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OK, where's that hum coming from?

7/4/2005 10:48 AM
Jag	OK, where's that hum coming from? I've got a BF Bassman (tube chart says AA864 but the circuit is actually AA165). I've been trying to track down the last bit of hum for a year now. Here's the situation; - With all preamp tubes, including the driver, taken out it has a low level hum. Not enough to be a major bother but more than there should be. - I've recapped the amplifier and even replaced the totem pole twice (second time I went with 2x220uf caps). - I've replace all coupling caps at least twice. - I've replaced ALL resistors including the power supply. - The filaments on the 6L6's are wired humbucking. - I've replace both the power and standby switches. - All grounds measure fine. I even star grounded it at one point. None of these changes have had any effect on the hum. As I said, the hum is not bothersome just more than it should be (I can hear the amp humming in the next room, although I have to strain to hear it). This amp was a MAJOR mess when I got it. It looked like it had been sitting in a greasy garage for years. I did remove the transformers and degrease the chassis with carb cleaner. The transformers seem fine but I haven't removed the end bells to have a look inside (all voltages seem fine and there are no shorts). Even though the outside of the amp was very dirty, the inside was very clean when I got the amp. There was some rust on the transformers but I have other Bassmen that are worse. I did spray some Tremclad on the laminates to prevent further rust. I did not brush or scrub off the old rust, just a light wipe down to remove dust. One other thing. This past weekend I rewired the Bass channel to match AA864 specs. This has increased the level of hiss, even with the volume on 0. I've rechecked all connections and replaced the plate load resistors for a second time. The hiss is coming from the Bass channel for sure. Removing the first two preamp tubes kills all hiss and the normal channel is fine. Comparing the schematics for the two versions, I'd guess that the AA864 version would have less gain, therefore should be quieter. Any idea where to look for this? BTW, this is by far the best sounding Bassman I have (I've got 3 BF and 2 SF). So I'm really hoping I can track down the last of the hum.

7/4/2005 10:48 AM

Jag

OK, where's that hum coming from?

I've got a BF Bassman (tube chart says AA864 but the circuit is actually AA165). I've been trying to track down the last bit of hum for a year now.

Here's the situation;
- With all preamp tubes, including the driver, taken out it has a low level hum. Not enough to be a major bother but more than there should be.

- I've recapped the amplifier and even replaced the totem pole twice (second time I went with 2x220uf caps).

- I've replace all coupling caps at least twice.

- I've replaced ALL resistors including the power supply.

- The filaments on the 6L6's are wired humbucking.

- I've replace both the power and standby switches.

- All grounds measure fine. I even star grounded it at one point.

None of these changes have had any effect on the hum. As I said, the hum is not bothersome just more than it should be (I can hear the amp humming in the next room, although I have to strain to hear it).

This amp was a MAJOR mess when I got it. It looked like it had been sitting in a greasy garage for years. I did remove the transformers and degrease the chassis with carb cleaner. The transformers seem fine but I haven't removed the end bells to have a look inside (all voltages seem fine and there are no shorts). Even though the outside of the amp was very dirty, the inside was very clean when I got the amp. There was some rust on the transformers but I have other Bassmen that are worse. I did spray some Tremclad on the laminates to prevent further rust. I did not brush or scrub off the old rust, just a light wipe down to remove dust.

One other thing. This past weekend I rewired the Bass channel to match AA864 specs. This has increased the level of hiss, even with the volume on 0. I've rechecked all connections and replaced the plate load resistors for a second time. The hiss is coming from the Bass channel for sure. Removing the first two preamp tubes kills all hiss and the normal channel is fine. Comparing the schematics for the two versions, I'd guess that the AA864 version would have less gain, therefore should be quieter. Any idea where to look for this?

BTW, this is by far the best sounding Bassman I have (I've got 3 BF and 2 SF). So I'm really hoping I can track down the last of the hum.

7/4/2005 9:40 PM

Dai Hirokawa

quote:
"- All grounds measure fine. I even star grounded it at one point."

you grounded every ground connection to one point? That could potentially be a problem.

7/5/2005 12:03 AM
Jag	Sorry, speaking before thinking. The filters for the preamp tubes and phase inverter were grounded to the same location, on the grounding buss. I moved the ground for the screen supply to the same location as the ground for the main power filter caps, over by the transformer. But, as I said, "at one point". All grounds are now exactly where they were originally.

7/5/2005 9:30 AM
R.G.	In no particular order: 1. Check the ripple on the bias supply. 2. Loosen, clean, then retighten every jack, pot, and ground connection screw to the chassis. 3. If it uses a CT connected to ground on the heater supply, replace that with two 47R 1% 5W wirewound resistors in series across the heater supply, then ground the center of the two resistors. 4. Set the thing up so you can insert a 1/8" thick plate of steel (like from Home Depot) between the tubes and transformer, and between transformers. Experiment with placement. 5. Check on the twisting of the AC power wires. They must be twisted tightly together, including the heater wires.

