A quartet of 6L6GC on top of a 12AX7 phase inverter, signal strength from the bank of tube oscillators was at a sufficient level to drive the amp directly. My aim is to tailor this amplifier for use with guitar, and as such a preamplifier is in order. That's a potential subject for another day, let's just have a look at the voltage supply & power section, as everything else is slated to be changed.
Power supply contains a parallel pair of 5U4 and a choke that would do some damage if it was dropped on a foot. I've found what appears to be a potential candidate for the schematic via search for "Baldwin PC", but it provides little data not already apparent by looking at this thing. I will, for example, have to measure out the choke to determine henries.
Did I infer above that the power section of this will remain untouched? Apologies. That was, in essence, bullshit. The twisted section of 6.3 volt filament pair incoming at lower right is the only twisted pair in the entire amplifier. Lead dress is a nightmare. I theorize that the air core coils at each anode are there to suppress supersonic oscillations that are bound to happen with a rat's nest mess like this. I further speculate that they got away with this due to the overall low gain in the amp. This is a quartet of 6L6, yet I recall seeing reference to an output of only 40 watts or so.
I'm okay with the idea of having a heavy and overbuilt amp for a scant 40-50 watts, I figure the output tubes will last forever here - and if it sounds good I'll leave it be. Of course, when I say leave it be, I mean live with the stock voltage supply and standard component values to support a cathode biased quartet of 6L6. Pretty much everything depicted above will be gutted and rebuilt, and while the spaghetti pile of wiring may have worked fine for many decades I'd like the ability to ramp up power and sensitivity without motorboating.
Another design change I plan on deploying is some variation on a cathode bias balancing circuit such as Blumlein's Garter Circuit so I can "use up" unmatched pull job & oddball tubes instead of saving them for single ended 6L6 amps (of which I think I only have one).
5 comments:
Leave the amp as is. Recap it using double the values for the filter caps. Add a 100uf 100v cathode bypass cap. Add an RCA jack right off the volume pot. Add binding posts and make sure that your negative feedback resistor (3.9k or 39k depending on which year yours is) goes to the 16 ohm tap (green if I recall.)
Install a bucking transformer to knock off 6.3 or 12v in it as these were designed to run on 110v-115v and they WILL cause the tubes to red-plate as they sit.
You will be surprised by the sound of the stock circuit, nothing wrong with them-find another and some good tubes, they are Mac killers.
This amp has been slated for use as a test bed for some instrument amp ideas I've had floating around since I first laid eyes on it. Structurally speaking, I'm doing little that can't be reversed or covered up with a wooden frame around the chassis walls; that said, the amp is gutted, choke/transformers are pulled & queued up for a coat of paint and I'm doing some cleaning up on the metal work.
Should I come across another one I will set it up as you describe and audition it in the living room to make an informed decision about how to treat a pair. Thanks for the good advice!
Hi, I've been looking at one of these for a lat '50's tweed project but was always curious about the 10 watts a piece from the 6L6s. If you don't mind my asking, what kind of B+ are you getting on the plates? Thanks, Skip
B+ is unknown at this point, I will post what I find when the amp is put back together..
B+ at the first filter cap is about 420. 410-417 on the 6L6 G plates Which is a bit high for them.Screens are at 320.I measured about 25v across the 250 ohm cathode bias resistor.The 40 w makes sense now with only 25ma.per tube.Use GC's.The hell with the RCA's GE or Sylvania work fine.
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