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71.70.154.214
As promised here's the schem. Old hands will notice that it started out life as JC Morrison's 3.5 micro (a 2A3 amp posted in Sound Practices some tine ago). Gary Kaufman saw the potential for use with the 71A and made the conversions to get it there. I changed a few cap values too I think. Anyway it's a cheap and simple build as drawn and it sounds great. Now that you have the schem I would appreciate any easy and non sound changing suggestions for booosting the gain a bit.
Thanks to JC and Gary!
Vince
Follow Ups:
How about using a pentodes like 717A, 6AK5, C3g, C3m [requires 20V filament].
Yamamoto San uses 717A very successfully in his 45 Amp which I built in the Kit form.
Sridhar Ganti
A 1.5V battery (rechargable) under the cathode should work fine too and restore some gain.
Ciao
Gianluca
You are right, I have used it on other amps and I liked it. I will have to give it a shot on this one too.
Thanks
Vince
Just flashed on that. What do you think? The sound might be interesting; I know it is one of my favorite tubes. I still use my diy parafed bugle all the time (thanks Gordon!)
Also, I think Gary did a 71A with a 417A for a driver. I will have to dig around in the vault and see what I can find.
Vince
that this little amp could be the perfect learning vehicle. If I study everything I need to know to understand how this little amp works and how to make changes too it, and why those changes do what they do, I will be well on my way to a much better understanding of tube design. I can't imagine a much simpler circuit to study. What the hey, it's worth a shot.
71A and 45 have same amplification factor and Miller capacitance, so 45 should work in this circuit too. However, if full benefit of increased power is to be obtained, B+ and idle current should be increased, and there will be much more heat generated by the cathode resistor. Will also take 1.5 times of driving signal swing.
Instead of parallel 6SL7 I would use a tube with more driving power, something like one section of 12AT7 or 12AZ7, choke load it, and feed it from a separate supply. Voltage gain will be about the same.
The most vulnerable spot of this circuit is cathode bypass cap. There is nothing remotely good in this value, so provided all parts are as good as it gets, this cap will dominate the sonic signature.
Hi sser2,
Correct, a 12AT7, etc can ONLY be used with a choke load because it's mu is not ideal for a two stage DC amp otherwise. A mu of 70 is the minimum you want, with a set of film caps, highest quality, across it's cathode. A mu of 100 is even better. Seventy or higher gives you the good old "jump" factor.
Contrary to our shared experience, high driving power is NOT needed or wanted in this type of circuit, basically a Loftin White. Anywheres from 1/2 to 1 mA will do fine, with GOOD wire in the whole amp, and particularly between the direct couple is a must.
See the Ansano Loftin White amp on the web, somewhere I saw the schematic. It uses 1/2 mA and half a 12Ax7 as I recall. Yes, it is surprising, but that type of mU and low current tube is ideal in a L-W sser2. ( ez jim ) See the Loftin White write up in Sound Practices, from the two Italians, an EXCELLENT article to have.
Jeff Medwin
50uf/200VDC auricaps should work just fine in that app...
For less money, some of the offerrings from SCR, Jantzen, and Parts Express should also do the job nicely.
-Paul
You will want to bypass any large films, with a .01uF Dynami Cap and maybe something between. Also, you need to compute the uF value, for 4 to 8 HZ being -3 dB.
Jeff Medwin
You sure have given me a lot to think about and try. I have a feeling this is going to be one of those "learning experiences" I get here from time to time. I will report back after I have played around a bit.
Thanks to all!
Vince
I ran some numbers on the alternatives. I assumed rp of 44k per 6SL7 half, a book value I have seen in real life. I used the mu of 70, although I believe the 6SL7 samples I have tend to run low on mu. Operating point would affect this and I did not figure yours out when doing this.
Your 6SL7 set up: Gain of 28
Bypass the cathode resistor only: Gain of 48
Eliminate the 24uF capacitor between the two 47k resistors (increasing RL to 94000): Gain of 40
Both bypass Rk and eliminate 24uF: Gain of 56
I don't know how important the 24uF may be to limiting PSU noise and ripple, so maybe you compromise with more plate resistance but not the whole 94k, retaining a dropping resistor and the 24uF. Guys below suggest this.
Hi,
Agree. Bypassing the cathode resistor on the 6SL7 is a give away to get the desired gain increase for usage with lower level signal sources.
I don't think it will affect sound to much in such a basic design as long as it is good qualite cap preferably parallelled with a 1/10 low voltage foil (or metalized) cap with PP or PPS or better.
(A few of those PP/PPS caps could do parallelling the cahode cap on the 71A as well.)
One could as well increase the load resistor after the cap and decrease the filter value resitor as well to keep anode voltage - though I haven't calced it I think 47K sounds a bit higher than usual, and I would guess 27k and so 68k as load. This would though decrease next stage driveability and shouldn't be done without decoupling of cathode resistor (as this increase the nex stage drivability).
mvh /Pär
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Me fail english, thats unpossible!
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Vinnie,
Just a couple of changes I would make to this. The Rp of paralleled 6SL7's would be about 25K. Therefore you may want to up the plate resistor and drop the dropping resistor to like 75K and 25K.
