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Heathkit A-8A Vintage Tube Amplifier Measurements

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Feb 2, 2023
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Hello everyone,

I am a vintage audio enthusiast who enjoys designing, modifying, and repairing tube amplifiers. I'm also an objectivist and a long-time reader of ASR, but I've always wished there was more tube-related objectivist content on this forum. As we all know, tube amplifiers are far from "ideal" amplifiers, with their high distortion, high noise levels, and low damping factor. But, in my opinion, this is what makes them enjoyable -- tube amplifiers are bad enough to actually sound different, whereas competently-designed solid state gear is all pretty much the same.

Anyways, I bought an E1DA Cosmos ADC and an SMSL M500 MK II so that I could do proper measurements (using REW) on all the gear I design and restore. Please let me know if anything in my graphs looks off; I am relatively new to measurements with REW.

Without further ado, the first amplifier to cross my bench since is a Heathkit A-8A tube integrated amplifier, which I restored and subsequently tested. I thought it would be fun for ASR users to see what kind of performance you could expect from a relatively cheap 1950s-era amplifier. Although, to be clear, there were much cheaper -- the A-8A uses nice components and is generally well-designed. It's also quite a rare amplifier, and I doubt I'll be able to find another for stereo use.

IMG_0531.jpg


The above picture shows the amplifier, post-restoration, in the configuration I tested. For any tube fans out there, from left to right: NOS 6SJ7, NOS 6SJ7, JJ 6SN7, and two factory-matched Tung Sol reissue 5881s. The rectifier is a used Tung Sol 5U4GB, not that it makes a difference. I chose these tubes because they offered the best measured performance (although the difference was only a few db THD).

As a disclaimer, many factors can affect the performance of vintage electronics. Even with a restoration, it's hard to know whether the results you're seeing reflect the design's optimum performance. With this amplifier, I believe the distortion results I'm seeing are representative. However, I suspect that rebuilding the amplifier with a better grounding scheme, shorter wire runs for the input jacks, and new connectors would reduce noise considerably.

Here are the measurements.

Spectrum @ 5 watts (the output level Amir uses for power amp comparison):

A-8A New 6SJ7 JJ 6SN7 New 5880 NG 5W.png


As you can see, THD is -50.2 dB and THD+N is -49.6 dB.

At lower power levels, noise dominates. There is an audible hum from the speaker with no input signal, but it's not really noticeable during actual listening.

Here's the spectrum at 15 watts, which is close to the maximum you can get with reasonable distortion (hard clipping begins at 17-18 watts):

A-8A New 6SJ7 JJ 6SN7 New 5880 NG 15W.png


Below is the frequency response. I'm still new to REW, so the units are wrong, but I verified with my oscilloscope that output power during the sweep was 1 watt:

A-8AFreqResponse 1 Watt.png


As you can see, there's some treble roll-off, but nothing too severe inside the 20-20k range.

For those interested, here's the square wave response at 10 khz with an 8 ohm resistive load and a .1 uf capacitor shunting the load. As the frequency response graph would suggest, it's not pretty, but the amplifier is quite stable -- adding the capacitor does basically nothing to the waveform.

20000101_005255.jpg


Anyways, hopefully this interests some of you. There's a longer write-up with pictures and additional measurements on my website, but I don't want to make this post excruciatingly long.

Subjectively, I enjoyed this amplifier, although it's clearly not the pinnacle of hifi. It pumped out way more bass from my small bookshelves than my solid state receivers, probably thanks to the low damping factor and strong LF response.
 
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Very interesting! Thanks for thanking the time to do this.
 
I think that with an amplifier like that, if it and the speakers get along well and it tickles your nun-handles just right, it's in, if maybe not with the most precise fidelity, but BIG TONE will carry the day.
 
Whoa, the metal jacket 6SJ7. I think I had one when I was a kid.
 
Is the design straight out of the RCA Receiving Tube manual?
 
In fairness, at least after the first few years that the "A-x" Heathkits were on the market, they weren't marketed as nor claimed to be "hifi" amplifiers -- presumably because by then Heathkit had some amp kits that were.

Full disclosure, I used to have a couple of A-7s myself. :)

EDIT: Here's -- sort of -- an example.
In this catalog from 1954, the A-9 is labeled a "high fidelity" amplifier...

1675481433449.png


... but the A-7 is not ;)

1675481523660.png
 
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The spectra scream, "Redo the grounding!" I bet it would be pretty simple to knock the noise down 20dB or more. A little tweak in the compensation cap should get rid of the ultrasonic overshoot.
 
For the E1DA, even if you are using the 2.5mm jack for high voltage, the ground pins for the XLRs can be used simultaneously.

It improved the noise considerably with my measurements.
 
In fairness, at least after the first few years that the "A-x" Heathkits were on the market, they weren't marketed as nor claimed to be "hifi" amplifiers -- presumably because by then Heathkit had some amp kits that were.

