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Post subject: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 10:35 am
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So this amp came in with stability problems and I rebuillt the power supply and sent it out.
It came back with the customer complaining, "Its too loud."
He plays harp and cant get it past 1 without clipping.
We plug a guitar into it and sure enough, its way loud at 1 as well.
There,s not much to this old amp. It's a rare version with miniature preamp tubes instead of octal. I can't find a schematic, but using the octal version as a guide, I think it's working properly.
It's built as a lead amp with sensitivity hotter than a HRDX normal channel.
So in order to tone it down, I didn't have much to work with internally, so I decided to try something new. The power stage has no feedback loop. I'm gonna try to make one work.
I connected a 100K to the AC input of the phase inverter and connected that to the speaker lead.
It squealed at first. Wrong phase,, so I switched it to the negative lead and it worked. In this amp, the speaker lead isn't grounded.
It suddenly went from 6 volts AC to 2 volts..
It seems very stable. I don't get 180 degree phase shift until >60KHz.
When the customer tried it out, he was pleased.
But we noticed a strange artifact at high volumes. When he blew a clean note, it's stable.
When he blows a chord, a tremolo effect appears. Its slow at higher notes and faster with lower notes.
I'm guessing it has something to do with power supply sag, but I really don't know.
The customer says he can use it to great creative effect. I just hope it doesn't go up in smoke..
If it does turn out to be a stable mod, I might try it on a few other amps and see how it works out.


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 1:21 pm
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TimsAudio wrote:
When he blows a chord, a tremolo effect appears. Its slow at higher notes and faster with lower notes.
I'm guessing it has something to do with power supply sag, but I really don't know.


It sounds like your mod may have induced some IM distortion into the output stage.

Arjay

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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 1:56 pm
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Intermodulation distortion. Hmmm. Yea.
Now the question is if its safe.
I guess we're gonna find out.


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 2:28 pm
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You might try a 220K resistor in lieu of the 100K that's currently installed. If it passes the smoke test it should be good to go.

Arjay

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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 4:19 pm
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Does the layout look like this schemo? (Gibson GA20 Crest amp):

http://bmamps.com/Schematics/gibson/GA- ... ematic.pdf

I'm guessing you have some coupling between stages. How old are the main PSU caps?


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 6:52 pm
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Well, Retro, I would rather drop the resistance of the FB resistor. I'm looking for a .1volt amp to compensate for his hot dynamics. It had to go out for a show this afternoon but if it comes back in you can be sure I'm gonna try different amounts of NFB.

Beemer, thanks for the schematic. I was gonna post it, buy couldn't find it at the time of posting.
The PS caps are new looking F&T.
The original problem I fixed was the 10K between the OT CT and the screen grids. They had a non-original 100 ohm in there. Not sure why. I changed it back to 10K. I think thats why it got louder.
The coupling caps are all mustard caps.
While the input tubes aren't pentodes and the circuit is changed to accomodate triodes, the input resistors are still a whopping 10Meg.
The grids are grounded as well. It has a ton of gain in that first stage.

I thought about putting another NFB loop in the input stage. Then cut the NFB to 220K as Retro suggested, to still get a .1volt sensitivity. Less compression on each stage seems to be a good idea. But then again, am I going to introduce more IM with another NFB loop?
So I think that if it comes back, I'm just gonna hook the input jack to the volume control and completely bypass the input stage.
I'll probably take off my gain crusher loop on the main stage and we'll see how it works out..


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 7:04 pm
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What are the input tubes? Seems that 6SJ7 pentode needs 5 to 10 meg-ohm grid-to-ground resistors. Fender following RCA Receiver Manual info, used a 5 meg ohm, on their early Champs and Champions.

http://ampwares.com/schematics/champ_5c1.pdf

If Gibson subbed out a 12AX7 duo-triode for those pentodes... perhaps they missed changing the G2G resistors to something closer to 1 meg ohm, for the 12AX7? That may account for the crazy overdrive nature of the front end of the amp.

I also noticed that Fender stopped using that input coupling cap and went to a 68k-ohm input resistor on the 12AX7. So, you may have both G2G resistor issues as well as impedance mismatch problems.

Champ Amp with 12AX7 input tubes.

http://ampwares.com/schematics/champ_5e1.pdf


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Sun Nov 27, 2016 11:07 pm
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BMW2002Ti wrote:
I also noticed that Fender stopped using that input coupling cap and went to a 68k-ohm input resistor on the 12AX7. So, you may have both G2G resistor issues as well as impedance mismatch problems.


