The Crate Palomino V32 212 Out of warranty, uncontrollable AC hum

The Crate Palomino V32 212
Out of warranty, uncontrollable AC hum after 2 years of light use. One of the sweetest and most articulate of
Class A nouveau boutique EL84 based amps ever made, and it is silenced by a design flaw… to save money,
the printed circuit board has a surface mounted ceramic cathode resistor that is located right in the middle of
the board, with no venting anywhere. It gets as hot as a furnace, and toasts all the surrounding components
before it burns itself out. Here’s how to fix it...
The Palomino rides again! I fell in love with my V32 212 the first time I plugged in; it has such a sweet and
gut thumping range in the clean channel, and the most musical, straightforward and fingertip-controllable
overdrive channel imaginable. The amp will show you new levels of expression in your normal playing,
improve your dynamic range, and possibly change your approach to the electric guitar; it rewards exploration
of pick angle, finger style, and re-acquaints you with the volume and tone knobs. I have a ’76 Les Paul
Custom, with the original humbucker in the neck position and an EMG active switchable dual to single coil in
the bridge position. Rarely did I take either tone knob below 10, nor the volume. All that is changed, now, I
have figured out how Larry Carlton makes the guitar talk with finger dynamics. I can use the volume knob to
make the transition between light crunch and ZZ Top lead, I can enjoy at living room volume all the tap,
harmonics, muting and feedback tricks I thought only worked at high wattage; and have rediscovered my preelectric (acoustic) technique. Finally, there is the warm articulation from the Les Paul to rival the easy quack
of a Strat. It is the amp I should have had when I started this journey over 35 years ago. Do I love this amp?
Let me count the ways.
Alas, this love story was savaged; snuffed by the inevitable onset of “the hum”. First audible only in the
reverb circuit; it got worse, and within a matter of a month took over both channels with the reverb off and
the gain at zero; so it was no longer a matter of the preamp or a bad cable. The Palomino was put aside. I
switched to a backup amp, then another. 2011 was a busy year... no time to dive in on the repair, couden’t
afford a professional to fix the Palomino (out of warranty). Stopped playing. The amp sat in the corner,
calling to me.
Heat is the enemy of electronic components. A design flaw in the V32 placed a furnace-like ceramic resistor
in an un-vented and highly sensitive position, directly underneath the control knobs which get hot enough to
burn your hand and fry numerous components nearby. This is all explained in a better way than I could, in a
web forum for amps; you can find it at http://music-electronics-forum.com/t11813/ . Kudos go out to the
forum members; especially to Barry Witt (bnwitt) who got this thread started, contributors Bob from
Norwalk (rockon1), charlie chitlins, and others who have added ideas and solutions. I had the advantage of
combing through their several tried and true fixes, which I have combined into a single mega-cooled amp
modification.
Here’s my step by step guide with pics.. and thanks to the good folks with the original ideas. I submit
with all humility three more ideas or extensions of ideas for your consideration, that (depending on your
personal belief system) will strike you as total overkill or way cool.
Pull the amp chassis, flip it upside down. Find R22.
There is is.. a burned out cathode resistor!
The source of all the trouble, and the root of all the hum and noise. In this picture, the resistor has already
been clipped from its leads at the highest possible point, to leave as large a stub of wire lead as possible
embedded in the board surface. You will see the resistor flipped over on its back; and the heat scoring on the
surface of the board in the printed outline for R22.
First mod: remove and replace the ceramic resistor with a higher heat-dissipating design, at a higher
resistance total of 75 ohm, vs. original 60 ohm (thanks to bnwitt)
I soldered a couple of red wires to the stub of leads left on the board when I clipped out the resistor,
and covered the lead ends with heat shrink tube. A hair dryer tightened them up nicely afterwards. Getting
the cathode resistor off the board entirely changes the whole dynamic, and removes excessive heat from the
other more sensitive components (thanks bnwitt, drewl and charlie chitlins)
Now, for New idea # 1, I run the wires (and the new resistors) outside the chassis, entirely. Drill a hole off to
the side of the circuit board, preferably well away from anything important.
Use grommets when you go outside the chassis.
New wiring viewed from the outside. Again, for this mod, hole location is not critical. There is lots of room
on this chassis. It could be argued that bringing tube wiring outside the chassis, or (God forbid) near a power
transformer is bad, but I will let more savvy people explain why this cathode resistor circuit mod does not
introduce the slightest bit of hum or hiss or AM radio signal. All I know is that it works.
Note that 2 150-ohm in parallel makes for 75 ohm total across the leads.. nice tip from bnwitt.
This will lower the voltage on the cathode, and run the "Class A" power tubes a little cooler
than the original 60-ohm resistor that got fried in the first place.
Mouser # 284-HS25-150F ...... $ 2.85 each
Now, This is what I call "Parallel" !!
This pair can be mounted anywhere on the outside of the chassis as long as you are protecting the exposed
leads from curious fingers or metal tools. I mounted the new resistors to the tube shielding cage. Mainly
because it puts the resistor right in the air flow path for Mod # 2, the next step.
Next step: add a fan (thanks rockon1).
buy some more parts...
fan = Mouser # 670-OA109AP112WBR ... $ 15.81
Wire guard = Mouser # 670-G10915A ..... $ 0.87
This fan is big enough to send cooling air directly into the chassis, and cooling air up into the cavity where the
tubes are located.
