Author Topic: Tube problem?  (Read 11437 times)

Offline jimbones

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Tube problem?
« on: January 15, 2013, 05:17:33 AM »
May have a tube problem in my SP8. changed all tubes (with old stock excluding the phono and PS tube) one at a time no improvement. PITA! So I just ordered all new tubes as balanced/matching to improve odds of having L&R channel the same but I have a feeling it may not be a tube. One channel is lower in volume and sounds a bit "muffled".  [-o<
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Offline richidoo

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Re: Tube problem?
« Reply #1 on: January 15, 2013, 07:58:46 AM »
Clean the volume and balance pots and switch contacts? Do not use Caig Gold on plastic pots, it leaves a film. Just use cleaner, or Caig pot cleaner. Spray inside, work it, rinse, work it, repeat a few times. I use RS contact cleaner spray.

Check B+ and inspect PS caps?  Small storage caps, just a couple 100uF electrolytics, but inspect those for burning or leaking. V7 is line stage B+ regulator, a 12BH7 tube making 402VDC. It has 640V on the plate. Have you replaced that? This would affect both channels tho.

Cathode resistor failing? Look at R23 and R28 and R25 on the failing channel.
R23 = 1.8k wirewound
R28 = 6.98k
R25 = 1.5k
If one drifts higher it can lower the volume.

They can be tested in circuit with preamp OFF, but verify storage caps empty (wait half hour after shutdown, then verify no DC voltage across R31) before testing resistors. I would not advise testing the B+ voltage yourself due to lethal high voltage when preamp is powered on. Since it is only one channel, the PS is probably OK.

Here is the amplifier schematic, in the pictures section:
http://www.arcdb.ws/SP8/SP8.html

Disclaimer: Don't believe anything you read on the internet! :D

Offline jimbones

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Re: Tube problem?
« Reply #2 on: January 15, 2013, 09:33:03 AM »
Clean the volume and balance pots and switch contacts? Do not use Caig Gold on plastic pots, it leaves a film. Just use cleaner, or Caig pot cleaner. Spray inside, work it, rinse, work it, repeat a few times. I use RS contact cleaner spray.

Check B+ and inspect PS caps?  Small storage caps, just a couple 100uF electrolytics, but inspect those for burning or leaking. V7 is line stage B+ regulator, a 12BH7 tube making 402VDC. It has 640V on the plate. Have you replaced that? This would affect both channels tho.

Cathode resistor failing? Look at R23 and R28 and R25 on the failing channel.
R23 = 1.8k wirewound
R28 = 6.98k
R25 = 1.5k
If one drifts higher it can lower the volume.

They can be tested in circuit with preamp OFF, but verify storage caps empty (wait half hour after shutdown, then verify no DC voltage across R31) before testing resistors. I would not advise testing the B+ voltage yourself due to lethal high voltage when preamp is powered on. Since it is only one channel, the PS is probably OK.

Here is the amplifier schematic, in the pictures section:
http://www.arcdb.ws/SP8/SP8.html

Disclaimer: Don't believe anything you read on the internet! :D

I like your disclaimer  :rofl:

I replaced the electrolytics (even though they didn't need to be, tube noise mistaken for PS problem) I cleaned the pts a yr ago will do again. I'll keep you posted. BTW I didn't replace the PS tube.
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Offline jimbones

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Re: Tube problem?
« Reply #3 on: January 15, 2013, 07:22:51 PM »
OK so I checked all the resistors that i could. I can't seem to find a 6.98K. Everything I did measure was accurate.  wonder if PP caps dry out/drift. there are 2 o.47 wonder caps that look a little tired. Looks don't mean it is bad, just saying. I have tubes on order so I guess I'll have to wait a couple of days.
Thanks.
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Offline richidoo

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Re: Tube problem?
« Reply #4 on: January 15, 2013, 09:07:02 PM »
The film caps should be OK. C9 .47uF is a coupling cap between 12AX7 gain tube and 6DJ8 current buffer tube. Good opportunity for upgrade there. As is C10, 10uF big boy, output cap. Both have styrene bypasses, so by upgrading you are changing the factory voicing which is pretty good. But there are better caps available nowadays.

I didn't see any cathode resistor bypass caps on the schematic, but I only looked at the line stage, and I am just a hack amateur.

The 6.98kOhm is 1/2W (smallish) metal film resistor labeled R28. Is the PCB labelled for part numbers? If not, look for it by the color code. Blue White Grey Orange Brown
http://www.digikey.com/web%20export/mkt/general/mkt/resistor-color-chart.jpg

Offline jimbones

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Re: Tube problem?
« Reply #5 on: January 16, 2013, 07:06:49 AM »
The film caps should be OK. C9 .47uF is a coupling cap between 12AX7 gain tube and 6DJ8 current buffer tube. Good opportunity for upgrade there. As is C10, 10uF big boy, output cap. Both have styrene bypasses, so by upgrading you are changing the factory voicing which is pretty good. But there are better caps available nowadays.

I didn't see any cathode resistor bypass caps on the schematic, but I only looked at the line stage, and I am just a hack amateur.

