Gibson GA19rvt another question/resistor???

[sidehatch]

So I have a 62(I think) Gibson Falcon GA19-rvt. It had the most lovely reverb I've ever heard. Its the type of reverb that changes in intensity when you change the volume but I got used to that.

Its been recapped-atoms,NOS tubes-everything. I put a celestion G12H30 in it and its a real amp now! I can gig it now but I noticed that with humbuckers in the neck position it didn't have enough highs.

I put a simple .001 cap over the volume pot and the tone is good now.
It can cut in a band situation now. Nice old school jazz amp and I can max it for a rock gig. I have a 47pf and 100pf silver mica cap I could try instead too.

The problem is that the reverb is now darker than the amps tone. Its there but I dont have the sweet crystalline reverb. Its more like a warm hall instead of a bright hall.

Is there anyway the added bright cap could take away reverb brightness or did I possibly make a mistake when I was inside of it?

I can take pics but its hard to get into and I'd like to learn all I can before I spend the evening working on it.

Thanks!!!

[Firestorm]

Why not put another .001 cap across the 2meg reverb pot?. If it worked for the volume control, it should work the same for the reverb tone.

You did keep the preamp cap value a 10uF right?

[sidehatch]

Why not put another .001 cap across the 2meg reverb pot?. If it worked for the volume control, it should work the same for the reverb tone.

You did keep the preamp cap value a 10uF right?

Yep, I didnt touch anything else .

After I installed the ceramic radio shack .001 I realized I had those silver mica 47pf and 100 pf caps that I like for a bright switch on my fender amps.

Do you thick I'd be better off with one of those as a bright cap over the .001?

Im not sure on my math and dont know which will add the most treble.

I like the idea about adding a cap on the reverb pot. I'll try that one tomorrow.

Thanks!

[Firestorm]

I'd think that 100pF would be plenty to overcome the treble loss in a 2M2 pot. Of course, at full volume the cap would be effectively be out of the circuit. Interesting that the amp got darker after the recap. It may be that the 10uF filter is high (they're rated for +50%, -10%). I assume that before the recap, that filter had probably gone low and the reverb circuit might prefer it that way.

[briane]

Interesting that the amp got darker after the recap.

I did not take it that way myself. What I think happened is the normal tone got a lot brighter, reverb stayed the same, and as a result the reverb seems darker.

That would be expected. The appropriate fix would be a smaller cap across the vol (say 100pf)...that would make the diff less noticable, and the other option (as suggested) is to add a bright cap across the reverb pot. Ideally yo would want the same value cap across the 2 pots, though tonally I am sure a match in brightness will require different values.

A better fix would be to mod the reverb so the bright quality matches the normal tone, though I am not familiar enough with this circuit to know the direction to head.

[sidehatch]

I never thought of that but I bet you are right. that makes sense to me.

I dont have a lot of values to try but Imi thinking about trying the 100 pf cap on the volume and a 47pf on the reverb because they are both silver mica caps and I love the 47pf in my pro reverb.

Those are literally the only high quality PF caps I have left. I do havea couple 120pf caps from the pro reverb when they were bright caps and I have a 100pf ceramic from radio shack.

The .001 cap I have is a cheapie from radio shack.

I guess I need to remove the chassis and just try some things and see what I like best.

I'll keep my findings posted. BTW: this Falcon was the best $140 I ever spent on an amp. For a 12 or so watt amp I like it as much as anything boutique.

Im trying to learn how to bias it since its cathode bias but Im running a 5r4 instead of the 5y3 and need to know the tranny is ok . If it could handle it Id put in a 5ar4. I know you read lug 8 and 3 but I dont know what number it needs to read at. Am i lost on this part or close at all?

Thanks

Shane

Interesting that the amp got darker after the recap.

I did not take it that way myself. What I think happened is the normal tone got a lot brighter, reverb stayed the same, and as a result the reverb seems darker.

That would be expected. The appropriate fix would be a smaller cap across the vol (say 100pf)...that would make the diff less noticable, and the other option (as suggested) is to add a bright cap across the reverb pot. Ideally yo would want the same value cap across the 2 pots, though tonally I am sure a match in brightness will require different values.

A better fix would be to mod the reverb so the bright quality matches the normal tone, though I am not familiar enough with this circuit to know the direction to head.

[sidehatch]

I just changed the .001 cap toa 500pf cilver mica and installed a 100 pf cap across the reverb and the reverb is brighter.

I need to play it for a few days to see what I think.

I just noticed that the amp has a 1 meg ohm resistor across the 1 meg volume pot.

Can anyone chime in on why thats there? On guitars I know it helps with the taper but I took it out and cant really tell any difference.

