I purchased a Fryette PS-100 in 2021–it appears to be a 2020 version. I had been having issues lately that appeared to be related to a bad power tube–redplating, blown fuses, etc. I ordered a replacement set of PSVANE 6550s ($150!) and installed them in the unit. It’s working okay, but I measured the bias on both tubes and one is running at 60ma while the other is at about 62ma. My understanding from reading other posts here is that the factory bias setting is 35ma. Unlike later versions I’ve seen online, I can’t locate any bias adjustment pot on the circuit board. Therefore, I don’t feel good about using the unit with the tubes running so hot. My only other thought is that the perhaps earlier units ran hot and this is normal? I’m a little confused, as the manual said no re-biasing is necessary when installing new tubes.
Hi Kenneth and welcome to the forum!
We get this question a lot, so I’m going to try and give a fuller response than normal. The caveat here is that this is my interpretation of Steve’s design decisions, so I stand by being corrected by someone from the shop.
Tubes operate in quite a wide operating range. Bias is adjusted to squeeze every last bit of power out of the tubes at a reasonable THD level. The Power Station assumes a matched pair in the “medium” range of your supplies’ anode current scale. This also solves the measurement-reference problem: tube vendors match at different plate voltages, so just publishing a mA value without full test values is kind of meaningless. So our advice to “buy a medium-matched pair” is always going to be actually more reproducible, even though it sounds less rigorous.
So why no user-adjustable bias? That trades a few percent of the theoretical optimum for three real benefits: no screwdriver settings bias to 70mA because “internet says it’s better” (I do not think you fall into this category, by the way); subsequent support and repair costs; no drift-prone pot in a high-voltage bias circuit (no pot-failing-open errors, i.e., red plating) , and no service when changing tubes.
If a specific tube pair sets the bias too far outside the window I’d consider returning the tubes and getting a matched pair more in the centre of the range. Either that, or find a local tech that will do the re-bias for you.
So I think the key point here is that it is really a design decision made by Steve. For almost all matched pairs, the Power Station will work fine for many years. It’s not a high-end tube amp where every Watt and THD matters. It is a workhorse which operates in punishing environments and is made even more reliable by clever design choices such as these.
Thanks for the response. I’m going to return the tubes because I assume 60ma is way too hot and may lead to issues. However, I’m not sure how to order replacement tubes as a medium matched pair, since none of the tube vendors I’ve found offer that kind of qualification. I guess I will find a local tech to work on it.
It’s probably not as bad as you think would be my hunch, because again, it all depends on how they measured them at what anode voltage. I’d dare say the anode voltage in the Power Station is much higher than the tube’s reference measurement. But, yes, asking them for something more medium and getting a tech involved (if needed) is a good way forward.