Re: Input Tube Selection for Amplifier
June 17th, 2013, 3:05 pm
The keys are needed drive voltage and the amount of feedback to be used. If it's an A2 PP KT88 type, then about 142V-PP drive is needed at each plate. This also assumes ~525VDC at the plate, triode mode, ~65mA idle current (-61V bias) So, allowing for excursions to +10V on the grid (which is probably extreme), I get ~142VPP.
If you DC couple the pairs, then the second plate will be somewhere around 300V (cathode around 150), so the + swing will bring the plate to +371V. Better have at least 425V B+ with plate CCS/choke loads, or >500V B+ with resistor loads.
The other application is G2 screen drive. Now we need 300V PP drive to each grid
So that 300V becomes 450V+ at peaks. 500V with choke/CCS loads, or >650V with resistors!
If you assume 3V PP as an input, then the screen drive amp needs a gain of 100 from the input stage after feedback; the A2 amp needs 47.3 after feedback. Another assumption comes in here, that with a good set of output iron, either amp will only need 10dB of feedback at most. If that pans out, then the gains needed become 316 and 150 respectively. I think.
So, with those requirements, and assuming and optimized gain 2nd stage consisting of a 6SN7 LTP (it can swing the volts cleanly!), with a gain of close to 20, then first stage gains needed are 16 and 7.5.
Since the fisrt LTP isn't fed a balanced signal, it's gain will be roughly 1/2 of mu. The 6SN7 works with the A2 amp (x10), but the screen drive needs 32+ (6N7, I think?).
Now there are a lot of "ifs" in the above, and that may mean a 6SC7 or higher gain tube. Only building, testing and listening will tell...........................................................................
Slick
If you DC couple the pairs, then the second plate will be somewhere around 300V (cathode around 150), so the + swing will bring the plate to +371V. Better have at least 425V B+ with plate CCS/choke loads, or >500V B+ with resistor loads.
The other application is G2 screen drive. Now we need 300V PP drive to each grid
So that 300V becomes 450V+ at peaks. 500V with choke/CCS loads, or >650V with resistors!
If you assume 3V PP as an input, then the screen drive amp needs a gain of 100 from the input stage after feedback; the A2 amp needs 47.3 after feedback. Another assumption comes in here, that with a good set of output iron, either amp will only need 10dB of feedback at most. If that pans out, then the gains needed become 316 and 150 respectively. I think.
So, with those requirements, and assuming and optimized gain 2nd stage consisting of a 6SN7 LTP (it can swing the volts cleanly!), with a gain of close to 20, then first stage gains needed are 16 and 7.5.
Since the fisrt LTP isn't fed a balanced signal, it's gain will be roughly 1/2 of mu. The 6SN7 works with the A2 amp (x10), but the screen drive needs 32+ (6N7, I think?).
Now there are a lot of "ifs" in the above, and that may mean a 6SC7 or higher gain tube. Only building, testing and listening will tell...........................................................................
Slick
Re: Input Tube Selection for Amplifier
June 17th, 2013, 3:08 pm
Don't overlook the EL84 in the second stage 
Re: Input Tube Selection for Amplifier
June 17th, 2013, 3:27 pm
Good idea, Roscoe. Great sounding tube. I'll have to give it another look in triode! 
Re: Input Tube Selection for Amplifier
June 17th, 2013, 3:39 pm
Or pentode... Also, if you're going to play in triode mode, don't use the similar SV83, it has a lower maximum screen voltage... 
Re: Input Tube Selection for Amplifier
June 17th, 2013, 3:55 pm
Or pentode...
OMG! As if a triode LTP with cathode current sources isn't complicated enough. And no........ I don't have $$$ to buy plate chokes..........
Re: Input Tube Selection for Amplifier
June 17th, 2013, 4:02 pm
If you use a 1K triode strapping resistor the SV83 can handle 300V on the screen. It doesn't do UL, however if you have a push pull amp.
Since 1998 Decware has been selling SE SV83 amps, and out of several 1000 sold, none have had any issues with output tubes failing due to low screen ratings.
The amp in my kitchen is one such decware amp, and I've used it daily for over 5 years with no issues, and depending on the rectifier I use, it will run the SV83 upwards of 320V.
Since 1998 Decware has been selling SE SV83 amps, and out of several 1000 sold, none have had any issues with output tubes failing due to low screen ratings.
The amp in my kitchen is one such decware amp, and I've used it daily for over 5 years with no issues, and depending on the rectifier I use, it will run the SV83 upwards of 320V.