Hi All,
Andy, I was pondering about the extra $90 US or so to upgrade from the Hammond to the Lundahl and if there was appreciable gain in sound quality, but I might just wait and get the Lundahl
Nicoch, I was considering getting some CMC’s wound, but I had a look in the shed and noticed I had some semi vintage A&R Australian made 1.5H 4 ohm 500mA chokes. If run in parallel in the + leg (750mH 1A) they may be useful in trying a passive LCL filament supply.
My current 26 filament supply has 60mH (2.5ohm) 36,000uF (ccs regulated). I modelled a combination of these chokes in psud2 under the following conditions:
13.5V 5A XF – 1N5822 – 60mH (2.5ohm) – 36KuF – 750mH (2ohm) – 10nF = 8.5V
I ended up getting 400mV ripple at the end. Thomas is using a similar supply with 2 LL2733’s in series. I modelled his psu and it also yields the same ripple at the end. I used 10nF to simulate the 26’s filament. If I get some time this weekend I’ll try it out
But…
I have a question in regard to the potential damage to the filament if the filament supply is on without any B+, since filament bias applies approx 6 times the filament voltage. Forgive my ignorance but in order to get the correct bias voltage and plate current (6mA) one needs to apply filament (bias) voltage (9V) and B+ (150V)? Any advice to clarify this would be most appreciated.
Rich
Andy, I was pondering about the extra $90 US or so to upgrade from the Hammond to the Lundahl and if there was appreciable gain in sound quality, but I might just wait and get the Lundahl
Nicoch, I was considering getting some CMC’s wound, but I had a look in the shed and noticed I had some semi vintage A&R Australian made 1.5H 4 ohm 500mA chokes. If run in parallel in the + leg (750mH 1A) they may be useful in trying a passive LCL filament supply.
My current 26 filament supply has 60mH (2.5ohm) 36,000uF (ccs regulated). I modelled a combination of these chokes in psud2 under the following conditions:
13.5V 5A XF – 1N5822 – 60mH (2.5ohm) – 36KuF – 750mH (2ohm) – 10nF = 8.5V
I ended up getting 400mV ripple at the end. Thomas is using a similar supply with 2 LL2733’s in series. I modelled his psu and it also yields the same ripple at the end. I used 10nF to simulate the 26’s filament. If I get some time this weekend I’ll try it out
But…
I have a question in regard to the potential damage to the filament if the filament supply is on without any B+, since filament bias applies approx 6 times the filament voltage. Forgive my ignorance but in order to get the correct bias voltage and plate current (6mA) one needs to apply filament (bias) voltage (9V) and B+ (150V)? Any advice to clarify this would be most appreciated.
Rich
Richard, when using filament bias, the resistor wired in series with the filament produces the voltage to bias the tube at your chosen point. There will never be 6 times the filament voltage applied to the filament. I can't see any issues arising from running without B+.
Because the bias, plate load and the B+ you apply are fixed, the only variable is plate current, which will settle to a value consistent with the other fixed parameters (refer to datasheet plate curves see what it will be).
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Richard,
Don't rule out the 01A as an alternative with low filament current 250mA - would suit your choke. It's very, very good - only very slightly bettered by the 26. My rough calculations for filament bias for this was 105v on anode from a 105v glow tube. 3mA through the tube, 18 ohm cathode resistor at 1 watt and 4.5v on the cathode (top of resistor). I tried this out and was very impressed - sound was very lively and engaging, just not quite as subtle as the 26. Supply voltage about 9.5v.
Andy
Don't rule out the 01A as an alternative with low filament current 250mA - would suit your choke. It's very, very good - only very slightly bettered by the 26. My rough calculations for filament bias for this was 105v on anode from a 105v glow tube. 3mA through the tube, 18 ohm cathode resistor at 1 watt and 4.5v on the cathode (top of resistor). I tried this out and was very impressed - sound was very lively and engaging, just not quite as subtle as the 26. Supply voltage about 9.5v.
Andy
Here is my experience with filament bias. It is pretty good and easy use method. Easiest I suppose. Gives an impressive boost to the sound than auto bias option. But the winner is grid bias for me. I have no doubts because I have tried all three methods in my 26 pre and grid biasing gave me the best results so far. It gave more control of changing tube parameters and sound is exceptionally life like and accurate. Andy, I would suggest you to try this if you have not tried it yet. Do not know whether the cap in the signal path does anything to the sound but I heard most accurate sounds of instruments out of this method. Vocals are simply magical.
