Ampeg SVT3 Pro Bass Amp
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
Ampeg has not been getting good reviews in the Customer Service Dept, (no parts)
some people have gone for a non orig. replacement from a different vendor,
i was thinking of sticking in a regular pwr trans, cheaper but still $$,
people will throw away the amp and but a new one if faced with a big bill for fixing this thing,
so what do we do? charge the poor musician $225 plus labor? they end up with a huge bill for an amp that will break again,
no, we can rewind the toriod xfmr, this is cheap and they get the original part that will not break, don't have to through out the amp -win-win situation,
luckily the offending winding is the last one wound, so we can leave the two primaries alone and also one of the low volt secondaries, the outer sec was meshed together with burnt insl. and a burnt up HV wind, so we had to strip it off,
tried a stick to rewind but ended up peeling the wire off the stick and just shoving the wire thru the toroid, as the wire is thick enuff to hold it's shape, >
some people have gone for a non orig. replacement from a different vendor,
i was thinking of sticking in a regular pwr trans, cheaper but still $$,
people will throw away the amp and but a new one if faced with a big bill for fixing this thing,
so what do we do? charge the poor musician $225 plus labor? they end up with a huge bill for an amp that will break again,
no, we can rewind the toriod xfmr, this is cheap and they get the original part that will not break, don't have to through out the amp -win-win situation,
luckily the offending winding is the last one wound, so we can leave the two primaries alone and also one of the low volt secondaries, the outer sec was meshed together with burnt insl. and a burnt up HV wind, so we had to strip it off,
tried a stick to rewind but ended up peeling the wire off the stick and just shoving the wire thru the toroid, as the wire is thick enuff to hold it's shape, >
Attachments
yes, the good, the bad, the horrible, ;D
we fix em all, and if we can't, we fix them so nobody else can,
that thing does have a few tubes in it if you look close,
that is what the HV wind is for,
i just hope it works after the xfmr rewind, you know, symptoms vs real cause ,
we fix em all, and if we can't, we fix them so nobody else can,
that thing does have a few tubes in it if you look close,
that is what the HV wind is for,
i just hope it works after the xfmr rewind, you know, symptoms vs real cause ,
Stupid thing with Ampeg and this amp...
They never matched the output MOSFETs and instead tried to strike a balance in the output biasing. This would result in some amps not really reaching their full output potential or over time getting low-level distortion as the bias was too cold.
I struck a "better" balance one last year only to have the one "hot" MOSFET short out under load and take out the whole output section.
As long as you're in there...You might want to check the MOSFETs and match up a set if they don't bias together.
They never matched the output MOSFETs and instead tried to strike a balance in the output biasing. This would result in some amps not really reaching their full output potential or over time getting low-level distortion as the bias was too cold.
I struck a "better" balance one last year only to have the one "hot" MOSFET short out under load and take out the whole output section.
As long as you're in there...You might want to check the MOSFETs and match up a set if they don't bias together.
