Just another update, ive changed the cathode resistor on the third stage down to 10K and what a difference!
Its also lowered the voltage on the anode to around 295V which is good, as thats in the proper voltage specs for the tube.
The anode voltage on the fourth stage is now 110v, which i expect is because its now drawing more current?
Not for an ECC82, a type that needs some current to improve linearity.
It is drawing a little over 5mA, so that is ok. Power dissipation is around half a watt, so no issue there either.
OK, so the current through the third is around 1mA now?
Divide the voltage across the anode resistor by the resistance of it. So it will be something like (352-295)/47000 = 0,0012A = 1,2mA.
As for the fourth stage, when the anode voltage goes up, that means that the voltage drop across the anode resistor has gone down. In other words: less current.
Yes thats true, it would mean less current, i guess the grid current has changed on the fourth stage as a result of lowering the cathode resistor in turn increasing the voltage drop?
Ive come back to the amp after it was running for an hour or so and the voltages have settled down now.
Its now reading 268V on the anode and measuring the voltage across the resistor is 68V, so its about 1.4 mA
Im alot more happy with the sound now, but i will have to add a bypass cap to the third stage.
Im thinking it may be a good idea to remove the bypass cap on the second stage, or at least lower it on the first stage.
I do feel its probably a bit muddy, although sounding alot better with more gain, the low notes, the E and A strings in particular sound very muffled, like there is quite a bit of distortion on the bass notes, so it might be a good idea to drop the cathode bypass cap to somewhere around 1uf.
Im not sure how multiple bypass caps will affect each other on each stage, as alot of amps im familiar with only use one on the first stage, but i guess i should try moving the tone stack across to the second stage and see how it compares.
Main thing is progress has been made which is good.
Im going to call it a night for now, but will work more on it tomorrow.
That "oscillation" sound had gone away, i thought that changing the cathode resistor must have fixed it, but its come back again, IDK if its something its picking up in the house? Ill check our appliances.
I will take a recording of it if its any help.
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Ive been doing more work on the amp today, ive added grid stoppers of 22K to the second and third stage.
I have not noticed any real difference doing this.
I have changed the bypass cap on the first stage to 1uf and this has cleaned things up quite a bit on the lower notes, but i still cant get the guitar as clean sounding as i would like.
Im still having hum issues, but not when the volume is down.
I cant find any 100 ohm resistors for the heaters, so i will have to get some when im in town next, but i might go through another box of components tonight that potentially contains some.
Im pretty sure there is oscillation happening, and i had the amp whistle when turning up the volume, i still have the sound i described earlier which i hope i can get rid of.
The hum will change when i move around the room and get closer to the amp etc.
I may need better shielding etc perhaps.
I have not noticed any real difference doing this.
I have changed the bypass cap on the first stage to 1uf and this has cleaned things up quite a bit on the lower notes, but i still cant get the guitar as clean sounding as i would like.
Im still having hum issues, but not when the volume is down.
I cant find any 100 ohm resistors for the heaters, so i will have to get some when im in town next, but i might go through another box of components tonight that potentially contains some.
Im pretty sure there is oscillation happening, and i had the amp whistle when turning up the volume, i still have the sound i described earlier which i hope i can get rid of.
The hum will change when i move around the room and get closer to the amp etc.
I may need better shielding etc perhaps.
Here is a youtube clip of the sound im getting, its very unusual.
Probably still a little too much hum for my liking, but its no longer humming when the jack is grounded, so im pretty sure its background noise.
Ive added extra shielded cable to the tone stack and that helped alot.
Before doing so, the hum would increase when getting near the amp,or even waving my hand over it!
It only does this nowif i put my hand close to the input jack.
In this video i have the first volume control up almost full and the second volume is about 1 third the way up.
https://youtu.be/ZbyNwG3YG7o
Probably still a little too much hum for my liking, but its no longer humming when the jack is grounded, so im pretty sure its background noise.
Ive added extra shielded cable to the tone stack and that helped alot.
Before doing so, the hum would increase when getting near the amp,or even waving my hand over it!
It only does this nowif i put my hand close to the input jack.
In this video i have the first volume control up almost full and the second volume is about 1 third the way up.
https://youtu.be/ZbyNwG3YG7o
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Another question, should i have negative feedback running on this amp?
I know most guitar amps usually have some form of it, this did originally have the OT tapped to the grid on the third triode through a 10K resistor, but this made the amp too quiet although cleaner, was no benefit so i removed it.
I know most guitar amps usually have some form of it, this did originally have the OT tapped to the grid on the third triode through a 10K resistor, but this made the amp too quiet although cleaner, was no benefit so i removed it.
A 'little bit of everything' sounding in that clip- hum (50Hz), buzz (100Hz) and perhaps some motorboating on top. Plus, I'd be surprised if there wasn't some oscillation at higher frequencies happening as well.
Have you wired up your input jack so it is shorting the input to ground when nothing is plugged into it?
Once that's done, the amp should be 'reasonably quiet' with both volume/gain controls wide open. That's the goal, anyway.🙂
I'd leave circuit changes like moving the tone stack and NFB till the noise problem is solved.
