Robben Ford Discussion

marinblues · Posted: Wed Mar 03, 2004 1:05 pm Post subject: Need help with my tube amp

Occasionally my amp just goes: Shreeeeeeccccchhhhhhh for no obvious reason. It is very loud and unpleasant. I can't replicate it and I don't why it's happening.

It goes away when I switch off the amp and switch it back again.

I checked the tube and all 5 of them are glowing and functioning.

Any ideas what it could be? Thanks

Marin
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scottl · Posted: Wed Mar 03, 2004 1:07 pm Post subject:

What amp?? What tube brands and types?
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marinblues · Posted: Wed Mar 03, 2004 1:11 pm Post subject:

The amp is this one:
http://www.engl-amps.com/engl-amps/amps/combos/screamer/default.htm

and from what I could see it has 3 x 12AX7 and 2 6L6. The brand of the tubes is Sovtek.

Thanks

Marin
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scottl · Posted: Wed Mar 03, 2004 1:34 pm Post subject:

Marin,

I am not an amp tech so I don't know if bad tubes cause your problem. The one thing I do know with 1000% certainty is that Sovtek tubes sound like crap. Really harsh and bad. Get rid of them immediately.

Try JJ tubes. In America, they are inexpensive. About $14 each for the 6L6 and only about $7 each for the 12ax7. They are sweet. Dumble uses them as well!! Well worth it and a very noticeable improvement. Do it anyway. Maybe it fixes your problem too....

Regards,

Scott
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Aeolian · Posted: Wed Mar 03, 2004 1:43 pm Post subject:

Is it microphone feedback kind or screech? Does the level change depending on the volume controls? If it is a master volume amp, is it affected by the setting of the input or master volume controls. Is there a presence control and does that affect the problem? Are any of the tubes microphonic? Take a drumstick (the famous Mesa "hammer" test uses the handle of the hammer) and tap on each tube with the volume at normal playing levels to see what noises are made. Does this tapping set of the ringing any way. If the tube are not overly microphonic, try tapping a little harder on the chassis. Unplug the amp, short the mains leads together, turn the power and standby switches on and leave it for a day, then open it up and Carefully look around inside for any burned parts. Be very careful not to touch anything. There is an old addage with tube gear, "one hand for the test probe, one hand for the pocket" The problem comes when you complete a circuit between some hot part and ground. I remember in the movie "Frequency" the guy was supposed to be a TV repairman, and he had one hand holding the top of the TV while he poked around with a test probe using the other one. No real repairman would ever do that, at least not for long! Shocked

The idea is to narrow down the possible culprits. Faulty preamp tubes will show up as varying with the control settings, a faulty phase splitter (the last small tube before the output section) won't but will probably react to a good slap upside the head. Output tubes will also go off regardless of the control settings. Presence controls introduce deliberate instability into the output section but in a controlled fashion, problems here can set up osscillation as can all sorts of fixed feedback loops within the amp.
Have fun!
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marinblues · Posted: Wed Mar 03, 2004 1:48 pm Post subject:

marinblues · Posted: Wed Mar 03, 2004 2:04 pm Post subject:

Aeolian · Posted: Wed Mar 03, 2004 3:31 pm Post subject:

Marin,

I looked at the ad for your amp and it looks like a fun toy, lots of preamp gain configurations. Does the problem happen mostly on one configuration? An unstable oscillation might occur if the relays inside got stuck and two configurations tried to be on at the same time. If they use LDR's like Mesa, the one or more LDR might have gone bad (common on Boogies) and you would have the same effect of two things being on at the same time. It's odd that the volume on the guitar would affect things, the only thing I can think of is some instability in the first gain stage that is affected by the input resistance (shunting of the guitar volume control across the input).

You can try replacing the first two tubes. The one closest to the output tubes would be the phase splitter and not a likely suspect.

You can also try taking out the footswitch and using the amps controls. Sometimes current consumed by the LED on the footswitch can trick an LDR into partially switching.

And yes, the bit about shorting the mains is to drain off juice stored in the filter capacitors in the power supply which is what can bite you. If you know what you are doing you can hook bleeder resistors to the caps and discharge them, if you don't know what you are doing you can end up looking like Don King. Shocked

Some of your clips have a really nice just barely broken up sound. Is that the low gain setting of the lead channel? Hope you can get it stable cause that little baby works for you.
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kirk95 · Posted: Wed Mar 03, 2004 6:20 pm Post subject:

It's most likely a bad tube. Switch them out one at a time. I had the same problem with my Two Rock EM50 and it ended up being a preamp tube.

Good luck!
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elizabeth · Posted: Wed Mar 03, 2004 6:27 pm Post subject: Screeeeech?

Check to see if your cat's tail might be under the amp (or your foot.) Forgive me, I just COULDN'T resist! (I can't help myself sometimes!) Smile

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Leftbender · Posted: Wed Mar 03, 2004 11:17 pm Post subject:

Can someone please give Lizzy some guitar lessons and buy her a tube amp, so she starts to understand the real problems in life???? Laughing

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Bluelobster · Posted: Thu Mar 04, 2004 2:45 am Post subject: bulbs

But you know how many singers you need to change a lightbulb. Wink

elizabeth · Posted: Thu Mar 04, 2004 6:21 am Post subject: Change lightbulbs?

Absolutely none! We only need to stand there, because the Universe revolves around us! Besides, can't you just buy a new lamp? Or a new amp? How about a new vamp?

(It's so much fun having this alter ego...sometimes I almost take myself seriously.....nnnnnnnnnnotttttttttt!)

OMG, lobster, you illustrated my point. Lefty, check out that photo of the cat with its tail under the amp....SCCCRRRRREEEEEEEEECCHHHHHHHH!

Rrrrrrrowwwwwwwwwwl. Smile

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Leftbender · Posted: Thu Mar 04, 2004 6:57 am Post subject:

elizabeth · Posted: Thu Mar 04, 2004 7:04 am Post subject: A tube amp is female?

Oooooooh Lefty, I'm not going to say what I am thinking about tube amps being female... Shocked

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