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Filament supply with no center tap

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Posted on February 22, 2017 at 13:17:28
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I am trying to eliminate the dedicated filament transformers to free up some room in the chassis and utilize the taps on the power transformer. So, on my power transformer, there are 2 x (2.5VAC 3A) taps. I would like to use these in lieu of the dedicated filament transformers that I currently have. The dedicated filament transformers have a center tap.

I tried connecting the power transformer 2.5VAC taps and while I do get 2.5V at the 2A3 tubes, I get no music. Is this because I need a center tap or in this case, a derived center tap by using 2 resistors tied to ground? If so, is 1k ohm resistors the typical value to use? I was using 1k on the dedicated filament transformer center tap. Also, I am trying to avoid using a potentiometer.

Any input would be appreciated.

Thanks, Pat


**************************************
UPDATE 2/23/17

Thank you for those who responded. I have implemented the "AFTER" schematic listed below. I will perform some critical listening to determine what sonic impact this had vs. the original dedicated filament transformers. IF there is no adverse impact, then I will free up the space within the chassis by completely removing the filament transformers. There appears to be little to no hum. I will measure shortly. At this point, no need for a potentiometer. Thanks again. Pat

***************************************

"BEFORE" SCHEMATIC



"AFTER" SCHEMATIC



 

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100K ?, posted on February 22, 2017 at 13:27:26
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 Keep in mind those resistors are the B+ current path. How much plate current are you going to get with 50K to ground? (100K / 2).

A low ohm pot with a suitable cathode resistor and bypass cap are needed.

P.S. if you can't stand a pot, the you need to hand select two filament resistors for lowest hum. That's going to be tough at those low values.

 

RE: 100K ?, posted on February 22, 2017 at 13:29:23
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Gusser - I mean 1K, not 100k. Pat

 

RE: 100K ?, posted on February 22, 2017 at 13:32:34
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 Well you would select that resistor value the same way as with a centertap transformer. Then add the "pot" resistance divided by two. But as the pot resistance is so low, like 100ohms, you could ignore it in your calculation.

 

RE: 100K ?, posted on February 22, 2017 at 13:34:52
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 So, in my case, I would end up with 1000 ohm resistor on each leg of the filament supply (ignoring the pot resistance). I have some wirewound 1k resistors that should suffice. Thanks!

 

RE: 100K ?, posted on February 22, 2017 at 13:46:17
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 No I would still use a pot or two low ohm resistors to form a centertap.

Though I have little practical SE experience, I would expect hum problems if you stray from conventional practice.

And consider how you would handle the cathode bypass cap with two resistors? Use half the cap value on each side?

 

RE: Filament supply with no center tap, posted on February 22, 2017 at 14:42:12
vetmedrobert@gmail.com
Audiophile

Posts: 453
Location: Ontario
Joined: September 20, 2007
 How is your 2A3 biased? If you are determined not to use a pot and your 2.5 vac winding has no CT then you need to make one using a pair of 50 to 100 ohm resistors. These need to be fairly closely matched and 3 to 5 watt ones should be sufficient. In turn this CT needs to be connected to a cathode resistor. If you don't use a resistor to bias the output tube you will likely destroy it in short order since you are not restricting the floe of current through it.
You should use a capacitor to bypass the cathode resistor unless you like listening to music without any bass.
You should have a separate winding for each tube. Good luck!

 

10-ohm pot with two 20-ohm fixed on each leg, posted on February 22, 2017 at 16:39:15
FenderLover

Like this 2A3?

 

RE: 100K ?, posted on February 22, 2017 at 17:05:08
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002

Make the cathode resistor whatever value it needs to be to properly bias the tube.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Filament supply with no center tap, posted on February 22, 2017 at 17:19:34
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Sir,

The cathode return of the 2A3 to ground is critical, if you expect to get best results.

Best results come from a separate Ef power transformer, 2.5 VCT, for each tube.

The end leads should be good wire, I'd suggest M22759/11 14, and the transformer's end leads needing to be about 6 inches long, so the Ef XFR is located AWAY from the 2A3 tube, so the Ef XFR's fields don't interact.

The center tap return is also critical, in that it needs to be as SHORT as possible a return to star ground, and as good as wire as possible.

I often will rewire the Ef XFR's Center Tap with three 18 or 19 AWG pure silver lead outs in parallel, maybe an inch long, and then go into my Rk power resistor(s). I would use the length of my power resistor(s) to " eat up " some of the distance, between where the Ef XFR exists, and where the star ground return is located. I might use 12 AWG wire, on the other side of the Rk, leading to ground, and for that M22759/11 12 will be OK.

I would never use a pot on the ground return of the 2A3's cathode. The crummy contact area, wiper arm to resistive coil, rectifies the signal, a diode effect, sounds like doggie - doo.

There are other factors in placing a EF XFR for a DHT finals tube, that I won't go into here. But one quickly surmises, a stereo amp can NEVER have an optimized lay out, its got to be two mono amps. hence, my next 2A3 build, in March 17, will be exactly that !!

