A Brief History of The Get*Set*Go

2nd harmonics for the masses

Postby Shannon Parks » Sat Nov 10, 2007 7:18 am

A Brief History of the Get*Set*Go

I started working on it while on a trip to New Orleans back in 2004, and I had a stack of old Sound Practices and some datasheets with me. Mornings were spent at Cafe du Monde and the rest of the day at Lafitte's Blacksmith Bar. I believe I came up with the Get...SET...Go... while at Lafitte's.

The PA774 was chosen for its relative cheap price, made here in Illinois and sold by Triode. But most importantly, it had dual 6.3V filaments, so 6B4s (which I have a stash of from my Ike PP testing) become the tube of choice. That fact that Russian NOS and that Sovtek monoplates are still available and cheap, made this very enticing. Plus, old 6B4s are still cheap on eBay compared to 2A3s. This was needed for the different biasing of channels. I figured that I had enough filament current left on each tap for one tube each, and fortunately most tubes are happy with a DC bias on their filaments (within reason). So the JE Labs design with the 6SL7 really became the basis at that point. The 6AX5GT is basically a 5Y3 with a 6.3V filament. It's filament requirements looked like they could squeeze in as the fourth tube, and its voltage drop was a huge plus. I needed the 380V B+ of the PA774 lowered to keep my max plate voltage at 275V (eg 325V B+ minus 50V at cathode), the rating for the old 6B4s. The first prototype used AC filaments on the 6B4s. With a fine tune hum pot, the best hum results I could get were 10mV of hum. So I modified the circuit with DC. AC ripple is still there with the simple CRC filter, and a virtual hum pot with the voltage divider resistors was used. Basically, the hum performance is as good or better than a 2A3 run on AC, around 500uV.

The amp can be modified using similar 6B4-types. One mod is a 300B mod, which I am using right now. It requires a UX4 socket and adjusting R19 & R20 for 5VDC. Currently, it is giving me about a half watt more power since it is more linear at max power than the 6B4s that I have. Those are 3W and the 300B is 3.5W, both maxing at 5%THD+N. The GSG really isn't a good candidate for hot rodding. The driver circuit - half of a 6SL7 - just isn't robust enough for a 380V B+ 300B SS rectified mod, IMHO. Possibly some triode-strapped EL34s might work well in this application, but I think 300Bs need a better driver, better outputs, etc.
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Postby Bob Lee » Sat Nov 10, 2007 11:22 am

Shannon,

Thanks for the explanation.

I noticed the driver tube 6SL7 plate and cathode resistor are different between JELab simple 45/2A3 vs get*set*go. Could you give us some insight how you arrive the value of the two resistor.

I am fooling around with a single triode driving a 45 tube. The 45 are much easier to drive, so I have a pick of 6P5g ( 76 in octal tube, love the sound ), 6L5g, 6J5 ( 1/2 6sn7).
My plate resistor is 33K, using a red LED for cathode resistor (1.56v), B+ is 320v.

I am looking for some more ideas.

Thanks
bob
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Postby Shannon Parks » Mon Nov 12, 2007 6:54 am

Bob Lee wrote:I noticed the driver tube 6SL7 plate and cathode resistor are different between JELab simple 45/2A3 vs get*set*go. Could you give us some insight how you arrive the value of the two resistor.


Good question. I did the testing with two variable resistors - one as the plate resistor and the other as a cathode resistor. While measuring in circuit I monitored the gain and distortion of the amp at both 1kHz and 20kHz. I monitored 20kHz during this process to make sure that the Miller Effect roll-off above 20kHz didn't hurt the performance too much. Ultimately, I decided on those values as a best mixture of those three things: gain, linearity and bandwidth. I did do tests with red LEDs, too. They worked fine, but I did lose .5 to 1 dB of gain compared to the resistor/bypass cap configuration.
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