I know that the topic on tube amp buzzing/humming has been well covered, but I'm hoping to get some answers specific to the issues that I encounter.
The amp is an old SE 6bq5 power amp from a stereo console. It has a single 12ax7 and a 6ca4 rectifier. I recapped the amp fully, and also moved the RCA inputs to a different location for convenience.
I'm getting significant continuous buzz through the speakers. I thought it may have been a ground loop issue with the RCA inputs, so I disconnected them, and now the buzz is even louder with very high frequencies hisses.
I don't have a tube tester to check the condition of the tubes, but I swapped the 12ax7 and 6ca4 with other tubes and the buzz is still there. I don't think the 6bq5 tubes could be the issue since the buzz is the same in both channels.
Please let me know if you have any experience with this and what the fix was. Thank you.
The amp is an old SE 6bq5 power amp from a stereo console. It has a single 12ax7 and a 6ca4 rectifier. I recapped the amp fully, and also moved the RCA inputs to a different location for convenience.
I'm getting significant continuous buzz through the speakers. I thought it may have been a ground loop issue with the RCA inputs, so I disconnected them, and now the buzz is even louder with very high frequencies hisses.
I don't have a tube tester to check the condition of the tubes, but I swapped the 12ax7 and 6ca4 with other tubes and the buzz is still there. I don't think the 6bq5 tubes could be the issue since the buzz is the same in both channels.
Please let me know if you have any experience with this and what the fix was. Thank you.
Short the inputs or the (triode) grids to ground. Use leads as short as possible and listen if the hum goes away.
I think the real issue is that your PSU really don't have much in the way of filtering.
Throwing in some cheap hamond or triad chokes might make all the difference.
Throwing in some cheap hamond or triad chokes might make all the difference.
Perhaps some more information on how the RCA inputs have been wired into the circuit?
(I presume you've checked the polarity of all the replaced electrolytics!)
(I presume you've checked the polarity of all the replaced electrolytics!)
I'm very new to tubes so I'll need some clarifications please.
Where are the inputs (triode grids) to be shorted to the ground? Where would the chokes go for PSU filtering?
I've moved the RCA inputs to the location of the optional external speakers since there were RCA connectors in place. I moved the wires from the 12ax7 to the positive RCA terminals, and I grounded directly to the chassis, instead of grounding to the C52/R55 as shown in the diagrams. Could this be the main issue? The buzz and hisses are even more pronounced when I don't plug anything in the RCA inputs.
Where are the inputs (triode grids) to be shorted to the ground? Where would the chokes go for PSU filtering?
I've moved the RCA inputs to the location of the optional external speakers since there were RCA connectors in place. I moved the wires from the 12ax7 to the positive RCA terminals, and I grounded directly to the chassis, instead of grounding to the C52/R55 as shown in the diagrams. Could this be the main issue? The buzz and hisses are even more pronounced when I don't plug anything in the RCA inputs.
Try grounding the RCA to the junction of R50/R52. Make sure you are using shielded cable.
The input grid (pin 2) may be shorted to the same junction.
Don't worry about chokes for now.
The input grid (pin 2) may be shorted to the same junction.
Don't worry about chokes for now.
I looked closer at the diagram and the amp now, and there are some differences in the ground locations.
The RCA inputs are grounded directly to the chassis far from the original r55/c52 location.
Also, the electrolytics capacitors (c60, c61, c62, c63) are grounded to the r55/c52 location instead of the original location on the chassis.
The RCA inputs are grounded directly to the chassis far from the original r55/c52 location.
Also, the electrolytics capacitors (c60, c61, c62, c63) are grounded to the r55/c52 location instead of the original location on the chassis.
Yes, rectify the above and let us know how it goes. Multiple grounding points are found in old equipment and lead to ground current problems.
> ...your PSU really don't have much in the way of filtering.
When new, it didn't buzz enough to turn-off buyers.
The filtering looks "commercially sufficient" to me. In DIY land, more is always better, but stock should not stink.
IMHO the problem is that the main filter caps and many joints have 50 years of rot and tarnish. It wants solder joint and filter cap overhaul.
When new, it didn't buzz enough to turn-off buyers.
The filtering looks "commercially sufficient" to me. In DIY land, more is always better, but stock should not stink.
IMHO the problem is that the main filter caps and many joints have 50 years of rot and tarnish. It wants solder joint and filter cap overhaul.
Out of curiosity, if you short the inputs is there a high-frequency component to the noise? Also, if you are able to figure out what frequency it is, that could help pinpoint the source.
Put the grounding back where it was and it is likely that the problem will be solved.
If not, exactly what did you do in 're-capping'? Were all cap changes strictly like-for-like, or did you attempt an 'upgrade'? Cap 'upgrades' can easily ruin a circuit.
The symptoms you have sound like HF parasitic oscillation, probably in the power stage.
If not, exactly what did you do in 're-capping'? Were all cap changes strictly like-for-like, or did you attempt an 'upgrade'? Cap 'upgrades' can easily ruin a circuit.
The symptoms you have sound like HF parasitic oscillation, probably in the power stage.
Last edited:
Thanks for the suggestions!
The capacitors were changed with the same values as the originals.
One of the newer cap only read 88 uF instead of the original 100 uF, but I don't think this should make such a big difference?
I've moved the filter caps ground and the power supply ground to the initial location but the buzz/hiss is just as bad.
Could it be a bad ground of the audio input?
The buzz/hiss seems worse when the preamp is not there (just the speakers connected to the amp, and powered on).
The capacitors were changed with the same values as the originals.
One of the newer cap only read 88 uF instead of the original 100 uF, but I don't think this should make such a big difference?
I've moved the filter caps ground and the power supply ground to the initial location but the buzz/hiss is just as bad.
Could it be a bad ground of the audio input?
The buzz/hiss seems worse when the preamp is not there (just the speakers connected to the amp, and powered on).
- Status
- Not open for further replies.