Need help on Resistor type

[rockpassion]

I have a pair of Quicksilver Mono-block amps, the 8417 variety converted to El-34.  I do not have the schematic for these so I am at a loss at what type of resistor I need.  I have found a 430k 5% resistor that appears to have overheated.  When I check it with my meter it shows being open.  Also it has a bunch of oily type material coming from each end that is brown and gooey.  It appears to be either a carbon film or carbon composition type resistor but I am not sure.  It is connected to the terminals of a large 320uf 450VDC large blue Mallory Capaictor that is mounted on the top of the amp.  Can someone tell me what type of resistor I should get and what is the difference between the two. 

Thanks
Richard

 

[mcandmar]

Can you show us a picture?  Its function is key to determining a suitable type, though it sounds like its just a bleeder.  They key question is why did it fail?

 

[Grainger49]

If it is attached to one of the power supply caps it is either for bleeding of the voltage when it turns off or to smooth out the power supply.

If it is a power supply resistor a wire wound is good.  If it bleeds down the high voltage at turn off a metal film will work well.

 

[mcandmar]

Found it, looks like just a bleeder. As above i would fit a metal film, 3W or greater, just check the voltage rating of the resistor you buy, if its a 450v cap i would use a resistor rated for that voltage.

 

[rockpassion]

Mcandmar, thanks for the picture and the information on the resistor.  I have one other question.  The large yellow capacitors going to the 12ax7 tubes appear to be auricaps.  Can you tell me what value you used and did it make a difference in the sound of the amps.  I figure while I have it open I might as well change these Caps to something better. 

Thanks

Richard

 

[rockpassion]

Mcandmar, to clarify a bit, I left the amps on all night. When I noticed that the bad amp was off I checked the fuse and it was blown. I replaced the fuse and was greeted with a very loud hum after I turned on the amp from the connected speaker. I changed the tubes to see if I it was a tube but no change. If the resistor is just a bleeder resistor I find it hard to believe it is causing the hum. I am not really technically oriented so any help would be appreciated. 


Thanks again

Richard

 

[mcandmar]

First off, that's not my amp, i just googled Quicksilver 8417 looking for pics of the internals.

In my limited experience cap rolling, and depending on what caps your amp has i'm going to say yes, almost certainly.  Grainger is the guy to talk to re cap rolling, hes a seasoned veteran at it!

I really dont know what the problem could be, BUT if the resistor has failed i would ASSUME it hasnt done its job and the capacitors may still have a lot of voltage stored in them so put a meter across them before doing anything.  I dont know of any reason why a bleeder resistor would fail, but the noise would suggest to me the possibility of a bad or shorted cap.

To be honest its probably best to wait for one of the BH guys to chime in, or take it to somebody who can diagnose the problem.  I've had far too much to drink this evening to be talking on the internet

 

[howardnair]

http://www.diyaudio.com/forums/tubes-valves/43124-quicksilver-8417-power-supply-caps.html

http://www.diyaudio.com/forums/attachments/tubes-valves/76520d1167232110-bias-adjust-installation-bogen-mo-100a-mo-100a-quicksilver-qs8417amp.jpg

above are some schematics that may help--it seems that you may have a failed power supply cap -

 

[Grainger49]

Do check the capacitors.  If they have been subjected to high voltage they often have the top, bottom or both bulging.  This indicated a damaged capacitor.  The resistor could be the second thing to go, i.e. once the caps open the resistor carries all the current.

As for Auricaps, they are very popular in Bottlehead equipment.  I've had several in my Bottlehead equipment.  Paully has had some in a ST-70 that he rebuilt and was very happy with.  And they are very reasonably priced!

Howard, I didn't find a schematic on the first link, the second one requires a log in.  I'm not a member there.

 

[howardnair]

https://picasaweb.google.com/100635555508988591604/Feb232014?authkey=Gv1sRgCO7g3s628M-v5gE

try this

 

[Grainger49]

Howard,

I'm having a bad time there.  I can call up one schematic but it doesn't have a 430k Ohm resistor on it.  Does the other schematic have a 430k Ohm resistor?  If so where is it?

