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Author Topic: HAMMARLUND HQ-129X 6SS7 SUBSTITUTE  (Read 1816 times)
AC0FA
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Posts: 298




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« on: July 26, 2007, 02:38:56 PM »

I finished putting new tubes in the rig last night and discovered something. The receiver schematic shows 4 6ss7 in the 1st 2nd and 3rd IF amps and the RF amp.

I found a 6AC7 in the 3rd rf amp. When I replaced it with a 6SS7 the audio dropped out. And did not return until I replaced the old 6AC7 back in the socket.

The most info I can find is the 6AC7 is listed as a replacement for the 6SS7. I am un-able to find any info if the socket must be rewired. Or even if the pins outs are diferent. I switched several 6ss7 in and out of the socket with no luck.

May be some one can remember. The receiver is only 60 years old and they made 10,000 of them Hi Hi.

AC0FA            
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KA5N
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Posts: 4380




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« Reply #1 on: July 26, 2007, 03:56:53 PM »

A google search will reveal that the pin outs are identical for the 6SS7 and the 6AC7.  The 6AC7 filaments draw .45 amps while the 6SS7's are only .15 amps.  Since the filaments are fed in parallel, this shouldn't make any difference.  Other specs are close enough that either should work (6SK7 is another sub). I would check the value of the resistors connected to this stage and the voltages with each type tube in place.  If this is an IF stage the tuning may have been adjusted for the difference in the tubes.  Complete IF alignment should be done.  
Allen
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AC0FA
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Posts: 298




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« Reply #2 on: July 26, 2007, 07:07:46 PM »

As always
Thanks Allen
John AC0FA
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AC0FA
Member

Posts: 298




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« Reply #3 on: July 27, 2007, 08:59:57 AM »

Ok,
This is still a widely circulated and readily available receiver at garage sales and ham fests.
So, I will follow up.

I checked the resistors and inspected the voltages and wiring. All was in order.

The rig sounds beautiful, really no complaints, finding a different part number on the tube was more of a what the heck? Than a real problem.I noticed a newer capacitor in the socket. It HAS been serviced  since 1947.

When the original 6SS7 didnt plug and play I was just kind of supprised.

The 6AC7 is the 3rd IF in a string of 3 low gain IF amplifiers for stability.

It seems to directly control the shape factor of the filter skirt.

I would expect a sweep generator and a scope to be required. To align the 3rd IF properly and get it to jive with the 6 selectivity filter settings as well as the 455khz IF.

I choose to stick in a new 6AC7. Why make it hard?

I don't have a service manual but it would appear that the adjustments to the 3rd IF can be made at the side of the rectangular tin tower next to the socket.

One thing I learned from My elmer.
Don't diddle with any adjustments if your rig sounds good.

Keep 'em glowing
John AC0FA


 

   
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KA5N
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Posts: 4380




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« Reply #4 on: July 27, 2007, 09:51:11 AM »

Sounds like a good idea.  The manual says use a sweep generator and oscope.  The difference in internal capacitance in the the two tube types is enough to upset the alignment.  You are correct in that the IF strip is stagger-tuned to give the desired response.
Without the proper gear you would probably do more harm than good by diddling.  "Don't keep repairing until you break it completely."
Allen
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