Hi all, I’ve been enjoying my first DIY amp quite a bit, bringing it out for several gigs, tweaking a few things here and there, and seeing a trajectory of steady improvement with the voicing.
However, there is an issue which I don’t know how to address. I’m hearing an extraneous noise accompanying certain notes, it’s a kind of crunchy rumble. Sometimes I think I’m hearing a distorted sub-tone an octave down.
It only happens on certain notes, and they seem to correlate to mechanical resonance which you can hear if you tap on the tubes or the chassis. The same pitches you hear when doing this, can sometimes get excited into a slow feedback type of noise. The same pitches, when you play them on the guitar, get this extra crunchy whoosh-y rumble added. It doesn’t seem to make a difference how I set the volume on my guitar vs on the amp.
These are behaviors I can notice while playing at home. They’re maybe a bit too subtle to be noticed on a gig scenario. I’m playing louder on gigs, but there’s also other musicians playing and it just gets buried. It would doubtless be a better if the amp didn’t do these things.
Attaching the schematic. V1 is a C3G and yes I’ve got it running at really high gain, but again the offending sound doesn’t seem to care how hard I’m driving the input.
V2 is 6SN7 and this one seems to be the most sensitive, microphonically. The resonance seems to be localized there. I’ve tried swapping in different 6SN7’s, they all act the same. Drivers are 6SL7 and outputs are KT66. Generally the amp sounds pretty good, I’m just pleased it works at all, let alone well enough to play out with.
I hope that someone recognizes the behavior which I’ve described as best I can. Thanks for your thoughts.
However, there is an issue which I don’t know how to address. I’m hearing an extraneous noise accompanying certain notes, it’s a kind of crunchy rumble. Sometimes I think I’m hearing a distorted sub-tone an octave down.
It only happens on certain notes, and they seem to correlate to mechanical resonance which you can hear if you tap on the tubes or the chassis. The same pitches you hear when doing this, can sometimes get excited into a slow feedback type of noise. The same pitches, when you play them on the guitar, get this extra crunchy whoosh-y rumble added. It doesn’t seem to make a difference how I set the volume on my guitar vs on the amp.
These are behaviors I can notice while playing at home. They’re maybe a bit too subtle to be noticed on a gig scenario. I’m playing louder on gigs, but there’s also other musicians playing and it just gets buried. It would doubtless be a better if the amp didn’t do these things.
Attaching the schematic. V1 is a C3G and yes I’ve got it running at really high gain, but again the offending sound doesn’t seem to care how hard I’m driving the input.
V2 is 6SN7 and this one seems to be the most sensitive, microphonically. The resonance seems to be localized there. I’ve tried swapping in different 6SN7’s, they all act the same. Drivers are 6SL7 and outputs are KT66. Generally the amp sounds pretty good, I’m just pleased it works at all, let alone well enough to play out with.
I hope that someone recognizes the behavior which I’ve described as best I can. Thanks for your thoughts.
Those inductors are small through-hole ones, looking like film caps but heavier. They’re within the box so I’d have to run the amp open to evaluate the question. Maybe I’ll try that later, thanks.
There’s not very much doubt that V2 contributes the resonance, if it gets to singing that E above middle C, you can damp it by touching V2. However, placing the amp farther away from the speaker doesn’t seem to lessen the crunchy rumble sounds.
Maybe that note is always humming at a level I can’t necessarily hear, and the rumbly sound I’m getting from playing certain notes is an intermodulation effect
There’s not very much doubt that V2 contributes the resonance, if it gets to singing that E above middle C, you can damp it by touching V2. However, placing the amp farther away from the speaker doesn’t seem to lessen the crunchy rumble sounds.
Maybe that note is always humming at a level I can’t necessarily hear, and the rumbly sound I’m getting from playing certain notes is an intermodulation effect
There isn't much info on the schematic, so this is a raw guess, but it could be a low-frequency resonance in the power supply. Try strapping a 20uF/400V cap at the 350V node to ground between the tops of R17 and R22 to decouple those two nodes. It might not do anything, but it's worth a shot.
