this one I am messing with. But that 18W has 6.3V 3A CT and 6.3V/5V 2A no center tap.

So on this one here we are all on about, I am only going to have 5VAC -> bridge rectifier -> 1000uF cap 5V relay coil. Done. I give up trying to get a 12AX7 on that 5V tap.
 
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RickyLee

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OK. So get ready for this long haired hippied of an idea for my next project of swapping my Jube 2555 over to the 18W transformers lol.

I want to join the two 6.3V secondary taps I mentioned above in series. 12.6 volt tap and that will then give me close to 5A total. Good enough for two 6V6 or even two KT66/6L6 and four 12AX7.

But I want to obtain a combo 12VDC for the 12AX7, but tap into the 6V section to run the two power tubes. An easier run on this and to avoid the center tapping/ground section of splitting the taps, would be just run all tubes at 12V. Run the two power tubes in series 12VAC. And run a bridge rectifier for 12VDC for the four 12AX7's.

My reasoning is: I would then take overall advantage of current capacity instead of just having one relay coil on a 3 amp tap.

Anyone want to chip in on my first idea if that is even possible splitting the taps after joining the two taps in series? lol

On this current Booger Slodowno project, could I have ran the 5VAC 3A no CT and 6.3VAC CT 5A in series for an aprox 12V? Can you run to different voltage output taps in parallel as well, and it would give you the average voltage out difference between them? I get 5.2VAC and 6.8VAC loaded up, so figuring you would net close to 12VAC.
 
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thetragichero

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5v tap with voltage doubler is likely to get you close to the +/-10% of the 6.3v heaters (y'ain't gonna get there with a bridge), although you'll want to use some pretty hefty diodes (i wouldn't use 1A 1n400x type diodes for that). you'll likely need a boatload of capacitance to accommodate the the current draw
instead of relay switching what about using vactrols in parallel with the grid resistor after the last stage of each preamp circuit? find the lowest off resistance you can to help eliminate channel bleed. over 1M on resistance should make them virtually "invisible" in parallel to 470k or 1m grid resistor

I've never made a channel changer (not really a fan of em) but i know soldano used vactrols on the slo
 

RickyLee

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Wow...that's an adventurer!!! :) Ok...I know you're trying to use a relay...have you thought about something like the Mojotone JFET switch...using mixing resistors for the channel junction and just using jFET's to short to ground the channel you don't want? Might save some current demands a relay coil consumes.

Right now I just have a double pole relay. I am using one pole to bring the two channels, CH1 on normally open, CH2 on normally closed, into common contact after there respective master volumes. I already experimented with using the other pole/section on the relay to mute each channel in strategic places. As in, either "mute" signal just before it enters relay or "mute" it further upstream as to silence previous gain stages. So far the one relay is working just fine for what I am doing. I would like to learn more, like your method. But it sounds like more parts and work, no?

Maybe yours has advantages of more contacts or mute points? How are the JFETS powered? A gate signal like a transistor?

I am sure I have a very slight loading of my first stage(s) where they separate. Think: Input -> V1A , then I have a junction where out of V1A plate is coupling cap for CH2. But also out of V1A plate is a tie direct to a tone stack of CH2 (Fender/Boogie). I could add another relay and separate here as well, but I am thinking the signal loss is very minimal.
 

RickyLee

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5v tap with voltage doubler is likely to get you close to the +/-10% of the 6.3v heaters (y'ain't gonna get there with a bridge), although you'll want to use some pretty hefty diodes (i wouldn't use 1A 1n400x type diodes for that). you'll likely need a boatload of capacitance to accommodate the the current draw
instead of relay switching what about using vactrols in parallel with the grid resistor after the last stage of each preamp circuit? find the lowest off resistance you can to help eliminate channel bleed. over 1M on resistance should make them virtually "invisible" in parallel to 470k or 1m grid resistor

I've never made a channel changer (not really a fan of em) but i know soldano used vactrols on the slo

I thought I already looked into the voltage doubler circuit? Hmmm. You have a preferred schem?

What if I went with half wave or tried another DC rectifier circuit, like just a single diode on one leg? Would induce noise into the circuit? I realize the voltage drops of the diodes in the bridge are keeping me from my close to 6VDC target.
 
