Traynor YGM-3 (early) - Selmer cab

[Reeltarded]

[cyclone_cj]

I didn't realize the physical layout was so different. Good to know, i went back and reread your posts on that. Those old Mullards always seem to be microphonic to a degree. They all read a hair lower than bogey on my tube tester, even the NOS or lightly used ones I sometimes find. Around 1100or 1150.

Good luck with your resurrection, I love mine, would sell everything else first.

[Barks]

I guess you don't know where your idle current is yet? You can measure DC resistance from the red OT CT to the power valve anodes (pin 7's) with power off, then measure voltage across the same two points and calculate current as V/R.

Hi Martin,

I've just done this measurement for idle current as suggested.

Pin 7's go to the OT correct?

EDIT - See post below, the following is incorrect due to my error.

R between pin 7 of V4 and the red lead of the OT measured 187ohms.
R between Pin 7 of V3 and the red OT lead measured infinite resistance (1 on my meter's scale)

When powered voltage was 5vdc for both between pins 7 and the red OT lead.

This equates to 0.0267 so 2.67ma

I also measured very carefully using the shunt method across the OT from both pin 7's and the measurement was 3mA.

Is this looking correct?

Apologies if I'm not understanding this.

[Barks]

...also

I've just remeasured the Cathode and the Anode again

Cathode is 6.4v

Anode is 428vdc.

Interestingly the first schematics that come up on the internet for the 3 input early YGM model like mine has 428/9v annotated next to the+398.

[Reeltarded]

Sorry no time to chattaboutit here is how shunt works

http://www.marstran.com/50W%20Bias.htm

[Barks]

I didn't realize the physical layout was so different. Good to know, i went back and reread your posts on that. Those old Mullards always seem to be microphonic to a degree. They all read a hair lower than bogey on my tube tester, even the NOS or lightly used ones I sometimes find. Around 1100or 1150.

Good luck with your resurrection, I love mine, would sell everything else first.

Thanks for the encouragement. It's also a great amp for Fender Rhodes this. Love it.

Again, I'd like to extend my sincere thanks to everybody who has helped out here.I'm loving this little journey.

I'd love to buy you all a drink/coffee/flowers sometime!

[Barks]

edit

[Barks]

OK - I screwed up by being on too low a setting for the resistance on the meter. Please ignore the above.

V3 pin 7 to OT CT reads 220 Ohms
V4 pin 7 to OT CT reads 196 Ohms

Vdc is 5.5 volts for both.

So I get 0.025 for V3, and 0.028 for V4 - 25 and 28mA respectively?

does this look correct?

thanks for your patience.

[martin manning]

Yes correct. Average current is 26.5 so average dissipation relative to max is 26.5mA*428V/12W = 95%. This is quite a bit higher than necessary, so adding the bias trimmer or increasing that 22k going to ground to dial it down to 70% or less is a good idea.

[Barks]

Great, I'm getting somewhere!

I'm detecting a slight fizziness after the note when the amp is pushed - could this be the cause?

Will get the trim and resistor tomorrow.

Thanks again.

[martin manning]

More likely some parasitic oscillation which is now occurring due to some leads having been shifted; prime suspects being the power tube grid leads.

[Barks]

Thanks.
There was fizz before the new PT. I was hoping it was partly due to the oversize PT
It is much better now but still there.
More noticeable when you stand in another room - a papery buzz.
I've just had another listen and it's detectable even when relatively clean and lower in volume.

quick vid:

http://s650.photobucket.com/user/katbar ... sort=3&o=2

[martin manning]

Sounds pretty good! Looks like there is a (wait for it...) fizz cap across the PI outputs, 47pf 1kV(?) ceramic. Maybe go a little larger? Very easy to try, just tack a second one across the existing one.

I also note you have no screen resistors on the output valves. The compression effect of some relatively small ones (270 to 470 ohms) won't be noticed, and they will help protect the EL84 screen grids too.

[Barks]

Thanks!
I'm assuming it is the one circled in the attached photo.

I have a 120pf 500v silver mica here - would that do?

[martin manning]

That'll work. I'd just tack it in place to see if it has the right effect. A ceramic would be best, if you decide to replace the 47pF.