Quad II fuse-blowing

[Henrys Cat]

I''ve not had any problems with using polypropylene MIT Multicaps for C1, C2 & C3 in my refurbished Quad II's. None have metal cases and I've not noticed any of the effects Keith Snook refers to (I am using 6L6G output tubes).

R12's power rating must be upgraded as 3 watts is far too low and it actually has more than 3 watts of power flowing through it under normal conditions!

I replaced C4 & C6 with 15 microfarad and 22 microfarad Solen polypropylene capacitors respectively rated at 630V. Because they're quite large they are attached to the base-plate using tool clips. I've not tried replacing the GZ32 with silicon rectifiers partly due to the much higher H.T voltages that would result, especially at switch-on before the output tubes start to draw current.  There's quite a big voltage drop across the GZ32 which is lost with efficient semiconductors so you're looking at a H.T supply of around 438V.  As I've mentioned in an earlier post my H.T voltage has always been higher than the stated 340V (around 370V) so with silicon rectifiers it might be over 450V!

If you can't get a replacement GZ32 easily I've used modern GZ34's without any problems.  Attached is a file from a website which I think is now defunct - Quad Pages - lots of useful information in the text though.  The images are missing, they display on my PC as the files are stored there but won't save in the archive for some reason.

 

[tpatton]

Last night both Quad II amps played beautifully for 30 minutes--I think I'm out of the woods, having started out with a bad rectifier tube and then, through missed solder joints, ruined another.  I have only two good rectifier tubes left, but they don't seem in danger of destruction any more.

I still want to go over to solid state rectifiers, and welcomed Paul J's idea of puttinhg a 100 ohms resistor in series with each diode in that SS rectifier.

What I need are instructions, on a level with those wonderful Bottlehead kit manuals we all appreciated so much, of precisely how to do this: which rectifier tube sockets to solder the resistors to (with the wire formerly going to those sockets now going to the other ends of the resistors).

I also wondered if 100 ohms was definitely the value to choose, or whether 120 ohms, say, might be a more conservative choice, but still allowing enough voltage to come through.

I've also asked another question once or twice, and would still like an answer: is capacitor C1, showing up close to the input on the circuit diagram, important to the sound?  Should I use Obbligato Premium Gold there instead of the F104K's that came with a Paramour II kit?

But the solid state rectifier question is the one I'd really like to get answered.  Thanks in advance for this, and a very big thanks to everyone who has helped me through the ups and downs of this long struggle to get this ancient pair of Quad II's working again--as I'm pretty sure they now are.

 

[Paul Joppa]

You can use a single resistor from rectifier cathode (pin 8) to the first capacitor C6. Naturally I don't have an example in front of me, so I don't know how they are now connected, but if there is a wire that can be replaced with the resistor that would make it easy.

The point is to limit the available charging current to the capacitor, so you can use either a resistor in each diode plate lead, or a single resistor in the cathode lead. It seems likely that the power transformer high voltage lines go directly to the plates (pins 4 and 6) so there is no convenient terminal on the plate side.

The average current through the resistor will be 147mA according to the schematic, which would be 2.16 watts if it were pure DC. However the current at that point probably has peaks of 500mA, and I would estimate the RMS current at twice that, or 4.3 watts. At 120 ohms, it's a hair over 5 watts. I would use a 15-watt rated resistor; I have found a safety factor of 3 or more times the actual dissipation to work reliably with resistors. Keep the resistor from touching any capacitors or other heat-sensitive components.

You will have to experiment with the resistance to get the target voltage, which should be equal to the voltage with a tube rectifier. My guesstimate of 100 ohms is just that - a guess.

Note that the extra 4.3 watts increases the heat under the chassis; that extra heat is normally dissipated by the tube above the chassis. Again, I don't have one of these in front of me so I can't assess how well such under-chassis heat is vented.

