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This topic comprises 2 pages: 1 2
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Author
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Topic: Valve amp designs
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David Buckley
Jedi Master Film Handler
Posts: 525
From: Oxford, N. Canterbury, New Zealand
Registered: Aug 2004
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posted 08-23-2005 05:37 PM
A pair of EL34s will easily produce 20W, most 50W guitar amps use a pair.
As I think you've sussed, the difficult part to get hold of will be the output transformer, you need to start raiding junkyards and stuff.
A couple of secs with google found http://www.beamecho.mcmail.com/rebuild.htm, which looks a pretty typical schematic for an ultra-linear push pull valve amp, he says, not having looked at something like this for many years...
12AX7 is the same as an ECC83 as used in the link I've posted.
I'd recommend you use a solid state power supply, rather than an old double diode valve. Also, you dont need initially to get all the way back to milivolts with valve gain, you can start with a phase splitter and a couple of output bottles and drive it from your hifi preamp, get that going, then think about if you want to have the small signal stages valved.
Good luck!
(Note to mods - yes I know about the link rule, but I'm bending it on this occasion as the link is to an example of the genre, replaceable with many others)
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Luciano Brigite
Master Film Handler
Posts: 277
From: Sao Paulo, SP, Brazil
Registered: Jan 2002
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posted 09-24-2005 07:56 PM
Ha! I don't know I missed this topic..
You can get 20W out of a pair with EL34 with low distortion somewhat easily,however the output transformer is 6600Ohm primary with taps for screen grid connection at 43% of primary turns and 4-8-16 ohm secondary.the distortion stated on the tech data is of about 0,8% at maximum signal. hte same configuration with a little more input will put out 37W with 1,3% distortion. The phase splitter is made with a ECC83 (12AX7). Most of the data sheets for tubes will have one or other suggestion for the tube, some also has a circuit diagram along with a table with component values to experiment. 6L6GC can also be used with the same output transformer (6600 ohm with taps)voltage ,currents and component values are a little different than the ones for a EL34 circuit.6L6 also can be used in Push Pull A class where it'll put out about 15W.
There are also single ended circuits, all in class A using only one output tube. distortion however is higher, reaching the 10% sometimes more,but is easier to do and costs less, hte only expensive part being the transformers. I made a few valve amps for myself, all of them single ended.one of them uses 2 ECL82 wich has the pre and output in one valve and uses not more htan 7 external components for each channel and a power supply common to both. If you have more than enough patience.. you'd try what I did,wind the transformers. It takes a lot of time ( since I wound all of them by hand) a bit of math to find out number of turns, core sizes and wires to use. the results will vary a lot. I'm pleased with the last pair I made wich took nearly 3 months to get done.
Maybe it's a good idea start with single ended circuits to get familiar with it then go to push pull. if you can get the 6550 valve at your local shop, it'll give you a SE amp that puts out 12~13W,I'm hopping to finish one with these tubes next month or when time and laziness allows me to finish the transformer winding
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Luciano Brigite
Master Film Handler
Posts: 277
From: Sao Paulo, SP, Brazil
Registered: Jan 2002
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posted 09-25-2005 10:37 PM
Paul, I wound all of my transformers, power and outputs, all of them by hand.the power transformer was the first of this kind I made and I'm pleased with it, it takes about 2~3 hours to get hot and some 3 more hours to reach the point where you can't keep your hand in it for more than a few seconds. For the outputs, it's another story. It took me a few times to come out with something I judged good ( for me) ,a lot of things were changed, from core sizes to winding sequence and wire gauges. the last pair I made have 8 secondaries in parallel and a primary split in 10 parts interleaved (??) with the secondaries. this arrangement was one of the winding methods shown in the radiotron designer's book . For the circuit it's only described, not drawn, in the data sheet for the EL34 from Mullard. In it the values for the split phase stage are pointed out for two circuits and the voltage,current and plate to plate load are also given. there are other suggestions for different power outputs with teir respective distortion ratio. I'll draw the schematic for it sometime this week and if time allows, revise the calculus for the output transformer I mentioned for it ( 6600 ohm prim./4-8 ohm sec).
I remember Mark once mentioning that output transformers aren't easy to do,indeed they are not, even the simple, small ones have their tricks,however it didn't make me leave it behind, nor did the first tries I made.Right now I have another circuit on the way, the power transformer is already made and tested ( it came out better than the first I made) and the outputs have started, for these I'll be trying the same arrangement I did for the last outpts I made . I don't have all the gear to test them but while playing with a portable RTA and a cd with test signals, the rta showed a full response when a swap signal was played tru the circuit ( and my ears complained when the swap reached the 2Khz and up, ouch!)
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Will Kutler
Phenomenal Film Handler
Posts: 1506
From: Tucson, AZ, USA
Registered: Feb 2001
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posted 09-25-2005 11:11 PM
Hi Mark
Very nice, clean work! I am curious about your finned case. Did you machine or purchase it?
Unfortunately I do not have the opportunity to listen to much hi-end gear. The last time was at a "big-boys toy show" in the Phoenix area. A local hi-end dealer had a Macintosh display under less than ideal acoustic conditions, but it sounded wonderful! However, while this was a "hi-end" dealer, I was not impressed with their lack of ability in answering technical questions.
Unfortunately, I really dislike going into "hi-end" audio stores. My experiences in both AZ and Southern Ca is that many of these stores are far beyond snobbish and arogant!
But Mark, I am finally in the midst of restoring my 1938 RCA 816K am/shortwave console. I've heard them restored, and they do sound nice! Unfortunately I was not able to get original style cloth covered wire in the needed colors. So, I compromised and am using 600 V rated 18 gage wire in solid colors and marking my own stripes with 3M FP301 Heat Shrink. But at least I am able to use the correct colors and chassis wiring layout. The other compromise is the modern cap and resistor styles, but at least they are all the correct original values and I am able to "stuff" the original top mounted E-Caps to retain original aesthetic appearance.
I'll try and post some pics.
My other piece of "hi-fi" gear is my Harmon-Kardon TA-230, which I believe was the first commerically available AM-FM Stereo Tuner. I had one years ago when I was in the service, but unfortunately had to sell it. I acquired another some time ago while in the clutches of an acute nostaliga attack, and this unit also awaits restoration. With a good set of speakers, these were very nice sounding units, also.
Cheers
K.
Cheers
K.
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