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Old 3rd May 2010, 08:01 PM   #1
rongon is offline rongon  United States
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Default DC Current-Regulated 300B filaments?

Hello there...

I'm thinking of building a new power amp. Will most likely use PP parallel 2A3's or PP 300B's. DHT's, of course. I would like to keep the hum down this time. I was reading some posts, found that voltage-regulated DC for DHT filaments doesn't sound good to most. Most prefer current-regulated DC for this.

Can anyone recommend a simple-but-decent DC current regulator circuit for something like 5VDC @ 2.5A (pair of 300B)? Or 2.5VDC @ 10A (quad of 2A3)?

Many thanks...
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Old 3rd May 2010, 09:41 PM   #2
tomchr is offline tomchr  United States
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I'm using an LM317 to power the filament of one JJ 300B. Set R1=120 Ohm; R2=360 Ohm and you're set (see fig. 1 of the data sheet). The LM317 will provide 1.5 A with proper heatsinking.

For higher output currents, look into the LM338. The LM338 will deliver up to 5 A with proper heatsinking. Again R1=120 Ohm; R2 = 360 Ohm for 5.0 V out.

~Tom
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Old 3rd May 2010, 09:56 PM   #3
Arnulf is offline Arnulf  Europe
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He's looking for current regulator. LM317 can be used in this role as well, put 1R (yes, one ohm) between output and adjust pin and connect load to the junction of resistor and adjust pin for 1.25A (a single tube). This will allow you to distribute the heat around by using multiple regulators.

A higher current version (LM338/LM350) should be substituted in the case of higher output current and current sense resistor should be appropriately adjusted.
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Old 4th May 2010, 01:18 AM   #4
tomchr is offline tomchr  United States
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Arnulf: It isn't clear to me if he wants a voltage regulator or a current regulator. I assumed a voltage regulator as the specs are given in volts.

But for a constant current source, the circuits you mention will work fine. One catch, though: The resistor from the output to the adjust pin (the one that carries the load current) will need to be a power resistor. I'd calculate Presistor = 2*1.25*Iout and round off to the nearest higher power rating. I use a factor of 2 to give a little margin on the resistor power. Expect the resistor to be at least 80~100 deg C even with this margin.

~Tom
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Old 4th May 2010, 03:53 AM   #5
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AAMOF, DC heating may be unnecessary, as PP topology is being employed. Cancellation of filament hum, in the O/P trafo, will occur. This is particularly true of the 2A3, which is known to work well SE, with AC heating.
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Old 4th May 2010, 04:33 PM   #6
kevinkr is offline kevinkr  United States
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I made the mistake of using AC heating in a line of commercial 300B PP stereo amps I manufactured about a decade ago, shortly after their release into the wild I had to recall and start retrofitting the units with dc filament heating in the output stages. The level of output "buzz" was a function of how closely the tubes stayed matched over time and the owner's willingness to adjust the filament hum null pots periodically. It proved entirely unsatisfactory..

I have found however that 2.5V dhts like the 45 and 2A3 work fine in both SE and PP amplifiers I have designed with AC heating. Higher filament voltages seem to be at the root of the problem, differing filament construction (symmettry?) may play a limited role as well.
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Old 4th May 2010, 06:07 PM   #7
rongon is offline rongon  United States
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Thanks for the replies.

A couple of people have mentioned that voltage regulated DC filament supplies can sound 'cold' or 'harsh.' I saw something KYW wrote, summing up DC vs. AC filaments for DHTs -- to paraphrase: softer, more distorted sound of AC fils can be compensated for by use of more aggressive sounding passive parts, while DC fils with more aggressive sound can be compensated for by using softer sounding passive parts.

However, I did build a PP 2A3 amp (two stages DC coupled, 6 watts per ch, but needs some gain from the preamp). I had some hum problems but a hum balancing pot in the filament of each pair of 2A3's fixed that well enough for my purposes. But there's still a tiny bit of hum down in there. I suppose it would be the same with 300B's, but I wanted to try to do better this time.

So let's say I use an LM338 as a 5A current regulator.

