I have a new build.
Schematic attached EL34, 12AX7 (preamp), 12AT7 (inverter)
There is a massive hum that changes pitch and loudness with the tone stack.
Trebble off, there is no hum, defiantly in signal path
With both preamp tubes out there is no hum.
With the 2nd tube in there is a little bit but not much at all.
When I put in the first tube the hum is there, and it is huge.
Master is up all the way, gain is off. ( first stage not the issue)
This has to be between the 2nd and 3rd stage.
My thinking is bad component or ground loop issue. Everything is new.
Currently R40 and D4 have been removed.
C22, C23, C13 are not installed yet
There is a grounding buss bar for the preamp section, the ground icons are where they connect to the buss, and I have noted how I have them grouped before connecting to the buss.
I have resoldered everything.
Not sure if this matters but C18 makes noise through speaker when tapped.
I changed that component, no change.
Amp does play
Swapped preamp tubes around but no effect.
Schematic attached EL34, 12AX7 (preamp), 12AT7 (inverter)
There is a massive hum that changes pitch and loudness with the tone stack.
Trebble off, there is no hum, defiantly in signal path
With both preamp tubes out there is no hum.
With the 2nd tube in there is a little bit but not much at all.
When I put in the first tube the hum is there, and it is huge.
Master is up all the way, gain is off. ( first stage not the issue)
This has to be between the 2nd and 3rd stage.
My thinking is bad component or ground loop issue. Everything is new.
Currently R40 and D4 have been removed.
C22, C23, C13 are not installed yet
There is a grounding buss bar for the preamp section, the ground icons are where they connect to the buss, and I have noted how I have them grouped before connecting to the buss.
I have resoldered everything.
Not sure if this matters but C18 makes noise through speaker when tapped.
I changed that component, no change.
Amp does play
Swapped preamp tubes around but no effect.
Attachments
You could skip the whole preamp section (modern sources require no extra gain at all) if you have modern sources and try with a normal 2Vrms max. source connected to a logarithmic volume potentiometer 10 .... 25 kOhm to C1. Start in lowest position of course. If you then have no hum but still have high volume on 1 o'clock investigate gain structure even further.
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+1
IMO it's unnecessary -especially the equalizer stages-.
BTW all of the potentiometer is the possible hum source.
The world went 2V rms max around 1980 so 45 years ago. HiFi amplifiers became 0.5 ... 1V input sensitivity for full output power. Stuff from before that time including the tubes was meant/calculated for other signal levels. The challenge is of course to accept that standard and design everything around that. It works pretty well. Fighting it, ignoring it or having a private standard does not but all sure are thread generators of proportions.
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Likely a guitar amplifier. If so, this thread should be in the "Instruments and Amps" section.
Are we still looking at the OP's issue? Then I suggest to follow up on Post #3.
Or is the thread already derailed after half a dozen posts? That must be a new record.
Jan
Or is the thread already derailed after half a dozen posts? That must be a new record.
Jan
The heaters have a CT that is grounded.
Also when I probe either side of C18 the hum jumps up like its shorted
No other cap does that
Either I have a short under the board or a bad filter cap
Also when I probe either side of C18 the hum jumps up like its shorted
No other cap does that
Either I have a short under the board or a bad filter cap
C18 is at very sensitive, high impedance nodes, so not surprising.
C14 would probably behave similarly, with the volume turned up.
Are all of the pot shells grounded?
C14 would probably behave similarly, with the volume turned up.
Are all of the pot shells grounded?
Is the value of R35 really 12K like it says in your schematic?
Could you post some pictures of your build?
Could you post some pictures of your build?
no they are not, never had that cause this issue before.
Also note:
If I disconnect the grid of stage two from the circuit and ground it, nothing chages.
If I disconnect the plate wire from satge two it stops.
Also note:
If I disconnect the grid of stage two from the circuit and ground it, nothing chages.
If I disconnect the plate wire from satge two it stops.
Yes it is, going for a cold cathode.
will have to do pictures later, have to go fix my mower
It is better to use a rectifier and capacitor for the V1 tube filament and make it completely DC. v1= first tube 12ax7.Of course, there are other things that need to be done to completely remove the hum.
I dont think anyones understand why I used the word "MASSIVE" in the title.
This is not a little hum that is bleeding though the heaters or another small issue.
This ia a holy crap what is wrong, the amp is totaly unusable situation.
I have built several amps the same way I built this, and never had this problem.
This is not a little hum that is bleeding though the heaters or another small issue.
This ia a holy crap what is wrong, the amp is totaly unusable situation.
I have built several amps the same way I built this, and never had this problem.