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Old 28th February 2012, 02:58 AM   #1
dgta is offline dgta  United States
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Default Heathkit A7E phono section

Has anyone measured or listened to the phono section of the A7E? It's a 12SL7 with the filter between the 2 sections. Pretty basic. Does it measure well? Does it sound decent? I've never owned one.

Just curious as to the state of affairs at that period in time.
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Old 28th February 2012, 01:34 PM   #2
djn is offline djn  United States
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I'll be watching this thread. I have two A7s on the shelve. They are both the last version sold with the phone AND line inputs. Does yours have the 2 inputs?
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Old 29th February 2012, 01:37 AM   #3
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Quote:
Originally Posted by dgta View Post
Has anyone measured or listened to the phono section of the A7E? It's a 12SL7 with the filter between the 2 sections. Pretty basic. Does it measure well? Does it sound decent? I've never owned one.

Just curious as to the state of affairs at that period in time.

Carefully compare the A7E schematic and the "classic" circuit out of the RCA receiving tube manual and you will see they are nearly identical. The 'SL7 and the 'X7 (7025) are closely related.

The big weaknesses of the RCA setup are POOR load driving capability (which the A7E finesses by having the phono section on the same chassis as the volume control circuitry and the use of a 1 MOhm potentiometer) and mediocre bass extension. Overall, it's decent and can be tweaked into outright good. AC heating in a phono section is asking for trouble.

The 27 KOhm resistor shown needs changing to the standard 47 KOhms, when mated with a modern MM level cartridge.
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File Type: gif Original RCA Phono Preamp Schematic.gif (55.3 KB, 92 views)
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Old 29th February 2012, 06:42 AM   #4
dgta is offline dgta  United States
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Yes indeed. I might breadboard it. A couple of CCS loads, a cathode follower, some LED bias and I wonder how much worse this would be compared to the fanciest present-day designs. With clean power and dc heaters, I bet there's nothing wrong with this 70-yr old very basic circuit.
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Old 29th February 2012, 08:22 AM   #5
DF96 is offline DF96  England
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They seem to use the same RIAA network between two slightly different stages, because one has cathode degeneration which will raise anode impedance. Therefore at least one of them is wrong, although the error may fall within the tolerance band of typical components of the day.
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Old 29th February 2012, 12:55 PM   #6
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I bet there's nothing wrong with this 70-yr old very basic circuit.

Yes sir, RCA's late 1940s circuitry stands up very well. All modern passive EQ designs descend from it. A tweaked version is attached below. Bass extension is improved by grid leak biasing the 2nd gain block, which lightly loads the EQ network. The DC coupled source follower takes care of load driving. Some MM level carts. and SUTs interact badly with the large CMiller at the circuit's I/P. Other than that, it's a winner. Undoubtedly, there are better designs available, but for SOLID performance in a comparatively easy build, it's a good choice.
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Old 29th February 2012, 02:48 PM   #7
dgta is offline dgta  United States
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Originally Posted by DF96 View Post
They seem to use the same RIAA network between two slightly different stages, because one has cathode degeneration which will raise anode impedance. Therefore at least one of them is wrong...
12SL7 has lower anode resistance than 7025, so with degeneration they will end up a lot closer, maybe even the same.
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Old 29th February 2012, 03:19 PM   #8
DF96 is offline DF96  England
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Yes, you may be right. The anode standoff resistor will help too. I guess this is about the simplest valve phono circuit which will work reasonably well.
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Old 29th February 2012, 09:28 PM   #9
smbrown is offline smbrown  United States
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I have listened to this in the A7E (actually a pair of them). The AC heating is an issue, but amazing it isn't more so! One of the really nice things, in my opinion, is the use of 12SL7's which are very inexpensive and great NOS examples are easy to find. I did find it necessary to add tube dampers, but otherwise, sounds really nice.
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Old 29th February 2012, 10:14 PM   #10
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I have listened to this in the A7E (actually a pair of them). The AC heating is an issue, but amazing it isn't more so! One of the really nice things, in my opinion, is the use of 12SL7's which are very inexpensive and great NOS examples are easy to find. I did find it necessary to add tube dampers, but otherwise, sounds really nice.

You can install a "hole shrinker" and Noval socket and switch to the Sovtek 12AX7LPS, which is a genuine 7025 equivalent. The difference between a 7025 and a routine 'X7 is the presence of a spiral wound, hum bucking, heater. Back in the day, RCA expected AC heating. Between the spiral wound heater and biasing pin 9 off B+ at about 75 V., the hum level might be acceptable. Of course, DC heating is best.

Yet another possible tweak is installing a Loctal socket and 14F7 in place of the 12SL7/Octal socket. The 14F7 is electrically equivalent to the 12SL7, but it's inherently less microphonic. Loctal tubes were intended for use in high vibration, mobile, environments.
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