AudioNervosa
Systemic Development => Amplification and Preamplification => Tubes => Topic started by: jimbones on January 15, 2013, 05:17:33 AM
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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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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 (http://www.arcdb.ws/SP8/SP8.html)
Disclaimer: Don't believe anything you read on the internet! :D
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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 (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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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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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 (http://www.digikey.com/web%20export/mkt/general/mkt/resistor-color-chart.jpg)
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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 (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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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.
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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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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!
My $0.02,
Pete
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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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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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I hope it all goes well. What tube complement did you get for your ARC SP8?
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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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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.
charles
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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.
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Good discovery!
With the sub disconnected is it merely "better," or is it totally cured, with no distortion and balanced channel levels?
Is the subwoofer something you added recently which caused the change in SQ or has it always been there?
If you are using the SP-8's tape output for the sub, then does the Monitor/Input switch position affect the problem? Maybe the tape amplifier is not on unless the switch is up. The tape output and the main output have separate cables, it is not the same signal, so there could be a tape buffer (current amplifier) circuit, which if not engaged or if broken could cause this.
The cable length will not affect the load significantly. If the cable is healthy it should not be an issue.
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Sorry Rich, I think I was editing as you were responding. It turns out it is not the sub. It is the pre. I will have no choice but to feed a signal in and measure the output across the frequency range and compare left and right. It's not a huge difference but is noticeable.
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No problem Jim
Measuring the output is a good confirmation that the problem lies in the preamp. Do you have a test tone? I can make one for you if you need it.
Did you even get a chance to find and measure the small 1/2watt cathode resistors? If there is level difference between the channels that would be a good thing to check, although likely hood of a film resistor being broken is small. It should be near the big wirewounds that you already measured.
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There are generator software as freeware online that I have used before. Thats what I'll start with. I'll used a constant input into my Aux 1 input of 1V? 500mv? It seems like the difference is in the mid/high end.
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I usually use 2Vpp for test signal, but CDPs make 2Vrms with a full scale signal, which is about 3Vpp. Try to use signal level typical of your own sources to start. If your source only makes .5V then use that to best simulate your actual situation. You can try up to 3Vpp input. The preamp can take higher input voltage but no consumer gear is louder than 3Vpp = 2Vrms.
I think the cathode resistor would affect the whole bandwidth, not just highs. And if a coupling cap shorted internally I think it would reduce the LF more than HF.
I don't know what else it could be other than dirty volume pot. At least you have AR service if you need them, or maybe a good local tech?
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OK so I pump in a signal and it begins a very slow roll off at 10K and by 19K it is pretty shot. Also the right channel has higher output than left side. I'll try again with different input level because this was a low voltage input.
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OK So I spoke to Scott at SG and he says he listened to the preamp, sounds fine, but when he measures it the left/right balance is off. The frequency response is fine though. He said it is slight enough that he doesn't hear it but can measure it and recommends I leave it alone. (which I will) He said the only thing to fix the balance problem is to install a ALPS Pot which would be very expensive.
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BTW, I retubed the SP8 while I was at it. went from Tungsol to Electro Harmonix. Me likey better :thumb: