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I don’t want to hijack, but I’m interested in DC. Are there any risks involved? What precautions (circuit mods) must be taken to remove the coupling cap?
Hey !!!! You are in KCMO and I am in Warrensburg, MO, just 55 miles away. !!!
Good for you. Its virtually impossible to out-perform a two stage direct coupled amp. Getting rid of the coupling cap is the BEST thing a person could do, if you seek high audio playback performance.
To DC an amp, the second stage must be elevated, typically by using about a 20 Watt ( usually higher ) power resistor, as a Rk or cathode resistor ( self bias here ) of the second stage. ( R6 above. )
In a DC amp, the plate voltage of the first tube, becomes the grid ( biasing voltage ) of the Finals tube.
This means you usually have an overall B+ that is higher, to feed the second "elevated voltaged " ( Finals ) stage. A typical
cap-coupled 2A3 amp may employ a B+ of about
300 - 320 VDC, whereas a
DC 2A3 amp will have a total B+ of
450 to 480 VDC.
Let me give you some numbers, you can refer to the highlighted schematic above.
Assume 150 VDC on the plate of the first tube, and hence, due to the direct couple, on the GRID of the Finals tube, a 2A3. Lets say you want to conservatively run 42 mA. at 250 VDC Plate to Cathode across the 2A3's plate, ( which is a great OP Point BTW ) !!
Lets say it takes - 50 VDC of grid bias, ( from the 2A3 tube's characteristic curve graphs ) at 42 mA
and 250 P-K.
OK, this means the Final's cathode needs to be 50 VDC higher than the grid, or at 200 VDC . This calls for a 4,761 Ohm Rk resistor value ( 200 V divided by 0.042 A , Ohms Law ). The plate of the 2A3, at 250 VDC P- K, needs to be at 200 V + 250 VDC or 450 VDC, on the plate. There will be an R times I loss, across the primary of the SE output transformer, to account for. Lets say the output transformer has a 100 Ohm Primary winding , so it loses 4.2 VDC ( 100 Ohms times 0.042A. )
This means your "overall" amplifier B+, to the Final tubes, has to be 454.2 VDC, to have it all balance out perfectly. ( 200 V on the Rk, 250 VDC across the plate, and 4.2 VDC more to account for R times I losses across the Output transformer's primary winding.
It best to use a tube rectifier in a DC amp, I like 5U4GBs, and a current inrush limiter ( called a Thermistor, a CL-90 ) after the fuse, on the Primary side of the Power Transformer. This provides a gentler start up, and long term reliability, reduces turn-on circuit stressing.
I have already written a comprehensive multi-page article on how to design and implement such circuits, Contact me privately at my email address, and I will email it to you, or anyone else who is interested. I HIGHLY recommend a two stage DC amp, with a high mu ( typically 100 ) Driver tube.
You travel on the "right track" with two stage Direct Coupled SE amps. IME, a coupling cap is sonically POISON, compared to a Direct Couple.
Dowto1000
