Quad II fuse-blowing

[tpatton]

I finally got external transformers for my European voltage Quad II's, and one of the amps works fine.  But the other amp works for about a minute and then blows a fuse--initially in the too-small 200 watt step-up transformers I had at first, now the fuse in the amp itself with the new 500 watt transformers being used.  I was looking at the insides when the last fuse was blown, and couldn't see anything going on before hearing the gentle noise of the fuse giving up the ghost.  Could a bad tube be causing this?  Can anyone tell me what I should be looking at to cure this problem?  Not easy to do at a distance, I realize.  I'd like to get both amps working before I begin my sweeping replacement of resistors and caps, both electrolytic and film.  Thanks in advance for any advice anyone can offer.

[Paul Joppa]

It may be a bad tube, or (probably more likely) leaky or shorted capacitors. Bad caps can cause tube damage, so I would not suggest you put the good tubes in the bad amp to see if that's the problem! Try to find a tube tester instead.

[JC]

And, if you don't have access to a tube tester, check your area for a guitar shop that works on vintage amps; they almost certainly have a tester.  Let them know that they may have to test the tubes for a minute or two; I have certainly seen my share of tubes that test fine initially until good and warmed up.

Then, as Paul says, if they find a bad tube, try to ascertain if another part may have caused it to fail, if you can.  It may help to make a little chart to indicate to you which position each tube was in before you remove them; that way, if one tests bad, you will know which socket it came out of to give you a starting point for further investigation.

[tpatton]

Thanks, both of you.  It looks as if I might do well to make all the replacements, including caps, and change all tubes, and see what emerges.  Right now I'm waiting for more fuses from Digi-Key.  But the fuse-blowing amp already had one major cap replacement, a pair of 16 uF caps, now Mundorf 15 + 15 "double caps" bypassed by 1 uF Solens.  The KT66 tubes have .1 uF film caps on two pins, which would be easy to replace, so maybe I'll do that too.  Thanks again.  I thought those peculiar symptoms--playing normally but only for about one minute--might point to something specific.

[Henry's Cat]

This sounds like it is a tube fault if the fuse is blowing a minute or so after switch on.  Do you get any sounds or music out of the speakers before the fuse blows?  Have you monitored the HT supply voltage to see what happens to it in the first few minutes after switch on?  It should rise gently and then drop, settling when the output tubes start drawing HT current to around 340 - 370V.  My pair have always had a HT voltage of around 360V even though the circuit diagram shows 339V.

[tpatton]

Thanks for the suggestions, Henry's Cat.  I'll follow up on them when some new fuses arrive.  I already have a CL-90 inrush surge protector in place.  Right now I'm proceeding with the resistor/caps changes, all resistors and caps, and when I next try the amp, I'll put in the tubes that work long term in the other amp.  If the problem amp then works fine, I'll know the problem stemmed from a tube, won't I?  I'll report on this, but without fuses on hand and being in the early stages of the resistor/cap changes, it won't be for a while.

[tpatton]

I got fuses today, and tried the troubled Quad II with the tubes from the good one--and that worked!  What a relief!  So it was a bad tube, and I now have to find out which one.  The GZ32 rectifier tube looks black on one side, but resistance readings are the same as on the other GZ32.  (I believe/hope that's the same as a 5V4G, a pair of which I'm pursuing on eBay.)  I may blow a few more fuses finding out which tube is bad, but with 10 on hand, that won't be too bad.  Meanwhile, I'm finishing up with the resistor/capacitor replacements, to get both amps "restored".  Before too long I'll be able to hear them as a stereo pair.  I'm sure they won't compare with either my SEX amp or my Paramour II's, but at least they kept me out of mischief for a while, and will make me remember my youth, and my first pair.

[tpatton]

My guess about the GZ32/5V4G rectifier tube was correct: I learned that at the $.41 cost of one more blown fuse.  With my one good rectifier tube, the amp played perfectly nicely.  I'm waiting now for new rectifier tubes from an eBay purchase, and all should soon be well.

I can't help still having my fingers crossed, but that's just me.  Thanks to all for sympathy and advice.

[2wo]

You had my sympathy at least:) Glad you got them going...John

[Henry's Cat]

Glad you've found the faulty tube.   The amps are well worth the effort of refurbishing.  Have a look at this site -  http://www.turneraudio.com.au/quad2powerampmods.htm.  The GZ32 has been replaced with silicon rectifiers with these mods.  Not tried them myself though.

