Hello! I owns a arc vt100mk1. I have hade two output tube failures despite the amp is recapped and is in good working order with matched Svetlana 6550c from a reliable source. I dont trust the single (potentiometer) bias adjustment for all 4 output tubes. The later versions (mk1,2) uses two and four respectivly bias adjustments. I have been in contact with swedish importer about this issue and the solution according to them is to buy "expensive" ARC output tubes. I am sure these are closely matched "but" if these output tubes begins to drift? I consider to modify this amp to use bias adjustment for each output tube but "convential" bias schemes dont seem to work here because the cathodes is connected directly to the sekundarys of the opt. Grateful for any input
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Actual separate bias adjustment for each valve would be tricky, because the output valves' grids are fed from the same source. In any case, you need a way to measure the current through each valve independently. This can be done by adding a small, maybe 10R, resistor in series with each cathode. Voltage across these small resistors will tell you their individual cathode currents. From this, perhaps you could match pairs of valves to operate in parallel.
The next refinement would be to give separate bias voltages to each pair of valves - two bias pots per channel. Anything beyond this would be major redesign, and not really necessary. But the first step is the ability to measure: the cathode current sensing resistors. Then you'll have some actual numbers, and know where you stand.
All good fortune,
Chris
The next refinement would be to give separate bias voltages to each pair of valves - two bias pots per channel. Anything beyond this would be major redesign, and not really necessary. But the first step is the ability to measure: the cathode current sensing resistors. Then you'll have some actual numbers, and know where you stand.
All good fortune,
Chris
Another option is to take a leaf out of Patrick Turner's book, adevelop an active protection circuit that monitors cathode current, then cuts off power when the cathode currents get out of tolerance if, for instance, one tube was starting to run away or they had drifted too far apart due to aging.
Dynaco ST70 and active protection
The starting point has to be matched tubes, and considering them to be service items that have to be replaced every 1000 hours or so, and factor that in as a running cost.
Dynaco ST70 and active protection
The starting point has to be matched tubes, and considering them to be service items that have to be replaced every 1000 hours or so, and factor that in as a running cost.
You could consider a quad of really matched tubes. Many vendors "out there" matches
by weight or on a historic tubetester at 40volt or less.
Good tubes properly matched will stay matched during use, at least that's what i have seen.
As you seem to be located in sweden the supplier is really close 🙂