Forgot to mention -- the B+ is currently at 393 Vdc and the negative bias voltage is -240Vdc.
Dan
Dan
I’m not sure I understand your meter scales and measurements fully, but it is important NOT to play music until you are certain the amp is biased correctly.
R18 (and R29) are not load resistors per se, but are inserted solely for the purpose of measuring the voltage developed across the known resistance by the current through each output transformer and 300b tube. You are correct - in order to measure current directly you must insert the ammeter into the circuit, which is inconvenient and risky with high voltages present, so instead sense or shunt resitors are used. Hence the “sensing resistors” (R18 and 29) that allows the measurement of the voltage across the resistor and then calculate the current though the resistor using Ohms law I=V/R.
You will use the “voltage measurement” setting on you multimeter(s), either 1V, or 1000 mV scale to measure the voltage across R18 & R29. Since the R=10 Ohm, 1V will represent 100 ma current according to Ohms law. I glanced at the bias setup instructions on www.Tubelab.com but could not (conveniently) spot the recommended current for 300b, however 60 Ma will be safe. That means begining with the tube in cutoff (at the most negative setting of the bias on pin 3), you adjust the R23 until you read 0.6V (or 600 mV) across R18.
Hope this helps. Ask if still unclear. Good luck
R18 (and R29) are not load resistors per se, but are inserted solely for the purpose of measuring the voltage developed across the known resistance by the current through each output transformer and 300b tube. You are correct - in order to measure current directly you must insert the ammeter into the circuit, which is inconvenient and risky with high voltages present, so instead sense or shunt resitors are used. Hence the “sensing resistors” (R18 and 29) that allows the measurement of the voltage across the resistor and then calculate the current though the resistor using Ohms law I=V/R.
You will use the “voltage measurement” setting on you multimeter(s), either 1V, or 1000 mV scale to measure the voltage across R18 & R29. Since the R=10 Ohm, 1V will represent 100 ma current according to Ohms law. I glanced at the bias setup instructions on www.Tubelab.com but could not (conveniently) spot the recommended current for 300b, however 60 Ma will be safe. That means begining with the tube in cutoff (at the most negative setting of the bias on pin 3), you adjust the R23 until you read 0.6V (or 600 mV) across R18.
Hope this helps. Ask if still unclear. Good luck
All,
Thank you all for reading and helping.
The amp works, but something's not right in it... I'd be grateful for any help. I've made a video and posted to YouTube:
In the video, I show that, using the turntable as the source, the amp can play at low volume. When I turn the volume up, I get a weird "pffthhhht" noise. I hope you can hear it ok in the video. Then, I lifted the needle and turned off the record player completely, and as I turn up the volume (without any music playing), I get the "pffthhht" noise again. And if I continue to turn it up, then you can hear some high pitched squealing, and other noises, etc.
Interestingly, this doesn't happen I use the streaming input (I think we have some sort of Amazon Echo that streams from Amazon music). I can crank the streaming input w/o any of this weird distortion happening, much louder than the record player can get before it starts making the weird noise. Sounds good! But the turntable would sound much better, if it would work correctly...
I've tried: connecting a choke and removing R4; and adding a supplemental capacitor in parallel to the short/fat/squat capacitor (C6, maybe???). Whether these modifications are installed or not makes no difference -- I get the weird "pffthhht" when using the turntable.
I thought initially the first time I heard it that this was some sort of microphonic problem in one or more the tubes -- it happens as the volume gets turned up, and just sounds sort of like microphonic problems I've had w/ other amps. But then I get confused by being able to play music at a loud volume w/ a different input.
I'd be very grateful if anyone has any thoughts or suggestions. I'm not sure what to try next... I've ordered some new output tubes (Shuaguang Black Treasure 300b-z), but they won't be here for another week. Could I try removing tubes to try to isolate the problem? I'm not sure if it's ok to play music through the amp with, say, just the rectifier and one channel's tubes installed.
Thanks for any and all help!
Best,
Dan
Thank you all for reading and helping.
The amp works, but something's not right in it... I'd be grateful for any help. I've made a video and posted to YouTube:
In the video, I show that, using the turntable as the source, the amp can play at low volume. When I turn the volume up, I get a weird "pffthhhht" noise. I hope you can hear it ok in the video. Then, I lifted the needle and turned off the record player completely, and as I turn up the volume (without any music playing), I get the "pffthhht" noise again. And if I continue to turn it up, then you can hear some high pitched squealing, and other noises, etc.
