Combining two HT secondary windings, would this work?

[Williamblake]

Hi!

So i have this powertransformer with two secondary HT windings and was thinking about getting more B+ Voltage as the 300V HT is dragged down to 280V at 80mA and maybe i could drive the 6V6 with 450V and have 300V for the 6BM8 driving the reverb. Maybe this is just a stupid idea as using just one HT winding is sufficient to power this amp but i was wondering if this would work. Also it would make better use of the transformer if it worked. Couldn't find something similar using google. Please roast attached schematic.

Regards
Jens

[jjasilli]

I think you can't get +HT from all 3 taps on the "top" secondary.  The CT must be (-) with respect to the 2 (+) outer legs.  So you can't run the CT leg to the other tube plates > cathode > common ground.

Also I do't get the KOV part of the circuit.


Provided that the 2 HT secondary windings are identical, then you can wire them in series for 2X the secondary voltage @ the same current rating.  But their CT's would be taped off, and have no ground connection.  The series connection must be properly phased. See, e.g., https://www.electronics-tutorials.ws/transformer/multiple-winding-transformers.html

[sluckey]

Silvertone did something similar to what you propose...

     https://el34world.com/charts/Schematics/files/Silvertone/Silvertone1484.pdf

They stacked one voltage doubler on top of another voltage doubler to produce a BIG B+ and a medium size B+. Same idea as stacking flash light batteries in series.

[kagliostro]

Which are the spec of your PT ?

AC voltages (and current rating, if you know) of the two HV windings

Franco

[Williamblake]

Thanks so far, so this doesn't necesserily blow up if the filtering is done right? Still dont know if i will really do this but whats in it for me: Less current and more power with two HT windings and still having slow startup, if i am not mistaken.

The Transformer is taken from a Nordmende UW958, it has a 300V CT HT that delivers approximately 285V @ 80mA so i am guessing i am taking as much as i can from that winding, the second HT winding has a little less voltage and the diameter seems to be about the same or slightly smaller, would have to look this up but it would only have to deliver 25mA at 500V wouldnt it?

[jjasilli]

This is not clear.  If the 2X HT secondary windings are not identical, they should not be placed in series or in parallel.  If they have different impedances, current draw will be out of balance, so one might get too much current draw.

You do not need more voltage for the 6V6 or the 6BM8 in SE. 

@285V the 6V6 draws about 42mA.  You have nearly double that current draw available.  You can use a voltage doubler off either or both HT secondaries.  That cuts their current ratings in half - which seems OK; but is over-voltage for SE operation. 

Why not just run each power tube (& small bottles), off its own secondary winding? It appears that there is no problem here, except the desire to over-complicate the use of this PT.

[Williamblake]

Not really needing more than 300V B+ is a valid point in my book, too. I don't really need more Voltage for the 6V6SE-RT practice amp this is. Actually it worked just fine running of the 300 secondary alone but i really don't have enough clean output-power to stand up to a drummer and a champ would. So right now i am rewiring the amp after testing it out some keeping the circuit the same and getting cleaner wiring and i am desperately waiting for parts to arrive and feel like thinking this through in the time being not knowing what purpose it may have but as of yet the transformer got warm some and i didn't exceed the heater current so these would already be two reasons to try this. Beeing the noob i am and still living to tell i would like your opinion on how to filter this, too. Regarding the max C for the EZ80 would be one thing and i do wonder what difference it would make to filter the B+ to ground instead of filtering it to the 300V rail. Wouldnt the 300V rail benefit from not getting that ripple? Albeit being SE this amp is too heavy to happily lug around anyway so i thought about what i attached. The 5k in the B+ might be 2k but i would like 400something on the OT.

[Williamblake]

Regarding the two secondary windings impedances i think i would be on the safe side of things as the 300V lower winding would give less current than it does now and the additional winding would give what it can, the load presented to it defining it's current. I will have to take another look with a gauge when things are disassembled some more but  peeking at the winding i am guessing it should do 25mA or more easily. Wouldn't the load presented affect any current drawn way more (like totally) than the winding impedances? 

[kagliostro]

If the winding has different current rating there is no problem to put them in series, but you can take as max current the current that is disposable on the weak winding of the two (or the weak winding will heat and burn)

Here is your original PS, unfortunately there aren't indications about AC voltages and available currents, if you make the count of course you can find the heater current, looking to the number and kind of tubes, but as to establish the HV current, I think the way is to add a load and increase it till the voltage drop of ~5%, that will be your max disposable current



as a way to have less drop voltage, have you think to try to use Solid State rectify instead of the Vacuum rectifier ?

this will result in a higher B+

Can you measure the AC voltages of the two windings and post it ?

Franco

[Williamblake]

This is my power transformer indeed. Thanks for looking it up and thanks to radiomuseum.org. Last time i measured it gave me 200VAV or so  unrectified to CT of the winding i am totally not using now.

I initially posted this thread because i am not aware of how voltage doublers work but i greatly appreciate your help on what i want to do this.

And adding silicon voltage above a tube rectifier seems like the perfect crime to me.

[PRR]

A wobbulator?? Does it even have enough iron to power 6V6es?

Also, that extra winding supplies very LIGHT load, some high-value resistors (using the line AC to sweep the frequency of an oscillator and swing a 'scope input).

Compare the DC resistances of the two windings. Maybe for their convenience they used similar gauge on each, so the current ability will be similar. If the DCRs are very different, then you can't draw much current from the extra winding without smoke.

You can stack as drawn; for some reason we usually rectify and filter each winding, the stack the DC outputs.

[Williamblake]

There is iron in this wobbulator. The powertransformer is 10,5cm x 8,5cm x 3,5 cm (or 9cm including the winding) 4,1in x 3,3in x 1,4in (3,5in with winding).

Looking closely at the two HT wirings they look absolutely the same and measure like this:

HT1 AtoB: 664VAC, 391Rdc, diameter 0,5mm, 1/5in.

HT2 AtoB: 364VAC, 1000Rdc, diameter 0,5mm, 1/5in.

HT1 is the winding i am using and it gives 285VDC at 80mA or 300VDC at 60mA.
The heater windings are of different sizes.

Wouldn,t the lower voltage winding want it's dc resistance to be smaller since it has less windings to it if they really were the same size?

[Williamblake]

Maybe i pull the tubes and take that mystery by itself, rectify and filter it and put 6k Load to it and if it yields 150V filtered DC i am fine? Maybe start with a 50k load and step down from there assuming 5mA wont fry thant winding immediately.

[Williamblake]

So i rectified HT2 with two 1N4007 and put a 22uF Filter to it and got 249VDC.
With a 45k load attached i got 239VDC.
With 22k load 229VDC.
With 11k load 218VDC. This is drawing 20mA.
With 6,2k load 201VDC. This is drawing 32mA.

[PRR]

I would not like to see over 10% drop on a transformer that size. Which suggests that 20mA is already a heavy load on that winding.

[Williamblake]

So HT2 behaves like you would expect. 182VAC to CT x 1,4 will give 254,8 V filtered DC and i am getting that whith no load attached.
But HT1 should give 332VAC x 1,4 = 464,8 V filtered DC and i am getting 300something V if i don't put too much load on it. Is this the EZ80 causing this?

Why would the windings be that different if they have the same diameter? So maybe they dont although they look like they do.

[DummyLoad]

the wire size was likely manufacturing convenience, however, the second HV winding was probably for a or other circuits that consumed less power. not uncommon design practice in old gear.

so, stack the PS rails as shown, filter the heck out of it, then use that higher output voltage as the B+ for your preamp circuits for > preamp headroom, or perhaps even a reverb driver. 


--pete

[Tony Bones]

I apologize in advance for not reading the entire thread right now (I have to run.) But, how about using a bridge rectifier on the 'top' winding for the 6V6? That should give you twice the voltage of the FWCT rectifier so you won't need to put the supplies in series. Assuming your drawing is close, you'd get ~400V for the 6V6.

[kagliostro]

@ Tony Bones

Seems that the winding has only around 20mA current available

Franco

[Williamblake]

What if i threw some more diodes at this and halved the voltage and the impedance on that lower voltage winding?  Would this allow more current? But maybe it cannot be done. Or could it? Or if it worked it wouldn't cut the impedance in half but maybe lower it?

[jjasilli]

To use a bridge rectifier with a CT secondary, don't use the CT; tape it off with NC (no connection).  The FWB rectifier circuit is grounded at the tail end of the bridge.

[Williamblake]

Thinking about this again, wouldn't this give half the voltage and a third more current on the diode rectified winding?

[DummyLoad]

What if i threw some more diodes at this and halved the voltage and the impedance on that lower voltage winding?  Would this allow more current? But maybe it cannot be done. Or could it? Or if it worked it wouldn't cut the impedance in half but maybe lower it?

adding more diodes is NOT going to change the physical characteristics of the secondary in question. it will add about a 1.5V drop for each pair.

stack the low current supply under the high current supply - this will work IF you're wanting greater B+ for the output stage. 


--pete

[Williamblake]

Maybe the diode on the CT to B+ isn't necessary and i mixed up B+ and B+1 in the last drawing,sorry but this could give enough current and voltage to power the 6V6 from B+ and and the 6BM8 and  3x12AX7 from B+1. Maybe power the 6V6 screen from B+1?

[Williamblake]

Ok it got "late" last night. The last drawing isn't different to normal two diode rectifcation, just going the other direction.
I still wonder how stupid it would be to bridge rectify from CT twice, using just one bridge rectifier from CT would probably give half the voltage but using two there are two paths from the winding tho B+ what if it worked would still be bad design i guess.
I might do fine with the one winding i have.
Thanks for all the comments.  Although i am not using this now i am glad i learned something i might use in the future.

[Williamblake]

So i think this is best i can come up with and i will give it a try.

