Help with tonepad Mctube

[whomeno]

ok I have been looking for days to find transformers for this build. No I do not want to use the P-T442.
I want to use what is called for 120v/12v 0.5a. back to back.
Dose any one know where to get them. no wall warts
Thanks

[danielzink]

https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166F12B?qs=sGAEpiMZZMvwUzoUXIIvyVNS5hMbZJqQDNkE7zPnCrs%3d


https://www.mouser.com/Power/Transformers/Power-Transformers/_/N-8u9n5?P=1yvimmoZ1yzuxsiZ1ydl0yx

[whomeno]

will the .03a be ok .it calls for .5 a?

[davent]

I've used flat pak Hammonds with pcb's in a couple tube projects. 164F12 is the 6VA, is on the page in the second link Daniel put up.

dave

[GibsonGM]

will the .03a be ok .it calls for .5 a?

Who - You are only drawing ~.15A or 150mA for the heater, and the circuit itself will only consume a few mA's.   So I believe you are MORE than fine with a transformer rated for 300mA  (it is .3A, rather than ".03A"...).   

[Rob Strand]

Who - You are only drawing ~.15A or 150mA for the heater, and the circuit itself will only consume a few mA's.   So I believe you are MORE than fine with a transformer rated for 300mA  (it is .3A, rather than ".03A"...).

When you put in the rectifier and filter the stresses on the transformer go up (approx 1.6x to 2x).
The other consideration is when you connect two back to back transformers, a magnetization current flows through the input side of the reversed transformer; even with not load on the output side.  It's a bit of an unknown and can vary quite a bit.

[GibsonGM]

Who - You are only drawing ~.15A or 150mA for the heater, and the circuit itself will only consume a few mA's.   So I believe you are MORE than fine with a transformer rated for 300mA  (it is .3A, rather than ".03A"...).

When you put in the rectifier and filter the stresses on the transformer go up (approx 1.6x to 2x).
The other consideration is when you connect two back to back transformers, a magnetization current flows through the input side of the reversed transformer; even with not load on the output side.  It's a bit of an unknown and can vary quite a bit.

It IS a bit of an unknown, but I am pretty sure it wasn't designed around a strict specification.  If in doubt, simply buy one rated at 500mA...or even 1A.  I made one once, my trafos were 1A as they are very common (RS in those days). 

[Rob Strand]

It IS a bit of an unknown,

Yes it varies a lot.

If you look at a small transformer, say < 15VA, which has a magnetization current (Imag) spec for the primary you will see  Vprimary_nominal * Imag_max is often more than the VA of the transformer!   For a 50VA transformer it might be about half the VA of the transformer.    It varies enormously.

To first approximation connecting two transformers back to back could actually use-up all the VA rating of the first transformer.  However when you look at this in detail the load on the first transformer causes a voltage drop at the output.   The current through the input side of the second transformer causes a second voltage drop.  The two effects both help to reduce Imag but just how much is pot luck.   You really need to measure it. 

Connecting two different transformers back to back isn't a good option.  If the secondary voltage of the first transformer is high then the magnetization current goes up in second transformer.  If you connect two equivalent transformers back to back at least you know the voltage drops will help reduce the magnetization current.

[whomeno]

ok guys this one seems to meet the specs tonepad has.
I would get 2.
what do you guys think?
https://www.alliedelec.com/stancor-p-8392/70213246/?mkwid=sUX5A4I5R&pcrid=30980760979&pkw=&pmt=&gclid=CjwKCAjwgabeBRBuEiwACD4R5qAeLxkmDF9VDLFt4wDE8rd-DsSdWcwv7mt58AgjMspKXCxRdFCeoxoCkUIQAvD_BwE

[GibsonGM]

It IS a bit of an unknown,

The two effects both help to reduce Imag but just how much is pot luck.   

So, what guarantees do we have that *some* 12V, .5A transformer is going to do the job properly, if  *some* 300mA might fail?  Is the .5A just enough overkill to assume you're safe?  Transformers have been, for me (and so many others, ha ha), a dark art.   It is unfathomable to me that setting them up as the McTube does, certainly drawing say < 170mA, can result in the negative effects - to cause failure - that you've laid out, Rob.  I know those phenomena do exist and that what you say is fact, but I find it difficult to raise a red flag.  That may just be ignorance, I'll be honest.   I don't really like doing the back to back thing, but it IS convenient, esp. to break into "real" tube circuits. 

Do you suppose Fred actually measured what was going on when he designed this oh so basic circuit?  Or just tried it, and when they didn't get hot...Bingo! ?  I've done a lot of pot luck myself...and I was telling Who there to go the same route.

[Rob Strand]

Unfortunately I've got no answer as far as a blind solution goes.  I don't think one exists for the man in the street.

