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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 11:29 pm
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Monkeyleg wrote:
I hate to go resistor by resistor and capacitor by capacitor, in part because I'm worried about really screwing up the board (have to learn to solder well first), and also because of the time. The amp sounds okay to me, but I know it sounded better.

Oh, yeah. I don't fart around people or guitar equipment. ;)


That's why differential diagnosis can be a better way to go. Note the symptoms, try to tie them to which section(s) could be at fault. Then, rule-out each section one-by-one.

You should not have to go though the entire amp, component-by-component to fix a single issue. Maybe, for a complete overhaul.


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Post subject: Re: What burns out power tubes?
Posted: Fri Feb 07, 2014 11:42 pm
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BMW, if I could tie the symptoms to a section, I'd be looking there. ;)


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 1:40 am
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monkeyleg, I hear you. Well, to narrow things down --- after a few blown resistors, fuses, and dingle-berries on the sockets --- prolly can get down to the poor bass response issue.

IME, what effects the bass response (in no order):

1.) Idle bias of the output tubes. Not too cold or too hot.

2.) The strength of the tube rectifier.

3.) The filtering ability of the main PSU caps.

4.) Correct voltages at critical points.

4.) The strength of the output tubes.

5.) Strength of the gain stage tubes.

6.) Setup of components of the gain stages.

7.) Leaky coupling caps.

8.) Health of the power and output transformers.

9.) Status of the speakers.


Prolly starting points, to rule-out causes of original problems.


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 8:21 am
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1. BMW, I've adjusted the bias from 32 on up to 38+. It sounds good at either. I don't think there's a major difference. It's hard for me to compare the sound, as I have to take the time to make adjustments, and I forget any subtle nuances to the sound.

2. The rectifier tube is new, and the old one was good.

3. The five filter capacitors have been replaced, and they read fine. I haven't checked either the old ones or the new ones with a capacitor tester, so I'm assuming the new ones are good.

4. I'm using two different sets of brand new tubes, and the voltages for them show that they're putting out more than enough power.

5. I've swapped out every tube with new ones (one or two at a time), and there's been no change.

6. #6, #7 and #8 need to be checked.

9. The speakers have been tested six ways from Sunday.

Just judging from the admittedly limited amount I've learned so far, I'd say that the vibrato section and the input section would be secondary for testing. The power supply, bias supply and output sections would be priority for testing. Agree?

The amp sounded good after you'd given me some suggestions on things to test, and then we swapped out power tubes. Then, when I turned the chassis upside down, it stopped sounding good. Maybe a bad solder connection, or just a bad connection of some other type?

I've tested my ugly solder connections, and so did the tech. Ugly, but they work.

Onward.


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 8:35 am
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Your amp appears to have some poor contact. Flip the chassis over on the bench and start looking for hincky contact points. Jiggle the tubes. Are they loose in the socket? Check all leads, cables, etc.

Good luck!


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 8:48 am
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BMW, that was back on January 23rd. For a few hours, before I tried to fix what I'd messed up. I should have just left the burnt wires and (maybe) zapped R70 and R71 resistors.


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 11:46 am
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OK, I recorded the amp at four volume levels, and posted it on Youtube at http://www.youtube.com/watch?v=xJMDIqB4 ... e=youtu.be

The first chords and strums are at volume 3. The second are at volume 4, the third at volume 5 and the fourth at volume 6. The bass and treble are at mid-range. The guitar is in the clean channel.

Bias is at 38. When I move from 33 to 38 on bias, I don't hear a difference, or at least I don't remember a different sound after the time needed to change the adjustment.

The distortion you hear increasing from volume 4 to 6 is present with every type of preamp tube and power tube I have.


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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 12:59 pm
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That sounds like a guitar problem to me. Not only out of tune, but maybe the pups too close to the strings. What kind of guitar and pups?

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Post subject: Re: What burns out power tubes?
Posted: Sat Feb 08, 2014 1:18 pm
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shimmilou, I've tested this with three different guitars. A Fender American Standard Stratocaster with Klein 59 pickups, a Fender special edition Stratocaster with HSS Fat 50's pickups, and a Squier Bullet Stratocaster with who-knows-what pickups. The Squier has heavy (.012 through .058) strings on it, the American Standard a bit lighter at .011 to .050, and the special edition with standard strings.

I have string heights set differently. The heavier strings have to sit higher. Even when I've had the strings set really high for using a slide, I never got this kind of distortion.


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