Heathkit A-8A Vintage Tube Amplifier Measurements

[mhardy6647]

At the beginning of the transistor era I remember manufacturers publishing transistor manuals, with the same aim.

I have my father's copy of this one

... but I digress.

 

[fpitas]

I have my father's copy of this one

View attachment 262313
View attachment 262314

... but I digress.

Single supply quasi-complimentary! Back when you couldn't get good silicon PNPs. Some thoroughly obsolete transistors there. They don't make them like they used to, and that's a good thing.

 

[mhardy6647]

1964.
Just sayin'.

 

[fpitas]

1964.
Just sayin'.

Yeah, I got a transistor or two to play with as a kid then. They were so bad I went back to tubes.

 

[Messing with Electronics]

Everyone is definitely right about the grounding, and I think the circuit is capable of much lower noise. These measurements were actually taken after I had done some work to address the ground situation, including cleaning connectors, adding tin foil underneath the amp (which is clipped to circuit ground), and adding a spade to improve the connection between chassis and circuit ground. Unfortunately, I think a significant overhaul (which is too much labor for me to justify) would be necessary to get noise any lower. And, given the rarity of the amplifier, I'd rather keep it cosmetically original in case a collector wants to stock, identical units.

Re. E1DA configuration, I know my measurement rig isn't causing the noise because the spectrum with the amplifier turned off has a very low noise floor, and I've measured a number of SS power amps with much lower noise levels on the output. The noise on this spectrum is definitely being produced by the amplifier.

 

[fpitas]

Everyone is definitely right about the grounding, and I think the circuit is capable of much lower noise. These measurements were actually taken after I had done some work to address the ground situation, including cleaning connectors, adding tin foil underneath the amp (which is clipped to circuit ground), and adding a spade to improve the connection between chassis and circuit ground. Unfortunately, I think a significant overhaul (which is too much labor for me to justify) would be necessary to get noise any lower. And, given the rarity of the amplifier, I'd rather keep it cosmetically original in case a collector wants to stock, identical units.

Re. E1DA configuration, I know my measurement rig isn't causing the noise because the spectrum with the amplifier turned off has a very low noise floor, and I've measured a number of SS power amps with much lower noise levels on the output. The noise on this spectrum is definitely being produced by the amplifier.

You might try a line-level transformer at the input to isolate grounds. Sometimes that older equipment didn't worry a lot about ground loops

 

[Blumlein 88]

Sometimes on those 2 pin plugs, just flipping the plug in the other direction lowers hum. I'm guessing you might well have tried it both ways.

 

[Messing with Electronics]

You might try a line-level transformer at the input to isolate grounds. Sometimes that older equipment didn't worry a lot about ground loops

I tried something equivalent to this by shorting the input jacks, but unfortunately it didn't lower the noise floor.

Sometimes on those 2 pin plugs, just flipping the plug in the other direction lowers hum. I'm guessing you might well have tried it both ways.

Yep, tried this as well and there was some difference but not night and day. I think that I remembered to keep the plug in the lower-noise configuration when I ran the tests, but honestly I'm not certain. I might verify sometime this week before I box the amp up to sell.

 

[firedome]

Heahkit did indeed start by purveying WWII surplus, amazingly kit airplanes were among their initial products.
Among their earliest amps were Williamson kits that also used surplus, notably ex-WWII surplus 807 "transmitting" tubes, very similar in function to 6L6GB, that they got by the crate, later moving to 5881s to eliminate the top cap for safety reasons. The A-8 and A-7 were their "economy" offerings.

 

[GXAlan]

Heahkit did indeed start by purveying WWII surplus, amazingly kit airplanes were among their initial products.
Among their earliest amps were Williamson kits that also used surplus, notably ex-WWII surplus 807 "transmitting" tubes, very similar in function to 6L6GB, that they got by the crate, later moving to 5881s to eliminate the top cap for safety reasons. The A-8 and A-7 were their "economy" offerings.

Out of curiosity, since a quick google search wasn’t effective, are there any benefits of a top cap? Seems like you’re just asking for high voltage to be easily exposed to the world

 

[fpitas]

Out of curiosity, since a quick google search wasn’t effective, are there any benefits of a top cap? Seems like you’re just asking for high voltage to be easily exposed to the world

A top anode connection allows higher voltage without danger of arcing. Back in the day with point-to-point wiring, that got to be important past maybe 500V.

If you use a good insulated top cap with a decent wire, the danger of electrocution is no worse than it would be for anyone poking around in a tube amplifier.

 

[egellings]

A top anode connection allows higher voltage without danger of arcing. Back in the day with point-to-point wiring, that got to be important past maybe 500V.

If you use a good insulated top cap with a decent wire, the danger of electrocution is no worse than it would be for anyone poking around in a tube amplifier.

The tube was also used as a radio transmitter final, and it was important to minimize capacitive coupling between grid & plate. Placing the plate connection on top, as in the 6BG6, would help with that.

 

[mhardy6647]

the danger of electrocution is no worse than it would be for anyone poking around in a tube amplifier.

That's true but somehow perhaps not as reassuring as it may have been meant to sound.

 

[mhardy6647]

Heahkit did indeed start by purveying WWII surplus, amazingly kit airplanes were among their initial products.

Indeed.
Heathkit's founder, Ed Heath, was - as you probably know - killed testing an aircraft.

PD-US, https://en.wikipedia.org/w/index.php?curid=37297670

 

[Saponetto]

Thanks a lot everybody for getting this post back on sight.
I appreciated it a lot...

 

[mhardy6647]

I - for one - harbor much affection for most things Heathkit.

 

[firedome]

Me too, I built a 7591 based mono integrated in 1964, my intro to Hi-Fi starting a near 60 year hobby.
The recv'r pictured like above were top rated in their day iirc.

The top cap high-voltage exposure was what they wanted to eliminate by going to 5881s in later models... as all those little baby boomers were crawling around by then, including yours truly and many of us.

 

[pjn]

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.

View attachment 261656

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):

View attachment 261657

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):

View attachment 261658

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:

View attachment 261659

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.

View attachment 261663

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.

That's great - what little I know of electronics (basically TTL logic) I can thank HeathKit for - I still have an oscilloscope built from a kit, and it still, amazingly, seems to work. Although it can't handle modern clockspeeds at all. Not even close

 

[MaxwellsEq]

A top anode connection allows higher voltage without danger of arcing. Back in the day with point-to-point wiring, that got to be important past maybe 500V.

If you use a good insulated top cap with a decent wire, the danger of electrocution is no worse than it would be for anyone poking around in a tube amplifier.

Ah yes, the joy of getting a shock off a top anode cap!

 

[egellings]

Me too, I built a 7591 based mono integrated in 1964, my intro to Hi-Fi starting a near 60 year hobby.
The recv'r pictured like above were top rated in their day iirc.

The top cap high-voltage exposure was what they wanted to eliminate by going to 5881s in later models... as all those little baby boomers were crawling around by then, including yours truly and many of us.

7591 has no plate cap. Were you referring to a different tube type?