Bipolar Cap Mult DC Heaters
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ruffrecords
Well-known member
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
merlin
Well-known member
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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.
Are you sure the DCR of the secondary is 0.892 ohms?
It accounts for a very large drop of about 4V.
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thor.zmt
Well-known member
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.
I was merely talking about heaters designed to be driven by constant voltage (low source impedance) because they will stabilise this way in series.
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?
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
thor.zmt
Well-known member
Why sop complicated
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
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
I didn't mention series connection. I was referring to the original schemo (posts 1 and 3).
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 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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I was extrapolating from another trafo. I will ask Antek and run this again.
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).
Because it's useless.
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 ,
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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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.
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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rock soderstrom
Tour de France
Interesting, I am also thinking about such a topology ATM, will you show the schematic?
Probably two would do here. There was a lower dropout regulator mentioned elsewhere, MIC29502
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.
I could do that within the cap mult too.
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Can of worms to post, but might someday once I iron out the trickier flourishes.
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 ,
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 ,
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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