Steve’s Return to Rehab
by Steve Graham
(Tube Amp Rehab, That Is)
I have agreed to help an audiophile in distress. A friend of a friend has amassed a large collection of audio gear, much of it vintage. Let’s call him Ace – a variation of AC, or Anonymous Collector. Though handy with a soldering iron himself – probably better than your faithful scribe – current circumstances and commitments have left Ace little time to curate his collection. I put my hand up to help two years ago, but we all know what happened to plans made in early 2020.
The amp rehab I’m performing will not be for profit. It will be purely recreational. There’s the therapy (mine) element too.
The Leak is about 80% refurbished. All capacitors and resistors have been or will be replaced with, for the most part, modern components. If the sticker inside the chassis is to be believed, the Stereo 20 was built on January 22nd, 1960. Waxed, paper capacitors, electrolytic capacitors and carbon composition resistors do not survive for six decades.
Haters and Trolls, Start Your Engines
We can’t turn the clock back. As much as we’d like to, audio gear is no exception. Sure, we can buy new paper caps and newly made carbon comp resistors – at a price – but they won’t be the same as the originals. Same with tubes. The chance of finding a NOS set of Mullard tubes for less than a thousand bucks is pretty slim.
So, we have to play the cards we’re dealt. Come on folks, it’s only audio after all. We’re not talking about touching up a Monet painting to make it a little crisper or rewriting some Mozart to make it sweeter on the ear. Audio equipment is a tool. We use these tools for musical enjoyment. A functioning tool, even if not original, is better than a broken tool. Sure, we can admire vintage gear for its own sake, sort of like train spotting for the sedentary, but it’s the music that counts. If you don’t like my philosophy, that’s fine. Tell me I’m wrong, but get in line. I have a significant other that tells me in her own sweet way, every day.
Some won’t like the component choices used in this amp’s rehabilitation and will no doubt think it heresy to use modern metal film resistors and poly film caps. I too might not like these choices (I mostly do), but I must defer to Ace’s wishes.
The Fishers are untouched, save for a rather rough re-cap done at least twenty years ago. I take electrical safety very seriously. Hard to imagine anyone messing around with multi-hundreds of volts in a tube amp, huh?
Troll alert, you won’t like this next bit either.
The Fishers have two-prong plugs on their two-wire power cords. The “death capacitors” have been replaced with slightly less unsuitable caps but all the same they must go. Search “death capacitor” for an explanation. A metal chassis demands a three-wire power cord for safe grounding. I’ve proposed to Ace that I remove the octal socket – used to power a preamp (though not in this instance) – and replace it with a three-prong IEC AC inlet. I can do this without any chassis surgery so if desired, a future owner may reverse the socket-ectomy. I’ll also install a power switch in the original AC cord hole as the amp was originally switched via a preamp connected to the octal socket.
What do you think, a green LED under each tube like a MacIntosh amp? Just kidding.
The 20-A’s circuit is very bare-bones, almost too much so. The Leak Stereo 20 has a circuit that could be and is built almost verbatim today. The Fishers, on the other hand, are a truly penny-pinching design. At the time of their build another fifty cents or so would have made for a much better design. Someone at Fisher should have taken a look at: Tube Circuits For Audio Amplifiers by Mullard, published in 1959. Maybe they did but someone in management figured that saving 50 cents per amp, calculated over ten thousand amps, would buy them a shiny new Porsche 356………. I’ll be putting those 50 cents back in, with Ace’s approval of course.
This pair of 20-As are not sequential. There’s a large serial number gap between them. There are component value variances, as suggested on the schematic I found online. One of the Fishers has a component that looks to be a factory error.
Mactone is a Japanese builder that has little or no visibility in North America. They are sort of like Shindos without the hair-shirt cosmetics. Perhaps Mactones don’t get taken seriously because they are too pretty, in a nineteen-seventies or eighties, mid-fi Sony sort of way.
One heft of the X-21 tells me that this Mac is more than just a pretty face. A peek under the hood revealed two power transformers that wouldn’t look out of place in a couple of Dynaco ST35s. The bank of filter capacitors almost look like something from my Audio Research REF 110 power amp!
But alas, the X-21 has problems. Intermittent noise that so far, tube swapping has failed to isolate. That same peek under the hood revealed some suspicious tube choices, and a very sloppy capacitor “upgrade” some time in the Mac’s past. There is a dearth of information on this and any other “Macs”, as far as I have been able to discover. Getting information from Mactone is a tricky process as well. If it were mine, I know what I’d do with it. Easy to say because I don’t have any skin in the game, as it were.
It’s not yet clear what the path forward on the X-21 will be. Depending on how things shake out I might get no further that a brief flirtation with Miss Mac.
Comment below if you wish. I won’t be taunted into a how-many-fairies-can-dance-on-the-head-of-a-pin discussion. Neither will I attempt to justify parts choices or potential circuit changes. I’m not a sentimental person, so I can’t be shamed for messing with history. Like the sign in the kitchen at a friend’s house says, “Dinner choices: 1 Take it, 2 Leave it”.
First up, the Leak Stereo 20. Watch this space.
Editor’s Note: Great to see you “kickstarting” the C/H/E Files again!
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