The 6.98kOhm is 1/2W (smallish) metal film resistor labeled R28. Is the PCB labelled for part numbers? If not, look for it by the color code. Blue White Grey Orange Brown
http://www.digikey.com/web%20export/mkt/general/mkt/resistor-color-chart.jpg

Is it no longer PC to say "Bad Boys Rape Our...."  :rofl:

Well, First I'll get the new tubes in. I bought balanced matched sets so hopefully that will resolve it but I am not feeling it.
What manufacturer cap would you recommend to replace the Wonder Caps?
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Offline richidoo

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Re: Tube problem?
« Reply #6 on: January 16, 2013, 09:09:49 AM »
I'd keep the stock caps. It's an SP8 and that's good. When you want more, upgrade to Ref 1 or Ref 3. Changing parts in such a finely balanced design will upset the cart. With clearer caps you'll hear issues that weren't meant to be heard.

If I had to mess with things I would try Jantzen Superior Z in the interstage and output caps, without any bypass. Maybe even Silver Z. I would not put VCaps because they are too clear, nor Duelunds because that's over-improvement and I'd rather put the money into a newer AR preamp with better everything, still balanced and musical.

The preamp only needs balanced triodes, not matched tubes.

Be sure to clean the contacts. Since you already tried replacing tubes, dirt is next most likely culprit. While you are cleaning, do the tube sockets (and RCAs) too. Either with a wood toothpick, or just wet the holes and wiggle the tube pins in the hole twss.

Offline jimbones

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Re: Tube problem?
« Reply #7 on: January 16, 2013, 10:00:19 AM »
I'd keep the stock caps. It's an SP8 and that's good. When you want more, upgrade to Ref 1 or Ref 3. Changing parts in such a finely balanced design will upset the cart. With clearer caps you'll hear issues that weren't meant to be heard.

If I had to mess with things I would try Jantzen Superior Z in the interstage and output caps, without any bypass. Maybe even Silver Z. I would not put VCaps because they are too clear, nor Duelunds because that's over-improvement and I'd rather put the money into a newer AR preamp with better everything, still balanced and musical.

The preamp only needs balanced triodes, not matched tubes.

Be sure to clean the contacts. Since you already tried replacing tubes, dirt is next most likely culprit. While you are cleaning, do the tube sockets (and RCAs) too. Either with a wood toothpick, or just wet the holes and wiggle the tube pins in the hole twss.

Actually i have a "kit" that includes cleaner and specialy tube socket brushes. I'll do that as well. I've got more than one issue I am dealing with in my system now so it kind of sux.
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Offline Triode Pete

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Re: Tube problem?
« Reply #8 on: January 16, 2013, 10:58:20 AM »

Be sure to clean the contacts. Since you already tried replacing tubes, dirt is next most likely culprit. While you are cleaning, do the tube sockets (and RCAs) too. Either with a wood toothpick, or just wet the holes and wiggle the tube pins in the hole twss.

Not too long ago, I had an issue with some noise in one of my channels... ended up being a dirty / oxidated tube socket and/or perhaps a tube pin on a Western Electric 417A triode in my preamp. Cleaned both with some Caig De-Oxit... No more noise!

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Offline jimbones

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Re: Tube problem?
« Reply #9 on: January 16, 2013, 11:34:28 AM »

Be sure to clean the contacts. Since you already tried replacing tubes, dirt is next most likely culprit. While you are cleaning, do the tube sockets (and RCAs) too. Either with a wood toothpick, or just wet the holes and wiggle the tube pins in the hole twss.

I guess you're called Triode Pete for a reason!!  :thumb:

Not too long ago, I had an issue with some noise in one of my channels... ended up being a dirty / oxidated tube socket and/or perhaps a tube pin on a Western Electric 417A triode in my preamp. Cleaned both with some Caig De-Oxit... No more noise!

My $0.02,
Pete
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Offline jimbones

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Re: Tube problem?
« Reply #10 on: January 20, 2013, 07:33:22 PM »
OK so I showed my SP8 some love this weekend. Actually I am doing housekeeping on the entire system with cleaning all connections, tubes, sockets etc with Caig DeOxit. Still awaiting tubes on order. I'll do a steady state pink noise test soon.
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Offline sleepyguy24

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Re: Tube problem?
« Reply #11 on: January 20, 2013, 07:45:21 PM »
I hope it all goes well. What tube complement did you get for your ARC SP8?

Offline jimbones

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Re: Tube problem?
« Reply #12 on: January 22, 2013, 05:01:04 AM »
I hope it all goes well. What tube complement did you get for your ARC SP8?

Well, i currently have some TungSol and Genelax so i decided to try Electro Harmonix and Jan Phillips. Wanted to see if different brands sound different. Bought balanced so hopefully L&R should be identical (or at least really close).

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Offline rollo

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Re: Tube problem?
« Reply #13 on: January 22, 2013, 09:16:48 AM »
   Having some tube issues myself lately. First the 5651 in the tube regulated power supply was oscillating. Removed tube put in another no issue. Then same ch the cathode follower buzzes away.
   When I jiggle the tube in the socket it stops. Thinking not a tube problem and something else. Thing is what ?
    Need some ProGold or equal and clean sockets. Maybe the socket pin solder joint is loose ? Any other thoughts ? Thanks.



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Offline jimbones

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Re: Tube No problemo
« Reply #14 on: January 25, 2013, 02:44:45 PM »
Ok so I get the new tubes put them in same problem. I'm thinking to myself what is the last freaking variable I can eliminate? The subwoofer connection! So I remove all unnecessary connections from the back of the preamp and at first I thought it's better but wasn't. I'll continue to work to figure this out.
« Last Edit: January 25, 2013, 03:54:48 PM by jimbones »
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