Bipolar Cap Mult DC Heaters

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I agree completely it’s worth considering the various price tiers of Meanwell switchers and consider cost/need ratio.
 
Wahrschau ...

ECC series tubes and the US equivalents (e.g. ECC8/6DJ8/6922) are not designed for series connection. It is not possible to guarantee correct sharing of the voltage. There were tubes specifically designed series connection of heaters, for the ECC88/6DJ8 the series heater connection version is PCC88/7DJ8.

Thor
Of course all bets are off for Russian made tubes in "current" production. Having said that, I have not had any issues with working pairs of EH6922s in series, but I would not add any more to the string.

Cheers

Ian
 
Wow, even at 1.2A, this doesn't work. Le sigh. Maybe I'm plugging errant info into PSUD2...or maybe ripple is over 4V coming off the trafo?

single supply.jpg

psud2 fail.png
 
Wow, even at 1.2A, this doesn't work. Le sigh. Maybe I'm plugging errant info into PSUD2...or maybe ripple is over 4V coming off the trafo?
The high ripple is caused by your reservoir cap having 1 ohms of ESR. That's way too conservative, especially when you have two caps in parallel in reality. Try 0.1 ohms.

I don't think you have enough headroom to use a MOSFET, since the gate-source voltage will likely be 3-4V. You're in BJT territory.
 
Uneven cath wear may be an issue with a direct-heated power triode, but not on a low-po indirect-heated tube.

No, but if one tube is at (say) 300mA @ 6.3V and the other at 400mA @ 6.3V, what is the result in series connection? Note that the heater is a tungsten filament with a temperature dependent resistance.

The tube that would like to get more current, will not get hot enough, so it's resistance will end up lower, which will make it's heater even colder and the one in the second tube even hotter.

Can you smell a potential "race condition" where one tube gets colder and colder and the other hotter and hotter? Where does it balance out.

Anyway, making the heater voltage balanced (referenced to what?) does not change anything to the supposed cathode wear, since the voltage gradient from one end to the other is the same.

I was merely talking about heaters designed to be driven by constant voltage (low source impedance) because they will stabilise this way in series.

In my experience, however, they do it accurately enough (at least if they are from the same manufacturer).

Yes. USUALLY it will work in practice.

Except when it doesn't.

😉 [EDIT to add: Does it matter if one heater in a string is 6.1V and one is 6.5V?]

Depends. The problem heater resistance is non-constant with temperature, heater power will depend on voltage and more voltage will make the filament hotter, raising it's resistance further while the second filament with less power will run colder and will thus have a lower filament resistance which in turn will mean it's resistance will be lower.

It is really a question of how big the actual variations are.

But, would you (for arguments sake) connect a 25W and 50W 6V light bulb (tungsten filament) in series and power them from 12V, because it is after all 6V+6V?

1/4 of HT, which is midway between the cathodes in a SRPP or WCF. That’s for minimizing stress between heater and cathode, rather than the (alleviated) concern about uneven heater wear.

Usually not a good idea with AC heaters (I know we are talking DC heaters here, but I thought I mention it). And normally even phono stages and microphone preamplifiers are fine with AC heaters IF they are constructed correctly.

Bottom line, run parallel filament tubes parallel, not series.

If that causes very high currents, you can always split heaters into multiple circuits, it is generally harmless.

Use AC where you can, tubes are designed that way.

Make sure to reverse-bias the parasitic cathode/heater diode which is what usually causes hum with AC heaters, even if correctly symmetric heaters.

If you must build totem poles using tubes, use a separate envelope and heater voltage for the upper tube, EXCEPT where your tube was explicitly designed to have significant opposite direction voltages. Often PCC tubes have heater structures more suited to such a setup and often it is specified if a tube needs to be in a specific position in the string.

Thor
 
Why sop complicated
Wow, even at 1.2A, this doesn't work. Le sigh. Maybe I'm plugging errant info into PSUD2...or maybe ripple is over 4V coming off the trafo?

Why so complicated?

An LM317 (times as many as you need in parallel) will do fine.

Cheap, indestructable.

Set 5.7V heater voltage for small signal tubes you will not exploit at more than 50% operational limit cathode current. It will allow longer tube life, with a slight reduction of initial transconductance, but a much flatter transconductance vs operation hours curve.

Include soft start (it's in the datasheet).

If the main PSU is external, I would put the 317 regulators for heaters into the signal chassis.