I'd start by looking again at:
Wiring errors
Grounding and layout- Are your jacks isolated from the chassis? Is the speaker secondary connected to ground? etc...
Getting the heater hum minimized. Any pair of resistors above 100 ohms will be a start. Another strategy would be a 'hum balancing pot' - 250 ohms and up- across the heaters with the wiper connected to ground. In my current project (a PP EL84 amp with 4 gain stges) I found the hum pot (aka 'humdinger') worked a bit better than attaching the heater center tap to ground or to an elevated voltage reference.
What is the scope showing?
Next (for me) would be to lower the gain by disconnecting/removing that preamp cathode bypass cap, to see if that helps.
It sounds like it has a lot of harmonics of the 50/100Hz as well. This could be from the power supply. Snubbers for the transformer windings and/or diodes will fix that, snubbers are discussed elsewhere on diyaudio.
It could also come from lighting, specifically fluorescent, cfl, or perhaps even LED light bulbs. Old fashioned incandescent light bulbs don't do that. If turning off the light substantially reduces the hum/buzz, a metal plate across the exposed "bottom" should shield it.
It could also come from lighting, specifically fluorescent, cfl, or perhaps even LED light bulbs. Old fashioned incandescent light bulbs don't do that. If turning off the light substantially reduces the hum/buzz, a metal plate across the exposed "bottom" should shield it.
Yes the jack is shorted to ground when unplugged, as is one side of the OT secondary. They are cliff jacks, and are plastic, so no part of its body makes electrical contact with the chassis.
I forgot to mention that in that video clip, there is a guitar connected.
Its super quiet when the jack is disconnected, with only a small amount of white noise.
If i remove the heater windings from ground, there is significant hum, even when the jack is grounded after the guitar removed.
I will try a humdinger if i find a suitable pot, but will wire up some resistors hopefully today to see what change i get.
I too are very interested to see my scope output, ive just shifted home and need to find its power cord!
Neon lights turned on or off make no difference.
What bypass caps do you recommend removing?
Im starting to question the other two though, i think i will try removing the second bypass cap.
Ive added 22k grid stoppers to the second and third stages, these did not do anything that i could notice.
I know the noise has to be coming from the first or second stage, or both.
I might try and put a metal plate across the bottom, this was something i was thinking of if its just a sensitive amp.
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Another update.
I think most of the Hum is relatively acceptable, although i would like to get it down as much as possible.
Im pretty sure the "oscillations" or "motorboating" were noticeable after lowering the cathode resistor in the third stage to 10K, however this made many improvements.
I can still hear it, although alot quieter when i disconnect the grid of the third stage from the preamp, so that must be happening in that stage.
Ive removed the cathode bypass caps on all stages except the first which is now using 1uf.
Below is a shot of the scope, the spike on the left seems to come and go with the "chattering" that i described earlier, volume 1 is up all the way here and volume 2 is not quite quarter the way up.
Alot of the 50Hz hum dies down when holding the guitar, but the oscillation is still present, although you dont notice it when playing, it still would be good to see it go.
Another thing i noticed is that the capacitor coupling the 3rd stage to the grid on the 4th stage is actually 4.7nF, not 47nF as in the schematic. Dont know if its cruical, but changing it to a higher value did not seem to make any real difference.
Either way, alot of improvements have been made, which is good.
Thanks for everyones input. I think that moving the tone stack might help with clarity possibly too.
I think most of the Hum is relatively acceptable, although i would like to get it down as much as possible.
Im pretty sure the "oscillations" or "motorboating" were noticeable after lowering the cathode resistor in the third stage to 10K, however this made many improvements.
I can still hear it, although alot quieter when i disconnect the grid of the third stage from the preamp, so that must be happening in that stage.
Ive removed the cathode bypass caps on all stages except the first which is now using 1uf.
Below is a shot of the scope, the spike on the left seems to come and go with the "chattering" that i described earlier, volume 1 is up all the way here and volume 2 is not quite quarter the way up.
Alot of the 50Hz hum dies down when holding the guitar, but the oscillation is still present, although you dont notice it when playing, it still would be good to see it go.
Another thing i noticed is that the capacitor coupling the 3rd stage to the grid on the 4th stage is actually 4.7nF, not 47nF as in the schematic. Dont know if its cruical, but changing it to a higher value did not seem to make any real difference.
Either way, alot of improvements have been made, which is good.
Thanks for everyones input. I think that moving the tone stack might help with clarity possibly too.
An externally hosted image should be here but it was not working when we last tested it.
OK, another update.
Been working on it all day, im no longer worried about the "hum" and potential "motorboating" IDK what the source is, but i fired up my marshall valvestate that i have not used for a couple of years and im getting the exact same noise as my tube amp!
So im now 100% confident that ive got everything sorted as far as noise goes.
I will have to try it in another area at some point and see how it goes there.
I also made some improvements to the tone stack, i think i was not getting much response with such a high bass cap, and such a low treble cap, and the low notes were muddy.
