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.

Agree with both above.  Looking back at how you called out the stages, you have to look at V2 as two stages, not one.

It appears you have sufficient drive at the power tubes and reasonable output power.  New power tubes properly biased will probably get you a bit more power out which should be in the ballpark of normal for this model.

One thing to note is that power tube grids can't be driven beyond the bias voltage, they will just clip the signal.  So for example if your bias is -30V, anything beyond 60Vp-p at the power tube grids will get clipped, even if the PI is capable of more output level (like you saw with power tubes removed).

  The cathode follower is not a gain stage, it is for driving lower impedances like the tone stack so it is fine.
Which side of the 22nF caps was the pot on?  At the PI plates or the power tube grids?
  It's acting as a master, but will only go to zero if the PI is perfectly balanced.  As that is not likely, you will still get some sound with it turned down all the way.
What did they use to replace the presence pot, just a 5K resistor with no cap?  That would be like having the presence turned all the way down all the time.
  Check with the owner if he uses that "master", if he does, I would recommend moving it somewhere and putting the presence circuit back to stock.

  The difference in bias of the power tubes could just be variation in the tubes, or the old ones could be faulty.  Once you change the bias circuit to the proper resistors for EL34 you can get a better idea of how much difference there is (how much difference in grid voltage to get the same cathode current).
  Yes, injecting your signal at the treble pot wiper bypasses V1&2.
So what is happening when you hit 60Vp-p?  The waveform is clipping?  Do you still have lots of room to increase the output of the generator?
Try it with the power tubes removed and see if you get more than 60p-p.

P.S. You said you were only getting 40Vp-p, but now you are getting 60Vp-p, which is double the bias voltage if it is at -30V.  Is this the case?
And when the PI is putting out 60Vp-p, what voltage do you get at the load?
Also, V2 pin3 appears to be a measurement error, recheck.

  The bias range will be greatly affected by the resistor Dr.Gonz mentioned, at the cathode end of the bias diode.  It and the 2 resistors at the power tube grids should be changed for the type of output tubes:  150K in all 3 positions for KT88, 220K for EL34.
  As far as the low volume, a good place to start is DC voltages for pins 1,3,6,&8 of V1,V2, and V3.

 So the orange wire is a lower voltage tap on the HV winding, you can ignore it.  I suppose if you wanted a lower B+ voltage you could use the orange instead of the red.
  Also, it seems the heater winding is grounded somehow, so no worry about that either.
  As far as the bias winding (brown), further reading suggests it is a little low for the range of bias needed, so this is why some people mod the units with the alternate bias circuit like you have.  So you can ignore the browns and stick with the bias circuit you have.
  Did you reverse the black and white at the power switch yet?
If so, I think you can probably start normal troubleshooting of the sound problems.
A complete listing of DC voltages at all the power supply points and at the tubes would be a good place to start.

  Any chance you have an x10 scope probe?
The PI plate signals (peak to peak on scope) should be about double the bias voltage to get full power.