Thor
 
Dude calm down, it's two tubes of the same type in series. Millions have been run that way. It's fine.
 
Try 0.1 ohms.
Will do. If I have inrush current limiting (it’s a toroid), can I add a 100n X7R in parallel to bring that down even further? I see that people tend to not do that on the reservoir cap but i don’t quite know why.
I don't think you have enough headroom to use a MOSFET, since the gate-source voltage will likely be 3-4V. You're in BJT territory.
I thought the dropout for a depletion mosfet used as a source follower was just RdsOn * current. No? That part is like $8 in 10 qty btw, you really pay for the low resistance.
 
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Will do. If I have inrush current limiting (it’s a toroid), can I add a 100n X7R in parallel to bring that down even further?
This is not how it works. A 100nF caps does nothing at 100Hz.
However a 6800uF cap should have an ESR of about 30mohm (0.03 ohm).
I see that people tend to not do that on the reservoir cap but i don’t quite know why.
Because it's useless.
I thought the dropout for a depletion mosfet used as a source follower was just RdsOn * current.
That is correct, it drops about 1V at 1.8A..
 
Ive seen tube amps with series/parrallel heaters become problematic ,
Not only do you have the see-saw effect of the voltages across the heaters but that in turn has a knock on effect on the number of electrons available at the cathode , how much anode current flows in the tube , amplification factor is altered , etc , if its a push pull quad your completely bolloxed . Your amp will suck from day one and go downhill fast .

It amounts to a built in unbalance even with perfectly good tubes that will only get worse as time goes on, the tubes tended to age appart under differing operating conditions .


I dont like grounding one side of a DC heater supply in tube gear ,
I prefer to tie each side of the heater to ground or its reference voltage via resistor center tap and have its own discrete +/-connection back to the psu .
50 ohms each leg and its worth using the meter to select a matched pair .
I place that at the PSU output end with a very good 0V connection direct to chassis ,
 
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I agree completely it’s worth considering the various price tiers of Meanwell switchers and consider cost/need ratio.
My bench version of this 4-channel preamp is probably gonna be a Meanwell for heaters, a Meanwell for phantom, and a prebuilt Maida for the HT off of a basic 25-30VA trafo, so I can focus on the circuit itself. But ultimately I’d like to see if it’s convenient to do a full linear supply with just that Antek hexfilar and everything on one PCB.
 
, if its a push pull pair or quad your completely bolloxed . Your amp will suck from day one and go downhill fast .
V1A - Common cathode
V1B - Common cathode
V2 - SRPP

…FWIW. Also, at quiescence and normal operating levels, V1 and V2 currents are roughly the same. 7.5mA total for V1 and somewhere between 6.5mA and 7.5mA for V2, depending on how much I care about max level into 600R. Does this rough balance aid with roughly equal heater aging?
 
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An LM317 (times as many as you need in parallel) will do fine.
Probably two would do here. There was a lower dropout regulator mentioned elsewhere, MIC29502
Set 5.7V heater voltage for small signal tubes you will not exploit at more than 50% operational limit cathode current. It will allow longer tube life, with a slight reduction of initial transconductance, but a much flatter transconductance vs operation hours curve.
I was thinking 6.1V, but that’s a new creative point I hadn’t heard before, cheers. I’m not doing more than 600mW in any section.

Include soft start (it's in the datasheet).
I could do that within the cap mult too.
 
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I suppose I have to qualify what I said by adding it was in a power amp I saw these bad effects ,

maybe less of an issue in preamps ,
still though for the sake of a slightly larger Meanwell or transformer ,why bother with series parrallel heaters ,
 
for the sake of a slightly larger Meanwell or transformer ,why bother with series parrallel heaters ,
simplicity. same reason i’d rather not do the step-down-for-heaters-and-back-up-for-ht thing. but if i can get the single 12.6V supply dialed, i’m gonna take what i learned (namely those schottkys and mosfet from yesterday) and see if i can squeeze out enough volts for all parallel heaters.

also, i’m having a heck of a time finding toroidal options in the US for HT around 170V-180V (pre-rectifier)…even without heater windings. that 175 i’m using doesn’t exist in one size up or down. there’s toroid.com, but they only have two standard models. toroidy.pl is clearly a resource, but with the euro-to-dollar mismatch, i don’t even wanna know the shipping. i’m hesitant to ask for a custom until i’m really ready to order 10+. does anybody stateside come to mind?
 
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