I have a 1969 Plexi 100W that I’ve been running with a 2017 PS-2 with no issues for a couple years. After 5+ minutes of playing, the fan (which is still operating normally) pulls a good bit of heat from power soak resistors which is normal for a blazing 100W. Yesterday, the tone changed a little, there’s a very slight buzz only on certain notes and there’s very little heat coming off the power soak resistors. The amp is still it’s normal ridiculous loud with the PS-2 off, and since I have a 2nd PS-2 for my 50W head, I substituted it and it gets hot just like the other one used to. I know electronics and fixed a cold solder issue on the PS-2 I use with the 50W head because they’re out of warranty but having no schematic it seems odd that the power soak is now staying cool. Any ideas anyone?
PS I do think it’s a PS-2 issue, I’ve checked cables, the amp, swapped another PS-2 etc etc.
It’s worth putting the front panel toggle switches into flat position. This will isolate the issue to the main 4 / 8 / 16 Ohm resistors on the bent metal assembly. If they are not getting hot with 100W entering, then it’s likely their value has shifted and your amp is not being loaded correctly.
Before you start, unplug the pre-2019 PS-2 disconnect everything, the power, all cables. Make sure front panel toggles are in flat position (this will not work otherwise). Now insert a jack into AMP IN and on the other end of the jack, measure the resistance with a multimeter probing across the tip and sleeve and change the AMP IN impedance in 2/4, 8, and 16 Ohm positions. If you see values not tracking expectations here, then, indeed, there is an issue with the main DC power soaking resistors. It could be a bad joint, or the resistors will need replacing.
I don’t want to give you too many steps to follow. Also, I’d recommend you drop this off at a local tech. if this gets more complicated than the above. But see how you get on with this.
Always unplug Power Station from the mains, wait for it to be cool and for the main power supply caps to discharge.
Just to baseline, these are pre-2019 PS-2 units, not a PS-100.
I performed your test with the power cord disconnected, switches both on ‘flat’ on the AMP IN jack. There is no change in DC reading (13.6 ohms) when changing the slider switch. However, since I have another PS-2 that is having no issues, I performed the same test on it and it’s reading of 13.9 ohms also did not change. For giggles I performed the test on the output jack of both units and also did not get any change in ohms.
I’m very good at tube amp technology having taken a year of EE courses before switching my major to CS and have worked on my own amps for years so feel free to give me all the steps you need. When we do find the issue I’d prefer to do the repair myself since I’m not impressed with local techs.
I appreciate the notation about loading my amp, which was also my main concern since my '69 plexi is a prized possession and I won’t risk blowing my OT since my experience is that nobody is making replacements that sound the same. I did test the amp with the PS-2 disengaged and it’s still ridiculously loud with no buzzing or issues.
Since I did have an issue with cold solder joints on the working (other) unit a while back I did re-solder all the connections on the resistor array under the fan to no effect. But I didn’t go further than that. Thanks again for helping with this- I love these PS-2 units.
It is not something silly like DC and AC mode in the multimeter? That is normally only an issue for voltage mode. Good to test against the working unit. The other PS-2 is a PS-2 and not a PS-2A? I am wondering if this measurement cannot be performed on this unit. Can you use the PS-2 in silent mode? With it unplugged and only using AMP IN, you get line-out audio. If so, then the measurement should work, and I’d say it’s some kind of measurement error.
Yes sorry I don’t know why I put ‘DC’ in there so please forget that- it’s ohms on a Fluke multimeter. Both units are PS-2, neither is PS-2A. I’m not understanding what you mean by silent mode- you describe getting line-out audio but say it’s unplugged. Please elaborate what exactly is the setup of ‘silent mode’ you’re talking about and how you want me to test.
Update: OK I figured out the measurement issue- to make this measurement you must not have anything plugged into the SPK OUT jack. So on the working PS-2 I get 13.6, 7.7, and 3.6 ohms when moving the slider. On the problem unit, I get 775, 7.6 and 7.6 when moving the slider, so clearly there is the issue and only the 8 ohm position has a correct ohms measurement. Can you instruct me on what next to look at? Perhaps it’s one or more blown resistors in the array but since I don’t have a schematic perhaps something else ahead of the resistor array or (hopefully not) the unit’s output transformer?
Update 2: I think I’ve got it- it looks to be one of the array resistors. I am going to de-solder them enough that I can get individual readings and will update this thread with the results.
Final Update: The 6.8 ohm resistor with the brown and yellow wires attached is open. And the pin came off another 6.8 as I was disconnecting the wire so I could test it. So (2) 6.8 ohm 50W resistors are needed. Do I buy them through Fryette or get them through an electronics supply house? Also, it appears from some data sheets that it may be possible to get 75W resistors to fit on the mount assembly. Are you guys still putting 50W resistors in? and should 50W be enough or perhaps a higher wattage?
Thanks for your assistance, the unit is now working again!
I’d like to hear your thoughts on the following: after 15 minutes of use with my '69 100W Plexi, the resistor array (as it did before) gets awfully hot. This is an older PS-2 pre-2019 at least since it has a switch for FX level instead of pot.
I noticed that at least by 2024 you guys added a piggyback board with (2) 3.3 ohm resistors above what people call the ‘inductor board’. I’m wondering if this was a design change due to the heat in the power soak resistor array. In other words I’m wondering if you guys made that change to remove more heat from the soak by moving resistors. Just spitballing ideas here on what I can do to improve the heat- perhaps a stronger fan, or substituting (2) 13.6 ohm resistors in parallel for each 6.8 that are getting too hot? Or do newer units have improved heat removal? Or should the soak not be getting that hot and perhaps there’s an issue to look into? Thanks in advance for your help.
PS I do have the means to buy a newer unit if the old units had a heat issue that’s been resolved. I just don’t want to chance blowing that Plexi output transformer because there’s something about them that I haven’t heard replicated- probably the iron.
Glad it is working thanks for coming back to let us know.
I’m not sure about exactly what revisions changed what, so Dave might have to comment. But in general small tweaks can happen, and they are not always in response to issues; maybe it is just preference and ear-turning. I think the fan in PS-2 is pretty good. You might be able to find one that has a higher flow rate in the same size. Sure that would keep things cooler. That sounds like an easier route than changing the design. If you wanted to mod the PS-2, if this is a persistent problem, I’m sure Dave would have some suggestions for you.
Always unplug Power Station from the mains, wait for it to be cool and for the main power supply caps to discharge. 
