Should I or shouldn't I get a Harman Kardon A500

[Curtwheeler]

Hi I have a chance to buy a Harman Kardon A500 for really little moola. It looks complete except the output tubes are missing. The face plate is nice, no rust on the chassis just oxidation. It even has the wood cabinet. I'm pretty sure I've talked myself into it already just describing it. I guess what I really would like to know is how involved would it be to change the output tubes 7355 to something less expensive, like the 6L6 ? I did see that Gordon did one awhile back, but I'm nowhere close to having is knowledge, but I am learning. I can recap and measure voltages, and follow a schematic.Is the 6L6 taller then the 7355, as I would like to use the cabinet. Are all the pin outs different for a 6L6. I haven't had a chance to go over the data sheets yet. I did see that the 7591xyz is a direct replacement, but I can't seem to find it and I saw it is taller. I guess the first thing to do is get it home and order a photofact. I'd really appreciated any help and or links for my future project. Thanks Curt

 

[captouch]

Can't help with mods, but definitely get it if it's cheap. Love my A300.

 

[byoungman1]

My A500 is by far my favorite integrated in my collection. I just completely re-capped it and WOW.
Re-designing around 6L6s is a waste of time IMO, as a matched quad of NOS 7355's is cheaper than a comparable matched quad of 6L6's.

 

[sony6060]

The HK is a good amp. No 6L6 tube swaps due to the filaments draw double the current and will ruin the power transformer. Biasing may be different too. Use 6V6GT tubes.

Oops- I was thinking 7408. 6V6GT is too limited electrically to perform in a 7355 circuit.

 

[Tom Bavis]

7591 is only one pin different from 7355, and that one pin MAY be unused but it's much larger than a 7355, and no longer available new... . Even 6L6GB or 5881 is probably too tall, and of course require rewiring. They are near identical otherwise.

Bob at findatube,com has 7355s for $23 new, $12 used.

Email for the Photofact - I should have it - 579-9.

<edit> Sony, you're thinking of the 7408 - used some smaller H-K amps. It's a variant of 6V6.

 

[Tinkerbelle]

The similar but lower power HK A-300 amp that I have sounds great, as does the HK TA-260 receiver that runs 7355 tubes. Though similar, I seem to prefer the 7355 sonic output to that of the 7591 tube.

7355 tubes are worth trying if you can find good sets. An alternative might be to re-socket (Magnoval), rewire and re-bias the outputs for the 7868 tube, which is currently available from new production and is in a relatively small form factor vs. 6L6 tubes.

 

[gadget73]

You're going to have a bit of a time finding good Magnoval sockets though, or I did anyway. I found lots of cheap new production ones with "gold" pins, but it took some doing to come up with 4 old ones. I think mine are Cinch or Amphenol, and I found some guy in Canada who had 4 for sale on that auction site. I paid a little more for them vs new Chinese ones but they're excellent quality. What I have them installed in is annoying enough to work on that I didn't wish to do it again.

 

[Curtwheeler]

Harmon Kardon

Hi all, Thanks for your input. I just got home with the amp this evening. The guy I got it from said he tried 7591 tubes. He got sound but one of the resistors started smoking so he shut it off. That's all I know at this point. I'll include picture of the underside tomorrow

 

[Tom Bavis]

7591 are one pin different from 7355 and will be biased near cutoff. Should work fine if bias voltage is dialed back - might need some resistor changes. But make sure there are no connections to pin 4 - unused on 7355, screen grid on 7591.

 

[Curtwheeler]

Hi. The seller said he did lift the wires off of the 4 pins. I took a quick look at it and it looks like he did. Looks like more investigation ahead.

 

[Curtwheeler]

Tom did you get my P.M.

 

[Jenkster]

I have the kit version of this amp that is currently in "the shop." Member GordonW has done many re-builds on this amp and is a very good source of information. I think he adapted one to use 5881 tubes with little modification if memory serves. I have about 7 good 7355's for mine that I lucked into. I just need to figure out the nagging issues with it. I don't think it will replace my Fisher KX-200 but I will keep an open mind.

 

[Curtwheeler]

Thanks again guys. i hope I luck into a set of 7355 but if not I'm absorbing all the information. Also I have learned so much from reading GordonW's threads, especially when I was working on my Magnavox amps.