[tpatton]

Henry's Cat, it was amazing that you posted this since I was just going to ask about a solid state rectifier.  My idea now is that my trouble was not a bad rectifier tube per se but rather a problem that lead to the destruction of my rectifier tube.

Progress today: after redoing some bad soldering (shame on me, I know), I had both Quad II's playing.  One, with an undamaged (as far as I know) rectifier tube, behaved nicely, sounding good.  The other, with a rectifier tube probably damaged by the bad soldering, acted somewhat strangely.  It came on and was playing nicely, but twice in six minutes the rectifier tube flashed brightly--white light--but kept playing, sounding like an LP surface defect, quick recovery.  It then played for ten more minutes, and then blew its fuse.  I didn't want to put my one remaining undamaged rectifier tube in before consulting this forum for advice.  So I'm maybe halfway there, maybe even closer.  One amp seems just fine now.

Here's what Tube Depot says about their solid state rectifier:

Note: Please be advised that this replacement will increase overall B+ (plate voltage) by 10-20%. If your filter caps are older than 20-25 years, we suggest you replace them before installing this solid state rectifier.


I'm not sure which the filter caps are, but I changed every cap in the amps, so none are too old.  Can I safely go ahead and order a pair of their solid state rectifiers?  I'll wait to hear.  I'm sorry to have been such a basket case with this Quad II restoration project, but grateful for all the help I've gotten.

[Doc B.]

In a new design I prefer SS rectifiers. But if you retrofit them into a tube rectified circuit you must know what you are doing. The increased B+ can not only overstress caps that may not be rated for the voltage, you may also need to adjust the operating points of the tubes.

There is a very simple way deal with old amps: replace all of the caps with caps of the same values, replace any resistors whose value has drifted, and make sure you have good tubes in the amps.

[tpatton]

Thanks, Doc B, for the cautions.  I'm okay on the voltage rating for the caps, which I think must be C4 and C6, now Mundorf M-Lytic 15 + 15 "double caps".  But I (as you'd expect) don't know how to adjust anything relating to the (output, I guess) tubes.

GZ32/5V4G rectifiers are usually quite expensive, and I'm getting tired of destroying them, though with luck that's in the past.  But with no spares left, I thought the solid state rectifiers, at $10 apiece, looked like a good, worry-free solution.  Guess not.  Could anyone tell me how to adjust--the points, was it?--for or on the tubes?  Change a resistor or two?  I could tell you where to find the circuit diagram again, maybe a good idea?  Or could you tell me in a general way, in terms of what that diagram says?

[Paul Joppa]

The current is not adjustable on the circuit you posted. It is set by the cathode bias resistor which is shared between the tubes. This arrangement is sometimes also called "automatic bias" and does, to some extent, compensate for voltage deviations from the design value.

(You must for the same reason use matched pairs for the output tubes, in order to maintain minimal DC current difference in the output transformer.)

The 5V4 is a relatively high perveance tube, so replacing it with a solid-state rectifier will produce a smaller increase in voltage, compared to (say) a 5U4. Neverthless, a change of 40-50 volts is to be expected. Thus the capacitors should be rated for 50v more than the original ones, unless you know for certain that the original parts were over-rated by 50v or more. You can add a 100 ohm resistor in series with each diode if you use solid state diodes, which should reduce the voltage to something close to what the tube gives.

However, if there is some problem with the circuit or the amplifier which causes 5V4s to blow frequently, then that problem has a good chance of blowing solid state diodes and/or other components with similar frequency.

[Deluk]

Hello Tom. My first post, another Quad II owner. Maybe it would be a good idea to carefully/safely check all of the relevant voltages at the points marked on Keiths circuit on your "good" amp and compare them with those on the other problem one. Getting them back as close as possible to the original design is probably the best idea at this point. Making non standard mods to an amp which isn't working properly now, won't make trouble shooting any easier.

As a footnote to Henry's cat, I took the bases off mine at the weekend to check the resistor values. Not been off for about 15 years. Some drifting up to about 25% on the 10 % ones and as you mentioned R12, I found one to be virtually dead short, about 6 megohm. Amp sounded fine and I can't say I noticed any change after I replaced it! Golden ears? Obviously not!