Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Anode voltage sag.
Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb |
#2
|
|||
|
|||
"Chris Berry" wrote in message ... Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb |
#3
|
|||
|
|||
Did I miss something?
cb |
#4
|
|||
|
|||
Chris,
I was reading your post when my 2 year old started wailing on the keyboard. :-) Sent an empty post. I suspect maybe a PS issue, caps maybe need attention. Vin "Chris Berry" wrote in message ... Did I miss something? cb |
#5
|
|||
|
|||
try another GZ34 ,but before that-try with different electrolytic cap(s)
after GZ34. -- -- -- .................................................. ........................ Choky Prodanovic Aleksandar YU "don't use force, "don't use force, use a larger hammer" use a larger tube - Choky and IST" - ZM .................................................. ........................... "Chris Berry" wrote in message ... Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb |
#6
|
|||
|
|||
"JVC" wrote in message ... Chris, I was reading your post when my 2 year old started wailing on the keyboard. :-) Sent an empty post. I suspect maybe a PS issue, caps maybe need attention. Vin That's put a smile on my face. Could well be. I'm investigating further tomorrow. Thanks. cb |
#7
|
|||
|
|||
"Choky" wrote in message ... try another GZ34 ,but before that-try with different electrolytic cap(s) after GZ34. Just tried a JJ GZ34 - same thing. It's a buzzing sound, a bit like when you switch off an amp and still have a signal going in. You all reckon it's power supply related then as I suspect? cb |
#8
|
|||
|
|||
Chris Berry wrote: Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb My first thought is that this amp may have never been "meant" to go into signal excursions beyond the clipping point. It very possibly has a negative feedback loop to help keep distortion low during normal operation in the linear range. Difficulty with such NFB loops, when they do break up they can do so quite unpredictibly. Try to find a schematic of your amp, and see if there is a resistor and/or capacitor connected from the speaker output back to an earlier point in the circuit (often the cathode of a preamp tube). Disconnect this feedback network, and see what it does to your sound. It should also significantly increase the amplifier's overall gain, which might be a bonus also. If you can't decide which you like better, add a switch and label it something funky like "cozmik overdrive". ;-) Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#9
|
|||
|
|||
Chris,
Blocking distortion, most likely, that amp probably isn't quite comfortable running WOT. Vin "Chris Berry" wrote in message ... "Choky" wrote in message ... try another GZ34 ,but before that-try with different electrolytic cap(s) after GZ34. Just tried a JJ GZ34 - same thing. It's a buzzing sound, a bit like when you switch off an amp and still have a signal going in. You all reckon it's power supply related then as I suspect? cb |
#10
|
|||
|
|||
Chris Berry wrote: Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb I would have no idea what "To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory." means. However, when the amp is grossly over driven, it isn't unusual to get a drop of B+ of 10%. Most amps when used without any overdrive or clipping will maintain their operating voltages. If the amp isn't a true guitar amp, and is a hi-fi amp, and has negative feedback, and high value coupling caps, then perhaps what you hear is paralysis. This is caused by the coupling caps chraging up due to the grid current flow, and the direct voltage across the caps is like over biasing of the output tubes, and they make the amp act like a class C amp. The only way to prevent this in an RC coupled amp is to minimise the size of the coupling caps. In a hi-fi amp, the output coupling caps are ususally at least 0.1 uF, with grid bias resistors of 220k or higher. Guitar amps often use 0.022uF and 100k which is a faster time constant, so that discharge of charge build up in the caps is much faster, and follows the fading note better. The is much less feedback in a guitar amp. Feedback makes paralysis occur much more severly, so reduce the amount of fedback for musical instrument use. Once the hi-fi amp has its feedback reduced, and the coupling caps reduced, the amp is then the equal of most guitar amps. The F response may be slightly reduced, but still be quite adequate for guitar. The changes of coupling caps *must* be accompanied by feedback reduction because the amp may become quite unstable, ie, oscillate at some LF tone if the feedback is left as is. Many hi-fi amps have 20 dB of feedback. Only a few guitar amps have no feedback at all, such as a Vox AC30, but quite a few have 10 dB, just enough to reduce the effective output resistance a bit. When making changes to feedback and coupling caps, the amp should still be stable with no load connected; if it is, then it will be stable with a speaker connected. Patrick Turner. |
