fender bassbreaker 45

[ilyaa]

wondering if anyone has any documentation/schematics on this amp?

everything works and ive got a solid swing coming from the PI to the power tube grids -

but the output section is not happening - im getting normal plate voltage but ZERO screen voltage and -3VDC or so grid voltage - problem is this amp has a pretty complex thing going on with the variable output and mosfet control B+ (or whatever it is!) -

i know schematics are hard to find for these - anyone have thoughts on a similar circuit i could use for reference?

[joecool85]

wondering if anyone has any documentation/schematics on this amp?

everything works and ive got a solid swing coming from the PI to the power tube grids -

but the output section is not happening - im getting normal plate voltage but ZERO screen voltage and -3VDC or so grid voltage - problem is this amp has a pretty complex thing going on with the variable output and mosfet control B+ (or whatever it is!) -

i know schematics are hard to find for these - anyone have thoughts on a similar circuit i could use for reference?

Did this amp just recently come out?  It must be under warranty.  If so, contact Fender.  If not, you'd still need to contact Fender lol.  Good luck either way!

[ilyaa]

its gone back for a warranty repair already -

the owner wants to make sure whatever happens doesn't keep happening!

fender is probably gonna give me the 'you're not an authorized service center' spiel and the cold shoulder

[ilyaa]

yeah - called em. no dice. amp is still under warranty.

he can send it back but i think he's a little hesitant to do so seeing as he has already - doesn't feel like going through the headache again and would rather have an amp that doesn't need to be mailed back to fender every few months!

i wonder: with the variable voltage thing going on in here - would it be possible to bypass the fancy  fet-controlled variable voltage stuff and just rewire this amp like a good old-fashioned normal tube amp? i think the variable voltage is just controlling the screen voltage and the bias (bias tracking or whatever its called) - anyone have any experience doing something like that?

[J M Fahey]

Google "power scaling"  and read what you finf, there´s various schematics posted online.

Usual (and the real thing)  varies the whole +V voltage, tube plates and screens , plus to compensate also varies fixed bias.
Of course MosFet dissipates a lot of power, gets very hot, etc. really needs a good heatsink and a fan, tube amp guts are very hot .

Cheesy versions *just*  vary screen voltage, (plus tracking bias), way cheaper, less dissipation, in my view they miss the boat.

So first read about the full one, then the screen controller, and compare that to what you have.

Worst case you will be able to bypass it, feed plates straight to +V (I think you already have that) , screens also to +V but a secondary one,you probably will need to add an extra RC section (1k 5 or 10W plus a filter cap) for them.

Can´t get into much detail lacking the proper schematic but that´s the basic idea.

Although I´d personally send that back to Fender and ask for a refund, since they can´t make it work. Period.

[DrGonz78]

Although I´d personally send that back to Fender and ask for a refund, since they can´t make it work. Period.

Yeah no doubt!! Crazy that they make these amps like this now a days!!

[g1]

A common failure with power scaling amps is that the power Fet shorts, leaving the amp stuck at full power all the time.
Too bad that does not seem to be the situation here, as that seems to be something the owner would accept in this case.
You are kind of stuck either drawing out the schematic, or finding it somehow.
I'm sure it is possible to bypass the power scaling like JM metioned.
Was the unit repaired or replaced last time (serial # would be different)?
If it's on the 'do not repair' list, they will just replace the unit, and there may not be a schematic.
If it's not on the DNR list, there must be a schematic somewhere, even if it's only available to authorized repair centers. (hint hint if you know of anybody  )

[J M Fahey]

Dear ILYAA: don´t know where are you from.

If USA, and within warranty period, send it back, period, and get a real JTM45 or build one out of a Ceriatone kit, a suitable prebuilt cabinet and your choice of speakers , no need to make everything from zero up.

If in  Country with no *real*  access to Factory resources and where getting a full refund is a problem (such as here in Argentina), then it may be worthwhile trying to reform it, but it won´t be a standard repair job , it will be reengineering , and definitely expensive. (not in parts but in applied knowledge).

Don´t understand why Fender Authorized Service could not repair it or worst case get a replacement unit from Factory.