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experimental 6L6G / 6BG6G amp design brainstorm

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After assembling and modding a S-5 K-12M, restoring some old Bogen tube 'booster power amplifiers,' I've been wanting to take the logical next step: design and build an amp from scratch.

Design Goals:

Intended purpose: hi-fi listening
Low noise, Low THD
Use undervalued NOS and pulled tube parts
Drive 4 or 8 ohm loads
Accept line-level or pre-level inputs
Learning/Experimental Amp. This will not be 'made pretty' as its design may be altered after initial assembly.

What I've got:

a pile of tested 6BG6G/6GB6GA tubes, as well as some 6L6s

power tran: Pri 117, tri-tapped secondary 320v.d.c 0.105amp DC, secondary 5v 2.0 amp, secondary 6.3v.c.t 3.5AMP

output tran: 2x Burstein Applebee Catalog, ultralinear. ~7000 ohms primary with 20% screen taps, Com-4-8-16 secondary winding

From what I've been able to find, the 6BG6G has a Zout of 6600 when operating in AB2 P/P. Problem is, with an anode current of 69mA, I would need about 300mA of B+ capacity for two channels and 150mA for a monoblock approach. The nominal plate voltage is reportedly 360v, so strike two against the power tranny in an AB2 P/P setup.

6BG6G as class-A p/p when triode strapped has a plate current at 42mA Va=325V, so I could see that power supply handling two SET single-tube channels, but I lack SET OTs.

My general feeling is to aim for good sound, not power output. As a triode-strapped class-A p/p, the 6BG6G has a lower plate voltage and current, a lower power out (6w) and a lower THD (0.6%).

If I wanted to go SET, I understand I would not be able to use an 'ultralinear class A' OT, as that would by definition be class-A pentode... SEP? My datasheet states the 6BG6GA class-A p/p has a Zout of 8000 ohms, and so that would be my ideal SET OT. I think the closest I've seen have been the Hammond 1629SEA at pri-6500ohms and their 1638SEA at pri-10K.

I'm not certain about the driver tube. I'm looking for something that's cheap/underrated NOS. I probably want to go with a 7 or 9-pin tube whose filaments accept 6.3v, like the 6BG6G, to simplify filament power supply. From other amps and projects, I have a supply of 7247/12DW7s, 6N6P, 5654/6AK5.. There are surely other ones I should be considering, though.

okay. uh. so, is my thinking reasonable thus far?
 
If I understand you right, you want to use as much as possible of what you have, which is that power transformer and a couple of single ended OPT's (you checked that they are single ended?)? First of all, if you want to make an SET with ultralinear OPT's, just ignore the ultralinear tap of the OPT and you are done. So it sounds like you can use all of your iron, although the PT sounds like it will be on the slim side.

For driver tube, there are countless choices as those power tubes are easy to drive.
 
I think your PSU will sag with 100 mA. I'd look for used iron on ebay from the start, because I have a feeling that you'll be doing it anyway.

As for SE, there's no reason not to try SEP - using the UL tap. You might be surprised by the outcome...more power and a distortion spectrum not too different from a SET.
 
Sorry my initial posting could have been clearer--- The transformers I have now are push/pull type ultralinear ~7200ohm primary G-4-8-16 continuous tap secondary. I'm looking at buying a set, probably new unless I can find something suitable on ebay. :)

I agree, I am not comfortable with pushing the power transformer I mentioned that close to the limit with quiescent currents. I wish I had two, it'd be perfect in a monoblock... I'll save that for another project. I may see if the specs on an old stereo Conn organ's power amplifier's transformer (7868 P/P x2, 12ax7 x2, tube diode, it may have at least 200mA B+ ability at a decent voltage) are up to it. I'd far rather burn my 7868 tubes in my David Bogen MO-30A amps. c_c

I guess my immediate task is to source SET or SEP OTs and start drawing up some schematics. :) It looks like 6L6G-based amps may be a good starting point for 6BG6G designs.

When selecting a OT whose primary winding doesn't match a tube's datasheet ideal, is it better to aim for the primary winding's resistance to be low, or high, for the typical loudspeaker load? e.g.: If the tube's app datasheet says class A P/P Zout 8000, but I can only found OTs with primary windings of 7000 or 10000-ohms, which would be best, or are there pros and cons of each?
 
