I Built a 5902 PP amp fed by an op-275 op-amp.
And John Smith just built one.
Now he's interested in making a 6V6 version of the same amp.
Tonight I started a few tests in Bread Board.

Here is what I have so far ( testing just one channel)


6AQ5 PP fed with op-275 preamp
B+ = +235vdc
SG = 190v
Cathode resistor = 280 ohms
Cathode bias voltage = 11.5v
20 ma per tube
8 ohm to 8K PP OT (Musical Power Supply inc OT-15 PP)

With the op-275 preamp fed from my Mp3 player... all sounds very good and loud.
( I am not getting much wattage out yet until refinements)

When I feed in a 600mv PP 400Hz tone ( just below the max threshold of distortion)

With an 8 ohm speaker load I get 12v PP out.

12v PP = 4.25vRms

with 8 ohm speaker load = 2.25watts .... for the moment.

Now get rid of the 22k and connect the screens direct to B+. Then lower the cathode resistor to 200 ohms.

...or else!!.....
I have an image of you guys with 3 sound systems in every room and double-blind testing underway 24-7.
When you find the perfect amplifier, THEN what?

Try to make an amplifier from recycled belly-button lint.

Double blind testing?
What does that mean?

Peter
Ok to just use the 6V6GT, instead of the 6AQ5?
If not I will plan on getting them.

Easy..
That means you find two blind guys... and then start asking them questions...

The schematic above will work but should have a screen bypass cap.

Hi Norm:
Thank you. I'll do that as a test step...
I don't know how to select the correct value.
I just read that the "rule of thumb" is the capacitor's reactance value should not to exceed 1/10 the value of the SG resistor.
But how does that work? Without going through all the calcs is there an easier rule or simple range of cap values for audio?... lol

If I interpret that correctly.. are they simply indicating that a bigger cap will be better since a very large value cap's reactance at low freq audio will certainly be very low?

REF:( http://www.rfcafe.com/references/popula ... ronics.htm )
"A rule-of-thumb used by circuit designers is that the reactance of the capacitor at the lowest frequency to be passed should not exceed one-tenth the value of the resistor it bypasses. Using this rule, we substitute known values in the equation developed for finding C:

C=1/(6.28 x 100 x 150) = 11 μf

An electrolytic capacitor is suitable for this purpose because its leakage resistance is not important and high capacitance is obtained in a compact size.

In some applications, such as high-quality audio amplifiers, the ratio of resistance to reactance at the lowest frequency passed is made 20 to 1 or even higher, but the ratio used in our example is adequate for most cases. Needless to say, the working voltage of the capacitor selected for any bypassing applications must be larger than the maximum voltage present. "

( I want to also try removing the SG resistor and go direct to B+.

I built this breadboard setup as an initial step just to see if I can get decent results with the B+ at 235v mostly because I wanted to attempt to keep my power supply caps at 250v rating. Otherwise if I take the B+ much higher I'll have to start looking for 300v or 350v caps.

I also in the final design expect to use an OT with UL taps. But the 8K OT I used for the test does not have those.

BTW:
I just saw some info online about SG by-passing where the guy talks about "output filter cap" ... What does that refer to? A cap across the primary of the OT?:
Ref: ( http://www.diyaudio.com/forums/tubes-va ... s-cap.html )
"With some types like the 6AQ5, 6V6, 50C5 the screen runs at the same DC voltage as the plate. In that case, the output filter capacitor provides the screen AC grounding. "

OK ... thanks I'll do that.
But you don't explain anything about what you expect that will achieve?
Greater output power I assume?

I don't know...lol
What does a retired surgeon do for fun... does he have to dig up dead bodies just to "keep his hand in"?? lol

When I built my 1st amp.. the 5902 PP stereo amp... it was a wonderful learning experience. Then I refined it a few ways over the next cpl years to where i think it's optimized.

I often thought about building another amp.. but as much fun as it seemed.. the value seemed silly ... because what will I do with it?
I'm already happy w/the 5902.
So.. do I keep building amps and throw out the old ones or start stacking them up in the living room and alternate usage each day?
Seems pointless.. right?

So I haven't built any more.
But I just helped John build the 5902 .... and he loves it too.
But he keeps hankering for more amps to build like a frenzied audiophile... lol
So we were mulling over the fun of trying a 6V6 output on the same op-amp preamp as the 5902... and so here we are!

I'm bread boarding it to try to keep B+ at 235v to avoid needing larger filter caps in the power supply. And also just to see how the op-275 will drive the 6V6 (6AQ5).

While he drills holes for his octal sockets he offered to drill for me as well.
So.. looks like I'll also end up with a (hopefully) nice 6V6 version of the 5902.

Hehe. Yeah, not too far off. Not any worse that a radio collector, though.

I don't think you have to worry about anyone finding the 'perfect' amp. I mean, it's not even definable, which pretty much guarantees an ample supply.

Yes. What's the point of having a 'bigger' tube if you don't use it?

That 22k lowers screen voltage for no reason, and lower screen means less power ability.

On a serious note, it has been found that when people know about the conditions or object of the test it influences the result, and that applies even to the giver of the test. It's not that it's necessarily 'intentional' either but subtle clues can be given off.

Double blind means neither the subjects nor giver of the test knows what's what so, in theory at least, they can't influence the results.

Using several 6080's or 6336 directly connected to a speaker should come closer to the perfect amp? No output transformer to limit frequency response.

Sounds like the "Futterman Amplifier"---we had a thread on this a while back.

Well that's intriguing Norm.
Is it real world practical? Are there any commercial amps using that?
I might like to see what the schematic is all about.
I see the 6336 is called a "DC amplifier" and uses 6.3v @4.5 amps for filaments

I didn't invent the 22k SG idea... I just borrowed it from the original design I was given for the 5902 amp.
Never got to know what the design idea was exactly.. unless it was to act a bit like UL.

We removed it from that amp too.

Here's the latest state of the amp:

When I used 200 ohm cathode res. the current per tube went up to 30ma and the bias was 12.5v
With that set up and SG connected to B+ , I was able to get approx 6.5 watts out into a speaker and about 5.5watts into an 8 ohm resistor.

I found that when I cut the current back to only 25ma/per tube everything still "sounded" terrific and it will make my power supply a little easier to build... but I will need to do some thinking about that yet.
I intend to make a voltage doubler and I hope it won't be too high in the first stages before I cut it back to +235 for B+. I don't want to have to use big wattage dropping resistors between filter cap stages to bring it into the 235v range.

Anyway.. I have now added individual 25ma CCS for each tube with 1000uf by-pass caps.
That requires the cathodes to be separated... and so ..then I re-coupled the cathodes together capacitively.

The greatest advantage of using separate CCS for cathode-bias, is that each tube will always auto-magically be biased to operate exactly at 25ma. Regardless of the tube age or brand!
... I will not need to have matched pairs and anytime the tubes are ever changed, the bias never has to be readjusted.

It's the perfect "auto-bias" .
Each cathode voltage is now at approx +13.5v which the CCS will automatically vary as needed in order to achieve the exact 25ma bias current.

With CCS as cathode-bias, the current in the OT will also always be perfectly balanced as well optimizing its performance.
What more could one ask for?

==>> Here's the FINAL Updated version which simplifies the CCS circuit & adds a pwr trans:<<==