Hello everybody,
This is my first post on DIYaudio, although I have been reading the post for many years. Today I bought a Harmon Kardon TA 224, labeled "Stereo Recital". It's an integrated tube amp with a AM FM tuner. I haven't seen it in person yet, but it was made in the USA, seems to have some decent iron in it. It's suppose to power up, the front panel looks good. I'm excited with the prospect of restoring it and finding out what it sounds like.
I'm trying to find out when this receiver was introduced and if anyone here has any experience with this HK model that they would like to share, it seems there is very little information out on the web about this one, I wasn't able to find any links on DIYaudio when searching for "Harmon Kardon TA224", it seems that it's somewhat an unknown.
Lou
Currently listening to a HK Citation V, JBL L65's, Apt/Holman Pre-amp, Technics MkIII, Logitech DAC
This is my first post on DIYaudio, although I have been reading the post for many years. Today I bought a Harmon Kardon TA 224, labeled "Stereo Recital". It's an integrated tube amp with a AM FM tuner. I haven't seen it in person yet, but it was made in the USA, seems to have some decent iron in it. It's suppose to power up, the front panel looks good. I'm excited with the prospect of restoring it and finding out what it sounds like.
I'm trying to find out when this receiver was introduced and if anyone here has any experience with this HK model that they would like to share, it seems there is very little information out on the web about this one, I wasn't able to find any links on DIYaudio when searching for "Harmon Kardon TA224", it seems that it's somewhat an unknown.
Lou
Currently listening to a HK Citation V, JBL L65's, Apt/Holman Pre-amp, Technics MkIII, Logitech DAC
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I’ve got an A224 integrated amp that I rebuilt and love. The electrolytic capacitors were shot as were a few of the carbon comp resistors. The 300 ohm cathode resistors in particular. Replace them with 2 watt Takman units. I used individual caps under the chassis to replace the can unit and left it there unconnected. I got lucky and mine came with a Mullard GZ34 rectifier. The small signal tubes were ok but the EL84 were shot so I used Russian 6p14-EV as they can handle the slightly elevated voltages caused by our higher line voltages. It wants to see 117 and I’ve got 123.
Mdpaudio, thank you for sharing your experience. It seems that the cathode resistors are expose to plenty of heat, I've had them crumble apart as I was going though a couple of amps. Will update this thread as to the condition I find the amp in when it arrives, I'm hoping that the tubes are ok.
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The H/K TA224 needs help in order to play MPX FM stereo. DIYer extraordinaire S. Bench has provided the 'phile community with a comparatively easy to implement MPX decoder.
The 7408 is an industrial version of the 6V6.
The unit uses a voltage doubler B+ PSU. Replace the OEM diodes with UF5408s.
The 7408 is an industrial version of the 6V6.
The unit uses a voltage doubler B+ PSU. Replace the OEM diodes with UF5408s.
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Eli D.,
I have a NOS Archer LM1800 on the way!
I've been researching the 7408, just by chance, I read a thread last night where old Soviet 6P6S tubes seem to have a lot people’s attention, I’m not sure if they would be a direct swap for the 7408, but it looks promising.
I have several UF5408’s here!
Thank you for your advice, it will save me several hours!
Louis
I have a NOS Archer LM1800 on the way!
I've been researching the 7408, just by chance, I read a thread last night where old Soviet 6P6S tubes seem to have a lot people’s attention, I’m not sure if they would be a direct swap for the 7408, but it looks promising.
I have several UF5408’s here!
Thank you for your advice, it will save me several hours!
Louis
6P6S is a direct swap for 7408 in HK amplifiers. I have two A300 amps, in which I used 6P6S for years, with no issues whatsoever.
The basic HK 7408 circuit has very interesting topology: local positive feedback within the global NFB loop. The sound of these amplifiers is refined and detailed, resembling that of triode amplifiers with no feedback. It is another world compared to typical bland-sounding Scotts, Fishers, and Eicos is of the time.
