Hi DIYaudioers,
I was fortunate enough to find a cheap hybrid tube amp on the local buy and sell. It's my first tube amp and I'm a big fan of the sound.
Johnson Guitars Its a made in china combo amp that has an 12AX7 preamp stage.
There is a hum which I'm told is normal. It's nothing major but its noticeable to me coming from solid state amps. I took it apart an looked at the amp circuit and it's pretty easy to work on. On some other forums they suggest increasing the 47uF filter caps to a 100uF.
Just wondering if you tube wizards have any thoughts on improving this amp. I know nothing about circuit design but am comfortable with a soldering iron. May try the cap swap but before I go to the local electronics store figured I'd ask here.
Johnson Audio was nice enough to send a schematic (attached)
Any thoughts? Hope you can help!
I was fortunate enough to find a cheap hybrid tube amp on the local buy and sell. It's my first tube amp and I'm a big fan of the sound.
Johnson Guitars Its a made in china combo amp that has an 12AX7 preamp stage.
There is a hum which I'm told is normal. It's nothing major but its noticeable to me coming from solid state amps. I took it apart an looked at the amp circuit and it's pretty easy to work on. On some other forums they suggest increasing the 47uF filter caps to a 100uF.
Just wondering if you tube wizards have any thoughts on improving this amp. I know nothing about circuit design but am comfortable with a soldering iron. May try the cap swap but before I go to the local electronics store figured I'd ask here.
Johnson Audio was nice enough to send a schematic (attached)
Any thoughts? Hope you can help!
Nice little amp.
In principle I would leave as-is, looks competent enough, no glaring errors.
There are some wild cap values in red, I guess they are Mods added at some Forum, hope they used the proper values.
Just in case check you have -- and not -- :
* C40 1n and not 224 which will make it intolerably muddy
* C22 22n and never ever ridiculous 100uF x 25V
* C24 22n and not ridiculous 100uF x 100V ... what were they thinking?
* VR3 : A250k or A500k is fine. B250k will work but hard to adjust.
* R4 is fine either 100k or 220k Higher value gives slightly higher gain.
* C38 - C39 22uF or 100uF same thing. 10V up us fine, up to 25V
* VR1: B10k or A25k are both fine.
* VR2 : A250k better (smoother control) than B100k.
A couple Mods shown make sense or harm nothing, most are wrong, follow what I suggest.
In principle I would leave as-is, looks competent enough, no glaring errors.
There are some wild cap values in red, I guess they are Mods added at some Forum, hope they used the proper values.
Just in case check you have -- and not -- :
* C40 1n and not 224 which will make it intolerably muddy
* C22 22n and never ever ridiculous 100uF x 25V
* C24 22n and not ridiculous 100uF x 100V ... what were they thinking?
* VR3 : A250k or A500k is fine. B250k will work but hard to adjust.
* R4 is fine either 100k or 220k Higher value gives slightly higher gain.
* C38 - C39 22uF or 100uF same thing. 10V up us fine, up to 25V
* VR1: B10k or A25k are both fine.
* VR2 : A250k better (smoother control) than B100k.
A couple Mods shown make sense or harm nothing, most are wrong, follow what I suggest.
R11 is kind of low at 100k. You might change it to 1meg so as not to load the guitar pickup so much.
Wow thanks guys! I'll pick up the parts and hopefully get to swapping some of caps/resistors this weekend. Really appreciate the help from you experts!
Out of curiosity without a scope is there an easy way to objectively measure these changes?
I will likely try to improve some of the wiring/grounding. I noticed the speaker wire isn't twisted, not sure if this will make a difference in the hum but why not. Maybe I've got some shielded cable in my junk bin and put that to use.
Thanks again! Excited to tackle this.
Out of curiosity without a scope is there an easy way to objectively measure these changes?
I will likely try to improve some of the wiring/grounding. I noticed the speaker wire isn't twisted, not sure if this will make a difference in the hum but why not. Maybe I've got some shielded cable in my junk bin and put that to use.
Thanks again! Excited to tackle this.
Don't use shielded wire for the power amp to speaker wiring. Not a good idea, may cause instability. As far as replacing R4 (100k) with a 1 meg resistor, you should hear more treble from your pickups with that setup. Most classic tube guitar amps run 1 meg grid resistors(Ampeg used higher values, but I can't remember ,maybe 5.6 meg on VT40's, V4's etc.???).
B+ is operating at <100V, and the 50/60Hz ripple may be noticeable from that aspect, and be getting past IC1A. Is there the same hum from speaker when V1 is in/out?
One option is to roll-off the signal level going in to IC1A at low frequency.
Another option is to full bridge rectify that supply rail to get lower ripple but that would increase A-A and require all the related caps to be changed to higher voltage ratings.
One option is to roll-off the signal level going in to IC1A at low frequency.
Another option is to full bridge rectify that supply rail to get lower ripple but that would increase A-A and require all the related caps to be changed to higher voltage ratings.
Thanks so much! I'm not sure what entirely improved the hum but it's much quieter. I'm only getting EMI interference when plugged in. I can crank the gain and not get a tonne of hum! I really like the volume pot change. Thank you!
Interestingly C40 was not what the schematic said. C40 was 1nF as you suggested. After seeing this i decided not to desolder C22 and 24. R4 was 220K. So what I ended up doing was:
-I changed C38,C39 to 100uF
-changed VR2 to A250k pot
-also twisted the speaker cable
-added another ground point/cable and cleaned up the grounding behind the faceplate
@BrunoP: I've kept R11 the same for now, but may switch to a 1Mohm if I want to tinker.
@Trobbins - I pulled the valve and no 60hz hum.
Really loving the sound of this think. Thanks so much!