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RickyLee

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Well, it sounds like the doubler circuit could work. But it states it has a disadvantage being difficult to smooth out a higher ripple voltage.
 

thetragichero

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there's no free lunch!

this is something i sketched up with hopes of running a 25v heater tube off a 12v heater, but I'd probably need a tripler and then I'd run out of heater current... should work for you but I'd use big big capacitance and also parallel with small film or ceramic (100nf). unloaded will be much higher voltage than loaded so I'd use an old 12ax7 (in the event it's way over voltage). if it's more than 10% over 6.3v a low value (but high wattage! with 600ma of heater current from two 12ax7 that's 3.6w, which you'd want a 10w resistor for!) resistor can be placed before the tubes. maybe another big cap after the resistor

rchTP8i.jpeg


i still think you should just use a power transformer with sufficient heater current capacity!
 

RickyLee

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5v tap with voltage doubler is likely to get you close to the +/-10% of the 6.3v heaters (y'ain't gonna get there with a bridge), although you'll want to use some pretty hefty diodes (i wouldn't use 1A 1n400x type diodes for that). you'll likely need a boatload of capacitance to accommodate the the current draw
instead of relay switching what about using vactrols in parallel with the grid resistor after the last stage of each preamp circuit? find the lowest off resistance you can to help eliminate channel bleed. over 1M on resistance should make them virtually "invisible" in parallel to 470k or 1m grid resistor

I've never made a channel changer (not really a fan of em) but i know soldano used vactrols on the slo

I tried the voltage doubler circuit I posted above using 1000uF 35V caps. Unloaded, yeah not a problem. I got 13.67VDC. Figured I should at least try for 12V lol. I wired in one 12AX7 for 12V heater and it dropped to 10.07VDC. So at least, it looks like 6VDC heaters can be a possibility. I have one 6V regulator in my stash.

Maybe I can try for V1 and the reverb tube V4 on this and all will be good.


I am stumped as Mickey worked with me last year and we got 5.8VDC to 6VDC range on two 12AX7's from a 5VAC tap full wave BR. Dam that River, at least that is how I remember it lol.

I will have to get that amp back out of the pile and check into that. Unless we went with the doubler circuit and I just do not recall it.


When I got this power xfmr, I figured I was plenty good with a 5A 6.3V tap. Plus I already had planned to mess with the 5V tap for the relay and at least one 12AX7 heater. I had figured I was going to get one 12AXT close to 6VDC from that 5VAC 3A tap and a relay coil . . no problem, so I thought.

The 18W iron set might have been a bad idea. But . . . . it was initially bought for something else. As I was waiting for it to arrive, I thought of that beast of a 100W Jube Clone and said WTF, it is getting the 18W make over lol.

I need to consider looking for different iron next time. I might need one more 20W power xfmr but this time, hopefully get one with around 5A heater winding.


Any way or oh well . . . .
 

RickyLee

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Added in another 1500uF to C1 (2500uF total) and 3300uF to C2 (4300uF total) and I am holding 6.1VDC on one 12AX7 heater and holding the relay coil as well. Using L7806 regulator. Will try this with a 3300uF on each cap location when I try installing.

Now just have to find a place in there to put all these parts . . . .
 

thetragichero

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if it's similar to the doubler i posted i would make sure you use equal capacitances (really, the same caps so that things like esr are the same) for the "top" and "bottom". 100nf or so film/ceramic cap in parallel to each branch is a good idea as well
 

mickeydg5

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@RickyLee
Sorry my head is not screwed on all the way. I forget how we got the voltage higher before exactly but it may have been due to a "+"tolerance transformer, you know putting out more than 5VAC at the secondary.

I just did not adjust for diode drop and gave that 7V number before. In actuality a 5VAC will yield about 5.2VDC with a basic capacitive filtered rectifier at nominal tolerances.

So a voltage doubler as @thetragichero indicated is the way to go if higher DC voltage is needed from the 5VAC secondary.

Why do you not just use the 5VAC for DC supply for relays and switching while adding another DC supply of the 6.3VAC for tube heater(s)?
 


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