Inexpensive Guitar Amp

[Eli Duttman]

One of the guys on my job is a guitar picker. What he owns for amps is unmentionable. I know nada about guitar amps. So, I'm starting with the Fender Champ 5C1 and looking for ways to hold costs down.

Fact, "12" V. versions of tubes cost less than "6" V. versions. With that in mind, I'm thinking 12SJ7 and 12V6 as the signal tubes, with heater power provided by "Rat Shack" catalog # 273-1365.

Another fact, the most cost effective way to obtain a decent 300 V. B+ rail is via a Greinacher ("full wave") Voltage doubler. For reasons of "sag", tube rectification is desirable in this design. I think I see a way to have the cake and eat it to. The 50 VA Triad N-68X isolation trafo provides enough current to energize both the heater of a 117Z6 and a doubler formed around the twin diode. A pair of 15 μF./250 WVDC 'lytics should be about right for the doubler stack. Perhaps an increase to 10 μF. in the reservoir position is appropriate.

Please comment on the PSU concept. Also, cabinetry and a speaker driver recommendations are needed. This will be a "combo".

[chrish]

I don't play, but built an AX84 pre and power amp combo for my son. It is more complex than a champ, but I used cheap Russian valves. 6P6S for the 6V6 and 5U4M for the rectifier. Check the usual eBay sources. I have had no problems with any of the ex-Soviet Block sellers...

I used a Celestion speaker in my project, but my son is not advanced enough o be able to tell me if it is good or bad, he is just impressed that he turn it up to 11 with 20 Watts and impress (not!) the neighbours! Maybe something like a Celestion Tube 10 might suit? They are about $45 at Antique electronics

[leadbelly]

I think the ripple will be a real problem.

I seem to remember you writing that you accept the tube construction itself as an acceptable form of galvanic isolation. Why not wire the Triad as 120:240 step up and run the 117Z6 off of the line side? That's what that tube was built for. That'll get you 300VDC without the ripple of a doubler.

I don't if cheap extends to the speaker, but if it does, you can use a PA driver rather than spending the cash for a real guitar speaker.

EDIT: Now that I think of it, why not wire the Triad as 120:240 and use TWO 117Z6s in a bridge heated using the secondaries. Then you'll have BOTH isolation and lower ripple.

[taj]

What genre of music does he play, Eli?

..Todd

[Eli Duttman]

The 117Z6 was intended for use as a doubler. Look at the data sheet. When the anodes are exposed to voltages above that found in North American residences, protective resistors are needed. Aside from other reasons, doubled forward voltage drop pretty much rules a bridge of 4X vacuum diodes out. BTW, turning a N-68X around, for step up, is not a good idea. The single winding designated as the secondary is sized with regulation effects taken into account. Don't believe for a single moment that all the windings have exactly the same number of turns.

There will be no more ripple with the doubler than in the Fender original. Leo used an 8 μF. 1st cap. in his CRC filter. A doubler working into a 2X 15 μF. has approx. = ripple. Any slight difference will be taken care of by increasing the reservoir cap. from 8 μF. to 10 μF.

[tubelab.com]

Another fact, the most cost effective way to obtain a decent 300 V.

I have been down this road before. You can wire a bunch of unusual components together and maybe save a dollar ot two on the initial build. If this is a one off, and you can fix it forever, this is OK. If your amp will ever wind up in the hands of someone that you don't know (like if you are going to sell them) you must face the first law of building guitar amps.

If it is going to be serviced by the typical guitar amp tech, it MUST not use any trick circuits or parts that you can't buy at the typical music shop. If you have ever spent some time with an "expert amp guy" you will know what I am talking about. Fact, they will NEVER understand screen drive, and they will rewire your screeming monster amp until it doesn't work even though it did work when it came into the shop. 6AV5's don't compute, the "expert" will stick 6L6GC's in and wonder why the smoke monster appears.

