[Texasrocker]

I've been building a custom "from chassis-up" scratch-build stereo-tube hifi amp the past several weeks in my spare time.
I thought up the tube layout, and applied my ideas in mixing together some of my good vintage-spares parts (all),.. into a orig 1947 RCA roll-over tube cage type orig old auditorium style orig PA chassis I recently acquired.Its a very-cool looking vintage homebrew rig if nothing else I suppose. Heavy duty chassis indeed!
I have the both amps on same chassis working now, and both sounds very good so far with a CD player playing thru it via separate amps 5k audio separate vol, treble, & bass controls, but Ive had to make some mods to get rid of excess distortion in my orig design. Im getting there, but still have very-slight distortion happening.
I also realize that since I basically dreamed-up the overall schematic as a "try-build", Im on my own out in left-field on this homebrew amp project! However, any your thoughts or suggestions would be appreciated in ridding the excess mimimal-distortion in this 2- monoblocks amps on one chassis project! Ive already looked for most obvious solutions, slight minimal distortion still present.

Heres the vintage parts & basic layout I used in this stereo amp project so far. Simple 5-tube lineup( for each individual amplifier) using 2 push-pull audio-tubes output ( ea amp) all on 1 chassis. I have no "hum" probs whatsoever,or any crosstalk,between individual amps, both quiet as a mouse between CD tracks playing thru both the amps out to matched vintage 8 ohm spkrs, both linear audio out trans properly matched in ohms to spkrs. This rig is absolutely " hum-free" & dead silent quiet, between CD player tracks! I used a half-wave AC to DC conversion diode bridge to power all the preamp tubes filaments chain by DC only, negative side chassis ground. The audio out PP tubes are 6.3 ac volt fils only. Both audio trans share the same B+ source.

The overall design is a cathode-biased amp ( all pin # 1 & pin 8 out to ground via (2)- 300 0hm 25 w power resistors worked, with separate 150 uf electrolytics strapped across each appropriately to improve smooth bass-response,for each set of pp output tubes sockets, that will accept any 6CA7/EL34, or 6550s, or 6L6s in push-pull. All work, as Ive subbed them in & out of the circuit. That the "beauty" of a Cathode-biased amp indeed! Most all tubes in this amp interchange, even the phase inverter, a 6SL7 with a 6SN7 ( I havent done that yet), but socket is ready. Going to a 6SN7 is going to lose my overall drive into the output tubes, as reason of my thoughts. In any case, the amps are working very well so for, and stable after many hrs power-up during my build via variac. The whole rig pulls less that 3 amps overall. I balanced the overall chassis DC voltages with the power trans centertap to ground via a 800 ohm 25 watt adjustable power resistor set now at about 150 ohms.

For the preamp tubes, I now have a 6C5 ( replacing my orig 6SJ7 idea front-end tube that was overkill on high-impedance selection) for aligning with a conventional home CD player. Fixed that prob. Bad idea from the getgo!

Now, my tube lineup is as follows,: 6C5 input tube feeds a 6J5 (same tube interchanges with 6C5) and so on on all schematics. That took care of the initial overdrive input (very high distort) prob on my orig design.

Total preamp tubes are now a 6C5, feeding the 6J5, into a 6SL7 phase-inverter setup, then into the pp audio tubes. I used some spare vintage early 1960s matched linear 20 watt audio trans off a old Sherwood S-5000 stereo integrated orig 6BQ5 in pp setup. Im using a heavy spare vintage power trans 450-0-450, 1 5U4 rectifier providing dc to both amps onboard,and main B+ is running consistent 403 vdc, and all 250 volts points out to preamp tubes are spot on using a 2-watt dropping-resistor back to chassis. The dc powered filaments on the preamp tubes ( running + & - ) are also spot on 6.3 volts dc via the high-uf / low voltage electrolytics.
Im almost homefree without any distortion in the circuit on this project! Still has a slight amt, and Ive carefully checked overall voltages! Any ideas?
Thanks in advance!!

[codefox1]

I've done a few of these myself, and when I see some ringing on the scope, (don't trust my ears any more.) it can come from a number of sources.

1. Did you use grid stopper resistors in the output section - Say, 1K 1 watt soldered really close to the first grid of the 6L6 ahead of the coupling capacitors. And don't make the coupling capacitors too large, .047 ususally is enough, you're asking for trouble if you use above .1.

2. Lead dress is always an issue. Nobody likes to cut the leads short on an output transformer, I know that, but ... Try a .002 cap (1KVor better!) plate to plate on the output tubes.

3. Play around with the negative feedback resistor value. Too much feedback is almost as bad as hooking the PP primary backwards, that 50-50 chance we all know about the first time you fire the thing up. Seems like I have a coin with two heads, (or two tails, don't know which,) but it never fails...

4. If you are using an ultralinear output transformer and you get the screen windings switched, you either get fireworks in the output tubes, or miserable sound. A lot of people also use stopper resistors on the screen grids as well.

5. Star grounding really rocks. EVERY GROUND TIE should return to that point where the the first electrolytic is soldered to the chassis, and the center tap of the HV winding is. At first this sounds silly, but prove it to yourself.

6. I really love the sound of 6SJ7 wired as a triode. Learned that trick from the Shulmerich Carrilon (Church Bells) amps I acquired. Now that's some really Old School Brick S**t house design. These things sat in dank basements or in belfreys and played on for decades after decades. There is a "Red tube" equivalent that is available from time to time on Ebay at reasonable cost, (5693 if memory serves.) They are a treat, if they are not microphonic, and most of 'em are not.)

7. You won't be able to tell what stage the distortion is coming from by ear. It can be anywhere from the input jack to the speaker terminal. What I like to do is hook up some non inductive resistors to the output transformers, put in a sine signal, get it to clip, and then look at each stage to find where it went south. It is not always in the final stage. Hint: The phase inverter is especially suspect.

8. If you're wondering, I have a little time to participate on the forum, but not enough time lately to get down to my own workbench where there is no clock, boss, GF, or family. Either way, please let me know if this helps at all.

[Texasrocker]

Hello Codefox! Thanks alot for all those tips! Just from your suggestions, I can see I missed a few steps! I will print out all your suggestions and take them to the workbench. The distortion is very minimal, but Im sure with enough time, will be able to find it. This is the first 2 amps all on 1 chassis Ive ever built. Mostly because the chassis was already punched & ready! My wife took some photos of the amp. Its still not finished, but will post the photos in a new thread in case anyone wants to see them.
Thanks again for all your help!!
Sincerely, Randal

[codefox1]

Thanks for listening to my rant. I pretty much like to start with some good "iron" preferably a pair of outputs identical or nearly so, a beefy power transformer, and a chassis big enough to hold everything, and a typical well discussed plan from the 1950's or 1960's using still relitively inexpensive and available tubes.

Sure cure for pesty harmonics is a very small value capacitor with a very large voltage rating across the primary of output transformer very neatly dressed. But you probably know that already, yes?