Knocking back B+ for 12AX7 tube project.

[digi2t]

Been working on a 12AX7 project of late, but wanting to stay away from mains power, I decided to try a small toroidal transfo, powered by 12vac instead. This is based on a project outlined at EL34;

https://el34world.com/Transformers/toroidal.htm

These small Amveco (now Talema) transfos are getting tougher to find. I did source one from Europe, but I started searching for alternates. I came across this one (as well as another one, but identical);

https://www.block.eu/en_EN/productversion/avb-1529/

It was in stock at Newark, cheap, and came in a couple of days. Today I decided to hook it up to test it's output. I have the primary and secondary hooked up as per the EL34 document, plus a diode bridge so I can test for DC voltage out. It's giving me a respectable 208vdc.

Granted, this is completely unloaded, so I expect that this might(?) drop somewhat once the tube is in the circuit? Otherwise, I'm shooting for a B+ of 165vdc. In the grand scheme of things, probably might not make any real difference in performance, but what would be a simple way to drop B+ in this case?

[iainpunk]

use a Zener diode to drop the voltage? maybe a 30v Zener would be able. how much current are you planning to run it? most can only dissipate 1W of power.

cheers

[anotherjim]

If you only have a bridge but no cap you're probably reading the average/RMS minus diode drops. A cap will charge to peak voltage which until loaded would be nearer x1.4 the RMS.

Would putting anti-phase diodes in the 12vAC supply drop your 9v traffy backward-driven output closer to the original 10v one?

[imJonWain]

You could just drop the B+ with a series resistor as done in the preamp of most every guitar amp?  I would assume with a RCRC filter on the output from the recitifer to your tube it'll drop the voltage quite a bit on that tiny transformer. 

[merlinb]

what would be a simple way to drop B+ in this case?

Why drop the voltage much at all? The 12AX7 is going to work exactly the same with the voltage you have already

[digi2t]

use a Zener diode to drop the voltage? maybe a 30v Zener would be able. how much current are you planning to run it? most can only dissipate 1W of power.

cheers

I think a 12ax7 is along the lines of 1w per side, so I suppose a 5w zener should do the trick?

If you only have a bridge but no cap you're probably reading the average/RMS minus diode drops. A cap will charge to peak voltage which until loaded would be nearer x1.4 the RMS.

Would putting anti-phase diodes in the 12vAC supply drop your 9v traffy backward-driven output closer to the original 10v one?

I don't want to mess with the supply, since it will also be used to drive a 12vdc and 5vdc sections as well.

You could just drop the B+ with a series resistor as done in the preamp of most every guitar amp?  I would assume with a RCRC filter on the output from the recitifer to your tube it'll drop the voltage quite a bit on that tiny transformer. 

I thought about this as well, but I guess I'm trying to account for possibilities as I get deeper into this project. That way if it goes one way or the other, I can adjust accordingly.

Why drop the voltage much at all? The 12AX7 is going to work exactly the same with the voltage you have already

Extremely fair question. The tube won't mind. I suppose I'm just being bent by online reports that lower B+ yields for a "gritter" tone, while higher makes for cleaner. Maybe just something I wish to explore, but yeah, in the grand scheme of things, probably not worth the effort. Then again.... fartin' around with this sure beats the crap outta mowing the lawn.

[vigilante397]

Why drop the voltage much at all? The 12AX7 is going to work exactly the same with the voltage you have already

I'm with Merlin on this one. I run most of my 12AX7-based pedals with a B+ of about 240V, I have some that I run at 350V. Feedback from my customers is that they prefer the higher. If I want more dirt I'll add more tubes rather than drop the voltage.

[PRR]

> what would be a simple way to drop B+ in this case?

It is a preamp. You WILL need ripple filtering. Possibly C-R-C-R-C. You should have a fistful of 1k and a fistful of 10k resistors to play with.

Note that "all" real g-amps feed preamps with 200V-380V, and some of the 380V ones are well-regarded; OTOH 48V tube preamps are OK as fuzz-gimmicks but not all-around amps.

[danfrank]

Digi2t said,
"The tube won't mind. I suppose I'm just being bent by online reports that lower B+ yields for a "gritter" tone, while higher makes for cleaner."

The cathode bias on the 12AX7 is what makes for "grittier" or cleaner sound out of the tube. Adjusting the value of the 12AX7 cathode resistor will determine this. For example, the "cold clipper" stage in a JCM800 uses a very high cathode resistor value in order to introduce asymetrical clipping (distortion) in that stage. Using a very low B+ voltage is just another way of making the bias of the tube less than optimal (no distortion).
I'd use anything in the neighborhood of 150-250 volts for your B+ power supply and just adjust the cathode resistor to get the type sound you want.

