Ampeg SS-70CA schematics

[Robert M. Martinelli]

Hi,
while I know Ampeg has a reputation for their tube amps, they ventured into building some SS amplifiers as well; a friend of mine owns an SS70-CA ( 70W 2X12 combo with reverb and chorus ) and we need the schematics to try to fix it; does anyone have an idea of where to get' em? I've been searching the whole internet and found very little about this amp, let alone the schematics....
Thanks in advance
Best regards
Bob

[bh12ax7]

The SS-70 is a very under rated amp! Was my main gigging amp for several years in the 80's and early 90's. I did the SS-140c too. Same pre, but with two 70w power amps. Cool, but didn't quite have the meat of the SS-70.

I too, would like a schem...

[Enzo]

Scan of a reduced scan, sorry. Is this of useful quality?

[Robert M. Martinelli]

Hi,
time ago I managed to obtain the SS-70CA schematics directly from St. Louis Music, they've been very kind and supportive. I just sent them an e-mail, asking their permission to post the schematics here. As soon as they' ll grant me their authorization, I'll gladly post the schematics in this section.
Cheers
Bob

[Robert M. Martinelli]

Hi,
well, I just got an answer from SLM tech support, they denied me the possibility to redistribute the schematic by posting it here, anyway, kind and cooperative as always, they allowed me to post a link to their e-mail address together with the instructions to get the schematics :

Please submit all requests to:

techmail@loudtechinc.com, enter “SCHEMATIC REQUEST” into the subject field and the model and serial number of the unit in question in the main email field.

You will be sent a short disclaimer that you must agree to, and then we will email the schematic (or access to) in the order received.

Hope this helps

Best regards

Bob

[bh12ax7]

Awesome! Thank you Bob!

[bh12ax7]

Thanks Enzo! Perfect!

[lowell]

They have a ton of schematics on their site now.. all downloadable.

I'm working' on the ss-70C now. No chorus.

https://supportloudtech.netx.net/lou...c/#/asset/7006

[lowell]

Anyone able to give an overall synopsis on the theory of operation regarding the chorus circuitry? There's the 3 ic's. 555 timer, 4013 flip flop, and 3007 BBD. Ive checked out each IC but curious how they all work together to create chorus. I also am guessing that a couple opamps are at play here as well in some sort of phase operation. Isn't chorus an auto-phasing effect? Or is the BBD simply adding a delayed copy of the signal?

[lowell]

Had one of my epiphanies last night in the finite space between wake and sleep. This amp is stereo, and the chorus is feeding both power amps, chorus is the sum of two different signals... so if I'm using 1 speaker/output on my bench for testing... it's not stereo. Maybe the chorus only works when the stereo signal is happening. Sure 'nuff, chorus works with both speakers/outputs.

So it works. But, the rate doesn't go as fast as I think it should on max speed. Any suggestions on how to make the Rate faster when it's up? Also, what purpose does the trim pot to the top left of the 3 chorus ICs serve? It looks as if to adjust the voltage to pin3 of IC7A.

Finally, the Right power amp output sounds fuzzy/distorted, like it has crossover distortion. I'll mess with the AP3 bias trim pot and see if it's a bias issue.

[lowell]

Ok dialed in the trim pot for the Chorus. It seems to be a calibration of sorts. It would only work in a specific position.

Right power amp was biased cold. Adjusted AP3 and it's sounded great. This amp is AWESOME!!

[Enzo]

Look at the switching. The preamp has two outputs L and M. each to a power amp. The preamp channel output is F, which feeds directly to the L output drive. When Q10 is on, it also branches over to the M output drive. That is the dry (chorus off) condition.


F also feeds into the chorus circuit, the chorused output of which leaves IC7b and trails down to Q11. Q11 also feeds the M output drive. So when chorus is on, Q11 is on and Q10 is off, and so chorus comes out M. Note output L is ALWAYS dry. M is either dry or chorus depending on your selection.

SO if you listen only through the left speaker, it is always dry - no effect. That is normal. If you listen only to the right speaker, you get either dry or chorus. But the chorus you hear does not include the dry. It is the play of the chorus against the dry out the two speakers that gives a solid chorus sound. If you listen only to the right speaker, the chorus effect is lessened because the dry is not included, but chorus ONLY goes to the right speaker.

The basic chorus is a bucket brigade IC, and it makes a variable delay. The delay is very shot though, enough to make a phase differential with the dry signal. Nor delay as in echo. The 555 is a timer IC, that is used as the clock for this circuit The faster and slower it goes means the faster and slower the signal runs through the BBD (bucket brigade device), the net effect being the phase changes. The555 sspits out a stream of pulses, but the 3007 BBD needs a binary digital clock, not a stream of pulses. So the 4013 is a CMOS logic IC that converts a serial pulse train into a digital count, see it is wired direct to the clock pins of the BBD?

Note that the BBD and logic ICs run on single sides supplies, so the couple op amps are also run on single supply. So the signal path through them is centered at a DC voltage half a rail. AP-1 trimmer is to set the signal path there to exactly half the rail so as to get the most headroom for signal.

[lowell]

Fantastic, thanks!

[lowell]

I've got another one of these in. The RIGHT power amp sounds fuzzy. Like cold biased transistors. I tested some voltages and the left Power amp has a 1 volt base to emitter bias. So I checked the right Power amp and it had 0 volts base to emitter. So I turned the trim pot and got it dialed in to 1 volt on the right Power amp but it's still got a slight fuzziness underneath the note. Less pronounced than before I adjusted the trim pot but it's still there.. I've tried another speaker and it's not the speaker it's definitely related to that power amp. Anyone got any tips on where to look first?

Here's the schematic https://ampeg.com/support/files/Sche...1Schematic.pdf

[g1]

Instead of B to E voltages, check the DC mV across the emitter resistors of the Tip141s & 146s (.47R5W). Compare between the left and right amps.