I have a Preso K8. I've replaced all of the capacitors and tubes. It works pretty well. I've already recorded many local bands with it. However, there is one consistent problem that continues to haunt me: when I turn on the amp section of the unit, once the tubes warm up, it starts pulsing slightly. You can see it on the VU meter and you can hear it in the recordings. I can't seem to figure out what's causing this problem. I have uploaded a video of it on youtube. If anyone cares to take a look at the video, if you have a suggestion of what might be causing the problem, please let me know. I'm a fairly new student of tube circuitry theory, so any advice/guidance is highly appreciated.
Thanks!
Presto K8 - Problem with pulsing amp
Moderators: piaptk, tragwag, Steve E., Aussie0zborn
Re: Presto K8 - Problem with pulsing amp
I guess it would help if I posted the link to the youtube video.
[YouTube2]http://youtu.be/f8yTM_9tstk[/YouTube2]
[YouTube2]http://youtu.be/f8yTM_9tstk[/YouTube2]
Re: Presto K8 - Problem with pulsing amp
Hello,
What does the pulsing sound sound like on the recordings? Is it something like a loose ground wire sound?
Was the pulsing sound there before you replaced all the caps? Also maybe one of the tubes could be bad?
If its a loose ground wire sound then maybe one of the capacitors might be loose.
What does the pulsing sound sound like on the recordings? Is it something like a loose ground wire sound?
Was the pulsing sound there before you replaced all the caps? Also maybe one of the tubes could be bad?
If its a loose ground wire sound then maybe one of the capacitors might be loose.
Re: Presto K8 - Problem with pulsing amp
Hi,
Seems like you have a low frequency oscillation present. That is called motorboating. The typical cause is due to bad or incorrectly installed bypass caps. The feedback path is via the B+ rail. Check the all of the B+ voltages on the low level stages to see if the voltage is pulsing. It should be rock stable. You should be able to isolate the stage that is causing the problem by pulling the pre-amp tube(s) (e.g. 12AX7). Start at the input work forward to the phase inverter. If the pulsing stops when you pull the tube, try adding a 10uf in parallel with a .1uf from the local plate B+ to chassis ground and see if that cures the problem.
This can also be caused if you have issues with interstage coupling capacitors. If the values are too low and in the audio feedback path, this can cause a similar result.
Since you re-capped, its possible that one of these problems might exist.
Hope that helps.
Mark
Seems like you have a low frequency oscillation present. That is called motorboating. The typical cause is due to bad or incorrectly installed bypass caps. The feedback path is via the B+ rail. Check the all of the B+ voltages on the low level stages to see if the voltage is pulsing. It should be rock stable. You should be able to isolate the stage that is causing the problem by pulling the pre-amp tube(s) (e.g. 12AX7). Start at the input work forward to the phase inverter. If the pulsing stops when you pull the tube, try adding a 10uf in parallel with a .1uf from the local plate B+ to chassis ground and see if that cures the problem.
This can also be caused if you have issues with interstage coupling capacitors. If the values are too low and in the audio feedback path, this can cause a similar result.
Since you re-capped, its possible that one of these problems might exist.
Hope that helps.
Mark
Re: Presto K8 - Problem with pulsing amp
When I unplug the preamp tube(s) the oscillation stops. I'm thinking that your diagnosis might be correct.
I'm a new student of tube circuitry, so I'm not exactly sure what the B+ rail is?
One thing I wanted to ask: is it possible that the original resistors may also need to be replaced? Some of the resistors look oddly shiny, while others are quite faded and dry in appearance on the surface. Since you mentioned adding resistors to the circuit, I wondered if the old resistors might be the problem? I hope these aren't stupid questions.
Thanks so much for your helpful insight.
It's hard to find people who are knowledgeable about these old units.
In a side note: My original Presto schematics are a bit strange too. There are some symbols that don't seem to be listed in any of the schematics symbols charts in the books I've read. Also, the pins for the tube sockets aren't labelled in the schematics which makes it difficult to figure out which pin certain caps/resistors/wires are supposed to be connected to.
I've built a couple of tube amps from kits with detailed instructions and they work great, but diagnosing and repairing circuits seems much more complicated.
Again, thanks a lot for your help.