#11
|
|||
|
|||
One thing to try is to (temporarily) add add'l filter capacitance. There is
a limit to the capacitance connected directly to the anode of a GZ34 but you can add add'l storage downstream if the amp has a filter choke or a pi network. Or, to continue a prior discussion, you can add a Weber Copper Cap and you shouldn't get much sag plus you can add a fair amount of capacitance right after the rectifier. It may be that your old caps have dried out a bit. I had an Eico St-70 that did wild wired stuff under load (B+ dropped a lot & there was a lot of distortion). Adding 30 MFD solved the problem. Permanently. Jon From: "JVC" Newsgroups: rec.audio.tubes Date: Thu, 15 Apr 2004 00:20:25 GMT Subject: Anode voltage sag. Chris, Blocking distortion, most likely, that amp probably isn't quite comfortable running WOT. Vin "Chris Berry" wrote in message ... "Choky" wrote in message ... try another GZ34 ,but before that-try with different electrolytic cap(s) after GZ34. Just tried a JJ GZ34 - same thing. It's a buzzing sound, a bit like when you switch off an amp and still have a signal going in. You all reckon it's power supply related then as I suspect? cb |
#12
|
|||
|
|||
The tone you want may not be available on that amp--guitar amps create the
'signature' sound through distortion laden circuits. Your Dyna amp is too clean. As for the voltage drop, you are probably just driving it hard and the power supply can't take the abuse. You could add a constant voltage or current source circuit (http://www.bottlehead.com/et/parts/parts.htm) to solve your problem, or beef up the power supply. Either one will make the sound cleaner and work against the blues sound a little, but you could solve that problem with a little distorted NFB a'la Fender design.. "Chris Berry" wrote in message ... Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb |
#13
|
|||
|
|||
"Jon Yaeger" wrote in message ... One thing to try is to (temporarily) add add'l filter capacitance. There is a limit to the capacitance connected directly to the anode of a GZ34 but you can add add'l storage downstream if the amp has a filter choke or a pi network. Or, to continue a prior discussion, you can add a Weber Copper Cap and you shouldn't get much sag plus you can add a fair amount of capacitance right after the rectifier. It may be that your old caps have dried out a bit. I had an Eico St-70 that did wild wired stuff under load (B+ dropped a lot & there was a lot of distortion). Adding 30 MFD solved the problem. Permanently. It's more likely to be the capacitors being a bit dry than them needing a bigger value. There's already 47+47 in there. I think I'll also test it with diodes to see if with a better supply this is greatly improved. The webber would simply take too long to get here for my liking although it is a possibility in the long run. Thanks. cb |
#14
|
|||
|
|||
"Fred Nachbaur" wrote in message news:Kckfc.18357$mn3.17840@clgrps13... My first thought is that this amp may have never been "meant" to go into signal excursions beyond the clipping point. It very possibly has a negative feedback loop to help keep distortion low during normal operation in the linear range. Here's the preamp circuit: http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif and the power amp: http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif I don't know why but I didn't think of that... You're right!. I think I'll clip the NFB resistor and put a 1Meg log pot in series with it and see how that helps. Difficulty with such NFB loops, when they do break up they can do so quite unpredictibly. Try to find a schematic of your amp, and see if there is a resistor and/or capacitor connected from the speaker output back to an earlier point in the circuit (often the cathode of a preamp tube). Disconnect this feedback network, and see what it does to your sound. It should also significantly increase the amplifier's overall gain, which might be a bonus also. Thanks Fred. I'll try it out and tell you how it fares. If you can't decide which you like better, add a switch and label it something funky like "cozmik overdrive". ;-) Or with a pot just label it Tight/loose with "Good Girl" in the middle.... cb |
