# stuck on a bassmaster



## parkhead (Aug 14, 2009)

I have a bassmaster on the bench 

when you flip the standby it burps, then if you play through it you get cutoff type distortion 
the note comes out all distorted and dies right away like a transistor is biased into cutoff 
there is also a ring modulation type sound but no hum 
all new tubes

voltages all measure good
grounds are good 

no leaking signal caps 
I have been subbing in filter caps and no change 

Ideas ... I have no scope and can usually track stuff down by ear

p


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## nonreverb (Sep 19, 2006)

Output transformer readings?


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## Wild Bill (May 3, 2006)

parkhead said:


> I have a bassmaster on the bench
> 
> when you flip the standby it burps, then if you play through it you get cutoff type distortion
> the note comes out all distorted and dies right away like a transistor is biased into cutoff
> ...


Hmm...when you say all voltages measure good, does that include cathode voltages in the preamp stages? It sounds like maybe a 1K5 resistor is open, or the bypass cap may be shorted. Either would shift the bias on that stage into silly territory.

A long shot might be one side of the OT is shorted. You would of course see plate voltage but that tube would be trying to drive a shorted load. Pete used big ass trannies, though. As I said, a long shot.

Wild Bill


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## dcole (Oct 8, 2008)

This sounds like what was happening with an Ampeg SVT Classic I just had on my bench. I thought I had it fixed up, sounded good on my test speakers but the owner just informed me that it has the same old problem once he got it home. Is the sound affected at higher volume or at any volume?


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## loudtubeamps (Feb 2, 2012)

I would take a close look at the 470 ohm /10 watt screeen resistor.


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## parkhead (Aug 14, 2009)

Wild Bill said:


> Hmm...when you say all voltages measure good, does that include cathode voltages in the preamp stages? It sounds like maybe a 1K5 resistor is open, or the bypass cap may be shorted. Either would shift the bias on that stage into silly territory.
> 
> A long shot might be one side of the OT is shorted. You would of course see plate voltage but that tube would be trying to drive a shorted load. Pete used big ass trannies, though. As I said, a long shot.
> 
> Wild Bill


yes cathode voltages are good, but I was thinking along the lines of a shorted or open cathode... I have set the amp aside to think for a bit 

My pop tests indicated that there was distortion in the output when popped at all pre-amp stages I have been looking at the inverter but should 
look a little deeper, I am missing something 

last time I had a problem amp it had a severed presence resistor... it would play great till vibration disturbed the connection then it would go wild 
2 or three techs missed it and in the end I just set the amp aside and thought about the noise till I knew where I had to look... even then I took some good 
digging to find the bad connection 

keep those Ideas coming 

BTW this amp has the 66 monster output transformer if its bad... it will be interesting 

p


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## Wild Bill (May 3, 2006)

parkhead said:


> yes cathode voltages are good, but I was thinking along the lines of a shorted or open cathode... I have set the amp aside to think for a bit
> 
> My pop tests indicated that there was distortion in the output when popped at all pre-amp stages I have been looking at the inverter but should
> look a little deeper, I am missing something
> ...


Sensible, PH! When you get a spooky problem sitting back and thinking it through is the only way.

This is what amuses me about some of the "techs" that crop up periodically. They never took the time to learn any theory. They just saw a video on youtube about how to bias an amp. So they set up in business and do ok with the really easy stuff. When they get a REAL problem they tell the customer the amp can't be fixed, or that it would cost too much and not be worth it.

I've found these guys don't last very long. Customers eventually start figuring them out. Still, it can be a pain in the ass when these guys scoop the easy stuff and you get all the tough jobs that raise your blood pressure!

Also, having to clean up some idiot's mess is never fun, either!

Oh well, there's worse to come. It's been announced that ABBA is making a comeback tour next year!

Oh, the humanity!

Wild Bill


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## parkhead (Aug 14, 2009)

Part of my problem is that I am not a tech... If I were working on amps daily I would have fixed my scope and would be faster at the tricky problems. 
I get drafted by buddies who usually have to wait forever for me to have "spare time" ... 
one brings me his collectable amps and says I know when you are done it will be fixed and look dead stock... 

I will always have a peak at a great sounding unique vintage amp ... and I have been lucky enough to have a good look 
at some killer vintage modded amps (old school & old guy mods ) that have provided some insight into some of the all time classic 
guitar tones...