Some generalized net discussion I've googled seems to suggest, for my money, I can do better than Hammond OTs, if I'm buying new iron. The 125ESE looked okay, and flexible!, but at least one review suggests there's quite a bit of inconsistancy between individial 125ESEs. One Electron's products might work, but they only make an SET OT with a primary resistance at 4500ohms. http://www.one-electron.com/trans.html
 
You mention in your first post that you would like to reuse some of the components that you already have. The output transformers that you have will work for a push pull amplifier using the 6L6's or the 6BG6's (these are basically the same tube depending on the versions that you have). You will need a new power transformer. These transformers have an ultralinear tap so they could be used in triode class A mode for 5 to 10 watts depending on the tubes you have. Ultralinear and pentode modes are possible too, and class AB bias could generate 25 to 50 watts, depending on the size and quality of your transformers. The tubes you have would probably do best at about 350 or so volts of B+. The 12DW7 is half of a 12AX7 an half of a 12AU7 in the same tube. It was designed for guitar amps and vintage versions are worth good money to someone with an old Ampeg. It could be used to drive a pair of 6L6's. The 12AX7 side is the voltage amp and the 12AU7 side is the phase splitter.

If you want to build a single ended amp, you will need new output transformers and a new power transformer. A single ended amplifier must be class A by design. A class A amplifier runs the output tubes hard, and they must be up to the task. There have been several types of 6L6's and 6BG6's made over the years, and they vary greatly in their power dissipating ability. The original metal cased 6L6 is the wimpiest and can make only 2 or 3 watts in triode mode. The 6L6G, and GA is a bit stronger. The 6L6GB is better still, and the 6L6GC is a serious step up. Good examples can make 5 watts in triode, over 10 in ultralinear. The older tubes can't handle much over 360 volts, but the later 6L6GC's and the new Russian (and even Chinese) tubes will work at 450 volts or more.

I have been building Simple SE's with 6L6GC's and I use a 5K ohm output transformer for triode and UL modes. I also run them at 450+ volts to get plenty of power. Older wimpy tubes will not work here, but the $6 Chinese ones will. So will EL34's and KT88's. Several builders have done the same thing with good results. The formula for lowest cost is an Allied 6K7VG power transformer and Edcor XSE15-8-5K OPT's. The Allied transformer is made by Hammond, costs less, and seems to work better than the Hammond 274BX. A better power transformer is the Hammond 374BX, but it costs a lot more. The Edcor CXSE25-8-5K is a far better OPT and really works good.
 
SY said:
I'm experiencing cognitive dissonance. If you want low THD, you want to go push-pull not SE. You should be able to get a solid 15 watts of class A triode from those tubes and that output transformer, though you'll need to get a better power transformer.

Well, that was always my understanding, as well. And I'm a little confused by the application data, which is where I'm getting the THD spec from. Though they say "class A P/P," which is.. again. Class A is single-ended, class-AB is push-pull. What is Class A P/P?

6BG6G TDS Tube data
 
SY said:
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.

Aaah, so this is Class-A with two tubes operating nowhere near cutoff during the entire waveform. Hrm. I wish this application data had a schematic; I guess the two plates are tied together and that hits the SET OT? If this arragement has a Zout of 8K, then a Class-A non-P/P single-power-tube arrangement would have a Zout of 16K? I don't think I've ever seen any new stock SET OTs with primary impedance so high, but I think I may be failing to grasp something...

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.
 
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.
 
I guess the two plates are tied together and that hits the SET OT?

No, each plate is individually connected to separate ends of the output transformer primary, with the center-tap being driven by the power supply (i.e., at AC ground). Topology looks identical to an AB push-pull; the loading and idle currents are different, with the plate to plate load generally being a bit higher.
 
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.
 
Miles Prower said:
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.
Interesting. I'm also sitting on an old quad of 6BG6G's I was given, and might want to throw an amp together at some point. Can you show what you mean in a schematic, though?
 
"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.
 
Eli Duttman said:
Chromal,PP O/P stages cancel internally generated even order harmonic distortion. There's the reduction you wondered about.
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, 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...

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".

Looks like I need to read and think some more about the long-tail pair, and how NFB would interact with the CCS requirements.

Don't forget to roll infrasonic noise off at the amp's I/Ps. [/B]

Hmm, 7hz highpass to prevent DC-like OT loading? Sounds like something I read in M.Jones and since forgot. :)
 
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