Traynor YBA-2B EL-84 Project

[jjasilli]

Rebuild: Traynor YBA-2B EL-84  – My other modding adventures to date have all exceeded my expectations.  This puppy is stubborn.  Though improved it just doesn’t bowl me over, and still has some bugs.  I’d like to blame the EL-84's, but the YGM-3 Guitar Mate sounds fantastic.  But that’s it for now.  On to other projects!

Mods:

1. Power Supply

   a) Carling progressive power switch: ON/Standby/OFF.
   b) Courtesy outlet added.
   c) Bucking Tranny mounted to amp chassis to reduce AC supply voltage
   d) Inductive power resistor added to drop B+ voltage & simulate tube rectifier.
   e) All B+ filter caps replaced/upgraded along with all resistors in the power supply, including plate resistors.
   f) Choke replaces B+ resistor to screen supply filter node.
   g) 470R screen resistors added.
   h) Diode added to “rectify” power supply after the screen filter node.
   i) Stock voltage divider removed at the 3rd filter node, as B+ voltage is now low enough.
   j) Power tube grid leak resistors 100K in F bias mode; 200K for K bias. (Couldn’t decide which I like better, and this solution happened to be easy to implement.) 

2.  Bias Supply – Switchable Fixed / Cathode Bias added

   a)  Dual F bias adjust pots & supporting circuitry added.
   b) SPTT SW added to accommodate the dual bias pots.  (Should have used SP4T SW to add the KOC SE mod, but didn’t think of that.)
   c) K bias circuitry added, switchable.  (In this amp, to my surprise, I hear little difference between F vs. K bias; or the value of the bypass cap!)

3.  PI – NFB

   A switchable, variable NFB loop was added; and the PI modified to accept it.  Basically plagiarized from the Traynor YGM-3 Guitar Mate schematic.  NFB has a noticeable effect.  But I can find no sweet spot or improvement per se, with or without various doses of NFB.

4.  Signal Path

   a) Input resistors changed to 33K
   b) 33K grid stopper added to 1st preamp stage (hoping to tame brittle tone)
   c) Tonestack: Amazingly this bass amp is known to be too bright and lacks mids.  Solution, through trial and error, increase cap sizes and reduce value of slope resistor.  Sounds better but not fabulous; there is now some tendency to blocking distortion with controls dimed.  Will live with it for now.
   d) Power tube grid stoppers were increased form 1K5 to 30K to reduce brittle tone.  Brittleness is significantly reduced but not entirely gone.

5.  Speaker upgraded to vintage 15" EVM with heavy gage cable.

[jjasilli]

Can declare success now. Got to play out loud with the wife away.  12AY7 in the 1st position works, but substantially drops overall volume. Tried a 12AT7 and kept it there (though 12AX7 is still a good choice for Tube 1.)  PI: tried a 12AU7; it works, but went back to 12AX7.  In the process removed a badly microphonic preamp tube; and swapped in a different EL-84 for a problematic power tube.  One power tube still rings a bit at hi vol. -- put 3 rubber dampers on ea power tube. 

NFB settings are now useful to tame things down, though I still don't think it changes tone per se.

The little '70's rock machine that lived inside this amp is now out!

[plexi50]

Now you got me thinking if i should try an 820 ohm in the guitar mates feedback circuit. Looks like you have done a lot of work on your YBA-2B

[jjasilli]

I haven't heard a stock Guitarmate, only the one that was already modded.  It sounded greatr even with the stock Marsland speakwer. I think KOC also recommends mods for this amp in TUT3. 

My stock EL-84 Bassmate sounded sterile.  I also have the 6V6 version which sounds fine stock. I wanted to keep the EL-84 close top stock.  The main ideas were to drop B+ voltage, and re-voice the tonestack for more mids. 

[jjasilli]

Some afterthoughts:

1.  NFB: There was some intermittent oscillation -- reason unknown -- with the NFB adj pot turned down, so I added another fixed series resistor.  (In this case adding a resistor was physically easier than replacing the existing one.)

2.  Added a .22uF bypass cap to the last B+ filter stage.  (Forgot to do this standard mod of mine earlier.)

3.  Added a 500pF bypass cap to the vol control.  This was done by trial and error with different values starting at 100pF.  The addition of this cap seemed to bring out more personality in the tone and power of the amp.  But, lower values were too bright over all vol pot settings.    This amp has a weird tonestack.  For some reason, 500pF was not overbright, yet preserved treble at lower vol pot settings. 

The glassiness of this amp, which I dislike in the living room, was not noticeable in a gig setting with bass & drums, etc.  It sounded real good and had plenty of vol & headroom.

The revised schematic is attached.

[phsyconoodler]

The bassmate is just that: a BASS amp.Change the slope resistor to a more guitar amp-friendly value and they come alive.

[jjasilli]

Change the slope resistor  Done already, per the schematic -- much smaller now -- the stock 220K was paralleled with a 63K = 49K.  Actually, this amp, stock, is known for being too bright!  This of course makes no sense, as it's a bass amp.  But my experience limited to this one particular amp bears that out.  Also doubled or more the size of the caps in the tonestack.  (The 6V6 version does not share these oddities.)

[phsyconoodler]

Quote:"Actually, this amp, stock, is known for being too bright!  This of course makes no sense, as it's a bass amp"

Not really,as it allows a wider range of frequencies through.I have modded a few of those amps now and none were too bright.Mine were the 6V6 versions of course!

[jjasilli]

I'm not sure I want to mod my 6V6 version.  It's one of the few things I haven't bastardized, though I bought it as a good but inexpensive vintage amp, ripe for eventual modding; and as a spare amp as I bastardized my Fender HRD.  Anyway, it seems the 6V6 Bassmate is now worth about $600, but I paid less than $200 for it some years ago.  It would be a shame to render it worthless by improving it. 

Still that unused half of the first 12ax7 is calling out for something.  /Undecided in Brooklyn.

[Cathode_ray]

Had a 6V6 YBA-2. Great bass amp albiet at low volume.  There is an unsued stage of the first 12AX7 in there I thought about using. Perhaps with series/paralell switch. That could realy jump the preamp gain  - make it more guitar oriented.