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Old 6th March 2009, 04:29 PM   #1
chromal is offline chromal  United States
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Default experimental 6L6G / 6BG6G amp design brainstorm

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?
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Old 6th March 2009, 09:48 PM   #2
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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.
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Old 6th March 2009, 10:16 PM   #3
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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.
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Old 6th March 2009, 10:28 PM   #4
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First it must be determined if the transformers are meant for SE use. How many wires on the primary. 5 = push pull. 3 = SE.
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Old 6th March 2009, 11:19 PM   #5
chromal is offline chromal  United States
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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?
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Old 6th March 2009, 11:50 PM   #6
chromal is offline chromal  United States
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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
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Old 6th March 2009, 11:57 PM   #7
Fenris is offline Fenris  United States
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Try EDCOR for new transformers. Very good quality and prices.
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Old 7th March 2009, 12:21 AM   #8
chromal is offline chromal  United States
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Okay, the Edcore GXSE10-4-8K and GXSE15-4-8K are both on the short list, with possible variations on the primary resistance depending upon whether I operate them in the SET or SEP mode... Great pointer!
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Old 7th March 2009, 01:55 AM   #9
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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.
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Old 7th March 2009, 01:59 AM   #10
SY is offline SY  United States
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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.
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