Latest Thread Images

Little Dot Tube Amps: Vacuum Tube Rolling Guide
Sep 22, 2013 at 4:51 PM Post #3,031 of 13,432
trolldragon said:
I have the pictures taken and a rough schematic made of the amp. Give me a few hours to assemble the info into a presentable format. I have some building maintenance to do first.
biggrin.gif
Click to expand...

beerchug.gif

 
If by any chance you could also get a clear shot of -what I think is- the cathode bias resistor along with the grid bias resistors, as per the pic below, that would just make my day lol. That day might be tomorrow though, as it 11pm where I live...
 

 
Thanks!
 
Sep 23, 2013 at 2:13 AM Post #3,033 of 13,432
Well the LD is a real pig to get out and put back in the case with the transformer wires coming down from the top...
 
Here is a very rough schematic of the Left Channel, the Right Channel would be a mirror copy.

 
Gain Resistors

 
Cathode Bias Resistor

Hope these are OK... 680Ω checked with meter and silkscreen value.
 
Sep 23, 2013 at 4:44 AM Post #3,034 of 13,432
trolldragon said:
  Well the LD is a real pig to get out and put back in the case with the transformer wires coming down from the top...
 
Here is a very rough schematic of the Left Channel, the Right Channel would be a mirror copy.
 
Gain Resistors
 
Cathode Bias Resistor
 
Hope these are OK... 680Ω checked with meter and silkscreen value.
Click to expand...

 
Awesome work TrollDragon! I can't understand half of it, but great work nonetheless lol!
 
Ok, so the cathode bias resistor is 680 ohms as I thought (I finally found a picture with an angle that kind of allowed me to see the color coding on that resistor late last night, but as usual, it was hard to be sure). So, that one's final and confirmed at least.
 
Now, looking at your schematics, I'm suddenly reminded that the gain switches are basically feedback switches... Can you actually tell which of the switch settings does what here (what resistors and pathways are used for each gain setting, basically), and whether or not there's a gain setting with no feedback?
 
Great stuff though! No idea how I'm going to make sense of this and use it to pick tubes, but most useful, especially that cathode bias resistor value.
 
"One small step for head-fi, a giant leap for LD users."
 
Edit: Nevermind, I think I figured out the gain switches myself. So, if I understand correctly, the only "bias" resistor for the driver tube grid (they call it a "grid stopper" or something right?) is that 10K resistor after the potentiometer?
 
About the cathode bias resistor: a 6922/6DJ8/E8CC(...) double triode takes that exact value, 680 ohms, as a reference to bias the cathode in each triode section. While it doesn't matter that much, it's a good start. I can't quite figure how to "calculate" a hypothetical grid voltage with the resistor value and input signal though... Then again, I'm not an electrical engineer in the first place; I have to rack my brains to understand this crap...
 
Sep 23, 2013 at 7:12 AM Post #3,035 of 13,432
Hey AFB!

Unfortunately my electronics knowledge was learned on the bench repairing audio gear for Radio Shack back in the day. Changing blown outputs in amplifiers and radios, cleaning, aligning and torquing cassette drives. Repair of remote controlled toys etc... No formal training here just a gathered "bench" knowledge.

I never learned anything at all about tube circuitry and after the repair shop closed I went straight into digital electronic repair for Commodore and Atari, mostly all TTL level gate's.

So I really can't be of any help analyzing the circuitry, I was pointed to a few good books on tube theory from the guys here and I guess its now time to delve into them whole hog... :D

I am going to try an do up a better schematic in Digikeys online cad program to straighten out the layout a bit. I had to remove some of the big caps from my board to follow some of the traces. That's why some things are sideways and not in the typical vertical layout. :D
:beerchug:
 
Sep 23, 2013 at 7:48 AM Post #3,036 of 13,432
See, I don't even have the practice part down lol, let alone the theory! I used to hate electronics back in my school days; but with perseverance and the drive for getting better audio, I am starting to understand parts of this. Like your schematics: after staring at it for 2 hours and looking a number of things up, it makes sense to me -for the most part :wink: .
 
The only symbol/part/thing I don't get is those capital A letters that the anodes are connected to (do pardon me if it's a silly question...). The rest I understand at the basic level.
 
Now, I just need some kind of software that would allow me to plot tube curves with cathode bias and grid stopper values... Not that I would know how to use such a program... We live and learn, I guess.
 
Sep 23, 2013 at 8:29 AM Post #3,038 of 13,432
siles1991 said:
  any strap recommendations for the RCA 6DT6A's? currently running ef95 no strap but bass a bit boomy
Click to expand...

 
As floating grid 3 on this tube type makes little to no sense, I'd recommend EF92 or EF95 with 2-7 strap. EF95 with 1-7 strap is a possibility too, but I didn't find it held much interest on 6DT6 tubes, so I'd stick with EF92 for sound quality and practicality.
 
Sep 23, 2013 at 9:27 AM Post #3,039 of 13,432
audiofanboy said:
  See, I don't even have the practice part down lol, let alone the theory! I used to hate electronics back in my school days; but with perseverance and the drive for getting better audio, I am starting to understand parts of this. Like your schematics: after staring at it for 2 hours and looking a number of things up, it makes sense to me -for the most part :wink: .
 
The only symbol/part/thing I don't get is those capital A letters that the anodes are connected to (do pardon me if it's a silly question...). The rest I understand at the basic level.
 
Now, I just need some kind of software that would allow me to plot tube curves with cathode bias and grid stopper values... Not that I would know how to use such a program... We live and learn, I guess.
Click to expand...

 
Sorry, I should have explained that part, the capital "A"'s are the traces that head off to to the power supply rail section which I made on another page and didn't get uploaded yet.
 
Have a look at this software here possibly.
http://www.glass-ware.com/GlassWare_Programs.htm
 
beerchug.gif

 
Sep 23, 2013 at 1:24 PM Post #3,040 of 13,432
Little glass goodies from T.Dot have arrived.
Time to test out my strapping switches I installed last night.

biggrin.gif

 
Sep 23, 2013 at 2:29 PM Post #3,042 of 13,432
Sep 23, 2013 at 3:13 PM Post #3,043 of 13,432
siles1991 said:
  any strap recommendations for the RCA 6DT6A's? currently running ef95 no strap but bass a bit boomy
Click to expand...

 
 
Hi siles.
 
Can certainly endorse AFB's recommendation - EF92 (mine via EF95 6-7 strap). Fabulous sound, and with the volume pot a good few notches down...
 
Am sure you've already found this out, though!
 
Happy rolling.
 
Sep 23, 2013 at 3:30 PM Post #3,045 of 13,432
Quote:
siles1991 said:
  any strap recommendations for the RCA 6DT6A's? currently running ef95 no strap but bass a bit boomy
Click to expand...
 
Quote:
hypnos1 said:
   
 
Hi siles.
 
Can certainly endorse AFB's recommendation - EF92 (mine via EF95 6-7 strap). Fabulous sound, and with the volume pot a good few notches down...
 
Am sure you've already found this out, though!
 
Happy rolling.
Click to expand...

 
EF95 6/7 = more forward and much more controlled bass then unstrapped
"     " 1/7 = More bass the above, but much more controlled then stock... more laid back and airy
"     " 2/7 = more similar to 1/7 except less bass impact and slightly more forward.
 
All good, just depends on what you like.
 
You must log in or register to reply here.

Users who are viewing this thread

Total: 6 (members: 0, guests: 6)
Back
Top