Full disclosure, I used to have a couple of A-7s myself. :)

EDIT: Here's -- sort of -- an example.
In this catalog from 1954, the A-9 is labeled a "high fidelity" amplifier...

View attachment 262104

... but the A-7 is not ;)

View attachment 262108
You may know, but most likely the designs were straight from the back of the RCA Receiving Tube manual. RCA invited people to use the designs. Which was smart, because then a lot of RCA tubes got designed in.
 
You may know, but most likely the designs were straight from the back of the RCA Receiving Tube manual. RCA invited people to use the designs. Which was smart, because then a lot of RCA tubes got designed in.
I do not know. It is certainly possible, but the use of the 12A6 output tubes seems abstruse.
Easy enough to peruse many of them at https://worldradiohistory.com/BOOKSHELF-ARH/Bookshelf-RCA.htm :)

My "understanding" of early Heathkit products is that the designs "leveraged" stockpiles of WWII military surplus odds and ends. That said, note, e.g., that the A-7 used tubes with 12V heaters rather than the (even then) more common 6V heater morphs. Besides the aforementioned 12A6, at least some of the A-7 variants used the 12SQ7 -- not exactly a common "hifi" tube, ever. ;) In this specific case, it looks (to me) like they had a stash of car radio tubes to use. ;)

The A-7 was a pretty bare-bones integrated amplifier. The (potted) power transformer used in most if not all of them was notoriously fragile.

1675537417115.png
 
So... yeah... the front-end could have easily come straight from the "resistance-coupled amplifier" charts.
Here's the introduction to the version found in RC-19 (1954). :)

1675537992336.png
 
SIY took the words out of my mouth. I once had one of these for awhile (belonged to a friend) which I retubed for him. It didn't have audible hum. I didn't have anything close to what you or us have now for measuring. I do remember a similar peak above 20 khz like you show here. I think my friend was using it with Klipsch Heresy speakers. Or maybe a clone of them.
 
I do not know. It is certainly possible, but the use of the 12A6 output tubes seems abstruse.
Easy enough to peruse many of them at https://worldradiohistory.com/BOOKSHELF-ARH/Bookshelf-RCA.htm :)

My "understanding" of early Heathkit products is that the designs "leveraged" stockpiles of WWII military surplus odds and ends. That said, note, e.g., that the A-7 used tubes with 12V heaters rather than the (even then) more common 6V heater morphs. Besides the aforementioned 12A6, at least some of the A-7 variants used the 12SQ7 -- not exactly a common "hifi" tube, ever. ;) In this specific case, it looks (to me) like they had a stash of car radio tubes to use. ;)

The A-7 was a pretty bare-bones integrated amplifier. The (potted) power transformer used in most if not all of them was notoriously fragile.

View attachment 262287
The 12SQ7 was used for All American 5 radios, as the AF and AGC detector and first AF amp. So, probably cheap at the time. The 12A6 seems to have been popular as the AF output, single-ended of course in a typical radio. The 12SN7 should need no introduction, since its 6SN7 brethren have somehow been enshrined as the best medium mu triode ever lol. And truth to tell, it was pretty linear if used correctly. They were also intended for 12V heater operation in car radios. I guess the high-end cars of the time had to have a radio...

On the subject of the higher filament voltages...remember the filaments were in series across the line in most home radios, so it paid to add up to somewhere near 115VAC.
 
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That's pretty impressive bass response; I guess Heathkit didn't skimp on the output transformers.
 
That's pretty impressive bass response; I guess Heathkit didn't skimp on the output transformers.
I owned a few Heathkits growing up. You generally got good value.
 
Those tube manuals were a marvel.
We'd probably have way less to talk about at ASR if more boutique "designers" used the example circuits from them rather than having inspiration come to them in a dream after swilling wine* by the fire, a la Kekule! ;)
1675539728923.png

My "Simple 2A3" (Joseph Esmilla's design, that is) came, in his own words, straight from the RCA resistance coupled amplifier tables. :)

The basic RCA MM phono preamp/EQ circuit, similarly, is perfectly good (although difficult to get quiet). There is some art in design - or at least craft. :)

_________________
* or huffing benzene, as the case may be... :oops:
 
We'd probably have way less to talk about at ASR if more boutique "designers" used the example circuits from them rather than having inspiration come to them in a dream after swilling wine* by the fire, a la Kekule! ;)
View attachment 262292
My "Simple 2A3" (Joseph Esmilla's design, that is) came, in his own words, straight from the RCA resistance coupled amplifier tables. :)

The basic RCA MM phono preamp/EQ circuit, similarly, is perfectly good (although difficult to get quiet). There is some art in design - or at least craft. :)

_________________
* or huffing benzene, as the case may be... :oops:
A lot of people thought Kekule was a nut, too. He didn't know about resonance, but he had the last laugh...

RCA went to considerable lengths to publish good circuits. And, they moved a lot of tubes because of it.

At the beginning of the transistor era I remember manufacturers publishing transistor manuals, with the same aim. Unfortunately, transistors don't need regular replacement like tubes, so the long-term profit wasn't there.
 
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