Question about Yamaha ra100 ss amp

[sergiomr706]

Hallo, I was looking for the fxs amps Pete Cosey was using with Muddy Waters and it seems that was using a bosstone, wahs probably a ton more, but his favored amp was a yamaha ra200,  I'd like to build the preamp from this amp, the ra100, as I havent been able to find the 200. Being solid state, seems straightforward, but I wanted to be sure first, if I send the signal from the Volume pin 2 to another solidstate power amp would this work as supposed to? I dont know what is the function of tr3 and tr4, these two trannies are out of the preamp? what are they doing? Thanks in advance.

[PRR]

> signal from the Volume pin 2 to another solidstate power amp would this work

Not well, if you need to go to a Power Amp. It gives preamp gain but also tone-stack loss. The full-up signal on the volume pot is about the same as the guitar. You want Q3, and really Q4 too to get a "power amp size" drive signal. You can skip the Reverb send and return.



This still may not be enough gain for "easy" playing with some power amps. I think the Yamaha power module (the octal socket) had higher gain than most box-amps.

[sergiomr706]

Thanks a lot prr, first i thought tone recovery, but having two transistors instead of one, like in a big muff, made my head explode. I ll look for the breadboard, not that im well with this, but i find circuit a little big to go perf and finding then , that it not works the way i was thinking. Thank you

[bool]

You can always rat-nest it first, and perf it later - when you're sure it will work.

[teemuk]

Yes, TR3 is a recovery gain stage that amplifies signal after its being attenuated by the passive tonestack. The stage has moderately high output impedance so its "buffered" by an emitter follower stage TR4, which has around unity voltage gain.

Gain of the first stage is approximately 12K/1K + 1, which is around 13x (ca. 22 dB). Gain of the recovery stage is around 22x (ca. 27 dB). You can punch the tonestack circuit to something like TSC or spice to plot the response and insertion losses. I have a hunch they are around - 15 - 20 dB and you have some more dB's of notch at mid-range frequencies.

I don't personally see the point to build it as is. The first stage likely will never overdrive under normal use, and you have NFB wrapped around it so it retains linearity up to point of clamping into rails, hardly. You could just substitute the stage with a plain non-inverting opamp and likely hear no difference. I can understand why they didn't do it in 1975 when opamps were not as commonplace and high fidelity as they are today.

The second stage is practically recovering attenuation introduced by the tonestack (which is passive "Vox-y" -type), and since signal levels are still in order of millivolts you likely won't overdrive this stage either. Nor will you overdrive the buffer, which has unity voltage gain. You can - again - substitute the voltage gain stage with a generic inverting opamp stage maintaining the proper gain ratio and you likely detect no difference whatsoever since you are not running to clipping. In addition, the opamp will have moderately lowish output Z so you can get rid of the buffer stage TR4 completely. (It's a "transparent" stage anyway).

just maintain proper input impedance so RC filters of coupling will work as supposed to.

Now you likely found out 80% of the components have no other use besides biasing those transistor stages so opamps will simplify things a lot. Now, you may wonder about clipping...? If it clips, it will be "hard" clipping, and looks to me the stages are both "center biased" so you have same maximum voltage swing to positive and negative directions, which means clipping will be fairly symmetric, just like with generic opamps. I really think you don't need to build it "as is" because its not 1975 anymore and there are other device choises in addition to discrete transistors.

In the end, tone of that amp is probably largely due to response of that tonestack. NOT TO MENTION, RA100 and RA200 are powered rotary speaker cabinets, similar to famous "Leslie" cabs, so the real magic of these amps is in the acoustic "dobbler" effect, which you naturally can't replicate by just copying the plain preamp section!

[PRR]

> I don't personally see the point to build it as is.

+1

> tone of that amp is probably largely due to response of that tonestack.

Which is in fact a Fender stack, impedance dropped about 10X, and drawn funny.



Note "slope resistor" dropped from 100K to 10K. Bass pot is 25K instead of 250K-500K. Mid resistor is 680 instead of 6.8K-15K. Volume is 50K rather than 500K. The Treb leg is even lower impedance. IMHO, they are trying to make-up the fact they can take gain only about 8 before the tonestack (to avoid clipping under 24V rail) rather than gain of 50 in a Fender tube amp (under 300V rail). Lower gain is lower signal level so relatively more hiss. Lower tone-network impedance is lower hiss.

Any "magic" here is, I agree, in the speaker or in the fingers.