Guitar Tube Power Amplifier Schematics and Layout

[Zarrir]

I guess many of us have had or may still have some kind guitar tube preamps that were very common in the 90s (Rocktron, Digitech and others). I personally have two Digitech 2101, great units. Although i can use the power section of some tube amplifier with an effects loop, I am looking for some DEDICATED guitar tube power amplifier plans and layouts, between 30w and 50w . I´d like to build my own. Thanks for any help!

[shooter]

DEDICATED guitar tube power amplifier plans and layouts

I've been stitchin amp parts together awhile now, pick ANY tube amp you want, take the PI, PA, OT, + PS and you have a dedicated Power amp.

[eleventeen]

If I understand you correctly, IMO you're probably more likely to find such power amps looking for hi-amps. Dynaco, Heathkit, EICO, Pilot, and the like. Those schematics are abundantly available. Hint: If you are 100% unfamiliar with these, go to google images and search for "heathkit [EICO,dynaco] power amp". From those results, make a list of the key model numbers, then search for same.


When you say "dedicated power amp", to me, you are referring to an item looking for about a 1 volt input and which in about 50% of cases has no input "volume" control.

[MakerDP]

Bottom of the page here....

http://ax84.com/corepoweramps.html

I built it (with a few tweaks) and it sounds amazing with KT77 tubes but it's just too darn loud and heavy for me. Doing the 20W PP design on that page with the 12AU7 input section would make it perfect. 20 Watts is really all you need for just about any situation.

If you are going to do a stand-alone power amp for use with a variety of preamps, then in my opinion, it should be made as versatile as possible. Variable NFB on a dial with a push-pull switch to defeat it altogether, cathode/fixed bias switching, bias circuit that can handle 6V6/6L6/EL34, pre-pi and post-pi master volumes. That way you can throw any preamp topology at it and it will match well. The amp also then serves as an awesome platform for testing custom preamp designs.

I have a couple old DigiTech tube preamps. They were pretty cool in their day but IMHO they don't hold-up well against a nice real tube amp. I used to put my 2112 through a Peavey Classic 30's effects return. It sounded decent. If you want a rack-mountable all-in-one unit they are still pretty groovy I guess. I did enjoy having preamp gain set to a c.c. footpedal and all of the effects' quality still ranks up there pretty high.

[HotBluePlates]

... I am looking for some DEDICATED guitar tube power amplifier plans and layouts, between 30w and 50w . I´d like to build my own. ...

Or you take any guitar amp schematic and lop off the preamp just ahead of the phase inverter (or before the pre-gain stage in front of a split-load inverter). Add an input jack, a resistor from jack hot to ground, and a coupling cap into the phase inverter (or pre-gain stage).

You'll know a pre-gain stage (if present) because there is a feedback loop from the speaker to the pre-gain stage. Everything within the loop should be considered the "output stage" of the amp schematic in question.

You might even add a volume control between the input jack & coupling cap.  Normally, this is a lousy idea at the input of a guitar amplifier's preamp, because it raises the noise floor.  But you might like having control over how hard the outboard preamp is driving the stand alone power amp, a la a Master Volume.

[Zarrir]

Wow, so many good ideas here!  Eleventeen, i probably misused the term DEDICATED. Although HiFi tube amplifiers are always on my mind for future builds, i am actually talking about guitar power amplifiers. MakerDP, this is exactly what i need. Strangely enough i googled many times and never found those plans. The 20W PP seems like what i am looking for. The layout there is very confusing but the schematics are clear as water. HotBluePlates and shooter, when you say ANY guitar amp schematic can i assume i may use cathode biased amps as well as adjustable bias?  Also curious to learn more on negative feedback loops. I see them very often in Hoffman amps layouts, are they absolutely necessary or only if you want more cleans (and headroom) out of a dirty amp?

[eleventeen]

Just as a matter of thoroughly confusing terms: We call "adjustable" bias "fixed" bias. Why, nobody knows. Fixed bias means you can turn a knob  and change it. Go figure.


Probably you should study a few articles on the general topics of NFB and fixed vs cathode bias.


http://www.londonpower.com/ has a few.


No doubt AX84 has some general articles


http://www.valvewizard.co.uk/


Those are probably the main ones folks around here repeatedly refer to.