Do not ruin your output tubes.

Jeff

 

Please, it's time learn tube basics., posted on February 22, 2017 at 23:19:55
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Pat, from memory, your 2A3 was once cathode biased. If your circuit diagram is correct, the bias circuit, which was once attached across the +/-ve legs of your filament, have been removed. As drawn, your 2A3 is not biased; no wonder your amp does not produce music. I vaguely recall explaining this in a past thread.

Making significant circuit changes requires basic knowledge - so you know what is safe, what should work, and how to ask questions. What you are doing now is a crap-shoot. It can be dangerous.

I am simply not confident that you have sufficient knowledge not to be putting your loved ones and yourself at risk. I can't responsibly provide any advice to you - I won't be complicit in what you are doing.

Take the time to learn some basics. Please.

Edit: I should add that for non-EEs, learning the basics can be challenging (people learn differently). I'm sure inmates, including myself (where I can), would be more than happy to help explain the key concepts where required.

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: 10-ohm pot with two 20-ohm fixed on each leg, posted on February 22, 2017 at 23:54:39
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Works well for me, using a 26-turn bulk foil pot. Easy to adjust for lowest hum (26 turns for a range of only 10 Ohms) and the pot can be of far better quality that carbon & wire-wounds (relatively very low Wattage part needed).

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Filament supply with no center tap, posted on February 23, 2017 at 05:38:53
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 PAT
The filament transformer is also part of the cathode circuit.If you are going to use existing ones on a multi-tap power transformer,the filament leads have to isolated from the other leads as if they were a totally separate transformer already.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Here's a reworked schematic, posted on February 23, 2017 at 08:29:39
sideliner
Audiophile

Posts: 208
Location: NYC
Joined: August 22, 2013
Contributor
  Since: December 15, 2023



These reworked component values are based on the voltages from the existing PSU.

The Schottky diodes are Cree part# CSD01060A and at 13mA they each drop approx. 0.85V. This info is taken from a Vf vs. If chart that was done based on testing performed by member maxamillion and posted on this forum some years back.

 

RE: Filament supply with no center tap, posted on February 23, 2017 at 15:17:06
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Mikey,

Just an FYI: I have it up and running using the PT supplies. Almost no hum and certainly less hum than before using the dedicated filament transformers. Cool!

Pat

 

RE: Filament supply with no center tap, posted on February 23, 2017 at 16:18:20
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 If you have disconnected the original Ef XFRs, that - will most probably be the cause of LESS hum. Your individual Ef XFRs were smack dab "on top of" both the driver tube, and the output tube, on each channel. ( N.G. )

Recall me discussing, Ef XFR located far away, using all 6 inches of ( redone and better quality ) leads ??

FYI, the best way to do any DHT execution IS with separate Ef XFRs, but that implies monoblocks, which is also the BEST and easiest, least-compromised way to do optimized lay out.
\
My new DC mono 2A3 amps should be up and playing within two months. Have fun listening and learning .


Jeff Medwin

 

RE: Here's a reworked schematic, posted on February 23, 2017 at 17:52:18
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 I do NOT think Pat has a center tap ,on his power transformer's Ef voltage, so he showed us a pair of 50 Ohm Rs in series, the joined leads acting as a Center Tap, to feed each 2A3's 1K Rk to ground.

ONE BIG QUESTION 330 VDC versus 322 VDC ????

Is there any decoupling of the front end B+, from the Final's B+?? None shows in any of these schematics?? The Finals B+, connected directly to the Driver's B+, will goof-up the stability of the Input stage big-time

If there is NO decoupling, the circuit would need a higher overall B+, to allow an R/C for the front end's B+, hopefully down to one mVAC or so. Then, the Rk of the Finals will need to be increased in value to maintain the same goal of 43 mA and 265 VDC P-K. All doable. Is Pat listening now with no front end decoupling ??


Jeff Medwin

 

Good pick-up Jeff., posted on February 23, 2017 at 18:47:42
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 The output and driver stages seem to be taken off the same node... but, the voltages out of the PS according the schematic are different. Not sure if Pat has either not updated the voltages or has not drawn in the dropping/decoupling resistances. It appears that the resistances have been intentionally removed at some point therefore my guess is the former (same node).

If my guess is correct - and this is where my knowledge starts to reach its limits - for this design, would the signal of the stages be at 180 degrees phase to each other and therefore draw from the same node remain stable(ish)? Also, Pat has created 150uF capacitance node, FWIW.

Note: I'd prefer to have a RC stage to decouple the driver as well, but perhaps it will work as drawn?

91.


"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Here's a reworked schematic, posted on February 24, 2017 at 06:41:17
sideliner
Audiophile

Posts: 208
Location: NYC
Joined: August 22, 2013
Contributor
  Since: December 15, 2023
 Good catch Jeff. I hadn't noticed that. I had turned my attention to the driver stage because the calculated plate voltage / plate current didn't jive with the tube's bias voltage as shown. That's why I reworked the operating point with B+ being kept the same.

