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General Discussions => Drawing Board => Topic started by: CJ on September 02, 2014, 03:09:24 AM

Title: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 03:09:24 AM
thought we could throw down some vac tube heater circuits all in one place,

there are a ton of them out there, AC and DC,

never had much luck with regulators, they do not like the start up currents,

and sometimes the high voltage plate supply finds it's way to the regulator,

here is a DC supply to get things rolling,
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 03:14:33 AM
here is another DC circuit, taken from Vacuum Tube Valley,

Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 03:28:07 AM
DC #3
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 03:35:04 AM
something a little different, just for kicks,

here is a preamble to a weird RF heating scheme>

Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 03:36:09 AM
here is the RF Heater circuit>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 04:40:48 AM
here is an AC supply with a cap in the CT>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 04:47:51 AM
here is a circuit with a 5 V-ac and 6.3 v-ac combined, AC/DC>

Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 04:53:34 AM
here is an imaginative approach, uses a Lamda DC supply in conjunction with the regular supply>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 04:56:56 AM
here is a 12 volt version of a DC supply>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:00:27 AM
this supply has the heaters biased up from the HV supply>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:04:17 AM
this is another elevated supply only DC this time>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:07:06 AM
one tube gets AC, one tube gets DC>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:10:00 AM
elevated 6.3  and hum balanced 2.5 V-ac circuit

individual transformers for the 2.5 V-ac tubes>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:17:21 AM
series resistor because heater voltage was 7 V-ac instead of 6.3>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:21:58 AM
stacking different 6.3 V-ac tubes to get correct voltage drops>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:28:19 AM
DC supply with single diode,

memory capacitors?  ???  maybe scrounged out of disposable cameras?  :o

Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:40:40 AM
elevated AC supply using cathode bias voltage>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 05:50:41 AM
infamous Marantz 10B tuner, has regular AC, elevated AC,

a and minus biased AC for the mini CRT which was used for tuning stations,
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 06:20:51 AM
infamous Fisher 500C has some weirdness (translate-do not understand)  :D

has a -22 V-dc series string on a few 12AX7's,

and some funny caps on the upper string>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 06:38:42 AM
Fender AC supply with hum balance pot>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 06:46:44 AM
this is my favorite, low hum, no frills, always works,

resistors burn up if you get a short plate to heater,

"save the transformer" foundation, join today!

Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 06:49:19 AM
getting back to basics>
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 06:54:33 AM
and finally the primitivo circuit,

can be made quiet if you are a 90 year old tech from the old school,
Title: Re: Heater Supply Circuits-AC and DC
Post by: johnheath on September 02, 2014, 11:10:07 AM
Great topic with a lot of answers for a lot of people …Thanks

/John
Title: Re: Heater Supply Circuits-AC and DC
Post by: mattamatta on September 02, 2014, 03:51:37 PM
memory capacitors?  ???  maybe scrounged out of disposable cameras?  :o

Caps out of a disposable camera would probably be smaller value very high voltage for the flash generation.  I would probably have labeled those "supercaps" on the schematic and that's a good search term to keep in mind for anyone attempting to find something like a 47,000uf 5V cap.
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on September 02, 2014, 07:15:10 PM
that schemo with  the memory caps was from Eric Barbour, maybe i will shoot him an email to see what he means,

could be computer caps, those big blue Sprague things that can be had for cheap,

Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on September 02, 2014, 10:45:25 PM
> Fisher 500C has some weirdness (translate-do not understand)  :D
> has a -22 V-dc series string on a few 12AX7's,
> and some funny caps on the upper string


The 22V (two under-heated 12.6V heaters series) is probably also a bias supply for the finals, which is why it is negative. (Rectifiers were too costly to throw-in another if one could do two jobs.)

The funny caps are probably the tubes in the IF string. There's a gain of maybe a million from IF in to IF out. Then you have wires running from one end to the other, eager to carry IF Out signal back to the In. So caps everywhere, and some smutz which may be inductors (beads)?