I'm a new student of tube circuitry, so I'm not exactly sure what the B+ rail is?
One thing I wanted to ask: is it possible that the original resistors may also need to be replaced? Some of the resistors look oddly shiny, while others are quite faded and dry in appearance on the surface. Since you mentioned adding resistors to the circuit, I wondered if the old resistors might be the problem? I hope these aren't stupid questions.
Thanks so much for your helpful insight.
It's hard to find people who are knowledgeable about these old units.
In a side note: My original Presto schematics are a bit strange too. There are some symbols that don't seem to be listed in any of the schematics symbols charts in the books I've read. Also, the pins for the tube sockets aren't labelled in the schematics which makes it difficult to figure out which pin certain caps/resistors/wires are supposed to be connected to.
I've built a couple of tube amps from kits with detailed instructions and they work great, but diagnosing and repairing circuits seems much more complicated.
Again, thanks a lot for your help.
Re: Presto K8 - Problem with pulsing amp
I just realized that you were talking about adding capacitors into the circuit and not resistors. Sorry about that. Like I said, I'm in the process of learning. Thanks for being patient.
-
Stevie342000
- Posts: 497
- Joined: Mon Dec 21, 2009 2:12 pm
Re: Presto K8 - Problem with pulsing amp
Ah yes some of those symbols such as mmF for capacitors equates to Pico farads the mm part may be expressed in the diagram as a small u with a tail too. In long hand mmF is expressed as micro micro Farad.
I have an tool on my desktop called Electronics Assistant, it is very useful it has a resistance code calculator i.e. you could punch in the value of the part from the schematic and it gives you the colour bar code, you can then visually match that to the part in the equipment. Oh yes it's a FREE Download or it was. It might be on duncan amps site. Try here: http://www.duncanamps.com/software.html There is valve data there as well, check it out. The resistor calculator is useful too.
I personally would change all the resistors, as for B+ if you stick your finger on it whilst it is on you will soon find out that that is the HT line which will be something like 300v DC at a few milliamps as it will give a nasty shock, it may be enough to kill you to.
As for valve sorry tube bases connections do a google search on valve/tube type and find the data sheet they are all pretty common tubes and work from that. The circuits are really simple and easily understood. Unlike modern kit which becomes obsolete this stuff does not as long as you can get hold of the tubes/valves.
Try here for valve/tube data plus lots more: http://www.tubebooks.org/.
Upload a copy of the schematics here, so it can be used for others to reference and talk you through fault finding.
You can not measure resistors or capacitors in circuit with a multimeter (LCR) and compare them to the schematic value without disconnecting one end from circuit. Personally I would change them all for modern carbon film 1/2 or 1 watt (check working voltage with supplier or data sheet).
I again would replace them all with 1 watt unless the circuit states a higher wattage rating. The bigger the resistor in power size the less noise you will get, remember this stuff was built to a budget and they used the smallest, cheapest component in each part to keep costs down to remain competitive.
In addition by using higher watt rated resistors they will run cooler, thermal noise becomes less of an issue and if they run cooler they are less stressed and may last longer. There may be debate on using metal film against carbon but if you want to keep it sounding the same as it did originally then carbon is the only option. You do not need any thing fancy capacitor wise either its a fool and it's money will soon be parted item and land of diminishing returns for money invested good quality orange dip will do and make sure that any capacitors in the picofarad range are close tolerance 1% or 2% and 5% at a pinch, they should be the best you can get polystyrene or silvered mica is usual choice here.
That about wraps that up oh and one final word of warning keep your fingers out of the kit when it is fired up if you are measuring any voltages as referenced to any that may be on the schematic. Keep you left hand especially away and behind your back. This stuff is not using hair raising voltages but 300 volts dc which will be up to about 150mA may kill you. My limit on DC voltages is 750 volts DC anything above that I keep away from. Most of the higher voltages are in the later stages of the circuit so be extra careful with those parts. Most vintage kit runs on 250 volts to 450 volts DC and some of it at higher amperage as well. Not trying to scare you just saying be careful that's all!!!
I have an tool on my desktop called Electronics Assistant, it is very useful it has a resistance code calculator i.e. you could punch in the value of the part from the schematic and it gives you the colour bar code, you can then visually match that to the part in the equipment. Oh yes it's a FREE Download or it was. It might be on duncan amps site. Try here: http://www.duncanamps.com/software.html There is valve data there as well, check it out. The resistor calculator is useful too.