When you first get past the point where you recognize the standard Williamson-style push-pull topology of any given amp, you then start to look at the injection point for the NFB, if any. There are several different "styles". You just have to look at a bunch of schematics to survey the various practices.


Some manufacturers, Ampex comes to mind, take the NFB way back farther before the PI than your typical Fender. Dyna does, as well.

[PRR]

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[MakerDP]

I've also built the AX84 20W PP amp but a paired with the Simple Preamp building block (amazing amp by the way) but  I added a NFB loop defeat switch. I didn't have any troue following the layouts. What is confusing to you about them? I'm sure we could clear it up.

[Zarrir]

Figured everything out, MakerDP, thank you !!

[shooter]

Nice premover

[MakerDP]

Figured everything out, MakerDP, thank you !!

My only guidance to you on that design would be if you are going to do the 20 watt version, you will want to go ahead and add the 12AU7 input stage from the 50 watt standalone so that it will take whatever you could ever want to throw at it. Also find a better output transformer. Using an Edcor or ClassicTone or even a Deluxe Reverb replacement will be MUCH easier to deal with and quite a bit cheaper as well.

If you want to experiment with variable negative feedback you can look at how it's implemented in the October Club amp on the site.

[HotBluePlates]

... HotBluePlates and shooter, when you say ANY guitar amp schematic can i assume i may use cathode biased amps as well as adjustable bias?  ...

I mean any.  Amplifiers are typically designed from the output section/speaker backwards towards the input, so there's no reason you can lop off a complete functioning power section (with any phase inverter present) for use.

You might want to ask for some of us to check/verify your plan if you do this yourself, so all the important bits get included.

You might want to ask for some of us to do a quick analysis of the drive signal required into the front of the new standalone power amp, which will set the requirements of output signal from your existing standalone preamps.  This will help insure they will match up well.

The image in this post is just brilliant!

...  Also curious to learn more on negative feedback loops. I see them very often in Hoffman amps layouts, are they absolutely necessary or only if you want more cleans (and headroom) out of a dirty amp?

It seems like if you want good, clear info on how feedback loops work, you're best off looking at "solid state opamps".  Then remembering that everything inside a feedback loop is essentially just an opamp.

As for seeing them often in Hoffman layouts: they only appear as often as they do in the vintage amps on which the layouts are based.

The loops in guitar amps mostly just give a bit more (clean) output power, and tighten up damping of the speaker (less speaker flap).  They do make the transition from clean to distortion (of just the stuff in the loop) a little more abrupt.

But guitar amps usually don't use a ton of feedback, and so the effects above usually aren't overbearing.  It is part of the reason a tweed Deluxe or AC-30 breaks into power amp distortion differently than a late tweed Bassman, 50w Marshall, or blackface Fender amp.

All of which to say you might want to think about which complete guitar amp you'd most like to mimic, and consider stealing the power section of that amp.

[Willabe]

The image in this post is just brilliant!

Yeah it is! 

[kagliostro]

The Preampmower man

 


Franco

[shooter]

i may use cathode biased amps

As you got by now, yup!  My last stitch took a plexi PRE, a Gibson PI, and a matchless (clubman) PA, (cathode biased).  Outcome, my best guitar amp yet.

I also did just like you're asking and made a power-amp only from a Vox AC30 (modified for EL84's), did what DP suggested and used a 6SN7 driver.  Outcome, good guitar, but even better audio, sold it to an audio guy.

[drgonzonm]

This forum has a great amp schematic library.  Under the Marshall section is a 100w power amp schematic (1969 JCM800) with four output tubes.  Use two output tubes, identify the transformer (probably can use Hammond's site), then double the reflected primary inductance on the transformer.  Say, with 4 tubes, the primary inductance is 2k ohms, then choose a transformer with 4k ohms. 

Good luck ,

[Zarrir]

This forum has a great amp schematic library.  Under the Marshall section is a 100w power amp schematic (1969 JCM800) with four output tubes.  Use two output tubes, identify the transformer (probably can use Hammond's site), then double the reflected primary inductance on the transformer.  Say, with 4 tubes, the primary inductance is 2k ohms, then choose a transformer with 4k ohms. 

Thanks for that!  Will go check it right now.