SA-300 inputs

Infidel

Infidel

New Member
I've inherited a SA-300, and can't find a manual for it. I can find a SA-100 manual, but it's not anywhere near the same.
The RCA jacks on it are marked IN Filter OUT. I've been listening it using a laptop with RCA plugs going into the IN and am leaving the OUT alone.
The preamp was built in a tiny bud-box. It's all point to point wiring. I can't use it at this point because the RCA inputs are too close together for modern cables. - and I don't understand it. It has two pairs of RCA jacks coming out. I think both were used. It appears to have a rectifier tube, a 6CG7, and what appear to be 2 12AX7's. I've tried to wiggle them free but suspect they may be soldered in place.

I would start building something like a 6SN7 preamp for it, but really don't know what I'm getting into if the inputs include a high / low filter.

So far, bass isn't too tight - or just different that what I'm used to hearing. That, and using a MacBook for a source isn't ideal.sa300.jpg
 
Register to hide this ad
A Mac itself should be a very good source. At the headphone out jack, frequency response is 20 Hz - 20 kHz +0.5 dB / -3 dB, s/n is >90 dB, thd is <0.007%, separation is >85 dB. The only issue would be the source of what is stored on the Mac.
 
Also....that center channel RCA plug between the left and right speaker terminals is supposed to feed a separate mono center channel amp. It's not an "input", it's a mono output.
 
Last edited:
Thanks for the schematic.
I don't have the center channel RCA, don't need it. I'm guessing I have a 1960-1961 SA-300, before the B was added. Serial #1001X
I see the difference between the two inputs in the schematic, but how do you choose which input pair to use? I'm working on a DCB1 buffer right now, but expect I'll need something with a little gain. I'm not really able to reach 100dB peaks using a MacBook. The cable run from phono stage is too long to test.
What is a reasonable amount of gain from a preamp to put in front of it?
Right now trying a JDS Labs Atom as a preamp.
My uncle had chosen to use common instead of ground for speakers.
 
Last edited:
Infidel said:
... It appears to have a rectifier tube, a 6CG7, and what appear to be 2 12AX7's. I've tried to wiggle them free but suspect they may be soldered in place.
...View attachment 2261287
Click to expand...
The tubes used are:

Rectifiers 5AR4 / GZ34 (2 ea.)
Gain: 12AX7 (2 ea.)
Phase Splitter / Driver: 12AU7 (2 ea.)
Output Tubes: 6CA7 / EL34 (4 ea.)​

Those amplifiers are really good sounding when they are in good operating condition. You have a nice treasure there.
 
Last edited:
Here are the preamp stages he built for it.
Yes, it's a pretty sweet amp. It does some things better, some not so much compared to my F4's with 6SL7 in front of them.
He also left behind a bud box which was in front of that preamp - but he admitted it had some hum and was a little bit of a gimmick. It has 4x 6267's, two nickel plate Mullard and two screen plate GE Holland. I found his notes in an old Mullard amp schematic book

Then the mystery preamp with two pairs of outputs, I'll have a pro look at it.

Also brought home a pristine 800c with half Telefunken smooth plate, half GE long grey plate as far as the 12AX7's go, Sylvania 7591 output tubes which would be impossible to replace. Haven't tried starting it, it's been in dry mountain pet / smoke free storage for over 20 years.
Probably around $100K in 1930's - 40's Radio and TV were donated to a museum near Durango.
I have a few bankers boxes of tubes, most not too useful without boxes, but some nearly 100 year old ones carefully wrapped and a few goodies. Need to find someone to take them off my hands.

IMG_9427.jpg IMG_9429.jpg
 
In regards to the SA-300 diagram and filtered vs not filtered inputs:
I'm an amateur...
If I have a preamp which doesn't have coupling capacitors on the output, I'm making a guess that the filtered input would be safest?
Time to listen to some tunes.
 
Rectifiers 5AR4 / GZ34 (2 ea.)
Gain and Phase splitter: 12AX7 (4 ea.)
Output Tubes: 6CA7 / EL34 (4 ea.)
Click to expand...

I don't think that's correct.
I can't speak for the SA300, but each channel of my SA300B uses one 12ax7 (ECC83) and one 12au7 (ECC82).
 
To backup what heyraz has said, the SA-300 only uses 2 12AX7 tubes, and uses 2 12AU7 tubes for the driver position. And importantly: Do not make the mistake of installing a 12AX7 in the driver position! This amplifier employs partially direct coupling between the driver and output stages. Installing a 12AX7 in the driver position would cause the output tubes red plate!

