Using PS-2A with old Marshall JCM 800

[Mikke187]

Hi!

Can I safely use the PS-2A with an old (1984) Marshall JCM 800 2203 100w amplifier in an 8-ohm configuration, where the AMP IN power rating is 150 watts, or would it be better to use a 4-ohm configuration with a 200-watt power rating?

In older JCM 800s (the Scandinavian model), it’s apparently quite common for the master volume to act almost like an on/off switch—meaning the amp can easily crank up even at lower settings (e.g., master volume at 2–3).

When cranked up, the JCM 800 can apparently produce peaks of up to ~180W, in which case 150W/8 ohms (continuous load) would likely not be sufficient.

The older model only has 4- and 8-ohm outputs.

A dummy load probably won’t go into “bypass” mode when overloaded; instead, the impedance will start to fluctuate, which could cause the amp to break (or at least blow the fuses)?

[dan]

@Mikke187 Welcome to the forum.

We don’t have any recommendation to use different Ohm settings with different amps, but the extra margin can’t hurt, and if you have the option, why not? The Power Station is rated for 150W to 200W average power. This would be really non-musical sounding. It’s basically a sine wave continuously blasing into the load. Music has breaks, pauses, loud parts, quiet parts (crest factor reductions), which reduce the average power down quite a lot. But you are right, the peak instantaneous power on attacking a note can be quite a lot, but it’s the average power that matters for thermals.

[Mikke187]

Thanks for the reply!

The same amplifier was tested with a dummy load from another manufacturer at full power (master and gain at 10/10), which did not trip the amplifier’s fuse. The PS-2A tripped the amplifier’s fuse even when the master volume was only at 2–3 after a couple of minutes of playing.

The amplifier worked flawlessly with the same master/gain settings even without the dummy load, without tripping the fuse.

Based on this, something is obviously happening on the PS-2A’s end in that situation, and I couldn’t think of anything else other than that the 150W power rating was being exceeded. Does exceeding the power handling rating cause impedance fluctuations or something similar that could explain the problem?

I’m also wondering if I should dare to try again with a 4-ohm connection once I get the amp back from service, or is my PS-2A defective? I’d hate to break my vintage amp because of this.

[dan]

Hi,

Need to be quite sure it’s like for like comparison, same settings, same playing etc. but assuming it is, I’ve not heard anything like this before. I would first suspect something in the amp. But let’s give benefit of doubt. Does this still happen in 8 Ohm setting with PA-2A?

[Mikke187]

Yes. With an 8-ohm connection, the amplifier’s fuse blew even at low volumes. However, on these older models, the volume knob’s setting apparently doesn’t always fully reflect what the amplifier is actually outputting. For example, I’ve heard that the “Master 2–3” setting can already be close to full power on some units.

I had just bought that Marshall, so when the problem arose, I sent it in for service to be checked, just to be on the safe side, in case there was an issue. With another manufacturer’s dummy load, the amp worked even at full volume, as I mentioned earlier, and without any dummy load at all, it operated at the same settings that previously caused the amp’s fuse to blow when used with the PS-A2.

All the fuses in the PS-2A were in good condition when tested with a multimeter.

So, since then, the PS-2A has been working flawlessly with my other amplifier (ENGL e670fe, 100w) in an 8-ohm configuration, but with this amp, the master and channel volume knobs operate more “linearly” and don’t suddenly boost the power at low settings.

Before trying the dummy load with the ENGL, I routed the guitar signal from the Quad Cortex to the PS-A2’s FX loop return to verify that it at least works without an amp. After that, I felt confident enough to try the ENGL with the settings I’d used previously. I’ve never actually tried the ENGL at full power with the PS-2A (at most, the master and channel volumes at about 5/10), in case the same problem would recur with an 8-ohm connection to the ENGL 100W amp (and I don’t necessarily want to try it).

If no problems or causes for this issue are found with the Marshall during servicing, I guess I’ll just have to try it again with the PS-A2, but with a 4-ohm connection. In that case, the only possible cause of the problem would seem to be exceeding the 8-ohm/150-watt power rating?

The bias on the Marshall’s EL34 power tubes is apparently set quite high, based on the markings on tubes (this will be clarified during service), and the mains voltage in my apartment is usually ~235V.

[dan]

@Mikke187

in case the same problem would recur with an 8-ohm connection to the ENGL 100W amp

Are you saying the ENGL is also blowing fuses with the PS-2A?

If this is the case, then we should so some debugging of the PS-2A. Please tell me if you have a multimeter to hand. Or if you have a local tech you work with, I’d ask them to take a look at the reactive load section.

What are the toggle-switching positions on the front panel? Please set both toggles to flat.

@Mikke187

If no problems or causes for this issue are found with the Marshall during servicing, I guess I’ll just have to try it again with the PS-A2, but with a 4-ohm connection.

So the Marshall went to the tech?

@Mikke187

The bias on the Marshall’s EL34 power tubes is apparently set quite high

Let’s see what happens but this could be an issue.