I personally would change all the resistors, as for B+ if you stick your finger on it whilst it is on you will soon find out that that is the HT line which will be something like 300v DC at a few milliamps as it will give a nasty shock, it may be enough to kill you to.
As for valve sorry tube bases connections do a google search on valve/tube type and find the data sheet they are all pretty common tubes and work from that. The circuits are really simple and easily understood. Unlike modern kit which becomes obsolete this stuff does not as long as you can get hold of the tubes/valves.
Try here for valve/tube data plus lots more: http://www.tubebooks.org/.
Upload a copy of the schematics here, so it can be used for others to reference and talk you through fault finding.
You can not measure resistors or capacitors in circuit with a multimeter (LCR) and compare them to the schematic value without disconnecting one end from circuit. Personally I would change them all for modern carbon film 1/2 or 1 watt (check working voltage with supplier or data sheet).
I again would replace them all with 1 watt unless the circuit states a higher wattage rating. The bigger the resistor in power size the less noise you will get, remember this stuff was built to a budget and they used the smallest, cheapest component in each part to keep costs down to remain competitive.
In addition by using higher watt rated resistors they will run cooler, thermal noise becomes less of an issue and if they run cooler they are less stressed and may last longer. There may be debate on using metal film against carbon but if you want to keep it sounding the same as it did originally then carbon is the only option. You do not need any thing fancy capacitor wise either its a fool and it's money will soon be parted item and land of diminishing returns for money invested good quality orange dip will do and make sure that any capacitors in the picofarad range are close tolerance 1% or 2% and 5% at a pinch, they should be the best you can get polystyrene or silvered mica is usual choice here.
That about wraps that up oh and one final word of warning keep your fingers out of the kit when it is fired up if you are measuring any voltages as referenced to any that may be on the schematic. Keep you left hand especially away and behind your back. This stuff is not using hair raising voltages but 300 volts dc which will be up to about 150mA may kill you. My limit on DC voltages is 750 volts DC anything above that I keep away from. Most of the higher voltages are in the later stages of the circuit so be extra careful with those parts. Most vintage kit runs on 250 volts to 450 volts DC and some of it at higher amperage as well. Not trying to scare you just saying be careful that's all!!!
Re: Presto K8 - Problem with pulsing amp
You might be on the right track. If you post a scan of the K8 schematic and indicate which tube(s) pulled made the problem go away, we might be able to narrow down some options. It would also be helpful if you post a picture of the chassis which will show the work you've done. You may have done some things in the replacement process that caused this problem.kirt1974 wrote:When I unplug the preamp tube(s) the oscillation stops. I'm thinking that your diagnosis might be correct.
I'm a new student of tube circuitry, so I'm not exactly sure what the B+ rail is?
One thing I wanted to ask: is it possible that the original resistors may also need to be replaced? Some of the resistors look oddly shiny, while others are quite faded and dry in appearance on the surface. Since you mentioned adding resistors to the circuit, I wondered if the old resistors might be the problem? I hope these aren't stupid questions.
Thanks so much for your helpful insight.
It's hard to find people who are knowledgeable about these old units.
In a side note: My original Presto schematics are a bit strange too. There are some symbols that don't seem to be listed in any of the schematics symbols charts in the books I've read. Also, the pins for the tube sockets aren't labelled in the schematics which makes it difficult to figure out which pin certain caps/resistors/wires are supposed to be connected to.
I've built a couple of tube amps from kits with detailed instructions and they work great, but diagnosing and repairing circuits seems much more complicated.
Again, thanks a lot for your help.
I wouldn't worry about replacing resistors at this point unless and until you find a defect. Going to higher wattage resistors is not needed. The design, as is, should have enough margin with the existing resistors that temperature rise (and noise increase) is not an issue. In a few key places, you might find that some higher wattage resistors have failed or drifted too far from the desired value. These will need to be replaced. You can also find cases of small wattage resistors in the signal path have become noisy and will also need to be replaced. It does not sound like you have this problem right now. Even given carbon comp resistor's lousy value drift performance over these many years, these circuits can tolerate quite a large shift in value with little or no loss of performance. The only reason that re-capping is necessary is that paper foil caps become electrically leaky over the years and electrolytic caps dry out an loose their effectiveness in circuit.
