Couldn't find any free schematics, but available for sale here:
http://www.hansenguitars.com/engine/inspect.asp?Item=422&Filter=Amps

Quote from: Roly on May 02, 2014, 02:20:16 AM
Another odd fault that old Fenders can exhibit when the electros are going down is that the volume goes down to zero at about 1 or 2 on the knob, then comes back up again from there to zero.

Roly, which electrolytics cause this, cathode bypass caps or supply filters?

  Excellent point Dr.Gonz.  I don't know if the ads here are enough to cover Joe's costs to keep the place running.  If he wanted to put on a "donate" button or run a fundraising drive, I'd be more than happy to donate, this site is greatly appreciated!

  Blending is not possible.
Switching is only easy if you turn off both amps.  Otherwise you need a fairly sophisticated switching box that makes absolutely sure the first amp is disconnected before the second is connected.
  Both amps connected to the same load at the same time usually ends in smoke, flames, and tears.

  Well, the only good news is I tried to buy the domain name through the godaddy link and it said the "domain is not available".
Dr.Gonz, thanks for the reminder about the paypal thing.  In case it's a financial issue I sent a donation via paypal to paypal (at symbol) firebottle.com.  Those donations are accompanied by an email to him titled "you've got money" so I think he will have it set up so those don't go into his spam folder  .  Added a note about hoping to see the site back up, so if he was unaware of the issue he should be notified now.

  Yes, I think that is the issue; the schematic shows 100V but he only has 80V.  Like Enzo said, don't worry about it.  In the world of tube guitar amps, that is actually close enough.  The current through that 270K is what sets the voltage there.  It will change a little bit for every individual 12AX7 you try.  If you have some other 12AX7's try them, you may find one that gets you closer to 100V.
You have 232V on one side of the 270K, and 80V on the other.  That is a difference of 152V.  Ohms law says I=V/R, so 152 / 270,000 = .00056, or .56mA  So we know the current through your tube is about 1/2 a milliamp.  But let's solve for their tube in the schematic, 232V on one side of resistor, 100V on the other side.  Difference of 132V.  132 / 270,000 = .00048 or .48mA  Very little difference, both their tube and your tube are running about 1/2 a milliamp and well within the variation we normally see with tubes (+/- 20%).
  And, like Enzo said, that DC voltage does not go to the 6V6, C6 blocks DC but lets AC (signal) through.  If there was DC getting through C6 to the 6V6, it would mean C6 was defective and (leaky).

Quote from: smackoj on April 28, 2014, 09:26:17 PM

Yes, you need polarized electrolyic caps to replace the main filter caps and the bias cap. Be careful to ensure you replace them with correct polarity observed.

And a reminder:  The bias cap has it's positive end going to ground.  This is opposite of all the other electrolytics and is normal and correct.

  They didn't change much till the boost version so I'll attach the standard schematic and layout which should be close.
  The bias filter cap is the white mallory on the little board by the pilot light.  Replace it with something bigger with a higher voltage rating, like 100uf@100V.  All other caps should be replaced with same value, but can be higher voltage rating.
  After you have replaced the main filters, and the bias filters, there are only a few more electrolytics, so you might as well replace them too.
  The blue ones are non-electrolytic.  They are being used as coupling caps.  They block DC voltage from getting where it is not wanted, but they let the AC signal pass through.  They last much longer than electrolytics and generally don't go leaky for any particular reason.  Some brands/types are more failure prone than others, the blue ones used in the old Fender amps are usually not problematic.

  The question is whether you want to replace just the electrolytic filter caps, or all electrolytics.
  As Doc mentioned, the main filters are on the other side of the chassis under the rectangular metal cover known as the "doghouse".
  The blue caps are non-electrolytic so are not prone to the same issues and are usually only replaced when they fail or go leaky.
  But the white mallory's with the + markings are also electrolytic.  The ones in the bias circuit should also be considered "filter caps" and replaced.  They may need to be replaced with higher voltage rated caps as they were sometimes under-rated and modern line voltages have increased.
  Other than that, there are some white mallories as cathode bypass  caps in the preamp, having gone this far you might as well replace them too.

Not quite sure which schematic/layout to post.  Does your amp have a "boost" pull switch on the volume control?

  This is an oddball Yamaha amp, I saw it mentioned on Roly's site, so thought I'd post the manual here, 2 parts.

  The speaker is supposed to be a 15".  From the looks of other pictures, there's not really room for anything bigger in there.  Have you measured it?
http://www.chambonino.com/work/gibson/gib7.html
http://www.vintageguitar.com/16351/gibson-ga-80t-vari-tone/
  Anyway, if you want to put a 12" in there, you will need a different baffle, so keep the original intact with the original speaker.

As far as the speaker goes, replace it with the same size, and be sure to keep the original.  The original speaker can be reconed, and would be required for any serious restoration.
  If you want to use a different size speaker it would be best to build a new baffle.  Save the original baffle and speaker.
  To me it looks like the amp is bone stock.  Although it looks rough, it is probably a great candidate for restoration.
  If you can verify that it works, a buyer will at least know the transformers are good. Even though the speaker is blown, you could probably get a decent price from the right Gibson collector.  Get an idea of the recone price so you will be better informed when haggling over a price for the amp.

  For the cathode bypass cap, you have the right idea, but it works opposite.  The ac voltage (signal) on the cathode resistor creates negative feedback.  By allowing the low frequencies to ground through the cap, there is less negative feedback and more bass.  So the larger the cathode bypass cap, the more gain for low frequencies for that triode stage.
V3A is common to both channels.  The path through R21 can go both ways, it allows Ch.1 signal to the reverb, but also routes Ch.2 dry signal into V3A.

  The 13.6VAC shown on the schematic is what you should get when you use the specified input signal shown at the input jack, with the control settings described in the notes section.  All AC voltages shown are under these same conditions.
  The 58W is the rated output at full power, with what ever signal level required.

Quote from: MTmopars on April 05, 2014, 10:08:39 AM
Every 3dB is doubling the volume. So to double the volume get a 3dB more efficient speaker, double the cone area, or double the power.

No.  It was established that it takes 10db to double the volume in the first post  .
The 3db boost in volume from the methods you mentioned does not double the perceived volume.
  In car audio competitions you measure SPL.  Does a given system with a 200W amp sound twice as loud as the same system with a 100W amp?
Double the power does not equal double the volume, you need 10db increase to sound twice as loud.