Amp Computer

The problem here was banana cables.   At some point, the ‘Asshole Audiophile’ community got a hold of banana connections and now it’s damn near impossible to get banana cables at a reasonable rate.   I wanted to use banana cables instead of say, posts and alligator clips.    These guys :  Circuit Specialists solved that for me.   I paid a buck and a quarter per 20″ cable.  Woot!  Back on that soon, but the first two parts are in fab now!    I haven’t decided on the mounting system yet, Cheap boxes or panel/rackmount.   Tough call actually.

Real McTube III

This got some love yesterday.   The power circuit is done but not boxed yet.   I am going to box it separately from the foot pedal part because hey, it’s 140/12 power supply and it’s not like I’m gonna take it on the road.   I wired up the heater stuff, LED and footswitch yesterday and may take a crack at the signal today.   More on that heater circuit later, but these are the first two amp computer style components.

Real McTube Calculations

This is gonna take a while, I did the numbers and they worked out, but now I can’t find the book I used and describing this stuff is really painful.   I’m back into the Rider book but that is the most annoyingly pedagogical book I have ever read.   I’d love to find a good clean book on tubes, with our without the math but it must have full data sheet reading info

SongBird

I’ve started buying parts and will rip the old stuff out of the chassis soon.   There may be some original Gibson Transformers on EBay soon.  Maybe I will recreate the Crestline in the amp computer style.  Maybe the 102C84 will get up there.  That would be ridiculously hard to find anywhere else.

I have purchased the best amp I have ever heard.    This is a 2006 Gibson GA 20 RVT.   It’s not perfect, I might have to mod it, but I am definitely in love.   My strategies about building the Songbird and other things are all up in the air now.

Let see why..

Once again.. this is the best amp I have ever heard.  I suspect that other people get better tone (T-Bone Burnett playing for Plant/Krause comes to mind) but I have never heard anything personally that even comes close.   In the first 20 minutes after starting this amp for the first time, I got a bit concerned.   It is voiced very dark and I’ve backed my Strat pickups off thestrings already to moderate Fender shrillness.  However, I very quickly came upon settings that were just magnificent.

Specs

Here are the specs from Gibson (with comments):

15W Class A   -   12AX7 pre-amps,  6V6 x 2 Output

The circuit really looks like it’s Class A.   No phase splitter.    12AX7 to 6V6, just like the old 50′s Champs and Skylarks.   The two output tubes are in parallel, but not push-pull (very rare .. last seen in the BR-9 I beleive), so I wonder if you couldn’t disable a tube for even more low volume fun.   I haven’t really pushed it very hard yet, so I don’t know how clean it would stay in a club situation yet.    Rumor is they use Sovtek or Chinese tubes in production.   I’ll replace with quality when the time comes.  It can only help.

Do I still need the Songbird?   Hmm..

Hand Built, Point to Point

I’m really not a purist when it comes to these things.    The Fender Hot Rod / Blue Deluxes are circuit board and they sound great.   My concern was that I would not be able to repair them myself.    Any fool can beleive he can repair a PTP amp.

2 channels plus mix input

Looking forward to trying the other channels someday :-)

Tube Reverb,  Tube Tremolo

The reverb is the most un-Fenderish reverb you can imagine.   It’s got a deep reasonant hall sound to it.. not bright and springy, but very very filling.   Hard to describe.. I’ll work on sound samples this weekend.  It’s a great sound and it’s nice to not be making the same sound everyone else does.

There is a tremolo but it could not really suck more.    This, to me, is amazing because the 50′s and 60′s  Gibson Tremolos sound fantastic.    They had an oscillator varying the power tube bias and were great.

Because of the fact that this has both reverb and tremolo, I’ll bet a lot of people bought it for either surf or very bayou swampy sounds and they were probably very disappointed in that.  Too bad for all concerned.   Gibson loses some luster with such a crappy tremolo (taking up two knobs and a full tube).

Again, the reverb is different but outstanding.

Pentode/Triode Switch

This is nice.   Cuts power and lets you reach a higher level of preamp saturation before you wake the kids.    Some tone change, but nothing untoward.

12??? Eminence Legend Speaker, Asymmetrically Placed

I am very interested in how this responds to other speakers.   It has various outputs so I’m wondering how it will sound against a classic Jensen Ultrasonic or the new Weber Alnico.    I’ve heard that this is a good first mod as these are probably not the made in Britain speakers.

Metal Film Resistors

Note :  How is that a feature?   Metal film is widely believed to cause sterility in amps.  This is balanced against the fact that the amp is silent even when dimed if you don’t play something.

DC Powered Filaments

No hum.. it must be working but it seems to be AC to the power tube filaments.    I wonder why.

Hand Wired Turret Board

Again, I’m not a purist, but this should make repairs/mods easier.

Two-Tone Vintage Brown and Tan Tolex

It does look good, but you can’t use right angle cables because the case is too thick and too close.