Mark
Re: Presto K8 - Problem with pulsing amp
Thanks. You guys are really great. Thanks for your time and attention. Here is a link some pics and the schematic.
https://www.flickr.com/photos/alexkirt/sets/72157645927448075/
https://www.flickr.com/photos/alexkirt/sets/72157645927448075/
Re: Presto K8 - Problem with pulsing amp
When I remove the 6N7 tube (V104 on the schematic) or either of two 6SQ7 tubes (V102 and V103 on the schematic) the oscillation stops. However, when I remove the 6SQ7 tube labelled V101 on the schematic, the oscillation continues.
Re: Presto K8 - Problem with pulsing amp
I can't download the resistor calculator that you recommended because I use a Mac. However, I found this cool resistor calculator that works as a Mac dashboard widget. It seems to work really well too!
https://www.apple.com/downloads/dashboard/calculate_convert/resistulator.html
https://www.apple.com/downloads/dashboard/calculate_convert/resistulator.html
Re: Presto K8 - Problem with pulsing amp
Hi,
I see you did not replace any electrolytic caps. Its a bit of a pain, but worth the trouble and it might be the source of the problem. You can get multi-section replacement cans, but they tend to be expensive. What most people do is leave the old cans installed for cosmetic purposes and add single axial lead replacement caps under the chassis. Note that you have to completely disconnect the cans from the circuit and perhaps add some terminal blocks to tie the new caps to. Make sure your replacement caps have the same working voltage or higher. Same goes for the capacitance values.
You might be able to prove that this is the problem by tack soldering a 20uf 450V cap to the terminal and chassis ground on the cap that supplies the B+ voltage to the plate of the tube that when removed, seems to stop the oscillation. I don't see the schematic or else I could give some better directions.
Mark
I see you did not replace any electrolytic caps. Its a bit of a pain, but worth the trouble and it might be the source of the problem. You can get multi-section replacement cans, but they tend to be expensive. What most people do is leave the old cans installed for cosmetic purposes and add single axial lead replacement caps under the chassis. Note that you have to completely disconnect the cans from the circuit and perhaps add some terminal blocks to tie the new caps to. Make sure your replacement caps have the same working voltage or higher. Same goes for the capacitance values.
You might be able to prove that this is the problem by tack soldering a 20uf 450V cap to the terminal and chassis ground on the cap that supplies the B+ voltage to the plate of the tube that when removed, seems to stop the oscillation. I don't see the schematic or else I could give some better directions.
Mark
Re: Presto K8 - Problem with pulsing amp
I don't see any electrolytic capacitors. All I see are orange drops where there used to be wax foil caps (the ones I replaced) and a whole bunch of resistors. I can't find anything that I would recognize as an electrolytic capacitor. Am I missing something?
Here's the schematic.
Here's the schematic.
You do not have the required permissions to view the files attached to this post.
-
Stevie342000
- Posts: 497
- Joined: Mon Dec 21, 2009 2:12 pm
Re: Presto K8 - Problem with pulsing amp
Hello
Yes C101, 105, 106, 107, 109 & 115 are all electrolytic, they will all we be about 50V except 106 & 107 which should 450 volt rating.
It is more than likely any of the capacitors in the cathode circuit or the two capacitors in the power supply I my opinion it is more than likely to be C106 & C107.
If that does not solve the problem then start to look at the anode resistors. We are all here to help
Yes C101, 105, 106, 107, 109 & 115 are all electrolytic, they will all we be about 50V except 106 & 107 which should 450 volt rating.
It is more than likely any of the capacitors in the cathode circuit or the two capacitors in the power supply I my opinion it is more than likely to be C106 & C107.
If that does not solve the problem then start to look at the anode resistors. We are all here to help
-
Stevie342000
- Posts: 497
- Joined: Mon Dec 21, 2009 2:12 pm
Re: Presto K8 - Problem with pulsing amp
If problems still persist then look at R109, 1144, 1155, 122 & 123 and additionally 102, 106, 108 113 and 120.
You problem should be resolved with changing those power supply and or cathode electrolytic capacitors.