Thanks

[Firestorm]

I'd definitely lose any added resistors on the pots. The schematic shows volume and reverb pots as both being 2M2. That's a hard value to source now so I'll bet someone replaced the volume pot with the readily available 1M (which works just fine). I don't know about the 1M resistor. Was it from the wiper to the input or the wiper to ground? BTW, where both pots (volume and reverb) replaced? That will affect your bright cap selection.
As to biasing the amp, you shouldn't have to (hence the term self-bias). but the schematic shows the 270R as being only 2 watts, which seems a bit light. 5W would feel more comfortable. I can't see any reason you couldn't try a 5AR4 (or a 5V4). B+ with a 5Y3 was supposed to be 300V. With modern wall voltages, that would be more like 325-330 and a 5AR4 would probably get you to 375 or so. Probably would sound pretty cool.

[sidehatch]

thanks for the help firestorm.

The 1 meg volume pot has been replaced. All other pots look original.

The 1 meg resistor on the volume was from the wiper to ground.

I wonder if they thought they'd get 2 meg out of it that way?

Either way it sounds pretty good now and is gig-able.

The only thing I wish I could change but its probably over my head is the reverb changing level/depth as you move the volume. If you set the amps volume high and set the reverb to taste and then lover the amps volume the reverb gets twice as loud. I have been told thats normal but if I could fix that it would be cool.

Thanks for the help.

[jjman]

I think you could re-wire so that the volume and tone controls comes 1st thus regulating the volume and tone of the signal that goes into the reverb circuit. Then adjust the Reverb control to set the ratio.

Something like this.

[Wayne]

thanks for the help firestorm.

The 1 meg volume pot has been replaced. All other pots look original.

The 1 meg resistor on the volume was from the wiper to ground.

I wonder if they thought they'd get 2 meg out of it that way?

...

It's not a linear pot, by any chance? That sounds like a trick to make a lin pot act more log-like. if that's the case, the solution is obvious!

W

[briane]

The only thing I wish I could change but its probably over my head is the reverb changing level/depth as you move the volume. If you set the amps volume high and set the reverb to taste and then lover the amps volume the reverb gets twice as loud.

I wonder if you could not replace the volume pot wih a dual ganged pot, then use the secondary set of lugs for controling the amount of signal fed into the reverb.

It sounds like the amount of signal fed into the RV does not change with the volume pot.

This would allow the RV to track the volume, though you really still need the RV pot to allow you to set the relative amount of RV. Basically you need to figure out where the RV signal is tapped off from, cut that wire and go the secondary set of lugs on he dual ganged pot. In theory that should work, though some tweaking will likely be in order.

[sidehatch]

All these ideas sound excellent but unfortunately I dont think Im far enough along to understand how to implement any of them.

Just in the past month of so I come a long way but I still cant read a schematic like jjman posted.

Can anyone point me to a book or page I could print out on learning how to read a schematic like that?

[Structo]

Well I think you just need a book on basic electronics for the symbols, or you can find all that on the internet.

For instance I googled electronic symbols then chose the image tab, and a whole bunch of images of symbols popped up.

Be aware there has been a gradual shift over the years as to how some symbols are drawn but they are basically unchanged for the last 100 years.

You could print this out.

Potentiometers or pots, are made up of a band of carbon or other resistive element that has a terminal at each end. The wiper, which is the middle lug, rotates on that band or strip thus changing the resistance between both outside lugs and the wiper.
A pot where only two terminals are used ( the wiper (middle lug)) and an outside lug is called a variable resistor or rheostat (archaic).

http://en.wikipedia.org/wiki/Potentiometer

Also to familiarize yourself with the electronic terminology a book or other source would be invaluable or you could just read a lot of threads here to pick up on it.
There is a bit of slang that is used in amps that is sometimes confusing to the beginner.

Caps- capacitors, electrolytic or non polarized.

Grid stoppers - resistor(s) in series with a tube grid

Swamper or swamp resistors- usually a resistor in series with a power tube grid.

Coupling or decoupling caps- caps used to separate stages in the amp or to keep DC voltage out of the signal path.

Rectifier - A rectifier is an electrical device that converts alternating current (AC) to direct current (DC).
Can be a tube or solid state diode.

Filter Caps- capacitors used to smooth ripple voltage from a rectified voltage source.

Ripple Voltage - AC ripple voltage where the voltage is not completely smoothed after being rectified.

Signal caps or tone caps- caps used to shape a frequency response.

Choke - a single coil inductor that is also used to filter DC voltage and reduce ripple voltage.

[img:557:820]http://www.hobbyprojects.com/general_th ... assive.gif[/img]

[img:524:806]http://www.everythingradio.com/images/symbols1.gif[/img]

The book Guitar Amplifier Handbook by Dave Hunter is a good book for a beginner.
He breaks down the various stages of an amp and explains components used pretty good.
http://www.amazon.com/Guitar-Amplifier- ... 087930863X

[sidehatch]

Thanks for taking the time to write all this for me Structo!

I didnt see it til today but Its printed out.

Thanks to this forum and you guys I think I'll be able to service my own amps in years to come. Just tonight I decided I'm done on the pro reverb and have it biased where I like it.

Thanks again!

Shane