mike stehr Posted January 26, 2017 Share Posted January 26, 2017 I posted this in 2 Channel on Dec 1st from a ongoing build thread for various amplifiers. _______________________________________________________________________________________________________________ ''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 think. 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."" ________________________________________________________________________________________________________________ The other amplifier "X" uses the Dynaco 6BQ5 PP circuit...a suggestion schematic for the A-410 OPT I would guess. After re-wiring amplifier "Y" with the pilfered Dave Gillespie modifications, (and peeking at a 2kHz square wave) I went ahead and hooked up amplifier "X" wired with the Dynaco schematic to a Cornwall with a CD player. One amp, one speaker. The GF and I listened to the amplifier for around 45 minutes. Right off the bat she gives me a meh look....no bass... But we listened on...midrange and highs seemed a little recessed, with the bass even a little more recessed. We concluded the amplifier sounded ok, but kinda boring. I went ahead and peeked at a square wave at 2kHz with the circuit before moving on to amp "Y" with the Gillespie modifications. One Cornwall, one side... The Gillespie modded amp had a rich fuller sound with a stronger bass, or a less attenuated bass for those little OPTs. I noticed that within the first minute of listening. I musta did something right, we listened to the amp for a hour and half with no objections in the sound. First scope pic is amplifier "X", with common loop feedback from the secondary to the cathode of the driver. 1000 ohm feedback resistor, with 1500 picofarad for feedback compensation. 2kHz square wave with a resistive 8 ohm load. The amplitude pot on the function generator is turned all the way down. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Author Share Posted January 26, 2017 Second scope pic is amplifier "Y". This is the Gillespie modded amp, 2kHZ square wave with a 8 ohm resistive load. Pardon the off center scope images, I was getting a better look at the left front edge. I'm no scope jockey, so bear with me. The bit of lead tilt in the square wave, which I would guess is just part of the LF (phase?) response of a small output transformer. I suppose a guy could noodle with feedback and square up the wave a bit more, but the amplifier sounds rather good...dunno... Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Author Share Posted January 26, 2017 The Gillespie modded schematic. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Author Share Posted January 26, 2017 Power supply. First cap goes to B+, the second cap to the screens, the third cap for 12AX7 de-coupling. Er, That should be one and a half amp to two amp fuse. oops I just ordered the parts to finish amp "X" into amp "Y". Mike Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Author Share Posted January 26, 2017 Amp why Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 26, 2017 Share Posted January 26, 2017 And it looks like you use maple/hardwood wheels under your. I have a box of them I use when putting finish on things. They work great for that. Nice looking amp, Mike. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Author Share Posted January 26, 2017 I bought the little wheels when the Ben Franklin used to be in town. The crafts section was immense, and they had a Radio Shack in the store...it was a audio dork paradise. Now it's a ACE hardware store...yay ACE does have the little white cabinet bumpers that make good feet for tube amps, and isolate better than the wood wheels. I'll need to get more of them... I poured a bit of time into these amplifiers, hickory bases, rustic umber paint job, polished transformer frames, ceramic sockets, etc...I'm just glad one of them (Y) finally sounds rather good and worth listening to, with hopefully (X) turning out the same. The GF said I'd better not sell them. Quote Link to comment Share on other sites More sharing options...
davesans Posted January 26, 2017 Share Posted January 26, 2017 Very nice work Mike look like your effort paid off. Quote Link to comment Share on other sites More sharing options...
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