6BG6-GA amp with EDCOR CXPP25-8-7.6K OPTs

[mesherm]

http://home.earthlink.net/~mikesherman/ ... GA-amp.jpg
http://home.earthlink.net/~mikesherman/ ... p-side.jpg

These are pics of my newest amp using Edcor OPTs. No pics of the underside yet as I am too lazy to unhook and open it up right now.
The chassis was an old ST70 crappy chassis that had been butchered by someone else. My friend who bought it on ebay had me rewire, upgrade and install a new chassis and I got to keep the old one. I couldn't let even an old chassis just sit and I had Shannon's ST70 driver board, a power tranny, tubes, and a C Chong ST70 cap board. All I needed to get was the Edcors.
The PT was from an old Fanon PA amp that used PPP 6L6s so it worked good except that it lacked a HV center tap. I had to use a diode full bridge which made the B+ a bit hot, 470v even with a inrush limiter on the primary. The voltage to the driver board was on the high side also running 320 and 365v which proved to be too much for the JJ ECC82s I first installed. I then used 12BH7s for the 12AU7s which worked fine while I waited for a pair of Philips-Jan 5814As. They are installed now. The 12AX7 is a Dynaco branded Telefunken that came with a junked PAS3x I rebuilt. I know now not to clean those Telefunkens with Windex as all the labeling came of with the grime.
I did have to hack the bias supply on the cap board as -51v was not high enough to bias the 6BG6s. The PT DID have a separate bias winding so I was able to full bridge the bias. I removed the zener and replaced the first filter cap with a higher voltage rating and cut the value of the bias resistor from 2.2k to 1.1k. Bias problem solved. Those of you who saw the pics from my previous 6BG6 amp will recognize the homemade bare anode clips. I reused the ones I replaced from the first amp for this one and just haven't had time at work to machine up a new set of custom Teflon caps as I did the other amp.
I used a 4H choke I had laying around instead of the normal ST70 choke and since it wouldn't fit inside I made an aluminum plate to mount it on and repositioned the bias pots to the tube and cap can spots.
As you can see in the pics, the Edcors are a nice metallic metalflake blue. They are rated at 25w but from their hefty size would probably do 35 or 40 without strain. They sound at least as good as the Hammonds I have used and look better. I was most pleased. I prefer the sound from the 6BG6 tubes over my stock circuit ST70 with EL34s and even over my KT88 triode mode amp. My usual test CD is Little Feat's Waiting for Columbus. The 6BG6s bring out every small detail of each instrument and is especially good with mid-bass and percussion (At least IMHO) which is strange considering its plate curves although I am running approx -63v bias (50ma per tube).

[erichayes]

Hi All,

Jeez Mike . . . -63V? You're practically in class C territory. Howzit sound if you start drawing around 65mA cathode current per tube? (I'm assuming your 50mA current was quiescent.)

Nice job.

[mesherm]

At first I had RCA 6BG6-GAs installed but even with full bias voltage available (-75v) they would draw over 100 ma and I could not get them equalized. I then tried a NOS set of Ratheons and had much better luck. My other amp uses old coke bottle style 6BG6-Gs and I have had them biased up to 55 ma but small areas of the plates would get dull red so I backed them off to 45 ma. From the plate curves I had seen I was expecting a much lower bias voltage. As near as I can figure there is a lot of difference in the brand of tubes used. Maybe its original use in horizontal deflection circuits has something to do with it. I have no equipment other than a DMM and a cheap digital scope to analyze the performance so I have mainly my ears to judge. I have a small apt so I never really crank my amps up very high and usually run my tubes on the low side current wise. My ST70 is only biased at 40ma a tube.

[Shannon Parks]

Hi mesherm,

I like it! BTW, what did you use for those preamp socket 'plugs'? Is that some standard part?

Shannon

[mesherm]

Those plugs came with the chassis. I wanted to remove them so I could install bias jacks but the previous owner had flattened the ears on the inside and siliconed them to the chassis. There was a lot of other silicone globbed about inside that I had to remove with a razor blade. Since the chassis had already been hacked up I felt no guilt in drilling extra holes for the Edcors and the bias jacks. The finishing plugs are available from McMaster-Carr. I have used them before for work projects.