[Rob Strand]

Been working on a 12AX7 project of late, but wanting to stay away from mains power, I decided to try a small toroidal transfo, powered by 12vac instead. This is based on a project outlined at EL34;

https://el34world.com/Transformers/toroidal.htm

These small Amveco (now Talema) transfos are getting tougher to find. I did source one from Europe, but I started searching for alternates. I came across this one (as well as another one, but identical);

https://www.block.eu/en_EN/productversion/avb-1529/

It was in stock at Newark, cheap, and came in a couple of days. Today I decided to hook it up to test it's output. I have the primary and secondary hooked up as per the EL34 document, plus a diode bridge so I can test for DC voltage out. It's giving me a respectable 208vdc.

Granted, this is completely unloaded, so I expect that this might(?) drop somewhat once the tube is in the circuit? Otherwise, I'm shooting for a B+ of 165vdc. In the grand scheme of things, probably might not make any real difference in performance, but what would be a simple way to drop B+ in this case?

The toroid transformers work well in reverse because they have a high inductance/low magnetizing current.

When you start using EI cores in reverse you can cook both the input transformer and the reversed transformer.  You can measure the no-load AC current to see how bad things are - it's not uncommon for the current to exceed the input and reversed transformer rating even with no load.   It turns out that small transformers are worse-off.  There's this old thread that goes through the problem,
https://www.diystompboxes.com/smfforum/index.php?topic=121216.0

The datasheet you linked says an EI30 transformer core - so it's a conventional EI and small transformer.

Without the resonating trick in the post:  If you don't need the full HV secondary voltage a good trick to help reduce the cooking problem is to feed the 12VAC input supply into the primary of a 15VAC transformer, of course your the HV will be lower by about 12/15 factor; in practice the drop is less because connecting 12VAC into a 12VA primary has a lot of voltage drops due to the high primary current.    I believe the most efficient connection (least stress on the input supply) is to drive the 12VAC into a 24VAC transformer winding but then you half your HV.    So what you can do then is use a 24VAC to 240VAC transformer.

Using different (and higher) secondary (LV) voltages on the reversed transformer is a way you can tune the +B voltage.

[puretube]

Been working on a 12AX7 project of late, but wanting to stay away from mains power, I decided to try a small toroidal transfo, powered by 12vac instead. This is based on a project outlined at EL34;

https://el34world.com/Transformers/toroidal.htm

These small Amveco (now Talema) transfos are getting tougher to find. I did source one from Europe, but I started searching for alternates. I came across this one (as well as another one, but identical);

https://www.block.eu/en_EN/productversion/avb-1529/

It was in stock at Newark, cheap, and came in a couple of days. Today I decided to hook it up to test it's output. I have the primary and secondary hooked up as per the EL34 document, plus a diode bridge so I can test for DC voltage out. It's giving me a respectable 208vdc.

Granted, this is completely unloaded, so I expect that this might(?) drop somewhat once the tube is in the circuit? Otherwise, I'm shooting for a B+ of 165vdc. In the grand scheme of things, probably might not make any real difference in performance, but what would be a simple way to drop B+ in this case?

Hi Digi2t: got a (small) supply of those (Talema/Made in India - 1999) trannies.
The E-H Toroidals were custom-made far-east clones of my originals (Original Talema 1996 that backindadayz were "Made in Germany"). They are/were used inside the EHX Tube-Pedal series. Even Aron used to sell them here!. PM me if interested.

[digi2t]

The toroid transformers work well in reverse because they have a high inductance/low magnetizing current.

When you start using EI cores in reverse you can cook both the input transformer and the reversed transformer.  You can measure the no-load AC current to see how bad things are - it's not uncommon for the current to exceed but the input and reversed transformer rating even with no load.   It turns out that small transformer or worse-off.  There's this old thread that goes through the problem,
https://www.diystompboxes.com/smfforum/index.php?topic=121216.0

The datasheet you linked says an EI30 transformer core - so it's a conventional EI and small transformer.

Without the resonating trick in the post:  If you don't need the full HV secondary voltage a good trick to help reduce the cooking problem is to feed the 12VAC input supply into the primary of a 15VAC transformer, of course your the HV will be lower by about 12/15 factor; in practice the drop is less because connecting 12VAC into a 12VA primary has a lot of voltage drops due to the high primary current.    I believe the most efficient connection (least stress on the input supply) is to drive the 12VAC into a 24VAC transformer winding but then you half your HV.    So what you can do then is use a 24VAC to 240VAC transformer.

Using different (and higher) secondary (LV) voltages on the reversed transformer is a way you can tune the +B voltage.

OK, wow, I had no idea. And here I thought I was off to the races here. What was really adding to the excitement was the much smaller footprint as well. If I have the math right, say I go with a 12vac xfo, that should drop the HV to about 160vac? Pretty much in the wheelhouse of where I wanted to be at the onset of this adventure. A 15vac xfo would drop it to about 125?

