How do you set your Musicman HD130?

[hollowbody]

My tech just called and said everything is fine with the head. I'm going to play a different bass for the weekend gigs and see if it just dislikes my EB0.

[Psycho Bass Guy]

High pitch oscillations, funny squeaking etc will still fit into the microphonic category.

Oscillations are usually symptoms of bad caps or stage to stage impedance problems. A microphonic preamp tube can set up an oscillation, but if it continues feeding back, there's a problem elsewhere in the circuit, too. The "whoosh" in power tubes is the momentary current spike caused by physical vibration shaking loose electrons from the cathode and overrunning the bias. I've never heard a power tube ring electronically. Not saying it couldn't happen, but I've never seen it.

Back in the late 70's I was having all sorts of issues with the set of valves that were in my Hiwatt, being terribly microphonic - also, weirdly, every time a police car went past it would pick up their broadcasts...

That's easy; the cathode to ground connection was acting as an antenna and the RF field of the CB was modulating it directly, basically turning the tube into a self-contained radio/guitar pickup. The microphonics were probably the result of poor shield connections to ground around the cathode and the shields themselves were vibrating.

I used to live about 5 miles from a 5 KW radio transmitter at AM 1250.

When playing my blackface '67 Bassman and 2x15 cab with connection via my coily cord, I often picked up the signal of that station.  I could hear it faintly but clearly through the amp.

In your case, the coily cord's capacitance is acting as the antenna. A small snap-on inductor clip at the amp end will cure it. It's the cable, not the amp. I live just a couple of miles from several high-powered TV and radio transmitters and my G&L's L2500's active setting reproduces 97.5 FM quite clearly, as does my Ibanez Musician to a lesser degree. Several of my passive basses also catch it faintly, but it goes away when I physically touch any ground point on them.

[amptech]

Oscillations are usually symptoms of bad caps or stage to stage impedance problems. A microphonic preamp tube can set up an oscillation, but if it continues feeding back, there's a problem elsewhere in the circuit, too. The "whoosh" in power tubes is the momentary current spike caused by physical vibration shaking loose electrons from the cathode and overrunning the bias. I've never heard a power tube ring electronically. Not saying it couldn't happen, but I've never seen it.

I didn´t say power tubes ring, nor did I want to become too technical - just point out that to find out if tubes are microphonic,
tapping them might give you a clue what the problem is. Drbassman requested a non-technical reply, so I tried to answer the question
without going into the circuit.

As I explained, I didn´t see in the text that the ´ringing´probably was just the tubes rattling physically, so please have me excused

[drbassman]

OK, I gave the noise a closer listen after letting the amp heat up for 90 minutes and it sounds like glass rattling, not ringing.  My mistake for describing it that way!  I am thinking maybe a tube or two isn't seated all the way?  I tapped them all lightly with a pencil and nothing happened.  I'm going to let the amp cool off and check how well the tubes are seated. 

The more I listen to it, it sounds like a vibration in the chassis area and it isn't coming through the speaker for sure.  It's probably something really simple (I hope).  I think the reason I can even hear it is because I am playing the amp at low volume here at home.

If it is something in the circuit, we have a great amp guy here in Rochester and I'll drag it over to hom for checkup, if necessary.

[amptech]

Let´s hope it´s simple, then! Another way to make the tubes sit better, is to tighten the contacts in the sockets.
But that requires draining of the caps, best done with the amp out of the cab. Your amp-guy will check for loose components first, but tightening sockets might be a good thing to do if it has not been done before.

A couple of years back, I had a guitar payer who bought a marshall stack new, never had it in for service. (serial nr. dated it 1983) He asked me to check out a rattling noise that got louder and louder over the years (!). The output transformer
was hanging all loose in there, with three loose screws. One screw was missing, with it´s nut and washer ´rattling´along inside the head! This is some scary stuff, I said. He just looked at me and said he had no ideas on the whereabouts of the screw

[drbassman]

Let´s hope it´s simple, then! Another way to make the tubes sit better, is to tighten the contacts in the sockets.
But that requires draining of the caps, best done with the amp out of the cab. Your amp-guy will check for loose components first, but tightening sockets might be a good thing to do if it has not been done before.

