Traynor YBA-1A MkII - Bringing It Back to Life

[rundgrenrules]

I just acquired a well-used and modded Traynor YBA-1A MkII.  The listing had it described as having the Plexi mods done to it. 

In addition to re-gluing the head cabinet which was separating at a joint, I have re-traced the schematic as best as I could to determine WHAT has been modded.  I would like to bring it back to stock as a starting point and only apply whatever mods to the circuit make sense to me.  I'm not wanting it to be a Marshall, I'm wanting it to be a Traynor (similarities and differences aside).  What I really want is the high headroom potential to be maximized (big Hammond iron is BIG), while keeping the tonality firmly in that middle area between Fender and Marshall that these amps are known to occupy and also to make sure that the previous tech didn't bungle anything up

So popping the chassis out to take a look, I see some very obvious changes:

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-The old "cigar" filter capacitors have been replaced, and the cap cans mounted to the chassis have been left there but disconnected.  In the place of those old
 caps are four newer 220uf caps, along with a:

-33uf cap to ground coming off of the 100k resistors (R8 & R9 on the schematic).  I cant find this on the schematic, although I see it in most gut pics of this
 amp online.
 
-There is an apparently new 3.3uf cap to ground between the 820Ω resistor coming from pin 8 of V2.  Dont know what that is doing.

-There are two 1K resistors coming off of pins 3 on both power tubes and connecting before heading to the filter caps.

-Phase inverter resistors are now 1M/10k/1M instead of the original 47k/6.8k/47k values.  (I think I'll keep these as is)

-.1uf bass cap now .022uf (I think I want to keep it that way).

-47k NFB resistor instead of original 100k. (might keep this too).

-470pf cap feeding pin 3 of the treble pot instead of original .001uf value.  (might want to step it down to 330Ω?)

-1M volume pots instead of 4M or 500k that I see in the two schematics floating around (might want to keep these too).

-270k mix resistors instead of original 100k (not sure what to do here, take 'em up to 470k?).

-10uf/150v caps in place of original 8uf/250v for C13 & C14 (might want to bump these up to 250v)

-V1 pins 3 and 8 are terminated to ground through a .68uf/2.7k and an 820Ω/25uf pair respectively instead of schematic values of 1.5k & 125uf?.  I am
 not sure what to do with these values.

-R31 43k instead of schematic value of 47k.

-Cant tell WHAT the two large high power resistors on the right side of the board are but my best guess is 20k and 1750Ω.

-The 470Ω R38 seems to be missing.

-Death cap gone - GOOD.

-Appears to be a choke added - cant tell if these came stock (yay, nay?)

-Bias adjust circuit added, 25k pot on back panel).

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The main thing that is concerning me right now is the power supply section.  I am not entirely sure how the original values of the cigar and cap cans translate to these new caps but aren't 4x 220uf caps WAY TOO BIG?  I want reasonable power handling, not values that might choke the amp up or be dangerous.

I know there is a ton of info here and a ton I don't know about tonestacks and proper values of components, and any help is greatly appreciated.

I have attached a file of the schematic that I highlighted and typed up the changes I see present.

Any suggestions?  Thanks for reading!

[rundgrenrules]

more pics, red-lined schematic:

[shooter]

4x 220uf caps

electrically = 100uf, twice.
a stiff PS gives you "tighter" voltage, less sag, less "variables" to worry about when making sounds.


the downside is the RC time, which might run into seconds, I don't want to do the math 
fwiw;
I typically use 4 X100uf in most of my builds that get close to 500vdc before the tubes start conducting


I would make it work, then decide if you want a saggier PS

[sluckey]

Fix it as is before you start "improving" it.

Oh yeah, look at this current thread...

     https://el34world.com/Forum/index.php?topic=27460.0

[rundgrenrules]

I kinda want bang down the middle of the road, no more filtering than was present at the original iteration of the amp.  The combination of cap cans and the chassis interior filter caps has me confused on what value those might be for these 4 currently 220uf caps and the other electrolytics (20uf, 33uf in the preamp).  Don't want it really tighter than stock.

What values do you think I should step them down to?

Fix it as is before you start "improving" it.

Oh yeah, look at this current thread...

     https://el34world.com/Forum/index.php?topic=27460.0

Thanks Sluckey, the amp is functional and sounds fine, a bit stiff.  I have not had the chance to really open it up.  My goal is to get it kind of addressed to where I know what the state of all these mods are at the moment, probably adjust the filtering, make sure Im cool with the "plexi" mods, etc

[thetragichero]

I've had the same philosophy with the yba1/yba1a amps I've had: keep em as traynor as possible. phase inverter grid/cathode resistors I'd swap for more "marshall-like" values and I'd add a bias trim pot, but not much else

i did feel like 90w was a lot to expect from a pair of modern el34-types (probably no sweat for the blackburn mullards that came in em) so i lowered the bias feed resistors to 100k and threw in some 6550s in

[sluckey]

It's a simple straight forward amp with easy to work on layout. Should just be a matter of following the schematic to get it back original.

