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Old 7th March 2009, 02:52 PM   #21
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Originally posted by chromal
The 'Class A P/P' really threw me for a loop. So, I can operate at class AB2 P/P with 1.8% THD at 26.5w Pout...
This is actually a Class AB1 (PP) spec. This is the one I selected for doing an 807 project. The Class AB2 spec is 80W out with THD= 3.0%. I just didn't need all that power. With the grid drivers I included, I'm getting a bit over 30W out since it can slip a bit into AB2 operation. The OPTs (Hammond, 30W rated) saturate before there's any clipping.

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Class A P/P with 4.0% THD at 24w Pout, or Class A P/P triode-strapped for 0.6% THD at 6w Pout. All this according to this application data I found on the net... I'm surprised the class A P/P has more THD than the class AB2. I suppose it could be because of the selected positions along the tube curves...
It's always about what the loadlines say. Class A isn't necessarily a "magic bullet" that insures sonic goodness. It also depends on the type and what sort of harmonic distortion it produces. Lots of folks say that 6V6s sound especially good, since this type produces mainly h3, which isn't so dissonant as higher order harmonics.

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Looks like I need to read and think some more about the long-tail pair, and how NFB would interact with the CCS requirements.
No big mystery about LTPs. Just set up your loadline, pick a Q-Point, and know that the LTP will give half the gain per phase. Your best bet with an LTP is active tail loading. This ensures AC balance, and also balances the harmonic distortion better than trying to force it with unbalanced plate resistors when you have a passive tail load.

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Hmm, 7hz highpass to prevent DC-like OT loading? Sounds like something I read in M.Jones and since forgot.
This is mainly to get very low frequencies out of the pass band. You get these infrasonics from such things as turn table "rumble" or the fact that vinyl disks aren't perfectly flat, or from the routine manipulation of guitar strings, etc.
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Old 7th March 2009, 03:11 PM   #22
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Looks like I need to read and think some more about the long-tail pair, and how NFB would interact with the CCS requirements.
When a bipolar PSU is employed, the non-inverting grid is grounded and the inverting grid is driven. Adding loop NFB is very easy. "Lift" the non-inverting grid and apply the NFB to it. Since it's in the cathode circuitry, the CCS is not involved.

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Hmm, 7hz highpass to prevent DC-like OT loading? Sounds like something I read in M.Jones and since forgot.
I'll go further. Set F3 between 15 and 17 Hz. O/P trafo core saturation, due to loop NFB error correction, is a MORTAL enemy. Don't let saturation happen! Remember, 31 Hz. is the lowest fundamental a double bass can produce.
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Old 7th March 2009, 05:23 PM   #23
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hey-Hey!!!,
On loading...SE is fairly simple; the load line is simmply the primary's reflected load, as in 3k, 5k etc. PP in class A, each tube sees half the a-a load, so you'd draw a SE load line at a-a/2. Now in AB the power stage cuts off one tube at higher signal levels and the load goes to a-a/4 or a 2:1 variation as signal increases. This load variation leaves the power supply needing to supply extra current( over the idle value ) and raises the stakes/difficulty on regulation of B+. I'd advocate keeping to class A...
cheers,
Douglas
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Old 7th March 2009, 06:22 PM   #24
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In an earlier post, I comitted a "fat finger" error. I said a LTP made from 6CA7s. I must be getting dyslexic. That should read 6AC7, which is a high gm signal pentode.

The 6CA7 is a beam power tetrode that's electrically equivalent to the EL34 power pentode. DUH!
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Old 16th March 2009, 09:55 PM   #25
chromal is offline chromal  United States
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Thanks to everyone who has replied thus far-- so far, you guys have been a great sounding-board in working through some of the pre-planning for my first DIY amp.

Right now, I've been testing out various power supply layouts in the time-honored plywood prototype approach. Since my 320-0-320 0.105A / 6.3-0-6.3 3A tranny can't really handle the B+ current for two PP channels, I've been playing with a monoblock design for now until I can source two new smaller power trannies, or a larger tranny for a stereo amp.

I've been playing a bit with power supply designs using 'Duncan Amp Pages' PSUD2 software. Wow, fantastic for the price (free!). Its ability to simulate made non-intuitive aspects of power supply design visually make sense (like how putting a Cap in front of a choke after the rectifier section can drastically increase the final B+ voltage). I have a tenative design I've come up from using this software for my monoblock class-A (triode) PP 6BG6G amp, which this thread has been about thus far... BUT!

I also just ordered a Tubelabs Simple SE, which I may play with as the basis for a second 6BG6G/6BG6GA (6DQ6A? another 6v heater tophat pentode?) amp. I'll probably post my power supply / OT setup ideas for that later on.
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Old 16th March 2009, 10:05 PM   #26
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hey-Hey!!!,
I'd put the ability of your power Iron into the Dynaco-marginal for its ability to run a single PP channel. Go 350-0-350 into an LC filter and bias the tubes to 80 mA each with a 6k6 a-a load.
cheers,
Douglas
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Old 19th March 2009, 02:46 PM   #27
flood is offline flood  India
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hey to all from bombay,

this is an interesting thread. i came across it while trying to figure out what to do with the 6BG6G pair I have lying around.

i thought it would be neat to be able to put them into an electric bass amplifier pushing about 50W - is this possible, with about 500V on the plates?

i can get quite decent toroids custom wound here according to my specs, so meeting the circuit requirements is not a problem. is this possible though? perhaps running 2 at a more conservative voltage?

thanks for any and all advice!
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Old 19th March 2009, 05:30 PM   #28
chromal is offline chromal  United States
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Quote:
Originally posted by flood
i thought it would be neat to be able to put them into an electric bass amplifier pushing about 50W - is this possible, with about 500V on the plates?
so I've heard different things. others please chime in: the original 6BG6G is a lot like an 807 tetrode, and maybe has less power handling capability than the 6BG6GA? Anyway, my Tung Sol 6BG6G sheet says the tube has a maximum plate dissipation of 20w, but I have some application notes that claim if you run it in Class AB2 PP you'll get a Pout of 26.5w, I believe -per tube-? Anyway, that's with an anode voltage of 360v, Vg2 @ 270v, grid biased to -22.5v. Anode current at 69mA, Zout 6600. THD==1.8%
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Old 20th March 2009, 04:27 AM   #29
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Quote:
Originally posted by chromal
so I've heard different things. others please chime in: the original 6BG6G is a lot like an 807 tetrode, and maybe has less power handling capability than the 6BG6GA? Anyway, my Tung Sol 6BG6G sheet says the tube has a maximum plate dissipation of 20w, but I have some application notes that claim if you run it in Class AB2 PP you'll get a Pout of 26.5w, I believe -per tube-? Anyway, that's with an anode voltage of 360v, Vg2 @ 270v, grid biased to -22.5v. Anode current at 69mA, Zout 6600. THD==1.8%
The 6BG6 is the octal version of the 807. (807 Application Report) The operating point mentioned here is for Class AB1 operation. Push 'em into Class AB2, and you get 80W at ~3.0% THD. You can get as much as 120W, but that's almost Class B, and is likely to sound horrible. Of course, that was for an AM plate modulator for a ham rig where you don't need good fidelity anyway.
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Old 20th March 2009, 10:48 AM   #30
flood is offline flood  India
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hey,

that's some useful information right there. i'm not too worried about THD, since i plan to use it for bass and actually want a bit of "character" to the sound... unless it's going to sound downright horrible, flabby and/or gated, i wouldn't mind giving it a shot

i'm just doing this amp for kicks. i think a class D design with an SMPS would be a little more forgiving on my back
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