Traynor Yba1a Mk II

[chris61]

I keep getting trouble with these amps!
going back to the MKII;
we talked about the varistor and adjusting the voltage across R25
now I see that pin 4 from the power tubes goes no where and the 4.7 K is not there at all.
shouldn't pin 4 be hooked up???
WTF?

both tubes are jumped at pin 4
it just goes to a bus bar and ends

[g1]

A few items to address here so bear with me. First, to clear up use of the term "varistor". You are talking about the bias adjustment pot, or variable resistor, properly called a potentiometer. A varistor is an electronic device sometimes used as a surge suppressor. This is what Loudthud referred to, the devices shown on the MKII schematic whose part numbers start with 6RS20 and appear across the primary of the output transformer. They are used for protection but often go bad causing low output or blown fuses. They can replaced by typical protection ("flyback" diodes) or just removed from the circuit.
Now, about the filter caps. The second schematic you posted is labelled YBA-1, not 1A. Is the amp it was in a 1 or a 1A? The 1 runs at lower B+ voltage (440V) so it uses single caps at each supply node. You may have a transition model using 80's instead of 40's? Also, some of the can type caps have more than 1 capacitor in each can. The MK11 schematic shows an 80uf and two 40uf in a series parallel arrangement for each the first 2 nodes. That equates to 40uf for each of the first two nodes but can handle higher voltage (up to 900V). This is necessary as the MKII B+ is shown as 540V, more than a 40uf 450V cap could handle.
OK, for the MKII, you say the pin4's of the output tubes are connected to each other but nothing else? Are the two 470ohm 10watt resistors (R25 & R38) in the amp? Is the amp working?

[chris61]

the second schem, is a yba1, this amp worked initially but I wanted to recap it, and was just wonder why they went to 80uf instead of the 40's shown in the schematic.

as far as pin 4 in the ybaia mkII nope there in no second 470 resistor, like I said the wire terminates at a buss bar and is not connected to any thing, I went over it several time with a continuity check just to be sure.

I fired the amp after replacing caps and tubes, and I can barley hear a distorted signal.
so i turned it off.

I went back over my work to make sure hadn't made any bad solder joints,
so Now I guess I have to give it a complete check over,
wish I had more complete info on this amp.
like a start up guide.

[chris61]

HA!

now I remember,
I totally forgot,
when I 1st opened the chassis, I found a transformer wrapped in a napkin stuck inside the chassis, and the lead wich were painfully small gauge wise; were bridged from this very spot on the buss bar,
so my guess is that they removed the 47k resistor and replaced it with this make shift choke.
not sure why though.

sorry, I have CRS sometimes.
now, what to do about all this?

like i said.
this site is the best for working things out.
you guys are the best!

[g1]

Some of the earlier versions used a choke, some guys say they sound better, so some later versions get "choked". I think it's a 470 ohm, not 4.7K or 47K .
Had to look up CRS, I think I have it too.

[chris61]

yea 470 ohm
so you think I should put this back stock or try to install a choke in the place of the pin 4 resistor,
mind you looking at the other el34 circuits,
none of that looks right.

[g1]

My personal preference is always stock, some people seem to have to have at least one mod, just to be able to say it isn't stock . Here's some more schematics from older versions, should be one with 6CA7/EL34 with choke: http://www.lynx.bc.ca/~jc/YBA1Schems.pdf

[chris61]

It's officially back to stock,
thanks for your help! everyone
and works perfectly.
man this thing is loud! you wouldn't think 2 el34's could be so loud.

[Gtr_tech]

I realize this is an old thread, but wanted to address an issue with the output section that causes problems. First off, make sure you have control grid stoppers installed. I use the std 5.6k value, but its not critical. I've used 4.7k and 10k as well. Lack of these can (and usually does) cause instability that crops up as a loud motorboating. Sometimes it happens as soon as you hit the stby sw, sometimes you need to get the vol up a little while play'n thru it to make it happen.

Second, I don't care for the common screen arrangement. I've seen way too many amps with common screens not work well with newer production tubes. The old Mullards may have been OK with it, but we don't have that option now unless you pony up for NOS. I will eliminate the 470r or 1K 10w in favor of 1k 5w on each screen connection at the socket. There seems to be some interaction between the screens that the new tubes can't deal with, so the individual resistors provide some isolation between them and I've not had any amp come back with problems after this was done. I have 3 old Traynors myself that I made the changes to and they all run great.

Somethin' ta think about....

[nevetslab]

I was curious how that YBA-1A Mk II came out with the KT-88's. I'm just starting into mine, after puling it out of storage, now with 44 years of age on the 'lytics. I will have the luxury of time to go thru full restoration and revisions.

I haven't decided upon a circuit topology for the Screen Supply on the KT-88's. It'll be about 1/2 the HT supply, each their own resistor as you mentioned. And, lots of other changes to make overall on the amp.

[Leo_Gnardo]

I was curious how that YBA-1A Mk II came out with the KT-88's. I'm just starting into mine, after puling it out of storage, now with 44 years of age on the 'lytics. I will have the luxury of time to go thru full restoration and revisions.

The owner was happy with it and I've done another since then. The second one has been used for lots of rehearsals, live shows, and a recording with a 3-guitar band that sounds HUGE! Sometimes the guitarist switches to a baritone guitar for even more gutteral chunk-tone.

I haven't decided upon a circuit topology for the Screen Supply on the KT-88's. It'll be about 1/2 the HT supply, each their own resistor as you mentioned. And, lots of other changes to make overall on the amp.

I'm sure it will turn out a beauty.

[gui_tarzan]

I have a YBA-1A that has the dual 40/single 80mfd filter caps in that parallel/series combination and it seems to be a strange setup but if it works I guess it works. I did not know what those two varistors were until now, would a bad one cause one of my 6CA7s to lightly red-plate?

Speaking of HUGE power transformers... wow!

[g1]

I doubt they would make a tube red-plate, but just disconnect one leg of each and see if it stops.
Swap the output tubes and see if the red-plate sticks with the tube or the socket. If it's a socket, coupling cap from phase splitter is suspect. Also check both screen resistors if it has them.

[gui_tarzan]

That's some really good advice. I will add those, mine has a single 470K 10W and no grid stoppers.

First off, make sure you have control grid stoppers installed. I use the std 5.6k value, but its not critical.

I will eliminate the 470r or 1K 10w in favor of 1k 5w on each screen connection at the socket.

[gui_tarzan]

It turns out the previous owner had some funky "repairs" done, I'm having to rebuild the filter cap section and bias section to specs so that's adding a bit of time to this project.