Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on December 17, 2021, 07:52:03 am
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Hi,
My cousin asked me to build me a clean, small 40w or so amp for use with his electonic keyboard. He's not a pro, but does play at coffeehouses now and then. I built him a Fender AB763 circuit with no reverb or trem; just treble/mid/bass and a raw switch so he can play through the amp and hear the pure piano tone without tone shaping. I have keyboard here and with a 12AU7 in V1, it sounds pretty good and is plenty loud. His line out is line level, with volume control and I have been playing it with the volume control pegged, then adjusting the volume on the amp manually.
My questions:
1. Will the high frequencies keyboards put out damage the speaker even if I don't run the amp super loud?
2. Would I be better off buying a PA speaker with a flatter frequency response and possibly, a coaxial design to provide a high frequency driver?
I know very little about keyboard stuff so am just looking for general guidelines and best practices.
Thanks!
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Keyboard should work just fine playing through a speaker designed for guitar amps.
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I belong to a Fender Rhodes FB group. (I like the repair/rebuild posts/threads. :laugh:)
A lot of those guys LOVE playing their Rhodes through a Fender BF Twin.
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Thanks for the responses! It sounds pretty darn good so I'm inclined just to leave it, as long as I'm not in danger of blowing anything.
I'm running the 6L6s cold but I just played the keyboard through it yesterday for the first time, and haven't taken a ton of voltage readings. I'm going to check the current with the volume up pretty good on the amp and piano, playing hard, and see what I'm getting. The transformer secondary is rated at 230ma so I've got a good amount to play with.
Thanks!
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there was a time when there weren't "guitar" or "keyboard (organ)" speakers, there were just musical instrument speakers. that's why i can pull out organ speakers and be generally assured they'll sound pretty good in a guitar amp (i have pulled some $$$ jensens out of a few organs, including the exact speaker that came in oooold princeton amps). with the switch from valve/solid state to digital tone generation ~1980s speakers were a lot more likely to be more 'hi fi'
that is to say you'll probably be fine. if your friend needs more/better amplification than your ~40w amp can produce a solid pa is the better route to go (1000s of pa watts are dirt cheap). but that's more likely needed in bigger venues that a coffee house
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Thanks. Yeah, he just wants something small for the occasional time when he needs amplification, and wants something tube-based so this should fit the bill well for him.
I just threw in a Celestion Classic Lead 80w in there for testing purposes. I'm inclined just to leave it. With a solid state rectifier, choke and a lot of filtering, it's got clean for days.
Thank you!
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+1
That sounds like a great amp for the job.
If he's desperate for more highs (the extended range of Keyboards) you could fit a compact piezo horn as well (Roland, Trace Elliot and others have done this). Just try it out first (externally to box), a piezo horn with Greencap say 1u (100V rated) in series and placed across your Celestion Speaker terminals.
Regards
Mirek
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.....those guys LOVE playing their Rhodes through a Fender BF Twin.
Rhodes has the same 'problem' as a (real) piano or guitar. String stiffness throws the highest overtones out of harmony with the fundamental. The classic tweak is a low-pass, 2kHz to 7kHz. Since the classic paper cone speaker can have a sharp peak/drop response, this can work good for piano or Rhodes and guitar.
Even more if using distortion. (Again, the high harmonics go sour.)
If you "need" 13kHz synth-squigglies like Unbroken Chain in Mars Hotel (or the Captain's Muskrat Love), then you will need a tweeter. In stage use, a substantial tweeter. To balance with a moving cone woofer, this may cost more than a woofer (yes, for just 2 octaves).