tictac wrote:I wouldn't split hair worrying over much about OT impedance. A Deluxe Reverb uses a 6K6 OT for a pair of 6V6's and a Princton uses an 8k OT both way off your 10K number; any OT made to operate a pair of 6L6's would be fine... somewhere around 4k.
Remember Jim Kelly amps are running those 6V's at 485 to 490V on the plates to achieve the claimed 60W output. Do you really want to do that?
With 420V on the plates your 20mA is just right (70% plate dissapation) you could bump it up to 23mA each but I wouldn't go beyond that....
TT
Hi
Good point (and frequently overlooked) about OT impedance. Kelley's OT was at circa 4k, although I have used 4k3. Kelley's plate voltage was high, but at full load the B+ drops to about 425V.
In any case, does the power transformer involved have enough current (around 330 mA) for 60W rms?
Jim Kelley was using 6V6GTA (the same as 7408) which are rated at on some specifications pages as 14W compared to regular 6V6 at 12 W (which I recall Sylvannia rated their tubes conservatively).
Hepcat wrote:Yes, 35 Watts is reasonable for a quad of 6V6s, I'm still having a bunch of trouble getting that. I seemed to have the amp stable and working using the 4 ohm tap with an 8 ohm speaker. Last night i was testing a guitar and the ting freaked out again. While I don't have another quad of matched fresh 6V6s laying around, I have about 10 used old tubes that will work as a test batch. I am stating to think that i have a possible tube problem. When my FM transmitters had unknown strangeness with all of the parameters seeming to be good, I'd put in a known good spare final tube. Often the problems went away and the original tube was shipped off to be rebuilt. I'll try the same in a few minutes and report what the result was. I have a 25 watts unit with almost the same circuit that works just great so I think I can make this work. Thanks for all of the suggestions so far.
ken
Hi Hepcat,
I just commented on another post in this thread, what is the VA of your power transformer? Or how many mA will the PT support on the Plate winding? To support 60W rms square you will need circa 331mA to support the load on the tubes with 423 Volts (full load dropped from 485-490V idle) on the plates.
You might want to check out Jim Kelley's schematic which is posted at this website, which I have attached to check out his PI. He used a voltage divided bias on his PI, a Concertina Splitter. His design goal was power tube distortion before PI distortion, before Pre-amp distortion.
(Note the Bias numbers and the amount of current in the PT as rated for 117 AC)
Best Regards,
Steve
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Crate used make 4x6v6, I think the opt was around 4k, the ps was relatively small for the 50w rig but they ran the plate V fairly high for the tube type. I've seen some old silvertone dano that biased 4 6v6 around the same ma. as 2 6l6, just a generalization but interesting to see in a schematic.
I have built a quad 6V6 amp with all bells and whistles - that is - power scaling, each pair SEPARATELY switchable between cathode and fixed bias and between pentode and triode mode.
Ideal tranny is a Hammond 1645 but I (like some above) used a spare 50W Marshall OT which I had on the shelf, Raa=3K4. I get about 30W out of it but then I'm running only a +350V rail. I did this (350V rail) deliberately so that I could use some 50 year old ST shaped 6V6G from the Sydney Australia AWV Factory.
The Output Tube anode and screen supplies are power scaled, and it has a tracking bias supply. A post Phase Inverter "limit" control is incorporated to allow control of how hard (if at all) you overdrive the scaled output tubes.
A Raa of anywhere between 3K and 5K will suit. If running rails in the 300 to 350V range tend toward the lower impedance (3K) end, if running rails at >=400V then you want to tend toward the higher end (5K). It really is'nt that critical and I defy you to pick the difference between a 25W and a 35W Amplifier. I can't and I've had lots of practice.
Cheers,
Ian
