#1 GK-71 - economical build.
Posted: Wed Jul 03, 2013 10:45 am
On the GK-71 thread in Valves I set out two operating conditions for the beastie for 12Watts and 25Watts output along with some example circuits. I then foolishly ordered two pairs of the beasties
So now I've started costing the build for the 12W design and decided it was getting too pricy for me at present with everything else I'm doing - not least getting the AERs repaired! What to do? Well, whilst investigating costs for OPTs I realised that the Hammond 125ESE is about as cheap as one can go and still have good power (please let me know if their are better or cheaper alternatives!) so I've based a design around that OPT. That sets a limit of 80mA on anode current and looking for a good op point for that I came up with the following conditions for the GK-71:
Load Line 5000 R
Power 9 Watts
Va 500 V
Ia 80 mA
Vg -72 V
Pa 40 Watts
Rk 900 R
P(Rk) 5.76 W
Ck 35 uF
F3 5 Hz
That looks good on paper - still 9watts out
The simplest circuit is the two valve one but on further analysis using the PL802 as the first valve has some problems associated with it - namely excessive millar capacitance that requires a 100R driving impedance - Ouch
I has flippantly remark at Owston that driving the GK-71 with 6AM4 would be good as this is a tiny, tiny valve designed for VHF/UHF use. However this is just what we need as it ensures low millar capacitance with high gain. But is it beefy enough? It will run 10mA anode current and that is marginal for 72Volts swing into the GK-71 grid but on the Quasars I am unlikely to run 9watts so I'll chance it - we'll see what it sounds like in the room at Owston!
6AM4 Operating Conditions
Va 200 V
Ia 10 mA/V
Pa 2 W
Vg -1 V
Rk 100 R
Ck 318 uF
F3 5
Rload 38000 R
Av 69
P(Rload) 3.8 W
B+ 580 V
Right on the max levels for a 6AM4 but its at least 5,000 hours at this level and they are cheap - at present anyway
The circuit is attached. I'm using Andrew Is filament supply idea (thanks Andrew for the inspiration!). The 110uF psu caps are series connected 220uF at 350V as these are very cheap! I haven't found a good source for the mains trannie yet that is cheap. If necessary I'll use a 230:110 transformer in reverse - not a good idea if other options exist but doable until the budget increases...
Now just to get all the parts and build the damn thing!
James
So now I've started costing the build for the 12W design and decided it was getting too pricy for me at present with everything else I'm doing - not least getting the AERs repaired! What to do? Well, whilst investigating costs for OPTs I realised that the Hammond 125ESE is about as cheap as one can go and still have good power (please let me know if their are better or cheaper alternatives!) so I've based a design around that OPT. That sets a limit of 80mA on anode current and looking for a good op point for that I came up with the following conditions for the GK-71:
Load Line 5000 R
Power 9 Watts
Va 500 V
Ia 80 mA
Vg -72 V
Pa 40 Watts
Rk 900 R
P(Rk) 5.76 W
Ck 35 uF
F3 5 Hz
That looks good on paper - still 9watts out
The simplest circuit is the two valve one but on further analysis using the PL802 as the first valve has some problems associated with it - namely excessive millar capacitance that requires a 100R driving impedance - Ouch
I has flippantly remark at Owston that driving the GK-71 with 6AM4 would be good as this is a tiny, tiny valve designed for VHF/UHF use. However this is just what we need as it ensures low millar capacitance with high gain. But is it beefy enough? It will run 10mA anode current and that is marginal for 72Volts swing into the GK-71 grid but on the Quasars I am unlikely to run 9watts so I'll chance it - we'll see what it sounds like in the room at Owston!
6AM4 Operating Conditions
Va 200 V
Ia 10 mA/V
Pa 2 W
Vg -1 V
Rk 100 R
Ck 318 uF
F3 5
Rload 38000 R
Av 69
P(Rload) 3.8 W
B+ 580 V
Right on the max levels for a 6AM4 but its at least 5,000 hours at this level and they are cheap - at present anyway
The circuit is attached. I'm using Andrew Is filament supply idea (thanks Andrew for the inspiration!). The 110uF psu caps are series connected 220uF at 350V as these are very cheap! I haven't found a good source for the mains trannie yet that is cheap. If necessary I'll use a 230:110 transformer in reverse - not a good idea if other options exist but doable until the budget increases...
Now just to get all the parts and build the damn thing!
James