Also I have changed my filament supply from SMPS to a TRAFO+LM317. This added some more to the overall result. I had a filament PSU kit in hand (JIMS audio@Ebay) and it worked perfectly. I wonder why I didn't try it before. Slight humming sound that SMPS was giving is now completely gone and it is dead silent now.
BTW, I found a new use for my 50W resister. I used it as heat sink for my HVPSU (SALAS) IRF840. It really worked well.
Also I have changed my filament supply from SMPS to a TRAFO+LM317. This added some more to the overall result. I had a filament PSU kit in hand (JIMS audio@Ebay) and it worked perfectly. I wonder why I didn't try it before
. Slight humming sound that SMPS was giving is now completely gone and it is dead silent now.BTW, I found a new use for my 50W resister. I used it as heat sink for my HVPSU (SALAS) IRF840
. It really worked well.
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Hi All,
Andy, I was pondering about the extra $90 US or so to upgrade from the Hammond to the Lundahl and if there was appreciable gain in sound quality, but I might just wait and get the Lundahl
<snip>
Rich
Save your money for the Lundahls, they are in most cases better designed and better made than Hammonds. IMHO This should be audible and measurable. If cost is an over bearing consideration you might want to look at Electra-Prints offerings as well - I've generally found them better sounding than their Hammond equivalents in my very limited experience.
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No problem applying filament voltage and bias prior to plate voltage, what you need to avoid is applying plate voltage without bias being present however derived.
Conventional cathode bias is safe too as bias will be developed almost immediately when plate current starts to flow.
The filament derived bias discussed here is perfectly fine IMHO and sonically far superior to conventional cathode bias, that said the idea of 9W of power in a resistor to create the bias for a 6mA operating point seems well.. inefficient..
Note that you should try to operate the 26 filament at as close to 1.05A/1.5V as reaonably possible no less than -10% is ok, but anything less and the transconductance starts to drop pretty drastically and in my long (10+ yr) experience with this tube both compromises tube life and most directly sound quality.
I much prefer grid bias with the 26/12/01 and a very high quality coupling cap. (I use Vcaps, but the Rel TFT is also quite good) 9V alkaline batteries can provide great service in this application and last many years. Do not use a rechargeable battery for grid bias!
My original 26 pre-amp article is still available at Positive Feedback Online, should be some insight for those interested in such things. (The design is now actually over 10 years old, although the article is more recent. More current versions of that design have been well received in Asia. Coolzero and I have chatted extensively about the 26 over the last year or so..)
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Joined 2006
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Hi Rod,
You posted the FET filament supply circuit back in Nov last year, but I seem to recall that there was a resistor missing in that circuit. (pardon me if that's not the case...)
Anyway, could you post the latest circuit again? And any plan for a kit on this soon?
Actually I am also interested in using 01A sometime in the future, and in that case, what would need to be changed?
Thanks for your help!
Cheung
You posted the FET filament supply circuit back in Nov last year, but I seem to recall that there was a resistor missing in that circuit. (pardon me if that's not the case...)
Anyway, could you post the latest circuit again? And any plan for a kit on this soon?
Actually I am also interested in using 01A sometime in the future, and in that case, what would need to be changed?
Thanks for your help!
Cheung
Hi,
This looks a bit foreign to my simple eyes. I can see -9v bias, but where does this go to ground? In the 100K pot that would precede this? Where is the input resistor, grid to ground?
Also, with 150v on the anode, 9v bias would give you over 7mA which is a bit strong isn't it?
Can you explain the circuit for my help!
Andy
This looks a bit foreign to my simple eyes. I can see -9v bias, but where does this go to ground? In the 100K pot that would precede this? Where is the input resistor, grid to ground?
Also, with 150v on the anode, 9v bias would give you over 7mA which is a bit strong isn't it?
Can you explain the circuit for my help!
Andy
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Hey Andy,
This was only done to show the principle of how it could be done. You are free to use whatever Ia or Ua you like. The only problem is that we can´t choose battery voltage so easy as the Lithiums are 3V.
As there is no grid current we can skip the cap.
Didn´t add the gridleak resistor as I thought it was so obvious it should be there. But it could also be the 100k pot you mentioned.
Below is how a 3A5 could be done. But I would go for a 4:1 on that one.
Just remembered that T recommended Silvermica for the 10n.
Attachments
Hi Kevin,
Parafeed is still lingering in my mind...
Was wondering if you have tried this topology in your 26 preamp?
Someone said sound coming from traditional series connection is more natural, while parafeed brings in more clarity but more towards ss kind of sound....
Thanks for sharing!