[Mikke187]

No. I just wanted to make sure first that there was nothing wrong with the PS-2A before I tried using it with another amplifier (ENGL).

I also checked the PS-2A’s “amp in” impedance with a multimeter, following instructions I found on one of your pages, and the measurement results were always slightly below the selected impedance settings (for example, with the “amp in” and “speaker out” impedances set to 8 ohms → measured result approx. 7–7.5 ohms).

Both switches on the front panel were in the “flat” positions.

I sent the Marshall to a service technician for inspection, just in case there’s something wrong with it. So it’ll be a while before I can test it again.

[dan]

Congratulations on finding that, yes, that’s the sort of check I was going to suggest next. The PS-2A is fine, don’t worry about it being a bit low or a bit high, it sounds normal to me. OK so we know that the flat position is fine. OK ,let’s see how it comes back. Maybe bias too high? We take it from there.

[Mikke187]

Well… I got a message today from the amp’s technician saying that everything is in order on the amp’s end.

They also tested that this particular unit delivers ~112 W RMS (clean sound) and, at full master/gain, 188 W at 8 ohms and 180 W at 4 ohms. The bias is 33 mA / 60%.

So, judging by all this, was the problem related to exceeding the PS-2A’s 150 W/8 ohms power rating, as I had previously suspected? The technician also confirmed that the master knob is quite sensitive—it already delivers nearly full power somewhere around the master ~3-position…

So it should be safe for me to use the amplifier in a 4-ohm configuration, in which case the PS-2A’s 200-watt power rating won’t be exceeded and no problems should arise?

[dan]

Yes, 4 Ohms will be fine. I’d also say 8 Ohm will be fine too because average musical power will be less than 188W.

I don’t think the problem was exceeding the PS-2A’s 150W / 8 Ohm rating. Because this would not blow your fuses, it would just make the resistor bank too hot in the PS-2A. You have verified that those values are fine, so it’s not that either.

Also what front panel toggle settings are you using? Do fuses blow only in flat or with them in all positions?

My best guess is that the fuses blew because the amp was being pushed beyond its limits, and that is possible to do quite comfortably with tools like PS-2A

[Mikke187]

During servicing, the amplifier was tested with a couple of different dummy loads (power handling capacity of at least 200 W, not a Fryette) as well as without any dummy load at all (with a speaker cabinet, of course) at full master/gain settings, with absolutely no issues.

The only situation in which the amplifier’s fuse has blown is when using the PS-2A with an 8-ohm connection and higher master/gain settings. If the problem were with the Marshall, the fuse would surely have blown with other dummy loads as well, or even with just a speaker cabinet at nearly the same settings.

My main point was to figure out the source of the problem so I can avoid it in the future. However, I still haven’t received an answer regarding how the PS-2A behaves when the power limit is exceeded — does it likely fail to maintain a consistent impedance in that situation?

[dan]

What you are describing does not sound like normal or expected behaviour of the PS-2A. It has been tested on classic amps like Hiwatts, Marshalls etc and does not show that behaviour. If you want to get your PS-2A checked out, maybe to verify any damage to the reactive load we have two routes to do that: local repair or RMA and return to Fryette.

More information here:

• Can I get my Fryette product repaired at a local repair shop?
• How can I send my product to Fryette for a repair (RMA Request)? - #2

[Mikke187]

Today I got to test the amplifier again with the PS-2A, and the amplifier’s fuse blows even when connected to a 4-ohm load. This happens after about ~15 minutes of playing, even with the master volume set to ~2.

The Marshall currently has a 1.6A slow-blow fuse, but apparently 100W Marshalls often come with a 2A slow-blow fuse? As far as I know, the amp is in stock condition (except for the input cable shielding), so that fuse is probably the original one in these Scandinavian “SEMKO” models.

I’ll contact the store where I bought that PS-2A earlier this year and ask if they can check the unit’s functionality.

[dan]

EU wall voltage is 230V, so 230V * 1.6A is approx. 368W. You recorded 180W at 4 Ohms (output), so let’s say the amp is 50% efficient; that would be drawing 180W / 0.5 = 360W. So yes, you are quite on the limit.

Sorry, I don’t have much knowledge about SEMKO, they seem to have a voltage selector on the back, though. Make sure that is correct for your wall voltage. I think 1.6A fuse is around right for 100W amp; sure, you could increase it.

[Mikke187]

The previous problem seems to have been resolved by replacing the amplifier’s 1.6A fuse with a 2.0A one (for those who are dealing with the same issue with SEMKO Marshalls).

Since I’m using multiple amplifiers, I’ve now tried using the PS-2A’s FX loop, which I’ve noticed causes high-frequency hum at higher volume and/or gain settings. I believe this is a ground loop between the PS-2A’s FX loop and the Quad Cortex (4 cable method), so I ordered a Lehle P-Split, which I hope will fix this problem.

[dan]

Hi @Mikke187

Great, glad you found a solution and thanks for coming back and letting us know. Sounds like that Marshall needs a lot of power