Home Brew Tube Stereo Amp

[Tom Bavis]

Edcor has some nice transformers and they're close to you - edcorusa.com. Some think that they're better than the Hammonds. I think the Triode part is a guitar amp design - where full frequency response is not needed (or even desired!). They DO have the very nice Dynaclone transformers, but at 7.6K/17W and 4.3K/35W neither is ideal for 6L6s.

The biggest problem with the Williamson is that negative feedback is used around four stages, and there's a LOT of gain within that loop. Without the best transformers, it may be unstable with some loads. Heathkit used this circuit with varying success...

You can do it with one less stage - for instance, 6SJ7 or half of a 6SL7 for gain driving a 6SN7 "long-tail" phase inverter.

[GordonW]

Rene, for what it's worth... I'm a great big fan of 6L6s used properly as hi-fi amp tubes. I've owned Heathkit W4s, built variations of Magnavox paraphase circuits into 6L6 PP amps, and currently own a pair of Eico HF-22 amps...

Given my experience... I have YET to hear a better-sounding 6L6 amp than the Eico HF-22. Circuit as seen here:

http://www.freeinfosociety.com ... hp?id=2340

Now, it does use the expensive EF86 input tube... but there are other tubes that can substitute. One is the very inexpensive 7C7 loktal-base tube. The loktal socket will easily fit within the hole for an octal socket. Driver tube is the ubiquitous 6SN7, and rectifier the 5U4.

With your power transformer, you could probably easily build a stereo amp using two channels of this. For output transformers, there's absolutely nothing wrong with Edcor... I've got a pair of these on the shelf myself, when I get around to building an integrated amp based on the HF22 and a Marantz 7 preamp all in one chassis:

http://www.edcorusa.com/Produc ... spx?ID=491

A 60 watt transformer may seem overkill... until you consider that the original output transformer in the HF22 was actually capable of 48 watts at full bandwidth! This is not much more than that... and the 60 watt Edcor transformer is really no more expensive than the 50 watt, for whatever reason.

As a Mullard circuit, it's much less sensitive to output transformer variations than the aforementioned Williamson circuit. Quite tractable, AMAZINGLY good sounding.

The HF-22 is only rated at 22w/ch... but that's from 20-20000 Hz. With VERY LOW distortion, across the band, when set up right. It's been able to run pretty much any speakers I've put it on... unless you've got some 87dB sensitivity or lower power hogs, you'll be quite OK...

Regards,
Gordon.
_________________
"It's the guys who think that attending meetings is real day's work that are the problem."- HepcatWilly (on AudioKarma)

[Rene]

I checked my stash of 6L6's and I have 4 very strong large ST 6L6's. I also checked out my power transformer and it appears to have what I want, including 400-0-400 HV estimated at 350 mA based on literature I have (ohms/volt vs current chart).

Those Edcor's look good and I think I'll give them a whirl. Hopefully they have power supply chokes too.

I happen to have Sam's 492-12 which is the schematic for the HF-22. I'll compare it with the schematic I have from "An Approach to Audio Frequency Amplifier Design" by the General Electric Co. of England. The particular circuit I'm looking at is a 30W design using 2 6SN7's and 2 6L6's. I'm going with the 5U4 rectifier with regulation using a 6AS7/6SJ7/OD3.

Now it's time to design and fab the chassis, and keep an eye on eBay for a decent stereo tube pre-amp. I see a number of Dynaco "PAS" types. Still looking for good speakers...so far I've heard of the Realistic Mach 1's and Altec A7's.

[bhamham]

[Rene]

I've ordered the output transformers from Edcor. I'm re-thinking the regulated power supply...the schematic I have requires a 500-0-500 power trans that ultimately delivers ~360 vdc. My transformer is a 400-0-400 so the regulation circuitry would likely result in B+ that is too low. The article I have on regulation made a good pitch but so many amp designs I see did not use regulation so I'll probably be OK without it.

I've read so many electrolytic cap horror stories (in all electronics) that I really don't want ANY in my amp. Chokes should work in the power supply but I'm thinking of somehow ditching the electrolytic in the cathode circuit of the 6L6's. The cap is parallel to the cathode bias resistor so I'm guessing any inductor would have to be in series with the cathode resistor. What's a good choke value? Has anyone ever done this?

[Alan Douglas]

The cathode should be at AC ground; an inductor won't do that.

You could use a zener diode, but low-voltage electrolytics are perfectly reliable.

[Tube Radio]

When you do build the amp use only Sprague Atoms capacitors in the power supply. I have some in use in the GEC912 plus amp that I built from the book "An approach to audio frequency amplifier design" and the amp has been run almost three years at an average of 8 hours a day with no troubles except for a bad output tube due to B+ being a little high.

[Dave Doughty]

The reason many amps don't use power supply regulation is because it usually isn't needed if the amp is running class A. This puts a constant load on the power supply and the current draw remains the same as long as you don't force the amplifier into clipping (distortion).

Regulation might be a good idea if other classes are used but, even then, it usually isn't necessary if the unregulated supply is designed well. Just my opinion.