I haven't tried it in my own, I have heard it in other applications, and an adaptation of my design. I'm really on the fence about the parafeed trend particularly in the case of line level applications, IMHO all of the disadvantages combined of chokes and transformers without the cardinal virtue of not running the BH curve of the output magnetics through the 0 crossing. (To some degree the use of exotic core materials can help a lot here.) In addition you have introduced the need for an additional coupling capacitor.. I've heard parafeed circuits that sounded good, but in general I prefer a really high quality SE transformer with a hefty primary dc current through it. Some small parafeed transformers made of extremely exotic core materials I have heard sounded great..(And were not IMHO good enough to justify their obscene price difference with a good conventional less exotic SE constructed with a similar level of care.) I am really not sure about the vaunted clarity advantage of parafeed per se, certainly haven't heard it frankly, but I think if you are not careful you can end up with significant odd order distortion at the 0 crossing of the BH curve just like in a good push pull transformer.
This is all very personal opinion in fairness and someone else's experiences and preferences can and should be different, and legitimately so.. So what works for me may not be the case for you and vice versa..
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Certainly not original to Thorsten, I and others in my immediate audio circle experimented with this as long as 10yrs ago and I found the sound not to my liking, by the time it was the battery bypass cap was of comparable size and quality to what I would have stuck in there in the first place with conventional grid bias, and the problem gets worse the higher the bias voltage required. The sound quality seems to be very dependent on the electrical characteristics/internal construction of the battery chosen, and with the 26 an effective bias depending on plate voltage (6mA) varies from -8 to - 10V and multiple cells in series seem to compound the problem.
I experimented with fixed bias using NiMH and NiCad batteries in the cathode circuit and found this sonically inferior to LED bias in most if not all cases. (I use LED bias, but no batteries currently in sight.) The Sanyo Cadnica batteries seem to have the lowest internal impedance of the batteries I tried and sounded pretty good compared to anything else I tried, but a simple LED trumped them too.. IMO..
Again my comments are preference/experience driven, I think at some point everyone should just do these sorts experiments and decide for themselves. A really good battery (varta?/Sanyo) should make a significant difference in outcome. IF nothing else the learning experience is FUN!
FWIW: I'm currently listening to Morris Day on Vinyl on a singularly unromantic and revealing SME 20/2 so what I do I know..
(I love it)Edit: Someone might want to try LED bias with the 26 in fact - should not be too hard to arrange..
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Hi Kevin,
I'm not sure what your preference is here - LED bias in the cathode or battery bias on the grid. I tried LED bias and found it rather hard and unforgiving, though very clear and bass was particularly good. Didn't have those smooth liquid mids as I implemented it. I haven't tried grid bias yet. You say grid bias is better than filament bias, but how does it compare with LEDs sonically, as you hear it?
Andy
I'm not sure what your preference is here - LED bias in the cathode or battery bias on the grid. I tried LED bias and found it rather hard and unforgiving, though very clear and bass was particularly good. Didn't have those smooth liquid mids as I implemented it. I haven't tried grid bias yet. You say grid bias is better than filament bias, but how does it compare with LEDs sonically, as you hear it?
Andy
Sorry, I seem to have muddied the waters a bit.. Yes, I am strongly in favor of absolutely conventional (fixed) grid bias with the 26 and all other DHTs, and use it extensively in my own designs. IMLE, yes grid bias is superior to filament based bias and I strongly encourage you to implement.. Easiest is a a pair of 9V batteries - one for each channel will last many years..
Andy,
I tried Mundorf M cap supreme, Russian PIO K75 and Auricap. Mundorf is mile ahead Auricaps comparing the sound. Well worth the money. PIO's are almost similar to Mundorf but marginally Mundorf wins the place because it’s overall performance. But PIO’s are much cheaper solution. Probably you can try Mundorf Silver oils for better results. I am using 0.15u for input cap (check the schematic I posted before). My initial choice was alkaline 9V batteries for bias supply which I recently replaced with a 15V SMPS. This gives me a room to adjust bias using a pot to experiment with different op points without changing the circuit. This goes well with SALAS HV PSU which has an adjustable B+. I havent noticed much of a difference (out of 2 or 3 I tried) with different brands of batteries for bias, but I thnk any decent 9V alkaline like Eveready Energizer would do the Job.
As Kevin mentioned, I tried all these with his kind assistance as well as many other’s valuable advices came through this forum. Thanks to all, efforts well worth after all. For sure, 26 is one certain way of having million dollar sound for a few hundred bucks.
Try the grid bias and see. You will understand.
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