Traynor YBA-3 - not very loud, bias testing

[Mick Bailey]

It still has the original Mallory Stock-O'-Dynamite caps, too. wonder how many are still being regularly used with those? I see plenty of Traynors that have never been recapped and still going strong, though time is against them.

[g1]

What kind of bias probe do you have? Does it require using mA or mV range?

[beedoola]

The Fluke 117 can most certainly measure current.
AC or DC.
http://assets.fluke.com/manuals/115C117Cumeng0100.pdf

Are you sure it can read MA? I've read that it cannot

What kind of bias probe do you have? Does it require using mA or mV range?

SRS bias tool I got off ebay. Mine is meant for MA reading. I used it before on my Ampeg v4, but the meter I have, that Cen-tech is kind of crappy.

I was getting 7.0 MA reading when measuring the YBA-3.

[g1]

Are you sure it can read MA? I've read that it cannot

Yes it can, see pg.14 of the pdf JazzP posted (actually pg12 of manual). You put the probes into different meter jacks and set for Amps DC.

[jastacey]

I looked at some of the old schematics and it appears to be a very early YBA-3 that had 7027A's in the power section ( they said they quickly changed over to 6CA7's ) and it was a fixed bias although I didn't see the pot for V3 , and it looks like the choke has been cut out of the circuit, based on what I see in your photo

http://www.0rigami.com/vb/traynor_cu...l_yba3_all.pdf

[g1]

The 7027 version would have been very early. The 6CA7 version was already out in '67. This unit has a sticker from a CSA inspector in '69.

[beedoola]

The 7027 version would have been very early. The 6CA7 version was already out in '67. This unit has a sticker from a CSA inspector in '69.

I was told it had been converted to use 6AC7s.

In the PDF linked by jastacey, mine has 4 plate resistors like the first of the three schematics in that PDF. The other schematics only seem to have 2 plate resistors.

[g1]

You mean screen resistors. Only the first schematic with 7027's has 4 and they are 470ohm. The next 2 schematics have two 47ohm screen resistors. Yours has 25ohm and they are much more modern construction than the amp, definitely not stock.
But we are getting off topic here, have you tried changing the bias? (as in post #26)

[jastacey]

I got to thinking and sent an e-mail to Traynor .... here's what they said

"This is interesting stuff. THis work is factory fresh and although I cannot believe it, it is a signal balance on the splitter , just as you thought. This is interesting because I have been around these amps for 51years and cannot remember ever seeing this before. Oh well live and learn

Regards Guy

Guy Beresford
Technical Services
Yorkville Sound
CANADA"

Seems this amp is when they just started using 6CA7's and haven't yet starting the measuring the voltage drop across the 470 resistor .... what I am wanting to know about is those 4 brown resisters, stacked together ... maybe a close-up to see the markings

[The Dude]

.... what I am wanting to know about is those 4 brown resisters, stacked together ... maybe a close-up to see the markings

By g-one:
"You mean screen resistors. Only the first schematic with 7027's has 4 and they are 470ohm. The next 2 schematics have two 47ohm screen resistors. Yours has 25ohm and they are much more modern construction than the amp, definitely not stock.
But we are getting off topic here, have you tried changing the bias? (as in post #26)"

[beedoola]

You mean screen resistors. Only the first schematic with 7027's has 4 and they are 470ohm. The next 2 schematics have two 47ohm screen resistors. Yours has 25ohm and they are much more modern construction than the amp, definitely not stock.
But we are getting off topic here, have you tried changing the bias? (as in post #26)

First, you're right; the bias resistor is 47k


So: Power tubes, closest to Can caps and moving toward the preamp tubes:

1: 0.060
2: 0.056
3: 0.016
4: 0.008

I swapped 1 into 4's spot and then got a bias of .045.

Putting 4 into 1's spot didn't change the bias - is that tube fucked up?

changing tube 3 into 1 or 2's sockets didn't change it's bias reading; .016

I checked to see if all the sockets and their wiring was good, all seemed to be.

The one tube with the low bias has a little bit of a brown spot/burn mark on the side - and it's not the getter, that is one the top of the tube.


EDIT*** I just measured around the bias resistor and I think one of those 2 8uf 250v caps may be blown - I'm getting an open reading on one. They look really old. I'm going to replace those, and maybe added a adjustable bias.

I got to thinking and sent an e-mail to Traynor .... here's what they said

"This is interesting stuff. THis work is factory fresh and although I cannot believe it, it is a signal balance on the splitter , just as you thought. This is interesting because I have been around these amps for 51years and cannot remember ever seeing this before. Oh well live and learn

Regards Guy

Guy Beresford
Technical Services
Yorkville Sound
CANADA"

Seems this amp is when they just started using 6CA7's and haven't yet starting the measuring the voltage drop across the 470 resistor .... what I am wanting to know about is those 4 brown resisters, stacked together ... maybe a close-up to see the markings

So what does it do, exactly? Thanks for emailing them!!!!

[g1]

Remove all the tubes. Choose one only and try it in each socket. The readings should be very similar for all sockets. If not, there is an amp problem.
Now choose one socket and try each tube in that socket. This will tell you how well matched the tubes are. It sounds like you may have a bad tube, or 2 pairs that are quite different from each other and will likely give poor results.
The phase inverter balance pot matches the 2 halves of the waveform (for the push and pull of the power amp) so they are the same amplitude. You need to understand what the phase splitter does before the alance pot will make any sense.

[beedoola]

Remove all the tubes. Choose one only and try it in each socket. The readings should be very similar for all sockets. If not, there is an amp problem.
Now choose one socket and try each tube in that socket. This will tell you how well matched the tubes are. It sounds like you may have a bad tube, or 2 pairs that are quite different from each other and will likely give poor results.
The phase inverter balance pot matches the 2 halves of the waveform (for the push and pull of the power amp) so they are the same amplitude. You need to understand what the phase splitter does before the alance pot will make any sense.

For the tests, do I need all the tubes in the amp or just the one I'm using for the testing?

Also, would those 2 8uf polar cabs with the bias resistor cause problems if they were bad/cause a pair to go out? I'm going to replace them, then do the tube tests.

[g1]

You should probably have the preamp tubes in, but just one of the power tubes. With all the tubes out the voltage may rise too high and the caps might not like it.
Replacing those bias filter caps is a good idea, remember positive end for those 2 goes to ground! They could be making problems but would not make a problem in only 2 tubes, it would be all 4 power tubes.

[loudthud]

It should be noted here that the bias voltage is connected to pin 1 on the power tubes per the original schematic. Since the amp has been repaired or moded, check this at each socket.