Some amplifier designs don't use a cathode capacitor at all in order to take advantage of the resulting degenerative feedback.

Dave

[Kevin Kuehn]

I wouldn't spend much time worrying about electrolytic caps causing problems. They've been used for over 60 years with much success. They do go bad with age, but its no big deal changing them out every 20-30 years. My 1964 Fisher amp runs daily with the original electrolytic caps in it. I'll change them out one of these days. Also hundreds of modern tube guitar amps use elec caps with no issues.

Kevin

[BigBandsMan]

Ditto Kevin. I have two 1964 Fisher receivers, recently rebuilt (threads below), and both of them made it this far with most of their original electrolytics in place. My homebrew job, "Big Bertha," still has all the electrolytics that went into it in 1992, and there are dozens. Some went in new, some were used.

As with any kind of parts, only one thing matters: The parts must be used as they were designed to be used, and operated comfortably within specs.

"Redlining" any parts (or designs) is simply asking for trouble.

Continued best regards, Larry
_________________
Glenn Miller forever!

[Rene]

OK, I'll use some electrolytics! I'll have to go back to the books to fully understand cathode circuit theory, but IIRC, the electrolytic helped "dampen" the voltage drop variation across the cathode resistor as tube current varied, in order to provide a relatively constant bias. Not ever seeing a choke in a cathode circuit beforechould have been one clue that it wouldn't work.

[Dave Doughty]

With Class A push-pull circuits, the current through the common cathode resistor doesn't vary. Current variations get cancelled because of the push-pull action. If a cap is used, it is there because the designer wanted to eliminate the degenerative feedback for various reasons.

Dave

[Kevin Kuehn]

Yep, the cathode bypass cap essentially holds the cathode at AC ground potential while the DC sets there and does its bias job. Without it you get negative AC feedback because the bias fluctuates with the signal level.

Kevin

[Mr. Detrola]

Since you mentioned that you are considering upgrading speakers, might want to take a look at Klipsch KLF30's or KLF20's. With a sensitivity of 102db/1M/1W they are exceptionally good on amps around 20-30 watts and have great response, you would not want a subwoofer. Various reports over the years have given the opinion they are among the best sounding speakers recently made with an original cost less than $10K and used ones are very affordable, often in the $1000 range.
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Dennis

[Rene]

OK, I think I have the cathode bias straightened out. Dennis, thanks for the info on the speakers. 1 k$ is a little more than what I plan to spend but sometimes stuff like this shows up at auctions for a decent price.

[Kevin Kuehn]

Don’t laugh, but the older Peavey SP2 PA speakers have sensitivities up around 102db/1M/1W and sound very good as home speakers. These have a 15” woofer and horn loaded mid/hi driver. Probably not the best looking in the living room, but can be had for cheap. Good for rockin' out in the basement.

Kevin

[BigBandsMan]

So long as we are talking about speaker systems, I will mention the EV S-80 studio monitors I installed in my synthesizer lab in 1992, and have used ever since. I am thoroughly pleased with them. They have long since been superseded in production by other models, but anyone would be well rewarded to acquire a pair of these.

Indeed, it is no bad idea to consider studio monitor speaker systems in general. They are built tough; they are built for smooth response across the spectrum; and as far as I have seen, their prices are not out of line with comparable "home entertainment" systems.

Apart from their power and impedence ratings, I have no idea what the specs of these S-80 units are. I was not interested in those at the time of purchase, because the sound sold me, not the specs. When it comes to speakers, the only specs that matter to me are those of my ears.

These units are small (hardly larger than bookshelf size), and fairly lightweight. I mounted them atop an equipment console which fills one wall of the studio from floor to ceiling. From there, the sound diffuses itself very nicely throughout the lab and audition rooms, with no sense of confinement. The sound is silky smooth and full-bodied. I get no sense of the sound being "colored" by the enclosures, and I have found no resonances in them.

Hope this helps.

All the best , Larry
_________________
Glenn Miller forever!

[Rene]

Regarding the front end/preampifier, is anyone familiar with the Dynakit PAS-2 or PAS-3? These are supposedly stereo tube preamplifiers but I can't seem to find them in my Sams index.

[Tom Bavis]

Manual is online here: http://www.curcioaudio.com/pas_mnl.pdf

Gary Kaufman has an online page with all Dynaco tube schematics, but it went down when Geocities shut down. I hope he finds another place to host it...

[Bill Thomas]

I am VERY familiar with the Dynaco PAS-2 and PAS-3 preamplifiers. Here's a link to a restoration article I did covering a PAS-3X preamplifier: http://www.audiocircle.com/index.php?topic=58900.0

The PAS-3 and PAS-2 preamplifiers are *similar* with minor changes to a few parts values and cosmetic differences in the case, face plate and knobs. These were designed to see a VERY high Impedance at the Input of the Power amplifier. (470,000 Ohms.) If they are loaded into anything with much lower than 100,000 Ohms Input Impedance, the bass response tends to roll off.

If you need more information, let me know.

Sincerely,

Bill Thomas