Hi folks,
I just finished a clone of the Supro Thunderbolt (with Fender tone stack followed by an extra 12AX7) and it all worked perfectly for some days. However, today it seemed to work at full volume although the volume pot was turned down.
I checked the tone stack pots and ran through all the connections in the amp, which seemed to be in good order. Most surprisingly, without the tone stack connected at all, it worked flawlessly at full volume too! So somewhere in the first 12AX7 dual triode there might be some shortcut for the signal, right? Is this maybe the preamp tube failing? The cathode voltages all seemed to be ok. Or maybe some ground loop?
Laurens
I just finished a clone of the Supro Thunderbolt (with Fender tone stack followed by an extra 12AX7) and it all worked perfectly for some days. However, today it seemed to work at full volume although the volume pot was turned down.
I checked the tone stack pots and ran through all the connections in the amp, which seemed to be in good order. Most surprisingly, without the tone stack connected at all, it worked flawlessly at full volume too! So somewhere in the first 12AX7 dual triode there might be some shortcut for the signal, right? Is this maybe the preamp tube failing? The cathode voltages all seemed to be ok. Or maybe some ground loop?
Laurens
Moved to Instruments & AmpsHi, I see that I have been unclear referring to the tone stack.
The sequence of amp parts of interest are: input -> first tube -> volume pot -> 3 pot tone stack -> second tube -> PI etc.
What I meant is that even without the volume pot and tone stack connected (so no signal link between tube 1 and 2, although in the same 12AX7) there is a clean signal coming through which appears to be at a considerate volume, that is, between the first tube and second tube there should be some kind of signal connection?
Thanks a lot for the replies! I'm going to rewire this section anyway, but the problem still remains vague to me.
The sequence of amp parts of interest are: input -> first tube -> volume pot -> 3 pot tone stack -> second tube -> PI etc.
What I meant is that even without the volume pot and tone stack connected (so no signal link between tube 1 and 2, although in the same 12AX7) there is a clean signal coming through which appears to be at a considerate volume, that is, between the first tube and second tube there should be some kind of signal connection?
Thanks a lot for the replies! I'm going to rewire this section anyway, but the problem still remains vague to me.
Probably best if you mark up a schematic with x's to show what you disconnected so that we can better understand your meaning
You deviated from the design. You're coupling at the power supply. Back up and tell us what you were trying to achieve.
What do you mean coupling at the supply? The idea was to add the Fender tone controls and compensate for the signal loss by using the second half of the 12AX7.
I should mention that the 240 Ohm resistor at the PSU wasn't used because B1+ was already 360 V which I found acceptable.
I should mention that the 240 Ohm resistor at the PSU wasn't used because B1+ was already 360 V which I found acceptable.
Note that in the original schematic each anode supply has it's own drpping resistor and filter cap. Yours doesn't.
Would recommend using a triode section as a cathode follower before the stack rather than a common cathode stage after the stack.
You need an additional ps resistor and cap at the least to address your complaint.
Would recommend using a triode section as a cathode follower before the stack rather than a common cathode stage after the stack.
You need an additional ps resistor and cap at the least to address your complaint.
Someone suggested the volume control connections. You are assuming there is a path past the volume and tone, but if the volume pot loses its ground connection, it no longer can act as a voltage divider, which is what a volume control is. And when that happens, it is the same as having it at maximum volume.
SO please verify that your volume control has a solid connection to ground at the zero end, and also that the volume control itself is not open at the zero end.
SO please verify that your volume control has a solid connection to ground at the zero end, and also that the volume control itself is not open at the zero end.
Ground VT2 with a short piece of wire ... does sound stop?
Lift that wire and now ground VT3 ... does sound stop?
Please test and post results.
Your problem must be a simple wiring error, nothing esoteric.
Lift that wire and now ground VT3 ... does sound stop?
Please test and post results.
Your problem must be a simple wiring error, nothing esoteric.
Hi there,
Grounding at VT2 doesn't stop the sound, with VT3 it does stop though. So I already suspected it to be located between VT2 and VT3.
Anyways, I just found the culprit: somehow the 1 nF capacitor at the first tubes anode (C6) was grounded 'wrong' (ground loop probably, because it was connected to a local node). At least I reconnected it directly to my star ground and all is fine now. Is this possible because the voltages at that capacitor are at anode level? Still interested in what's causing this.
Thanks for all your replies! Many things to reconsider🙂
Grounding at VT2 doesn't stop the sound, with VT3 it does stop though. So I already suspected it to be located between VT2 and VT3.
Anyways, I just found the culprit: somehow the 1 nF capacitor at the first tubes anode (C6) was grounded 'wrong' (ground loop probably, because it was connected to a local node). At least I reconnected it directly to my star ground and all is fine now. Is this possible because the voltages at that capacitor are at anode level? Still interested in what's causing this.
Thanks for all your replies! Many things to reconsider🙂
- Status
- Not open for further replies.