Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Cabinets-Speakers => Topic started by: dogfish on July 13, 2014, 11:40:40 am
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For better or worse I am planning to use a champ amp for home bass and guitar practice. There are obvious limitations and it's not ideal however it's just the right size for my apartment. Plus, I have big amps at my practice space. That said is there any advice you can give me on tuning the champ cab or otherwise modding it to make it more ideal bass and guitar. I'm planning on using an alpha eminence 8" PA speaker.
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... it's just the right size for my apartment. ... any advice you can give me on tuning the champ cab or otherwise modding it to make it more ideal bass ...
I suggest forgetting the notion of "tuning it for bass" and just accept it for what it is.
A Champ cab and an 8" speaker really don't reproduce a guitar's bass well, so they're not going to do the extra octave down any better. But you have bigger, better amps & cabs. So the Champ doesn't need to be "good & loud" just "good enough for apartment practice." So I wouldn't do anything special for this speaker & cab.
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The bass can tax the O/T because of the single end amplifier. Check out the specs for replacement O/T's and most do not handle the low frequencies very well.
To be clear, the output transformer won't care either way, meaning it is not "being stressed" by playing lower frequencies.
However, an OT that isn't rated for lows simply won't pass the full output power of the amp to the speaker at those frequencies. Think of it like a fixed EQ (low roll-off) imposed by the OT iron.
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... "aren't SE transformers subject to saturation at lower frequencies, and therefore, SE transformers that handle lower frequencies larger by design ...
ALL transformers have a low-frequency limit imposed by the size of their cores, so to go lower they need to be bigger. This is about power through-put, so if you reduce the applied power by an order of magnitude, the low end extension increases by an order of magnitude. So it's really a power-bandwidth product.
What is being used up is the ability of the core to handle/transfer magnetic force created by the power applied to the primary winding.
... and therefore, SE transformers ... require air gap design considerations?
All SE transformers use up a portion of the core's magnetic capability in the unbalanced direct current through their primary, which is the output tube's idle current. The air gap helps matters, but doesn't eliminate the unbalanced d.c. issue. OT manufacturers usually specify a maximum unbalanced d.c. for their SE transformers.
The follow up, doesn't a transformer that is saturated gets hotter?
Not necessarily. Or at least, not due to the saturation.
Heat is caused by I2R losses, also known as copper loss. "I2R" means "current2 times resistance" (of the copper winding), which is one variant of the equation for power. So high current through a high resistance winding implies large power dissipated as heat (which is not usually an issue in typical guitar amp transformers).
Eddy currents in the core can result in heating of the core, but that is why the cores are usually made of laminated steel plates.
So really, core saturation will cause the transformer to be unable to pass more power through it, or to have its bandwidth restricted, but doesn't necessarily create heat without some other fault occurring at the same time.