FrankenstAmp build

[Paul P]

I've worked a bit on the chassis :



Rear panel showing the controls for the output tubes : test points across
a 1 ohm resitor, and bias switching between fixed and cathode bias. The
pots on top are for adjusting the fixed bias to each tube :



Paul P

[Paul P]

Next time I'm going to build something simple

I've finishing installing the bias circuit. This is made up of two parts,
the bias voltage supply and the switching between cathode and fixed bias.

Here are the schematics :



And in real life :



The bias supply is the circuit board and the pots, and the rest is the
switching. There are four wires still missing, the bias tap from the
power transformer and three ground wires. Since I'm using a star
grounding scheme the question came up as to how to ground this
circuit since it straddles the signal and power circuits. What I've
decided to do is ground the wire from the 220K grid lead resistor
(when cathode bias is selected) to the PI decoupling capacitor
and the wire from the cathode and the ground from the bias supply
to the power section ground (PT, OT etc).

Paul P

[tubeswell]

Gosh that looks awesome so far Paul. Impeccable workmanship!

[Paul P]

I installed the preamp for the normal channel today :



Just missing B+, ground and heater wires, and the signal wire leading out.

Paul P

[Paul P]

Moving right along...

It's starting to look like an amp :



I've added the PT, OT and choke, the decoupling capacitors, the little mixer board,
and mostly finished wiring the output stage apart from the heater wires.



Paul P

[Paul P]

The big black cable is the AC mains going to the power on/off switch. I was
a bit worried about hum coming from this cable so I used a piece of doubly
shielded power cable, with both braid and foil shields. The twisted red wires
are B+ to the standby switch and back :

[ATTACH]1889[/ATTACH]

At the back on the left can be seen the main ground point. I also replaced
the 1 ohm resistors on the power tube cathodes. For some reason I went
with 5w wirewounds which are extreme overkill so I changed those back to
the 3 watters I had originally intended to use. There's very little power
dissipation through this resistor. Besides the big black ones were ugly and
blocked the view of the bias pots :

[ATTACH]1890[/ATTACH]

Here I've hidden the mess under the first filter stage :

[ATTACH]1891[/ATTACH]

Heater bias and balance, as well as B+ to the decoupling caps :

[ATTACH]1892[/ATTACH]

So the rear part of the amp is pretty much done, just need to add the
heater wires to the power tubes.

The transformers are pretty heavy and cause my chassis vise to rotate so
I added a chain to compensate :

[ATTACH]1893[/ATTACH]

Paul P

[Casey4s]

Looking fantastic Paul looks like you are getting close to fire up time.

Is that the biggest chain you could find? LOL

I am getting anxious to see the final product, I wish my current project was anywhere near that neat.

[tubeswell]

Well its a solid looking jig anyway. Very Heavy duty. A sign of the amp to come?

[Paul P]

Today I finished up the last bits of phase 1 of my amp, adding the heater
wires and the signal wire from the normal/clean channel preamp. I could
have made the signal wire shorter but this is a bit of a test for when I
go to a push-pull output stage and need a long wire to the phase inverter.

[ATTACH]1899[/ATTACH] [ATTACH]1900[/ATTACH]

I didn't continue the heater wires all the way to the pilot lamp because it'll
be easier to work on the reverb circuit without them in the way but I may
just add them anyway so I can tell that the heaters are getting power.

It'll be a while before I build up the courage to turn this thing on. With all
the parts and wires there's got to be an error somewhere. At least I didn't
have any "oh sh*t !" moments though I came close a couple of times. The
worst was the bright switch for which I had not taken into account the fact
that there's only one way to install it if you want "on" to be "up" and you'd
better determine beforehand where the lugs'll end up . I was saved by about
1/16" (<2mm) clearance to the chassis. And I had to move the big black
power cable down a bit because the way I first installed it the screws to
hold the chassis coverplate on would have screwed right into the cable

Next will be to go over everything wire by wire, checking things against my
schematics and then throw together a "dim bulb" circuit.

Paul P

[dobsont]

Incredible thread. I dig detail. Watching intently!

[Paul P]

dobsont, I read with interest your experiences with grounding schemes. Now
you have me worried about mine .

I still haven't powered up my amp. I didn't have to look far to find an error in
the wiring. Somehow, even armed with a schematic and a drawing of the
parts in question, I still managed to miswire my DPDT switch for selecting
between fixed and cathode bias. Currently it grounds the grids and the
cathodes at the same time...

I made a dimbulb circuit. For those that don't know what that is, it's a way
of limiting the current into the amp when you first power it up in case there's
a short or the like somewhere. You take the hot wire from the AC mains and
pass it through a light bulb before it gets to the amp. The light bulb acts
like a resistor to reduce current and gives some visual indication of what's
going on. So it's like a variac with a single setting. You can vary the amount
of current going through by changing light bulbs.

Here is the schematic :



and in real life :



Paul P

,

[Paul P]

With a 15w bulb in the Dimbulb it will glow somewhat with the amp turned on
with no tubes installed. With just the rectifier tube it will grow brightly but
the rectifier won't do anything useful. With a 100w bulb the bulb does not
glow at all and the rectifier works properly. So I'm in business.

