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#11 | |
diyAudio Member
Join Date: Feb 2005
Location: Evergreen, CO
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Quote:
6BG6G TDS Tube data |
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#12 |
On Hiatus
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Aha, there's the confusion. Class A can be done in push-pull. All it means is that the idle current is the same as the peak current, i.e., neither tube cuts off.
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#13 | |
diyAudio Member
Join Date: Feb 2005
Location: Evergreen, CO
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Quote:
I blame the datasheets I've found for the 6BG6G/6BG6GAs. Now, folks have compared the 6BG6G with the 807, and the 6BG6GAs with the 6L6G*s, and I do see differences in the application data. FWIW, the 807's app data seems to match the app data for the 6BG6G, with the exception that they provide true class A app data for it: Class: A S/E Va: 500 Vg2: 200 Vg1: -14.5 Ia: 50 Ig2: 1.6 Ra: 39000 S: 5.7 Rk: 280 Zout: 6000 Pout: 11.5 THD: 12 Holy crap. So going from Class A P/P (triode) to Class A S/E brings the THD from 0.6 to 12. *scratches head* what's so magical about Class A P/P with respect to THD, I wonder. |
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#14 |
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
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Chromal,
PP O/P stages cancel internally generated even order harmonic distortion. There's the reduction you wondered about. Those O/P trafos have 20% primary taps. That number argues in favor of the E-Linear topology Doug Piccard (Bandersnatch) vigorously advocates. You run in full pentode mode, with regulated g2 B+. A LTP splitter/driver made from 2X 6CA7 signal pentodes will provide ALL voltage gain needed. The LTP anode load resistors connect to the O/P trafo taps, not B+. Doug doesn't like loop NFB around the O/P trafo, but I think a few dB. are an excellent idea, given the shaky provenance of your "iron". Don't forget to roll infrasonic noise off at the amp's I/Ps.
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Eli D. |
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#15 | |
On Hiatus
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"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is." |
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#16 |
diyAudio Member
Join Date: May 2005
Location: USA
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The 6BG6 is the octal version of the 807, for use as a TV horizontal deflection final. A couple of things to watch for when using these. They like to make higher order harmonics, and can definitely sound pretty nasty when run open loop. You will need local NFB to tame those beasts, and O. Schade recommended feeding back 10% of the AC at the plate back to the control grid. I tried that, and the recommendation looks to be spot on. Another 6.0db(v) of gNFB cleaned up the remaining messes nicely, and took the edge off a too-agressive sound. Used right, these can sound very good indeed.
The other thing: you will definitely be needing screen stoppers. 807s like to Barkhausen oscillate when they go into cutoff. 1K5 stoppers mounted close to the screen pins stopped that. If you decide to run them in pentode mode, an active screen regulator is also a good idea, as pents run more linearly with the screen voltage nailed down and returned through a low AC impedance. Active regulators do both better than a series dropping resistor or a voltage divider. |
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#17 | |
diyAudio Member
Join Date: May 2006
Location: Berlin
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#18 |
diyAudio Member
Join Date: May 2005
Location: USA
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"Can you show what you mean in a schematic, though?"
Vixen Main Schemo The local feedback is from the plates of the 807s to the grid of the 6SN7 cathode followers. This is a simple voltage divider that is scaled for 10% feedback, the impedance being formed from R33-34, and the parallel impedance of R11-12, R15-16, and the r(p) of the 6SL7, this being 74.5K from the loadline of that stage. C9-10 are simply DC blocking capacitors. |
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#19 | |||
diyAudio Member
Join Date: Feb 2005
Location: Evergreen, CO
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#20 | |
On Hiatus
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Quote:
That said, I run my pentode amps in AB1, my triode amps in AB2.
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Thread | Thread Starter | Forum | Replies | Last Post |
Experimental loudspeaker design -need criticsm | BMD | Multi-Way | 12 | 15th October 2011 08:55 AM |
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6L6G Amp recommendations | 77seriesIII | Tubes / Valves | 13 | 21st November 2007 03:42 PM |
Anyone build/use 6BG6G? | kegger | Tubes / Valves | 12 | 28th February 2007 08:12 AM |
Best driver for an experimental design | BMD | Full Range | 12 | 12th May 2006 11:30 PM |
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