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Topics - FleshOnGear

Amplifier Discussion / Modding an Ampeg SS70
September 20, 2023, 01:38:56 PM
A couple months ago I bought a solid state Ampeg SS-70. Even when it's stock the amp sounds pretty great. Channel B has a really nice clean that goes into a decent crunch at higher gain settings, and channel A does the metal thing really well. The only problems with the design are that the bass and treble controls on Ch. A are pre-distortion, Ch. A is too noisy, and the pre-distortion tone filtering takes out too much treble leaving a parked wah sounding top end (even with the treble dimed) that sounded particularly dull when rolling down the guitar's volume.

Here's the schematic:

I replaced C57 (47n) with a 4n7 cap, which opened up the top end, and replaced C20 (220n) with a 330n to extend the range of boost a bit lower. Unfortunately the extended treble caused oscillation at higher treble settings, so I wired a 10k resistor from P4's CCW tab to the wiper so that C13 drops a bit of treble at higher treble settings. Now I get oscillation only at the very highest treble and gain settings. I can probably increase C57 a little to mellow the top end a bit, but I don't run the treble very high, anyway.

I replaced IC1 and IC2 (RC4558) with very low noise LT1124 op amps. This reduced hiss significantly, even with the treble boost.

I wired a 68n cap in series with an 820 ohm resistor from the node where R19 meets R20 to ground. This drops the mids and highs relative to the bass. Before that the Ch. A tone was pretty thin, but this beefed up the overall tone a lot. I also increased C15 (560p) to 1000p, to retain a bit more presence on lower mid settings.

I'm really pleased with how this turned out. When I first played the amp I could tell it had potential, that it just needed some changes to the tone filtering before and after the distortion. I wish I could change the tone controls to be after the distortion, but it would require serious rewiring, and it's kinda cool being able to control the way the lows and highs cut through the gain. The post distortion filtering I added pretty much negates the need for post EQ, anyway. With these mods I get a distortion channel that balances well with the clean channel, and the distortion cleans up really nicely from the guitar volume.

Out of my solid state amps, this one has a unique midrange voice that I really like. My Marshall 5215 has a lot of girth to the tone, fat and corpulent; the Peavey Bandit has a smooth, creamy, but punchy midrange; this Ampeg has kind of a stiff, cutting midrange that could probably tear through any band mix. I'm a fan of all of it!
Greetings! This is my first post here, and I have a lot of questions about the possibility of modding my Marshall Mosfet 100 Reverb 5215. I tried searching for threads that touch on the relevant topics, but I didn't see much that answered my questions. I apologize if these topics have been discussed and I missed the info.

Basically, I'm thinking of modding my 5215 to add current feedback, in the hope that it will add some depth and presence to the tone. In its stock form, it sounds a bit dry, and dull on top. Here is the schematic of the power amp. I have experience with vacuum tube electronics, but less with solid state, outside of preamps and pedals.

Going into this, my main concern is that replacing some of the voltage feedback with current feedback will give the amp more gain at the extremes of the speaker's impedance curve, and that this might cause clipping with higher signals. I don't have enough experience with SS power amps to know if clipping is safe within the safe operating area of the MOSFETs. Is this a valid concern?

While researching that, I did some math on the stock power amp. It appears that it only takes 591mV peak signal to drive the power amp to full 100WRMS output into a 4 ohm load. The preamp should be able to put out almost 3x as much peak signal. Is this common? On top of that, it appears the current protection doesn't start to work until the amp is putting out 7.6A of current, which is way past the rated power of the amp into a 4 ohm load. Is this normal? Is the protection only for shorts?

It looks to me, in my limited knowledge, like the amp is already built to melt itself down if the user is just a bit too exuberant. Would using current feedback only make it more likely to be pushed beyond its limits? Can I beef up the protection circuitry to make it safer?

Thanks for reading, and thanks ahead of time for any insights.