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Possible? You May Have Misinterpreted My Question

Posted by FlaCharlie (A) on August 8, 2007 at 07:59:24

In Reply to: Ping FlaCharlie posted by Russ57 on August 8, 2007 at 06:29:59:

Hey Russ - First, I hope all goes well with your surgery and wish you a speedy recovery.

I'm wondering if you may have misinterpreted my question. When I referred to two 6SN7s or four 2C22s - that was referring to the entire stereo amp, not just one channel. So, either a single 6SN7 or a pair of 2C22s per PP 6B4G channel. Using a preamp.

I've seen the schmatic you linked to. The schmatic of the Eico HF-87 I posted earlier is similar in that it also uses one triode section (half a 12AX7) direct coupled to a 6SN7 for each channel.

Here's the text from the "Is It Possible?" post Russ is referring to below so nobody has to scroll down again:

> > > Guys - So I'm still wondering if it's possible to accomplish this:

Run PP 6B4G in Class A (no feedback), cathode bias, using only four 2C22s (equivalent to one 6SN7 per channel) ? Keep in mind that I will be using a preamp that provides gain, not just a passive.

Paraphase or LTP with negative supply is fine with me. I don't want to use an interstage transformer since I'm trying to keep the cost down. I see how Poinz' LTP is built using a negative supply. And mikeyb explained how I could use the Eico HF-87 schematic and adjust the voltages downward to keep the same operating points by using a negative supply. And I did check out the PP1 schematic that Russ pointed me to which uses a CCS. My understanding is that a negative supply and a CCS are two methods of accomplishing the same goal. Correct?

Russ' comments help me understand what I need. . . I think. Are my interpretations correct?

"Pete's circuit has half of V4 operated as a voltage amp, which is directed coupled (actually, isn't it cap coupled? see C8) to the other half, which is operated as a split-load invertor. This alone is enough to drive tubes like a 6v6 or el84 but not a triode like a 6b4g. The split-load invertor is cap coupled to V5 which is operated as a push-pull amp."

So Pete's circuit uses V5 to provide additional gain. Will my use of a preamp be able to make up for the proposed elimination of this stage?

Also: "Notice one other big (and important) difference. V4's first section is running at 3.6ma (actually 3.0 I think - see R13), its second section at 4.5ma and V5 is running both sections at 6ma. The schematic you have posted is running about 1.3ma. This is important because it takes current to make a voltage change in the face of capacitance. The grids of the 6b4g have a good deal more capacitance the the grid of a pentode"

So I need more current than the 1.3ma provided by the paraphase. How much is needed for PP 6B4Gs in Class A? Is the current sent to the outputs the sum of the currents from those previous stages?

Can this be provided by the four 2C22/two 6SN7? In the Eico HF-87 schematic I posted the single 6SN7 per channel is running about 7ma. Is 7ma enough?

I've learned a lot from this discussion and I can build from a schematic but, frankly, I get confused by some of the math and charts. Ohms law I understand and I've read about drawing load lines in my tube manual and some online but it still confuses me. I'll keep trying but math is definitely not my forte.

So, is it possible? Can anyone suggest some decent operating points? < < <

Thanks . . . . Charlie

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