Someone has to set-up two transformers (either two the same or even two different) then do all the measurements and convince themselves that both transformers are operating withing their limits.
Also, the test really needs to be done at the highest mains voltage as the magnetization current can actually increase rapidly with voltage.   Even then the samples you have may not be representative of worst-case!

If you go through this measurement process then it is OK to use two different transformers.  And it may be possible to have a larger one for the first transformer and a smaller one the for second (the backward one.)  A larger first one would be able to supply the magnetization current and some additional load current.

Other tricks would be to add a power resistor between the two transformers or to use a higher input voltage for the second one (eg. mains-> 12V feeding a 15V->mains).   This helps reduce the magnetization current but it also drops the HV voltage.  I've seen some people doing the opposite like mains-> 15V feeding a 12V->mains. That's just asking for trouble and it makes the whole problem worse.   That's not to say someone hasn't found a golden combination of transformers that lets that method work!

So, what guarantees do we have that *some* 12V, .5A transformer is going to do the job properly, if  *some* 300mA might fail?  Is the .5A just enough overkill to assume you're safe? 

Even the rating cannot save you.  It's more to do with the rating vs the current required for magnetization.
In very broad sweeping terms larger transformers tend to have a lower magnetization current to rating current ratio so they will give you more probability of success of back-to backing two of the same transformer but is by no means a guaranteed recipe for success.

Transformers have been, for me (and so many others, ha ha), a dark art.   

The fine details are very complicated.    When you buy a transformer a lot of evils are hidden from the user.  When you start running them in reverse it opens-up a whole nasty can of worms.   The supplied specs simply do not cover enough details to let that work.

It is unfathomable to me that setting them up as the McTube does, certainly drawing say < 170mA, can result in the negative effects - to cause failure - that you've laid out, Rob.  I know those phenomena do exist and that what you say is fact, but I find it difficult to raise a red flag.  That may just be ignorance, I'll be honest.   I don't really like doing the back to back thing, but it IS convenient, esp. to break into "real" tube circuits.

Do you suppose Fred actually measured what was going on when he designed this oh so basic circuit?  Or just tried it, and when they didn't get hot...Bingo! ?  I've done a lot of pot luck myself...and I was telling Who there to go the same route.

I don't know!    He does make a point of spelling out those transformers.  Maybe he found a working combination.    However for people that want to use something different they are oblivious to the fact they could be handling unworkable combinations of transformers.

If you were building products you would have to work out the magic combination in development then stick the same parts in production and probably do some testing in production.   That might mean designing your own transformer, one that works and is not constrained by using off the shelf limitations.

The other way is to give up on the iron transformer and use a solid-state HV booster.

[thermionix]

https://www.amplifiedparts.com/products/transformer-hammond-plate-filament-or-bias-120v87ma

[GibsonGM]

Thanks Rob, very interesting reading! 

Thermionix:  exactly. 

[Rob Strand]

Thanks Rob, very interesting reading! 

Here's an example.
For the purpose of analysis the transformer is set-up as a 1:1 transformer.
What this lets you see is the relative currents of primary and secondary better.
The actual secondary voltage doesn't affect the results.

Simple back to back transformer connection
Assumes transformer inductance is linear

240V 50Hz input
Transformer rating 1.8VA
Imag ~ 7.5mA
Rp  ~ 2k ohm
Assume Rs'  = 1.5 Rp  ; more copper in primary
(In actual tx, the o/p V is higher than rated V when unloaded  but drops below rated V when loaded.)

1) Normal operation: Unloaded
  Vs_unloaded = 239V
  Ip = 7.5mA
  Pp = 0.11W

2) Normal operation:  Full load 1.8VA at output
Vsloaded = 196V   ; referred to primary, approx -18% cf unloaded.
Ip = 11.6mA
Is = 9.4mA
Pp = 0.269W   ; dissipation in primary
Ps = 0.265W   ; dissipation in secondary
Pt = 0.53W      ; total power dissipated by transformer

3) Unloaded Back to Back  (Tx1 -> Tx2)
Vout_p2_no load = 231V
Ip1 = 14.5mA
Is1 = 7.2mA
Is2 = Is1
Ip2 = 0
Pp1 = 0.42W
Ps1 = 0.16W
Ps2 = 0.16W
Pt1 = 0.58W
Pt2 = 0.16W

What is very interesting for the back to back case is
the secondary winding current of the first transformer Is1 is
below the rated current secondary yet the dissipation in the
primary  is higher than the normal fully loaded case.
Total back to back power dissipation in Tx1 exceeds
that of single transformer under full load despite
having no load on the second transformer.

Anyway have to do stuff for the next few days.

[Rob Strand]

Here's my solution thread to reducing the power dissipation in TX1,
https://www.diystompboxes.com/smfforum/index.php?topic=121216.0