gibson ga-20 coupling caps

[john_t]

I,am pretty sure the coupling caps are leaking on my (new) 52 ga-20. 4 inputs 1 mic - 3 The amp is plenty loud. Even has a bit of head room. When you push much past 6 o clock however  on the volume you actually lose sound. Does that fit leaking caps

[stratele52]

Defective coupling cap make noise , DC voltage pass trough. This is not your case as I read.

Your problem could be a defective tube(s) , bias of output tube or .......

[Fresh_Start]

The easy way to test for leaky coupling caps is to check for DC voltage where there shouldn't be any.  In some places in the circuit, you may need to lift the end of the cap away from DC in order to test.

Chip

[john_t]

Tried new tubes.  6sl7 and both 6sj7. Both channels sound pretty good good up to about 9 oclock. as you push pass it starts to over drive. Still sounds good.  by 12 on the pot the sound starts to brake up in a bad way. As  I continue on, I actuality start to lose volume. I guess I could live with as is.  Also, on both channels the opposite volume control is acting as a presence control for the opposite channel.  I thought this was only the case when using the mic channel. I try to check filter caps next.

[stratele52]

Look to work "normaly" for me .

[john_t]

Stratele, are you familiar with the ga-20. perhaps that is normal. I am not sure what the normal clean range is. It is fairly loud between 3-4 oclock as hands on a clock go.

[Willabe]

by 12 on the pot the sound starts to brake up in a bad way. As I continue on, I actuality start to lose volume.

I don't think that's right, your amp shouldn't loss volume.

Also, on both channels the opposite volume control is acting as a presence control for the opposite channel.  I thought this was only the case when using the mic channel.

The volume controls on a GA-20 are wired the just like on a 5E3 and are interactive. Their working correctly.


                      Brad     

[Twangmaster]

The preamp in the GA20 is gridleak biased and very interactive. I have had 2 and currently have 1 GA20 and both have had a 6SC7 in the inverter position factory stock. Hope this helps!

[john_t]

Twangmaster Thanks. My amp a 1952 base on speaker code  and  6sj7 preamp tubes. I have read some came with 6sc7 tubes. I am assuming mine is pinned for the 6sj7.  Does you amp lose volume when  the pot is turned past the half way point. The amp sounds good up to that point. Just not sure that should happen.

[mageerc]

Have the filter caps been replaced in that GA-20?  I recently worked on one and it's not really a clean amp unless you use the instrument inputs at a fairly low volume.  The microphone input is a lot hotter...  I think the lack of clean headroom is due to the paraphase phase inverter circuit used in that amp.

[john_t]

The caps in the power supply are new. The filter / by pass caps appear original. There are not that many caps to replace really. Would you just replace all the caps as the amp does work. I,am just not sure it is working as it should. My fear is changing all the caps will adversely affect the tone. Also how does that affect value. It would basicly be a new amplifier at that point.

[Twangmaster]

john_t,my GA20 just gets  thicker and more distorted past halfway. It once sounded bad like you describe when I tried a 6SL7 in the inverter position.

[Willabe]

Be careful in swapping the octal preamp/PI tubes in that amp.

I'm not sure that they all have the same pinouts.

Check a tube manual first to be shure.


                 Brad     

[john_t]

twangmaster, thanks for the help. I  ordered a ESR meter to check the caps in circuit. I am going to leave things alone meanwhile. The amp sounds good as you describe to about halfway. So maybe thats the norm. A little dark but nice tight distortion. It runs clean around 1/4. I was a little concerned about wrecking something else on the amp due to a bad or leaky cap. (tranny).  

Thanks John

[john_t]

Willabe, I will check that Phase inverter 6sl7. That is the tube the amp came with. I am assuming it is correct.

Thanks

[HotBluePlates]

... As  I continue on, I actuality start to lose volume. ...  Also, on both channels the opposite volume control is acting as a presence control for the opposite channel.

Willabe already explained this. There's an 85% chance your amp has nothing "wrong" with it.

The Gibson GA-20 volume controls are wired like a Fender 5E3 Deluxe. They are backwards from "proper" volume control wiring, and work by loading down the signal.

But if you do not have the unused channel's volume all the way off, as you turn up the volume for the channel you are using, your sginal partially bleeds back through the other volume control. This tends to change the tone of the channel you're using.

I have had 2 and currently have 1 GA20 and both have had a 6SC7 in the inverter position factory stock.

This very well could be stock. But as shown in the schematic I linked, at least one version had a 6SL7 phase inverter. But Gibson is notorious for changes without updated schematics, and for amps with the same model number and wildly different circuits.

Look at the data sheets for the 6SC7 and 6SL7. They are largely the same tube, but the 6SC7 has a shared cathode and different pinout.

The quickest deciding factor is what pin 1 is connected to. If no connection, or it's connected to ground, it's a 6SC7. If pin 1 is being used for a grid connection for the phase inverter (compare to what else connects to a phase inverter grid on the schematic), then it's a 6SL7.

My guess is if the amp works at all, you probably have the right tube in the socket.

[john_t]

HBP Thanks, Some times it takes a bit to sink in. That made sense to me. Thanks everyone for the help. Gona enjoy my amp the pressure if off.