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Re: DC vs. Cap

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Well, I got into this comparison via building "Darling" 1626 amps. Here's what I have found : the DC amp is somewhat more difficult to get set up properly, but not as bad as I had feared. The presentation is more detailed and faster in comparison to a similar amp with Solen coupling cap.

The main concern (very important) is having a high enough B+ so that you do not compromise the operating points of the two tubes. If it's too low, you are forced to choose between lowering the p-k voltage of one or the other tube, or lowering the plate load R of the driver, or lowering the Vbias of the power tube (probably the best solution if you are forced to choose.)

Here's how my simple 2A3 (8532 driver) amp works.

B+ is about 415VDC. 2A3 is running about 245 V plate to cathode and about 42Vbias. So the grid of the 2A3 sits at 128V : this must also be the plate voltage of the 8532. I was able to get this to work using the following parts values : cathode R of the 2A3 is 2.5K. Note this part is dissipating 11 and a half watts : you MUST get a VERY beefy part here, a heatsinked-aluminum body or big ol' ceramic/wirewound 25 watt resistor is necessary.

(Fair Radio has a good selection of suitable parts : my 2.5K/25 W units look just like Harley Knucklehead cylinder heads, I mounted them on top of the chassis as much because they look cool as to keep them running cool.)

The cathode R is bypassed with 1000uF/200V TSHA 'lytic cap. The plate of the 2A3 is loaded in the conventional manner (unconventional around these parts I guess) with an AudioNote T-144 tranny, 2.5K:8 ohms. The 2A3 pulls 68mA through these; they are rated for 90mA.

Plate load for 8532 is 26.5K, with a 100 ohm cathode resistor bypassed by 4700/6.3V ELNA. This results (using selected/matched 8532's, mine were all over the map unfortunately) in 10.8mA throught the 8532, dropping the necessary 287V through the plate load. 8532 Vbias is 1.08V, not as much as I'd like, so I bypass the cathode R for most gain. I do have a pair of current source boards I plan to listen to when I get a chance.

I built a very similar amp using 8532 driving a pair of 1626 in parallel single-ended configuration. 3K/20W (shared) cathode R and around 33K plate load for 8532.

Both amps sound delightful, with the 2A3 amp a winner. But I have some work to do on the "Double Darling" power supply before I write it off.

-j






Topic - DC vs. Cap - bp 12:40:41 09/28/99 ( 19)