Comments

This thing streeted for 1k in 2006 when it was made.   Honestly, I wasn’t even shopping for an amp, but I saw that come up on ebay for 300 to start, I bid 300 and until the last few hours, I was the only bidder.   I got it for $406.01.  For a hand built PTP amp, that is a phenomenal deal.

So far I have only barely touched the “crunch” and mix channels and really haven’t taken it out of Triode mode much either.  I literally stop the backing tracks all the time now and just listen to the amp play the notes.   I’ve never enjoyed hearing a guitar more!

As I said, it is voiced very dark.   You have to shape your tone a fair bit (which I am not really used to doing) but it’s glorious once you get there.

Drawbacks

• Tremolo is completely useless to me.
• Very loud pop when switching to standby.
• No master volume.
• Only one control for reverb.
• The channels share pre-amp tubes, so you cannot swap tubes in a channel

Nothing end of the world here but unsurprisingly, I am starting to think about some …

Mods

• Yank the Tremolo and add a master volume.   You could get one per channel since the Trem takes two knobs.
• See if there is a place in the reverb circuit to adjust the dwell.   Pop a pot in there and bring to front if I only use one master volume.
• revamp the pre-amp topology so that I can use a 12A?7 to fiddle with the sound on channels

Truth be told.. I might never do any of those mods.     Maybe I’ll just leave this pristine and wrap up the McTube and the Songbird.

Now many of the options that I was considering are simply to try them out, with all other things being equal.   I have no real idea what effect they would have…  I want to hear them for myself.

Here is a quick list of things I am considering :

Input

• Fender vs. Gibson style grid stoppers,  Old school cap and grid.

Pre-Amp Stage 1

• Switchable bypass cap, Tweakable bias voltage,  Bias negative warning light, The Dave Hunter small cap mod, Other plate voltages

Pre-Amp Stage 2

• Variable NFB, Switchable “Tone Expander Cap”

Output Stage

• Switchable Bias Cap, Master Volume Control, Triode/Pentode switch

Power

• Standby Switch, Hum balance vs. rectified heaters, choke vs smoothing resistor.

That’s a lot of options and I’m pretty sure I’ve missed some.   So my plan is now somewhat different.   I am going to build the amp as an analog computer first.   The person who gave this blog it’s name calls it bread boarding, but I think of that as using the board with all the little squares.

I will create component boxes for all the pieces I need.  I’ll put banana jacks on them for connections and wire them up as need to try things out.   I’ll have generic parts, like decade boxes, alligator clip leads,  octal and nine pin tube sockets, pots etc. and I’ll make up custom stuff like switch sets, and transformer holders.

I’d like to have them all sitting on something stable.   Maybe I’ll put velcro on the bottom of all these boxes.   I think I’ll do the Real McTube II like this first.   Woo hooo.. this sounds like fun

Tube Biasing – 1st Stage

As I understand it from my readings, the standard way of biasing, self biasing, is a complex reaction.   Essentially as the power drawn from a tube increases, the bias goes more negative, somewhat thwarting the increase in power.    This could be a very good thing.   To my mind, this sounds like a plausible explanation for why compression happens.   Now, as you all know, my understanding of what’s going on in these amps is mild, at best.    My thoughts for biasing are actually three fold.   Here is the first stage of the GA-5 :

The Fender Champs (5E1 and 5F1) and original Skylarks biased at 1.5V.  Interestingly, the schematics from Crestline Skylarks (GA-5 and GA-5T) are annotated with voltages and despite the fact that these are exactly the same circuits up through the second stage, the ’5 shows a bias voltage of 1V while the ’5T shows 1.5V

I want to use a potentiometer here, to play with various values.   Note that setting the bias is not really an issue for 12AX7 / 6EU7s in this case.  The circuit is self biasing.    I just want to see if fiddling here would change the tone.

Additionally, I want to be able to switch the bypass cap on and off.   The 5E1 Champ had it but the 5F1 champ did not.  Finally.. and here’s the kicker,  I am considering a straight bias option.  Sending a fixed voltage to the cathode.   This is how tubes used to be biased, with a battery or other source directly supplying the right voltage.  So this would be a complex little bit of business, just so I could play around with biasing options.   Knowing me I’ll find a setting I like and never change it.   Dave Hunter’s Two Stroke amp has a three way switch with two different caps and a no cap selection.

But still, that’s not complex enough!   Next up an LED/diode combo to tell me when the grid voltage is higher than the bias voltage  (Actually when Vg + Vb >= 0).   No point in buying a lot of extra tubes if I can give myself a warning before I cathode strip them.

More Coming…

Unsurprisingly, I am starting with a Gibson Skylark.   To be specific, I am starting with the pre-crestline GA-5.   This is a lovely little class A, single ended amp, rated at about 5 watts.   She uses a 12AX7 as a pre-amp, 6V6 power tube and a 5Y3 rectifier.

This was a student model amp, and a direct competitor with the Fender “Champ” amps.  Fender started making the champs in ’48 (I’d kill for a 5B1 schematic, but I’ll bet it’s damn close the ’52 Les Paul amp).    By the early to mid 50′s these student models were all pretty damn similar.

