Here's what I've done to mine (in addition to 3 prong cord):

Replaced the filter caps with LCRs (no longer in production but still
available.)  They fit in the same holes as the old Mallorys.

Changed volume pots to 1 meg (gives you more usable adjustment at low volumes).

Replaced bias resistor with pot/resistor combination to allow adjustable
bias and repalced the bias electrolytic caps.

Removed .001 cap on bright channel.
Changed cathode cap/resistor on other channel to 820ohm/250uf for a bassy sound.

Sovtek tubes.

I use Celestion G12M-70s in a closed back cab.  Sounds good.

Lee


Well I bought the YSR-1 and YT-12 cabinet.  Paid $200 US for the 
 pair.  Guy threw in a new Celestion G12 speaker as well because there was a slight buzzing in one of the speakers.  Now I have a couple of questions.

The tubes on this amp are mismatched and need replacing.  Which tubes  would be a good fit for this amp?  I understand that the tubes run a little hotter in the Traynors.

What mods would be considered essential for this amp?  Only thing I 
plan on so far is replacing the 2 prong cord with a 3 prong.

There is a tiny bit of crackling on the first notes when the tremolo is
turned 

I thought the bright channel was way too bright at low volumes with the
.001uf  cap.  Removing it makes it just like the other one.  Another
option would be to use a small cap.  Fender uses 120pf cap for their bright switch and Marshalls use 470pf (about 1/2 of a .001uf).  You have to decide what you like best.

My schematics was a 2nd or 3rd generation copy that Yorkville sent me.  It is also fuzzy but readable.  Are there specific values that you can't read?  I'll try to help if I can.

Hi, my ysr-1 used to overheat the power tubes when I'd turn up the 
trem.(even if biased
cold) Turns out the trem pot was bad. The trem wags the bias on these> 
check your power
tubes for glowing plates (mine turned bright red) Hope this 
helps.

A typical values would be .001uf to 270pf.  My YSR-1 used a .001 and it >made that channel too bright IMO.  It would go between the wiper and the non-grounded leg.

Agreed. My YGM-3 and YBA-1 schematics show .001uF (1000pF). Increasing the value lets more highs pass "around" the volume control (at lower volume settings...at max volume the cap is out of the circuit). The classic Bassman circuit uses 100pF. Most blackface Fenders use a 120pF cap which is switched in/out with the bright switch. Another variation is the Deluxe Reverb which uses a hard-wired 47pF on the vibrato channel.

I find the .001uF is much too bright as well...I'll probably settle for
100-120pF after some further experimentation

I have a YSR-1 which may or may not have the same biasing scheme as your YBA-1.  In the bias circuit there are two resistors, one in between the two caps and one with one end to ground.  I'm pretty sure the one that is grounded sets the bias.  In my YSR-1, it was 39K.  I replaced it with a 33K resistor and a horizontal trimmer 25K pot.  That allowed me to dial in the bias I wanted.  I set the bias current at about 35 ma.

If you want to keep the fixed resistor, you could set up the pot, dial in 
the current you want, then replace it with an equivalent fixed resistor value.