Russian Roulette
Stage 1 (Evaluation)
Pick A Tube, Any Tube
So, I guess I did not find audio nirvana with my 5687 Version I preamp. I went back to the solid-state Buf-03 passive line stage for a short while to break in my Frankenstein Horns and reassess the old preamp. Although the Buf uses the famous Alps "Black Beauty" pots, I always felt that passive attenuators sounded better. Unfortunately, I had taken mine apart for its parts. I recently started to clean up the basement in preparation for remodeling, and I noticed my old Zen preamp chassis sitting there with my precious stepped attenuators... I said to myself, "What a waste!" Although I can't reuse the chassis, I can certainly reuse the attenuators (I'll just put a resistor in front of them) by dropping them into a circuit to see what happens. This got me to search the Internet for a suitable schematic, and I remembered that I once saw a really simple circuit similar to my first 5687 tube preamp, and simple enough for many other tube types. For newbies, here is a must-read article about building a test circuit that will eventually lead you to finding your own preferred sound, and tube. Check it out. From the many tubes mentioned, I randomly chose to use an inexpensive 6N1P since it is still in production. I took the original schematics (without permission) and modified them with my chosen parts:
According to the PSU Designer II software from Duncan Amps, the original values didn't work out... so I substituted using the little knowledge I possess and came up with a low value cap after the initial choke. This 47uf Solen is the same value I used in my prior project. If it worked there and there was no smoke, why not again? All three inductors are Hammond open-bracket chokes (157J). Although they don't look nice, they're cheaper than the closed models. I don't know if I will chop up the first preamp for parts as I can use that one in another room with a second system. The output cap will probably be a Hovland MusiCap, and since I'm spending so much here it bothers me that the rest of the parts might be too plain. Maybe I really do need that Elna Cerafine and silver wire? An Elna Cerafine cap now costs ~$70 a piece. I got my first for $35 on sale. Who knows, I may steal it from the original preamp anyway.
Here are the original schematics (posted here without permission).
New PS Changes (added April 26, 2003)
I just happened to visit JE Labs for some tips on building a 2A3 amp and I saw some cool designs for Joseph Esmilla's preamp power supply, which was in a separate chassis. Here is a link to his schematic. I'm thinking I can do the same and create new preamp modules to connect to the same power supply in the future. I'll also use a 5AR4 again so that I don't have to buy an extra 6.3v tranny for the EZ80. My friend "Tube Guy" tells me it's absurd to think a GZ34 or 5AR4 sounds bad in a preamp. I don't know... but it would certainly keep things consistent for me. The parts list does not reflect these changes yet but I am certainly considering these hefty rectifiers (usually found in an amp). I'll probably decide on a different power tranny, too... Maybe at 300-0-300.
I checked around for the GZ34, but they seem too expensive for me... average cost was as low as $60 and went over $100+ for those special NOS tubes... So, I guess it's another Sovtek 5AR4 for me... cost is ~$12.
Another change - only two chokes instead of three. With a separate supply chassis, I'll always have room to add another if needed. Same goes for B+ filter caps.
Here's a schematic of the power supply now:
The numbers 1, 2, and 3 go to the preamp chassis via a standard IEC power cord, where I will also add a cap for better filtering. This may change as I think I need to extend both wires from the filter supply to avoid a ground loop.
Tube pin numbers of the 6N1P:
Spec sheet for the 6N1P (PDF) and sample circuit (PDF)
Spec sheets for EZ80 rectifier (PDF1, PDF2) This tube uses a B9A socket, the same 9 pin mini as the 5687 and 6N1P.
Spec sheets for 5AR4 here (PDF). GZ34 is the same, here.
If anyone has any comments about the values, please feel free to email me your suggestions. I would like to know if you think my project will go up in smoke at turn on. Thanks in advance!
dancetracks@yahoo.com
Stage 2 (Parts list updated May 18, 2003))
Quantity - Description - Source - Price (if available)
Purchased so far...
1 - Hammond 272DX 123VA, sec.
300-0-300 - Angela - Total $45
2 - Hammond 158M 10H, 100ma, 262 ohms, 400VDC. - Angela - Total $26
8 - Four pair RCA jacks - Triode Electronics - cheapo gold jacks. These feel
cheap, but look ok. Total $24.
1 - toggle switch for two inputs only. had this in parts bin.
1 - mini 6 amp toggle for power supply. had this in the parts bin. has small
solder lugs but will try it anyway
3 - 1200uf 100v Panasonic TS-HA. scrap parts from old Zen power supply. I can
use these for the filament supply, although I will still use one 10,000 uf cap
in this fashion - 2 x 1,200uf > resistor > 1 x 10,000uf > cable to preamp >
1,200uf > 0.01uf ceramic cap on the tube socket.
1 - 5AR4 - Telsa/JJ-Electronic - Triode Electronics - $15 - I won't be using the
EZ80, and I already have the new rectifier warming up in my old 5687 preamp... sounds
different than the Sovtek I had in there but it might just need more burn-in
time to settle down...
2 - 9-pin, unglazed, cheapo, ceramic sockets - Triode Electronics - Total $6 -
These look and feel like a step down from the Angela sockets. I'm trying to cut
costs but it hurts looking at these. Sonics shouldn't be affected...I hope.
2 - 5687 Tung-Sol - www.tubeman.com - Total
$20 - People say these are good... well, I'll be trying them in the old preamp
real soon...
1 - bridge rectifier (3 amp is ok as filament current draw is less than 2 amps)- Mouser
Parts to come...
Resistors
2 - 100k - 0.5 watt Riken - on input - Parts Connexion
2 - 1k - 1 Watt Riken - on grid - original value is 100R - Parts Connexion
2 - 15k - 12w Mills - for plate - Parts Connexion
2 - 10k - 12w Mills - for plate - value for other tube types - Parts Connexion
2 - 150R - 1 watt Riken - cathode - Parts Connexion
2 - 100R - 1 watt Riken - cathode - value for other tube types - Parts Connexion
2 - 1 meg - Rat Shack - in personal stock
1 - 0.3R - 1 watt - filament supply - Mouser or Digikey
4 - 10R - 1 watt - HV snubber - Mouser
Caps
2 - 220uf 350 volt - Panasonic ts-ha or hb - Digikey
1 - 10,000uf 10v - Panasonic ts-ha or hb - Digikey
2 - 2200uf 10v+ - Panasonic - Digikey
4 - 0.01 uf ceramic for filament snubber - Mouser
4 - 1uf film/foil for HV snubber - Mouser
2 - 2uf Hovland MusiCap - Parts Connexion or cheaper Auricaps
2- 20uf Solen 400v - Parts Connexion for preamp chassis
Tubes
4 - 6N1P - These are cheap enough to buy four and choose the best two - Parts
Connexion
Next: Stage 3: Construction