There are many great techs around now. when I was starting to do my own stuff 1991 it was triggered by an incident where I took my deluxe reverb 
to the the local store and asked their tech to change the filter caps. I paid for the work then when I got home decided to open up the cap can and check the workmanship...
the original filters were still there in all of their leaky glory. Within a year that store was bankrupt, I bet the owner just wrote me out a bill and took my money... since the amp still worked ... 

I could already solder... having replaced pickups in guitars... the rest was background reading and sourcing parts 

P


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## nonreverb (Sep 19, 2006)

I've always been willing to take a loss on a repair from time to time. It becomes a personal challenge if it's a complex problem or, a discreet one I haven't seen before on a common model . Ultimately you win twice in this scenario as the customer is very happy (particularly if they've taken it elsewhere with no results) and it's a new problem to store in the data banks for the next one with a similar problem.
Consequently, I have enough work to keep me busy for a long time which I'm thankful for.:smile-new:


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## parkhead (Aug 14, 2009)

Ok I should have known better

here's the list so far 
Bad coupling cap in the output 
really bad bias caps = ring modulation sound 
really bad filter caps = secondary ring modulation sound 

normally if it were my amp 
I would have started with all of the electrolytics and 
then only had to find the bad coupling cap 
but since it was a "look see" situation for a friend I didn't want to invest this much time 

oh well 

p


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## Wild Bill (May 3, 2006)

parkhead said:


> Ok I should have known better
> 
> here's the list so far
> Bad coupling cap in the output
> ...


Yeah, a "look see" just doesn't work with electronics. Parts can look perfect on the outside and be totally snackered inside! 

That's the trouble with doing such for a friend. In a regular customer situation you shouldn't do it at all! Many players don't understand that it's not like a car, where you can often see something is broken. With an electronic circuit, you have to take measurements until you find something screwy and then spend some time thinking about what would CAUSE the screwiness!

This can take a LOT of time, as this amp you're working on has shown!

With electronics, unless you are really lucky and the problem is "Homer Simpson" obvious, an effective "look see" might as well be done with a ouija board and psychic powers. You are guaranteed to be sucked into a spiral of more and more time spent on the problem.

My policy has been to charge a minimum 1 hour labour and to give the customer a "best guess" at the start, with a promise that if after crawling inside the amp I find things looking much uglier and more expensive than we thought I will stop and call him to let him decide about taking things further. If he's a decent sort I usually accept a "spooky" amp problem as a learning challenge and not charge for most of the extra time.

If he's a mean and grumpy sort I smile and plead a heavy backlog, while handing him a card from the worst tech in town! I figure they deserve each other!:sFun_dancing:

99% of my customers are good people and have been great to me. They come first! There is no law saying I have to fix an amp for an unreasonable crankbag.

That all being said, I too sometimes make the mistake of a "look see" for a friend. Murphy's Law says it ALWAYS turns into a "time sucker"!

Still, it's part of my hippy roots. If you're too ruthless in business you will reap such ruthlessness right back at you!

Once again, as said by Ted "Theodore" Logan and Bill S. Preston (Esquire) - "Be excellent to each other!"

Wild Bill/Busen Amps


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## WCGill (Mar 27, 2009)

parkhead said:


> Ok I should have known better
> 
> here's the list so far
> Bad coupling cap in the output
> ...


Bad as in leaking DC?


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## nonreverb (Sep 19, 2006)

Ah yes, the two edged sword! I almost always accept the challenge of an amp with a spooky problem. Problem is, you become the go to guy for spooky amp problems. Can be challenging to make much money off them however, my conscience compels me to do so and I learn something...always a good deal.



Wild Bill said:


> Yeah, a "look see" just doesn't work with electronics. Parts can look perfect on the outside and be totally snackered inside!
> 
> That's the trouble with doing such for a friend. In a regular customer situation you shouldn't do it at all! Many players don't understand that it's not like a car, where you can often see something is broken. With an electronic circuit, you have to take measurements until you find something screwy and then spend some time thinking about what would CAUSE the screwiness!
> 
> ...


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## low4 (Oct 7, 2013)

Just wondering is it fixed now?


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## parkhead (Aug 14, 2009)

yes... If I had started with a cap job I wouldn't have got stuck 

p


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## bolero (Oct 11, 2006)

Wild Bill said:


> .
> 
> ......If he's a mean and grumpy sort I smile and plead a heavy backlog
> 
> Wild Bill/Busen Amps


hahaha..man, I sure hope you don't think I'm "mean and grumpy"!!


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