Quad II fuse-blowing

[Doc B.]

Any progress reports on the Hammond?

Yes, a fair amount has been repaired so far. The Type F amp in one of the D-20 speaker cabs (actually Webster-made PPP 2A3s) had the power trans replaced at some point, with something that was running 100V too low. So in lieu of the proper power trans I modded the LCLC filter to a CLCLC and got back about 60V. The wax caps throughout the organ are averaging about double the nominal capacitance and many are in low pass filter circuits, so they are being replaced as I go through the various stages - maybe about 80 pieces total. The Model E has not one, but two tone generators (and two expression pedals and two tremulants, practically two organs in one box). The regular tone generator was running OK, but the AC power to the chorus generator (that big monster with all the magnet shafts sticking out the back in the photo) motors had been disconnected at some point and the wheels seemed stuck. Desoldered about 60 wires holding it in place, flipped it on it's side, shot about a half a can of WD-40 into the bushings and after about 30 minutes of gently working the tone wheels shafts back and forth they broke free and it seems to be running at the proper rpm. Have to restring the missing oiler threads and then it will hopefully continue to run as expected.

Last night I charted the tonewheel output levels. There are definitely some weird variations that I will have to adjust after I replace 50 or 60 wax paper low pass filter caps in the generator (this weekend's fun). Then the chorus generator can be reconnected and adjusted and I will be running out of excuses for not practicing my scales.

Cabinets look bad, will probably redo the lacquer finish next summer.

[Paul Joppa]

Finally got a chance to look at C1 again. It's used in an unusual way; I don't recall ever seeing that in any other schematic. So it's impossible for me to tell what effect it might have on the sonics. Fortunately, 0.1iF caps are plentiful! C2 and C3 are definitely in the signal path.

[JC]

Oooh, thanks for posting the pic!  Now, if anyone didn't already know why these little gems weigh so much, they will have some idea!

I look forward to hearing the clips down the road!

[tpatton]

Paul J, if you can't predict the bearing that C1 might have on the Quad II sound, I bet no one, with the possible exception of Keith Snook could.  If I get a rest from daily setbacks--see my 6L6 tube trouble in a new post asking about equivalents to KT66--I'll someday put an Obbligato Gold Premium in as C1 in one chassis and see if I can hear any difference between it and the other, on good mono material, one amp at a time.  More fun!

Again, thanks for putting so much time into helping me with my Quad II adventures/problems.

[tpatton]

I emailed eBay "amplifier_surgery", who restores Quad II amps, and here's his reply:

Hi Tom, I would not bother with the silicon Rectifiers. In my opinion the unit sounds lifeless after that "upgrade". We have performed that work in the past and I have always felt that the tube rectifier sounded more detailed. Regards, Hamad.

That decides it for me.  Now to get rid of two solid state rectifiers, thru Audio Asylum, maybe?

[tpatton]

I asked Hamad at Amplifier_surgery (eBay) about C1, and he said this:

Dear tomp961,

Hi Tom, C1 may have been an attempt to eat DC spikes or produce a warmer sound. But it will effect sound if a poor capacitor selection is placed there. Any high quality polypropylene (low leakage) cap would work well there. Regards, Hamad

No wonder Paul J was puzzled.  Once I have all tubes matching in the two Quad II amps, I'll listen for a difference between Obbligato Gold Premium and Bottlehead F104K.  I don't expect to hear a difference, but with the two amps set up for the test, I'll try it.