Lightweight guitar amp ideas... Now with compactron! (and schematic, more ?s)

[soapfoot]

Since you already have an amp that is what you want, but too heavy, what if you put that head in the lightweight wood cab with the Neo speaker, how much weight would you save?

A good suggestion, however I'm reluctant to do that. This amp that I'm already using is an amp that I like exactly as it is in its current configuration. I use it a lot for recording, etc., and would rather have something else. Besides, you can never have too many amps.

 

[soapfoot]

OK, so here's something I've tried.  Maybe someone can tell me why it won't work.

I realized that without the loss of a tone stack (which I don't want, nor need), the first stage would probably overload the second stage rather dramatically and drive the grid into conduction.

Now this may not be the right way to deal with this, but what I came up with was to use a 6K11 instead of the 6C10.  The 6K11 is somewhat like a 12AX7-plus-half-a-12AU7.  I decided to use the medium mu section for the second gain stage.  Looking at the transfer curves of a 12AU7 (can't find exact ones for the 6K11, though they should be similar), it should bias up to about 10v right in the middle of its range (as opposed to about a volt and a half for a 12AX7 section).  This would allow 20v P-P input into the med-mu triode before grid current flows, correct?

OR would it be better to use the medium-mu section for the first stage, and just not hit the volume control and second stage as hard?  The reason I did not do this is that I figured noise performance, etc. would be better with the hi mu stage first.

Any thoughts?  Perhaps I'm going the wrong direction here, but it's all about the learning for me.

 

[PRR]

BTW, note that 6DZ7 is a very tall tube. In some Champ packages, it may block the speaker. Yes, 12" cab and Neo mag do offer some space over a 6" ceramic-mag package. However I had to do a full sketch-up to fit 6550 (1/8" taller) into a 12"-Champ box.

> the first stage would probably overload the second stage rather dramatically and drive the grid into conduction.

You have a volume control between. Turn down!!

Draw a gain/level diagram.

With volume pot full-up, 20mV at jack should make full power.

50mV-500mV from Vol pot wiper should make full power. Any higher gain, it will hiss even full-down. Any lower gain, V1 is liable to strain when not full-up.

Your low-Mu V2 plan seems to need quite high level at the vol-pot, but the overall gain is correct, and we know V1 is a Fender Classic stage which will work. Fender usually used a Hi-Lo input jack-pair; perhaps the guitar's volume knob makes this unnecessary.

The bias supply..... 

> Damn those bridge circuits combined with bias supplies.

Multi-output rectifier _generally_ are brain-sprains. And often stressful to parts. IMHO, it would be better to wire a 120:12 backward across a 6V line to get a simple -60V supply. But here the weight would be objectionable.

> I've never had even a single course in electronics

Makes little difference here. I would not expect a fresh EE to (without SPICE) design that low-volt supply; some would say it didn't work. Without experience, only the brightest would spot the other issues. (I'm not so bright, but I have smoked a lot of parts.)

I don't mind if you connect stuff wrong as long as you can afford the smoke (parts, amp, or house).

Bias supplies generally:

A) If the A (heater) supply fails, the amp goes cold.
B) If the B (plates) supply fails, the amp goes cold.
C) If the C (bias) supply fails, the amp BURNS UP in minutes.

A B or C end the gig

A or B, usually only one part to fix.

C means unknown damage to power tubes, OT, PT, and possibly other parts. Damage may be gross or hidden. Bias supply must be reliable.

Therefore I like to see bias supplies drawn "obvious", left-right in their own ample space. Piazza's is good. Yes, most commercial schems jam and twist the bias supply into odd corners. One Fender mixes power switch, safety ground, and bias in a snake-orgy.

If you must trim-bias with a pot, you should ask what happens WHEN (not "if") the pot wiper loses contact. Some supplies use the pot as voltage-divider potentiometer. When wiper breaks, grids have NO voltage reference. They will float to whatever voltage the grid-current likes. Small cool tubes may float to cut-off, but big hot tubes often flaot to melt-down.