#15
|
|||
|
|||
After looking at the poweramp scheme, I'd say, since your using this for a
guitar amp delete the 1meg resistor on the inst input and connect it to the top of the 500k log pot or even straight to the tube input. I'd put a 1k - 5k resistor right at the tubes input or as close as possible either way. - Dyslexics of America Untie! keithw... "Chris Berry" wrote in message ... "Fred Nachbaur" wrote in message news:Kckfc.18357$mn3.17840@clgrps13... My first thought is that this amp may have never been "meant" to go into signal excursions beyond the clipping point. It very possibly has a negative feedback loop to help keep distortion low during normal operation in the linear range. Here's the preamp circuit: http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif and the power amp: http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif I don't know why but I didn't think of that... You're right!. I think I'll clip the NFB resistor and put a 1Meg log pot in series with it and see how that helps. Difficulty with such NFB loops, when they do break up they can do so quite unpredictibly. Try to find a schematic of your amp, and see if there is a resistor and/or capacitor connected from the speaker output back to an earlier point in the circuit (often the cathode of a preamp tube). Disconnect this feedback network, and see what it does to your sound. It should also significantly increase the amplifier's overall gain, which might be a bonus also. Thanks Fred. I'll try it out and tell you how it fares. If you can't decide which you like better, add a switch and label it something funky like "cozmik overdrive". ;-) Or with a pot just label it Tight/loose with "Good Girl" in the middle.... cb |
#16
|
|||
|
|||
"dcoff" wrote in message ... The tone you want may not be available on that amp--guitar amps create the 'signature' sound through distortion laden circuits. Your Dyna amp is too clean. Supposedly clean... There's masses of gain there - it starts overdriving on 1 - 1.5.... Just removed the NFB and it's the same phasing sound - most probably power supply related. Will take a closer look this afternoon. As for the voltage drop, you are probably just driving it hard and the power supply can't take the abuse. You could add a constant voltage or current source circuit (http://www.bottlehead.com/et/parts/parts.htm) to solve your problem, or beef up the power supply. Either one will make the sound cleaner and work against the blues sound a little, but you could solve that problem with a little distorted NFB a'la Fender design.. I'm guessing but those EL34's are being pushed quite hard indeed - certainly more than the 40W the manufacturer states for this amp but that's the clean rating. Luckily, I'm using a hot plate (output attenuator) to get this moving though and won't have dead ears at these volumes. With no NFB it's sounding a little better. I think the problem might not be leaky capacitors after all but that the capacitor is too large! This would take longer to charge and hence cause my phasing effect. cb |
#17
|
|||
|
|||
"Keithw" wrote in message ... After looking at the poweramp scheme, I'd say, since your using this for a guitar amp delete the 1meg resistor on the inst input and connect it to the top of the 500k log pot or even straight to the tube input. I'd put a 1k - 5k resistor right at the tubes input or as close as possible either way. - Dyslexics of America Untie! keithw... Thanks Keith. That won't affect the phasing problem so when I get round to improving the preamp, I'll take that into consideration. cb |
#18
|
|||
|
|||
