Bottlehead Forum
General Category => General Discussion => Topic started by: tpatton on October 19, 2011, 06:37:29 AM
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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You had my sympathy at least:) Glad you got them going...John
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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.
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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.
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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.
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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?
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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.
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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!
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I just hit a wrong key and lost a lengthy reply to Deluk and Paul J, and will say it all again.
For any who want it and don't yet have it, here's where to find the circuit diagram: www.keith-snook.info >> Quad II Information >> here >>Quad II Power Amplifier
On that diagram, I see no voltage higher than 340V. My C4 and C6 are rated at 450V, my C2 and C3 at 650V, my C5 at 50V, but no high voltage there (but lots of current: Keith S recomments higher than 3W for R12 and I have a 5W Mills).
Would a solid state rectifier need any circuit change for the amp to handle it?
I could put a 100 ohms resistor in series with each diode if I knew in physical terms where to put it. In terms of rectifier socket pin numbers, maybe?
On the circuit diagram, there's this scary note (bottom right):
Note: The original capacitors for C2 and C3 had insulated metal bodies which were connected to ground, the small capacitance to ground controls the open loop frequency response and thus the stability with feedback; change those capacitors with caution!
This is all Greek to me. My Obbligato Gold Premium C2 and C3 have nonconductive bodies. Am I in trouble with them? Is there anything I could/should do?
One last question: is the quality of C1 important to the sound? I'm using a capacitor that came with one of my Bottlehead kits, rectangular and orange-brown. I could use an Obbligato Gold Premium, but guessed that in that early position it wouldn't help the sound. Any advice on that?
Thanks to everyone who's gotten involved with this project of mine and its problems.
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I posted last night and it never got here--it says I posted, but no post of mine appears. I hope this one works out better.
I didn't follow Paul J's suggestion at first, then figured out that the diodes that could have a 100 ohms resistor in series with them are, in a solid state rectifier, like the diode bridges us Bottlehead kit-builders used to mount on a terminal strip. I see four wires going to four rectifier sockets, and am supposed to make them connect to 100 ohms resistors instead, which then go to those four sockets.
When I feel able to take the pain of ruining my last good rectifier tube, I'll put it in, and with terrific luck the amp will play troublefree for more than 10 or 12 minutes, with no bright flashes from the rectifier tube. But I want to change over to solid state rectifiers anyway. Has anyone looked at what GZ32/5V4G tubes cost? It's one of those tubes that some audiophiles go crazy over, and even the cheapest aren't anything like cheap.
I had another question that failed last night to get posted. Is capacitor C1, which separates resistors R2 and R3, important to the sound? If it is, I could use Obbligato Gold Premiums there. But I assumed that it wasn't, and used a cap that came from my Paramour II kit, a F104K. Should I upgrade there?
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I now know about Page 2 in these threads: what I wrote last night did get posted after all. Sorry to be so dumb as a Forum contributor, maybe I'll be okay from now on.
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I was about to order a pair of Tube Depot solid state rectifiers, when I made the mistake of looking at the schematic for them: http://site.tubedepot.com/pdf/ssr.pdf. And confused myself badly: there are only three rectifier tube sockets used, but I have wires going to four. I realize that a tube needs current to allow it to work, provided by at least some of those wires. Still, I'm back to not knowing where to put the 100 ohms resistors in series with the diodes, and it seems that there are only two diodes, not the four found in the diode arrangements used in Bottlehead kits, which puzzles me too.
Can anyone help, Paul J in particular? I'll remember you in my will.
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Have you looked at the link that Henry's Cat posted for you? It shows one guy's approach. Note the Wattage ratings on the resistors he used. Basically, the little drop-in SS replacements for tube rectifiers just use SS diodes to replace the two diodes found in the tube version, and leave the heater power unconnected. This generally results in, as you have learned, a significantly higher B+. So, the conventional approach is to waste some of that extra Voltage by dropping it across a resistor or resistors.
Perhaps the first thing to do is to find out how much your rectifier tube typically drops the Voltage. You can hopefully get that info from a tube manual. You can then get an idea of how much higher your B+ is going to be if you use SS rectifiers instead. That will allow you to calculate a resistance and Wattage for the resistor you will need to drop the B+ back down to within spec.
Getting this info together is part of what Doc was referring to when he replied to you; you are essentially redesigning the B+ supply to a certain extent. Another thing to be aware of is, when you stop using the 5 V rectifier heater winding from the transformer, your other secondary windings might creep up a bit on you. Maybe not much, but you don't really want to exceed the heater Voltage rating on the other tubes by more than 10 or 15%, so it is important to check.
Meanwhile, is there perhaps a less expensive substitute for your rectifier tube by any chance? It might pay you research that as well. If there is, you could maybe get one or two for testing purposes or temporary "in a pinch" replacements.
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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.
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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.
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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.
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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.
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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.
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Mouser has Xicon 100 ohm, 15 watt wirewound resistors for 64 cents each. Antique Electronics has 5V4s for $15-$20.
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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.
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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.
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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?
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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.
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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.
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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.
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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!
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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.
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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?
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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.