Harman Kardon A50K

[rknize]

My father-in-law gave me his old Harman-Kardon A50K, which I believe is the kit version of the A500. It's spent the past few decades in a garage, so it is a little crusty/rusty. It still has the original tubes, so it seems. HK-branded 7355's and "Made in West Germany" 12AX7s. It obviously needs to be recapped and generally fixed-up before applying power.

The 7355's have some getter left, but definitely look worn. He used this amp for a long time and liked it a lot, so I imagine they are tired. The 7355 seems all but extinct.

Any advice? Is this amp worth fixing-up (assuming that it works)? Should I mod it for 6L6's? I assume the OPTs are good quality, so should I just build something better around them down the road?

[rknize]

I guess the 7591 will drop in place of the 7355. They may be too tall though. I did find the schematics for the A50K. Nothing special about the amp design...simplified Williamson with a concertina driving the output tubes as pentodes. Pretty much what I have in my Thomas amp right now, though a bit more power I suppose.

[vinylkid58]

Originally Posted by rknize

Any advice? Is this amp worth fixing-up (assuming that it works)? Should I mod it for 6L6's? I assume the OPTs are good quality, so should I just build something better around them down the road?

Could make a dandy "El Cheapo" Grand project, provided the irons ok. Definitely something with a better PS and driver stage.

jeff

[Jim McShane]

When I get a request from someone who wants to buy 7355s (now VERY hard to find, let alone find matched ones!) I recommend the Sovtek 7591XYZ. Those were the stopgap tubes made for a while until the EH 7591A came out. The XYZ tubes were Sovtek 5881/6L6s pinned out to match the 7591. As a result the XYZ is a virtual drop in sub for a 7355. Just one caution...

H-K A500/A50Ks used pin 4 as a tie point on the output tube sockets since the 7355 tube didn't use pin 4. They usually connected one end of the grid resistor there. But on the 7591A or 7591XYZ there is a screen connection at pin 4! So anything in the amp tied to pin 4 MUST be relocated and pin 4 left empty - or you'll have the screen voltage applied to the control grid!

That's the only change needed to use the 7591XYZ. They sound nice, pairs/quads is no problem - and they're cheap too! I did an A500 with a set of XYZs not long ago - it turned out great!

[rknize]

I did some measurements and it looks like the EH 7591A would fit...just. The JJ 7591 is about 1/4" shorter. Thanks for the heads-up about pin 4, Jim!

A few pics....

[Jim McShane]

Originally Posted by rknize

I did some measurements and it looks like the EH 7591A would fit...just. The JJ 7591 is about 1/4" shorter. Thanks for the heads-up about pin 4, Jim!

A few pics....

Not the 7591A!! The 7591XYZ. The XYZ is much closer electrically since it's a 6L6 type. You'd have to redo the bias circuit to use 7591As.

And the 7591XYZs fit fine.

Remember - the 7591XYZ is the tube you want, NOT the 7591A.

[rknize]

OK...well...the XYZ is out of production. It's been replaced by the A.

[Jim McShane]

Originally Posted by rknize

OK...well...the XYZ is out of production. It's been replaced by the A.

I have XYZ in stock and can obtain plenty more if I need them. There is no shortage of them. Whether they will run them again in production - who knows?

[rknize]

Thanks Jim. I see them on New Sensor's site too. The "normal" vendors have stopped carrying them, but that makes sense since they are actually "weird" tubes that don't quite work properly in amps that expect 7591s. They look like 6П3С tubes, so they do align well with 7355's, spec-wise.

[rknize]

I popped the bottom off. After vacuuming out all of the bird seed that was packed in there (mice?), it looks pretty good. Whoever built the kit did a nice job. I see one repair where an old film cap was replaced. Everything else looks original and in good condition. I think I will attempt to reform the caps on the variac and see what happens. If it works, maybe I'll keep it. I think my father in law would be a bit heart broken if I dismantled it.

Do you sell any cap "kits" for the A500, Jim? The power supply is a bit lacking. BTW, I don't see the 7591XYZ listed on your web site.