Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Misc Schematics and Layouts => Topic started by: Willabe on February 24, 2015, 09:55:59 am

Title: GA-55V
Post by: Willabe on February 24, 2015, 09:55:59 am

(I'll move this in a day or 2 to schematics after it gets seen.)

I've been going over this schemo and there seems to be a couple of flaws in the drawing and with Gibson schemo drawing there might be more.

1, is the B+ string looks like it's missing a 'dot'? (Red circle.)

2, is the wire connecting from the junction of the vibrato channels volume pot wiper and the modulator/mixer hi pass filter output to the junction of the PI's plate and the 'pull' 6L6's coupling cap. I think it's wrong. (Blue circle.)

3, does anyone see anything wrong with the 6SN7 modulator circuit? (That's my only interest in this amp.)

                   Brad    :think1:

Title: Re: GA-55V
Post by: shooter on February 24, 2015, 10:51:02 am

I think they did forget a dot in the red, and the line in blue doesn't look right to me, but, I will have no problem being wrong on that guess!  It doesn't seem right to take a plate into a pot to ground.

Title: Re: GA-55V
Post by: Willabe on February 24, 2015, 10:59:36 am

Quote from: shooter on February 24, 2015, 10:51:02 am

It doesn't seem right to take a plate into a pot to ground.

Well with the pot turned to '0' that's a direct short of dc plate voltage to ground. Got to be wrong.


                 Brad     :laugh:

Title: Re: GA-55V
Post by: eleventeen on February 24, 2015, 01:18:41 pm

I dunno, some people like that smell.  :icon_biggrin:

Title: Re: GA-55V
Post by: Willabe on February 24, 2015, 01:56:17 pm

 :laugh:

Title: Re: GA-55V
Post by: octal on February 24, 2015, 03:13:08 pm

That modulator looks broadly similar to the one in the Gibson GA83s. So you could check against that for the basic topology.

Title: Re: GA-55V
Post by: Willabe on February 24, 2015, 04:44:49 pm

Yes it's similar but not the same.

I'll have to re-draw the 55V schemo so it's less twisted up. It will make it easer to compare.

On 1 hand you'd think they just took the working vibrato circuit from the 55V and just doubled it up for stereo but who knows? And you'd think the 55V was made 1st then the 83S but......      :dontknow:   

Gibson schematic drawings are often wrong.    :BangHead:       :cussing:   


               Brad    :icon_biggrin: 

Title: Re: GA-55V
Post by: kagliostro on February 25, 2015, 10:32:50 am

I think you can obtain more comparing the GA55V schematic with the Vox AC30 than with the Gibson GA83s


Franco

Title: Re: GA-55V
Post by: Willabe on February 25, 2015, 11:00:53 am

Yes, I'll be looking at both.


            Brad    :icon_biggrin:

Title: Re: GA-55V
Post by: Willabe on February 26, 2015, 12:10:52 pm

I'm not sure that Gibson drew it up correctly? So I re-drew the GA-55V modulator/mixer circuit hoping to untangle it to see the circuit a little better, not sure it helped.

Here's 2 versions. The 2nd version I did because I'm thinking that the resistive dividers of 510K/1m should be as close to the 6SN7's grids.

Do you think this circuit will work?

                 Brad    :dontknow:

Edit; I added a re-drawn Wurlitzer circuit to compare.

   

Title: Re: GA-55V
Post by: Willabe on February 27, 2015, 10:03:43 am

I bought the 2nd GA-55V schematic on line and it turned out to be the same schematic.   :BangHead:

The seller ($5) had a tube incorrectly listed as part of the tube line up, which is why I thought there was a 2nd version that Gibson made.

He did refund my $.   :icon_biggrin:


               Brad    :dontknow: