3dzapper Posted April 8, 2006 Share Posted April 8, 2006 That respect thread went to Hades in a handbasket so I'll update my preamp project here. It breathes after some wild rides. First the regulated power supply put out a waveform on the scope that looked like an AM radio modulation envolope. That had to go!!! I built a more conventional power supply that is basically a LCRC design with a 2uF film cap off FRED diodes into a 3H@200mA choke to 90uF, dropping resistor and a 720uF bank. This configuration eliminates the distorttion of the rectified wave caused by a large input capacitor, storing more energy in the choke. The output from this configuration is a flat line, undectable ripple even at the most sensitive setting on the scope. There was still an undesirable buzz/hum. Scoping the filament supply, therewas the usual sawtooth waveform seen with regulated power supplies. I inserted another bridge on the 15V winding and fed it to the 3 12xxx tubes. The noise was still present. In the end I took out the R core transformer. Replacing it with one from a Dynaco PAS, adjusted the dropping resistor value to get my 250V, 3 15KOhm 1 Watters in parallel did the trick. The PAS tranny filament winding is designed to run 4 12xxx tubes plus a rectifier filament anda pilot light bulb, so I tied in the 12xxxs to the rectifier for the 6922. I left the regulated supply to the 6922 as it was not contributing to the noise. Some residual noise was still present. I one of the bypassed the input relays direct to the main board that was it. Another regulated power supply noise eminating on the input board. See any coincidents here? I wired the selector with shielded teflon cables and fired it back up. No noise working properly. All systems go. Dean wouldn't like it though. With a RCA clear top 12AU7 a pair of Tele 12AX7s and a CCa output, thesound was too warm for my tastes. Way too "syrupy" for Dean.[]. That soupy "audiopile" sound. The output coupling caps design calls for 10uF. I figured that was where I would start. Sure enough, a pair of (cheap for now) 4uFs livened up thesound nicely. I to use wanted 5-6 but did not have any on hand. Here is another pic. The large white caps are the 10s that I replaced. There are still a few loose ends to tie up. (like drill a hole fro a screw on the tranny. Rick Quote Link to comment Share on other sites More sharing options...
lwhaples Posted April 8, 2006 Share Posted April 8, 2006 Wow! Talking way over my head! Sure looks good to me. Larry Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 8, 2006 Share Posted April 8, 2006 Hey but anyone can do it ask Erik [].................................. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 8, 2006 Share Posted April 8, 2006 See any coincidents here? Yea but rather then cause a war I just erased three paragraphs. Of course Rick I'm sure we are both on the same wave length anyway. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 8, 2006 Share Posted April 8, 2006 Nice work Rick. It seems from your post you had to do a bit of work to get noise levels acceptable. I cobbled up a one sided linestage using a single 12B4A, it works pretty good. The 12B4A has some bass grunt compared to 6SN7, IME. Almost worth trying a stereo version........ This old Admiral PP 6BQ5 console stereo amp I've been cocking with used a 12AU7 cathodyne splitter driver circuit. The amp had a raspy kind of unglued sound to me in stock form. It seemed ok at first, but after some listening you could really notice. Nothing really seemed to sound that great on the amp. I tried the driver circuit for the Dynaco low power (6BQ5/6V6) amplifier schematic, with the A-410 output transformer. (Paraphase inverter, if I recall.) The Dynaco schematic uses the screen taps of the A-410, the A-410 has a primary impedance of around 8K. I'm using the stock Admiral OPT's, which have a primary impedance of 6.7K. No screen taps, I'm using a screen supply from the PS. Copied the schematic part for part, right down to the 1000 ohm feedback resistor. The schematic calls for a .001 uF compensation cap, I'm using a 1060 picofarad sliver mica. The PS is basically stock, CRC, with a extra RC for the driver tubes. The voltages were too high with the original 5U4, so I swapped to a 5Y3 and that helped drop the plate voltage to a more acceptable level.. The 5Y3 seems to be holding up..... I figured the amp wouldn't even work, oscillate or sound real bad..... The thing sounds rather nice, a helluva lot better than it did. I'm using some strong Mullard 12AX7's and EI yugo 6BQ5's. PS noise is very quiet considering the primative supply. The amp seems nice and smooth and open on the Cornwalls, with plenty of power. The bass seems fine, with no noticed roll-off, same with the top-end I guess I got lucky....... Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted April 8, 2006 Share Posted April 8, 2006 Rick, what kind of chassis/case are you building on? Is that something you picked up with the diy boards? Looks good. Quote Link to comment Share on other sites More sharing options...