"Chris Berry" wrote in message ... "Fred Nachbaur" wrote in message news:Kckfc.18357$mn3.17840@clgrps13... My first thought is that this amp may have never been "meant" to go into signal excursions beyond the clipping point. It very possibly has a negative feedback loop to help keep distortion low during normal operation in the linear range. Here's the preamp circuit: http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif and the power amp: http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif Marvelously simple design ! If you use 'INST' input, just try a 1nf cap in lieu of the 10nf at the input. Of course, the tone stack is not what is usual for guitar. I don't know why but I didn't think of that... You're right!. I think I'll clip the NFB resistor and put a 1Meg log pot in series with it and see how that helps. Difficulty with such NFB loops, when they do break up they can do so quite unpredictibly. Try to find a schematic of your amp, and see if there is a resistor and/or capacitor connected from the speaker output back to an earlier point in the circuit (often the cathode of a preamp tube). Disconnect this feedback network, and see what it does to your sound. It should also significantly increase the amplifier's overall gain, which might be a bonus also. Thanks Fred. I'll try it out and tell you how it fares. If you can't decide which you like better, add a switch and label it something funky like "cozmik overdrive". ;-) Or with a pot just label it Tight/loose with "Good Girl" in the middle.... cb |
#19
|
|||
|
|||
Hi Chris,
By looking at the schematic I would think that your amp would work pretty well for a guitar and should not respond a whole lot different than your average 2 X EL34 push pull AB design. The 40 v sag is normal - under pretty severe clipping anyhow. If you are sagging your supply down that much at low power though, then something is drawing too much current. For sure check or replace the filter caps but from what you are describing I think maybe you have a bad tube. You might try lifting that level indicator tube off the 4 ohm output tap just to be sure something there isn't leaking and screwing up the OT's load. What are the DC voltages at your power tubes? How old are those tubes anyways? Todd |
#20
|
|||
|
|||
"Patrick Turner" wrote in message ... Chris Berry wrote: Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb I would have no idea what "To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory." means. It means that there's a little break-up that's usable when much more would normally be expected. However, when the amp is grossly over driven, it isn't unusual to get a drop of B+ of 10%. Most amps when used without any overdrive or clipping will maintain their operating voltages. When overdriving at about what I expect to be the levels I want it to, B+ sag is 30% - 290V If the amp isn't a true guitar amp, and is a hi-fi amp, and has negative feedback, NFB's been removed and a little improvement has resulted and high value coupling caps, then perhaps what you hear is paralysis. This is caused by the coupling caps chraging up due to the grid current flow, and the direct voltage across the caps is like over biasing of the output tubes, and they make the amp act like a class C amp. The only way to prevent this in an RC coupled amp is to minimise the size of the coupling caps. In a hi-fi amp, the output coupling caps are ususally at least 0.1 uF, with grid bias resistors of 220k or higher. Guitar amps often use 0.022uF and 100k which is a faster time constant, so that discharge of charge build up in the caps is much faster, and follows the fading note better. The is much less feedback in a guitar amp. Feedback makes paralysis occur much more severly, so reduce the amount of fedback for musical instrument use. Once the hi-fi amp has its feedback reduced, and the coupling caps reduced, the amp is then the equal of most guitar amps. The F response may be slightly reduced, but still be quite adequate for guitar. I'll check that later. The changes of coupling caps *must* be accompanied by feedback reduction because the amp may become quite unstable, ie, oscillate at some LF tone if the feedback is left as is. Like I said, with NFB removed completely, there's a slight improvement. Many hi-fi amps have 20 dB of feedback. Only a few guitar amps have no feedback at all, such as a Vox AC30, but quite a few have 10 dB, just enough to reduce the effective output resistance a bit. When making changes to feedback and coupling caps, the amp should still be stable with no load connected; if it is, then it will be stable with a speaker connected. Here, the hot plate is my friend. I must be getting 1/2 a watt out with the amp at full powergiving the amp a 16 ohm load. Thanks for the tips. cb |
#21
|
|||
|
|||