You are right, there should be a resistor between the main B+ filter stage and the PSU filter for the driver. If we assume a ~9 mA draw as per the OP's original schem, then this series dropping resistor should have been 880 Ohms for that 8V drop.

 

RE: Here's a reworked schematic, posted on February 24, 2017 at 07:37:03
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Jeff / Sideliner / 91derlust - I just remeasured the voltages and updated my schematic to reflect the voltages. The B+ going to the driver tube and final tube are the same. I have not heard about decoupling the driver from the finals, so any input would be appreciated.

Pat

PS - 91derlust, I am going to lookup driver decoupling online but just remember that I am admittedly not the quickest learner regarding circuitry. For me, baby steps. :-)

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 07:45:22
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Please see my response to Medwin's post. Also, Here is what I found online regarding decoupling resistors/cap:

Isolating one stage from another by adding resistor in series and a large value cap to prevent oscillations and other noises that may feedback thru the power supply connections. It also provide further filtering.

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 08:07:00
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 You got it, basically. Read on "Time constant" to get an idea of what values of R and C are ideal for your circuit. And the other point Jeff made is that you may need a higher B+ ahead of the RC decoupling network, so to account for the necessary voltage drop across the new R. That's if you want to keep B+ to the output stage the same as it is now.

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 08:30:57
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Lew,

Thanks for the TC comment. I just read on Time Constant. Here is what I came up with:

Fc = 1 / (2*pi*R*C)

I want to target an Fc of roughly 2-4hz.

My C is already determined by my existing caps which are 50uF. So, if I pick an R value of 1000 ohms, this gives me an Fc = 3.18 hz. This will drop the B+ going to the driver, but I think that will be OK because I will reduce the existing 22K Plate Resistor that drops the Plate Voltage to 133V. I can work on that offline.

let me know if this sounds correct.

Thanks,
Pat

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 08:45:35
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Yes Pat. Now, you need to figure out what sized resistor to use, and how much B+ voltage you need to drop across it, to get ABOUT 1 mVAC at the "C3" spot with the higher driver current you use. "C3" can be a DC LINK WIMA 50 uF, bypassed.

So, your entire supply will be like so: L1/C1/L2/C2/R1/C3. C3 can be shared L-R or split into two R1s and 2 C3s, you likely don't have chassis room to do that splitting.

So, model the supply in PSUD, and change the value or R3 with 50 uF held steady as C3, till you get 1 mVAC at C3. That will be the value, "ball park" needed of R1 ( AND a voltage drop across it ) that you will need.

So Pat, lets say you need to drop 50 to 100 VDC across R1, ( for 1 mVAC at C3) where will you get an extra 50 - 100 VDC , with a fixed power transformer and tube rectifier ???

Easy, Model "645 mHy at 10 Ohms" for L1, ( Hammond 159Y ) and SEE what that does to raise your C2 B+ VDC value. THAT is how you get the extra B+, with a smaller value, lower mHY, lower DCR L1. The VDC at C2 will then dictate the R1 value you need.

Also, Pat, with eg: a 50-100 VDC higher B+ at C2, you will need to use a higher value 2A3 Rk, to maintain 265 VDC at 43 mA. P-K across the JJ 2A3-40.


Go for it. But when you are doing this, remember one thing, one truth. A cap couple is the worst way to couple two stages. A transformer couple is better than a cap couple, easily. And, a direct couple is THE best way to couple two stages. Have fun.

Jeff Medwin

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 08:53:20
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 No !! You let us know !! :-)

Plug the entire filter into PSUD, and tell us what you need to get 1 mVAC at C3. That would be L1/C1.L2/C2/R1/C3. C3 equals 50-51 uF. Do not worry too much about a low time constant, but concentrate on 1 mVAC of ripple at C3, as your design priority.

Low Mu

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 09:49:19
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Jeff - I would like to use PSUD, but I have a MAC computer and PSUD is for PCs only. I got the Time Constant to work with an Fc = 3.1 hz by using a 1k R1. Pat

 

RE: Good pick-up Jeff., posted on February 24, 2017 at 10:15:12
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Don't tell soul I told you this, but it can be three times or more higher, and NOT be a problem. But I want 1 mVAC at C3. Call me some times, and I will ask you for data to input, and I can PSUD supply for you.

Is the new Pontiac GTO running well ?? Still liking the purchase??

JM

 

RE: Filament supply with no center tap, posted on February 24, 2017 at 11:32:57
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Pat
I looked at your transformer schematic and the filament leads are totally independent of your other leads so that's good.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Here's a reworked schematic, posted on February 24, 2017 at 12:58:42
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Pat, I am not a fast learner either and I don't have much time to spare these days... and it is getting less.

keep at it.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

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