The resistor you indicated as missing on the schematic has been replaced with a choke.

[rundgrenrules]

It's a simple straight forward amp with easy to work on layout. Should just be a matter of following the schematic to get it back original.

The resistor you indicated as missing on the schematic has been replaced with a choke.

Thanks again Sluckey!  I am going through the board and changing the values back to stock except in a few locations where I want to preserve the changes (bright cap, bias adjust, etc).  What would be a more appropriate value for these large filter caps (instead of 220uf) considering the original 40/40 cap cans and the 80uf cigar caps that were there originally?

Also it appears that the bias splitter resistors are 100k, would I benefit from stepping them up to 220k?

I have gone through the schematic some more labelling the changes I have observed, someone correct me if Im wrong in my labelling:

[rundgrenrules]

Am I off base in thinking these 4 220uf caps that replaced the two 40uf/40uf cap cans and three 80uf cigar electrolytics are just way too big?

The amp originally would have had a combined 400uf of filtering and it now has 880uf.  Surely I could knock these 4 caps down to 100uf and be much closer to original spec?

[sluckey]

I would use four 100µF@350v axial caps to replace C15, C17, C18,and C19. Small, compact. I would use original values for C20 and C21. Leave the original cans just for looks.

What are the numbers on the choke?

[rundgrenrules]

The choke is a Mercury MC10H.  10 Henry/250mA.

Im going to order 4 100uf caps, one 80uf, and one 10uf.  I am not sure what the filter cap to ground between the plate resistors in the preamp is for, if I should pull it out or not.  It isn't on the schematic.

Also Im not sure if I should just leave the two wirewounds in place (20k & 1750 ohm).  The schematic calls for two 10k values here.

One of the problems is that I cant find a version of the schematic that exactly matches my revision (schematic on the amp interior has crumbled over the years) so I'm left with a few head scratchers.

Those two (what I think are) diodes in the schematic (RS20SP20 & 6RS20SP7B7) are nowhere to be found....I see they were used as OT protection in case of surges and that they are advised to be clipped out if they fail.  I guess I'll leave em out.

[sluckey]

If you are gonna return back to original specs quit wondering about all these "improvements" and just follow the schematic.

[thetragichero]

Those two (what I think are) diodes in the schematic (RS20SP20 & 6RS20SP7B7) are nowhere to be found....I see they were used as OT protection in case of surges and that they are advised to be clipped out if they fail.  I guess I'll leave em out.

they look like the really old cardboard tube electro caps. from what i understand they fail pretty spectacularly

[rundgrenrules]

I looked it up and they are tunnel diodes that were originally put in place to protect the OT.  They were either not installed on this iteration of the amp or were clipped out but either way I’m planning to leave them out!

[rundgrenrules]

Can anyone advise me as to what the electrolytic cap to ground between the plate resistors in the preamp section might be for?  It’s not listed on the schematic but I see lots of pictures of otherwise stock YBA-1s and YBA-1As with an apparent 20uf-50uf cap to ground at the intersection of the plate resistors.  It was added on my amp (33uf) at some point.  I could tell it wasn’t stock.  Wondering if I should leave it out or keep it in.

Thank you!

[2deaf]

Can anyone advise me as to what the electrolytic cap to ground between the plate resistors in the preamp section might be for?  It’s not listed on the schematic but I see lots of pictures of otherwise stock YBA-1s and YBA-1As with an apparent 20uf-50uf cap to ground at the intersection of the plate resistors.  It was added on my amp (33uf) at some point.  I could tell it wasn’t stock.  Wondering if I should leave it out or keep it in.

Thank you!

Isn't that C21?

[rundgrenrules]

Can anyone advise me as to what the electrolytic cap to ground between the plate resistors in the preamp section might be for?  It’s not listed on the schematic but I see lots of pictures of otherwise stock YBA-1s and YBA-1As with an apparent 20uf-50uf cap to ground at the intersection of the plate resistors.  It was added on my amp (33uf) at some point.  I could tell it wasn’t stock.  Wondering if I should leave it out or keep it in.

Thank you!

Isn't that C21?

Oof.  I see that now!  The way this amp is wired there are jumpers from place to place that make it a little more difficult for me to trace out.  To see.  Thank you!