dude you are a mind reader,
i was just getting ready to post up on that,
this thing had a weird distortion problem that was compounded by the silly Tube Gain control,
so i check all the test points, replaced a shorted coupling cap in the preamp, (C18 .1 100V poly) things get a little better, then i find out the guy had a 12ax7 in the 12au7 socket, too much gain = more distortion, so i fix that, things get even better, but still that weird fuzz at the end of the notes,
recheck the test points, scour the web under "Google-SVT3 problems", and up pops the drama you are talking about, there is even a 2007 thread in the Lab about this,
touchy bias trim pot, badly matched fets, cold bias current, and even incorrect power rails due to the toroid pwr xfmr being wound with a 100 volt primary,
and we have a "hot" fet, 7 read about 10-15 mv across the .47's, the hot guy reads 35 mv, it is all here>
blog #1
http://music-electronics-forum.com/t10203/
blog #2
http://www.talkbass.com/threads/svt3pro-bias-setting.437406/
here is the pwr section, do not understand how it works as there is no phase inverter, both banks get the same signal, in phase, it does have good rail centering with the DC servo as there is no output cap, this amp has been a learning experience to say the least, and a real PITA, ;D
but there is light at the end of the tunnel, as soon as i rewind the xfmr to get +/- 65 instead of +/- 80,
and yes, the fets were reading about 1 mv across the .47 resistors, which is why we had the cutoff distortion,
was wondering if we could match the fets by playing with the 47 ohm gate resistors, otherwise Mouser will sell me 10 ea. for $1.79 a piece,
http://www.mouser.com/Search/Refine.aspx?Keyword=IRFP-240
here is a chunk of the pwr amp>
i was just getting ready to post up on that,
this thing had a weird distortion problem that was compounded by the silly Tube Gain control,
so i check all the test points, replaced a shorted coupling cap in the preamp, (C18 .1 100V poly) things get a little better, then i find out the guy had a 12ax7 in the 12au7 socket, too much gain = more distortion, so i fix that, things get even better, but still that weird fuzz at the end of the notes,
recheck the test points, scour the web under "Google-SVT3 problems", and up pops the drama you are talking about, there is even a 2007 thread in the Lab about this,
touchy bias trim pot, badly matched fets, cold bias current, and even incorrect power rails due to the toroid pwr xfmr being wound with a 100 volt primary,
and we have a "hot" fet, 7 read about 10-15 mv across the .47's, the hot guy reads 35 mv, it is all here>
blog #1
http://music-electronics-forum.com/t10203/
blog #2
http://www.talkbass.com/threads/svt3pro-bias-setting.437406/
here is the pwr section, do not understand how it works as there is no phase inverter, both banks get the same signal, in phase, it does have good rail centering with the DC servo as there is no output cap, this amp has been a learning experience to say the least, and a real PITA, ;D
but there is light at the end of the tunnel, as soon as i rewind the xfmr to get +/- 65 instead of +/- 80,
and yes, the fets were reading about 1 mv across the .47 resistors, which is why we had the cutoff distortion,
was wondering if we could match the fets by playing with the 47 ohm gate resistors, otherwise Mouser will sell me 10 ea. for $1.79 a piece,
http://www.mouser.com/Search/Refine.aspx?Keyword=IRFP-240
here is a chunk of the pwr amp>
Attachments
You could build a jig, buy a box of MOSFETs and match them yourself. I wussed out and bought a matched set from an Israeli guy on eBay. It was kinda pricey...But so was buying a box of MOSFETs hoping I could find enough to make a match.
I have another one coming in that I'll probably have to do this for as well.
I have another one coming in that I'll probably have to do this for as well.
did you get some humm when you biased up the fets?
nightmare #2:
this shorted cap shown in the pic had the test points reading funny, but we did not know about the shorted cap yet,
so we check the schematic and see that the resistors in the cathode-grid circuit do not match the ones stuffed in the board. silkscreen mix up?
so we change the resistors to match the schematic and now the amp does not work at all.
so we find the shorted cap, fix it, and find that the amp still does not work,
we analyze the circuit and see that the board is right, the schemo is wrong,
so we change back the resistors and the thing works now,
weird luck on that one,
at this point, we pray for the next job to be a Fender Twin Reverb, ;D
nightmare #2:
this shorted cap shown in the pic had the test points reading funny, but we did not know about the shorted cap yet,
so we check the schematic and see that the resistors in the cathode-grid circuit do not match the ones stuffed in the board. silkscreen mix up?
so we change the resistors to match the schematic and now the amp does not work at all.
so we find the shorted cap, fix it, and find that the amp still does not work,
we analyze the circuit and see that the board is right, the schemo is wrong,
so we change back the resistors and the thing works now,
weird luck on that one,
at this point, we pray for the next job to be a Fender Twin Reverb, ;D
Attachments
I was not wrong about this amp then?
Ampeg used to have a good name at one time. I guess accountants got hold of the company and kicked the engineering guys out.
The old style phase splitter on this amp used to be one of its attractions to give a bit of edge, same old story, they had to fix something that wasn't broke!!
