MB Subway Rocket Overall Volume Seems Low for 20w

[chunkitup]

A friend brought over a rocket that he bought online. Someone had installed the mercury mags aps output tranny. He is complaining that the volume is not as loud as other 20w amps. I have to agree. I can dime the amp and easily stand by the 4x12. It gets somewhat loud but how can you determine? The volume taper is even the whole sweep of the pot. Is this the way the amp should be? Some amps just seem louder but I dont know anything about this amp. I put in new sovtek el84's and no volume change. Next step is to take the chassis out but I need some advice from the forum.

[Jazz P Bass]

What is the cabinet rating (ohms)?
Which output tap are you using?
Most amps are rated at clean output.
Is this true for the Mesa SR?
I do not know.
Hook up a dummy load & a scope & test the amp.
A 20W clean output into an 8 ohm load will read as a 12.5Vac output signal.

[Enzo]

Loud is not usually a matter of power. 40 watts will only be 3db louder than 20 watts, for example. What usually passes for loudness is the gain of the preamp. "This amp on 4 is louder than that amp on 6."

[chunkitup]

Well I am running a 4ohm 4x12 thru the 4ohm tap. The volume doesnt really change with new el84's. I just think this amp was designed to be a bedroom/recording amp and doesnt get as loud as other 20w amps. The pa can be biased for 20w static but if the signal coming from the PI is not as large as another amp the volume will not be as loud. Am I correct in this thinking?
By the way this amp sounds really good!

[olddawg]

Well I am running a 4ohm 4x12 thru the 4ohm tap. The volume doesnt really change with new el84's. I just think this amp was designed to be a bedroom/recording amp and doesnt get as loud as other 20w amps. The pa can be biased for 20w static but if the signal coming from the PI is not as large as another amp the volume will not be as loud. Am I correct in this thinking?
By the way this amp sounds really good!

What does it sound like with a boost pedal? If it was set up as a clean amp it may scream with a pedal. My 18 watt Marshalls are a little pedestrian until I tickle them a little.

[chunkitup]

Sad to say I dont have any boost pedals. I believe this amp is set up for lead. It doesnt have a clean channel. It has a rhythm channel and lead channel. The rhythm channel is louder by a little bit.

Quick edit:
The sound does not change at all as I roll thru the master vol. This amp has el84's and they would definitely start to break up and color the sound if the MV was dimed. Correct? Unless it was designed to keep the PA clean. Correct?

[Jazz P Bass]

Measure the output with a volt meter.

[g1]

Have you tried with a patch cord connected between FX send and return, with FX mix turned up?

[chunkitup]

Alright I am finally back to this after the holiday. I tried the patch cord and no difference. I did not measure the output but I did check voltages on the PI and power tubes. I am trying to follow this scheme:

http://tubefreak.com/subway5.gif

and I definitely have a problem with the PI. I am not getting any voltage on pins 2 and 7. In fact pin 7(input according to the scheme) hums pretty loudly when I put the lead on it. On the scheme there is a a presence also and there is no presence on the amp. Hard to follow the path on the pcb.

[chunkitup]

Can someone explain the PI in the scheme please? Perhaps 0v on pin 7 and 2 is right since pins 3 and 8 are only at 2.3v.

[g1]

Those voltages are ok if 3 and 8 are good, you don't need to worry about 2 and 7.
You need to figure out if your problem is in the preamp or power amp. If you have access to another amp with an FX loop, run the fx send (or preamp out) into the rocket's fx return (or power amp in). Then try the rocket's fx send into the other amps fx return. This should help you narrow the problem down to preamp or power amp.

[chunkitup]

Signal Voltages

Signal voltages finally done. I have finally gotten back to this amp and recorded the signal voltages at each stage. I have attached the whole scheme. I am using the lead channel. Here is what I have:



Input: V2 pin 2: .022v
V2 pin 6: .1v
V2 pin 1: .1v
V3 pin 1: 33v the lead gain @ 5
V3 pin 6: .375v the lead master @ 10
V4 pin 1: .33v
V4 pin 6: .38v
4ohm tap: .6v

All the voltages after the lead master were taken with the control @ 10

Here is a link to schematic heaven for the scheme
Mesa Boogie Amp Heaven Schematics - Caliber Angel Heart Breaker Lonestar Mark Maverick Nomad Rocket

Just editing. The fixed eq for the contour has only -15v supply instead of the -30v stated in the schematic

[chunkitup]

Checking the power supply for the contour eq and the scheme has it fed from the blue lead on the sec. and that reads correct at 73Vac. On the scheme it goes to a diode then 4 1k resistors in parallel. But on the board it goes thru a power resistor. The board says 220ohm5w in paranthesis and 120ohm2w not in paranthesis. Perhaps import voltages. Problem there is someone put in a 560ohm7w resistor and I only get 26v after that which I havent been able to trace where it goes after that.

[g1]

Don't worry about those resistors, 26V coming out should be ok.
You need to figure out whether this is a preamp or power amp problem before you do anything else. See post #11.

[chunkitup]

I have no other amp to use. Isnt following the signal just as good? The signal before the contour eq is big and the signal after the eq is less than a volt ac. V3a signal is good then it goes thru the eq and the signal with master on 10 is only .375Vac