So going with what you've pointed out, I see these offerings over at Newark...

https://www.newark.com/block/vb1-5-2-12/isolation-transformer-1-5-va-1/dp/26M4778

https://www.newark.com/block/vb2-3-2-15/isolation-transformer-2-3-va-1/dp/26M4785

https://www.newark.com/myrra/44093/isolation-transformer-1-5-va-1/dp/25M3903

Also, higher priced, but this line as well;
https://www.farnell.com/datasheets/2603262.pdf

The crux here is that this may eventually result in a board for the DEFX shop, so I'm looking not only for economical, but readily available as well. The center pin on the secondary is also a bonus, since it would help people from loading it onto the board backwards.

Recommendations?

Hi Digi2t: got a (small) supply of those (Talema/Made in India - 1999) trannies.
The E-H Toroidals were custom-made far-east clones of my originals (Original Talema 1996 that backindadayz were "Made in Germany"). They are/were used inside the EHX Tube-Pedal series. Even Aron used to sell them here!. PM me if interested.

I might take you up on it, if this avenue turns out to be a dud. I see that Banzai has them in stock as well, but like I mentioned above, cost and global availability are hard considerations here, hence the allure of these Block or VTX mini xfos.

[merlinb]

If you're tyring to keep the footprint that small does it make sense to use back-to back transformers? Would a 555 boost converter work just as well, with more easy to source parts?

[puretube]

If you're tyring to keep the footprint that small does it make sense to use back-to back transformers? Would a 555 boost converter work just as well, with more easy to source parts?

Yes, my Wizard - "Nixie"-supply will work in most DIY-cases.
But if he goes commercial, he is facing the usual regulations to be obeyed.

The other "small footprint"-thing with magnetical 50/60Hz transformers is the hum-induction/radiation. (Much less/negligible with toroidals).
Even from the 12V AC-wires (or PCB-traces) to the tranny!!!
(That`s the reason why the EHX tube-pedals got their supply-socket on that exotic side of the box ...).
Twist the wires to the tranny tightly!

[merlinb]

Heck, since the current is so small you could use an ordinary octupler rectifier on the 12Vac and get about 130Vdc. Just diodes and caps.

[digi2t]

If you're tyring to keep the footprint that small does it make sense to use back-to back transformers? Would a 555 boost converter work just as well, with more easy to source parts?

What worries me about going this route is the potential for noise. Insofar as "going commercial" is concerned, I don't plan on building complete anythings, just DIY PCB's is the plan, but again, availability, cost, and simplicity are the primary factors, with footprint watching the back door.

@Rob... from what I read from the link you posted, am I to understand that adding a cap across the output of xfo 2 would smooth out the feathers??

EDIT: BTW, I see that they're  bit bigger, but how about these "semi-toroidal" transformers? Better than EI in this case, or will they present the same issue?
https://belfuse.com/resources/datasheets/signaltransformer/ds-st-lp-series.pdf

[davent]

A wall wart feeding a backwards transformer in the enclosure would shrink the box footprint and eliminate the 120Vac from the box.

Article, In praise of wall warts

https://audioxpress.com/assets/upload/files/stamler2874.pdf

[imJonWain]

Have you considered sourcing an order of custom transformers and including them with the boards?  I assume you're ordering boards via panels and in fairly decent quantity. 

It probably wouldn't be that expensive to have 50x or 100x transformers custom made.  You could probably even sell them as their own thing since they aren't an easy to find COTS part but are common to HV tube pedal builds.

[merlinb]

What worries me about going this route is the potential for noise.

People have successfully built phono amplifiers with that boost converter, so it shouldn't be a problem. Transformers in the same box are usually worse for noise (hum).
http://www.hagtech.com/pdf/cornet3.pdf
https://hagerman-audio-labs.myshopify.com/collections/phono-preamps/products/cornet3

[Rob Strand]

I guess there's a few details to fill in.  I guess the main points are:
- predicting the output voltage
- tweaking the output voltage
- avoiding the transformer cooking

OK, wow, I had no idea. And here I thought I was off to the races here. What was really adding to the excitement was the much smaller footprint as well. If I have the math right, say I go with a 12vac xfo, that should drop the HV to about 160vac? Pretty much in the wheelhouse of where I wanted to be at the onset of this adventure. A 15vac xfo would drop it to about 125?

That's the basic idea.
                                 Case 1                             Case 2
Transformer:                115VAC to 12VAC               115VAC to 15VAC
Turns ratio:                  115/12                             115/15
Input to transformer:     12VAC                              12VAC
Transformer Output:      12V *(115/12) = 115VAC      12V*(115/15) = 92
DC (rectifier and filter)   Sqrt(2)*115 = 163V             sqrt(2)*92 = 130V
The DC output is expected to drop under load.

However, in practice you will find the output voltage ends-up somewhat lower than that!

