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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Wed Dec 07, 2016 9:26 am
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Joined: Thu Aug 07, 2014 3:25 pm
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Good tip. Most of them will be coming out during the preamp rebuild, so I'll give them a close eye.
There will be a few left over, so I can select the best ones.


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Fender Play Winter Sale 2020
Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Wed Dec 14, 2016 10:42 am
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Joined: Thu Aug 07, 2014 3:25 pm
Posts: 1023
UPDATE 12-14
So here are the after mod pics

Image


Image

I chose a 6g6 Bassman input circuit. You see, I had an extra tube section available and I always wanted to see how one performs. This is the model of Bassman head that Mesa stole from Leo and put their name on it. It was the 60s and the hippies thought everything should be free. I wondered what they saw in this circuit.
I had one come through the shop. The only (and significant) change was to install a nickel OT to raise the gain and put their stickers all over it.


The number 1 input jack is stock Fender with a 68K and 1M to ground. Channel 1 has a 22uF/2.7K on the cathode. Channel 2 has a 2.2uF/2.7K for bass rolloff.
The preamp is a direct-coupled 12AX7 with output from the cathode of the second tube section. As expected, it had a ton of gain. So I built a couple voltage dividers for that cathode circuit.
The 100K to ground was split into a 68K/33K on the first channel. I used a 82K//18K for the second channel. This produces a 3:1 and 4:1 gain reduction for each output.
Both are routed to through the number 2 jack switch to their respective volume control wipers.
So the second jack bypasses the first stage altogether and goes directly to the volume control wiper.

The performance is impressive. It is clean up to a certain point then starts clipping the bottom of the wave. There is a lot more headroom before the top starts clipping.
The second input is still strong enough that I can get it to clipping with a standard mic.
I think his harp mic is a lot hotter, so I don’t think he’ll have any problem getting the tones he wants.
The surprising part was the apparent reduction in feedback. That has been a major problem with all of his amps. I’m not sure if it’s a phase issue or just that I finally got the amount of gain reduction that I knew would improve the situation.. I removed the original mod feedback loop to open thepower stage back up. Even so, the amp now has 1/6 of the original gain.


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