#26 pre amp
I am in the process of completing Phaeton RC 26 preamp published on http://www.vt52.com/ site (Jim de Kort) as my latest DIY project. But still I am little puzzled with few things with my current project. Hope someone could provide an expert advice to me to figure out those.
One thing I am confused with is the hum coming out from B+. I figure out it is something to do with the grounding however. Lately I found out that if I ground the filament negative with B+ negative with 470uf capacitor, the hum is almost disappearing. But not sure whether this method is right or safe because what I did was sheer coincidental . Need advice on this. I used separate power rail using a SMPS for filament and both filaments were connected serially with 3V supply. Only connection for B+ negative from this rail is 470uf negative to negative (capacitor + to filament's negative and capacitor - to B+ negative).
Second thing I am puzzle with is the bias supply. Should I use separate 12 V supply for bias for 26 tube or should I use voltage divider to get the bias supply? Where exactly should I connect the bias voltage in this schematic?
Greatly appreciate if someone could clarify these for me.
Added as requested by djn -
Jim de Kort's site
Kevin Kennedy's PF article
Keven Kennedy's Schematic PDF
Where to begin? At the beginning of this thread
mach1 sneaks in a 12B4A schematic
Richard's double-fisted 156C choke load
coolzero's hum avoidance diagram
andyjevans's 10Y preamp
regal's 26 driving 71A
Rod Coleman's DHT linestage
Rod Coleman's DHT linestage with power supply
andyjevans 26 with AZ1 rectifier
mogliaa's 26 testbed
mogliaa's 26 source follower
andyjevans still can't find the rotate 90į command
Rod Coleman's 26 with a filament-on-filament bias with IDHT output
mogliaa version 2
dhtrob 2 (now with grilled pencils)
andyjevans 26 with VR tubes.
andyjevans 26 with VR tubes and CCS
The 26 is notorious for humming if the filament supply is not up to scratch.
The cap from negative filament to ground is safe.
What bias voltage do you mean? The design you posted is auto-bias. Therefore the bias you refer to is probably the voltage across the cathode resistor.
The hum is ripple in your filament supply. What you did "by mistake" was twofold. You "star grounded" and added a "C" filter.
You can actually incorporate this into your filament supply circuit.
Maybe increase the value of that capacitor also. 1,000uF??
Make sure the B+ capacitors and chokes are of good quality, but I doubt the hum is coming from there.
By the looks of it the "bias" of that tube is actually more around -15V (~6.2ma thru a 2K2 cathode resistor.
It appears he took the operating point for the 26 straight out of the RCA manual. Vp 180 Grid Bias -14.5V Current 6.2mA
What you might consider is placing the 2K2 Cathode resistor and the 470uf on the center tap of a 50 Ohm 1W potentiometer. The two outer tabs of the potentiometer then go to the filament pins.
This is a "hum balance" pot.
Check your voltages proper operation should be at idle (short the input)
~194 - 195V at plate connection to ground
and pretty close to -14.5V grid to ground.
He is running the 26 kind of "hot" the other designs I have seen show it running more at Vp135 current 5.5mA
Find a copy of RCA RC-10 and look at specs.
Good luck, I am working on a 27 preamp right now.
Thanks BH. I am suspecting grounding because as soon as I put the cap, hum is disappearing almost to zero. I ordered two of IXCY CCS from Kevin@K&K audio (http://www.kandkaudio.com/accessories.html) to replace the 33K anode resistor which I change to increase the gain a little bit. I tried simple CCS using a IRFBC840 and it gave good result. 150V B+ gave the best sound out of this schematic. Now I can enjoy it without hum since you help me to clear my doubt. Thanks again.
Thanks coldcathose for all the suggestions. Sorry for my limited knowledge of electronics first of all. I haven't had much of an idea of what I was doing by connecting cap from filament negative to B+ negative, that is the reason I told it was by mistake. BTW, it get rid of all my hum issues and now it is zero hum :). Also I used simple CCS and 10000uf filter cap after the SMPS output to feed the filament which I assume sufficient to get rid of the filament hum issue. I use JJ caps and Hammond chokes (14H-->4H) for my B+.
BTW, I like the idea you gave of this "Hum pot" which I will try soon.
About the OP. points. So far what I found was 150V (with 33K anode resister) is best sounding. Smooth treble, tight and tuneful base and I am in heaven:). I had opportunity to listen to many of 1st class Audiophile equipments including audio gears like LAMM mono blocks with Wilson speakers connected to; but this little project is a serious contender from the sound and musicality standpoint for those, even it is running with my NAD C 372 SS amp and DALI Evidence 470 speakers (at least for my ears, this preamp change the sound of my audio gear drastically. Tone, image, PRAT whatever I could think of). Really glad that I did this project and best I did so far.
Good luck for with your 27's. Isn't this tube a "IDHT" btw? I tried few IDHT pre-amps schematics as well but DHT's are winners hands down if compares.
Finally I have completed the pre-amp. Attached the schematic (sort of Kevin Kennedy, Jim D'Cort mixture). Thankful to Kevin for his great advices to come up with this final circuit which work nicely and worth all the efforts. Used a PSU with Hammond 372 X (which I had already otherwise an overkill I suppose) +5U4GB+10K(5W)+OA2/OB3 tube regulators+IXCY CCS (kit module K&K Audio -http://www.kandkaudio.com/accessories.html) to produce 250V hum free B+ which I built out of randomly picked up items I had already. Otherwise, any simple PS schematic would work I assume. For me, use of regulated B+ helped towards reducing the hum drastically. Hum with unregulated B+ was unacceptable to me (even though it sound great). For filament, I used a cheap China made external DVD SMPS which worked greatly with no hum at all. :)
Attached a picture of the final product (pardon for my poor craftsmanship:no:).
Ask you my question?
what is the PS seats? Used 10M45S ?
Looking at the 220nF coupling cap at the output; isnít that generally too small?
In normal (not DHT) linestages this is usually 2-4uF. Are there special considerations with DHTs?
A very nice build btw., coolzero.:cool:
I am confident that a well constructed 26 linestage is one of the best linestages money can buy... regardless of price.
I have not jumped on the DHT linestage wagon myself yet, but I have a lot of 26 tubes.
Think I will try a 27 linestage first. I have collected a small truckload of Arcturus Blue 27 (tipped).:cool:
Hi there! A 26 preamp is indeed the highest of high end. Hard to better!
I have one built which sounds pretty good - I use a glow tube to get 150v B+ and then a plate choke - the cheap Hammond 156C (150H). A resistor on the cathode right now, but will be replaced by something better.
However, a better solution is a cascode IXYS active plate load. I believe you have one already. Then I would try some red LEDs as cathode bias. Each is 1.7v, so say 130v plate, -8.5 (5 LEDs) for 6mA. I don't know what that would be as a sense resistor on the IXYS cascode - must try it and see.
For filaments I use a 22,000uf cap, a LM1084 chip as current source with a 1.25 sense resistor (min. 6w but better 12w). That's the "end bit" of a Ronan Reg. You can add a voltage reg before it but you may not need it. The current source is the important bit - better sound. You'll need a heat sink about 3" by 3" for the LM1084 - gets hot depending on the voltage going in.
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