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This will be the third time I have built this amp section so I know it works fine, and is normally very quiet.
I normally don't use a choke but did this time.
But if I ground R1 the amp hums/Buzz very loud along with some whooshy noise.
I tried grounding C2 directly like I have seen done and the amp hums exponentially louder until shut off.
The amp works without this grounded at all as long as the negative feedback is connected.
Test I have done:
All preamp tubes removed, No change.
Preamp section disconnected from amp section no change.
Phase inverter tube removed, hum & buzz gone. Just a normal low hiss
Changed Inverter tube to one I know works, no change.
Grounds test (several points) show less than 1 ohm all the way to the power outlet.
I don't have a scope, but my meter shows less than 2 volts of AC at the power caps. B+ 473
Plugged another amp with the same amp circuit to the same power outlet, with no issue.
Other symptoms:
When turned off the speaker takes a bit to stop making sound (can't remember if that's normal)
The power caps instantly drain almost completely at shutoff. Normally it takes a couple of minutes with a drain resistor attached.
This will be the third time I have built this amp section so I know it works fine, and is normally very quiet.
I normally don't use a choke but did this time.
But if I ground R1 the amp hums/Buzz very loud along with some whooshy noise.
I tried grounding C2 directly like I have seen done and the amp hums exponentially louder until shut off.
The amp works without this grounded at all as long as the negative feedback is connected.
Test I have done:
All preamp tubes removed, No change.
Preamp section disconnected from amp section no change.
Phase inverter tube removed, hum & buzz gone. Just a normal low hiss
Changed Inverter tube to one I know works, no change.
Grounds test (several points) show less than 1 ohm all the way to the power outlet.
I don't have a scope, but my meter shows less than 2 volts of AC at the power caps. B+ 473
Plugged another amp with the same amp circuit to the same power outlet, with no issue.
Other symptoms:
When turned off the speaker takes a bit to stop making sound (can't remember if that's normal)
The power caps instantly drain almost completely at shutoff. Normally it takes a couple of minutes with a drain resistor attached.
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Are heaters grounded or elevated some few DC volts above ground?
Regarding delay, it will depend on lytics. If they are large, as heaters are still boiling electrons due thermal inertia, the amp will continue sounding until one or other cuts anode current flow.
Regarding delay, it will depend on lytics. If they are large, as heaters are still boiling electrons due thermal inertia, the amp will continue sounding until one or other cuts anode current flow.
Try unground the heaters, and refer them to, say, 20VDC above cathode voltage. Making or using an already existent bleeder. Also you can put a 200~500 ohm preset between heaters and the wiper to such a tap in the bleeder.
If it is a recent build, you can also try other phase inverter tube: 6BQ7, 6BZ7 or similar as they are designed with better heater/cathode isolation in Cascode circuitry. In such a case, ground pin#9, the internal shielding.
If it is a recent build, you can also try other phase inverter tube: 6BQ7, 6BZ7 or similar as they are designed with better heater/cathode isolation in Cascode circuitry. In such a case, ground pin#9, the internal shielding.
I am not understanding how the heaters are causing the issue.
I have another amp with the exact same circuit and tubes with no problem
I have another amp with the exact same circuit and tubes with no problem
I did change the tube as stated in my post.
So to understand you, you are saying that leaking from the heater would cause the 150ohm resistor (R1 in schematic) to make the amp buzz extremely loud when grounded.
So to understand you, you are saying that leaking from the heater would cause the 150ohm resistor (R1 in schematic) to make the amp buzz extremely loud when grounded.
My prediction: You have made an incorrect connection somewhere, or used the wrong component value.
I checked my connections three times as well as the values.
Might have to pull the components and check for bad ones.
Might have to pull the components and check for bad ones.
When you ground R1 you reduce negative feedback. So gain goes up and rejection goes down. Double whammy.
Heater circuits seems worth a check? Pretty much any incorrect value around 12AT7 - that circuit relies on balance for rejection. Soldering iron too near the circuit - pickup from tfr in solder station or simlilar on the bench? Long test leads, grounding of signal generator or source?
HTH! - LD
Heater circuits seems worth a check? Pretty much any incorrect value around 12AT7 - that circuit relies on balance for rejection. Soldering iron too near the circuit - pickup from tfr in solder station or simlilar on the bench? Long test leads, grounding of signal generator or source?
HTH! - LD
Just seen R12, speaker load 8R - it shunts R1, except at rf/hf. So maybe its surprising R1 influences hum so much...... is R12 grounded appropriately ?
Edit: Ooops that's not right my bad. Still a check on R12 ground scheme might be good?
Edit: Ooops that's not right my bad. Still a check on R12 ground scheme might be good?
I have every thing around the amp turned off other that over head lighting.
The preamp section isn't even connected so signal isn't an issue.
When I say hum/buzz I mean it's freaking loud.
What is the name of that resistor i.e. function?
Since we are talking about heaters, If I am running two heater lines 6.3 for Power tubes and 12.6 for 12a_7
Should I run them apart (twisted) or twist them all into one?
Same PT secondary split into two taps
The preamp section isn't even connected so signal isn't an issue.
When I say hum/buzz I mean it's freaking loud.
What is the name of that resistor i.e. function?
Since we are talking about heaters, If I am running two heater lines 6.3 for Power tubes and 12.6 for 12a_7
Should I run them apart (twisted) or twist them all into one?
Same PT secondary split into two taps
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The heaters must be ground referenced or it will hum like crazy (grounded centre tap, artifical centre tap, whatever you prefer)
The HEATER winding needs a center tap, grounded. Or somewhere between ground and whatever the cathode sits at, if the cathode voltage is high. Whatever you do, it can’t just sit there and float unconnected to anything. Sometimes they wont hum when you do that. But MOST of the time they will.
You increased the NFB figure and thus decrease the gain of overall amp. This don't mean the issue is solved, it is shadowed.
I guess the amplifier is oscillating for some reason.
Btw, what do these diodes in parallel with the output's cathode resistors do?
Best regards!
Btw, what do these diodes in parallel with the output's cathode resistors do?
Best regards!