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Mikey If We Are Talking Concepts...

210.84.179.233

Posted by Ejam on May 18, 2004 at 00:38:02

In Reply to: Way Too Much Iron on that Chassis posted by mqracing on May 17, 2004 at 17:30:31:

Guys

Was looking at Bas's and Keto's EXO-173 PP 2A3 amp and was thinking that a pentode driver instead of a triode driver might be worth a go. High transconductance pentodes run at high current make really nice drivers. Sorry no drawing package but if you want Bas you can draw this up for Version 3. Really appreciate it.

Take a 6688 pentode and using the data Pete Millet posted on his website: B+ 300V, Vpk = 130V, Vg2 = 150V, Ip = 21mA, Ig2 = 5mA and Rcathode = 50 ohms and Rplate = 8k (I used 8k2) operate it as the driver.

Use a DN2540 current source as use the load for a shunt regulator consisting of two VR150 in series. Set the current source at 41mA (21mA for the 6688 and 20mA for the VR tubes). Might have to heatsink your current source. The shunt regulator delivers 300V. Hook this up to the 8k2 plate load resistor. Taking the voltage from the first VR150 tube for your screen voltage of 150V. Cathode resistor is 51 ohm, no bypass. Input resistor 50k pot.

Using the EXO-173 phase splitter topology, that is single ended driver with EXO phase splitting for push pull 2A3 output stage. You will have to raise the B+ for the output stage a tad so as to get sufficient voltage drop across the current source, say 30V. As a suggestion operate the 2A3 at Vpk = 275V, Ip = 60mA and Vgk = -50V. Along with the transformer voltage drop this should give you about 330 - 335V, hence the required voltage drop for the current source. Now you can use a current source for the 2A3 but I like KYW suggestion of tying a 430 ohm resistor back to the 6688 cathode. In this case, adjust the 6688 cathode resistor to 9R1 (a 20R trimmer would be nice to dial it in) I know that it has been mentioned but ultrapathing the output stage with a 20uF oil cap between the B+ and the junction of the 2A3 cathodes works really well. So there you have a pentode driver. I can see the hairs standing up on some of the punter,s necks! I will let you guys in on a little secret, the 6688 is a great tube and produces a simple and very enjoyable PP amp. Listening to it now....fresh off the bench.

Regards

PS. forgot the power supply, 6DM4 damper diodes to an LC filter (5H-470uF). AC filaments.

PPS. iron count 4 and holding......

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