 

[tpatton]

Thanks very much, Paul, for all this.  I can easily put a resistor between rectifier pin 8 and C6.  It's clear from the circuit diagram, isn't it, that the power transformer high voltage does go directly to pins 4 and 6 (now that I know, from knowing where pin 8 is, which pins these are).

Experimenting with 15 watt resistors would be both expensive and call for voltage measurements, which I'm chicken about, only doing them when absolutely necessary (as with the Bottlehead kits) and with my wife there ready to pick me up off the floor and call an ambulance.  So despite the fact that I've already ordered a pair of solid state rectifiers, I might go back to looking for 5V4G tubes that don't cost an arm and a leg.  It boggles the mind what lots of people are willing to pay for them!

I've become neurotic about rectifier tubes and their ability to last--less than 12 minutes, in a couple of cases--but maybe I'll get over that if the present ones continue not to fail or to cause fuse blowing.

 

[Henrys Cat]

Re. C1's effect on the Quad's sound.  I've not had experience of the what effect it may have on the sound simply because I've not tried different brands or dielectric type.  You may have to experiment.

120 ohms as a dropper resistor seems rather a low value to reduce the H.T voltage.  If 147mA is flowing through then it only drops 17.6V DC and about 25V with A.C current. 

Re. the Tube Depot SS rectifier.  It's a full-wave rectifier equivalent of the GZ32. The anodes connect to the A.C windings of the HT transformer and the third connection is the equivalent of the cathode and supplies the rectified DC.  The centre tap of the transformer is the ground connection.  You could replace the wires from the HT transformer to each anode connection on the tube socket (pins 4 & 6) with dropper resistors and perhaps another dropper resistor from pin 8 to the smoothing capacitor. Like Paul Joppa says you will have to experiment with resistor values but remember, the higher the value the more power is dissipated and heat generated.  You could always try GZ34's which are cheaper than GZ32's.

 

[Paul Joppa]

Mouser has Xicon 100 ohm, 15 watt wirewound resistors for 64 cents each. Antique Electronics has 5V4s for $15-$20.

 

[JC]

If these 100 Ohm resistors are for use in front of the anodes of the diodes, then I think from a look at the schematic that 5 Watt resistors would be just over double what they will see in operation.  In the past, I have found that putting these resistors in that position when using a SS replacement rectifier package is a good idea, both for dropping some B+, and also for  current limiting for the diodes during turn-on and resisting any little diode switching transients on their way back to the high Voltage winding of the transformer.  Additional resistance can be added between the junction of the cathodes and the first filter to tweak the B+ further, and similar attention must be paid to the Wattage of the resistor.

IIRC, the last time I bought a 5V4 from Groove Tubes, it was around $20.

 

[Doc B]

Antique Electronics has 5V4s for $15-$20.

Yeah, I was thinking "since when are 5V4s that expensive?" I'll stand by my suggestion to replace all the stock caps with similar values - and any resistors that are out of spec, and keep the tube rectifier as the best course of action for you.

 

[JC]

Well, the only thing I have been able to find is that Dan Torres has sold a lot of NOS ones, and has gone to Sovtek now.  I haven't bought any of that type for a while, and tubes can be like the commodity markets sometimes, I guess.

Any progress reports on the Hammond?

 

[tpatton]

Doc B, I've replaced ALL resistors and capacitors, but am leaning toward your recommendation of sticking with rectifier TUBES, to avoid complications I'm not competent to handle, not the least of these being the heat Paul J warned about.  I'd guess that the Quad II chassis is especially bad in that respect.

I guess I'll have to experiment with a "boutique" C1 instead of the Paramour II's F104K.  I thought it would be easy to tell from the circuit diagram whether it would affect the sound quality--guess not. 

Paul J, your efforts to tell me how to handle a solid rectifier, with its several cautions, and your latest on where to buy, are much appreciated, even though I may not go for a solid state rectifier in the end.

Will a GZ34 work perfectly well in these amps?  Has anyone tried them?  I would have thought probably not, since they don't come up as equivalents.

 

[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.