"Memory Caps" are "Super Caps", activated carbon electrolytic, used for low-low-tiny current drain to keep CMOS memory alive when power is off. Traditionally they have huge ESR, make poor 60Hz filter-caps. Recent ones are far better.
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on October 06, 2014, 03:17:34 AM
another bone for the stew pot,

 Peavey Roadmaster heater supply has a 25 volt DC bias on the tubes,
Title: Re: Heater Supply Circuits-AC and DC
Post by: mjrippe on October 06, 2014, 11:49:31 AM
How does the cap lower the required wattage rating of the pot?  Isn't it there to block the +25v DC?  Also, shouldn't they be using 35v caps ;-)
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on October 06, 2014, 06:36:57 PM
good catch! yes the cap blocks the DC , posted too late in the evening,  :o

still gonna have 6.3 V-ac across the pot, so wattage does not change,

that cap might pop in a plate to heater short unless all those fuses work first,

bias supply could take a hit also,
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on July 25, 2015, 07:50:03 PM
another heater circuit that uses 47 ohm 1/2 watt resistors to create a virtual center tap,

normal value here is 100 ohms each, so this is a little stiffer of a CT,

dissipation is 6.5/47*2=69.15 ma * 6.5=0.45 watts/2=0.225 watts in each of the 47 ohm resistors or about 1/4 watt out of a 1/2 watt which is ok, possibly, we would probably go with 1 resistors over here,
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on March 10, 2016, 04:06:01 AM
HP 400D AC voltmeter from the 50's has DC heaters,

this is because it has a 1 mv input range

clever series resistor scheme,

adjustable also>

Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on March 10, 2016, 04:24:49 AM
Cool stuff CJ. Where can I buy 58,000uF capacitors!! I like the idea of using a single beefy diode on the dc side to drop a fixed voltage - neat.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on March 10, 2016, 05:23:24 PM
> Where can I buy 58,000uF capacitors!!

http://capacitor.tedss.com/58000uF

I have no idea who that is, just came up first in Google.

Appears to have once been a Standard Value, because old NOS show up in many places.
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on March 10, 2016, 05:34:08 PM
> Where can I buy 58,000uF capacitors!!

http://capacitor.tedss.com/58000uF

I have no idea who that is, just came up first in Google.

Appears to have once been a Standard Value, because old NOS show up in many places.

Odd indeed. 68000 I could understand but 58000 is a new one on me. I wonder where that value came from - I notice a few of the caps are 75V rated and appear to be 'telecoms' related - old telephony types? They all seem to be in the US. Google UK gives some on UK ebay but they are all from the US.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on March 10, 2016, 06:02:12 PM
Totally strange....

I again Googled "58,000uFd capacitor" and this time one of the links is to THIS thread.

"Thursday, November 20, 2014
Lee Marvin & Pussy"

http://twogoodears.blogspot.com/2014/11/lee-marvin-pussy.html

(http://2.bp.blogspot.com/-ozhQIGmX8Kw/VG4q6qgtBmI/AAAAAAAAPCU/L92NnYkzx-M/s1600/10441125_749129125142074_2405325668291612872_n.jpg)

Top link in left column "TGE's Picks - My Blogs and Sites List" is (at the moment) this thread.
_____________________

The few non-audiophile links for {that size cap} mention "ringer", so it may be a Ma Bell compatible ringtone generator part.
Title: Re: Heater Supply Circuits-AC and DC
Post by: CJ on March 11, 2016, 04:55:50 AM
well,  he has a big organ but does he know which knobs to pull?
Title: Re: Heater Supply Circuits-AC and DC
Post by: saint gillis on June 20, 2017, 06:43:00 AM
Awesome information collection CJ!

A friend of mine told me that DC in the heaters could after a long period of time polarize the heaters which was not good, and he said it was good to have a switch to reverse the 2 heaters supply pins time to time , to avoid heaters polarization, what do you  think of that?
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on June 20, 2017, 11:19:39 AM
Awesome information collection CJ!

A friend of mine told me that DC in the heaters could after a long period of time polarize the heaters which was not good, and he said it was good to have a switch to reverse the 2 heaters supply pins time to time , to avoid heaters polarization, what do you  think of that?

I think it is rubbish. LOL

There is a whole group of tubes designed for battery radios that had dc heaters. They never had any problems and the heaters were not made any different.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: Andy Peters on June 20, 2017, 01:21:11 PM
A friend of mine told me that DC in the heaters could after a long period of time polarize the heaters which was not good, and he said it was good to have a switch to reverse the 2 heaters supply pins time to time , to avoid heaters polarization, what do you  think of that?

"Polarize the heaters?" I'd love to hear the explanation of that phenomenon.
Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on June 20, 2017, 06:07:31 PM
When you ran fluorescent light tubes from DC (not a common thing), you got better life if you swapped the ends occasionally.

