RA100 noisy reverb

Go figure, I took out the new =C= EL34 (one or two of these were rattle makers, disappointed basically), even tried a known to be good EH EL34 and had same results. However, I wanted to retry the KT77 again (fist time due to miss aligned tube, sockets were modified by original owner or defect in casting, found the hard way that tubes can be installed in any orientation) {I am good at making run-on sentences :roll: }..... more to the point, No noise what so ever with the KT77. Could be related to tube type?

Thanks for the info.....please keep it coming....

I would love to get my hands on the schematics for this amp. Not quite sure what preamp tubes run on AC heater circuit. More than likely V6 may be an AC heated circuit. Some preamp tubes will have isolation issues or may be susceptible to cross over transients which causes pickup of hum or static like sounds from the AC supply. The Reverb tank by itself is an antennae. Because of the powder coating or paint of the metal enclosure, circuit ground is not complete. The suspension springs are grounded that attach to the enclosure but seems that the box may not be due to the thin insulating film of the paint or coating. I have removed the tank for inspection, it is as reported the same tank used in the Mark V and Mark IV as well as other amps. The only difference, there is not lid on the box. This in itself may also be part of the issue. Wood does not make for a good shield. Also there was no aluminum foil heat shield on the top surface of the head unit. Those that have the combo, you should not have much of an issue with the noise since the box is complete and sits much farther away from the power transformer.

It is also possible if you have leaky power tubes (power tubes will be AC heated) that may cause the electrical transient noise to be broadcast in the AC heater circuit. Only one tube can cause this issue. Heaters are basically resistive, but there is an associated inductance in the heater element. Triodes and or Pentodes have high capacitance as well as other impedances depending on small signal model. There are many factors that could contribute to the noise level, intensity, or signal degradation. There are volumes of books on this subject. It is next to impossible to condense such that it would make any sense. It would be easier to do the trial and error approach than to understand the physics of the devices and associated affects of electrical transients. Another source of the noise in audio equipment would be the rectifier diodes in the power supply. They are susceptible to damage in the event of no load conditions on the output, the high induced voltage could cause damage to the diodes. In short, noise issue may be related to improper shielding of the reverb tank (lack of signal ground) or related to typical transients (most likely tube related). A faraday box or shield surrounding the reverb tank would be one approach in attempt to reduce the associated noise issue. Wire mesh (chicken wire), metal screen, perforated sheet metal. once constructed, it cannot touch any part of the Reverb tank since it will be earth grounded to the chassis. Signal ground may not be the same as earth ground so to avoid any ground loops (which will add more problems) chassis grounding of the Faraday box or cage and not connected to the ground of the input signal would be best. I may make a project out of this and get a perforated aluminum sheet to make the faraday box and see if there is any difference.

Also to note: I now have all Mesa branded (current production) 12ax7 in the amp (V3 is still the original 12at7). All I hear now is the typical hum that you get with any tube amp. It just sounds different due to the tone of the RA100 when compared to the Mark Series amps. Clean channel is the most noticeable with the hum than the higher gain channel. I can accept what I am hearing as it is not as dominant as it was before. I may do the Faraday box just for kicks. Another note: the amp chassis is a bit more tricky to get back into the head once removed. I found 1inch thick boards (they are actually under one inch in thickness) long enough to stick out of the head cabinet by 4 inches help to work as chassis installation guides when installing the chassis into the shell.

Hi - any resolution to the noise revery problem?
I just picked up a used RA-100 and I'm getting the exact problem.

One question on the head layout: my head shell was custom built (a beautiful hardwood shell) and the reverb connectors face the front of the head. Is this the correct layout? Or should I re-install the reverb unit so the connectors face the back of the head?

Thanks.

My reverb tank has not been removed as far as I can tell, tank jacks are facing front and sit above the power switch.
I get noise with my amp but not quite a reverb issue. However with the reverb on it does seem to enhance it. I have a hall light that is controlled by a dimmer switch. When the hall light is on, the amp is very noisy. When it is off I do not hear it and just get the typical hum I would expect from a tube amp.

If you invert the reverb tank, you may wind up with wires against the tubes.

Would love to see a Hardwood RA100. Can you post pictures of it?

It's so nice that it's too nice... meaning I'm afraid to put a beer on top - even with a coaster.


Thanks for the info on the reverb tank.
I also found out that engaging the power soak on each channel significantly reduces noise.

That is really cool. I did not expect to see a tight fit on the chassis. I like the floating face plate too. Have you thought about placing the Mesa Logo on it or is that taboo at the moment.

One thing I found that caused more noise in the amp other than external line noise issues was V3 (SEND/RETURN).(I would not use a 12AX7 in V3, check to make sure V3 is a 12AT7). The Chinese 12AT7 seemed a bit flakey to me. I did get a Mesa replacement but also bought a few NOS 12AT7 since they were the same cost if not less than the Mesa tube. I like the RFT 12AT7 as it seems to be less noisy than the JAN GE 12AT7 or the MESA Chinese 12AT7. It did not seem to matter what tube I used in V6 (reverb circuit) as that did not seem to contribute to the noise.

Thanks - I'll give that a try!

On the faceplate question, not sure what I'll do at the moment. I see someone is selling the full RA-100 faceplate, but I'll have to ask him to measure so I don't end up with too tight a fit. We'll see. I'm still looking for a full Mesa head shell so I can take this guy out of my house, too.

Based on how tight the chassis is to the custom cab, the RA100 face plate will not fit. With the RA100 shell, there is almost 2 inches between the chassis and each side of the shell. Not quite wide enough to stuff the Roadster chassis into it but almost (thought that would be interesting but no fit). I am wondering if the RA100 chassis would fit a Stiletto shell as it does look similar in shape. Without dimensions to compare no way to tell if it would.

I have grown fond of the RA100 shell. There is also a rear panel that wraps around the back of the chassis similar to the front plate that fills in the gap between the chassis and the shell.

Ah, you're right. I see what you mean about the distance being too tight for the faceplate. Hey, you saved me like $75, so thanks.

Your welcome.

I measured the width of the RA100 shell, inside edge is 25 1/4 inches. Definitely much wider than the amp chassis which is around 22 1/4 inches. I could probably fit the Roadster chassis inside the RA100 shell with the exception of the faceplate will get in the way. Also the depth is different too between the two chassis. I believe the RA100 chassis does not have a common ancestor like some of the other Mesa amps.