My attempted repair of the broken choke failed. The magnet wire was frayed and broken in a couple of spots, and the lead ends that were still there were far too short to resolder. I have one good one to experiment with, but it looks like I will have to either use the Hammonds or do without for stereo.
No worries, you can remove/cut coil rapping paper and get hold of the broken wire. After repair you can fix the paper with glue.
This is a simple amp, for POC (prof of concept) you don't really really need any choke and such things. Supply voltage is only +/-275 and load current is +/-40mA, for this requirement this much headache will slow your project.
My advice: Go ahead build it with whatever parts are available with you listen carefully for 2/3 weeks and then if you like the sound rebuild the same using better/appropriate parts onward teak the circuit for finals. For POC build you need only two iron PT and OPT-that's it.
Regards
This is a simple amp, for POC (prof of concept) you don't really really need any choke and such things. Supply voltage is only +/-275 and load current is +/-40mA, for this requirement this much headache will slow your project.
My advice: Go ahead build it with whatever parts are available with you listen carefully for 2/3 weeks and then if you like the sound rebuild the same using better/appropriate parts onward teak the circuit for finals. For POC build you need only two iron PT and OPT-that's it.
Regards
Yes, I will start simple and not bother with the chokes for now. I did manage to repair the broken choke, but it’s not an exact match to the other. How close do they need to be? One is 2240 Ohms and the repaired one is 2070 Ohms. Using the same method as earlier to approximate the inductance, the repaired one seems to be around 250H, while the other is 350H.
I guess I was confused about your intentions re the chokes. I assumed they were to be used in the PS, which is why I was questioning how such a small choke could possibly measure hundreds of H unless its current rating was very low, which would make it useless in the PS.
If you're thinking of them as plate chokes, I personally wouldn't consider using two such disparate values even if they were in good condition. They really need to have the same specs. As minhaj says, just build it with a resistor load first.
The resistor will need to be a higher value than the DCR of whatever plate choke you're considering. Typically 3x to 5x the plate resistance (maybe more?) is used. If you switch to a choke later you can then lower the supply voltage in order to maintain the same operating point.
minhaj, could you please explain your rule of thumb: 20H per Ri? I'm not sure what you mean by Ri.
I'm using the cheap (~$15) Hammond 156Cs because I wanted to experiment with choke loading. They're 150H with maximum 8 mA and seem to work well.
While I haven't researched it extensively, it seems that cost increases tremendously if you want to step up to anything better. I'm all about "bang for the buck" so I tend to shy away from paying 5x to 10x more for something that's twice as good.
If you have any suggestions for other plate chokes that offer similar value, I'd love to hear your suggestions. Please post links if you can.
I recall reading that Andy tried using a pair of 156Cs in series but that it introduced a lot of hum. Not sure why.
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I don't know about "a lot of hum" but this particular choke seems a little hum prone depending on placement. It has been used in a "humbucking" setup where 2 are linked bottom to bottom and connected out of phase. Using 2 in series also sounds a little better.
I've used a few Hammond chokes. The 126C is quite nice - it's actually an interstage but I was using the primary as a plate choke. Then there's the 157G which Hagerman uses in his headphone amp. There's a few more useful ones, like 155C and maybe 158L.
''20H per Ri? I'm not sure what you mean by Ri.''
Sorry, should have phrased as 20H/1k of Ri (internal plate resistance).
For 26 Ri is more or less 8K as such inductance of the coil should be 160H minimum.
For 26/other small signal dht while implementing filament bias if somebody connects 10ohms R from filament to ground and applies filament bias voltage across filament pins the tube should not take long time to die.
Regards
Sorry, should have phrased as 20H/1k of Ri (internal plate resistance).
For 26 Ri is more or less 8K as such inductance of the coil should be 160H minimum.
For 26/other small signal dht while implementing filament bias if somebody connects 10ohms R from filament to ground and applies filament bias voltage across filament pins the tube should not take long time to die.
Regards
Thanks for the clarification about hum and the 156C.
The 126C costs about 5x more than the 156C. Is it that much better?
Looking at the specs, the only advantage I see is that it can handle more current than the 156C (15ma vs 8mA) and the DCR is much lower (only 375 ohms vs 3700 ohms) although I'm not sure how important that is. But it actually has less inductance (only 106H vs 150H).
The others you mentioned look more like PS chokes to me.
Thanks for the explanation. So it looks like the cheap little 156C is pretty close at 150H. It's main limitation seems to be its 8 mA current limit. I'm only running ~5.5 mA so it works in this situation.
Two in series will increase the inductance to 300H but the current rating wouldn't change, correct?
"if somebody connects 10ohms R from filament to ground and applies filament bias voltage across filament pins the tube should not take long time to die."
Why?
Do you know any technical information, that we don't?
BTW, I used my #26 preamp for five or six years (daily 4-5 hour listening) in this arrangement without any problem.
The globe Majestic #26 tubes (from 1934) as good as at beginning.
My graphite anode 801 (from that time also) tubes also living after 5 years continuous operation in filament biased preamp.
Why?
Do you know any technical information, that we don't?
BTW, I used my #26 preamp for five or six years (daily 4-5 hour listening) in this arrangement without any problem.
The globe Majestic #26 tubes (from 1934) as good as at beginning.
My graphite anode 801 (from that time also) tubes also living after 5 years continuous operation in filament biased preamp.
A new development. Yesterday I received an Aphex 124A 10/4 Audio Level Interface that allows me to go back and forth between pro level XLR and home consumer level RCA connections. While fiddling with the 124A and my XTA DP200, I figured out that my XTA DP200 could deal with with the level and gain issues that were causing noise in my speakers without using the 124A. This noise is due to too much gain. This got me to thinking. If I have such an excess of signal available in my setup, are there other very low amplification DHTs that haven't been considered so far that could work as driver tubes in my particular application?
Rod Coleman regulator has constant current.
In the case of #26 this must be set to 1.05 A (+/- xxmV).
It's irrelevant, that R.C. has 7V (not filament biased) or 15..20V input PSU, the tube's filament "sees" only the CCS current.
Please excuse me may be I am wrong.
Let us assume filament is across pin 1 and pin 2. For filament bias we connect + lead of filament supply to pin 1, ground pin 2 with a 10ohms resistor and connect -lead of filament supply to ground. Now if somebody accidentally or otherwise connect + lead of filament supply to pin 1 and -lead of filament supply to pin 2 while pin 2 is connected to ground via a 10ohms resistor excessive current will flow through the DHT.
Regards
Let us assume filament is across pin 1 and pin 2. For filament bias we connect + lead of filament supply to pin 1, ground pin 2 with a 10ohms resistor and connect -lead of filament supply to ground. Now if somebody accidentally or otherwise connect + lead of filament supply to pin 1 and -lead of filament supply to pin 2 while pin 2 is connected to ground via a 10ohms resistor excessive current will flow through the DHT.
Regards