Not applicable to vacuum tube heaters.
Title: Re: Heater Supply Circuits-AC and DC
Post by: saint gillis on June 21, 2017, 08:40:59 AM
  Well this guy did surely get his bad concepts from this light fluo tubes thing ..
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on December 08, 2017, 11:18:18 AM
Switched Mode Power Supplies (SMPS) are much better than they used to be, with switching frequencies well above the audio band. The addition of hiccup mode short circuit protection designed to fire up into large capacitive loads means they are now suitable for consideration as heater supplies. I have started using them myself in tube lunch box projects that require 12V at up to 3 amps. Inrush current is still an issue so I overate the SMPS so for 12V @ 3A I will use s 100W rated unit

My favourites right now are mad by MeanWell. Despite their origins they are are sizeable and well respected company and often used in telephony/GSM operations where longevity and reliability are paramount. At present, the one I use most is the MeanWell LRS-100-12.  It is available at low cost from TME:

https://www.tme.eu/gb/details/lrs-100-12/industrial-power-supplies/mean-well/?gclid=EAIaIQobChMIk9qMhun61wIVSrvtCh0w0AOXEAAYAiAAEgIPLfD_BwE (https://www.tme.eu/gb/details/lrs-100-12/industrial-power-supplies/mean-well/?gclid=EAIaIQobChMIk9qMhun61wIVSrvtCh0w0AOXEAAYAiAAEgIPLfD_BwE)

Datasheet is here:

https://www.tme.eu/gb/Document/5adfc6ee9376b871796d0b1e9b29d2ad/LRS-100-SPEC.pdf (https://www.tme.eu/gb/Document/5adfc6ee9376b871796d0b1e9b29d2ad/LRS-100-SPEC.pdf)


I will add other types as I test them.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 09, 2017, 07:44:52 AM
A friend of mine told me that DC in the heaters could after a long period of time polarize the heaters which was not good, and he said it was good to have a switch to reverse the 2 heaters supply pins time to time , to avoid heaters polarization, what do you  think of that?
Hum can noticeably increase in some input stages if the heater-to-cathode leakage resistance falls (it's typically many many megohm) - which is usually fixed by tube-rolling.  The heater to cathode interface could degrade differently over time if a DC heater was used - perhaps due to metal dendrites or other changes in the alumina baked insulation.   So sounds plausible to me.
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on December 09, 2017, 07:53:12 AM
Hum can noticeably increase in some input stages if the heater-to-cathode leakage resistance falls (it's typically many many megohm) - which is usually fixed by tube-rolling.  The heater to cathode interface could degrade differently over time if a DC heater was used - perhaps due to metal dendrites or other changes in the alumina baked insulation.   So sounds plausible to me.

I have often come across poor hk leakage especially in NOS tubes. I am pretty certain the vast majority of them were operated on ac heaters so I see no evidence dc heaters are to blame.  Most of my designs require elevated heaters and they very soon show up poor hk leakage in a tube, I am sceptical of dc being a cause. Pro audio mixers from the 40s often used dc heaters. Those guys knew what they were doing, I have all tube mixers with customers that have been operating for several years on dc heaters and I have never had a complaint about hum.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 09, 2017, 08:17:28 AM
There are many  degradation mechanisms of hk resistance it seems, and many are due to quality and variation in the heater insulation coating across manufacturers and years of production. So I wouldn't exclude it.
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on December 09, 2017, 12:24:47 PM
There are many  degradation mechanisms of hk resistance it seems, and many are due to quality and variation in the heater insulation coating across manufacturers and years of production. So I wouldn't exclude it.

I won't exclude it but neither will it be anywhere near the top of my list.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: EmRR on December 09, 2017, 01:27:52 PM
Pro audio mixers from the 40s often used dc heaters.

This I have never seen.  Love to have an example pointed out.  Only thing I can think of is 1st stage gain reduction tubes.  Literature of the era points out that, given capacitor size/quality of the time, DC filaments can often have more hum than a proper AC filament. 
Title: Re: Heater Supply Circuits-AC and DC
Post by: ruffrecords on December 09, 2017, 05:46:12 PM
This I have never seen.  Love to have an example pointed out.  Only thing I can think of is 1st stage gain reduction tubes.  Literature of the era points out that, given capacitor size/quality of the time, DC filaments can often have more hum than a proper AC filament.

Altec 250 SU. Its 535A power supply provides 6.3V DC at 13 amps. Peak to peak ripple at 13A is 1.5V.