wrote in message ... Hi Chris, By looking at the schematic I would think that your amp would work pretty well for a guitar and should not respond a whole lot different than your average 2 X EL34 push pull AB design. The 40 v sag is normal - under pretty severe clipping anyhow. If you are sagging your supply down that much at low power though, then something is drawing too much current. For sure check or replace the filter caps but from what you are describing I think maybe you have a bad tube. You might try lifting that level indicator tube off the 4 ohm output tap just to be sure something there isn't leaking and screwing up the OT's load. That's it 40V at mild clipping is what I'm getting. What are the DC voltages at your power tubes? How old are those tubes anyways? Todd -40V bias 400V B+, Vg2 330V Valves are new and tested with avo valve tester. Thanks. cb |
#22
|
|||
|
|||
400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA through each EL34. I haven't checked what happens to the anode voltage under full load or overdrive. Maybe your GZ34 isn't up to spec anymore, or the filter caps need to be replaced. The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes. Corné Chris Berry wrote: Hi all, I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83 valve complement) which seems to be misbehaving... OK so I'm using it as a guitar amp as well but that shouldn't be much of a bother. I've noticed that when playing music through it, the anode voltage seems to drop faster than I'd expect. For guitar, where I'd be deliberatly trying to get the amp to distort nicely (there is such a thing,,,) as soon as the anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd phasing noises and this is in the early stages of break-up. To give you an idea, I can get an in-between clean-overdriven bluesy tone but not get anywhere near a blues/lead territory. I know this isn't the "right" newsgroup for guitar amp stuff but I must say that my recent experiences here really do make it a much better place to post this than alt.guitar.amps. Thanks guys. cb |
#23
|
|||
|
|||
"Corné Janssen" wrote in message ... 400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA through each EL34. I haven't checked what happens to the anode voltage under full load or overdrive. Maybe your GZ34 isn't up to spec anymore, or the filter caps need to be replaced. The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes. Corné Which one do you have? Mine's the 225-102 but I have a few of their circuits if you need them (supplied by Dynacord themselves) I'm using a JJ GZ34 and EL34's made by Halltron (well Branded at least). cb |
#24
|
|||
|
|||
The one I have is a 125-102. A few years ago I scanned the circuit diagram and Duncan Munro put it on his site at: http://tdsl.duncanamps.com/pdf/eminent.gif If you didn't know the TDSL site has a lot of tube data and amplifier circuit diagrams. TDSL can be found at: http://tdsl.duncanamps.com/ I'll check anode voltages at full load and overdrive conditions. If I can find the time I'll do it this weekend. Corné Chris Berry wrote: "Corné Janssen" wrote in message ... 400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA through each EL34. I haven't checked what happens to the anode voltage under full load or overdrive. Maybe your GZ34 isn't up to spec anymore, or the filter caps need to be replaced. The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes. Corné Which one do you have? Mine's the 225-102 but I have a few of their circuits if you need them (supplied by Dynacord themselves) I'm using a JJ GZ34 and EL34's made by Halltron (well Branded at least). cb |
#25
|
|||
|
|||
Here are the measurements These measurements are taken at 1kHz Quiescence anode voltage : 435V anode voltage @ 40W clean : 382V anode voltage @ 50W over driven : 365V When th amp is overdriven hum from the powersupply gets into the output signal this is clearly visable on an oscilloscoop. This may explain the phasing noise you hear. Corné |
#26
|
|||
|
|||
"Corné Janssen" wrote in message ... Here are the measurements These measurements are taken at 1kHz Quiescence anode voltage : 435V anode voltage @ 40W clean : 382V anode voltage @ 50W over driven : 365V When th amp is overdriven hum from the powersupply gets into the output signal this is clearly visable on an oscilloscoop. This may explain the phasing noise you hear. well that problem is now sorted out and I reckon you're very right about the ripple getting through. Maybe the supply is still weak but with less current draw through the valves it's stable and in the region you're getting. Now I've a different problem with the bias on one valve dropping from -40V to -48V while the other rises 1 or 2 volts max... cb |
#27
|
|||
|
|||
unmatched tubes or grid cap leak?
-- -- -- .................................................. ........................ Choky Prodanovic Aleksandar YU "don't use force, "don't use force, use a larger hammer" use a larger tube - Choky and IST" - ZM .................................................. ........................... "Chris Berry" wrote in message ... well that problem is now sorted out and I reckon you're very right about the ripple getting through. Maybe the supply is still weak but with less current draw through the valves it's stable and in the region you're getting. Now I've a different problem with the bias on one valve dropping from -40V to -48V while the other rises 1 or 2 volts max... cb |
#28
|
|||
|
|||
Hey Chris-
From a few posts ago you said your voltages at your EL34's we -40V bias 400V B+, Vg2 330V I assume you are taking these measurements at idle with zero signal input? These voltages are the same at both tubes? By the looks of your schematic - there is something wrong with your Vg2 level. It should be about the same as your plate voltage (at idle). I think you should replace your filter caps before going any further with any of this - they are likely the main cause of all your troubles. Once you have new filters -including the bias filters, go ahead and change the screen grid resistors (100 ohm) and make sure the 1k grid series resistors are good - or just replace those too. Mount both the Rg2 and the Rg1 as close to the tube socket as possible. By what you describe in your last post - your schematic shows a 25 k trim pot for each output tube- are you saying that one of these voltages is drifting after they have been set for equal currents? Do you have reasonably equal amounts of bias voltage at each tube when you initially set for matched (push/pull) current? Or - are you taking this measurement with signal applied? Todd "Chris Berry" wrote in message ... "Corné Janssen" wrote in message ... Here are the measurements These measurements are taken at 1kHz Quiescence anode voltage : 435V anode voltage @ 40W clean : 382V anode voltage @ 50W over driven : 365V When th amp is overdriven hum from the powersupply gets into the output signal this is clearly visable on an oscilloscoop. This may explain the phasing noise you hear. well that problem is now sorted out and I reckon you're very right about the ripple getting through. Maybe the supply is still weak but with less current draw through the valves it's stable and in the region you're getting. Now I've a different problem with the bias on one valve dropping from -40V to -48V while the other rises 1 or 2 volts max... cb |
#29
|
|||
|
|||
wrote in message ... Hey Chris- From a few posts ago you said your voltages at your EL34's we -40V bias 400V B+, Vg2 330V It's risen with grid resistor changes in the output stage to 430V ish and the bias supply has increased (but not to schematic values - close though) with the changing of the bias supply diode. I assume you are taking these measurements at idle with zero signal input? No. The measurements above are but the sag is with a strong signal input. These voltages are the same at both tubes? At idle yes - not under load. By the looks of your schematic - there is something wrong with your Vg2 level. It should be about the same as your plate voltage (at idle). I think you should replace your filter caps before going any further with any of this - they are likely the main cause of all your troubles. Replaced most of the capacitors thus far - still need a 22uF+22uF for the main supply though. Once you have new filters -including the bias filters, go ahead and change the screen grid resistors (100 ohm) and make sure the 1k grid series resistors are good - or just replace those too. Mount both the Rg2 and the Rg1 as close to the tube socket as possible. Already done that. Improved the supply but the bias does it's funnies. By what you describe in your last post - your schematic shows a 25 k trim pot for each output tube- are you saying that one of these voltages is drifting after they have been set for equal currents? Set for 40V as per the schematic. They drift under load - Left one - rises to -39V, the right one drops to 48V Do you have reasonably equal amounts of bias voltage at each tube when you initially set for matched (push/pull) current? Well I'm setting for voltage - not current but with 2 sets of tubes (matched at one point) it's always the right hand socket that produces this behaviour. Or - are you taking this measurement with signal applied? Setting it is with no signal drift is measured under load. Thanks for taking a poke... cb |
#30
|
|||
|
|||
I would suggest fixing one thing at a time. Let's clarify: What are the DC
voltage measurements at your output tube sockets with no signal applied? |
#31
|
|||
|
|||
wrote in message ... I would suggest fixing one thing at a time. Let's clarify: What are the DC voltage measurements at your output tube sockets with no signal applied? Todd, I'll be able to tell you tomorrow or the day after when I'm back with the amp and have a moment. Thanks though.. cb |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
rec.audio.car FAQ (Part 1/5) | Car Audio | |||
Cathode curves | Vacuum Tubes | |||
New cathode bias method. | Vacuum Tubes |