To the left is a  classic Fender Champ 5E1 schematic.  12AX7, 6V6, 5Y3 and almost the exact same thing as the  …

… Champ 5F1 schematic.   In fact, the choke has been replaced with a resistor (much cheaper) and the bypass cap has been removed from the 1st stage preamp cathode.  If you look carefully, you can see that the cap was simply erased, as the dots are still there.

And here is the GA-5 Skylark schematic that I will be using for a baseline.  There is really almost no difference.   Fender has a 1MΩ resistor at the input, the bias resistors are slightly different on the preamp…    just little stuff mostly.    However, don’t think you can just swap the OTs on these things.    Fender uses 3.2 or 4 Ω speakers and Gibson almost always uses 8 Ω speakers.    The GA-5 also uses 10 and 20 μf capacitors for power conditioning compared to 8s and 16s in the Fenders.   I’ve read that this is expected to mellow out the highs.   I don’t disagree, I just doubt that I would notice.   Who knows though?   There is never any trouble getting ampophiles to argue over which part is most important.

A little caution is in order when discussing the Gibson Schematic.  They are prone to small errors.   Note that V3 is listed as a 6Y3  or arguably a 6V3 if you want to be kind, but either way, it’s actually a 6V6 of course.

Why the Gibson?

To my ear, the Gibson sounds just a little warmer, and the Fender is a bit harsher.   Some ears will prefer the earlier transition to overdrive/distortion on the Fender.   I prefer that ‘barely holding it together’ feeling I get from the Skylarks.   When you back off with the pick it’s plenty smooth, but hit it hard and it will bite back.

Since these are all 5 watt amps, they can reasonably be dimed in the studio (or your garage).   Try bringing your Marshall stack into a little studio and see what happens.   The Skylarks I play at home right now are 8-9 watts and Push-Pull instead of SE.   This Skylark will give me more overdrive at even lower volumes.

Later on, when I discuss power tubes in more detail I’ll drag in the Gibsonette schematic, which just uses two 6V6′s in a class A, SE configuration.   This could give more power .. although I’m not really sure that is a goal.

So.. to wrap it up.. these are classic Studio/Bedroom amps.   Great tone and the ability to push it hard even with the kids in bed.   This is what I want to build.

The major goal of this project is to get me a Champ/GA-5 (Pre-Crestline) style amp for far less than the 500 or so for an original. I don’t like any of the re-issues, both the Gibson and Fender re-issues sound very thin to me. Also, I don’t think they use a tube rectifier, but I could be wrong there.

These are real Class A Single Ended amplifiers.   Three tubes, one knob and just fantastic sounding.   Although they are very similar, I am going to build off the Gibson GA-5 schematic.  The basic reason is that I like the Crestline GA-5 I have so I trust the Gibson.   Not that the Fender would sound bad, they are far too similar for that.   Thus the name “Songbird”.  I will be basing this on the original Gibson Skylark.

The primary motivation is to get that sound into my arsenal.

So thats the key motivator, but secondary considerations abound.  This is a simple amp,  5Y3 rectifier, 12AX7 pre-amp and 6v6GT power amp.  Maybe 30 parts in all.   If this was just a straight copy of the old amps, I’d be done already.   I have some ideas that I’d really like to try.  Most of the things I want to do different were design decisions in the original.

I point out that the original GA-5 was the lowest priced amp in the Gibson line.   It’s purpose was to get an amp that could be played at home or in the dorm room into the hands of as many kids as possible.  When you are building an amp the two basic ways to cut costs are to put fewer parts into it and pay less for the parts.   Gibson and Fender both did as much of the first as possible and then started trading off quality parts to keep the costs down.   Nothing wrong with that, but I am trying to capture the sound, not save money.   I am going to use high quality parts (although I won’t be buying 80 dollar capacitors, no matter how freaking good they are).

I want to improve the circuit that Gibson made by doing the things I think they would have done if cost were not an object (as can be seen in the more expensive amps).  I have a few ideas of my own too.   Some new options, and every damn thing I can think of to keep it noise free.

Power 

This amp was only designed to push about 5 watts.   You can still piss off your housemates with a 5 watt amp, but the neighbors might just resent you rather than develop outright hatred.  This became the genius of the amp when that overdriven sound got popular and people started realizing that big amps do not work well in the studio.

I want to keep the max power the same, but I am considering a Triode/Pentode switch.  In theory this would allow me to push the thing into overdrive at even lower volumes.   Probably with some changes to the tone, but who knows it might sound better.

Also, I am strongly considering separating the PT and rectifier circuit completely from the rest of the chassis.   Essentially, I would have two chassis’ :  one for audio and one for power.   I expect to run the power and heater lines to the bottom of the chassis and punch them in exactly where they are needed.

Heaters

I am almost certainly going to rectify / condition the heater voltages.   This will not be tube rectification, of course.

There is way more..  stay tuned.