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Old 24th November 2010, 02:59 AM   #11
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Quote:
Originally Posted by hidnplayr View Post
Can someone explain to me what happens when I use a too small feedback resistor?
Reducing Rf has the effect of increasing the level of plate voltage fed back to the input. This has the effect of increasing the loading on the driver. Take a look at this loadline (attached). Without that local NFB, the load on the 6SL7 would simply be Rp || Rg:

220K || 1MEG= 180.33K

Connecting the local NFB makes that load look like 144K effective. In this case, the THD estimate is barely acceptable, however, since this is designed as part of an LTP you lose the even harmonics, and the 6SL7 should mainly be producing h2, so that works out. In the end, it worked out just fine. Adding more local NFB would have been pushing everything to the extreme, making even more distortion as the loadline grew more vertical, and 6SL7s don't have a helluvalot of current margin to spare.

Quote:
Originally Posted by Allen_Wright View Post
I have experimented with anode/anode fb many times, but no matter the improvements in measured performance, the amps always sound "thin"... Anyone else experienced this?
No. Not my experience at all. With the 807-based design, the 807s like to make lots of higher order harmonics that sounds nasty running open loop -- Ozzy sounded really nasty. Fingernails-on-a-blackboard nasty. In the "white paper" O. Schade (the type's developer) on the 807 recommended feeding back 10% of the plate voltage to the grid. That's what I designed for, and that recommend was spot-on. It cleaned up a good deal of that open loop nastiness, and all that was required was some 6.0db(v) of gNFB to clean up the rest. The end result didn't sound thin at all.

May be if you over do it, then you might have problems, either due to excessive driver overloading, or may be 807s just don't like more than 10% fed back (there must have been a reason Schade recommended 10% as opposed to some higher figure).

Vixen Main Schemo
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Old 24th November 2010, 03:17 PM   #12
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Originally Posted by artosalo View Post
By the way: You can get up to 70 W from an AB1-class PP-pair of 807.
Makes the whole construction much easier...
Hoo! That's pushing it. YOu really meant' AB2 ? 6.5K a-a; anode 600V screens 300V.Then you expect 80W.

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Old 24th November 2010, 03:42 PM   #13
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Quote:
Hoo! That's pushing it. YOu really meant' AB2 ? 6.5K a-a; anode 600V screens 300V.Then you expect 80W.

richy
No. I mean AB1. With the following values:

- anode voltage 750 V
- screen voltage 300 V
- bias voltage -35 V
- g1 drive voltage 70 Vpp
- plate to plate load resistance 12k
- quiescent anode current 30 mA
- output power 72 W

This specs is from RCA 807 data (NOV, 5. 1954)
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Old 24th November 2010, 03:55 PM   #14
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Here is one example of such design, Geloso G.3262. (75 W @ AB1)

http://digilander.libero.it/iw2dgs/b...lif/G-3262.png

There also an AB2 class version; Geloso G.3272 ( up to 100 W @ AB2).
You can see the cathode follower to drive the 807-pair.

http://digilander.libero.it/iw2dgs/b...lif/G-3272.png
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Old 24th November 2010, 07:00 PM   #15
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OK- I have to admit- I don't get it.

This is the first time I've encountered feedback from the plates of the output to the plates of the driver- I'm not sure what this is supposed to do- What is the primary objective?

I've designed an amp that's differential throughout with 20 dB overall FB and was wondering if there'd be any advantage in keeping the O/P xfmr out of the loop by taking the overall feedback in differential mode, from the plates of the KT-88s to the grids of the input diff amp -but what I see here isn't that... but kinda-sorta.

Could someone take a moment to enlighten me, please?
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Old 24th November 2010, 07:30 PM   #16
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The feedback really isn't to the plates of the drivers (though it may look that way). It's really back to the grids of the output tubes- the plate impedance of the driver is pretty high, so the feedback ratio reduces to the feedback resistor divided by the driver plate resistor.
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Old 24th November 2010, 07:38 PM   #17
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Short feedback with minimal phase shift that linearizes output tubes and lowers output stage's output resistance, VS long path around the whole 2-3 stages plus transformer itself. Do you hear me now?

However, in my amps I use more than one feedback, but this local feedback around output bulbs helps a lot.
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Old 24th November 2010, 07:39 PM   #18
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Quote:
Originally Posted by artosalo View Post
No. I mean AB1. With the following values:

- anode voltage 750 V
- screen voltage 300 V
- bias voltage -35 V
- g1 drive voltage 70 Vpp
- plate to plate load resistance 12k
- quiescent anode current 30 mA
- output power 72 W
807 operating points like these were intended for AM plate modulator duty where sonic performance is not a consideration. That's likely to put the finals close to Class B, and that'll give a lot more x-over distortion. They're after lotsawatts to AM modulate a ham rig where fidelity is neither needed nor legal.

Quote:
Here is one example of such design, Geloso G.3262. (75 W @ AB1)

There also an AB2 class version; Geloso G.3272 ( up to 100 W @ AB2).
You can see the cathode follower to drive the 807-pair.
These are schemos for PA systems: another application where the premium is lotsawatts, not fidelity.

807s aren't gonna give you that much power and do it while sounding good.

Quote:
Originally Posted by kruesi View Post
OK- I have to admit- I don't get it.

This is the first time I've encountered feedback from the plates of the output to the plates of the driver- I'm not sure what this is supposed to do- What is the primary objective?
Local NFB represents a continuum between pure pentode operation and full-on pseudotriode operation. The local NFB serves two purposes: it linearizes the pentode final(s), making its AC plate characteristics more like that of a triode. It also serves to get the internal impedance down. Pents have very large r(p)'s that make for Hi-Z outputs, which you don't want in an audio amp since there is less damping of the speeks.

Local NFB is needed for certain power pents since these like to make too much of the higher order harmonics and can sound quite nasty running open loop. The 807-oids would fall into this category. O. Schade, the developer of the type, recommended local NFB. Other types (e.g. 6V6-oids, 6BQ6, possibly other TV horizontal deflection finals) don't make all that nasty higher order harmonics, and don't really require this extra help.

Quote:
I've designed an amp that's differential throughout with 20 dB overall FB and was wondering if there'd be any advantage in keeping the O/P xfmr out of the loop by taking the overall feedback in differential mode, from the plates of the KT-88s to the grids of the input diff amp -but what I see here isn't that... but kinda-sorta.
The only reason to keep the OPT out of the gNFB loop is because you're either using a lousy OPT, and should replace it with a better one, or you're trying for insane levels of gNFB because you really like that solid state sound. I tried 20db of gNFB with one project, and, yup, it sounded no different from any Big Box store solid state amp.

Another project incorporates adjustable gNFB (0 -- 13db). Dialing in the full 13db is definitely tending towards that solid state sound. That might be OK for some types of music, but really makes Techno and Hard Rock and Metal sound "subdued". Around 6.0db is just about right.
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Old 24th November 2010, 07:41 PM   #19
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Quote:
Originally Posted by Miles Prower View Post
I tried 20db of gNFB with one project, and, yup, it sounded no different from any Big Box store solid state amp.
But did the output sound like the input?

Just to put things in perspective, Futterman amps had upward of 40dB of feedback.
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Old 24th November 2010, 08:03 PM   #20
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Wavebourn, Sy- Thanks a lot.
That's actually what I thought at first (said to self: "Why, all this is nothing more than local FB around the output stage!) but then I made more of it that there really is/was.

Not that it's easy to implement, impedances being what they are here.

Thanks for clarifying- I'll stay tuned to this interesting thread. -!
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