Cheers

Ian
Title: Re: Heater Supply Circuits-AC and DC
Post by: EmRR on December 09, 2017, 05:55:00 PM
Altec 250 SU. Its 535A power supply provides 6.3V DC at 13 amps. Peak to peak ripple at 13A is 1.5V.

Cheers

Ian

1960's, and probably the only commercially produced American console I can think of that did.   

Now, up until about 1940 they may have used batteries for filament.
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 09, 2017, 06:18:35 PM
Most of my designs require elevated heaters and they very soon show up poor hk leakage in a tube, I am sceptical of dc being a cause.

Ian, you would be elevating the heater to avoid stressing the hk voltage beyond rated limits.  The hk insulation in that situation is then stressed towards but hopefully under rated limits due to signal swing, and the likely large elevation levels.  Can you elaborate on how you then distinguish a 'poor' valve - is it from a constant high level of mains frequency leakage current (due to the heater AC waveform), or an ad-hoc pop-corny style breakdown of that insulation, or some other observation?

It's also worth noting that many use a Vdc elevated heater voltage (to a much lower elevation level, where hk voltage is not stressing the insulation) to minimise hk leakage resistance - which results from an interesting variation of hk resistance when hk voltage passes through 0V.

As a general comment, its much easier nowadays to distinguish mains rectifier related noise artifacts in audio signals due to modern spectrum analyser software and very low noise floor adc equipment/soundcards.  To observe those hum artefacts back in the 50-60's, the hum level would need to be significant, and I guess the S/N of recording media would inherently mask hum from earlier in the signal processing path.
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 09, 2017, 09:52:29 PM
Speaking of unusual heater circuits ...

I've just overhauled a late 60s japanese stereo integrated tube amp with pp el84 finals..

The heater supply for the phono preamp section is the cathode current return from the finals .. and thence to providing bias at the finals' grids !

ie.    el84 cathodes -> some resistance  -> phono section AX7 heaters (series heaters) -> el84 grids

It all works out well - the EL84 cathodes are around +20V @ 60mA for each pp pair - the stereo pp pairs are connected together, after some rheostat adjustment, giving a total of 120mA current at +20V or so.

That feeds a pair  AX7s with dc (under) heating (series tubes with series heaters) for stereo phono preamping....

with around [email protected] for each AX7 tube then to ground   ... *also* then with a connection of the second AX7 heater  (+10Vdc) parallel sent to the grids of the pp EL84 stereo pair  ... as a 'fixed' bias   (each with 250K resistances grid to ground)

Quite roundabout, for sure -  but one does wind up with combo fixed/self bias on the EL84s of around +11Vdc cathode-to-grid  *and*  dc heaters for each of the phono AX7s  of +10Vdc at 120mA.

I was amazed but it does make sense when one one wants underpowered dc heaters on phono preamp section without the cost of an additional heater winding on the traffo  plus rectifying/filtering  :)

The el84s are thus biased very hot, at +11Vdc cathode-to-grid, tending towards the class A pp mode - not too efficient, resulting in some 10Wrms into 8ohms each channel at 3% THD.

It surely does sound good - at lower power of 5Wrms or so, THD drops to around 0.5%.


Those crazy 60s Japanese hifi tube designer gurus for low cost solutions!

...

I recapped where necessary, rebiased for lower sensitivity on the aux inputs and got the hell out!

It was way too sensitive to use with a modern computer audio interface, so I set  up the 'low sensitivity'  aux inputs  to handle around 8Vpp for max power ...  I left the tuner/tape and phono mag/crystal, mic, tapehd  inputs as they were.

One could rework that whole 'high gain' section to be a killer guitar pre, but I resisted  and left the old RIAA, NIB weird feedback eq'ing as is   :)

This  'Encel' el84 amp featured an elaborate speaker-with-feedback-coil capability, called  'Motion Feed Back' and a complicated 'Damping' control to boot .....  I replaced all that with a simple variable nfb circuit.

Replaced the original tube set  for some fresh ehAY7 (preamp),  jj5751 (cathodyne) and russkie 6P14P ev finals ....

The amp was running the original australian made 'milliwatt el84' finals at some high supply  ..  like +370V plate, +320V screens   :o   

Amazingly, these tubes, circa late 60s tested very well!

I replaced with some nos 70s 6P14P ev   ..   which are a good sounding and cost effective rugged el84 sub ...  they are rated for 400V plate, 300V screen with 14W max plate dissipation .. a bit of a 'super el84/6bq5'.

