What would cause an amp to do this?

[Johnny Q]

Hello All:

I have had a Rivera 100 Duo 12 for about 4 years. Its a two channel amp, one channel voiced for an "American" Fender-ish sound the other a more "British" Marshally sound. Anyway, I am not certain for sure if this happens on both channels, but it definitely happens on the Brit channel. I'll turn the thing on, and it sounds real thin and clanky, then it will fluctuate while I am playing, like someone is turning the bass knob up and down. One time the volume started fluctuating up and down, and another time, even with the master volume on 8, you could barely hear it. Power tubes are about a 1 and 1/2 years old, pre tubes are brand new (it did it with my old pre-tubes btw).

The volume fluctation leads me to suspect the power tubes, but would defective power tubes cause the fluctuations in tone, from tinny to full??

Another little tidbit, some jackass modded the amp without my permission, he didnt like the way the Svet EL34's I supplied biased up, so he installed a "cap" to get the bias to where he thought it belonged.

Any ideas????????

JQ

 

[aleclee]

Re: What would cause an amp to do this?

Sounds like the power tubes are going, especially if the mod increased the bias current.

 

[Lewguitar]

Re: What would cause an amp to do this?

Problems in a tube amp are almost always tube related. Almost always...though not always. Still, that's what I'd suspect. And it could be any tube!

If you're hearing whistling or squeeling noise it's usually it's a preamp tube.

In your case it sounds like it might be a bad power tube though...

You need to keep an extra set of tubes on hand and then you can substitute them one by one until you find the bad tube.

You could also have a bad capacitor somewhere too...or a bad solder connection somewhere that makes and breaks connection as the amp heats up. Lew

 

[marvar]

Re: What would cause an amp to do this?

Problems in a tube amp are almost always tube related. Almost always...though not always. Still, that's what I'd suspect. And it could be any tube!

If you're hearing whistling or squeeling noise it's usually it's a preamp tube.

In your case it sounds like it might be a bad power tube though...

You need to keep an extra set of tubes on hand and then you can substitute them one by one until you find the bad tube.

You could also have a bad capacitor somewhere too...or a bad solder connection somewhere that makes and breaks connection as the amp heats up. Lew

Yeah, what he said!

 

[Johnny Q]

Re: What would cause an amp to do this?

Sounds like the power tubes are going, especially if the mod increased the bias current.

Yes sir - this is exactly what it did. The prick (excuse my language) that did this insisted I buy his overpriced Ruby Tubes (which I didnt do) rather than supply my own (which is what I did), so when I went to pick the amp up, he told me the tubes I supplied wouldnt bias properly and he had to install a cap to get them within range. To make matters worse, he wrote "not guaranteed" all over my receipt.

Anyway - time to bring it to someone who can put it back to stock, retube, rebias and maybe I can then forget this whole thing happened.

JQ

 

[Antihero]

Re: What would cause an amp to do this?

Is it a combo? Im drawing a blank on this amp...

If it is did you check to see if the speaker connections are loose? Maybe also the input jack is loose?

Im not saying that either is definatly it but they are a lot cheaper than a re-tube/taking it to a tech.

Good Luck.

 

[Johnny Q]

Re: What would cause an amp to do this?

Is it a combo? Im drawing a blank on this amp...

If it is did you check to see if the speaker connections are loose? Maybe also the input jack is loose?

Im not saying that either is definatly it but they are a lot cheaper than a re-tube/taking it to a tech.

Good Luck.

Yeah - I tried just about everything, including new pre-tubes. Yep - the R212, Hundred Duo Twelve is a 100 watt combo.

JQ

 

[Johnny Q]

Re: What would cause an amp to do this?

Sometimes I wish my amp problems would be as simple as "just stopped working" because this is driving me NUTS!!:yell:

I had this amp checked, re-tubed, rebiased and they tell me there is nothing wrong with the amp, but there is!! Although, I have the nasty feeling just because the service center couldnt recreate the problem, they didnt even bother to diagnose it. Funny, I recreated the problem the 1st 5 minutes I had it home.

Now I have noticed that when the sound thins out (the primary problem), I can hear crackling and that crackling can be made worse by wiggling the input jack, speaker output jack, anything plugged into the effects loop, OR by tapping on any of the tubes (both pre and power tubes). Whats odd is when the sound isnt thinning out (ie when the amp sounds normal) none of this happens.

Then sometimes I get real bad hum, that can be reduced by placing a finger on something metal, like the metal piece at the end of the guitar cable, or speaker cable. And finally, sometimes I get a hideous ringing through the speakers, even with nothing plugged into it. Again, tubes are new and have been swapped out countless times, so I am convinced its not them. All of this happens when the sound thins out, which is the primary problem, it will drive you crazy! One moment your tone is big and full, the next, its thin, tinny and buzzy.

 

[greendy123]

Re: What would cause an amp to do this?

Did you try other cables yet? The ones your using now could be defective. If thats not the problem I would think maybe bad solder points somewhere inside amp.

 

[Lake Placid Blues]

Re: What would cause an amp to do this?

Does this amp have an effects loop? More specifically does it have a seperate loop for the "Brit Channel"?

 

[Lake Placid Blues]

Re: What would cause an amp to do this?

Thinking about this, it may have something to do with the R/C network between the cathode and ground on one or more of the preamp triodes. If it uses a small electrolytic there, and it's going bad, this type of thing could happen. The cap drains out so it cuts the bass out of the tone, but as it recharges, it allows a thicker tone..up and down....

 

[Johnny Q]

Re: What would cause an amp to do this?

Thinking about this, it may have something to do with the R/C network between the cathode and ground on one or more of the preamp triodes. If it uses a small electrolytic there, and it's going bad, this type of thing could happen. The cap drains out so it cuts the bass out of the tone, but as it recharges, it allows a thicker tone..up and down....

Ah-ha!!! That makes a lot of sense, I will suggest that to the next tech I bring it to. Too bad the last place, a Rivera authorized service center in Northern NJ bother to think like some of the folks on this forum.

To answer some of the other questions:

Yes - I replaced all the cables, speaker and guitar - same deal. I replaced all the pre tubes multiple times, new power tubes, swapped guitars, took all the effects out of the chain, turned off the reverb etc etc..

Regarding the effects loop: Yes, it has a loop that I run a Boss Ge7 EQ through, I took that out of the chain, put it back, tried new cables in the loop, no luck - same deal. Just one effects loop for both channels btw.

I plan on bringing this amp to a guy named Jim Somma of Sommatone in NJ, I have nothing but rave reviews about his amp chops, as well as being a decent guy to deal with.

JQ