So, I have learned that if you are going to sell it, it has to use tubes that are in the Sam Ash, Guitar Center, or Musicians Friend catalog. That is a rather small list. So for a small amp, you are stuck with the 5Y3 (and maybe a 5AR4 for a louder sound), a 6V6GT or a 6K6 for the output tube, and a 12AX7 for input duty. Yes, Fender used the 6SJ7 years ago, and mister wonder tech can't possibly stuff a 12AX7 into the socket, so it's probably OK. I have used other tubes in small amps (the 6W6) but you have to assume that someone WILL stuff in a 6V6GT or even a 6L6GC down the road. Funky combinations of transformers will confuse mister wonder tech too.

So for my "Champs" I use the Allied 6K56VG. You get about 300 volts with a 5Y3 and 320 volts with a 5AR4. For the big guys (50 watt P-P) I use the Allied 6K7VG, 430 volts of B+. I have been using these transformers in guitar amps for at least 15 years, and I haven't seen a failure yet. They are Hammonds so they do get hot, but they don't blow up. I built a few "Turbo Champs" that used a 12AX7, a KT88 a Hammond 125CSE OPT, and a 6K7VG power transformer. They were screamers.

Amp experts, and most users have no idea what impedance is and it is not too uncommon to find several additional speakers connected up in random fashion. I used the 125 CSE to allow for some means to achieve a match. I used a 4 way switch to select the tap on the OPT's these were used as tone / distortion controls.

Part of the "Fender sound" comes from the fact that Fender always used an under sized OPT. Saturation was part of the sound, but is was also part of the reason that Fenders ate tubes. There was a "fact" that was widley told in the 60's that stated that "a Bandmaster will blow up if you play a bass guitar through it." I can say that I saw this fact verified in flames in the 70's. It sounded cool, but after about two hours one of the 6L6GC's went nuclear and sparked out. So, the message, the OPT controls the sound. A Fender replacement" of the correct size is needed for the correct sound, but a cheap Edcor or Hammond 125 series will give a less distorted sound at full crank but be easier on the tubes.

If you build the 5C1 and the player uses a stomp box that puts out a bit more than "guitar voltage" you can get some unusual distortion by trying a 6SK7 in place of the 6SJ7. It was a common trick that I used years ago. I quit using that circuit about 15 years ago since the 12AX7 is just too common in the guitar amp world.

[leadbelly]

Originally Posted by Eli Duttman

The 117Z6 was intended for use as a doubler. Look at the data sheet.

Well the 12SJ7 was intended for RF, so by your logic you can't use it in your amp because that's a different service

Originally Posted by Eli Duttman

When the anodes are exposed to voltages above that found in North American residences, protective resistors are needed.

The secondary voltage will fluctuate with primary voltage. How do you avoid needing that same resistor on the secondary side if you need it on the primary side?

Originally Posted by Eli Duttman

BTW, turning a N-68X around, for step up, is not a good idea. The single winding designated as the secondary is sized with regulation effects taken into account. Don't believe for a single moment that all the windings have exactly the same number of turns.

What does it matter if it's not exactly 1:2?

[Ty_Bower]

With respect to "turning around" a transformer:

I've been told that supplying a winding with higher voltage than it was designed to handle is bad mojo. I've got some oddball transformers where I thought I could "turn them around" and run my 120VAC wall voltage into a secondary, and take something useful off the primary. Apparently this kind of foolishness doesn't work. Maybe the secondary doesn't have enough turns to provide the inductive load (is that something I made up?) required to support a higher voltage. Instead of exciting the core you just end up drawing a lot of current in a low resistance winding, burning a lot of heat and (hopefully!) blowing a fuse.

[tubelab.com]

Apparently this kind of foolishness doesn't work.

It seems to work OK with the Triad N-68X, The regulation sucks though. I use a N-68X to make + and - 140 volts with a SS bridge, It works OK.

[Eli Duttman]

George,

Thanks for your comments. I'm a HIFI guy. IF this project happens, it will be a 1 off cheapskate special.

Thanks for the heads up about the O/P "iron". As it turns out, Triode Electronics has Champ O/P trafos with 8/4 Ω speaker taps and they cost less than $20.

I still need advice about cabinetry and speaker selection.