3dzapper Posted April 8, 2006 Author Share Posted April 8, 2006 It's all fun Mike! That sounds like a good project. you could always get your screen voltage off the plates and try the 6BQ5s in triode mode. Should do about 8 watts PP. Craig, There will always be enough people out there in the world who can't, won't or don't want to do this sort of thing to keep you very busy into your old age. There are many times I question myself: "What the heck are you putting yourself through this for, buy one or pay someone to do it." But, like you, I'm mule headed. Can I design and build something like a Blueberry or a VRD? No, not a chance. Am I as meticulous as Erik? Just the though of comparison makes me chuckle. Can we all have fun? It's our hobby. If it ain't fun, why bother. Dee, the case costs a grand total of $37.50 + shipping complete with 5 pairs gold plated RCA jacks, the IEC jack, feet, a pair of aluminium knobs, on/off switch and selector switch from DIY club. The front panel alone could cost much more around here. This whole pre-amp can be built for $250 or so by a knowledgeable person. One of the things I like about it isfor just over a hundred dollars, I can put an entirely different board and sound in it. I have intentions of trying a 6H30 board in the near future. I've read so much good about that tube that I just have to taste it. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 8, 2006 Share Posted April 8, 2006 "It's all fun Mike! That sounds like a good project. you could always get your screen voltage off the plates and try the 6BQ5s in triode mode. Should do about 8 watts PP." The amp seems to sound pretty balanced. I dunno if strapping to triode would help make things sound better. It might give a better damping factor. It wouldn't be to difficult to try, I've never tried PP 6BQ5 strapped in triode. Cobbling up tube circuits is fun....... Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted April 8, 2006 Share Posted April 8, 2006 There are still a few loose ends to tie up. (like drill a hole fro a screw on the tranny. ------------ Detail, Details... Even in my mostly ignorant state; i still love reading these posts! Quote Link to comment Share on other sites More sharing options...
thebes Posted April 9, 2006 Share Posted April 9, 2006 Well I'm glad to see you amateurs are following in my footsteps. Monoblocks are more fun though. Twice as many chances to electrocute yourselves. Also who needs these modern circuit board designs when you can try something real interesting like taking some 45 year-old piece-of-crap public annoucement amplifiers and make them sing. Yes folks we're talking about the universally disregarded Bogen PA amps. I cogitated for quite awhile over these things and then decided why not convert them to EL34's. So I bridged a couple of the the valve pins, shifted some resistors, replaced others, rewired some grounds and with constant experimentation upped the negative B to the appropriate voltage for an amp running at 630 volts on the front side. Then I got out the organge drops for the couplers, beefed up the SS rectification diodes, doubled up the power supply with new top-side cans and direct coupled the HIZ terminal eliminating the speech function of the amps. Since these have a "fixed" bias aproach I took some readings and matched up the 4 EL34's per mono for each side of the circuit and set the balance pot using an ohm meter. Dropped in new pilot lights and fired them up. Like you guys I had a slight hum issue but I solved that by putting a probe in the wrong place, grounded something and was treated to a nice display of blue flame and sour-smelling smoke. Replaced the toasted safety resistor and the flames having magically realigned various molecules in the amp. It now has no hum and only the smallest hint of static sounds caused by random electrons making there way through the universal ether. I strongly recommend this approach to solving hum issues. From purchase to completion including tubes about $450 to $500. Only thing left is some top-side chokes and new rca terminals. Oh yes and I may bificarburate the whatsits, but then again I may not. Piece of cake. I have no idea what all the mystery is all about. After all electrons are our friends, they mean us no harm and what could possibly go wrong? Quote Link to comment Share on other sites More sharing options...
3dzapper Posted April 9, 2006 Author Share Posted April 9, 2006 See Craig ANYBODY can do it!![] I think Ole Thebes learned tube electronics from a 40 YO NRI correspondence course with no one to correspont with. So he talks to himself a lot. Mark, The only regulated supply I kept in is dropping the 12 VDC filament supply down to 6 for the 6922. Even with large caps, the sawtooth was still there on the 12Vs. When I switched trannies, I no lomger had the 15VAC necessary for the 12V regulated circuit so went with a "normal" capacitive filter. The AC ripple remaining on the filaments is inaudible and immeasurable on the outputs with the rustic equipment I have. (Scope and a Fluke) Rick Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 9, 2006 Share Posted April 9, 2006 Craig, There will always be enough people out there in the world who can't, won't or don't want to do this sort of thing to keep you very busy into your old age. Rick, I have absolutely no concerns about running out of work actually the reverse is true. That's not my point at all with the "anyone can do it" statement. In fact I get a steady stream from the DIY market segment they actually creates more work in many cases. Craig Quote Link to comment Share on other sites More sharing options...
3dzapper Posted April 9, 2006 Author Share Posted April 9, 2006 In the garage business we had a saying:, "Fix'em on Sunday, tow'em in on Monday!"[] Quote Link to comment Share on other sites More sharing options...
mike stehr Posted April 9, 2006 Share Posted April 9, 2006 I remember the Scott LK-72 I had.........I respect Craig too much to send him that hacked- up abortion to fix. That would be rather cold. A kit integrated amp that must had 50 billion hands soldering in the circuit over the years, it was a real mess. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 9, 2006 Share Posted April 9, 2006 I remember the Scott LK-72 I had.........I respect Craig too much to send him that hacked- up abortion to fix. That would be rather cold. A kit integrated amp that must had 50 billion hands soldering in the circuit over the years, it was a real mess. Ahhh that stuff doesn't even phase me anymore. Just get the wire cutters out and redo it all expensive way to go about it though. Besides Mike I'm sure you could tackle the stuff yourself you have the natural ability[] Craig Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted April 9, 2006 Share Posted April 9, 2006 In the garage business we had a saying:, "Fix'em on Sunday, tow'em in on Monday!"[] LMFAO! Quote Link to comment Share on other sites More sharing options...
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