Stph
Stph
Oh hi, thank you for chiming in. Sorry my schematic doesn’t include the power supply nor part values. There is a big cap at the supply node there, but I agree that isolating those two stages from each other might be worth doing. That’s on my “next time” list.
But having spent more time thinking about the issue, I’m sure it’s not doing me any good having drivers (V3a&b) with a bunch of gain, after the phase inverter (V2b). If there are microphony problems with V2, they are sure to be a lot worse down the pipe. I’m going to try a different arrangement.
Last night’s gig was a jam session situation, and I got a chance to hear a couple other players going through the amp. Not bad!
But having spent more time thinking about the issue, I’m sure it’s not doing me any good having drivers (V3a&b) with a bunch of gain, after the phase inverter (V2b). If there are microphony problems with V2, they are sure to be a lot worse down the pipe. I’m going to try a different arrangement.
Last night’s gig was a jam session situation, and I got a chance to hear a couple other players going through the amp. Not bad!
Part of the reason I put the 6SL7 as output drivers, rather than before the PI, was simply to have a use for both triodes.
It does sound kind of like power supply resonances as has been said. There is no harm in increasing the ps caps a little except the first cap after a tube rectifier, it must be no larger than the spec sheet says or you’ll use the rect tube! It also might be something with the middle tone adjust frequency selector. They might create a funky(technical term) resonance the circuit doesn’t like.
Thanks all. I’m using a silicon rectifier bridge, and the first cap is 680uF. The B+ node where R17 and R22 join has a 100uF cap. I went big because I wanted it quiet, but I will definitely accept any lessons as to why bigger ain’t better.
The good news is I’ve never blown the fuse on startup, haha.
The good news is I’ve never blown the fuse on startup, haha.
I’m not certain exactly how power supply resonances might arise, but the two 6SN7 stages are drawing a pretty healthy current. There is a cap at the B+ but it is shared between these two stages. I wonder if further resistive decoupling might be a good idea. Like, run one stage at a little lower voltage with its own R/C “branch” from the power supply.
I’ll add this to the list.
I’ll add this to the list.
Jaybee writes: "I’m not certain exactly how power supply resonances might arise...."
For some insight into how power-supply feedback can affect high-gain amplifiers, take a look at this resource:
Terman, Frederick E., "Regeneration in Multistage Audio- and Video-frequency Amplifiers," Electronic and Radio Engineering. New York: McGraw-Hill Book Company, 1955, pp. 360-363. (PDF, 565K).
I keep a copy of this document on my thermionic.info site, it you need to find it again.
For some insight into how power-supply feedback can affect high-gain amplifiers, take a look at this resource:
Terman, Frederick E., "Regeneration in Multistage Audio- and Video-frequency Amplifiers," Electronic and Radio Engineering. New York: McGraw-Hill Book Company, 1955, pp. 360-363. (PDF, 565K).
I keep a copy of this document on my thermionic.info site, it you need to find it again.
Cool resource, thanks for that info.
The various B+ voltages for this amp’s power supply are coming from the usual sort of cascade of RC filters. I’ve also been pretty zealous about connecting grounds separately as well. V2a and V2b do share the same B+ node/cap so I’m not sure how decoupled we consider that, but both these stages are near unity gain. If there is some kind of regeneration going on, V1 would be the likely culprit.
I am planning to rebuild the thing and I think I’ve got a lot of ideas which are all going to help with the less desirable behaviors.
The various B+ voltages for this amp’s power supply are coming from the usual sort of cascade of RC filters. I’ve also been pretty zealous about connecting grounds separately as well. V2a and V2b do share the same B+ node/cap so I’m not sure how decoupled we consider that, but both these stages are near unity gain. If there is some kind of regeneration going on, V1 would be the likely culprit.
I am planning to rebuild the thing and I think I’ve got a lot of ideas which are all going to help with the less desirable behaviors.
Part of me wonders whether this is just normal tube amp idiosyncrasy going on. The problem is not really noticeable on gigs. At home it’s less easy to overlook. I’ve never been a big tube amp guy, I have played through solid state amps happily enough for most of my life. I don’t think an amp should make these noises, so I’ll try some remedies…