7/5/2005 9:30 AM

R.G.

In no particular order:

1. Check the ripple on the bias supply.

2. Loosen, clean, then retighten every jack, pot, and ground connection screw to the chassis.

3. If it uses a CT connected to ground on the heater supply, replace that with two 47R 1% 5W wirewound resistors in series across the heater supply, then ground the center of the two resistors.

4. Set the thing up so you can insert a 1/8" thick plate of steel (like from Home Depot) between the tubes and transformer, and between transformers. Experiment with placement.

5. Check on the twisting of the AC power wires. They must be twisted tightly together, including the heater wires.

7/5/2005 3:52 PM
loudthud	Sounds like something is out of balance in the output stage. Unfortunately, that's probably what makes it sound good. Do you have the bias pot setup like the AA864 or the balance pot of the AA165 ? I assume you've tried a different set of tubes. Any difference there ? How do you set the bias ? Check the 6L6 sockets for good tention. Check the voltage across the 470 ohm screen resistors and the voltage on the grids. If you can't find something that's out of balance, the output transformer is suspect.

7/5/2005 3:52 PM

loudthud

Sounds like something is out of balance in the output stage. Unfortunately, that's probably what makes it sound good.

Do you have the bias pot setup like the AA864 or the balance pot of the AA165 ? I assume you've tried a different set of tubes. Any difference there ? How do you set the bias ?

Check the 6L6 sockets for good tention. Check the voltage across the 470 ohm screen resistors and the voltage on the grids. If you can't find something that's out of balance, the output transformer is suspect.

7/5/2005 9:19 PM

Eric H

quote:
"2. Loosen, clean, then retighten every jack, pot, and ground connection screw to the chassis."

Clean the shorting-contacts on the jacks, too.

-Eric

7/5/2005 11:02 PM
Jag	Thanks for the replies, it's much appreciated. "1. Check the ripple on the bias supply." I don't have a scope but, reading with my DMM, the DC voltage seems really solid. Fluctuates between -58.6 and -58.7 volts. "2. Loosen, clean, then retighten every jack..." Yup, did that a while ago. "3. If it uses a CT ..." I was thinking about that. I'll give it a try tomorrow. I might even positive bias the filament supply. Right now it reads 3.64v on one side and 3.12v on the other. "4. Set the thing up so you can insert..." I'll try that too! "5. Check on the twisting..." Did that already. "Do you have the bias pot..." Actually, the bias pot on the AA165 is already correct. The AB165 is when Fender changed to the balance circuit. Yes, I've tried a new set of tubes, same result. I bias for 70% dissipation using the method described at http://marstran.com/50W%20Bias.htm. "Check the 6L6 sockets..." Tension's good. Here are some measurements; - Bias Supply: Fluctuates between -58.7v and -58.6v - Filament Supply: 3.64v one side 3.12v other side - Voltage across screen resistors: 1.13v and 1.26v - Grid voltage: 39.5v on both - Plate voltage: 437v on both - Screen voltage: 436v and 435v - OT Primary resistance: 49.65 Blue to CT, 46.58 Brown to CT. - Plate current: 47.5ma I'll break out the soldering iron tomorrow and rewire the filament supply and replace the screen resistors. Thanks, Jag

7/5/2005 11:02 PM

Jag

Thanks for the replies, it's much appreciated.

"1. Check the ripple on the bias supply."
I don't have a scope but, reading with my DMM, the DC voltage seems really solid. Fluctuates between -58.6 and -58.7 volts.

"2. Loosen, clean, then retighten every jack..."
Yup, did that a while ago.

"3. If it uses a CT ..."
I was thinking about that. I'll give it a try tomorrow. I might even positive bias the filament supply. Right now it reads 3.64v on one side and 3.12v on the other.

"4. Set the thing up so you can insert..."
I'll try that too!

"5. Check on the twisting..."
Did that already.

"Do you have the bias pot..."
Actually, the bias pot on the AA165 is already correct. The AB165 is when Fender changed to the balance circuit. Yes, I've tried a new set of tubes, same result. I bias for 70% dissipation using the method described at http://marstran.com/50W%20Bias.htm.

"Check the 6L6 sockets..."
Tension's good.

Here are some measurements;

- Bias Supply: Fluctuates between -58.7v and -58.6v
- Filament Supply: 3.64v one side 3.12v other side
- Voltage across screen resistors: 1.13v and 1.26v
- Grid voltage: 39.5v on both
- Plate voltage: 437v on both
- Screen voltage: 436v and 435v
- OT Primary resistance: 49.65 Blue to CT, 46.58 Brown to CT.
- Plate current: 47.5ma

I'll break out the soldering iron tomorrow and rewire the filament supply and replace the screen resistors.

Thanks,
Jag

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