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8/14/1999 3:59 PM
Just picked up a Harmony H430 amp, two 10" jensens  
and reasonable iron. Looks like it is worth restoring.  
The amp uses three 6973 tubes which are a bit  
pricey. I looked through the chart of a drug store  
tube testers with a bunches of test sockets, and  
found the base diagram for the 6973. I then found that  
the 6CZ5 had the same base diagram. Who says tube  
testers are worthless? I had serviced a bogen PA head  
a few years ago that used two 6CZ5 for output tubes.  
The 6CZ5 seems like a possible substitution.  
My RC-25 RCA tube manual specs suggests the two tubes are  
extremely similar but 6CZ5 is not listed as a substitute  
for the 6973. 6CZ5s can be purchase for $6 while the  
6973s are $25+. Two of the 6973 are the output tubes and the other one I am not sure about. I suspect it drives the reverb. I have not pulled the chassis and do not have a schematic handy.  
Any second opinions?
8/14/1999 8:20 PM

Yer real close but the basing diagram isn't identical - note pin 8. So make sure that the screen grid supply is connected to pin 1 on the socket 'cuz fer all practicable purposes the only difference is the dual screen grid connections of the 6973. In fact, a couple of years ago a friend brought me an "Orpheus" amplifier that belonged to his father. He has installed new IEC 6973s and the amp just didn't work and several service shops had passed it on as "unrepairable." When I got into it I noticed that there was no screen voltage 'cuz IEC had rebranded a 6CZ5 as a 6973 but the amp used pin 8 on the socket as the screen connection - so I just moved the wire to pin 1 on each of the sockets.  
8/15/1999 4:54 AM

Yes, the odd 6973 is the reverb driver tube.  
I think I found the H430 schematic in my old copy of Jack Darr's amp book.  
If Steve A. will help me.... hint...I can scan a copy (with some minor mods to make it sound a little more hip) and email it to you or him.  
Even for a decent DIY'r, it's an "unusual" amp design and circuit board layout that produces some serious head scratching.  
It takes some concentration to reverse engineer it so try and get a copy of the schem.  
I just refurbed one for a customer last week and the retail price of the two 6973s was over $50.00 so, while he wasn't looking, I rewired the sockets for EL84s and they worked just fine too.  
But since I had three NOS 6973, and he wanted to keep it mostly original, I redid it and installed the 6973s.  
Dont waste too much time on this tube issue and just rewire the sockets to take a different but similar pentode tube.  
Also carefully remove the cheap cardboard box cover that houses the reverb springs and clean the RCA jacks and plugs.  
This is a source of weird reverb problems on a few of these old timers.  
8/15/1999 8:28 PM

Thanks to Graywater and Bruce for helpful hints. I  
had caught the grid being attached to both pins in the  
6973 but the problems it could cause had not  
occured to me. Sounds like I need a copy of the  
schematic Bruce! I will have to think long and hard  
about whether to go with the 6CZ5 or the EL84 conversion.  
Also, I just found a whole series of Univox/Unicord  
tube amp schematics from around 1970.  
Pretty standard stuff but I would be happy to  
share a set of copies with any of the folks who maintain FREE web pages with schematics for posting.  
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