Express 5:25 mystery

[tmac]

Hi, I just bought a used Express 5:25 head which I really like, but there is something fishy about the 5W mode: the sound is much weaker than expected and with too little bass. The 30W mode is working fine.

Then I had a look inside the amp, and noticed a difference compared to a picture in a review of the 5:25 in Guitar Player July 2007: Very close to the 5W/30W switch, I have a suspicious resistor. This resistor is not present in the picture in the magazine. I have attached pictures below.

I am wondering if this could be related to the problems I am having with the 5W mode? But I don't want to remove it before I have some idea about what it does... Btw, I am currently also waiting to hear what Mesa/Boogie thinks about this.

The previous owner also bought it used, and so didn't know anything about the resistor. So either it was put there at the factory, or the first owner put it there for some reason unknown to me.

Here is a picture from my own amp, and part of the picture from Guitar Player:

Best regards
Torquil
Norway

 

[Don]

My 5:50 has the same resistor. I assume it's there so that the impedence will be the same going from two power tubes to one.

 

[nomad100hd]

it's only 39Ω and it's on the card with the OT secondaries. Might be part of a recording out or something similar.

 

[tmac]

Thanks. It's probably supposed to be there, since Don has it too. Then there must be some other reason for the faulty 5W mode. Hopefully it will just be the tubes...

 

[nomad100hd]

well do you have a recording, an EL84 in SE mode will be a lot thinner sounding than in PP mode.

 

[phyrexia]

Could be you have one bad tube. Have you swapped power tubes around to see if the problem persists? I could see you not noticing it in full power mode.

 

[tmac]

Sorry, I won't have access to my speaker cabinet for maybe a week or so, or even longer.

But the most obvious sign that something is wrong in the 5W mode is the low volume. If I have the gain and master about halfway up, I get a volume similar to normal speech. It is still possible to communicate without shouting even if I then turn the master up to the maximum setting. A similar setting on a 5W Blackheart or a Tiny Terror in 7W mode would be really loud...

In 30W, everything works fine, I then must of course have the master somewhere close to zero to have a similar volume (at the same gain).

Anyway, for the price I paid I am still very happy with just having the 30W mode, which works like a dream. But fixing the 5W mode would of course be really nice.

The amp is originally from the USA, so I won't be able to use the warranty to get it fixed here in Norway.

 

[tmac]

Good idea, phyrexia. Just tried it, but unfortunately it made no difference at all. Btw, the tubes in it came from the previous owner, they are two JJ EL84 power tubes, and the preamp tubes are marked (going from V1 to V5) Elektro Harmonix, Ruby, then 3 x Mesa Russian.

I will soon have a chance to try a few different types, both power and preamp tubes. So then I will see for sure if this is a tube problem or not.

 

[Don]

Something is definitely wrong then. My 5:50 is still very loud in 5 watt mode though there's not much head room at 5 watts (a good thing).

I'd suspect a tube before anything else.

 

[tmac]

Hi again, today I took one more look inside. When I looked underneath the card where the output jacks are connected, I noticed what looks like a broken diode located between the two 4 Ohms outputs. I don't have much practical experience in electronics, but this diode must have been through an ordeal...

When I use the diode-measuring setting on my multi-meter, it fluctuates a lot but is in the range 10-100 both ways. I know that I should really disconnect one end to get a real measurement, but I din't want to do that just yet.

 

[Don]

That looks like something mechanical happened to it, like someone messed with it.

 

[tmac]

Yes I agree. But I don't have any experience with such things so I couldn't say if this could is a result of something electrical or mechanical... Maybe it could happen if it got really hot.

Anyway, I got an answer in an electronics forum where I also posted this picture. Apparently this is a 120V 5W Zener diode used in relation with the B+ on the power tubes. So now I know exactly what it is and will probably be able to find a replacement for it.

Hopefully, the 5W setting will work after I replace it

 

[nomad100hd]

that could have happened from a voltage spike, like if someone unplugged a cab while the amp was on.

if you go to mouser.com and do a search for 5380b you'll find the part you need. Luckily you can read it off the part. 32¢ not a bad price to get yoru 5w working.

 

[tmac]

Hi, I'm in Oslo, Norway, so I found a local electronics store, and will get a few of those tomorrow. It will be interesting to see what happens when I put in a new one.

Btw, I have no experience with the tube voltages, but I saw somewhere that they should not exceed around 300V for en EL84 tube in class A/B mode:

http://www.ampage.org/td/vtd6bq5.html

In mine I have one of the pins at 405V and one at 340V, so maybe this faulty component also affects the 30W mode by resulting in too large voltages on the tubes? Or maybe it is normal for an EL84 amp to use these high voltages?

Also, in 5W class A mode, the voltages on the two tubes are not the same. Only one of the tubes has a negative voltage on a pin (-11V). At the corresponding pin on the other tube socket there is 0V.

But anyway, I hope I will have a working amp this weekend after replacing the zener-diode

 

[nomad100hd]

Did you measure that with the tubes in or out, if the tubes are not in the voltages will be higher since there nothing loading them down.

You can exceed 300v on the B+ of an el84, but the screen grid needs to be below 300V. That's a a general rule, but not a law. 25W is a little high for 2 el84 so maybe they are running the B+ really really high. Or they are talking peak power!

The -11v is the bias voltage 0V would shut down the other tube, thus being in 5w mode at least i think 0v would shut down the other tube. Just make sure you discharge the amp before you stick your hands in their.

 

[tmac]

Aha, if 0V shuts it down then it is natural that I get 0V on one of the tubes, since the amp is only supposed to use one tube in the 5W mode. And now I know which one it is

I only measured without the tubes, since that was sort of trivial to do. I'm quite careful when doing anything inside the amp, those big capacitors look scary! I didn't go as far as only using one hand, as we were instructed to do in our physics laboratory classes when dealing with high voltages...

 

[nomad100hd]

i don't know if it works on the express, but on my amps if you unplug it from the wall. and turn both the power and standby on it discharges it.

 

[phyrexia]

i don't know if it works on the express, but on my amps if you unplug it from the wall. and turn both the power and standby on it discharges it.

while that works sometimes it is not a good idea to tell people to do that as it is not generally a safe practice.

 

[nomad100hd]

i did do the bad thing and assume people would check the voltages after doing that!

 

[tmac]

Got an answer from Mesa that the 39 Ohm 5W resistor in the original picture is definitely not from the factory. I have another one i parallel below, so it's 19.5 Ohm. The original resistor 120 Ohm 2W, and should be located under the board.

It determines the tube bias in 5W mode, so this is probably the reason my amps 5W mode is faulty; I will try to install a 120 Ohm resistor. I am also replacing the zener-diode, since it looked damaged (even though it could be OK).

Best regards
Torquil Sørensen