Think of a transformer powered from mains.    If you measure the secondary voltage with no load you will get a voltage higher than the rated voltage.   When you put a load on the transformer equal to the transformer's rated current the secondary voltage drops to the voltage on the label.   In the above case the rated voltage would be 12VAC (and case 2 15VAC).

So just how high is the unloaded voltage above the rated voltage on the label?   The spec which captures this effect is called regulation.   Regulation specs are not consistent across transformers or brands.  For large transformers say 30VA to 50VA the unloaded voltage is about 10% higher.  However for lower VA ratings the unloaded voltage can easily be 25% higher than the label rating.   For very small transformer it can be even higher in fact for transformers below about 2.5VA it gets silly.  In the old days transformer manufacturers often put a cap of how high the output got, that means using a larger core.  However these days people want small and cheap and you will see transformers which have regulations worse than 25%.

The regulation affects the predicted output voltage.

If a 115AV to 12VAC produces 25% higher output voltage the no load output is 12*1.25 = 15VAC (roughly).  That means the turns ratio is really 115/15 not 115/12.  When we run this 12V *rated* transformer in reverse and feed 12V into the secondary we get 12V *(115/15) = 92VAC rms not 115VAC.

If we had a 115V to 15VAC transformer and feed it with 12V AC then we need to account for the regulation and the fact the we are feeding a "15V" transformer with 12V.      No load voltage 15V*1.25 = 18.75V.     Expected output with 12V in 12V * (115/18.75) =73.6V rms.

At this point don't think of this is good or bad.  All it's saying is because of the way transformer voltages are specified a reverse transformer will produce less than the mains voltage out when you feed it with the 'rated' secondary voltage.

The transformer cooking issue is completely separate from this.   However, if you feed 12VAC into a transformer with a 15VAC rating transformer it will be less likely to cook than feeding 12VAC into a 12VAC rated transformer.    If you feed 15VAC into a 12VAC rated transformer then you will increase the chances of it cooking.

The biggest point about the transformer cooking or not, in any of the scenarios, is it's difficult if not impossible to predict from the manufacturer's info.    The only safe way is to build a prototype and measure the transformer currents to make sure all is OK.   If you discover this is a problem, one solution is to use a 15VAC transformer to reduce the secondary current but that comes at the expense of a lower output voltage.    If that's not acceptable then you have to find a another transformer - which one?  no idea it's pot luck.

The advantage of using toroids is the chances of success are *much* higher.     I will add that the voltage regulation points above still apply to toroids.   You might find the voltage regulation is a little better.

Don't forget, whatever extra no-load current the transformer needs the input supply must have a big enough rating to supply it!

@Rob... from what I read from the link you posted, am I to understand that adding a cap across the output of xfo 2 would smooth out the feathers??

Yes.  If you want to try to recover the situation with a given transformer then that trick might help.  Especially, if you don't want to give-up output voltage by trying 12V into a 15VAC transformer.

So going with what you've pointed out, I see these offerings over at Newark...

https://www.newark.com/block/vb1-5-2-12/isolation-transformer-1-5-va-1/dp/26M4778

https://www.newark.com/block/vb2-3-2-15/isolation-transformer-2-3-va-1/dp/26M4785

https://www.newark.com/myrra/44093/isolation-transformer-1-5-va-1/dp/25M3903

Also, higher priced, but this line as well;
https://www.farnell.com/datasheets/2603262.pdf


The crux here is that this may eventually result in a board for the DEFX shop, so I'm looking not only for economical, but readily available as well. The center pin on the secondary is also a bonus, since it would help people from loading it onto the board backwards.

Recommendations?

I understand where you are coming from here.

It's not possible to work out if these transformer will cook in reverse.   I suspect if you use a higher rating transformer it will:
- have better regulation
   If you look a the datasheet some of those transformers have very high
   "no load voltage (ca * factor)" spec
   I'm reading that as regulation.  So the no-load output voltage is 1.35 times higher then the loaded voltage.
   That will make the output voltage lower when used in reverse than my 1.25 factor examples.
- be more likely to succeed in reverse.

If I can summarize the problem.   It's highly like you can get a set-up to work.   I guess you could start off with an ideal set-up with a small cheap transformer.  If that works, all is good.  However, if it doesn't then the back-up plan would be to use a different transformer and/or lower the output voltage.   If you were developing products to sell this process would be part of development.  However, building one-offs it can work out expensive trying a lot of transformers.   That's where using the resonating cap trick can pull things back into line without stuffing around too much.

FWIW,  I'm not against using DC/DC converters.  I guess the issue here is you need to choose your inductor carefully and do a good layout.   It's possible you could have noise issues that you need to debug.   That could take a few iterations to get right and possibly try different inductors or add more filtering.

So I guess both avenues have the potential to go right first time or to cause headaches that need debugging and fine tuning.