 

[GordonW]

I finally got around to writing up a short instruction set for changing over from 7355 to 5881 (6L6 pinout) on these units:

7355 to 5881 Conversion- Harman Kardon A50K/A500:
-Plate (pin 3) and heater (pins 2 and 7) remain the same between 7355 and 5881.
-Temporarily remove 1K and 270K resistors from tube sockets (7355 pins 1, 4 and 6). (BTW: Recommended to replace with new resistors (same values) instead of reinstalling old ones, due to frequent drift of values with age.)
-Move Grid wire (one lead of coupling capacitor) from 7355 pin 1 to 5881 pin 6.
-Move Bias wires (either white/black stripe or black/white stripe, depending on year) from 7355 pin 4 to 5881 pin 1.
-Move Screen wire (orange) from 7355 pin 8 to 5881 pin 4
-Move Cathode wire (yellow) from 7355 pin 5 to 5881 pin 8.
-Install 270K resistor between pin 1 and pin 6 of 5881.
-Install 1K resistor between pin 5 and pin 6 of 5881.

The socket will look something like this once you finish the mod (viewing the amp chassis with the front closest to you):

(NOTE: In the picture, I have added two 10 ohm 1/4watt resistors in parallel- to make 5 ohms- in series with the cathode lead. This allows measuring effective cathode current through each individual tube, by taking the voltage across 5 ohms, and dividing it by 5 ohms. This is optional, but a good idea.)

Do this, and get a set of Sylvania, RCA or Tung-Sol 5881 tubes (old stock or reproduction), and you can reinstall the top cover with no problems. These tubes are within 1/8" of the same height as the original 7355 tubes.

You can also use 6L6GB, 6L6GC, 7581A or Soviet-made 5881 tubes in these- but the lid will not go back on, due to these tubes being about 3/4" taller than the original 7355s. Metal-can 6L6s could be used- but be careful if you do this, as the tube metal shell will have about 25v on it (bias voltage). Not lethal, but it could lead to a nasty surprise if you shorted the tube metal shell to the chassis!

All of these tubes will bias up just like the original 7355s. No other changes will be necessary, and the amp will sound pretty much exactly as it did originally.

BTW: I would NOT recommend EL37 or European KT66. Those draw too much heater current, and will bias up differently.

This will allow ready long-term use of these fantastic amps. Using them every day is no big deal, with the multi-vendor proliferation of 5881 tubes out there. No problem finding those, either now, or in the future, most likely!

Regards,
Gordon.

 

[byoungman1]

Still curious- why go to all that trouble when NOS quads of 7355's are under $100?

 

[Curtwheeler]

Harmon Kardon

Thanks sooooo much Gordon for your time. That was just what I was looking for. Changing pin out with great instructions, diagrams and even pictures. Sort of like painting by numbers. I'm sure others will appreciate it in the future. As for finding inexpressive 7355 tubes. I haven found them yet.and if I knew I could always find them, I would not go to the trouble of switching out tubes. I know, I only got the amp less then 24 hours ago and that is why I'm asking for opinions. You guys know a lot more then I do.This is going to be a long proses as I'm not in a hurry, I have other good tube amps to listen to.and it's my new hobby I'll keep you posted on the project and add a picture of the under side later today. Thanks again. Curt

 

[buickguy62]

I've restored several (more than 10) A500's and except for one broken tube, have found the 7355's to test very strong, and all appeared original to the amps. I understand that HK did not drive them very hard so they seem to last and last. Also, on that large auction site, a quad of Sylvania's (said to be NOS) just sold for $115. Unless you are looking for a different 'sound' I would use the 7355's. Just checked, there are 19 7355's for sale on that auction site. If it were me, I'd watch for a reasonable priced quad, buy them for backup and expect to be OK for decades.
Tom

 

[Curtwheeler]

Harmon Kardon

Thanks Tom That's good to know about the longevity of the 7355 in the Harmon Kardon amps. I'll be watching the auction site. I do like the ability to have more choices with the 5881 with GordonW's pin out mod.
I'll probably be doing E-caps and coupling caps , diodes before I have to make the final decision on what tubes to use. I do appreciate your help. Here's a picture of the underside.

 

[Curtwheeler]

Here's a picture of where the previous owner temporarily lifted the pin 4 leads to try some 7591 tubes he had.

 

[Curtwheeler]

And here's the resistor that started smoking when he tried the amp with 7591 tubes. He then got discouraged and I got it, without tubes. The smoking resistor is the blue/green smoky colored one in the center of the picture. I won't know the value till I get a photofact as I can't make out the color code.