I was hoping to get some ideas/recommendations on the DC heater supply for the 6922 preamp I'm building.
I was originally set upon the technique Joe Rasmussen uses in the tube buffered GC, an LM7808 feeding a series 5R6 resistor. Low parts count, addresses turn on surge, doesn't limit current (which some say is bad). But I have 4 tubes, so 300mA x 4 is more than 1 7808 can handle.
So, any favourite techniques out there? I MUCH prefer sand for this.
I was originally set upon the technique Joe Rasmussen uses in the tube buffered GC, an LM7808 feeding a series 5R6 resistor. Low parts count, addresses turn on surge, doesn't limit current (which some say is bad). But I have 4 tubes, so 300mA x 4 is more than 1 7808 can handle.
So, any favourite techniques out there? I MUCH prefer sand for this.
Thanks SY.
I don't think I will go that route though, I have a transformer already and its winding is too low in voltage to put them in series.
So I guess you don't buy the current-limiting-is-bad argument. Any pitfalls to just using an LM338 to regulate to 8VDC and using 4 resistors, 1 to each heater?
I don't think I will go that route though, I have a transformer already and its winding is too low in voltage to put them in series.
So I guess you don't buy the current-limiting-is-bad argument. Any pitfalls to just using an LM338 to regulate to 8VDC and using 4 resistors, 1 to each heater?
leadbelly,
You can use the trafo you have and a series heater string. Rig up a "full wave" doubler from 2X Schottky diodes. You'll do fine, as long as the caps. in the doubler stack are good sized. TANSTAFL, you get 2X the Volts and 0.5X the Amps. The RMS current rating of the trafo should be 4X the DC draw at the doubled voltage.
BTW, Schottky diodes in the filament rectifier are a good idea no matter what topology you use. Stamp out switching noise.
You can use the trafo you have and a series heater string. Rig up a "full wave" doubler from 2X Schottky diodes. You'll do fine, as long as the caps. in the doubler stack are good sized. TANSTAFL, you get 2X the Volts and 0.5X the Amps. The RMS current rating of the trafo should be 4X the DC draw at the doubled voltage.
BTW, Schottky diodes in the filament rectifier are a good idea no matter what topology you use. Stamp out switching noise.
Please check my CCS method,
http://www.headphoneamp.co.kr/bbs/zboard.php?id=diy_sijosae&no=217
http://www.headphoneamp.co.kr/bbs/zboard.php?id=diy_sijosae&no=217
Use LM350T, heatsink it, adjust the right voltage with two fixed resistors and use two big electrolytics on the input and on the output of the regulator (>20000uF) and pair of protection diodes over the chip.
But the truth is AC heating - more musical, more 3D and less compressed sound - especially for 6922 tube.
But the truth is AC heating - more musical, more 3D and less compressed sound - especially for 6922 tube.
I have built about 10 preamps and headphone amps with 6922 and 5687 tubes and all they sound better with AC heating and center grounded pot. I don't know why, but the difference is huge:
- AC - musical sound and 3D soundstage
- filtered DC - dull sound and flat soundstage (with >30000uF caps used the soundstage is getting better but not much to like it)
- regulated DC - dull sound and compressed dynamics
I'll be very grateful if somebody explains this phenomenon.
- AC - musical sound and 3D soundstage
- filtered DC - dull sound and flat soundstage (with >30000uF caps used the soundstage is getting better but not much to like it)
- regulated DC - dull sound and compressed dynamics
I'll be very grateful if somebody explains this phenomenon.
reaction said:
fdegrove said:
Hi, rather that start a new thread, I thought I would follow up on this one.
I plan to start a E88CC/6922 SRPP preamp with EZ81 and I am wondering about the heaters.
I did an extensive search and there are a few people who say that the 6922 does not need DC heaters and that AC sounds better. Is there any fact to this?
My transformer is ~6.3VAC so voltage reg. will not work with LM317/LM350 (too much voltage drop). However, current regulation is possible. Otherwise, DC with CRC... filtering is also possible.
Perhaps I should keep it simple and start with AC and change only if required. What do others think?
Cheers,
Gio.
Well, since this was originally my thread, I'd thought I should at least comment.
1) That preamp I built with 6922's was dead quiet with DC CCS fed heaters.
2) I am finding out the hard way that at least some new russian tubes, which I use a lot, hum like the dickens on AC heaters even with careful wiring, routing, etc. So I don't think I'll attempt AC heaters again until I have an all-NOS project.
1) That preamp I built with 6922's was dead quiet with DC CCS fed heaters.
2) I am finding out the hard way that at least some new russian tubes, which I use a lot, hum like the dickens on AC heaters even with careful wiring, routing, etc. So I don't think I'll attempt AC heaters again until I have an all-NOS project.
leadbelly,
Thanks for the reply.
How did you implement the CCS? I have a 6.3VAC winding, so after rectification, 8V. Did you drop the voltage using CRC before going into a regulator?
With your CCS DC heaters, did you use one LM317 per tube? Otherwise, if you go with one regulator and one tube heater goes out, the other will see a pretty large current.
The tubes I have are new E88CC/6922 made in Slovakia.
Any chance you could share some additional information about your heater circuit?
Cheers,
Gio.
Thanks for the reply.
leadbelly said:
How did you implement the CCS? I have a 6.3VAC winding, so after rectification, 8V. Did you drop the voltage using CRC before going into a regulator?
leadbelly said:
With your CCS DC heaters, did you use one LM317 per tube? Otherwise, if you go with one regulator and one tube heater goes out, the other will see a pretty large current.
The tubes I have are new E88CC/6922 made in Slovakia.
Any chance you could share some additional information about your heater circuit?
Cheers,
Gio.
Attachments
gmilitano said:
My heater circuit was a plain-jane 1 LM317 per 6922, no voltage dropping stage at all. Can't remember what the unregulated voltage was beforehand, it doesn't matter much, the tube will take care of itself. For your 6.3V winding, I would use a voltage doubler and a really big cap with the LM317, or I would try full wave and an LT1086.
rdf said:
I played with it quite a bit, including tweaking the routing and DC biasing. I gave up. I don't have enough experience and enough tubes to be 100% sure that the issue is new russian tubes, but it sure seems to be.
EDIT: To be clear, I never tried the Sovtek 6922 with AC heaters, I built them with DC heaters off the bat. It was only Sovtek 6H8C that I gave up on AC heaters with.
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