I dropped the screen supply to 300V at first, but then returned it to 320V ...  for a little more power at 3%  ....  the robust russkie has a generous screen dissipation and seems to have no trouble with the higher than spec supply. 

The bias setup doesn't lend itself to tinkering  >:(  I tried with no success  .. always seemed to 'equilibriate' back to +11Vdc cathode-grid but with higher THD, so I gave up!

...

Happy with the result - it's equally at home with low and high senstivity inputs with a lot of adjustability in between.

I'm using it as a stereo FX amp with hifi speakers, but it really sounded good and loud! on a 12" jensen alnico cab too,  with a DI box on some guitar   8)

It's super quiet ... absolutely no hum even at full power  ..  about the best hum performance I've come across in a setup with significant gain - it has those old  style steel alloy close fit shield/heatsinks on the phono AX7s.

Quite a feat really, considering it has a voltage doubling HV supply circuit, which usually give more ripple, *and* the ancient main resevoir caps showed no sign of degradation ...  I subbed with new JJ multicaps with no measurable improvement, so I kept the originals.

I did replace all the bias electrolytics and all the coupling caps in the cathodyne. A lot of the old carbon comp resistors were all over the place, so replaced them as required too.
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 09, 2017, 11:00:37 PM
Here's a top pic ..
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 09, 2017, 11:02:09 PM
and front ..

The various mono and stereo modes are interesting to play with ...  great for comparing Beatles mono and stereo mixes   ;D
Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on December 10, 2017, 01:47:15 AM
> Those crazy 60s Japanese hifi tube designer gurus for low cost solutions!

Fisher did the same many years before for a VERY expensive four-6V6 mono console.

You can find it all over, if you look.
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 10, 2017, 02:07:49 AM
That makes sense .. I had the impression the 'Encel' was based on a Fisher or a Scott integrated amp of that period.

I also think there was some connection with Sansui, like an oem done for the Australian market for a Melbourne based importer.

I will check out some more Fisher schematics.
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 10, 2017, 06:38:52 AM
The earlier Japanese import X1212 from Encel (dating from Sept 1964) didn't use that heater filament loading technique.

AWA used that technique in at least the PA872 and PA1005  PA amps - the earliest I can track those amps is the PA872 from circa Nov 1963 (first observed adverts in RTV&H).   AWA engineers would have had quick access to any new techniques of interest to them.

PRR, I couldn't easily identify the Fisher quad with heater filament loading - I looked through about a third of the schematics on http://www.fisherconsoles.com/
Title: Re: Heater Supply Circuits-AC and DC
Post by: PRR on December 11, 2017, 03:32:44 PM
> identify the Fisher quad

Similar to the 1950 Coronet chassis R1:
http://www.fisherconsoles.com/r1%20console.html
http://www.fisherconsoles.com/r1%20console%20service%20manual%20page.html

Had a distinctly long quiet warm-up.

The one I saw had a Damping control. Ah, I see that in the 1955 Custom Electra I, which also has the quad-6V6 and preamp heaters for 6V6 bias.
http://www.fisherconsoles.com/custom%20electra%20I.html

Fisher Executive II Console (1958) is VERY different (styling, stereo), but if you trace it out the four EL84 in the power amp feed three 12AX7 in the control chassis.
http://www.fisherconsoles.com/executive%20II%20service%20manual%20page.html
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 11, 2017, 06:19:48 PM
Doh, don't know how I missed those - thanx. 

Ok, so the 1950 R1 has the cathode bias provided by three heaters in series, and a parallel 4k7 resistance (to provide fail-safe cathode voltage rise so as not to stress hk voltage rating), and a local 50uF bypass cap to constrain signal currents to the local output stage region and provide some possible distortion reduction (as sometimes found when bypassing the Williamson common cathode, and depending on the tube type used).

The 1955 K-15 appears to use some level of cathode winding feedback, and provides a fixed bias from what looks like a 2.2k to 1k divider off the bypassed DC heater supply - with the divider also providing fail-safe rise of hk voltage if an input stage valve is pulled, or gets a poor heater connection.

The 1958 Executive 2 is similar to K-15, but doesn't use the cathode winding feedback, and in addition elevates the ac powered heaters to the output stage cathode DC  supply derived from the preamp heaters.

A nice timeline and set of changes indeed!
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 11, 2017, 06:29:22 PM
Interesting ...


