Tarheel Posted January 24, 2014 Share Posted January 24, 2014 Very tidy wiring and a handsome amp. Would love to hear it! Okay Marty.....your turn Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted January 24, 2014 Share Posted January 24, 2014 What! No metal film resistors??? Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 24, 2014 Author Share Posted January 24, 2014 (edited) What! No metal film resistors??? Heh Nope. Ain't been flush enough lately to buy 4 dollar resistors. Ironically, I have another Magnavox console amplifier that is pretty much the same amp/circuit, except a bit older. Except it uses the Caddock resistors, and Mills power resistors. Russian K40Y coupling caps are used instead of the Rel-Caps. The fun part is finding out which amp sounds better, and justifying why I spent that much on resistors. But sometimes one wants to try something to hear what happens. The other Magnavox amplifier does sound nice as well. There is a difference, but maybe not enough to justify the cost. A good 5% carbon film or metal film is probably good enough. Edited January 25, 2014 by mike stehr Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 25, 2014 Author Share Posted January 25, 2014 Mock up/start of the next project. 6V6 PP mono amplifiers. Hopefully, I can use the 5V4GA instead of a 5Y3GT. I don't why I couldn't...the data is rather close. If not, I don't mind a good strong GE black plate 5Y3...I just like the indirectly heated approach. The Admiral output transformer is a bit larger than the original Magnavox OPT, ( probably because the two secondary windings, 16 and 8 ohm) and it's crowding the 12AX7. I'll have to do a little hogging on the mounting holes to get the OPT a bit more away from the tube. These should look kinda cool. Quote Link to comment Share on other sites More sharing options...
toolz Posted January 25, 2014 Share Posted January 25, 2014 You have been busy with Stormin Normin is keeping a watchful eye on the project. What did you use to paint the chassis? Quote Link to comment Share on other sites More sharing options...
erik2A3 Posted January 25, 2014 Share Posted January 25, 2014 (edited) Nice! I had a pair of vintage Radio Craftsman PP 6V6 monblocks -- a staggered purchase since it was hard to find a second matching mono integrated amp. 5Y3 worked fine for them, though you should be able to use other qualifying 5volt rectifiers, maybe with some minor adjustments to resistor values, and/or inout to filter cap values (if the circuit has a capacitor immediately after the rectifier) in the PSU if you want to maintain the same operating paremeters -- all of which I know you already know. After some reasonable time spent on these old amps, and having worked on or built a number of modern counterparts, I realize that there are perhaps a few new things happening in tube audio design, but maybe not as many as one might think. Many of what some may consider as being a recent innovation, such as the well-known "parafeed" output topology (more traditionally known as parallel or shunt feed) and the so-called "ultrapath" connection -- are antiques! Edited January 25, 2014 by erik2A3 Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 25, 2014 Author Share Posted January 25, 2014 Nice! I had a pair of vintage Radio Craftsman PP 6V6 monblocks -- a staggered purchase since it was hard to find a second matching mono integrated amp. 5Y3 worked fine for them, though you should be able to use other qualifying 5volt rectifiers, maybe with some minor adjustments to resistor values, and/or inout to filter cap values (if the circuit has a capacitor immediately after the rectifier) in the PSU if you want to maintain the same operating paremeters -- all of which I know you already know. After some reasonable time spent on these old amps, and having worked on or built a number of modern counterparts, I realize that there are perhaps a few new things happening in tube audio design, but maybe not as many as one might think. Many of what some may consider as being a recent innovation, such as the well-known "parafeed" output topology (more traditionally known as parallel or shunt feed) and the so-called "ultrapath" connection -- are antiques! Thanks Erik. Yeah, I love the looks of the old Radio Craftsman amplifiers. The triple chrome plated thing going like the EH Scott radio and phonograph consoles. I'll start with the PS xformer, measure DCR, measure it unloaded, determine the ratio, and do the math to input into the Duncan PS program. Use a 5V4 in the program, and go from there. I would think there is less voltage drop using a 5V4, which can be adjusted for. Parallel shunt feed was a idea from Western Electric? Who knows, it may have been around before WE...I wouldn't doubt "parafeed" topology predated tetrodes and pentodes. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 25, 2014 Author Share Posted January 25, 2014 You have been busy with Stormin Normin is keeping a watchful eye on the project. What did you use to paint the chassis? America's Star. Rustoleum rustic Umber. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 27, 2015 Author Share Posted January 27, 2015 Okay, so I finished the two mono PP 6V6 console amplifiers as explained in post #18. Magnavox chassis and power xformer, OPTs from a Admiral 6BQ5 console stereo. It's the Dynaco circuit, but a screen supply is used, with changes in feedback. I stayed with a 5Y3GT rectifier, because the voltage drop puts things more in line with the B+ voltage I wanted. The wiring job isn't that fancy. Those chassis' are tight, and it was bit of a chore in layout/wiring. I just need to mount the wood frames, snake in the power cords, and mount binding posts. Both amplifiers work, and sound good with a junk speaker and junk portable CDP source. With square waves,10kHz looks good, square with minimal ringing. At 20kHz things are starting to look a bit rounded, and they are done by 30kHz. I looked at sine waves, but never determined the power each amp puts out. Tomorrow, I'll lash them up with the Cornwalls and go from there... Mike Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 28, 2015 Author Share Posted January 28, 2015 (edited) The amplifiers sound rather good with the Cornwalls. The bass is there, not the strongest bass, but it does adequate considering the output transformers used. Now for matching power cords, and mounting the wood frames... Edited January 28, 2015 by mike stehr Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 28, 2015 Author Share Posted January 28, 2015 Thank you, Mark. The wood frames will make them a bit more attractive. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 31, 2015 Author Share Posted January 31, 2015 Done for now...I'll get new polarized power cords when I'm more flush. 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted January 31, 2015 Share Posted January 31, 2015 Mike those look very nice. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 19, 2017 Author Share Posted January 19, 2017 Back working on these 6V6PP mono amplifiers again. After listening to them (last spring or whenever) with the circuit used and the Admiral output transformers, the GF and I weren't very impressed. So I swapped back in the original Magnavox output transformers, and adjusted for feedback the conventional way. Better, but not by much. Most the issue is hash/hiss noises. These original Magnavox 6V6 PP console amplifiers had shields for the driver tube. One had a 6AN8 driver tube with a shield. The other had a 12AX7 which had a strange lead cover for it...a type of shield. So I have one amplifier with a new socket and shield, amp "Y"...another thing is I had each plate of 12AX7 up around 285 volts for each amp, with a cathode voltage of 2.3 volts. Probably a bit excessive. I also reworked the driver circuit using the Gillespie driver circuit for the Magnavox 9300. I may have to noodle with a couple values for the feedback and HF compensation for the 12AX7 when I look on a scope. The Gillespie driver/paraphase (floating) circuit uses a higher value plate resistor for 12AX7 (actually 6EU7, same thing but 6.3 volts). That dropped the plate voltage down to 170-173 volts, with cathode voltage at 1.1-1.5 volts. That should help with noise I would guess. The original Magnavox circuit for 164BA has two 100 ohm resistors forming a virtual CT/divider, and the voltage is taken from the center-tap and applied at the cathodes of 6V6. I omitted this with the Dynaco circuit I copied. With amp "Y", and went ahead put the divider back in, with the voltage from the CT to the cathodes of 6V6. If I recall, this same technique is used with my Maynard 6AQ5 SEPP amplifier. Mike Quote Link to comment Share on other sites More sharing options...
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