Your ver30 plan may work with value-changes. Again, this is NOT ameniable to EE training. All such plans are "designed" on breadboard.

 

[soapfoot]

Thanks.  I fully agree that it's probably time to put the pen and paper down, and pick the breadboard up.  I'm much more comfortable working experimentally, anyhow.

When I can afford to buy a few parts (sometime over the next few weeks), I'll start with my version 3.0 and see what happens when I start moving parts/values around.  I'm liking the idea of the 6K11 because it's a bit cheaper and more readily available than the 6C10.  If I decide I don't like the medium-mu triode as the second stage, perhaps I'd like it as the cathodyne PI.  I know many Ampegs used a 12DW7 or 12AU7 for the PI.  The higher current would probably drive the grids of those pentodes nicely.

For bottle height of the 6DZ7-- I'm actually planning on using a somewhat larger cab-- the Blues Junior size (made with Paulownia).  Using Paulownia, cabinet weight will already be small, so I can afford to have a cab that will make the 12" speaker happiest, and leave me enough room to have the larger tube in there (It's actually also the exact perfect width to fit through a subway turnstile with a handle on either side... I checked).  The reason I planned on the smaller chassis was for weight savings.  It will 'live' perfectly fine in the bigger cab, although as I finalize the circuit I might find I have to make the chassis bigger to fit everything.  Time will tell.

All your other points are well taken.  Thanks again for everyone's help and patience.  I'll be sure to update as I get the project rolling.

 

[lassoharp]

This will require a PT with bias tap (both Edcor and Hammond make them).  The general plan is simple and should be reliable.  I've owned 5 of these for many years. Solid performers.

http://www.triodeel.com/al1568a.gif

 

[jeffrey_burr]

soapfoot, just a word of encouragement, as I've been hacking away on a similar line of thought.  Go man!

I also came across the paulownia and thought it would be the lick.  I haven't built the cabinet yet but getting my hands on the wood, I'm no longer convinced it's going to make a pleasant sounding box without some further treatment.  It is a "hardwood" but it's awfully soft, especially for something that's going in and out of the car frequently.  I'm considering some kind of stiffening substructure, and/or maybe a transparent fiberglass layup on the outside.  Did your cabinet maker have anything to say about it?

I'm not a "true believer" with the vacuum tubes so I can't really speak to what you've got going in these schematics.  I've gotten a pre-built class-D power amp module and SMPS from Hypex to mate with a preamp of my own design.  Together these units are about 1 lb!  Maybe I'll fill the box with helium and carry my amp to the gig on the end of a string...

 

[soapfoot]

soapfoot, just a word of encouragement, as I've been hacking away on a similar line of thought.  Go man!

I also came across the paulownia and thought it would be the lick.  I haven't built the cabinet yet but getting my hands on the wood, I'm no longer convinced it's going to make a pleasant sounding box without some further treatment.  It is a "hardwood" but it's awfully soft, especially for something that's going in and out of the car frequently.  I'm considering some kind of stiffening substructure, and/or maybe a transparent fiberglass layup on the outside.  Did your cabinet maker have anything to say about it?

I'm not a "true believer" with the vacuum tubes so I can't really speak to what you've got going in these schematics.  I've gotten a pre-built class-D power amp module and SMPS from Hypex to mate with a preamp of my own design.  Together these units are about 1 lb!  Maybe I'll fill the box with helium and carry my amp to the gig on the end of a string...

Thanks for the thoughts.  I was considering giving it a coat of wood hardener (the stuff they use to shore up rotten wood) before applying some sparkle naugahyde in the 'tolex' role, like I did to another amp one time.  My cabinet maker didn't say anything about it, just quoted me a price.  He had to look around town to find some paulownia, so I'd wager he's never done one before.

--b

 

[jeffrey_burr]

Well, I'd be very interested to see what you wind up with.  Best of luck!