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I am restoring one of these, using vintage 7591 tubes one channel gives me about 34 ma across a 10 ohm cathode resistor (added for measuring bias) and around 25 ma across the other pair, which tested weaker than the first pair. From what I see here the first pair is doing well @ 425v on plate with, combined with the cathode current, gives me around 14.5 watts per tube. The second pair though is giving me around 9 watts, unlike other Scott amps, there is no bias output adjust, although the voltage at the grids are the same at around -18.? per tube. Does it appear that I need to replace the second pair (or buy a new matched quad) as I cannot get the required cathode current in the second pair? Thanks!
Follow Ups:
Hi, I redid one of these years ago and with the help of the kind souls her.e I installed two bias pots. one for each side. You can read about it in the below thread.http://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=244707
Good luck.
Dan
Edits: 12/12/16
Thanks, Jim and Michael, yes, I meant per tube, not per pair, oops! And that is right Jim, I have the 340A model which does not have adjustments for the bias for each pair, just the DC balance between individual pairs. Looks like I need to order up a quad from you, Jim!
You must have a 340A model since the 340B had bias adjustment pots - one for each channel's output tubes - along with the DC balance adjustment.
You need a matched quad for you amp. The pair in the first channel is in great shape running 34 ma and 14.5 watts dissipation. The second channel tubes are either weak or a very cool biasing pair (meaning less current at a given bias point).
Regarding the bias voltages at the tubes - here is a section from my FAQ on "Tube Problems" that should explain that:
Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!
Since you have no way to set the bias voltage differently in each channel you need to have tubes that bias properly at the voltage you have. Or change the bias supply to have adjustable bias for each channel.
That was a great link. The only thing I would add is that sometimes, cleaning the tube socket is not enough. Sometimes they are just shot. Both small signal sockets on my Dynaco Mark IV's were shot and had to be replaced.
Dave
Thanks Dave!
I agree with you regarding the need to change sockets when they are simply worn out. But I want people to be a bit proactive - clean the sockets whenever you change tubes or maybe once per year! Why not avoid trouble with a bit of TLC before trouble sets in?
Randy
When you say 25ma per pair,I assume you meant 25ma per tube? If you can't lower the negative bias voltage enough for the weaker tubes,you will have to change them if you want the left and right to be balanced.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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