This one
With its original Harman Kardon 7408.
But not in as good condition as this one.
As said before, I have several, two of which are in new condition.
I'm still surprised that these small output transformers can go so low in the bass.
With its original Harman Kardon 7408.
But not in as good condition as this one.
As said before, I have several, two of which are in new condition.
I'm still surprised that these small output transformers can go so low in the bass.
This would be a very interesting project from my perspective. When I was assessing which way to go with my Pioneer SM-R150 (similar to HK A300, but lower league I guess) I was looking at that circuit for inspiration.
There is a good write up of an HK A300 restoration on the Angelfire site - Restoring a Harman-Kardon A-300 Amplifier.
Is the goal to keep a Phono stage? I tried to get confidence in that HK design with simulations of different designs in LTspice, but could not get a model to work for it.
There is a good write up of an HK A300 restoration on the Angelfire site - Restoring a Harman-Kardon A-300 Amplifier.
Is the goal to keep a Phono stage? I tried to get confidence in that HK design with simulations of different designs in LTspice, but could not get a model to work for it.
Not necessarily, I have more in reserve.
the goal is not to lose the spirit of this amp, which is charming but above all, it's still a playground and as long as the output transformers are safe, the rest is of little importance.
That was a great site. I'm glad I grabbed a bit of information from it on various amps. Wish he'd revive it, and do his proposed book on Williamson amps.
A300 is a worthwhile mod project. This is the only vintage tube amplifier known to me that has really good sound. One worthwhile mod is replacing 7408 output tubes with directly heated 2E24. The replacement will necessitate modifying screen supply and auto bias.
A300 small transformers are good quality, but bass limited. A300 needs subwoofer help for good bass. TA230 uses almost exactly same circuit, but has bigger output transformers. Good DC balancing of power stage goes a long way in improving LF response.
I believe that good sound of A300 comes from positive feedback nestled inside the global NFB loop.
I believe that good sound of A300 comes from positive feedback nestled inside the global NFB loop.
I don't have a 2e24 but I have two 2e22 quads.
on the other hand I do not agree on the bass of the A300, ok it has super tight and well-kept bass but on the other hand it goes down very low and very flexible.
it just requires speakers with one or two speakers and above 93db
a real treat with my L100 century or my JM lab db20.
on the other hand I do not agree on the bass of the A300, ok it has super tight and well-kept bass but on the other hand it goes down very low and very flexible.
it just requires speakers with one or two speakers and above 93db
a real treat with my L100 century or my JM lab db20.
It is not that I am disappointed with A300 bass. It is good. But in comparison with Scott and Fisher amplifiers that have big OPTs, the A300 doesn't have the same umph. Mids and highs of Scott and Fisher are nothing like A300 though.
I was very happy with A300 driving ADS300 bookshelves, tweeters upgraded to Scan Speak Revelators.
I was very happy with A300 driving ADS300 bookshelves, tweeters upgraded to Scan Speak Revelators.
What's the reasoning behind this recommendation, please? Where are the advantages of directly heated tubes over more recent indirectly heated ones?
Best regards!
On another A300, I played a little bit, I grafted this configuration (A500 cathode balancing potentiometer) (1 per side) and a choke on the screen power supply, it works really well.
In my experience, directly heated output tubes sound more refined and natural compared to indirectly heated ones. Both triodes and pentodes.
I did what could be considered the ultimate DH-IDH comparison with the A300 platform. 2E26 is a plug-in IDH equivalent of 2E24. 2E24 sounds better hands down.
I can forsee the kind of response to this - no proof, this is my subjective experience. Let's not get into this kind of argument because it leads nowhere. There are many highly respected DIY, on this forum and elsewhere, for whom the sonic advantages of DH tubes, such as 01A, 45, 2A3, or 300B, does not need any proof.
I did what could be considered the ultimate DH-IDH comparison with the A300 platform. 2E26 is a plug-in IDH equivalent of 2E24. 2E24 sounds better hands down.
I can forsee the kind of response to this - no proof, this is my subjective experience. Let's not get into this kind of argument because it leads nowhere. There are many highly respected DIY, on this forum and elsewhere, for whom the sonic advantages of DH tubes, such as 01A, 45, 2A3, or 300B, does not need any proof.
I guess the response is partly because this thread is to stimulate the restoration of old 50's to 60's amps, and plugging in an obscure DH tube is not part of the normal beginners' YouTube video.
This isn't as easy as it looks. In A300, the four output tubes share the common cathode bias resistor, which consists of a string of 12AX7 heaters. It will be necessary to add a low ripple 12AX7 heater power supply with your approach.
I agree. But the thread wasn't "Restoration of Golden Era Tube Amplifiers for Complete Newbies". Of course, you begin with resto to original condition, but once you are there and impressed with the result, you start thinking of how to make it sound even better.
After a lot of experimentation, my system of choice uses DHT 01A preamp tubes and DHT 833C amp output tubes, because they were the best sounding to me.