If after replacing any resistors (R109 - 123 are likely to have drifted over time) the problems still persist look at the tubes. Which more than likely are out of spec by now as well, they may be an issue as well. The only one which may be expensive is 45, you could check out to see if there are any equivalents, they may be similar to 2A3 but then you have a different heater voltage. They are American tubes not sure what UK equivalent if there was one would be.
Another issue might be that the tube pins need cleaning try some de-oxit on them. Check tube base connections and any changed components against circuit. It should be fairly simple to fix...oh yes check out the insulation on any wires as well and if it perished replace it, these things are built to last but there are several bits that age at different rates and they all have a finite lifetime.
You problem should be resolved with changing those power supply and or cathode electrolytic capacitors.
If after replacing any resistors (R109 - 123 are likely to have drifted over time) the problems still persist look at the tubes. Which more than likely are out of spec by now as well, they may be an issue as well. The only one which may be expensive is 45, you could check out to see if there are any equivalents, they may be similar to 2A3 but then you have a different heater voltage. They are American tubes not sure what UK equivalent if there was one would be.
Another issue might be that the tube pins need cleaning try some de-oxit on them. Check tube base connections and any changed components against circuit. It should be fairly simple to fix...oh yes check out the insulation on any wires as well and if it perished replace it, these things are built to last but there are several bits that age at different rates and they all have a finite lifetime.
Re: Presto K8 - Problem with pulsing amp
Hi,
Looking at the schematic and given the fact that pulling V102 and V103 causes the oscillation to stop, I'd look at the 10uf C109 as the first guess. That capacitor is the B+ bypass cap that feeds both of those tubes plates. I'm assuming that pulling V101, the first tube in the chain did not stop the oscillation. Try tacking in a second 10uf 350V or better cap at that circuit point and see if that kills the oscillation.
Mark
Looking at the schematic and given the fact that pulling V102 and V103 causes the oscillation to stop, I'd look at the 10uf C109 as the first guess. That capacitor is the B+ bypass cap that feeds both of those tubes plates. I'm assuming that pulling V101, the first tube in the chain did not stop the oscillation. Try tacking in a second 10uf 350V or better cap at that circuit point and see if that kills the oscillation.
Mark
Re: Presto K8 - Problem with pulsing amp
Hello Trolls,
I've got wonderful news! Mark was correct, it was the C109 capacitor. Here's what happened: when I replaced all of the capacitors, I mistakenly installed a non-polarized orange dip capacitor rather than a polarized electrolytic cap. Being new to the electronics repair game, I was unaware that the stripe on the side of the original cap indicated that it was polarized. So I simply removed the orange dip cap and reinstalled the original. Presto! (pun intended) the oscillation is completely gone!
In my rural hillbilly area, nobody keeps 10uf 450v caps in stock, so I ordered myself a 10 pack online. Until the new caps arrive, I hope this old wax foil dinosaur cap will hold together.
I want to thank all of you for walking me through this. It's been a great learning experience with a prize at the end!
Now I'm going to dive into restoring my Rekokut Imperial II. I'm sure I'll come whining back to you when I get stuck (which is pretty much guaranteed.)
Best wishes and a great weekend to you all. I'd buy you each a cold beverage if I could.
Thanks again,
Alex
I've got wonderful news! Mark was correct, it was the C109 capacitor. Here's what happened: when I replaced all of the capacitors, I mistakenly installed a non-polarized orange dip capacitor rather than a polarized electrolytic cap. Being new to the electronics repair game, I was unaware that the stripe on the side of the original cap indicated that it was polarized. So I simply removed the orange dip cap and reinstalled the original. Presto! (pun intended) the oscillation is completely gone!
In my rural hillbilly area, nobody keeps 10uf 450v caps in stock, so I ordered myself a 10 pack online. Until the new caps arrive, I hope this old wax foil dinosaur cap will hold together.
I want to thank all of you for walking me through this. It's been a great learning experience with a prize at the end!
Now I'm going to dive into restoring my Rekokut Imperial II. I'm sure I'll come whining back to you when I get stuck (which is pretty much guaranteed.)
Best wishes and a great weekend to you all. I'd buy you each a cold beverage if I could.
Thanks again,
Alex