Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: valvetones on December 31, 2021, 08:53:58 pm
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Hello Friends, I’m new here, glad to be on the site.
So I got this awesome 300mA power transformer but it’s got a center tapped 12.6 volt winding, and I need 6.3 volts.
What can be done?
Can I use 1 leg and the CT as a pair to get 6.3 volts?
If so, I’ve got a few questions.
1. Will the current handling capabilities of that winding be reduced if I use the CT and one Leg?
2. Can I simply do the artificial center tap with two 100 ohm resistors, one on one leg and the other on the CT that I’d be using as a leg?
Thanks in advance.
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i would run the little bottles off 12.6v, ground the center tap, run the power tubes one leg to each side with the other heater pin to ground. i've usually just run 6v heater power tubes with their heaters in series when i've used power transformers with a 12.6v winding and no adequate 6.3v winding (gotten a few from old organs where they ran 439063849085 12au7s)
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center tap 12 volts winding is that : Picture.
Update
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Use the 2 outside wires solder together to have twice the current if you need, for one side for tube heaters.
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That would be a short across the secondary :huh:
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Use the 2 outside wires solder together to have twice the current if you need, for one side for tube heaters.
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That would be a short across the secondary :huh:
Update
You may be right
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Use the 2 outside wires solder together to have twice the current if you need, for one side for tube heaters.
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That would be a short across the secondary :huh:
No, each half of the secondary will be in parallel.
Reading ohm will show
Just try it and you'll see!
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I don't have PT like this one on hand.
I must admit Sluckey you are always right :worthy1:
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If the two halves of the winding are separate, and IDENTICAL, you may connect in parallel for twice the current.
When the winder permanently connected the windings as Center Tap, connecting the ends is a short.
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Alright I’m confused now.
Safe or unsafe to connect the outside wires?
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UNSAFE!
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Alright I’m confused now.
Safe or unsafe to connect the outside wires?
You are confused because you don't know Sluckey knowledge. Trust him, always.
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As PRR told
if you have two separated windings joined externally to the transformer (you see two different wires joined forming the CT)
you can manage the transformer disconnecting the two wires (that forms the CT) and put the two separated windings (pay attention to the phase) in parallel to double the current handling (two 1A windings joined in parallel will give you 2A of available current)
But if your transformer has the two windings for the CT joined internally to the transformer and so inacessibles
you must use only the CT and one of the two extremities and you obtain half the voltage at the same current (es. if you have a 12.6V with CT rated for 1A using the CT and only one extremity will end in 6.3V @ 1A current, the current of the other half winding is not usable)
(https://i.imgur.com/eCWVtKt.jpg)
This may not be what you wanted / hoped for as 6.3v filaments require more current than 12.6v filaments
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Can I simply do the artificial center tap with two 100 ohm resistors, one on one leg and the other on the CT that I’d be using as a leg?
I think you can do that
(https://i.imgur.com/ZTZo1iN.jpg)
(I mean a 6.3v tube for each side)
The current available on each half winding will be the same of the full winding connection (12.6v) but HALF the voltage (6.3V)
What establish the available current, at end, is the wire thikness, not the number of turns of the wire
I think (if I remember well) that our friend TIMBO can say something more about the use of a 12.6v CT transformer to feed 6.3v heaters
Franco
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Thank you all for your time educating me on the options.
I’ve actually been scratching my head on a different idea.
What about getting one of those little deals that regulate DC voltage.
If I use some diodes to rectify the 12.6 AC to 12.6 DC, filtering with let’s say,, 12000 mfd,, then take the 12.6 DC to a board that I can adjust the voltage down to 6.3, then I’ll be set yes?
Any objections?
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Feasible but ....
12.6v AC rectified will result in around 17v DC
17v dropped to 6.3v Is a lot of energy dissipated in heat
this Is a solution I don't like much
Franco
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12.6v AC rectified will result in around 17v DC
Yes, using a FWB rectifier. But he has a 6.3-0-6.3 PT. So, use a conventional 2 diode full wave rectifier to produce 8.9vdc which is more typical to use with a three terminal 6V regulator chip. The chips are typically available in 1 amp or 3 amp packages. If you need more current, you will need additional circuitry. This could be a good solution for a few 12AX7 or similar type tubes, but may not easily power a full amplifier. You stated that you have an "awesome 300mA power transformer", but it's unclear if that is the 12.6 rating or a HT winding rating.
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+ 1 for the 300mA HT winding (🤞)
I'm waiting to see if this is or is not confirmed
Franco
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So long as you have an even number (at least 2) of each type of tube that uses a 6.3V heater, you can wire those in series pairs. You just have to make sure that whatever 2 tubes you want to wire in series has the same current draw in the heater supply. To simplify that, just stick with the same tube types for your pairs.
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Oh yes I’m sorry, the 300mA is the HT winding.
It’s a 300-0-300 @ 300Ma
5vac @ 4 amps
And 12.6 CT @ 4.5 amps.
Plenty of current capabilities on all windings.
And!! If I’m not mistaken , if I buck it down to 6.3 volts, I then have Twice the current rating on that winding right? 9AMP instead of 4.5 right?
I’m thinking I could possibly push 4 el34s and a mess of preamp tubes if I could just figure out the best solution for the high 12.6 secondary.
