Wow, a reply from the man himself! I am blown away. I should have the amp finished tomorrow, can't wait to hear it. It just so happens that I have a couple of lovely Lundahl plate chokes that I used for a build of Jeremy Epstein's "Free Lunch", so that is likely to be next direction for me.
Thanks again for the thoughtful reply!
John
Thanks again for the thoughtful reply!
John
Hey, the 6C6 can draw a screen current of ~0.5mA. The 0.5 uF cap hanging off the screen can help to buffer instantaneous screen changes, but with a 1.5M resistor feeding it, it will take a while to fill up again. I would expect the screen voltage to wander around with sustained drive. Try feeding a steady sine into the input of your finished amp and monitor the 6C6 screen.. I would expect the voltage to sag, especially as there is no global feedback to limit the 6C6 plate excursion. Just sayin'...
You posters are making me think about using a Class A Pentode input tube . . .
1. Quiescent Screen Current; no test tone; no music
2. During 1/2 of a sine wave test tone, the screen current is larger than the quiescent screen current.
3. During the other 1/2 of a sine wave test tone, the screen current is smaller than the quiescent screen current.
I am estimating the results when an average electric bass plays at the bottom of its range, about 40Hz.
At 40Hz, 0.5uF, Xc = 8k Ohms.
Let Quiescent Screen Current 0.5mA
Suppose the screen current increases to 0.8mA peak, delta current is 0.3mA during the 1/2 cycle, 0.3mA x 0.637 = Average 0.19mA extra current.
Delta 0.19mA x 8k Ohms = screen voltage Reduces by 1.5V.
During the other 1/2 cycle, the screen voltage Increases by 1.5V.
For a 40Hz sine wave, there is just as much time for the 0.5uF cap to re-charge, as there is time for the 0.5uF cap to dis-charge.
Class A works great.
An estimate is just that.
A measurement confirms or denies the accuracy of the estimate.
1. Quiescent Screen Current; no test tone; no music
2. During 1/2 of a sine wave test tone, the screen current is larger than the quiescent screen current.
3. During the other 1/2 of a sine wave test tone, the screen current is smaller than the quiescent screen current.
I am estimating the results when an average electric bass plays at the bottom of its range, about 40Hz.
At 40Hz, 0.5uF, Xc = 8k Ohms.
Let Quiescent Screen Current 0.5mA
Suppose the screen current increases to 0.8mA peak, delta current is 0.3mA during the 1/2 cycle, 0.3mA x 0.637 = Average 0.19mA extra current.
Delta 0.19mA x 8k Ohms = screen voltage Reduces by 1.5V.
During the other 1/2 cycle, the screen voltage Increases by 1.5V.
For a 40Hz sine wave, there is just as much time for the 0.5uF cap to re-charge, as there is time for the 0.5uF cap to dis-charge.
Class A works great.
An estimate is just that.
A measurement confirms or denies the accuracy of the estimate.
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Having said that, for such a physically large tube, the 6C6 doesn't seem good for a lot, with a plate dissipation of only 0.75W and screen dissipation of only 0.1W in pentode mode.
The 6C6 is a beautiful tube.
It is a small signal tube, not a power tube.
The filament is only 300mA.
It provides plenty of gain for a 2 stage SE DHT (DHT with lots of bias voltage, requiring lots of peak to peak drive voltage).
It is inexpensive for a Vintage NOS tube.
What is not to like . . .
Well, you probably do not have a 6 pin socket on hand; and you do not have the right sized grid cap on hand.
It is a small signal tube, not a power tube.
The filament is only 300mA.
It provides plenty of gain for a 2 stage SE DHT (DHT with lots of bias voltage, requiring lots of peak to peak drive voltage).
It is inexpensive for a Vintage NOS tube.
What is not to like . . .
Well, you probably do not have a 6 pin socket on hand; and you do not have the right sized grid cap on hand.
Simple -at least 1W- 2W -I use wirewound- potentiometer.
The dissipation is 0.125W (2.5V heater voltage).
The dissipation is 0.125W (2.5V heater voltage).
I probably do have a few 6 pin sockets on hand as I bought a few by mistake, when looking for sockets for 1625.
Hi John,
Your chassis reminded me of the last pair of Flesh and Blood 300B monoblocks Herb Reichert built.
JE
That pair of amps were commissioned by Herb's friend in the late 90s. He built them exactly as described in his Sound Practices article using the 5K pri Z Tango Hirata XE60 OPT. Sadly his friend passed away a couple of years ago. As it turned out, the new owner is my Instagram contact. Small world for DIY/SE-DHT enthusiasts.
I had to search my HD for the scanned film photo when I saw your amp project. Road signs repurposed as tube amp chassis is such a cool idea!
I had to search my HD for the scanned film photo when I saw your amp project. Road signs repurposed as tube amp chassis is such a cool idea!