I’ve got a SSE on the bench for checkout. At this point I’m only operating with the 12AT7 installed and I’m operating it off a regulated power supply connected at the choke terminal. As such, I’m taking some measurements around the CCS and driver tube OPs at various B+ values. I’ve run up against something interesting and wondered if it’s been discussed previously.
First, I’m guessing that R14/24 is there to dissapate some heat and take some of the load off the CCS heat sink. Is that correct George? It wouldn’t have occurred to me to use that resistor in the design and I’m wondering it’s purpose.
To the point now, I’m noticing that the AC wave form observed at the plate looks pretty horrible until the B+ voltage is dialed up to about 375 volts. Then it begins to look terrific. That’s not a problem for most users obviously, since a higher B+ is typical, however it is a curiosity to me.
I placed a DMM across R14/24 and observed that voltage drop across that resistor was about 86 volts and constant with B+ from 500 volts down to, you guessed it, 375 volts - after which the voltage drop (and current) begins to decline. With lower B+ the CCS appears to fail to control current as it looses its required driving force. I’m thinking R14/24 then starts acting like a load resistor and suddenly the 12AT7 begins to operate across diagonal load lines rather than the horizontal load lines of a CCS load; hense the ugly wave form.
Assuming for a quick moment that I’ve got my head around this correctly, and that might be a stretch, one might want to reduce the value of that resistor if one planned to operate at a B+ value much below 400.
What say those who know?
First, I’m guessing that R14/24 is there to dissapate some heat and take some of the load off the CCS heat sink. Is that correct George? It wouldn’t have occurred to me to use that resistor in the design and I’m wondering it’s purpose.
To the point now, I’m noticing that the AC wave form observed at the plate looks pretty horrible until the B+ voltage is dialed up to about 375 volts. Then it begins to look terrific. That’s not a problem for most users obviously, since a higher B+ is typical, however it is a curiosity to me.
I placed a DMM across R14/24 and observed that voltage drop across that resistor was about 86 volts and constant with B+ from 500 volts down to, you guessed it, 375 volts - after which the voltage drop (and current) begins to decline. With lower B+ the CCS appears to fail to control current as it looses its required driving force. I’m thinking R14/24 then starts acting like a load resistor and suddenly the 12AT7 begins to operate across diagonal load lines rather than the horizontal load lines of a CCS load; hense the ugly wave form.
Assuming for a quick moment that I’ve got my head around this correctly, and that might be a stretch, one might want to reduce the value of that resistor if one planned to operate at a B+ value much below 400.
What say those who know?