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Tried doing this one too while I was at, and really ran into trouble. It screamed like a banshee when I fired it up and I have not been able to get it to do otherwise. I would really like to try this one since it is a dht unit.
There is a lot of similarity to the 27, and I like the sound of that.
Edits: 08/02/15Follow Ups:
I built my first 26 preamp in 2008 - circuit pretty much like the one Vinnie posted. By 2009 I was using two 156C plate chokes in series - good sound but a bit hummy. Then I eliminated the cathode feedback cap and went to filament bias. Huge improvement - ditch those cathode bypasses. I was using single op-amps at that stage for the DC filament supply. By 2010 I'd started using Rod Coleman's first filament designs - big improvement. By 2011 I was using Lundahl LL1660/5mA as outputs and that took the sound up to another level. That was pretty much the end of the road for the 26 preamp.I then went over to the 4P1L with the same configuration - Rod Coleman filament supplies, filament bias, Lundahl LL1660/25mA. That's where it's been since and I haven't found anything better. Very satisfied. This stage is now the first stage in my amp, with PSE 4P1L outputs. So I only have 2 stages of amplification. I feed this from my DAC and into Alpair 10 full-range columns. That's it - I can't build anything better and it's been like this for 3 years.
The 26 preamp dates back to 2011, but here's the circuit.
Edits: 08/04/15
Besides the filament bias, this is what I've been telling you.
Start by modifying your current circuit with the Rod Coleman supply, then try the output transformer (this will set the freq response so you don't have to worry about the input impedance/coupling cap as much. From there, change the bias scheme (BTW, thats a dang good idea, never thought about it on a preamp)
Interesting circuit. May have to give it a whirl after I have finished playing with the current one.
Thanks
Well wouldn't you know one of the biggest problems was I was using a VERY microphonic 26. Just my luck to pull what is probably the worst 26 out of my stash. I discovered this by accident by just saying let's try a different tube for yucks. Low and behold I got music out the other end. So then I put the old tube back in and it played for just a minute. As soon as I touched the chassis of my test mule to adjust the volume it went beserk. Just the slightest little vibration would set it off. Must go into oscilation or something like that. Very unpleasant.
Anyway..... it is playing now with the new tube and very nicely at that.
I have also been trying out some of the suggestions deafbykhorns and others posted. I reduced the cathode cap to 47uf instead of 470 (didn't have 68uf), reduced the voltage to the plate to 150. I tried a 2uf cap and a 1uf cap in place of the 220nf one, but they did not sound as good, so the 220nf stays for now.
Everything else is the same as the schem, except that I decided to try a hum pot and see how that would work since I am using ac on the filament. I paralleled the resistors and the cap shown in the schematic and tied them to the center wiper of a 20 ohm pot and took the other end to ground. The hum is just barely audible abot 5' from the speaker now.
I am very impressed with this circuit to say the least. It has a very smooth, clean sound and seems to mate well with my 91A diy clone. I do believe I may have to build one of these as it really meets my philosophy of having the minimum number of parts between the music and the speakers. To my eaers it sounds better than my current 76 preamp with way less parts. Going to have to pull out some charts for the 26 and see where I might do a little tweaking.
As it stands right now I have 4.5 mA going through the cathode resistors.
Any suggestions anyone might have for fine tuning would be welcome.
Thanks to everyone for all the help with these two (27 & 26 ) preamp circuits, I appreciate it.
Edits: 08/03/15 08/03/15 08/03/15 08/03/15 08/03/15
if you cant hear the difference with that cap unless I'm doing something wrong with SIM. What is your amps load impedance? I think some use a coupling transformer at output so the freq response doesn't change with amps input impedance? The picture below is a 50K input, the higher the better low end. Probably why some people experience a certain "synergy" with preamp/amp combos.