Some more information to help your project:

1. The input sensitivity of the SA-300 is 0.90 vac for each amplifier to reach full power output. Assuming a nominal sensitivity of 200 mV for your preamp's high level inputs, the line/tone stages within it should have a gain of about 5X with the tone controls set at flat for best operation with the SA-300. This would allow the input level controls of your SA-300 to operate wide open, which always delivers the best performance when such controls are provided. Many if not most vintage preamps of the day had a line/tone stage gain of X10, so the SA-300 was designed to be able to work with preamps with this much gain or more due to the inclusion of the level controls, but using a preamp with the proper amount of gain that allows wide open operation of such controls will always deliver the lowest S/N level, especially under low level listening conditions.

2. The filtered and unfiltered inputs were provided because the SA-300 was offered at the height of the (then) new electrostatic speaker panel craze. As such, the amplifier was designed to be quite stable into highly capacitive speaker loads, but also included a filtered input, that lopped off both the lowest end of the audio spectrum as well knocked down some of the high end as well. The filtered input was used when panel speakers were employed because they could not handle high levels of bass frequencies, and really weren't capable of producing the highest part of the audio spectrum. So, the filtered input was offered to make the most of electrostatic speakers. But, the panel craze didn't really catch on, so in the B version of the SA-300, the hi-pass portion of filtered input was dropped (allowing full bass response), but the low pass portion remained, to curtail the high frequency response. Why Fisher chose to alter the filtered input performance in the B versions as they did, who knows. Maybe they had improved the low frequency performance of the panels by then. In any event, if you are using conventional speakers, then you want to use the "Out" input jacks, which provides a straight in connection to the level control, and full response from the amplifier.

The amplifier will always operate best when driven from an active device or preamp that has a low output impedance. That's because even when using the unfiltered (Out) inputs, the output of the internal filter circuitry appears at the unfiltered input. If a passive preamp were used (which displays a relatively high output impedance), then the filter circuitry would impact the unfiltered input signal by loading it down at high frequencies. By using a source with a low output impedance, this prevents any such interaction from happening.

I hope this helps!

Dave
 
right, the "filter" is a band pass filter. 47k into 120pf is the low pass part, -3db point is at 28khz. The .047 into 270k is 12.5 hz.

Thats in addition to the caps and resistors that the un-filtered input works through so the actual response of signal getting to the first tube grid will be different than just those 2 pairs of components would imply. Looks to me like adding the filter into the mix would make the roll-off steeper, not create a filter where there was nothing.

I am a little curious about C14 here though. The Fisher schematic linked is for the 300B, but it shows a small cap from the top of the level pot to the wiper, probably for Miller compensation so it doesn't lose HF with the pot below max. This one shows a cap to ground, which would be adding extra HF roll-off if the level pot is anywhere below max. Wonder if thats a Sams error, or another change in the filtering. The B schematic does lose three 0.047uf caps between the input and the grid vs the Sams above so it may very well be on purpose.
 
heyraz said:
I don't think that's correct.
I can't speak for the SA300, but each channel of my SA300B uses one 12ax7 (ECC83) and one 12au7 (ECC82).
Click to expand...
You are correct. I miss-remembered. I have corrected my earlier post. Thanks for the catch.
 
gadget73 said:
right, the "filter" is a band pass filter. 47k into 120pf is the low pass part, -3db point is at 28khz. The .047 into 270k is 12.5 hz.

Thats in addition to the caps and resistors that the un-filtered input works through so the actual response of signal getting to the first tube grid will be different than just those 2 pairs of components would imply. Looks to me like adding the filter into the mix would make the roll-off steeper, not create a filter where there was nothing.

I am a little curious about C14 here though. The Fisher schematic linked is for the 300B, but it shows a small cap from the top of the level pot to the wiper, probably for Miller compensation so it doesn't lose HF with the pot below max. This one shows a cap to ground, which would be adding extra HF roll-off if the level pot is anywhere below max. Wonder if thats a Sams error, or another change in the filtering. The B schematic does lose three 0.047uf caps between the input and the grid vs the Sams above so it may very well be on purpose.
Click to expand...
I pasted a SAMS schematic (couldn't get a good image from The Fisher Service Schematic). SAMS always seems to mess things up.
 
yeah its confusing, just curious if its accurate and indicates one of the changes in the filters, or if its a mistake. Frankly I could believe either.
 
You must log in or register to reply here.
Back
Top Bottom