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Old 16th August 2012, 04:14 PM   #1
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Default 7199 replacement in AMPEX 6973

hi

I am rebuilding an AMPEX 6973, all new everything (except the transformers), however I am now having to decide on what tube for the front end. Originally this was specified as a 7199, which are now scarce and quite expensive (circa $40 ea), so I have been researching alternatives.

I have found the the conversion wiring diagram for the family if tubes 6GH8A, 6BL8, 6U8A which seems to support the other following tubes as well 6AN8A,ECF80 and the 7059 and 7687.

I have attached the circuit for reference (although the actual unit has an additional 2.7k resistor between pin 8 of the 7199 and C4 and R4 and R20 are in fact 30k)

The question is what tube is likely to sound best, and are there any tweaks the circuit that will significantly increase the sonic qualities of the unit. All the replacement components are top quality, and I would appreciate any advice and considerations.

I have read that those with Dynaco amps have faced this issue and that the 6GH8A is reckoned to sound the same as the 7199, though others suggest the 6BL8 is sonically very good, and tubes and more propose the 6U8.

Please feel free to assist in any way you can

Thanks in advance


Peter
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Old 16th August 2012, 04:33 PM   #2
20to20 is offline 20to20  United States
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With adapter sockets I've used 6GH8A and 6EA8 in an ST-70 with no discernable difference to 7199.

I've also modded a Magnavox 12ax7/6bq5 amp to use a 7199 VA/cathodyne PI driver circuit on one channel but rewired the socket to use the 6GH8A/6EA8 and can't tell any difference in fidelity compared to the stock other channel.
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Old 16th August 2012, 04:42 PM   #3
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Default rewiring

Hi

thanks for the response

I am happy to rewire the sockets to suit 6U8A/6GH8A (and in fact have done one channel-but thought it wise to make the decision re tubes before doing the other), though given the differences in the tubes (according to my RCA manual), I am wondering if there are any variations in component values etc

Peter
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Old 16th August 2012, 04:57 PM   #4
20to20 is offline 20to20  United States
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Quote:
Originally Posted by pwdiya12 View Post
Hi

thanks for the response

I am happy to rewire the sockets to suit 6U8A/6GH8A (and in fact have done one channel-but thought it wise to make the decision re tubes before doing the other), though given the differences in the tubes (according to my RCA manual), I am wondering if there are any variations in component values etc

Peter
When the Dynaco ST-70 Series II came out with the 6GH8A there were no changes to the driver circuit from the original. Could there be some tweeks to the operating point between the 6U8 and the 6GH8? Probably small. I can't speak to the 6U8 as a long term replacement because I've read some rather negative posts about that tube's robustness and decided the 6EA8 was better to roll in the 6GH8A socket for kicks.

HTH
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Old 17th August 2012, 02:54 PM   #5
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If you really are stripping the unit, I don't see why you wouldn't separate the input tube from the driver/phase splitter. In other words, replace the first tube with a 12AT7 and the second with a 6CG7. The 12AT7 will have less gain than the pentode front end but you can also reduce the GNFB to compensate. This will require some re-engineering, but I think the improvement in performance would be worth it.
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Old 17th August 2012, 09:06 PM   #6
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Measure the cathode voltage on the triode phase inverter and adjust the pentode screen resistor value if necessary to get the voltage to about 1/4 of the supply voltage. This applies to the7199 as well... higher than that could cause rapid tube failure from heater-to-cathode breakdown, usually showing up as serious hum. And this operating point should be near the lowest distortion as well...

I replaced 7199s with 6GH8s in one amp with voltages coming out exactly per the schematic without changing any resistors... or maybe I should say AFTER changing all the off-value resistors.... you should be checking 'em...
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Old 18th August 2012, 01:58 AM   #7
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Default 7199 vs 6GH8A

thanks for the replies....

I now have the added conundrum of having just bought a stock of NOS tubes (2 full TV repairman service boxes full) have spotted that I now have 5 GE 7199 which appear to be new....

sooooo do I go with the original 7199's or move the amp to the 6GH8A (which I also now have a supply of RCA and Sylvania

any thoughts?

Peter
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