A couple of years back, I had a guitar payer who bought a marshall stack new, never had it in for service. (serial nr. dated it 1983) He asked me to check out a rattling noise that got louder and louder over the years (!). The output transformer
was hanging all loose in there, with three loose screws. One screw was missing, with it´s nut and washer ´rattling´along inside the head! This is some scary stuff, I said. He just looked at me and said he had no ideas on the whereabouts of the screw

Yeah, I'll make sure the tubes are in tight and go from there.

[drbassman]

OK, things are straightening out nicely.  Pulled the back off the amp cab and guess what I found in the bottom?

Yep, one of the big transformer mounting screws!  I put it back in and tightened the others.  Would it be wise to put some star washers under these things?  I think so.  The rest of the screws in the chassis seem to be tight.



As for the tubes, they are fine also.  Nice and tight.  Dumb question:  are the bent metal rings supposed be holding the tubes in?  Cuz if they are, they are barely contacting the sides of the tubes' bases.  Just wondering.........



I played the amp after tinkering and it sounds fine.  If I stick my ear next to the back, I can hear faint vibrations when I hit a note at volume, but nothing significant.  When play at a moderate volume and sitting in front of the amp, I can't hear anything.  So, I think that's cleared up.

So far, so good!  Going to check the chassis screws in my other MM amp next!

[amptech]

OK, things are straightening out nicely.  Pulled the back off the amp cab and guess what I found in the bottom?

Yep, one of the big transformer mounting screws!  I put it back in and tightened the others.  Would it be wise to put some star washers under these things?  I think so.  The rest of the screws in the chassis seem to be tight.



As for the tubes, they are fine also.  Nice and tight.  Dumb question:  are the bent metal rings supposed be holding the tubes in?  Cuz if they are, they are barely contacting the sides of the tubes' bases.  Just wondering.........


I played the amp after tinkering and it sounds fine.  If I stick my ear next to the back, I can hear faint vibrations when I hit a note at volume, but nothing significant.  When play at a moderate volume and sitting in front of the amp, I can't hear anything.  So, I think that's cleared up.

So far, so good!  Going to check the chassis screws in my other MM amp next!

Washers might be a good idea.

The clamps can be tightened, probably the amp had tubes with bigger sockets at one point.
Glad it turned out well!

[drbassman]

Yep, but now the amp is putting out static every time I hit a note and any volume.  So, it's off to the shop.  Damn.

[hollowbody]

I ended up bringing my EB-0 and Ripper to the gig on Saturday. After playing about 20 min at home I thought that everything was good to go. That was not the case. It turns out that this mudbucker puts out too much signal for the amp to handle. The bass volume knob had to be set at about 6.5 to keep it clean. My settings with the EB were treble and mid at full, bass at 4, bright switch on and deep switch off. I did use the Ripper for the last set and everything worked beautifully.

The amp also had no problems with the 67 Rivoli that I picked up today. The EB-0 pickup must be very hot.

[Dave W]

I ended up bringing my EB-0 and Ripper to the gig on Saturday. After playing about 20 min at home I thought that everything was good to go. That was not the case. It turns out that ...

The amp also had no problems with the 67 Rivoli that I picked up today. The EB-0 pickup must be very hot.

There's no difference between the pickups. If you had the "baritone" switch engaged on the Embassy, that would make a big difference.

[hollowbody]

I know that they are the same pickup. I guess it may be premature to say that it's just the EB. Mabye the Rivoli will overdrive the pre as well once I turn the amp up to stage volume. I will try it after I set up on Thursday's gig.

[Psycho Bass Guy]

As for the tubes, they are fine also.  Nice and tight.  Dumb question:  are the bent metal rings supposed be holding the tubes in?  Cuz if they are, they are barely contacting the sides of the tubes' bases.  Just wondering.........

When the amp was made, it used US-made fat bottled 6CA7's which had larger bases than EL34's.

[drbassman]

When the amp was made, it used US-made fat bottled 6CA7's which had larger bases than EL34's.

Ah, figures!  Thanks for the info.