heathkit aa-151 advice please

jntit

AK Subscriber
Subscriber
I was thinking of setting a small system in my bedroom when I remembered I had this. It's pretty clean, was in a box. The tubes are 6anb GE Electronic tube, 6au6 Daystrom Nichmen, 6eu7 all it says is Great Britain, the el84 have no insignia on them, someone wrote 6bq5 Ray on them, (could the ray mean raytheon), the gz34 just has Great Britain on it. Should I go ahead and fire it up. Any advice would be greatly appreciated. Thanks John
 
has to be at least 7 years.
Not a good idea to just turn it on. You should bring it up slowly to see if anything is problematic. 7 years is a long time for the capictors to dry out and loose their values or have a short. You could cause it damage. Do you have a dim bulb tester or a variac?
 
About 17 years ago, I acquired an AA-151 which had spent several years in someone's garden shed. It worked without (much) hum, though two of the output tubes tended to red-plate intermittently; the issue went away after I swapped the pairs of output tubes, so it might've been dirty socket contact(s). After this many years, it's likely that the filter capacitor cans have dried up, and the molded-paper capacitors probably oughta be replaced as well. You could use discrete capacitors to replace the two filter cans, or you could see if Hayseed Hamfest has something which would work (60/25µF@500/450V, and 30/30µF@350V, according to this page). Good luck!
-Adam
 
I own two AA151's and they are good amps, but after close to 60 years, they should be reconditioned by replacing the caps and any out of tolerance resistors to get them to perform correctly and to have decent reliability. I've done that to both of mine as the filter caps were toast and most of the smaller caps were leaky too. Hayseed Hamfest is a good place for electrolytic caps along with Antique Electronic supply which is where I got the caps for mine.
 
Here is a pic of the underside.View attachment 1360023
You're (mostly) in luck! Someone has replaced (most of?) the molded-paper capacitors with orange drops, which are excellent capacitors. However, the can capacitors appear to still be in circuit, as does a tubular electrolytic in the top right corner of that photo. I'm not sure about the identity of the two black tubular components on the far right edge of the photo, but they could be some sort of capacitor. In any case, I'd probably replace the tubular electrolytic with a modern one of the same value, then bring up the amp slowly on a metered variac with a pair of speakers connected to the output terminals, making sure that it's not drawing too much current or humming too loudly. Good luck!
-Adam
 
Like AdamAnt316 says, you're in luck with the small signal coupling caps being replaced with orange drops which is what I used in mine. The electrolytics should be replaced along with checking for out of tolerance resistors. Both of my amps needed a bunch of the resistors replaced because they had drifted way high over the years. I used carbon film resistors in mine as that is what I had on the shelf, but you could use metal film resistors too. One of my amps is used in my garage system connected to a pair of Bozak speakers and a Kenwood tuner which to me sounds pretty good and can be on for several hours at a time and it hasn't caused me any problems other than an occasional tube replacement in the years it has been out there.
 
Thanks for all the info and advice. I'll check out the caps that AdamAnt316 mentioned and order replacements. I'll try to find the 60/25 & the 30/30 cans. I should be able to find a parts list with the resistors, looks like this is going to be a bigger project than I though, but that's ok. Again thanks to kaplang, Big Harry, ant AdamAnt316. John
 
Check hifiengine.com in their manual library section. They have the construction manual available for download which should include a parts list. If that doesn't work, I have the construction manual and can scan the parts list and schematic for you. Hayseed Hamfest can build the electrolytic caps for you or you can buy them from Antique Electronic Supply.
 
Well I found the manual here in the AK data base, it's a lot of resistors. The 60/25 can is being harder to find, so I'll go to Hayseed for it. I think I have the caps that AdamAnt316 noticed figured out. I guess I'll start ordering in the next couple of days. Thanks again, John
 
You don't need to use exact values for the replacement, just close values. If using discrete capacitors, you could use 47µF or 68µF for the 60µF section, and 22µF is close enough to 25µF. As far as can capacitors go, Hayseed should have something close enough, but barring that, you could get a 30/30/30µF@500V can, and parallel two of the sections (connect two of the lugs together).
-Adam
 
Well I got everything ordered. The small black caps that Adam noticed are listed in the parts list as 50uf 10 volt miniature, I found some sprague 50uf 16 volt which might be a little bigger, but should be ok. I had, I don't know why, a 50uf 250 volt sprague atom, which will probably be way too big, but I should be able to use it. I emailed Hayseed about the cans. The resistors should get here first, so I'll start on them. Thanks again guys for all the help & info. and have a Merry Christmas. John
 
Well I got everything ordered. The small black caps that Adam noticed are listed in the parts list as 50uf 10 volt miniature, I found some sprague 50uf 16 volt which might be a little bigger, but should be ok. I had, I don't know why, a 50uf 250 volt sprague atom, which will probably be way too big, but I should be able to use it. I emailed Hayseed about the cans. The resistors should get here first, so I'll start on them. Thanks again guys for all the help & info. and have a Merry Christmas. John
Definitely a good idea to replace those black capacitors, as they're almost certain to be bad by this point. The 16 volt parts should work fine. A 250 volt part would work fine at lower voltage, but electrolytic capacitors tend to 'form' at the voltage they're used at. So if you used a 250V capacitor on a 12 volt supply for a long period of time, then tried to use it at 250 volts, it may not work very well. I know this isn't a likely concern in this case, but it's something to think about, iMO.
-Adam
 
I did not know that about the electrolytic caps. It's always good to learn new things. Thanks, John
 
Well I finally was ready to turn it on and it appears no power to the 6eu7's. Probably should have turned it on before I started, now I don't know if it was like that or if it was something I did. I guess I can check the out put on the other Xfmr and compare. Isn't this hobby a blast? :D
 
By no power to the 6EU7's do you mean that the tube filaments do not light or there is something like no plate voltage or the tubes won't pass a signal?
 
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