Mesa Subway Rocket Low Volume

[Pedro Vecino]

Original value is 1uF like the Nomad and others. I always replace them with one of polyester preventively. Usually are tantalum, but if is electrolytic, with more reason.

[chunkitup]

Thanks I did just read the thread. My current cap is 1uf in there. It looks like you replaced yours with a 0.1uf correct? If so, were did you find an electrolytic that small?

As everyone stated it is 1uf. I used a electrolytic but I was tempted to throw in a 1uf Mallory 150. Would last alot longer.

[chunkitup]

1uF is fine and a proper value, that's not the problem.

But it might be dry/open so replace it with another, same type.

If any, there's a schematic error.

FWIW Mesa is known to sprinkle a few here and there, maybe to deter copiers.
cloner who never had the original one in his hands will follow the schematic (what else?) and build a poor performing copy.

There were lots of discrepancies between the schematic and actual amp. It was usually just the value of the compnent. The schematic was definitely needed as long as you didn't rely on the value of the part. Replace what the actual part is.

[Pryde]

Sumbitch that was it!
1uf coupling cap after the EQ. Changed it out with another and the amp is nice and loud. This thing had me pulling my hair out. Thanks to everyone for all the help and guidance.

Amp was made in 95 so I suppose a full cap job is in order soon.

[J M Fahey]

Glad you sorted it.

a full cap job

Just the electrolytics.

The plastic ones (polyester, polypropylene, etc.) will last ages, thanks God they are not those leaky paper in oil from the 50's .

[Pryde]

Of course just the electros.
I will wait until it acts up again before doing a full cap job. It sounds fantastic right now

[Pryde]

One more thing while I am at it is the bias current. The amp has a pair of JJ EL84s in it with vP of 370 and drawing 40 m/a bias current. Way excessive it appears but the tubes are not red plating and sound great. I don't know what current mesa had in mind with designing this one as there is no info available.

Would you folks reduce the current or just leave it be? I could parallel a resistor to the 33k bias feed if so. Thoughts?

[Justin Thomas]

Dang it, lost my reply! Anyway, don't be like the jerk who did a "cap job" on my 62 Concert. Funny, 95% of the original CC resistors are still there... hmmm. IF it acts up again, find the bad cap and replace IT, not "them."

Re voltages: "EL84s are the bitches of guitar amp design." -Chuck H- One, they're cheap. Two, IF your tubes are not melting, AND your trannies are not melting, AND the amp sounds great, leave it. If it's broke, fix it. If it isn't broke, fixing it WILL break one of the three above named conditions. You fixed the amp. Enjoy it until it breaks again, IMHO.

Justin

[The Dude]

One more thing while I am at it is the bias current. The amp has a pair of JJ EL84s in it with vP of 370 and drawing 40 m/a bias current. Way excessive it appears but the tubes are not red plating and sound great. I don't know what current mesa had in mind with designing this one as there is no info available.

Would you folks reduce the current or just leave it be? I could parallel a resistor to the 33k bias feed if so. Thoughts?

I'm going to assume you are talking 40mA total or 20mA per tube? If so, that is fine.

[Pryde]

I'm going to assume you are talking 40mA total or 20mA per tube? If so, that is fine.

Nope 40 m/a per tube. measured on a single bias-rite socket.

[The Dude]

IMO, I would back it down or you are going to have a "tube eater".

[Pryde]

IMO, I would back it down or you are going to have a "tube eater".

Hey there. I added a 33k parallel resistor to the 33k bias feed resistor (yielding ~16k resistance) and it had no effect on the bias current m/a at all? It remained at 39-40 m/a.

I then clipped in another parallel resistor knocking total resistance down to 10k and it still had now effect? WTH? I am positive I located the correct bias resistor as I traced it back to the 1000uf cap, etc. Not sure what is up. Any ideas?

[52 Bill]

Hey there. I added a 33k parallel resistor to the 33k bias feed resistor (yielding ~16k resistance) and it had no effect on the bias current m/a at all? It remained at 39-40 m/a.

I then clipped in another parallel resistor knocking total resistance down to 10k and it still had now effect? WTH? I am positive I located the correct bias resistor as I traced it back to the 1000uf cap, etc. Not sure what is up. Any ideas?

Adding a resistor in parallel to that resistor will not lower the bias voltage, it will increase it. You need to add a resistor that will shunt some voltage to ground. Be careful as there is also a -3 volt supply that can be effected by the change to the -16 volt bias supply.

[g1]

Adding a resistor in parallel to that resistor will not lower the bias voltage, it will increase it.

That is what he wants, to increase bias voltage and decrease idle current.

Maybe the cap is bad?
Or some issue with the bias probe? Does it use voltage range or current range on your meter?

[Pryde]

That is what he wants, to increase bias voltage and decrease idle current.

Maybe the cap is bad?
Or some issue with the bias probe? Does it use voltage range or current range on your meter?

Could be a bad cap, not sure on that. Unfortunately I don't have a spare 1kuf to try. I know my bias probe works fine as I tested it on another amp. Think the cap is the most likely culprit as I can't see why the exrta resistor does not have an effect. Hmmm...