In Reply to: Re: 300B switching conversion for Moondog posted by mobile homeless on December 21, 2000 at 13:47:36:
Wow you have quite a memory, or did some searching in the archives !
Here are some specifics on my modification and my own very subjective opinion of the sound.
I am running the 300B plates off the C1 power supply voltage with a 100 ohm resistor and a 10microfarad solen capacitor to provide some isolation. This gives about 425 volts at the plate with a voltage accross the cathode resistor of about 71 volts. So the 300b is seeing about 350 -355volts from cathode to plate. I have used switches for both the filament a nd plate voltages, so with the unit switched off, I can change back to the original configuration.
I have been doing my listening through Lowther PM2-A's in medallion II cabinets. I originally felt I heard a fuller but somewhat fatter base response using JJ-300b's and the original equipment Phillips 6sn7wgta driver tubes. The amp is equipped with the standard carbon comp resistors, and Hovland coupling caps. I would call the mid and upper mid range a little less detailed (but very nice) when compared with 2a3's - a little bit more like the detached lush sound that some 300b designs are noted for. I did notice an odd, occaisional, upper frequency hardness with this configuration. (noticeable on some female vocals)
After my initial listening, I did some additional tube rolling. I changed the driver tubes (both stages) to Hytron 5962's. This makes the bass seem both tighter and more extended. The high frequency hardness seems to be gone, and there seems to be more detail too. The JJ300b's now have around 350 hours on them, which could also account for some of the change in sound. I was doing my critical listening with around 175 hours with the original configuration.
I preferred the sound of the AVVT-2a3 meshplates before I added the Hytron drivers, but right now I am really enjoying the 300b's. I think the amp is more sensitive to brand of 6sn7 tubes with the 300b's installed. I noticed a similar high end hardness in antique sound labs amps using the Phillips drivers with 845 power tubes. In those amps I found that using GE, RCA or mid to early vintage sylvania 6sn7's made a large improvement to the entire sound range.
With the 2a3's installed in the moondogs I thought the phillips 6sn7's sounded good, without the hardness, but I haven't really tried any of the vintage tubes for comparison.
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