Edits: 08/03/15
I actually haven't been playing enough music on it yet to tell. I have the 26 feeding into the input of my 91A clones. They are 300B amps with Magnequest FS030 opts. I think those are about 3k impedance if that is what you mean.I still have some issues to work out before I can give them a real run. I had one channel working well, so I decided to go ahead and bread board the other channel too. Now I have some pretty heavy hum again and even though I put another hum pot on the other 26, neither of them seem to be able to adjust the hum like the single one did. I am going to have to figure that out first.
It really was sounding good with just the one 26; I was listening to a mono recording, but it didn't have a lot of bass in it to be able to tell. My speakers are Altec Granadas, and they have plenty of bass normally.If I ever get back to no hum I will start working on the other stuff.
Got any hints for why the hum came back with the second channel? I don't think it is a ground loop, but it might be.
Looking at the schematic it looks like you have some components out of place. The cap should be between the 50k and the 470 k resistors and their positions should be swapped.
Edits: 08/03/15 08/03/15 08/04/15
n deaf's schematic the 22k plate resistor is just drawn different.
The 470k to ground after the coupling cap is the same and the 50k resistor is just what deaf is guessing the input impedance (first stage grid resistor) of your power amp might be.
The impedance that the coupling cap "sees" is the 470k in parallel with the input impedance of your power amp.
If the coupling cap is to small in value it won't pass all the bass notes.
If we knew the input impedance value of your power amplifier we could calculate what size coupling cap you need to pass all the bass.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
This is where I usually say ohhhhhhhhh, and slap my forhead.I am assuming the grid resistor is the one that is between the input pot and the grid of the first stage tube (ef37a). It is a 220K resistor and is tied to ground on one end.
Also, for the second 26, do I need a separate hum pot or can I run both off the same one and change the resistance of the cathode reistor?
Thanks
Edits: 08/04/15
Having a grid resistor and a volume control means that the input impedance changes when you adjust the volume control.As an example let's say the volume control is 220k.
If you turn it all the way up then you have (as an input impedance for the power amp) the 220k ohms of that pot in parallel with the 220k ohm grid resistor (110k).
If you turn the volume pot to the resistive mid point, 110k from input to wiper and 110k from wiper to ground, then you have 220k in parallel with 110k (73.3K) with the 110k of the wiper to input in series, so you end up with an input impedance of 183.3k ohms.
You don't need the grid resistor. The volume pot will serve as one.
Without the grid resistor the input impedance of the amp is always just the value of the pot no matter where it's adjusted to.
So then we would have a fixed impedance that we can use to calculate the needed capacitance value for the output cap of the preamp to make sure all the bass frequencies are being let through.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/04/15
The volume pots are 100k alps audio taper pots. I will take the grid resistors out, so figure it that way.
What about the hum pots?
In a perfect world we would want 20Hz to be unaltered.That would mean a high pass filter with a -3db point a full decade below 20Hz. So for a -3db point of 2Hz you would need a .8uf cap assuming a 100k input impedance.
In the real world the phase shift in the lowest frequencies does not cause much sonic harm and most people shoot for a -3db point of 3 to 5Hz.
For a -3db point of 3.4Hz you would need a .47uf cap and for a -3db point of 4.8Hz a .33uf cap will do.
Edit; I didn't take the 470k resistor that's part of the preamp into consideration. 100k in parallel with 470k is 82456 ohms so the .33uf gives you 5.8Hz and the .47uf gives 4.1Hz. Close enough. BTW 1uf gives 1.9Hz.
If you had a filament transformer for each tube then you can use a separate hum pot for each tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/04/15
Well, I think we are making progress. I have now been able to get each channel to work individually with almost no hum by using a humpot. I still get hum when I try to use one pot for both channels though. I think I am going to try and use another filament tranny I found with a rheostat to get the voltage down and see if that will do the trick. It would not be a permanent solution, just long enough to listen to it a while to make sure I want to go to the trouble of building one. If I do I will use separate filament trannys of the right size.
So far it sounds pretty darn good through one channel with a mono source.
Oh, the 0.47uf seems to work nicely by the way. Good bass and a generally overall smooth sound. I also modified the cathode resistor and cap by removing the two 20 ohm resistore and just using a 2.2k resistor paralleled with the cap. Top end to the center wiper of the humpot, bottom end to ground.
