Low gain in Heathkit SA-2 phonostage - diyAudio
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Old 13th March 2013, 10:15 PM   #1
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Default Low gain in Heathkit SA-2 phonostage

I am fixing up an old Heathkit SA-2 for a friend, and I have got the thing working quite well. The problem I can't seem to sort out is that the phono (mag phono) input does not have sufficient gain. When playing any other source material, you get lots of volume. With Vinyl and a moving magnet cartridge the volume is very low. So. I replaced almost all the components in the phono-stage with new ones. 20% coupling caps got replaced with 5%ers and all the 10% resistors got chucked in favor of new 5% resistors. I replaced every part in the signal path up to the second screen, and still no luck. (Many of the resistors had drifted 20%) I also cleaned the selector switch. I think my next plan is to measure the AC voltage out of my phonostage, (at the selector) then measure the Ac voltage from the Tape deck, and make sure that I am seeing the same AC voltage from the output (plate) of the phonostage as I am getting from the tape deck (without amplification).

here is a schematic.The Free Information Society - Heathkit SA2 Electronic Circuit Schematic

If i find that the Ac voltage from my first triode is insuficient, what is the best way to bring it up? I have already tried 3 sets of known good tubes so I know it is not bad tubes. Any guesses?
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Old 13th March 2013, 10:27 PM   #2
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I hate to say it, but the circuit is whacky by modern (post-1960's) standards. One triode stage just isn't enough to do phono preamplification and equalization from a magnetic cartridge. Probably your best bet is to make conventional phono equalizers from the two 12AX7's. Should still be plenty of gain for line level sources.

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Old 13th March 2013, 10:30 PM   #3
smbrown is offline smbrown  United States
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Did you notice that all the other inputs are significantly padded down by a voltage divider at their input? Are those parts still in place? If someone took them out, and routed the input of those sources directly to the selector, they will seem signficantly louder than the pnono input. Just a thought. Otherwise, maybe post some voltage measurments?
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Old 13th March 2013, 10:57 PM   #4
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I did notice, all the inputs are padded by a resistor, I replaced the ones on the phono-stage. I am thinking Chris is correct and one half of a 12AX7 is just not enough to bring the signal up to line level. I think I am going to recommend a stand alone tube phono-stage, unless someone has a brilliant way to correct this that I am missing... I am going to (after consulting with my friend) probably wire it for 4 channels of line level input, and build him a separate phono-stage. The crystal phono input is useless anyway so they all may as well be made into line level inputs...
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Old 13th March 2013, 11:08 PM   #5
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As the previous posters indicated, the SA-2 schematic provides us with a lot of info.

The gain structure IS out of whack. From a S/N perspective, it makes no sense to pad line level sources down and then amplify the resulting reduced signal. Bring the line level sources directly to the source selector and have the source selector directly drive the volume control. V1b and V2b should serve only in the 2nd gain blocks of the RIAA preamp section.

Rework the phono section to use a known good performer. The tweaked RCA setup I've uploaded is a strong contender for the job. Even with decent circuitry in place, it is common for phono to require a different volume control setting than the line level sources require. The overall gain of the tweaked RCA setup can be increased by CCS loading the 2nd gain block.

Notice that the phono preamp section employs AC heating. The only tube to use there, unless a switch to DC heating is made, is the Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater.
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File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 85 views)
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Old 13th March 2013, 11:10 PM   #6
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One approach would be to remove the padding on the line level inputs, take V1A and V1B out of the signal path, and run the switched line level signal directly to the top of the volume control. This would free up two triode sections, enough for conventional phono equalizers, with two triode stages per channel between mag phono input and line level switching.

All good fortune,
Chris

edit: looks like Eli's got it covered.
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Old 14th March 2013, 01:34 AM   #7
kevinkr is offline kevinkr  United States
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Actually the phono stage front end is switched directly into another gain stage which is also switched to line sources, because of the high gain it is not an optimum arrangement for line stage use and hence additional padding is required.

I've heard properly restored SA-2 and they are surprisingly decent sounding enough so if you like vintage sound I hesitate to suggest you gut it and start over.. It should have more than sufficient gain for a typical MM if everything is working properly.

It's a bit of history in addition..

And no I wouldn't use one of these for serious listening....

I scratch build so what do I know..
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Old 14th March 2013, 02:30 AM   #8
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Kevin,

The schematic does not tell us how good the O/P "iron" is. Does the SA-2 have decent O/P trafos? A 5AR4 certainly has the "stones" to support all the signal tubes. While a "reference" piece is "not in the cards", the unit should be tweakable into the highly enjoyable category, provided the magnetics are decent.
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Old 14th March 2013, 03:56 PM   #9
smbrown is offline smbrown  United States
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This uses the 51-29 output transformers which are also in the AA-151 (same circuit essentially). I'd put them on par with the transformers from the Dynaco ST-35. I've restored a few AA-151's and once brough up to speed, as Kevin points out, surprisingly good sounding.
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Old 14th March 2013, 03:59 PM   #10
smbrown is offline smbrown  United States
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Transformer test results shown here on the first page, wave forms on the 4th page: Push-Pull Transformer Test
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