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Old 31st July 2011, 02:52 PM   #1
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Default 807 monoblocks with big PSU

Here's a schematic for a pair of monobloc amps I just completed, using the RCA 807 power tetrode.

Not a triode anywhere in the circuit yet it's beautiful sounding thing though I say so myself.

The PSU is a massive affair, with a 6AS7 providing regulated B+ to the output stage. The supply is effectively silent, giving 0.06 mV at the speaker outputs.

The PSUs are based on some I had powering a set of PL519 sweep tube SEP amplifiers with each mains transformer capable of supplying half an amp.

Backed up by this big supply, the 807 sounds completely unstressed and is capable of high levels of clean sound through my open baffles.

Whatever the argument may be over the need for a supply of this capability and complexity, to feed a "mere" 807, there's little doubt in my mind that this is a great sounding piece of kit.

I'm toying with the idea of regulating the screens on the outputs with a couple of 0A2s or VR150s in series. That might be the next logical move.

Click the image to open in full size.
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Old 31st July 2011, 10:56 PM   #2
ryuji is offline ryuji  United States
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I use three oc3's for regulating my 5 channel 807 amp screens. I run 400v b+, however on my 807
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Old 1st August 2011, 07:45 AM   #3
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Hmmm

That 5 channel amp must look cool when lit up

Steve
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Old 1st August 2011, 07:55 AM   #4
ryuji is offline ryuji  United States
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yes, it does. i dont have a good 'lit up' picture, but at my 400v b+ (soon will be upgraded to 500v) all the tops of my 807's glow blue
http://ryujiwarui.com/tube/forum_friendly/close_up.JPG

im right now using 12au7 anode+cathode follower as a preamp and AT7's as drivers for the 807's. me and my friend recently were dicussing perhaps using 6SN7 aikido's and same triode for drivers, but doing napkin math, it looks like the 6au6 all by itself takes line input and drives the 807's. wonder if that's an equivalent/superior result, you certainly get pure 'pentode sound' that way. Have you experimented at all with other ways to drive your 807's?

opps, typed 6au7 instead of 6au6

Last edited by ryuji; 1st August 2011 at 08:02 AM.
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Old 1st August 2011, 11:45 AM   #5
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Nice looking amp with all those 807s lined up like that.

Re driving 807s:
I started off with an Alex Kitic RH84 single ended pentode amplifier with a 12AT7 driver.

Over the years I had become dissillusioned with single-ended triodes and to be honest the RH84 was a revelation. It sounded far more like music to me than my SETs had done.

After the RH84 I built a sweep tube amp using PL519s which was better still.
Whilst working with SETs I had used pentode drivers such as the C3g and 12GN7 with good results. They seem to have somehow fallen out of favor these days, but I still like them.

It was a leap of faith to drive a pentode with another pentode in single-ended configuration. It was an even bigger leap of faith to use global feedback. With the PL519 I had tried every scheme under the sun in order to avoid the "dreaded" GNFB; plate to grid, plate to cathode, transformer secondary to cathode, the lot. But after working out and setting up GNFB on the PL519 amplifier, eveything fell into place, soundwise.

The 807 amp is a development from there and uses GNFB and pentode driving pentode. The results for me are stunning. The whole amplifier philosophy is diametrically opposed to the SET method of doing things, yet for me it is the sound that best represents music.

Either I don't have the skill to build a good SET or SET sound is not the thing floats my boat.

I'm currently gathering the bits together to build an 813 single-ended tetrode amplifier. This will of course need an intervening power cathode follower driver direct coupled to the output tube and a negative supply for said follower and grid bias for the 813, but the principle of pentode input driving pentode output, with GNFB remains the same. That project is some way off yet though. In the meantime I'm enjoying the sound of the 807 immensely.

Steve

Last edited by Steve Cresswell; 1st August 2011 at 11:57 AM. Reason: add a bit more info
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Old 1st August 2011, 05:44 PM   #6
ryuji is offline ryuji  United States
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so schade style local feedback wasnt enough when using pentode drivers?
this is the circuit im using and it sounds wonderful to me. havent really listened to anything else yet tho. from a engineering pov, as long as you overbuild your power supply to have low to zero ripple the circuit is near-perfect from the point of view of having BARE minimum components in the signal path to add distortion and/or noise.

your circuit here kind of reminds me of the same topology principle

my friend did a lot of experimenting and has same audio tastes, he designed the output/driver circuit and said this is the best he has come up with atm
http://ryujiwarui.com/data/7868SE%20...7%20driver.png
http://ryujiwarui.com/data/12au7_preamp.gif

Last edited by ryuji; 1st August 2011 at 05:48 PM.
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Old 1st August 2011, 06:48 PM   #7
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Quote:
Originally Posted by ryuji View Post
so schade style local feedback wasnt enough when using pentode drivers?
Well I wouldn't say that. It was perfectly fine and seemed to be the best local feedback method I tried, in terms of overall sound quality.
I just preferred the GNFB set-up in the end. It wasn't night and day but GNFB
just somehow sounded right to me, with my amp, in my system.

YMMV of course.

Steve
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