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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?
 
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.

All good fortune,
Chris
 
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?
 
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...
 
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.
 

Attachments

  • Full Tweaked RCA Phono Stage.jpg
    Full Tweaked RCA Phono Stage.jpg
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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.
 

kevinkr

Administrator
Paid Member
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.. :D
 
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.


It's quite well regarded, (output transformer I mean) and IMLE at least as good as the ST-35 iron as smbrown mentioned.
 
The B+ has a rather minimalist filtering topology for the PA stage. Looks like Heathkit uses a single 60uF cap on V11 (5AR4) output, then puts B+ straight to the OPT in the PA.

Earlier stages get filtered by the string of RC networks, so presumably by the time the B+ gets down to the input tubes it's very well filtered. But it looks a little sporty with just a single modest-valued cap as the only B+ filter to the output stage.

I know Midnight wasn't reporting any hum issues, and Heath likely sold a bunch of the SA-2s to happy users, just wondering if the output is as hum-free as we expect these days?

Of course, I also live by the adage, "If it ain't broke, don't fix it" :D