Thanks,

 

[howardnair]

no it does not-the schematic shows a  330k 1 watt resistor across each power cap-and doing some reading -not sure how relevant it is but it seems the 330 uf cap is too large for the gz34/5ar4 rectifier a mod shows disconnecting the 330uf and putting in a 22uf to 100uf 500v cap-not sure if this will post but here it is
http://www.triodeel.com/8417.htm

toward the bottom  of  the page the 330uf are paralleled and a  22uf  to 100uf500v  to ground
before the choke
also try this
http://www.bonavolta.ch/hobby/en/audio/8417.htm#8417%20Push-Pull

 

[Grainger49]

Ok, both of those links worked for me.  The 330k Ohm resistors are bleeders indeed.  They could just as well be wire wound resistors.  Metal film is good too.

The inrush into the first cap is what controls the RC charging rate.  The second cap can be much larger without damaging the rectifier or blowing the fuse.  I have heard this from PJ many times and circuit wise this makes sense.

 

[howardnair]

Grainger- knowing that the first cap is too large and a simple mod is in order and hopefully no damage to the rectifiers--leaves a question-does Richard understand this and can he implement it-also how  could this cause what we perceive to be cap failure-Richard!!

 

[Paul Birkeland]

What the schematic calls for and what is in the photos embedded in this thread are pretty different.

The embedded photos show a single carbon composition bleeder across one of the high voltage caps in each amp.  I would suggest going with the ~330K value in the schematic, and perhaps using a Vishay PR-03 resistor in this position. 

It is possible that the 2x330K language in the schematic means to put them in series to get 660K (less power supply load), or in parallel to get 165K (faster bleed down).  Since you have "stock" amps, you might be able to peed in there and tell.

I agree that the first cap is huge.  I'd be looking for a 100uF motor run to stick in its place. 

 

[Doc B]

peed in there and tell.

Whoa! Make sure it's unplugged first if you do that!

 

[Paul Joppa]

Just saw this thread. Nobody has mentioned how odd it would be to see an oily, gooey mess exude from a resistor - so odd, that I expect it is from the capacitor not the resistor. Especially since it's an old electrolytic capacitor. Maybe I'm wrong, I can't make enough sense out of the picture to see what the rest of you seem to see.

The maximum capacitance for a rectifier tube actually depends on the power transformer resistances - the real limitation is the peak current on a cold start. Sometimes you can find a more detailed specification sheet for a given tube that will go into more detail on this than the usual sheets.

 

[howardnair]

Paul-yes i wondered about that-but- i did not say a word--there are no pics of the amp in question the only pics are of a amp that mcandmar found as a reference-besides somebody peed in there

 

[rockpassion]

I am trying to get some pictures of my amp and I want to thank everyone for the suggestions.  One area to clarify is these were modified by Quicksilver to run EL34's since the 8417 were becoming so scarce and had become rather expensive.  I believe that is the reason for the large first capacitor, the value is 850 MFD/475 VDC.  I currently run KT77's which I find to be very nice.  I have never had any problem with these amps and use them as stand ins when my Dynaco MKIII are on the fritz (which they are now).

I did find part of my problem which was the reason for the loud hum.  My input connectors from my CD player were touching.  They are 90 degree connectors and on occassion they twist and touch.  Needless to say these will be changed this week.  I listened to amps briefly today and sound wise they still sound good.  Not as good as my MKIII's but close.  I did get new Auricap caps to replace the caps that are connected to the 12AX7's and the 12BH7's.  I also matched the values of the replacement with the values in my amp that is good, which is a .22uf/600VDC.  I think Quicksilver may have made a mistake because the current value in the problem amp is .22uf/250VDC.  Anyway, I believe this cap change may give this amps a little more detail and clarity, at least I hope so they are a little laid back. 

Again, thanks for all the help, I did learn a few things so my problems turned into a learning experience which I am thankful for having this forum to turn too.

Richard