[Williamblake]

Or rather like this?

[PRR]

Why do you think you need HIGH voltage on 6V6? Higher V forces lower I to stay within the 12W-18W dissipation limit, which then means a higher impedance OT for a good match.

[ECV02]

I agree with PRR here. Changing the voltages on the power tubes by this much is inviting trouble, and trouble will come. I've burnt up quite a few tubes (ironically, one being a 6BM8) by not paying careful attention to AC plate load impedance. Learning loadlines has helped tremendously  .

If you REALLY want more voltage, a DeLon is usually my go-to. If designing this amplifier from the ground up, I'd use the original 6v6 secondary with a FWBR (as opposed to the two-phase installed now), and choke-feed the amp from it. That might still be an option here, but chokes are getting more and more expensive, and they are bulky. This will still require an output transformer replacement, or some careful manipulation (if it's even possible).

In my humble opinion, it's not a good idea to mess with PT windings that were made to be separate in the first place. I wouldn't be surprised if the PT blew up setup like that, even if it's theoretically safe.

Kind regards,
E

[jjasilli]

Ditto.  This has all been said earlier in this Thread, which seems to be going in circles.

[kagliostro]

Seems he isn't so interessed to a higher voltage, but the transformer is low in current availability and he is trying to obtain more from it

the way he is doing that is to have a higher voltage level that is supposed to drop near to the wanted voltage when in use (due to circuit current consumption)

I think I can understand but I don't like this way to do (lower current consumption on power tube of course require to use a higher voltage but also a differet load impedance (OT), splitting the PS to feed the reverb section and using for it the unused PT winding will only save a very small part of current from the principal winding

Franco

[Williamblake]

Yes, it just doesn't do any good so i'll just not use that winding. Was looking into using the 450V for the 6BM8, too which appearantly for the same reasons is not a good idea either. Thanks for everybody's patience while pointing this out.

[Williamblake]

On a totally unrelated side note: The smaller winding at 219V 20mA would power the pentode of a 6BM8 to nearly 4W plate dissipation. Maybe a _little_ more. The reduced current draw on the other winding would let it's voltage rise above 300VDC.
No voltage doubling needed anymore.

[Williamblake]

Or maybe i will not do that, the as of yet unused winding won't even power just the preamp HT sufficiently giving 225V at 12mA so i moved the 6BM8 pentode up and the 6v6 down on the powersupply and i am drawing less current now using the windining i always used having 11W and 4,5W plate dissipation. This is a lot ado just because i overestimated the powertransformer and i surely will do a load test on any powertransformer i might use before going into this again. But than again probably not.

[Williamblake]

So this is not another spin around the same corner. After considering screen current i realized i wasn't dawing nearly enough power to use a 6V6 and a 6MB8 so i changed the 6BM8 for a EL95 and planned to use the EL95 on the little winding but boy did the voltage go up drawing 10 or 15mA less. I am around 60mA now and i am getting 350V with the 6v6 at 12W and the EL95 at 5W using one winding only. Will have to tweak the load on the reverb transformer as it still sounds a little funny but i wouldn't have expected a difference like this. In the wobbler the winding was giving just 310V.
So this thread should rather be called "Making do with an old powertransformer". Just wanted to state to whom it may concern repurposing old test gear is still going somewhere. Regards.

[kagliostro]

Consider that your PT will be good for a complex preamp (can feed many filaments, low B+ current available)

Franco

[shooter]

I am around 60mA now and

just for fun you might wanna check the 1hr temp test, if you don't loose skin I'd shipit 

[Williamblake]

I have had it running for hours with more current and it was "fine" and then i pushed it some more and could still touch it for at least several seconds. Also it has more voltage than in the wobbler it was build for so i am feeling quite safe with the current i am drawing now. Just because i measure with centimeters doesnt mean this is little iron.

So with the EL95 at 300 to 330 Volts i should use 12-15k load? Reverb still sounds a little dull with 12k. Couldnt find anything on that. But the drip is back which dissapeared since i dialed in more power to the 6V6.

Juggernauting about i thought i could use the low power winding to supply the EL95 screen only and its all wired up that way and voltages read "in the ballbark". Will be checking the sound tomorrow.

[Williamblake]

So i adjustet the voltages and have the EL95 at 4W plate dissipation and below 300V plate to cathode and the srceen to cathode at 230V and at the moment it sounds best with one ME4 reverb tank with 2,5Ohm dc resistance. Had two of these in there but will leave the tanks and reverb output transformer subject to change, just glad a load from an old radio suits fine, the reverb OT is 1:41 ratio, so i am having 7k load?
I can turn up the dwell two 2/3 before the EL95 starts distorting with the screens on the little windinding, before i did that i could turn up the dwell to 1/3 and it distorted. Grid resistor is 500k. I dind´t change how i drive the EL95 but the difference seems a little too big to be true, maybe that's just me thinking all the hours messing with the powersupply led to something useable after all.