Also the 470k input is not a good idea when you parallel you need to derate the 1M max imput impedance accordingly. I would drop it too say 250K and then grid stop each section with like a 1K or something.
I am not a fan of paralleling tubes. You maybe better off using some of the obscure single triodes.
I love the 71A using it all over the place have like 300 of them now.
Thanks
Gordon
J. Gordon Rankin
I'd also recommen changing the 71A's cathode bypass cap to a 10 uf ASC or GE oiler.
Good post Jim, hey we all generally agree !! Wowser.
A CAP in parallel with 6SL7 680 Ohm resistor also increases gain, and distortion...
Sorry if someone already suggested this, I am green in this matters, but learning every day... ;)
Antonio Melo Ribeiro
Your schematic shows a 30 ohm/15W resistor in series with the 5V4 filament. Did you mean to put that resistor betw output of rectifier and 10uF cap? (I hope so.) Otherwise I'm missing something here. If so, care to elaborate?
Add a 10K resistor in series with the top 47k and move the 24uF from where it is to the 10K-47K junction. That should give a bit more gain (~2 dB).
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.. . .Oh!. . .Remember: Modifying things voids their warranty.
Hi,
Do I read you right??
From the main B+ from the output stage, convert the existing 47 K drop down resistor to 10K, and then put the 24 uF film cap at the juncture of what will be effectively a 94K plate resistor ( 47K plus 47K ) and the new 10K drop down R. Is that correct V.S.??
If so, that would make sense to me. Hell, in the first thread on this, I was congratulating them for having 470K as " swinging good " plate resistor, but 47 K, ten times less, is not too cool at all for swing.
The 'lytic on the 71A cathode needs to become high quality bypassed film.
Hey, what about trying a large bypassed film on the 6SL7 Rk, as I previously suggested. Hell, I'd do one half of a 6SL7 at a mill or less, like Ansano did in his, to get BOTH a higher Rk and Ra, and with this lower current, it gets more swing.
Jeff Medwin
Using globes or ST's? Wondering what your impressions are of the two if you've tried.
I recently discovery the beauty of the DC scheme and will stay. I'm running a globe 56 DC globe 46 amp as I prepare for an IT globe 50 output. I'm actually so thrilled with the first two stages that I'm delaying the final stage.
Thanks for the circuit. Enjoy. Joe.
Hey Joe
I tried a set of globes and they sounded nice, but they were an old set and not too strong, so I went back to the st's. I will keep my eyes open for a strong set of globes though, because I think they would sound realy nice. I am tending to go with "less is more" these days. The simpler circuits with first rate parts and tubes sound best to me.
Vince
STOP!!! Where you are !!.....with the third stage, you lose a transparency and immediacy, that you gain with a 2 stage direct-coupled amp.
You get gain with three stages, at the sacrifice of the ultimate a 2 stage can offer, IF done well ( and most are not )!!
Look up the Loftin White by Ansano, the one by the Italians in Sound Practices, etc. Requires 98 dB speakers.
Jeff Medwin
I'd be foolish to stop. You might be very right but, of course, I need to hear it to believe it.
Vinny
You can always add a 6922 in there to boost gain..HA HA HA HA yea rightttt. Anyway its saying the 6sl7 is paralled. I wonder why the second section wasn't drawn in? I would add another tube myself but Im wondering if the 680 ohm cathode would be right for a paralleled tube..Im assuming they are jumping pp and cc and gg or maybe not.
Vinnie and others -
This was a simple "on paper only" conversion of JC's 3.5 Micro to use 71A's. I've never built it myself, but am thrilled that Vinnie has (and that it worked out nicely). I'll have to follow his lead one of these days - I have lots of 71A's. I originally didn't want it posted until I was sure it would work!
The driver stage is straight from JC. It does sound very good. The two triodes are used in parallel.
JC's original 3.5 Micro (2A3) is wonderful. To quote the original article
"The Micro 3.5 W amp is as simple as it looks. It's a perfect partner for your Siren (Song Preamplifier). Built it as proposed and it will work great. Given the ultra-simple direct coupled signal path the output transformer will determine the ultimate sonic performance of the amp. Excellent iron = excellent results. Build it and enjoy!!"
The original schematic is above:
I just adapted the cathode resistor to correct the voltages to give an op point of 180v plate/20ma/-40v bias.
Hey Gary
For some reason your photo link isn't working for me. Any idea what might be wrong?
Vince
I got ya..Is that cathode resistor value the value for paralleled sections?
Hi Gary,
How do you insert tube socket(s) into express PCB? Do you have to make a custom item?
-Paul
I have an ExpressPCB file you can copy and paste from. It has 9-pin sockets. You can select the 9-pin socket (you may need to group the pads and silkscreen bits into a component first if they are not already grouped) and save as a custom component. Then you'll always have it in your library.
It was designed to specifically fit Antique Electronics 9-pin ceremic socket: P-ST9-214
The file is linked off the bottom of this page:
Paul -
You have to set up a custom item.
I have patterns for 7 and 9 pin minature, compactron and octal sockets. Email me privately and I can set you up with them.
- Gary
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