low string nasty

[kollantai]

I just picked a 59 bassman clone for super cheap. 500 bucks. My problem is that it sounds great at low volume but when i crank up half way it gets some pretty nasty distortion coming from a couple of the speakers. I'm thinking that it's speaker related having gone through the amp resoldering points and doing some bias and voltage measurments. Although it seems like it's dissapating alot of power. 445 on the plates and -46 to the grid. I used the resister shunt method and was getting some pretty high calulations compared to what I get from my JTM45. like 33 watts or something. Any suggestions fellas?

I feel pretty strongly a speaker or two is done.??!! What about that dissapation??

[Plexi]

Hmm its biased a little on the hot side. Best way is to try different speakers,and see if it is the speakers or the amp.. If its only heard in 2 speakers. Now..things that might also be causeing it.. You can have a part vibrating when you crank the amp,and it can make a buzz noise that sounds like a blown speaker. And may not do this till you crank the amp some. So try the amp out of the cab..

Loose speaker spade terminals can cause a speaker to sound like its blown.
You may have an oscilation when you turn up, you may be able to hear it,or see the tubes react to ultrasonics when you turn up..
The last ..the OT can do exactly this..sound ok when low volume.crank the amp,and hit guitar chords pretty hard..low chords like E..You can hear them crap out.

Weird things..some tubes will vibrate and cause noises in the spekers..
metalic sounds can be a cathode cap or resistor off value.

So there are some things to try..do the easy one first.

Richie

[kollantai]

Hey thanks Richie. I'll try the easy stuff first.

[kollantai]

well I think the speakers are fine and i checked the resistor values and all seems well with that. The degree at which it does this was lessened somewhat by removing from the cab but i tried a different set of 6L6's and got the same result.

The nasty distortion continues, like I said, the thing sounds great when turned down. Turn it up and sounds like crud, the distortion continues as the guitar volume is backed off and the chord that was played sustains. How could i find a bad cap? I looked for shorts found nil.

How would i get rid of any oscillation if this is what I've got? Tye all the grounds together? resistors somewhere?

[Plexi]

59 clone.. is it useing 12AY or do you have 12AX7 in the preamps?
Both channels doing this? I'm sure you have tried different preamp and driver tubes?

To cure oscilations.. Shielded cable from the inputs..with the grid stopper on the tube pins. Shielded cable from mixer resistors.. 270k and one should may have a bypass cap. use shielded cable to the pin 2 of V2

Then again its hard to say without hearing it,and just guessing on things here. If it made it a little better out of the chassis,you might also look for loose connections wires etc.sometimes they are overlooked or hard to see them. Then hopefully its not a bad OT. Thats kinda what it sounds like. Have you measured the OT primary sides?

Richie

[kollantai]

it's using the 12ay7. i've switched out all the tubes aside from the power tubes.

I measured 50ohms on one side of the primary OT and 45ohm on the other. It's an axiom with the single 2ohm secondary.

I have a share pair of 6l6's I might try. It has some old RCA's in it now.

[kollantai]

new power tubes didn't work either. I'll go through it one more time to look for loose connections before I come to grips with a bad OT.

[Plexi]

Sometimes the OT measurements can tell if something is screwy,sometimes not,i've seen them almost measure dead on,and be bad,and those were ones that sounded good clean,but crank the amp,or hit a hard note string..and it crapped out lost power.

Those OTs are good ones that is in the amp.but you never know how the amp was used or abused before.
I don't know if you have one to just sub in and try for a test. Not good advice,but you can disconnect it,and just use jumper leads to wire in a sub OT for testing.

Have you checked voltages through the amp? that can sometimes tell if you have a problem. The PI/driver check those resistors,they do go out sometimes,the 100k and 82k that section. If one of those is way off,or dead,you only halve half an amp. Screen grid resistors..and grid stoppers/bias

Richie