With the amp off the Dimbulb, a JJ GZ34 installed and no other tube I get :

B+ +448vdc
Filaments 6.6vac (3.3--3.3)
Filament bias + 48v
Bias supplies -55vdc to -35vdc

I didn't know whether the 50v bias tap on my PT was specified in peak or rms
volts. A search on the net led me to believe that it would be in rms but
I wasn't sure. The bias voltage after the first filter cap of my bias circuit is
-70vdc so it must have been in rms. This has raised the range of the circuit
so I'll add a dropping resistor up front or fiddle with the following resistors
to get it down to -25 to -45dc or so.

With the preamp's 12ax7 installed, no power tube, off standby I get for B+ :

+442 after the first filter stage
+442 after the choke
+430 after the first dropping resistor
+407 after the second dropping resistor
+270 to the plates of V1A and V1B

+2v at the top of the preamps cathode caps

So next I'll fix the bias supply and get the preamp B+ down to around 190v.

I can see that I'm going to have to play continuously with the dropping
resistor values as I add more circuits to my amp.

My voltage measuring setup :



Paul P


,

[Paul P]

After a bit of tweaking I have a working amp. Voltages are a bit higher
than I expected, even using a NOS RCA 5Y3GT. Partly I guess because
my 320-0-320 power transformer is putting out 334-0-334 instead, on
a 120vac line.

Here are some numbers, off standby :

B+ first filter... 384 vdc (2 vac)
6v6 at plate ..... 378 vdc
screen node ...... 383 vdc
6v6 at screen .... 375 vdc
preamp node ...... 307 vdc
V1a,b at plate.... 205 vdc

6v6 cathode current at idle :

cathode bias.......... 48 ma
fixed bias at -51 vdc, 48 ma

I plugged my daughter's strat into the amp and hooked up one
of my family room sound system speakers (2-way, sealed, 6"
acoustic suspension woofer, probably less than 90db efficient).

Sounds pretty good so far. There's lots of harmonics stuff going on.
And there's really no hum, just very faint with everything turned up to
max, and this is with the amp open a few feet underneath 14 fluorescent
tubes. So I'm very happy with that.

My grounding scheme is a star ground system with the following
properties :

• all jacks are isolated from the chassis
• each preamp jack is grounded to its section ground
• each preamp section is grounded at its decoupling capacitor
• the grounds from the decoupling capacitors are collected together and a single wire goes to the bottom of the 1st filter stage (two 20uf/600v caps) where the grounds from the heater bias circuit and the power tube bias circuit are also collected
• then there's a wire running from this collection point to a bolt on the chassis right at the AC inlet where are attached the mains safety ground, the PT centertap, the speaker jack ground and the power tube cathode ground

After reading an illuminating article provided by dai h. :

http://ampgarage.com/forum/viewtopic.php?t=220

I decided to twist together the B+ and ground wires from each section to
its decoupling capacitor and run the negative wire of each filter cap along
the cap to the positive end where the two wires are then attached.
The idea is to have as little loop area as possible to pick up interference.
This also makes for neat wiring.

Paul P

[Paul P]

Since this thread is turning out to be quite long I'm going to keep things
free of debugging stuff here and post it in the "debugging your build"
forum. The process of firing up my amp for the first time can be found at :

http://music-electronics-forum.com/s...ead.php?t=6222

Thanks to loudthud I discovered why my bias and idle currents were way off.
Now, using fixed bias, I've got 10w dissipation with -29v of bias and 25ma
of current through the tube at idle. I'm going to have to change my cathode
resistor to 750 ohms or so to get something similar using cathode bias.

And the amp sounds fantastic. This is my first tube amp and my only experience
till now has been my daughter's cheap ss amp. This is something
else. I just played some power chords with the volume close to max and
wow. Beauty ! I can't wait to hear it through a real speaker.

So on to phase two which is add the second channel preamp, then the
tremolo and reverb circuits.

Paul P

[Paul P]

I'm going to add the second channel preamp and the tremolo together, get
that running, and then do the reverb last.

Here are the drilling templates for the two boards :

[ATTACH]1949[/ATTACH]

The boards turreted :

[ATTACH]1950[/ATTACH]

The second preamp is just like the first except for one additional capacitor.

The tremolo is the pièce de résistance of my amp, at least I hope it will be.
This is Fender's patented tremolo from the Brown era amps. You can see
the patent at :

http://patimg1.uspto.gov/.piw?Docid=...iew+first+page

There are three versions of this circuit that I know about. The first is the
one in the patent itself which I don't believe was put into production. The
second was used in the 6Xn amps (and used two tubes) and the third in
the 6Xn-A amps like the one I used, the 6G7-A Bandmaster, which uses
three tubes (though half of one is unused). It can be seen here :

http://www.schematicheaven.com/fende...6g7a_schem.pdf

I've never heard one of these for real but everything I've read about it
suggests that it sounds great. I had to find out for myself. The circuit
starts with a regular oscillator circuit that is then passed through a phase
inverter to produce two opposite sine waves. The high frequencies are
then superimposed on one wave and the low frequencies on the other.
[something happens, I'm not sure what] then the two signals are recombined
and the two oscillator waves cancel out leaving only the now modified
signal.

The board with the jumpers installed underneath :

[ATTACH]1951[/ATTACH]

The jumpers installed above :

[ATTACH]1952[/ATTACH]

The finished board :

[ATTACH]1953[/ATTACH] [ATTACH]1954[/ATTACH] [ATTACH]1955[/ATTACH]


Paul P