Sorry for the delayed response fellas , thanks for your reply’s and thoughts.
VT
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Oh yes I’m sorry, the 300mA is the HT winding.
It’s a 300-0-300 @ 300Ma
5vac @ 4 amps
And 12.6 CT @ 4.5 amps.
Plenty of current capabilities on all windings.
And!! If I’m not mistaken , if I buck it down to 6.3 volts, I then have Twice the current rating on that winding right? 9AMP instead of 4.5 right?
Nope. In fact, in any scenario other than the transformer being wound with 2 separate windings to allow you to connect them in parallel, your going to have losses in any attempt to get 6.3V, which will have to be accounted for to make sure you dont exceed the rating.
Why are you determined to use 6.3V? 12.6V has some benefits in lower noise, and it’s easier to wire.
EL34 heaters draw 1.5A at 6.3V. If you wire each pair of ‘34 heaters in series, two EL34s will draw 1.5A at 12.6V.
Edit... fixed quote
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Ummmm, I dunno. Good question! Lol
Maybe I’ll give it a try. This particular transformer is a ways away from being used.
But just curious, any good power pentodes that are cheap that are designed to run on 12.6v ?
I know there’s a 12V6 that is in all other ways identical to a 6v6 except the heaters. Any equivalent for EL34 or 6L6?
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6L6 equivalent with 12.6v heathers and not very expensive ?
Easy the 1625 tube (0.45 A heater current), it is a 12.6v heater 807 and 807 is a Top Cap 6L6
Very good tube, only needs his socket, but it is available on the web
https://www.tubesandmore.com/products/vacuum-tube-1625-beam-power-amplifier (https://www.tubesandmore.com/products/vacuum-tube-1625-beam-power-amplifier)
https://www.tubesandmore.com/products/socket-7-pin-ceramic-1625-bottom-mount (https://www.tubesandmore.com/products/socket-7-pin-ceramic-1625-bottom-mount)
http://www.r-type.org/exhib/aaa0118.htm (http://www.r-type.org/exhib/aaa0118.htm)
http://www.r-type.org/pdfs/1625.pdf (http://www.r-type.org/pdfs/1625.pdf)
For other info about the 1625 see this 807 datasheet
http://www.op316.com/pdf/tube-p5b/807-stc.pdf (http://www.op316.com/pdf/tube-p5b/807-stc.pdf)
with a pair in PP with your PT rectified with a diode bridge you will have around 50W of power on a 3200 Ohm a-a Load (see datasheet)
Other option (cheap tube but very very strong, consider it a rugerized KT88) can be the russian GU50 (0.76 A heater current) (the russian version of the german LS50 military tube used in Airplanes and Tank apparatus - many people say the russian version is also stronger construction than the german version)
(https://i.imgur.com/CYY84SG.jpg)
with your PT and a load of 5000 Ohm a-a a pair in PP will result in near 50W
A friend is building a PSE with a pair of this tubes
(https://i.imgur.com/3InVrU3.jpg)
Franco
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Thank you sir!
Those look wild! First I’ve heard of em.
Any comments on sound quality?
Of either the 1625 or GU50 ?
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1625 tubes will play similar to 6L6 tubes
GU50 are more used by HiFi guys and there are lot of good feedback
on the guitar PSE prototipe of my friend at the moment they show a bit weak on bass, but the project is not yet completed
Franco
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So I got this awesome 300mA power transformer but it’s got a center tapped 12.6 volt winding ...
... I’m thinking I could possibly push 4 el34s and a mess of preamp tubes if I could just figure out the best solution for the high 12.6 secondary.
Two EL34 heaters in series: PT Wire ---> EL34 1 Pin 2 ---> EL34 1 Pin 7 ---> EL34 2 Pin 2 ---> EL34 2 Pin 7 ---> PT Wire.
The bold arrows are the actual connections to make. Now you have a pair of EL34s eating 12.6vac to light up.
Wire your 12A_7 tubes as: PT Wire ---> Pin 4 ---> Pin 5 ---> PT Wire.
If you only use pins 4 & 5, your 12A_7 tubes eat 12.6vac to light up.
Make life easy & skip that mess with DC heaters & a bunch of extra complication.
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Take what I say with a small grain of salt, but I agree with HotBluePlates. Seems your best bet is run the heater's in series and avoid any extra complications. Is there any down side to that idea? I'm interested in knowing.
G.
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Hmmm🤔 well it’s either between hot blue plates suggestion or the DC headaches, OR using 1625s or 12v6’s for the power section, and this really obscure and unheard of dual triode called a 2C52, which allegedly has a MU factor of 100 but is an octal base tube that has a 12v filament.
I happen to have some 12sj7’s too.
As it is the only 12v winding I have on a PT, and being as how I already have 2c52’s and 12sj7’s ,, I may try to pick up four 12v6’s and cathode bias those little buggers and use the 12 volt octals.
Who knows🥸. I’ve got another vibroverb to build, a bassman to finish, a single ended 6l6 amp to build, and a hifi stereo to complete. So this particular PT with the 12v winding probably won’t see any action untill next year. But I really appreciate all the suggestions and will consider the different possibilities carefully when the time comes.
VT