Acoustic amp

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Acoustic amp

Postby johnf » Thu Mar 20, 2014 2:30 pm

I'm thinking about an acoustic amp (combo style but with the amp in it's own space above the speaker) with tone controls to drive a 16 litre bass reflex cab with a Celestion TF0818 (94dB) plus tweeter.
I've tested my guitar with Fishman preamp through a +20dB preamp and ST-35 into a similar sensitivity speaker and feel tha overall gain is ok.

I thought of using a PP EL84 circuit with a Baxandall tone control preamp at the front (both using 6SL7) The tone circuit has around 30dB of gain at tone flat. I figure 6SL7 will be slightly less gain and I could always lose the cathode bypass caps to lower it further if necessary. The tone circuits came from here. ... one-A.html.
Would use a solid state PSU

Push-Pull-EL84-6BQ5-6V6-6AQ5-Dynaco-A-410-Tube-Amp-Schematic.jpg (136.88 KiB) Viewed 2665 times

jt_consolette_01-18-06.jpg (104.68 KiB) Viewed 2665 times

Amp-Tone-4-A-A.jpg (57.11 KiB) Viewed 2665 times

So it is really an audio amp
Questions are to do with joining the two together and where to put the volume control. I've looked at a couple of versions of the Ampeg Portaflex circuits which use 6SL7 but they put the tone controls like the diagram below in one version with the volume after that and one version with the volume after the first gain stage and the tones between the second stage and the phase splitter. They also have a 120k input resistor with a 2-5M resistor to ground at the input.
Should I input via the usual 68k grid resistor and 1M to ground and lose the 0.047 cap?
Also thought of running the 6SL7 heaters from DC as the tranny has a 12.6v winding. Also possibly sub 6V6
Any comments on the circuits / project welcome.
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Re: Acoustic amp

Postby Jerryz1963 » Sun May 11, 2014 10:04 pm

I have a schematic I can share with you that beats those you are considering.

Hi fi tone controls aren't necessarily optimal for a guitar amp because a guitar's frequencies are more limited. That's why you don't see hi fi tone controls in guitar amps. There is a free program on line that lets you change the values in typical guitar amp tone stacks and it shows you the response curve as you vary the controls. You might want to look at that.

The question is, what are you using? a microphone? pickups? That may have some bearing on what you decide to implement for an input stage. I realize this is a tube site, but I believe in using the right tool for the job, and a JFET is pretty hard to beat as the input device. they are also extremely easy to work with.
The advantages are: Low noise (tubes are noisy). they sound like tubes (look at the curves). high gain (you won't need so many gain stages), and very inexpensive and very easy to implement. when you have a high gain amp like a guitar amp, power supply hum is more problematic. Having a JFET for the first stage with a well-filtered power supply helps somewhat in that regard.

That first schematic you posted, I wouldn't waste my time.

The EL84 became ubiquitous because its CHEAP. It was DESIGNED to be CHEAP. Cheap to purchase, and cheap to implement (easy to drive, you can use cathode bias). but it has horrible specs. Good news is, anywhere you see an EL84 being used, you can use a 6V6. The only alteration necessary is the cathode resistor. Everything else can stay the same.

If you were making an electric guitar amp, I would tell you there are lots of differences between an electric guitar amp and a hi fi amp and those differences are there for a reason. since you are talking acoustic, you can disregard some of it. But it might fall in the "good to know" category as you design your amp.
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Re: Acoustic amp

Postby Greg Smith » Mon May 12, 2014 10:58 am

I think the biggest problem with the power amp you are proposing is that your clean headroom will be limited - i.e. the amp won't be very loud. Amps of this size have become popular among guitarists because, when driven into overdrive, they sound significantly louder. But overdrive isn't usually desirable in an acoustic amp, so you may want to consider a higher-power amp (the Dynaco Mk III/Poseidon come to mind). It all depends on how loud your amp needs to be.

Then again, if you're going for lots of clean headroom, you might want to think about (dare I say it) a solid state power amp (in addition to the jfet-based preamp previously mentioned). For guitarists, the ream magic of tubes lies in their overdrive characteristics. In the clean range, the differences are subtle, and in a live performance situation, few if any are going to notice those subtleties. You can build a solid-state power amp in the 50-100 watt power range for a lot less than a tube amp. Look at the chip-based Gainclone projects that are all over the Web.

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