Mesa boogie 5.25 low output

[pontiacpete]

This amp came to me with all new tubes. The complaint is that it's not as powerful as would be expected. I measured the output in the clean channel and I'm getting 6.2vac , which is about 9.6watts.
Now it just so happens I have another boogie here but seems to be a slightly older version of this amp, the F-30, lucky me
Anyway they have a very similiar tube configuration and all but the F-30 is measuring about 26watts out of the clean channel.
I think the 5.25 is definitely under powered..
Tubes are cathoded biased and pulling about 32mA. Voltages in the power supply are looking good but about 7-10v lower than schematic
Any ideas where to go from here?

edit: sorry this is a fixed bias circuit.

[Enzo]

Do you really mean it is less loud? Power is not loudness, so how did you determine the power stage is the issue? A weak signal into the phase inverter, for example, will also result in low output. Or weak signal from the first stage, or anywhere else in the circuit. The difference between 10 watts and 20 watts is only 3db.

[pontiacpete]

Sorry I guess I miss spoke. Shouldn't have jumped to "definitely under powered" what I meant to say, the F-30 is a lot louder than the 5-25. And when measuring the output I get 7.5vac for the 5-25( 7.5^2/4=14w) and the F30 is over 10.5v both into a 4ohm load. There might not be anything wrong with this amp. Perhaps my customer had it on the 5watt class A setting and not the 30watt,.. which it does seem to be making either.
Shouldn't I be seeing 30watts on the output or something close to it like the F30?

[J M Fahey]

This amp came to me with all new tubes. The complaint is that it's not as powerful as would be expected. I measured the output in the clean channel and I'm getting 6.2vac , which is about 9.6watts.

6V into 8 ohms is about 4W ... check it´s not stuck in the low power mode.

Anyway they have a very similiar tube configuration and all but the F-30 is measuring about 26watts out of the clean channel.

25W from a couple EL84?
That waveform must be very distorted, almost squarewave.

Tubes are cathoded biased and pulling about 32mA.
edit: sorry this is a fixed bias circuit.

Both: push pull fixed bias in high power mode, cathode bias single ended reduced screen voltage resistively attenuated (2k7 25W in series with plate) in low power.

There so much stuff being switched in/out that just one of those failing will mess things big time.

As a side note; schematic shows 8 or 10 patents , plus "many more pending" , supposedly applied to this circuit. WTF?

Maybe ONE , but 10+?
Busy working now, just stopped 5 minutesto brew some tea, but late night I will check those USPTO numbers.
Expect to find quite a few silly ones.

[pontiacpete]

Called Mesa boogie and the tech took me through a series of tests in which we discovered that the amp was in partial mute. 12v were not getting to the "J175 mute"(that's what it's called on the board). There are a few J175s on this amp but 2 had been clipped out already, although that didn't seem to matter to him. It was this main 'J175 mute' fet that was shorted. I could describe some of the components that we lifted before hand to get to that conclusion but they might not be on the schematic. Even, his layout seemed to be different from this board.
Anyway when clipping out the 'J175 mute' the amp started working at the right volumes but of course the switching is now noisy without it.
Not having this particular jfet, I tried to sub in a J111., but this didn't work being that it is an N-channel, come to find out. Then I found one J174, a P-channel fet. Tried that and it didn't work either, amp went into complete mute, no sound. Now I wonder if there might be something else wrong with it.
But I will wait for my J175s to arrive to see before calling him back.
I must say they are very helpful over there at mesa boogie.
BTW he said that you should expect to see only about 14 watts on the output of clean channel, which is what I'm seeing now after hitting one of the switches by the contour pots.

[J M Fahey]

Cool
And of course

14 watts on the output of clean channel

means 14W RMS, just reaching clipping, the normal definition of RMS power and what real World EL84 can supply.
The "25W" bit implies

25W from a couple EL84?
That waveform must be very distorted, almost squarewave.

which is of course cheating, just marketing BS.
Oh well.

[g1]

With the J175 removed, check the gate voltage.
I think a good J174 should have worked there, they are not that much different.

[pontiacpete]

G1 that's what i thought. The J174 is practically the same. I didn't check the voltage when I had the 174 in. With the original 175 the gate was at 2.9v, with it clipped out it was ~12v where it should be. Maybe I'll try the 174 again in a few days.

[Enzo]

The J175 might be CAUSING the amp to be quiet, but that doen't mean it is defective. Might be, might not. The JOB of that transistor is to mute the amp. If it shorts, it mutes, of course. But if the circuit that controls it is boffo, then it mutes when it ought not, but not the fault of the JFET.

In case: JFETs are ON until a gate voltage turns them off. That means that two of the legs will have a low resistance between them. Oh I don't know, 100 ohms or something comes to mind. Not zero, and not open. So when you check them, keep that in mind, they may not be "shorted".

Also, JFETs do not have standard pin arrangements Types that are electrically similar can have different pinouts. So I always check with a meter. The two legs with low resistance are source and drain, which are interchangeable. The remaining pin is the gate. I never trust my memory on JFETs.

[The Dude]

I'm not sure which FET we're talking about. Is it the one in front of the PI? I'll share what I discovered with a Boogie that I worked on recently and it was possibly a completely different amp so take this with a grain of salt. It will require some investigation on your part. The FET is a 30V FET. With a large signal present added to the existing supply, the FET's voltage was far exceeded in the amp I worked on- destined to fail. I could see no reason to replace it. It served no necessary purpose. The shorting jack on the input is sufficient. I just clipped it out and left it out. IMO, it's just another thing that could go wrong and isn't necessary.
Of course, some FET's are necessary for switching purposes. If it's one of those, ignore this post.

[Gregg]

I had a similar problem with 5:25. It was that J175. The difference between J174 and J1745 is their ON resistance. The gate should see something like 12V.
Concerning the power output I also wondered if there was something wrong but I wrote to Mesa and hey confirmed that 14W is OK.

[pontiacpete]

Thanks Gregg for your input. The J175 does work!

[g1]

So I take it the amp is good now and the J174 is not a suitable sub in this application?
I don't think the ON resistance is the culprit as it would seem the J174 is not turning OFF.
In any case, good to know J174 will not work here.