Have swapped the 250pf with a 470pf and the 0.1 with a 0.022
Only issue remaining is sometimes there is a very slight glow on the plate of the tube, weird thing is its only visible sometimes. But its alot more noticeable on my NOS mullard, which i quickly switched off after noticing.
Will increasing the cathode resistor to 470K be enough to drop the voltage across the tube?
I believe this is a common issue with single ended amps drawing too much current when idle.
I want to add a second high sensitivity input, any suggestions on doing this would be appreciated.
I may do something similar to how fender do it, but there may be better ways.
Thanks for everyones help. Im happy with what ive now got 🙂
Been working on it all day, im no longer worried about the "hum" and potential "motorboating" IDK what the source is, but i fired up my marshall valvestate that i have not used for a couple of years and im getting the exact same noise as my tube amp!
So im now 100% confident that ive got everything sorted as far as noise goes.
I will have to try it in another area at some point and see how it goes there.
I also made some improvements to the tone stack, i think i was not getting much response with such a high bass cap, and such a low treble cap, and the low notes were muddy.
Have swapped the 250pf with a 470pf and the 0.1 with a 0.022
Only issue remaining is sometimes there is a very slight glow on the plate of the tube, weird thing is its only visible sometimes. But its alot more noticeable on my NOS mullard, which i quickly switched off after noticing.
Will increasing the cathode resistor to 470K be enough to drop the voltage across the tube?
I believe this is a common issue with single ended amps drawing too much current when idle.
I want to add a second high sensitivity input, any suggestions on doing this would be appreciated.
I may do something similar to how fender do it, but there may be better ways.
Thanks for everyones help. Im happy with what ive now got 🙂
Yes, increasing the cathode resistor on the EL34 to 470 ohms will drop the idling current and I think will be sufficient to stop the slight red-plating.
The input you have is already max. sensitivity for the circuit. You could add an additional lower sensitivity input (a la Fender) if you wanted (but that's not much different to turning down the guitar vol. control).
I may have missed a change from the initial schematic, but that shows a single 56K input resistor (which may now be wired as a grid stopper?).
Adding a second Fender-style input would put a second 56K in parallel, dropping the input resistance, and 'increasing the sensitivity', I suppose.
Thinking along somewhat the same theme as Malcolm, I now just put in a 'high sensitivity' input and turn down the volume on the guitar if I think the pickups are too 'hot' for the amp first stage. However, I don't have active pickups on any guitar I own.
My approach would be to put a smaller grid stopper (10K or less?) right on the tube pin and then drop the 56K resistor value, and forget about drilling another hole for a second jack.
Of course, that may bring back an oscillation/noise problem.
To add a markedly higher amplification input, I think another gain stage would be needed.
A 10k grid stopper (instead of the 56k) will reduce the hiss level somewhat (but increase the risk of radio station pick up and oscillation). However, it doesn't really change the sensitivity - the current into the grid is extremely small (unless we overdrive the input and produce grid current), so any voltage drop across the grid stopper is negligible.
As Francis Urqhart famously said in 'House of Cards':"You may very well think that".....🙂
Millions of Fender amps with input resistors of 68K and alternatively 34K (68s paralleled for greater 'sensitivity') argue otherwise....or perhaps I don't understand what you are saying?
At any rate, it's a quick experiment for anybody who has a handful of resistors and a soldering iron.
In the Fender set up, one jack puts the two 68ks in parallel giving a 34k grid resistor and a 1M grid leak - that gives full sensitivity.
Using the other jack forms the two 68ks into a voltage divider, so that the grid receives half the input voltage.
Using the other jack forms the two 68ks into a voltage divider, so that the grid receives half the input voltage.
OK, that confirms my thoughts on the output stage.
Yes, your right, i was really referring to adding a lower sensitivity input, since i have pretty high gain as it stands.
I usually find its better changing sensititivy at the amp if possible, i always find that turning the volume down on the guitar changes tone somewhat and can make it muddy, depending on the guitar. some musicians find this as an advantage.
The 56K resistor in the schematic has always been wired up as a grid stopper right on the tube socket itself.
I dont think the fenders have a grid stopper (not what ive seen myself anyway) and instead both resistors are wired on the jacks themselves.
I was thinking of doing something similar and wire both resistors as gridstoppers together on the valve socket and connect with shielded cable to the jacks.
Ive already got a hole in the chassis with a jack installed, so i may as well use it .
I dont think I will ever need another gain stage with this amp, as its got tons as it stands!
Adding a low gain, was really what i meant, and would be useful to tone it down.
A low sensititivy input may very well help me get a cleaner sounding guitar if i dont want to overdrive the amp or use an accoustic guitar perhaps.
Ahh, yes your right, that is how the fenders are configured.
Would say using a 4.7K grid stopper be a better idea and then wire both input jacks in a similar fashion to a fender amp?
I probably would stay with 56K instead of 68K
I also see some amps appear to have a low pass filter with a 470pf cap across a 470K resistor that is in series with the grid and input, do these serve as a noise suppressor?
I see other configurations with caps across the input to ground, but not sure what purpose these serve, as i found i got more noise adding them.
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