New idea #2: add a filter to the fan, to prevent dust and dirt accumulation all over the amp
Filter web is from Home Depot.. pack was about $5, for a 50 year supply Replace when the filter gets visibly
dirty (it will be visible once mounted).
I wanted to ensure a very solid mounting for the fan, to handle future transportation/handling issues.
Here’s what I came up with..
3" x 5/8" Home Depot "3-Surface Corner Brackets"
(it is part number 608191 at Home Depot). About $4 for 4 of them.
Build a bracket out of two of the "3-Surface Corner Brackets" Each corner unit is half of the bracket.
The fan mounting holes form a 10.5 x 10.5 centimeter square. This bracket supports the “top” of the fan, and
will attach directly to the chassis.
10.5 cm width for top of fan
(a) For
Mount
to
chassis
(b)
New holes drilled (to attach halves)
Put 2 bracket halves together, as shown above. Slide the brackets against each other at the base, to get the
existing holes at the top of the brackets at 10.5 cm apart. Temporarily scotch tape the two bracket halves
together as shown.
Each bracket half has one (inner) hole at the point of intersection that is not aligned with corresponding
point on the other half, refer to the red arrows above. Mark & drill through the respective overlap points to
make complete through-holes for both halves on these inner surfaces. Then, use two short round-head
screws with their heads facing down to permanently attach the 2 bracket halves together at the intersection
points. I used lock washers. Be sure to face the screw head down; it will not interfere with the next
operation, and provides an almost perfect height adjustment to ensure the 10.5 cm square is positioned
properly.
You will note the extra holes drilled at the tips of the bracket (added later as the final drilling) That was to fix
the mistake I made by not being quite precise when lining up the existing bracket holes to 10.5 cm on first
attempt. Nor was I certain of the spacing between bracket and chassis for the 10.5 cm height spacing
exercise. In retrospect, I could have measured the (top) spacing of the existing holes twice, and added a few
more washers under the round heads of the 2 (red arrow) screws to facilitate the perfect 10.5 cm square
before drilling the pattern in the next slide. Don’t fret, though, it all works out in the end using washers for
the shims on the final height adjustment. They are cheap and solid, and nobody will ding you for it. And if all
else fails, you can re-drill the tips!
With tubes facing up, drill the hole pattern below. Use the bracket as a template, and mark the holes
indicated by the yellow arrows (a) and (b) both the bracket illustration above, and the chassis illustration
below. Note that the chassis slots will be directly in the path of the fan, and the tubes are roughly centered
in the fan path.
(a)
(b)
10.5 cm width for bottom of fan
Note the holes for the bottom of the fan are drilled out in this picture. Depending on your confidence and
the quality of your ruler and square (mine was cheap plastic and hard to read), you could simply (1) mark and
drill all holes at once, or (2) pre-assemble parts before marking and drilling the bottom of fan holes. The key
is to keep all holes at 10.5 cm spacing.
Viewed from the inside.. gotta be careful how deep that drill goes!!!! (here, with the screws, nuts and lock
washers..)
and mount the bracket.
Use a few stacks of flat washers between the bracket and the chassis to get the height spacing right, and
complete the 10.5 x 10.5 cm square on the drilled holes receiving the fan.
10.5 cm
10.5 cm
While you have the drill out,
I: Drill a hole for a new toggle switch to turn the fan on and off. Great feature, thanks again to rockon1.
When you want the ultimate quiet, just shut the fan off entirely.
II: Drill a hole to route the power leads from the toggle switch to the fan. Note the heavy card paper I
slip between the chassis and the circuit board to collect metal filings when I drill. One could argue you
should remove circuit boards before doing such things. I took a different approach. Decide for yourself.
Clean it up, thread wires and add grommets
Wire power to the switch, tapping from existing push-on connectors to the circuit board. It’s a little hard to
see here, but just make sure your tap on the hot (black) wire is AFTER the fuse, not before.
Make sure to tap into the 110 V at the lug downstream of the slow blo fuse to get
power to the switch. Here, black is hot, yellow is neutral. Grommets in new hole out of chassis..
New Idea # 3: a dimmer speed control for the fan, between the fan kill switch and the fan.. for very cool low
noise operation. Using a standard solid state dimmer common to light fixtures, the fan can be regulated from
a whisper quiet slow breeze to a full-on turbine. I have found my perfect setting for balance of airflow vs.
quietness. Although the dimmer also has an on-off function, I prefer the toggle switch. It is in easy reach,
and I can set the dimmer to an ideal flow, then activate/deactivate without touching the dimmer knob (which
tends to be quite sensitive at low RPM’s).
Switch, $3
Dimmer, $6
Oh, and this is just neat.. when working on the bench, you can flip the entire amp over, tubes and all, and it
makes a stable 3-point stance. Nice for working on the internals.
cut 3 little cooling slots in the back (credit rockon1’s solution, + 2 more for fun and safety)
She's still really pretty from the front!!!!
All total, about $36 of parts plus wire, screws, nuts and washers.
the BEST part of all.. the amp sounds fantastic, not a bit of hum anywhere, clean and powerful, and I can say
that the 75 ohm modification has not hurt the power whatsoever in the crank-it-up region.. I may be fooling
myself, but if anything it gets a little bit of smooth compression going at an ever so slighlty lower volume. I
don't like it when my ears bleed anyway. After about 2 hours of playing, I put my hand on the knobs.. they
were friggin cold. No exaggeration. I do mean metal-cold knobs, with the fan running at a whisper. Next
time I will run with the fan off. Just to convince myself that I didn't accidentally convert it to a zombie
vampire amp.