Good call, and a distinct possibility.

Arjay

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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Mon Nov 28, 2016 9:54 am
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Wish I'd taken pictures.
The 10 megs are soldered to the grids on the sockets. So are the cathodes.
The socket punchouts are sized for miniature tubes.
I'm starting to think they made one run of these with the modern tubes and decided it was a bad idea. I don't know how much gain difference the 12Ax7 has over the pentode, but its unworkable here.
It has an input grid for each input jack. Each input is identical. Can you imagine four guys plugging into this thing. OMG.


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Mon Nov 28, 2016 10:43 am
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TimsAudio wrote:
Can you imagine four guys plugging into this thing. OMG.


I remember four of us (two guitars, a bass, and a microphone) plugging into a Silvertone Twin Twelve back in the day.

:lol:

Arjay

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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Tue Nov 29, 2016 8:53 am
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TimsAudio wrote:
Wish I'd taken pictures.
The 10 megs are soldered to the grids on the sockets. So are the cathodes.
The socket punchouts are sized for miniature tubes.

I'm starting to think they made one run of these with the modern tubes and decided it was a bad idea. I don't know how much gain difference the 12Ax7 has over the pentode, but its unworkable here.
It has an input grid for each input jack. Each input is identical. Can you imagine four guys plugging into this thing. OMG.


IME, there is a big difference between properly loading a pentode versus high-mu duo-triode. There really is no 9-pin mini tube near-equivalent to the octal 6SJ7. IMHO, you'd need to re-vamp the impedance load on the grid and change that G2G resistor to 1 meg, to get a 12AX7 to work. The cathode bias resistor/bypass prolly also needs re-working.

You still have access to the amp?


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Tue Nov 29, 2016 10:35 am
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Yea, you are convincing me that the "engineers" at Gibson really didn't know what to do with these new tubes.
I can just picture the Boss coming down to engineering and yelling that they're in the new Fenders and they're damn well better be mini tubes in Gisbons.
"Yes, Sir. Right away Sir."
Now I get to clean up the mess.
Thinking back, I did hang 1Meg resistors on each of the input jacks to G. It didn't change much.
I wonder if putting them on the grid side of the caps would lower the gain by changing the DC operating point.
On the other side of the cap it won't do that.
As to wether I'll see the amp? He's gone through a number in his tone quest. He likes his Gibson Bluebird, but it was underpowered. Then he bought a Harpgear or something like that. That had an interesting interstage transformer for the PI. Then he went to a 59 Bassman. It was finally enough power, but too heavy. This amp he really likes the tone and he may settle down with it. But....who knows. I guess its like other relationships. The hindsight of coulda, woulda, but now its gone. I'm used to it now. There's always something new on the horizon to explore..


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Tue Nov 29, 2016 12:05 pm
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Haha. Gibson engineers. I still can't understand why they had such a hard time keeping their guitars in tune. Tone is great. Holding a tune, on some models... legendarily poor. :|

I think adding a 68k-ohm resistor on the 12AX7 signal grid may also help. I'm sure the internal resistance & capacitance (Miller effect) is much different between the 12AX7 and the 6SJ7.

I've seen Blues Harpists used a tiny amp (like a Champ or smaller) --- cranked to 10. Then, mike it to a larger amp or house electronics --- for volume.


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Tue Dec 06, 2016 9:01 pm
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UPDATE Dec 6
Well, that Gibson came back. Its still too loud.
So I get to try out your ideas for converting the front end of this thing.
I also am going to set up the input jacks different.
The high sensitivity jack will route its output through the second jack for a low sensitivity input. I won’t use two of the grids that way.
I’ve seen a number of Marshalls wired this way
Any other suggestions before I start?

Image

Image

Image

Image
That big green resistor is a 100 ohm I replaced with the 10K specified.
I also changed the grounding on that rack of F&T caps. The original was a loose multi-cap with a ground wire to the center of the chassis. It cut the hum down.

Image


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Post subject: Re: Gain Crusher FB for Gibson Crest GA20
Posted: Tue Dec 06, 2016 9:26 pm
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Did you check for DC leakage on those Astron caps? I can't remember, if you did. They are notorious for leaking, if past 10 years old.


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