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Old 22nd June 2007, 01:28 AM   #1
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Default SE EL84 project for my friend

I have in my possesion, at the moment, a SE EL84 stereo amp c1959 out of a zenith console. A friend of mine who is a music fan, not particularly insane about flat response, etc loves to use this thing. It has the stock alnico 12" woofers in it (which are fairly good) and the original "tweeters" (5 inch open backed cones). We upgraded the tweeters to a set of CTS cheapos which made a pretty decent improvement by itself.


I brought home just the amplifier, because it, well stinks. His intention is to use this in his fairly small home for a main system. He recently purchased a Dynaco PAT4 (SS) preamp that he wants to use with it. The preamp works perfectly, again, not audiophile quality really, but it does what he wants, as far as giving him more inputs, and a phono stage.

This particular amp had real tone controls, a 12au7 on the input, and a 12ax7 after the tone stack driving the el84's.

The amp didnt work on one channel at all. I got both sides to work, but distortion is fairly high, and the thing just lacks any kind of quality sound. Somewhere along the line there was a bizzare balance setup that combined the channels, almost like a stereo to mono blend rather than an actual left/right balance.

I bypassed all the tone controls with a .1uf cap and it sounds, well, VERY tubey, but still pretty distorted (and this is using a dynaco PAS tube preamp that I have).

The controls are no good on it, and most or all of the resistors have deviated from their original values, and the caps are shot. The chassis is in great shape, but it's small and there isnt much room to work.

The iron however, is pretty large, and I'm assuming are typical 5K primaries/ 8 ohm (Because that's what the speakers are) secondaries.

I'm fortunate to have the ability to fabricate a very nice chassis at work, so I'm thinking of starting from scratch on this and just building a very SIMPLE power amplifier.

So, I realize there are many many schematics for EL84 SE stereo amps out there, but I'm wondering if anyone has a very simple and very effective design out there that I could use for this. I don't really want to spend a lot of time, and I'd like to go about this as quickly and simply as possible.

I need to know also how well the SS preamp is going to drive this thing when all is said and done. It doesnt have to be super loud, but it was fairly loud before it was removed, but it sounded bad. I'm thinking of copying the Eico AF-4 if anyone has seen or heard of that. It has a simple single pot tone control that looks like it would be easily bypassed, and It's had good reviews.

Here is the schematic of that one:

http://i73.photobucket.com/albums/i2...all442/af4.jpg

any suggestions are welcome,

Aaron
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Old 22nd June 2007, 01:58 AM   #2
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Aaron,

You can get good sonics from that O/P "iron" (without NFB) if the reduced power O/P associated with triode mode is acceptable. Follow the general model of the DECWARE SE84.

You are going to employ a line stage with gain. So the mu of the voltage gain twin triode can be modest. Suitable 9 pin mini types are the 6CG7/6FQ7 and 12BH7/6GU7.

EL84s make decent triodes, but the 6V6 makes a more linear triode and it works with EL84 "iron". You could do an all Octal amp. with a 6SN7 and a pair of 6V6s. That's a LINEAR combination.

You should have no electrical problems using a SS preamp to drive a tube power amp. Unfortunately, the Dyna PAT4 is nothing to write home about, from a sonic perspective. You can contact AVA to see if they still have stuff to upgrade a PAT4. The current web site does not show anything, but I know for sure that PAT4 upgrades were offered in the past.
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Old 22nd June 2007, 12:21 PM   #3
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Is there a chassis number? (Zenith was usually # letter # format - first # is number of tubes, letter indicates year)

Zenith had a few weird models with three output transformers and center-channel output, and a few with "matrixed" stereo... both use right and left channels in opposite phase.
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Old 22nd June 2007, 12:37 PM   #4
Sherman is offline Sherman  United States
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I built this schematic a couple years ago. Hard to imagine anything simpler!

As drawn it makes 1.9W in triode mode and about 4W in UL. I did have it running in pentode mode for a while with a 100R resistor between the grid and anode. In pentode mode it made about 5W.

If I were building it again I think I'd use a 6922 or 6N1P instead of the 12AU7, though a 12AX7 would work as well.

I use it with a "passive preamp" but I imagine the PAT4 would do the volume control/source selection job just fine.
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Old 22nd June 2007, 07:56 PM   #5
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THANKS!!! these replies have been great. I know the PAT-4 isnt anything to write home about, in fact, I own one myself, and its in my garage, driving an old Knight 6bm8 PP amp with the internal preamp disconnected (functioning only as power amp) Actually, it sounds pretty good for what it's doing.

My buddy who bought it purchased it from a person who used to work at the dynaco factory, and it's been modified to improve the sound (I have no idea how, but its really not bad).

Anyway, the chassis number is: 5B29 (I think... it's on a paper tag) and I imagine the stamp 9 59 is a date code for september of 1959.

I am familiar with some of the strange stereo setups zenith used, I have (found on side of road!!! woohoo!) one zenith amp that did have the 3 opt's, a bass channel and two out of phase stereo channels that hooked up in an interesting way within the console to the speakers. I just dumped the garbage and put it together to run straight out as a stereo amp. It sounded like garbage until I added some NFB, it did not have any low end response because I imagine they didnt worry about it since it had a sort of psuedo P/P bass channel.


Should I use input caps on this? or can the SS preamp go right into the tubes?

Thanks for the replies, I'm looking forward to getting more!
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Old 22nd June 2007, 08:29 PM   #6
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Schematic for 5B29 is online at http://techpreservation.dyndns.org/s..._5B29_166.djvu

You'll need the DjVu plug-in from lizardtech.com or IRFANVIEW plus its DjVu plug-in.

Check for positive grid voltages (sure sign of leaky caps) - check other voltages per schematic. Get that 2 meg volume control out of there - 100K is better - if you want any treble response. There's probably more gain there than you need - without tone controls, a stage could be dropped.

Odd that the output cathodes aren't bypassed - it would increase max output, decrease output impedance. The bean counters won, I guess.
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Old 22nd June 2007, 08:37 PM   #7
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THAT IS AWESOME!!!!!!!! THANKS!
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Old 23rd June 2007, 04:28 AM   #8
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I can heartily recommend the RH84...

http://www.tubeaudio.8m.com/RH84/rh84.html

dave
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Old 23rd June 2007, 01:34 PM   #9
dmason is offline dmason  United States
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Mmmm 6V6... For about 4.5 watts in UL, what kind of output in triode could one expect with this tube...Eli, the 6SN7/6V6 full Octal treatment sounds pretty sweet. Have you heard/built this combination? Is there a readily available schema for it? tx

The only one I have ever seen, commercially, would be the Sun Audio 6V6, and I believe Jef Larson (Abraxas Audio) made a few, and really liked them, as I recall him saying.
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Old 23rd June 2007, 03:32 PM   #10
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Have a look at this...John

http://boozhoundlabs.com/6v6hifi/
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