Thanks
Thanks
Edits: 08/04/15 08/04/15 08/04/15
Seems like you're having fun and are on the right track.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Ok, I will try 0.47uf and see how that works. Only have one filament tranny at the moment, so I guess it's one pot. Thanks.
The higher the resistor(impedance), the more bass you will get out of the preamp. Your original cap may assume you have a really high input impedance.
You would need a second hum pot unless you can find two identical tubes.
Trust me on this, get rid of the AC heated filaments for the 26 and order a couple Rod Coleman kits, they are cheap and you can use them on a variety of tubes by changing a couple parts.
Trouble is I have always liked the sound of AC heaters, plus I am not a big fan of sand electronics. There is another method I might try using; a bridge and some caps and small chokes. First I want to see if I can get the ac heatrers low enough on hum that it is tolerable like it was when I just had the one 26 hooked up.
Edits: 08/04/15
I completely understand but the 26 is a different animal with noise and microphonics.
If your using the bridge and choke, the Coleman will work and sound better.
Read the thread link below
The one thing that I am still holding out for is that I have been able to get the hum down to inaudible past 3 ft with AC using a hum pot on an individual 26. I have done this with both channels individually but have not been able to do it with both of them on one hum pot. Going to try separate filament tranformeres and hum pots and see how that goes. What can I say, I really prefer the sound of AC on the filamnet, I always have.
I have that linked bookmarked and have been going through it as time allows. Lots of good stuff in there.
Thanks
Edits: 08/04/15
Yes, but you have never tried a Coleman, its not the same......
As I said, if I go as far as I can with this one, I may try one of those for comparison.
The 26 is fairly easy to get quiet if you have ST shaped tubes . All the globes I have tried were horribly microphonic . I would suggest looking at the filament voltage , it may need backing off . Are you using AC on the filaments ?
Al
Yes, I am using AC. It worked fine on the 27 circuit. How do you back off of 1.5 volts?
Edits: 08/03/15
Suggest going to DC fils , AC fils on a DHT linestage is never a good idea . By backing off I mean reducing , get a DC fil supply going and try at 1.5V , if this gives similar issues use a dropping resistor and try at 1.4V etc . I find that with some DHT this is the only way of killing microphony , with AC fils I would imagine there will be a lot of filament noise which will be difficult to get rid of . Easiest way is to use a high capacity 1.5V alkaline D cell and see if this helps mattersAl
Edits: 08/03/15
..... I would use (2) 26's..
Willie
Sorry about that
Willie
I thought this was yours. Must have bookmarked the wrong link. Do you have a copy of your 26 preamp circuit you would be willing to post?
Edits: 08/03/15
You have any of Rod Coleman supplies laying around?
My 26's drive a 2A3, I'll never go back....
Your cap is too small again, change to 1.5 or 2mfd
I would run the plate voltages lower, I think mine are between 140-150 at the plate.
The cathode cap is overkill as well, try around 68mfd.
You can get about 4v drive with less than 1% dist. Much better than the 27 schematic.
The thing aboutthe 27 is it is sooooo simple and it sounds pretty darned good. Of course i have not compared it to the 26 yet, so I may change my mind.
Vinnie,
I just received a quote from Rod for a couple of his regulators. Use the Bartola web site as suggested. A lot of the Inmates have used the Hammond 156C to load the 26 plates with good results...they are also relatively inexpensive.....
Stuben
Don't even know what they are, let alone have any. Is it some sort of voltage regulation?
Do you have a schem of your 26 pre you would be willing to post?
http://lyrima.co.uk/dhtreg/dhtRegIntro.htmlhttp://www.bartola.co.uk/valves/dht-filament-regulator/
My 26 drives a 2A3, its the same circuit your using, same cathode resistor, just a lower plate voltage. I'm using one of magnequests BC16 grid chokes just after the 26 for some extra magic.
Edits: 08/03/15 08/03/15
DITTO on the Rod Coleman regulator. Use a 3 stage model.
Eli D.
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