Traynor YGM-3 (early) - Selmer cab

[Barks]

Ok Here it is.

[IMG575]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/IMG_20150706_113338250.jpg[/img]

I cannot see the grey lead that is supposed to go to earth?
8 on primary, 10 on secondary

[IMG575]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/IMG_20150706_112824522.jpg[/img]

*edit*
I get it - it's inside connected to the end shell panels - so must make sure the paint on the four legs is cleaned off to make a good contact with the amp chassis?

[martin manning]

It seems that feature has been eliminated, or perhaps the shield is internally connected to the core?

[Barks]

Yes, I just peaked inside.
It's there on the secondary side connected to a grip washer that one of the bolts goes through.

[Reeltarded]

man... we so close..

I'm excited.

[Barks]

Hi all again,

just doing preliminary checks etc, and thought it best consult about the fixed bias setup on the amp.

According to the schematic the two resistors should be 15k (R28) and 27K (R41) (ignore the 38K here as this a version I found on the net) its labelled as 27k on all the others.

[IMG:500:359]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/Bias%20part.jpg[/img]

I've found this part on my circuit (circled)though on mine it looks like the 15k is correct but the other (R41) in parallell with C28 is 1M Ohm? The arrow shows the lead that connects up to the Tremolo Intensity.
*EDIT* SORRY! LOOKED AT THE WRONG RESISTOR. THE 1M IS CLEARLY MARKED ON THE SCHEME. THERE IS A RESISTOR BETWEEN THE TWO BLUE CAPS I HAD NOT SPOTTED, JUST VISIBKE, WILL CHECK BACK WITH ITS VALUE


[IMG768]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/IMG_20150508_133449506_1.jpg[/img]

Does this make sense? I think mine has been beefed up to deal with the current high voltages.

[rp]

They built it for 400V with -20V bias. w/o the tubes you just need to check that you are getting near the spec'd 20V with a decent range up and down with the pot. If you're blessed -20V will be in the middle. If so then the circuit should be fine. Max out the bias voltage when you start up and tweak it from there with some 1 ohm resistors on the power tube cathodes or using dissipation calculations. When you power up make sure you have visual access to the power tubes, dim the lights or better turn them off and look for red plates, check the plate seams, they're sneaky. Down the road you might want to replace the bias caps, they look original, if they fail you get runaway tubes - note that they are backwards for the -V.

Good luck I'd be kind of nervous trying 400V on EL84s. If not nervous fatalistic that they'd red plate.

[didit]

Good luck I'd be kind of nervous trying 400V on EL84s. If not nervous fatalistic that they'd red plate.

Several amps I've seen including these Traynors have high voltage fixed bias el84 at 400v or more. It will need a cold bias, with resulting clean to brittle tone. Really suggest heeding that advice to replace the electrolytics. Sad bad things can happen.

Best .. Ian

[Barks]

Thanks for the advice.

It's actually been kinda ok with it's tubes over the last 20 years even with this 456v. I just needed to replace them to get some definition back every 6-9 months when I was using extensively. I certainly wouldn't describe its tone as brittle!

[Barks]

Ok, here's a better picture of the biasing circuit, and of what I believe to be R41 between the blue caps:

[IMG575]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/IMG_20150709_185028541.jpg[/img]

The colour code seems to point to it being 22k?

Also any ideas what this rather ominous red (and not in character!) red lead is doing that is disappearing under the fibre board (centre of pic)

*edit* it seems to just go to the chassis under a piece of black electrical tape - looks like an extra ground mod.

[IMG768]http://i650.photobucket.com/albums/uu222/katbarks/Traynor%20-%20Selmer/IMG_20150508_133453426.jpg[/img]

[martin manning]

Yes, 22k. As I said above, I believe you will be fine starting up after the PT replacement with the bias circuit as-is. You can then check voltages with no tubes in to start, and then adjust the value of that 22k to dial the bias in. You may want to consider adding a trim pot there, and 1-ohm current sensing resistors in the power tube cathode grounds. That red wire is likely the output transformer primary center tap since it comes off the first filter cap.

[sluckey]

The bias supply seems to match this schematic...

http://el34world.com/charts/Schematics/ ... e_ygm3.pdf

The red wire is probably the OT primary center tap.

[Barks]

Thanks Martin, that makes sense. It does indeed appear on the other side going into the OT

I apologise for these newbie questions - I just want to make absolutely sure when I see something I don't understand.
Thanks for the patience - I'm thoroughly enjoying this learning!

[Barks]

Thanks Sluckey.

Those are much better resolution - I see the 22k now.
On the ones I've found for the early 3 input (1968) model it looked like 27k.

[Barks]

Martin - I think I'd like to implement that bias pot later once I have the PT in.
Really appreciate help how it hooks up.

Could you explain the 1 Ohm current sensing resistors?

I've just disassembled the entire amp and will be removing the old PT tomorrow

[martin manning]

I agree, do the PT swap first and get it up and running again.

For the current sense resistors, the power valve cathodes are grounded through 1 ohm resistors (as shown schematically below) to make it easy to measure the idle current flowing through each valve. With this set-up the voltage measured at the cathode pin will be numerically equal to the current in amps, e.g. 35mV = 35mA.

You could add a terminal strip to hold the resistors, or if you don't mind poking more holes in the chassis, you could add some banana jacks so you can measure the voltage without removing the chassis from the cabinet.