Traynor YSR1 Bias help

[Smokebreak]

I'm changing caps, tubes on a buddy's YSR1 . This schematic is the closest I could find : http://www.lynx.bc.ca/~jc/720822_YSR1.gif
This particular amp has reverb tranny and choke, but the schematics with choke and tranny have different PS that this one I have.

Anyways, I don't know much about tremolo circuits, but know this one has a bias-modulation happening. I don't have a footswitch to turn it off, so I just have the speed and intensity all the way down, as I'm just trying to bias up some new tubes. I took some readings on a few diff pairs of EL34s, and here are my dillemmas:

-Even with trem controls down, the Tubes are fluctuating in current draw, anywhere from 50-60mA! ....430 plates.

Do I just pick a median point in the fluctuation to set bias? BTW the neg voltage feeding grids is NOT fluctuating.

This is obviously too much current, too, so I'm trying to figure if it's best to reduce the 47K(between the caps in bias circuit....I know..it's kinda a mess AND sideways) or if I should change the 39K to ground? No trimpot here...


-Now, when I turn the trem controls, the grid voltage varies, as it should, as well as the current the tube is drawing, but it's fluctuating below and ABOVE the idle current...say 10mA to 100mA.

As little as I know about trem, I thought that the bias modulation would happen below the tubes set idling point, with the "top" of the bias shift drawing say 55mA(the way it is biased right now)

Is this just the way this kind of trem works? Isn't that dangerous for the tubes?

Thanks,
J

[Firestorm]

Might be too hot. Just short the footswitch jack and adjust the bias by changing the resistor(s) in the bias circuit. The trem won't sound good if the bias is set wrong. BTW, you can adapt almost any footswitch to work there.

Sorry to be imprecise. Check the series resistor, but unless it's way wacky, change the 39k shunt resistor to something larger. Best of all would be to parallel a 100k cermet wired as a rheostat with a 100k to ground (in case the pot opens) and you should be able to adjust your bias voltage wherever it needs to be.

[Smokebreak]

Thanks. Turns out the 47K bias resistor had drited up quite a bit. A new 47K has got me close. I couldn't figure out how to kill the trem, so I just pulled the tube and bias is steady. I'm not sure if this is an accurate method though...

I'm still interested in how this trem works in the sense of how it is "ok" for the current draw to swing so wildly high above appropriate idle settings. Seems this puts it in a constant state of cutoff-redplate. That sounds like torture !
It actually is quite hypnotic

[Firestorm]

I just edited my last post. See that. These trems swing up and down (though some are skewed) so the bias voltage moves. But if you think about it, the signal itself changes current through the tube in a similar way and since higher frequencies tend to ride on the lower ones (so say the Fourier transforms) its an elegant way to achieve tremolo.

[Firestorm]

As to how hot is too hot: it's an average. In Class A, a tube idles near max and swings almost double to almost off. Tubes are very forgiving.

[Smokebreak]

Aha I see. Thanks a bunch for the explanation.
40K series got me there for bias.

Now for mods, my buddy kept saying "plexi mods!". I think he must have been reading about YBM with the cathode follower and all. This one's all plate fed with a tonestack I don't recognize, and I'm convinced this is what is making the amp sound restricted at breakup. Cleans are great and loud btw.

I was thinking about a Fender stack with fixed mid. Anyone have any experience with this particular model? Regardless, I'd be interested to hear how you would modify this amp, even if just looking at the schematic.