by EWBrown » Thu Oct 11, 2007 7:38 am
If you replace the 6AX5 with SS rect (easiest mod is to solder in some uF4007s to the appropriate pins in an empty octal tube base), you should kick the voltage up by another 20-25VDC or so. WIth 300Bs, every little bit of B+ helps...
I did some voltage measurements on the still-in-process "300B on an ST70 chassis project", getting 455V B+, 435 VDC on the 300B plates, and 72-73VDC across the 1K cathode resistors 2 2K 10W in parallel, each).
A quick SE CAD simulation indicates this should be good for just a little short of 7 WPC.
This was strictly an "idle current" test, as I have built the SRPP driver board, but haven't yet installed it. The OPTs are some "Transcendar" 15 Watts, 3K primary 300B SET, that I scored on E-bay last year. Their DCR is about 280 ohms, so it appears that Ohm's Law still holds true Yellow_Light_Colorz_PDT_08
AES claimed that the socket and turret board that I bought and used was appropriately sized for an ST-70. It isn't, and the "ST70" board in the 2007 catalog is still the wrong size, too...
I'll need to do some mechanical wizardry / hackery to make it fit, and not look like a totally sloshed Homer Simpson built it.
I powered the 300B filaments with 5VAC, 2A trannies, one for each tube. Along with a 50 ohm WW "balance" pot. This gets all the 60 Hz hum out, I do get a tiny bit of 120Hz ripple. (Just from the 300Bs, with no other signal input).
Note: there is no electrical connection between the circuitry and the chassis at this time, except for the OPT secondary "cold" end. I am going to experiment with what point on the chassis yields the least hum / noise.
Since this IS the get*set*go topic, I have finally fired up the 6B4G version, and it sounds great!
Some day, will be a "shootout" between it and the "fancied up" Bottlehead Paramour IIs. The BH amps do run the B+ voltage higher, and the split rail scheme puts around 320VDC from P to K (ok, F). But then, Bottlehead specifies 1100 ohms for the 2A3 cathode resistors.
The next G*S*G incarnation, for the second board, will use 6C4Cs s and some Magnequest RH-60 OPTs. I ran the SE CAD on this combo and it looks really good. Since the OPTs have a 5K primary, the power out is a tad less, but the 2nd harmonic distortion falls off fairly significantly. I could "cheat" and use the 16 ohm secondary tap, and make it look like a 2.5K OPT, for more power output and higher 2nd harmonic content.
Then there are some 6BL7s t experiment with.... And 6CK4s, etc... Yellow_Light_Colorz_PDT_07
/ed B in NH
Last edited by
EWBrown on Tue Feb 19, 2008 11:17 am, edited 3 times in total.
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