DIY (guitar?) amp building - sans preamp?

[RustyBrooks]

I have a small selection of amps that I use at home to record. Most of them are too loud, even the 5w head that I use (although it's not too bad). So I'm looking into building a nice tube amp of very low wattage. Perhaps something on the order of .5 to 1 watt.

Something I was thinking is that I'd like it to be as simple as possible. And it occurred to me - why does it need a preamp? I have several devices that would be suitable to take a high-impedance guitar signal and turn it into line level (DI boxes, preamps etc).

This way I could use the amp as a line-level effect for re-amping, and for other line level sources (keyboards, etc). I could even use the amp as an "effect" (bringing the return back in with a mic and preamp).

Is there any reason not to do this? Are there any designs out there which don't include a preamp? How hard would it be to take an existing amp design and excise the preamp?

I have a lot of familiarity with electronics, soldering, etc, but I have almost no experience designing, or understanding the design of, audio components. Any pointers to resources, or thoughts, would be greatly appreciated.

[RustyBrooks]

For the sake of usability it would probably be nice to have the amp in question have balanced inputs, although I don't think that would be strictly necessary.

[helmuth]

http://www.ax84.com/index.php?pg=projects&project_id=firefly

[RustyBrooks]

I've seen those kits/plans before. They are meant for high-impedance guitar signals, though, as far as I understand it. Was just wondering if anyone fooled around with line-level amps like these (no preamp built into it)

[brianroth]

What you're seeking is just a power amp (the audiophiles call these "monoblocks"). In the case of the "firefly" amp linked earlier, you could eliminate V1a, V1b and V2a, and insert your signal into the top end of the 1 Meg volume pot. That high Z input is NOT a problem since it will act as a bridging input for your signal source. In fact, I'd suggest reducing that pot to, say, 50K for this application. If you also desire a balanced input, then just hang an audio transformer ahead of the volume pot. The Edcor iron is a great bargain, at around $10.00, since a Jensen, Sowter, Lundahl or Cinemag would be overkill in this situation.

More than a few years ago, I stumbled a whole bunch of links somewhere on the net for tiny guitar amps, and any of those could be readily modified to be line-level-only by eliminating the preamp stages.

Bri
_________________
Brian Roth Technical Services
Oklahoma City, OK
http://www.brianroth.com/

[RustyBrooks]

Would doing such a modification save money and effort (enough to notice?) It seems like it would. I wonder why such thing are not talked about more. Almost anyone who is going to mic an amp (and that seems to be the target audience for super low wattage amps) is going to have their own preamps, with tone stacks, clean boosts, etc. Why build a preamp in, especially when it's generally going to be super simple (single knob for gain is common in these)

[RustyBrooks]

Take a look at this schematic:
http://home.cfl.rr.com/dbhammond/Firefly_sch_R1.gif

This is an earlier revision of the firefly. Revisions past this were modifications to the preamp (to offer different preamp tones).

It looks like I could cut out everything from the input jack/resistors to C7/R8 (which looks like a high-pass filter?). Is it really that simple? 2 tubes, a couple of resistors and capacitors, and some transformers?

I'm trying to find Doug Hammond's email address, I'd love to ask him some questions. I wish I knew more about this stuff, I'd love to take a class or something. Embarassingly, I have a degree in electrical engineering but a) they don't touch tube design with a 10 foot pole and b) they don't really even do much analog design beyond REALLY basic stuff. Unless you get a job in circuit design, you pretty much never encounter this stuff.[/url]

[brianroth]

[brianroth]

[brianroth]

One other thing on that Firefly schematic:

http://home.cfl.rr.com/dbhammond/Firefly_sch_R1.gif

If you're running the line level signal into the VR1 pot, eliminate C5. That is a "bright" cap which shunts HF around the pot. On Fender amps, that cap was switched in/out via the...."bright" switch!

You might also keep looking around for other "tiny amp" designs.

Bri
_________________
Brian Roth Technical Services
Oklahoma City, OK
http://www.brianroth.com/

[RustyBrooks]

Thanks for your help and suggestions. I've done a lot more looking around on the web and I think I'm going to make a trip to the local library.

Something that occurred to me today is something I learned from amptone.com back in the day... a lot of commercial amps have crappy preamps and people often short-circuit around them by plugging into the effects return (using their own preamp, which they like). So I've been looking at plans for amps that have effects loops - I can chop off the whole thing prior to the effects loop return, and there's a line level amp.

Things have definitely changed. When I got my EE, my senior design project was to design a simple CPU. I modelled a lot of it in the tools of the day, but in the end I laid it all out by hand (well, with a computer. But I ran all the pathways and made all the components myself). A friend's brother recently graduated and they were allowed to use auto-layout tools (where you define the *operation* of the processor and it pretty much does all the physical design for you). Kids these days!

[brianroth]

LOL...ok, then stick with "simple" analog circuits.

I was recently looking at the designs of things such as computer motherboards.

Geeez...six or eight (or more) layers for the circuit board. Gotta have a computer to design THAT!

I thought I was a "designing stud" when I manually laid out a double-sided board back in the 1970's! <g>

Bri
_________________
Brian Roth Technical Services
Oklahoma City, OK
http://www.brianroth.com/

[RustyBrooks]

So I went looking for schematics for the epiphone valve junior that I have, and here they are:
http://www.valvejunior.com/img/misc/ValveJunior/ValveJunior.png

This looks a lot like the firefly except with no push/pull stage at the end.

Could I give this amp an "effects loop" by putting in a send/return jack right before (or after?) the volume pot? Then I could bypass the preamp when/if I felt like it.

I've ordered some books on tube amp design. Really interested in figuring all this stuff out. Thanks for the help in this thread.

[lacquer_monkey]

There are a couple things in that schematic that look pretty wrong. I'm not saying that that is not how the circuit is built, just that it doesn't make sense. At the input is a voltage divider that reduces the signal from the guitar by half. So much for optimizing signal to noise. Not much sense there... Secondly, after the first stage ther is another voltage divider that reduces the output of the first stage by 66%. I don't know why you would want to do that. The 1 meg resistor in parallel with the 1 meg pot seems a little weird, too. Maybe they just wanted it to be really clean and never distort...
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uh... what??

[RustyBrooks]

Yeah, that's been noted on various Valve Junior mod sites. A lot of people remove or drastically reduce the second voltage divider to get more gain. Given that at about the 12:00 position on the volume dial it starts to break up, I'm guessing they were trying to get enough room to have some variation in the clean section? I've considered putting in a switch to bypass the second voltage divider.

There are some other common mods... a shunt capacitor before the second preamp tube, like the bright switch mentioned above, for example.

Just out of curiosity, if I put an input jack where the volume pot is, and plug from my own preamp into the power amp section, does it do any harm to have the preamp section sitting there attached to nothing (no load)? There won't be any input signal either.

Man, I know so little about all this stuff.