One thing I found a little unexpected in the Encel, was that the el84s, with their fixed grid bias + cathode resistance bias 'combo' ..   had no cathode bypass capacitors.

I added  1000uF to ground, at each of the cathode pins in the pp finals ....   made very little difference to the 3% power at the speaker terminals.

Usually when I bypass the cathodes, as I did in my 6v6 pp amp, I get a big increase in gain at the finals resulting in quite a bit more power to the speaker terminals. 

In the 6V6 with full cathode bias, the increase in power was substantial . .. like 60% or so. 

In the encel el84, with the combo bias,  it was more like 5%.

Again, I left it as it was, figuring it wasn't worth bothering with. I suppose that one needs full cathode bias to make bypassing effective.
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 11, 2017, 06:43:53 PM
The hi-fi Encel would be set up for class A, so bypassing is likely to have no major effect, and depends on how close the valves are pushed to grid conduction, and the valve's characteristic.

Amps set up for class AB operation need a bypass capacitor to try and maintain a fixed bias on the output stage during higher signals levels where one PP side stops conducting current (and hence a common cathode voltage would vary significantly if it was just resistance to ground).
Title: Re: Heater Supply Circuits-AC and DC
Post by: alexc on December 11, 2017, 07:35:42 PM
Thanks for the clarification ....   that makes sense.

And PRR, that Fisher is similar to the Encel with the exception that it appears to be a full cathode bias, with bypass cap.

The thing I wonder, is what practical difference is made  with the 'combo' bias, with a fixed grid voltage providing about half of the cathode-grid dc voltage, and the cathode circuit resistance providing the rest.

My understanding is that

- 'fixed bias', sometimes also described as being 'Class AB' in the data sheets example setups, provides a less programme dependent bias (more stable)  and is generally of a higher efficiency and therefore power.

- 'self bias' or 'cathode bias' does vary the tube's operating conditions more with  the higher rms programme and usually results in a lower available power.

Fixed bias sometimes  is described as be more 'hifi' and cathode bias more 'compressed', leading some to favour one for hifi application and the other for instrument amplification.

Surely a simplification, but is 'combo' bias the 'best of both worlds'  ?

I generally use finals with fixed bias so I can closely achieve my desired operating conditions; cathode bias and combination of both is new to me. 

The only other time I've used 'combo' bias  it is with my vari-mu limiter builds, especially in the fc660 and it's local derivative, the poorman 660, where one is trying to set operation on a specific part of the remote cuttoff transconductance curve, where it has significant 'bendiness'.

----

Just found another popular 6BQ5/EL84 integrated amp using 'fixed + cathode' or 'combo' bias - the Scott 222 series.

 - it takes the lowest HV rail, neon bulb regulates it to +48V and uses that as fixed bias to the grids of the finals, which also have a +60V cathode bias circuit; giving a +12Vdc cathode-grid net bias voltage.

- that same +48Vdc neon regulated rail is also used for 4x series heaters of 12V each for the input stages (4x 12AX7)

(the remaining heaters use a 6.3Vac winding with one end connected to the ground line - I don't think I've seen that before; an ac winding seemingly 'ground referenced')

The Scott 222 is a most interesting circuit for anyone interested in an integrated amp design with AX7s and EL84s.  There is a write up in Vacuum Tube Valley  Issue 13   :)
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on December 11, 2017, 09:00:19 PM
A few have found combo bias of benefit for their particular application  Gingertube has had benefit with hifi use in reducing distortion.
Title: Re: Heater Supply Circuits-AC and DC
Post by: janos.hardi on June 19, 2020, 09:19:20 AM
Hi guys,

Trying to resurrect this topic for selfish reasons :) I know this has been discussed million times (AC vs DC heaters) but I have PCBs that expect DC voltages and I cannot change that. So to me this is not about the effectiveness / noise / efficiency debate.

I tried to "implement" some of the designs posted here in PSU designer but usually failed. The numbers just don't add up :) I'm very inexperienced with this obviously and I miss something to get the desired result - at least approximately.

The circuit visible on the attachment is the B+ from this post https://groupdiy.com/index.php?topic=57166.msg728679#msg728679

Thank you for being generous with your time!
Title: Re: Heater Supply Circuits-AC and DC
Post by: trobbins on June 19, 2020, 09:46:51 AM
Turn off the soft start option.
Title: Re: Heater Supply Circuits-AC and DC
Post by: janos.hardi on June 19, 2020, 10:15:30 AM
Turn off the soft start option.

Thank you sir! Much closer :)