I have a 6.3V 5A winding on the power transformer. That's just enough current for four 300B filaments (1.25A * 4). Rectification minus voltage drop across the diode bridge would yield about 7.5 to 7.8V, correct?

So, how much voltage drop would I expect across the LM338 when used as a current regulator? About 2.5V? That right there would get me to my 5V for the 300B's.
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Old 4th May 2010, 06:15 PM   #8
rongon is offline rongon  United States
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Quote:
shortly after their release into the wild I had to recall and start retrofitting the units with dc filament heating in the output stages.
Kevin, I hope this is OK -- can I ask what type of DC supply you used for the 300B filaments? Anything special? Passive w/ choke? Some kind of SS voltage regulator?
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Old 4th May 2010, 06:34 PM   #9
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by rongon View Post
A couple of people have mentioned that voltage regulated DC filament supplies can sound 'cold' or 'harsh.' I saw something KYW wrote, summing up DC vs. AC filaments for DHTs -- to paraphrase: softer, more distorted sound of AC fils can be compensated for by use of more aggressive sounding passive parts, while DC fils with more aggressive sound can be compensated for by using softer sounding passive parts.
Yes ... and Homer Simpson said: d'oh !

Quote:
I have a 6.3V 5A winding on the power transformer. That's just enough current for four 300B filaments (1.25A * 4). Rectification minus voltage drop across the diode bridge would yield about 7.5 to 7.8V, correct?

So, how much voltage drop would I expect across the LM338 when used as a current regulator? About 2.5V? That right there would get me to my 5V for the 300B's.
"Under 3V", according to datasheet. Pretty realistic figure, but then again you're forgetting the 1.25V reference drop that must also be subtracted from total. As in: not going to happen in your case, you're approximately one volt short even if you used schotty rectifiers

Hooking up multiple filaments in parallel to a single CCS seems like an extremely bad idea anyway since they weren't manufactured for series operation and might therefore differ significantly in their parameters and therefore also in current consumption (and consequently emission). Guess what happens when the most thermally stressed filament (one of them will always take the brunt at the pwoer-up) gives up with such arrangement ? Why yes, the 5A current is shared by three remaining filaments, provided that CCS's supply rail is high enough (*). When next filament moves south there remaining two share 5A. Then another one goes farming and it's just last one dealing with 5A until it goes up in smoke as well

*: CCS only has to supply something in excess of N*If current, where N = number of filaments still alive and If = current each filament requires to keep the cascade going, it needn't (and with your supply arrangement indeed couldn't) actually supply full 5A until the bitter end to take out all four filaments.
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Old 4th May 2010, 06:56 PM   #10
tomchr is offline tomchr  United States
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Using a 6.3 V winding may be a bit optimistic. I'd use the following worst-case estimate for the drop-out voltage:

Vrectified = 0.9*Vnominal*1.3-2*Vd = 0.9*6.3*1.3-2*0.7 = 5.97 V.
--> Vdropout = 5.97-5 = 0.97 V.

The mains voltage may vary +/-10 % in many places, hence the factor of 0.9 for worst case. I use a factor of 1.3 rather than the theoretical sqrt(2) to calculate the rectified voltage. After all the conduction angle is non-zero. A diode drop of 0.7 V is assuming a fairly beefy power diode. Note that many diodes approach 1.0 V drop at high currents due to their internal resistance.

With 970 mV available for the drop-out, you're looking at something other than an LM317/338 as these require about 2.5~3 V drop if used as voltage regulators, 3.75~4.25 V if used as current sources. Linear Technology has some regulators that can work with the low 970 mV drop-out, but they're quite pricey ($7~8/each). I think Sharp has one too.

Personally, I'd use one LM317 per 300B and run it as a voltage regulator. The 1.5 A current limit of the LM317 will provide a controlled in-rush current. I'd power four 300B's with a 9 V, 50~60 VA transformer. The transformers from Antek are quite good and very inexpensive. If you insist on using a current source to power the filaments, I'd probably go with LM317's and a 10 V, 50~60 VA transformer.

Note that 3~4 W will be dissipated in each LM317. You'll need to provide adequate heatsinking to get rid of the heat.

~Tom

Last edited by tomchr; 4th May 2010 at 06:58 PM.
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