The basic HK 7408 circuit has very interesting topology: local positive feedback within the global NFB loop. The sound of these amplifiers is refined and detailed, resembling that of triode amplifiers with no feedback. It is another world compared to typical bland-sounding Scotts, Fishers, and Eicos is of the time.
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SSER2, I'm looking forward to hearing the TA224, almost everything I have read, which isn't much, the writers praised the sound. One guy said that the RATED distortion measurement was to high, that it sounded terrible and he threw in the garbage.
Mdpaudio, I saw that also, it had me wondering why? I believe the A224 lacks the radio, otherwise they look similar.
Mdpaudio, I saw that also, it had me wondering why? I believe the A224 lacks the radio, otherwise they look similar.
It is reassuring to have a variac and a dim bulb tester, if you do not have these already. It is very useful to be able to power up an old tube device to some non-destructive point - say just after the valve rectifier starts conducting, and take voltages and get a feel for what is working, and what is not.
Plus, being able to ramp up the voltage slowly keeps the option open for reforming the capacitors you have, assuming the unit has been dormant for some time.
I had a Knight mono amplifier, with 6973 finals. The unloaded B+ was well over 400V, and the amp was dependent on a healthy current going to the output tubes [there was a 450R 20W resistor in the first place in the power supply]. Well, they only had 5% of emission, so by not pulling down the B+ the whole amp would have ended up seeing a much higher voltage than the components were originally specced for. By being able to find a happy spot with the variac, I could at least determine I had bought a pig in a poke ;-/
Plus, being able to ramp up the voltage slowly keeps the option open for reforming the capacitors you have, assuming the unit has been dormant for some time.
I had a Knight mono amplifier, with 6973 finals. The unloaded B+ was well over 400V, and the amp was dependent on a healthy current going to the output tubes [there was a 450R 20W resistor in the first place in the power supply]. Well, they only had 5% of emission, so by not pulling down the B+ the whole amp would have ended up seeing a much higher voltage than the components were originally specced for. By being able to find a happy spot with the variac, I could at least determine I had bought a pig in a poke ;-/
OldHector,
Thank you for the good advice, I'm going to do precisely what you advise. Looking at the capacitors, they at least look good, no swelling or spillage. I'm going to see if they will reform, the amp actually doesn't seem to have many hours on it, I believe it still has the original fuse. Sorry to hear about the Knight amp.
Thank you for the good advice, I'm going to do precisely what you advise. Looking at the capacitors, they at least look good, no swelling or spillage. I'm going to see if they will reform, the amp actually doesn't seem to have many hours on it, I believe it still has the original fuse. Sorry to hear about the Knight amp.
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Y'all are right, I wouldn't be comfortable with them, I would constantly be worried about them starting a fire. Just ordered the caps from Mouser, the individual ones, I going to do as mdpaudio did, just hook them up under the chassis, leaving the original cans in place. The cost for the caps with shipping came in at about $25, not bad really.
Finally powered up the amp, cleaned all the controls, built a dim bulb and used it in conjunction with a Variac, started at about 35 volts, eventually getting up to 90 volts, no sparks, the amp started doing it's job and it sounded ok, after about an hour the sound coming from my circa 77 newly capped Heresy's was very good, I didn't want to stop listening. I did turn the radio on but no luck, I have no idea as to what's going on there but I'm thrilled with the pre-amp and the amplifier. I do have a problem with the Loudness/Balance control, it have two potentiometers back to back, but the tabs on the rear one, that attached it to the front pot, were fatigued, only 2 of the four tabs were still intact, I bent them open and they just came off, the tensile strength of the metal tabs was down to nothing. I held the potentiometers together with some plastic ties, but that's not a satisfactory solution, anyway that will need to be corrected.
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Good for you! I think it is worthwhile recording all voltages on all valve pins, so you have a record and can just compare with any service information you find. It is reasuring to know that everything is within tolerances, or if not, you can catch it early.
Some tubes from the good ol' days are more tolerant and were abused a bit in their specifications. That can also be good to identify.
Some tubes from the good ol' days are more tolerant and were abused a bit in their specifications. That can also be good to identify.