I have a weird oscillation problem:
Check the drawing for reference.
I have built a 2 channel amp, and the channel can be switch to play each one individually or jumped together.
When I play the channel 1, everything is good but when I switch to channel 2, there is oscillation happening with the gain 3/4 up and more. The pitch of the oscillation vary depending on the gain setting.
Also the cathode bypass capacitor marked xxx are on a 4 position rotary switch and this is the value per setting:
1: c7=unconnected c8=0.33uF
2: c7=1uF c8=0.68uF
3: c7=0.68uF c8=1uF
4: c7=0.33uF c8=2uf
Changing the position of the bypass capacitor also change the pitch of the oscillation.
Jumping the 2 channel removes the oscillation almost completely.
I highly doubt the lead dress or component placement are the issue here as I made extra sure everything was close to perfect and every critical part of the circuit that needs to be close to one another is (ex: grid stopper resistor and grid).
Since the pitch change depending on the selection of the bypass capacitors I suspect it could be it but at the same time it is nothing crazy to have bypass caps.
Also I scoped the b+ and it is smooth as f but interestingly enough I have a weird 7volts peak to peak weird ac wave riding on the b+ right at the anode of R11 and R16 but on the other side of those resistors the b+ is smooth.
I'm really puzzled here and anyone who could pitch ideas as to what this is would be really appreciated.View attachment Oscillation problem.pdf
Check the drawing for reference.
I have built a 2 channel amp, and the channel can be switch to play each one individually or jumped together.
When I play the channel 1, everything is good but when I switch to channel 2, there is oscillation happening with the gain 3/4 up and more. The pitch of the oscillation vary depending on the gain setting.
Also the cathode bypass capacitor marked xxx are on a 4 position rotary switch and this is the value per setting:
1: c7=unconnected c8=0.33uF
2: c7=1uF c8=0.68uF
3: c7=0.68uF c8=1uF
4: c7=0.33uF c8=2uf
Changing the position of the bypass capacitor also change the pitch of the oscillation.
Jumping the 2 channel removes the oscillation almost completely.
I highly doubt the lead dress or component placement are the issue here as I made extra sure everything was close to perfect and every critical part of the circuit that needs to be close to one another is (ex: grid stopper resistor and grid).
Since the pitch change depending on the selection of the bypass capacitors I suspect it could be it but at the same time it is nothing crazy to have bypass caps.
Also I scoped the b+ and it is smooth as f but interestingly enough I have a weird 7volts peak to peak weird ac wave riding on the b+ right at the anode of R11 and R16 but on the other side of those resistors the b+ is smooth.
I'm really puzzled here and anyone who could pitch ideas as to what this is would be really appreciated.View attachment Oscillation problem.pdf
There is a gain of about 3,000 in the lower chain.
Relays as drawn, the input node R6 is *unconnected*, begging to catch *any* small hint of signal from the output and amplify it again and again... oscillation.
Relays as drawn, the input node R6 is *unconnected*, begging to catch *any* small hint of signal from the output and amplify it again and again... oscillation.
I highly doubt the unconnected relay pin is the problem since any connected one could pick up a signal as easily, but my layout leads me to believe that it is very very unlikely. The gain of 3000 I didn't knew :s
I will go through and calculate this tomorrow and check if it is in fact that high.
I will go through and calculate this tomorrow and check if it is in fact that high.
It isn't the "unconnected relay pin" per se', it's the floating grid without a reference to ground that invites problems, assuming the drawing is accurate. Simply tack solder a 1 meg resistor (or lower) to the grid and ground and see if stops the oscillation.
I came to the same conclusions, very high gain (bottom path) and floating input.
I would put the 1meg on the other side of the relay 2.
I would put the 1meg on the other side of the relay 2.
The last valve in the OD channel looks too similar to a phase shift oscillator for comfort. A wee bit of stray capacitance and some coupling to the input and bobs your uncle.
I will try to solder a 1meg resistor as you say but here's the thing, there is no oscillation when on my clean channel (relay state are as on the drawing). The oscillation is only present when on my channel two (relay 1 switch state and in turn connect the grid to ground via the 1meg resistance).
I will still solder an extra 1meg to the grid and see if does anything but I doubt it since it would only parallel the 1 meg already in place when the channel is activated.
I'll do that tonight and report back if it did anything (fingers crossed).
It looks like an oscillator because of the rc filter back to back but there is no feedback loop 🙂 at least not an intentional one...
Moved to I&A per forum policy. Well thank you all you beautifull basterds! Soldering a 1 meg resistor from R6 to ground and R2 to ground did solve the problem! I never would have guess that!
It's too bad we all live in places that are spread apart cause I would invite you all for a drink on me!
It's too bad we all live in places that are spread apart cause I would invite you all for a drink on me!
I got caught out with a TDA7294 amp with valve front end.
To make up for valve gain I had reduced the gain of the TDA7294. I didn't know that with low gain the TDA oscillate.
So my amp oscillated nicely but not only the TDA but the valve too !
With the valve being high input impedance it was picking up the radiation from the output and feeding it back into TDA.
So I not only had oscillating amplifier but a radio transmitter too.
Of course the valve internals sticking up are a nice aerial too.
To make up for valve gain I had reduced the gain of the TDA7294. I didn't know that with low gain the TDA oscillate.
So my amp oscillated nicely but not only the TDA but the valve too !
With the valve being high input impedance it was picking up the radiation from the output and feeding it back into TDA.
So I not only had oscillating amplifier but a radio transmitter too.
Of course the valve internals sticking up are a nice aerial too.
- Status
- Not open for further replies.