I know my 12Ax7's are good. I will try a 7025, and failing that, replace the tube socket.
I know it can't be a said with 100% certainty, but failing the above test, at this point does it appear that the tube socket is the last remaining culprit?
I really appreciate the help, thank you everyone
I know it can't be a said with 100% certainty, but failing the above test, at this point does it appear that the tube socket is the last remaining culprit?
I really appreciate the help, thank you everyone
New socket, swapped in multiple 12AX7/WA's and a 7025, still hearing this hiss. I'm going to record and post an audio clip so you can have a listen for yourselves...
Also, someone previously changed the bias balance circuit to the earlier adjustable bias. The 220K bias resistors and 1.5K grid stoppers are close to spec value, I'm thinking the tubes are likely not closely matched since they read about 10mA apart (30mA/40mA @ 460V), just thought I'd mention it.
Also, someone previously changed the bias balance circuit to the earlier adjustable bias. The 220K bias resistors and 1.5K grid stoppers are close to spec value, I'm thinking the tubes are likely not closely matched since they read about 10mA apart (30mA/40mA @ 460V), just thought I'd mention it.
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https://www.sendspace.com/file/26p9wj
The recording is in order - Bass channel, followed by Normal channel.
EQ is all set to 10. Deep off, Bright on. I roll the volumes from 0-10-5-0.
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The recording is in order - Bass channel, followed by Normal channel.
EQ is all set to 10. Deep off, Bright on. I roll the volumes from 0-10-5-0.
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Moderator note... its very exceptional for moderation to edit another users post but we have received a report that the link downloaded unwanted adverts and unwanted browser add ons.
So use at own risk
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Looking at the AB165 schematic I see the Normal channel's second gain stage doesn't get the added filtering that the Bass channel's second stage gets. The second gain stage's plate load supply of the Normal channel is taken from the same node as the third gain stage common to both channels. Also since that second gain stage on the normal channel has a higher supply voltage than the Bass channel there will be more gain to amplify the 120Hz ripple.
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All the filter caps look to have been replaced in the 90's. Do filters really play into this if the Bass channel does not have reproduce the hissing? I.e. the filter in question also supplies the common 3rd gain stage - so the Normal channel would likely be affected by a bad filter as well. Also, any point in augmenting the value of the voltage supply to the 2nd gain stage?
Thanks again
Thanks again
The cap might not be bad, I was just saying that because of the higher voltage in the second stage of the normal channel it will have more gain (in comparison to the bass channel) to amplify the supply ripple. And on top of the increased gain that stage has less filtering than the bass channel.
I am merely pointing out design flaws that would cause the normal channel to inherently be noisier than the bass channel. The third stage that is common to both channels has a good amount of local feedback so less gain to amplify supply ripple.
Parallel a cap effectively increasing filtering on that node and see if that helps, or tie that second gain stage of normal channel in with the first gain stages.
I am merely pointing out design flaws that would cause the normal channel to inherently be noisier than the bass channel. The third stage that is common to both channels has a good amount of local feedback so less gain to amplify supply ripple.
Parallel a cap effectively increasing filtering on that node and see if that helps, or tie that second gain stage of normal channel in with the first gain stages.
Hullo! I have shameful secret - I have a scope and signal generator but I definitely can't get the most of them because I just don't know how to interpret the image. I learned how to use the scope's controls, but I don't know where to probe and what to look for. So far I haven't found any good references to learn from.
FMB - Eureka! Moving the 2nd triode of the Normal 12AX7 into the last filtering section (with 22K resistor) really helped get rid off the hiss!!!
Not sure if this is related, but I believe it is a new problem - when the 'bright switch' is activated, I lose all treble frequencies if the treble pot is turned to 10. Could be a pot issue, but this doesn't happen with the bright switch off.
Thank you very much for waiting it out and all the helpful suggestions (everyone, really).
FMB - Eureka! Moving the 2nd triode of the Normal 12AX7 into the last filtering section (with 22K resistor) really helped get rid off the hiss!!!
Not sure if this is related, but I believe it is a new problem - when the 'bright switch' is activated, I lose all treble frequencies if the treble pot is turned to 10. Could be a pot issue, but this doesn't happen with the bright switch off.
Thank you very much for waiting it out and all the helpful suggestions (everyone, really).
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Great ! I guess we'll never know why, but I'd venture that change was enough to upset self-oscillation.........in any event 'scoping now wouldn't show anything I suspect - but when things really don't make sense it's a good bet that oscillation's happening, not just for noise.
And getting your head around the 'scope seems worthwhile - only tip for working on valve gear is to watch out not to exceed max dc I/p rating at the scope input, not that I've ever lost one that way. Hey you could use it to investigate the new odd presence problem - if it don't make sense think 'oscillation' - all the best !
luckythedog,
Did you listen to the audio file? To me it just sounds like 120Hz hum. I would revert back to the normal configuration and replace the 20uf decoupling cap after the 4.7k dropping resistor and see if it resolves the hum.
I do agree that using the scope is a invaluable tool. I always check every amp for stability.
Lets look at the problem again. The normal channel hums/hisses with increased volume, so this points to the input stage. In my experience when there is an oscillation in the gain stage and I turn the volume up it takes a little time for the oscillation to build and finally end up in some sort of audible positive feedback or squealing. Some oscillations are inaudible and can chew up power and cause distortion and damage to speakers.
Did you listen to the audio file? To me it just sounds like 120Hz hum. I would revert back to the normal configuration and replace the 20uf decoupling cap after the 4.7k dropping resistor and see if it resolves the hum.
I do agree that using the scope is a invaluable tool. I always check every amp for stability.
Lets look at the problem again. The normal channel hums/hisses with increased volume, so this points to the input stage. In my experience when there is an oscillation in the gain stage and I turn the volume up it takes a little time for the oscillation to build and finally end up in some sort of audible positive feedback or squealing. Some oscillations are inaudible and can chew up power and cause distortion and damage to speakers.
After a google search I turned up two forums talking about the same problem with the normal channel.
Bassman AA165 musings..... - AMPAGE Archive
'67 AB165 Bassman - Normal channel hum/buzz - The Gear Page
Others have suggested what I have, which is poor design and 120Hz hum in the normal channel.
Looks like replacing the 20uf decoupling cap probably won't fix the problem and keeping that gain stage with the others in the last filter section is still probably your best option as I originally suggested.
Bassman AA165 musings..... - AMPAGE Archive
'67 AB165 Bassman - Normal channel hum/buzz - The Gear Page
Others have suggested what I have, which is poor design and 120Hz hum in the normal channel.
Looks like replacing the 20uf decoupling cap probably won't fix the problem and keeping that gain stage with the others in the last filter section is still probably your best option as I originally suggested.
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