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In Reply to: RE: Magnavox Rebuilds and Fultons (longish) posted by JJ Triode on April 05, 2014 at 14:48:08
JJ
I was wondering about that because even tho the 6EU7 which is a 12v 12AX7 is a great sounding tube,I'm not crazy about it as a driver.The direct-coupled concertina is a wonderful idea and I think Pete would like it as well.
What tube did you use? I actually like the Mullard pentode-triode combo even tho its implemented in a long tail pair but the DC coupled concertina is worth a shot..
Some change these over to the 12AT7 like Jim McShane suggests in single driver tube amps and that works very well indeed.
BTW,you dawg..I really miss my Fulton J's but I will gladly take those RTR6s off your hands..I have a pair as well but I needed an audio transformer for one of them..I finally found an NOS one for 30 dollars on Ebay...Are those incredible tweeters or what? Those and the Heil Air Motions should be reissued IMHO.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 04/06/14Follow Ups:
Mike,
Sorry, my RTR/FMI tweeters are not leaving my house until I either die or go deaf. The Fulton version only runs from 5K up but most instruments and voices image at the height of the tweeters.
The 100s run full-range since the lowered position (only few inches off floor) and limited treble dispersion of the Peerless mid-tweeters effectively low-passes the main speakers.
I use RCA 6CG7s in my PP amp for the front end. Not knowing any better, I just followed an old Williamson schematic (I will try to find it for Fitero) with 47K plate and 470R cathode resistor on the first stage, direct coupled first-stage plate to second-stage grid, 22K above and below the second stage. Then second-stage plate and cathode cap-coupled to output tube grids (no third driver stage, unlike Williamson) with 0.1 µF Dynamicaps, 220K to ground. It sounded very nice to me so I have not altered the topology since then (about 2 years ago.) No doubt it could be tweaked more. I only use 4-6 dB of feedback so it is enough gain.
As Mikey said, I would be interested in perfecting what I have built....
However, when you said "a direct-coupled concertina scheme" I looked at that like cows look at trains.....
If it is in schematic form, then I can read it, and possibly build it, otherwise I am lost, and Mike gets badgered with my pestering calls and messages.
Fitero,
You can see an example at
http://www.preservationsound.com/wp-content/uploads/2012/01/WilliamsonAmp_1947.jpg
V1 and V2 are two sections of a 6CG7 or 6SN7 twin triode (the 6CG7 will fit the noval sockets in your amp but has a completely different pinout from the 6EU7.) As mentioned in my answer to Mike's post, I used 0.1 µF caps to couple V2 to the output tubes (stage V3/V4 in the schematic deleted). The very small-value coupling caps used in the Magnavox schematic will roll off your bass badly if you reduce or eliminate the feedback.
One difference from the schematic I reference: I replaced the parallel power supply decoupling filters R2C1, R6C2 with a single filter with 3.3K, 22 µF feeding both first and second stage plates. I did not have room for two filters, and the lower power supply voltage in an EL84 amp (and higher current drawn by a 6CG7 relative to a 6EU7) mandates a smaller resistance. There is some room
for variation in the other resistances, but R3, R4, R5 and R7 should all scale by about the same proportion if you want to change them. R5 and R7 need to be high-precision or closely matched.
Good Luck,
JJ
Thank You for posting the link and taking the time to explain the pertinent info and deviations JJ. Food for thought.
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