Interestingly, this doesn't happen I use the streaming input (I think we have some sort of Amazon Echo that streams from Amazon music). I can crank the streaming input w/o any of this weird distortion happening, much louder than the record player can get before it starts making the weird noise. Sounds good! But the turntable would sound much better, if it would work correctly...
I've tried: connecting a choke and removing R4; and adding a supplemental capacitor in parallel to the short/fat/squat capacitor (C6, maybe???). Whether these modifications are installed or not makes no difference -- I get the weird "pffthhht" when using the turntable.
I thought initially the first time I heard it that this was some sort of microphonic problem in one or more the tubes -- it happens as the volume gets turned up, and just sounds sort of like microphonic problems I've had w/ other amps. But then I get confused by being able to play music at a loud volume w/ a different input.
I'd be very grateful if anyone has any thoughts or suggestions. I'm not sure what to try next... I've ordered some new output tubes (Shuaguang Black Treasure 300b-z), but they won't be here for another week. Could I try removing tubes to try to isolate the problem? I'm not sure if it's ok to play music through the amp with, say, just the rectifier and one channel's tubes installed.
Thanks for any and all help!
Best,
Dan
Thanks, I don't think I've encountered motorboating in an amp before.
This is the weirdest thing. When I stream "Smokin' at the half note" and i can turn it up as loud as the amp can go and no distortion. But when I play the record of "Smokin' at the half note" I get the weird noise.
I tried using another phono preamp, as well, but same problem (although slightly different sounds...)
Thanks again in advance.
Best,
Dan
This is the weirdest thing. When I stream "Smokin' at the half note" and i can turn it up as loud as the amp can go and no distortion. But when I play the record of "Smokin' at the half note" I get the weird noise.
I tried using another phono preamp, as well, but same problem (although slightly different sounds...)
Thanks again in advance.
Best,
Dan
The motorboating could originate in the phono stage, depending on various factors, including grounding scheme.
Is the phone stage connected to earth ground? Do you have both components plugged into the same power outlet?
OK, thanks so much! I think you are on to something...
I said previously that the issue persisted when I used a different phono preamp, but I was wrong about that... I'm (again) ashamed to say it, but I forgot to attach the ground wire from the turntable to the phono preamp. Now, with ground connected, I can play records as loud as necessary w/o hearing any of that weird "pffthhht" stuff!
I have 2 phono preamps - a Wyn Palmer phono preamp, which is built into my preamp/line stage (also the WP preamp 🙂, and a Schiit Mani. I much prefer the WP phono preamp to the Schiit, but the Schiit is working now, so that's what I"m using. I suspect the issue in the WP phono preamp is something grounding related because the noises the Schiit made when connected incorrectly (w/o the ground) were pretty similar to the noises the WP phono preamp made initially.
I'd of course be very grateful to hear any suggestions or advice, but I'm going to investigate the grounding of the first phono preamp next.
Thank you!!!
Dan
PS - the amp sounds great! my records sound wonderful. Listening to Charles Mingus now (trio), and there's tons of space around the instruments, cymbals sound wonderful, and the big guy himself sounds very ... authoritative? Love it. Thank you all, and especially Mr Tubelab, George (if I may)!
I said previously that the issue persisted when I used a different phono preamp, but I was wrong about that... I'm (again) ashamed to say it, but I forgot to attach the ground wire from the turntable to the phono preamp. Now, with ground connected, I can play records as loud as necessary w/o hearing any of that weird "pffthhht" stuff!
I have 2 phono preamps - a Wyn Palmer phono preamp, which is built into my preamp/line stage (also the WP preamp 🙂, and a Schiit Mani. I much prefer the WP phono preamp to the Schiit, but the Schiit is working now, so that's what I"m using. I suspect the issue in the WP phono preamp is something grounding related because the noises the Schiit made when connected incorrectly (w/o the ground) were pretty similar to the noises the WP phono preamp made initially.
I'd of course be very grateful to hear any suggestions or advice, but I'm going to investigate the grounding of the first phono preamp next.
Thank you!!!
Dan
PS - the amp sounds great! my records sound wonderful. Listening to Charles Mingus now (trio), and there's tons of space around the instruments, cymbals sound wonderful, and the big guy himself sounds very ... authoritative? Love it. Thank you all, and especially Mr Tubelab, George (if I may)!
I’m not familiar with the WP, but search for grounding in that thread:
https://audiokarma.org/forums/index...wyn-phono-line-warp-headphone-pre-amp.946223/
https://audiokarma.org/forums/index...wyn-phono-line-warp-headphone-pre-amp.946223/
In particular this post: https://audiokarma.org/forums/index...e-headphone-amps.937686/page-31#post-14458686