CJ, your long service medal for working on crap amps is in the post
best
DaveP
Ampeg used to have a good name at one time. I guess accountants got hold of the company and kicked the engineering guys out.
The old style phase splitter on this amp used to be one of its attractions to give a bit of edge, same old story, they had to fix something that wasn't broke!!
CJ, your long service medal for working on crap amps is in the post
best
DaveP
Hey CJ....Just caught something....You're saying that all of the MOSFETs are the same, but I don't think that's right. You should have four N-Channel and four P-Channel which is why you don't need a phase inverter.
http://www.ebay.com/itm/8-PIECES-MATCHED-IRFP240-IRFP9240-MOSFET-FOR-PASS-F5-TURBO-AMPLIFIER-/150494316878?pt=LH_DefaultDomain_0&hash=item230a29094e
Something like that should get you going.
You might be able to fuss with what you have and make it work...But for long, healthy life and less smoke...I'd advise an upgrade.
http://www.ebay.com/itm/8-PIECES-MATCHED-IRFP240-IRFP9240-MOSFET-FOR-PASS-F5-TURBO-AMPLIFIER-/150494316878?pt=LH_DefaultDomain_0&hash=item230a29094e
Something like that should get you going.
You might be able to fuss with what you have and make it work...But for long, healthy life and less smoke...I'd advise an upgrade.
youre right ! doh on me, ???
IRFP9240 and IRFP280R
thanks for that, saved a nuclear meltdown, ;D
IRFP9240 and IRFP280R
thanks for that, saved a nuclear meltdown, ;D
But we *learn* from meltdowns
> check this weird heater supply, they use DC, a series string of two tubes for 24 volts, and tie it into the fan and temp circuit
Rube Goldberg would be proud.
Someone wanted to be designing iPods or cellfones, got stuck at Ampeg, and decided to string-together as many different functions as possible.
True, there IS a lot of stray crap and various power demands in a big over-stuffed amp, so sometimes you want to merge stuff.
Ah, I see.... "+65V" is the sand-state power rail? So that was handy.
I'm really upset that removal of V1 (perhaps for testing) will kill the fan. "Maybe" the output bottles won't overheat without their blow, but in a "testing" dilemma, who knows?
It is also pretty fussy about the specific fan used. 150ma must make a good wind. A lower-current fan will tend to over-volt; a higher current fan will not spin well. Since we "always" buy fans by VOLTS, this is "wrong".
They did have to spot a large film-cap across the fan to reduce fan-has around V2 wiring.
Failure of D23 would be bad too.
At least we have pure right-Volts DC at V1 V2 heaters.
Hey! I d not recall overheat lamps, vari-fan, or DC heat in a 1970 SVT. Did the music suck so bad that we need these modern complications?
I don't even see a need for fan modulation. A CHAMP can play louder than a fan. If fan-whirr offends you, you are not *using* the SVT the way it was made to be used (LOUD!!).
Rube Goldberg would be proud.
Someone wanted to be designing iPods or cellfones, got stuck at Ampeg, and decided to string-together as many different functions as possible.
True, there IS a lot of stray crap and various power demands in a big over-stuffed amp, so sometimes you want to merge stuff.
Ah, I see.... "+65V" is the sand-state power rail? So that was handy.
I'm really upset that removal of V1 (perhaps for testing) will kill the fan. "Maybe" the output bottles won't overheat without their blow, but in a "testing" dilemma, who knows?
It is also pretty fussy about the specific fan used. 150ma must make a good wind. A lower-current fan will tend to over-volt; a higher current fan will not spin well. Since we "always" buy fans by VOLTS, this is "wrong".
They did have to spot a large film-cap across the fan to reduce fan-has around V2 wiring.
Failure of D23 would be bad too.
At least we have pure right-Volts DC at V1 V2 heaters.
Hey! I d not recall overheat lamps, vari-fan, or DC heat in a 1970 SVT. Did the music suck so bad that we need these modern complications?
I don't even see a need for fan modulation. A CHAMP can play louder than a fan. If fan-whirr offends you, you are not *using* the SVT the way it was made to be used (LOUD!!).
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