Balance control measurement

[GuyNoir]

Hi there:

I have a weak right channel on my Sherwood - according to the balance control scale, it would be down about 8db, I tested all resistors from the output tubes, to all the way up to, but not including the front panel controls and numerous resistors. They were all in tolerance. I tested the balance control, not really knowing what I'm doing. From the outer left leg to the middle leg, pot turned full counterclockwise, I measured 122 meg-ohms. From the center leg to the right leg, full clockwise, I measured 108 meg-ohms.


Could this account for my channel imbalance?

Thank you!

C

 

[Dave451]

Have you tried switching the tubes from side to side to see if the weakness follows L vs. R channel tubes?

 

[GuyNoir]

Yep - I tried that early on. I swapped output tubes, phase splitters and front-end tubes.
If it were only a slight imbalance, I could live with it. However, the left channel has a crazy amount of gain, while the right...not so much.

 

[primosounds]

Hi there:

I have a weak right channel on my Sherwood - according to the balance control scale, it would be down about 8db, I tested all resistors from the output tubes, to all the way up to, but not including the front panel controls and numerous resistors. They were all in tolerance. I tested the balance control, not really knowing what I'm doing. From the outer left leg to the middle leg, pot turned full counterclockwise, I measured 122 meg-ohms. From the center leg to the right leg, full clockwise, I measured 108 meg-ohms.


Could this account for my channel imbalance?

Thank you!

C

I don't think that the difference between the 2 sides of the pot is enough to account for a big difference in the balance, although i don't think you got the resistance value right. On tube equipment the volume pot is usually 100k to 500k ohm and maybe a balance pot can be 1 megohm So, it seems like you are off by some decimal points.

 

[primosounds]

Yep - I tried that early on. I swapped output tubes, phase splitters and front-end tubes.
If it were only a slight imbalance, I could live with it. However, the left channel has a crazy amount of gain, while the right...not so much.

Can you explain how your amp or preamp got to be this way? did it happen suddenly or gradually? Did you just have it rebuilt? Anything unusual happen before your issue developed.

 

[GuyNoir]

Can you explain how your amp or preamp got to be this way? did it happen suddenly or gradually? Did you just have it rebuilt? Anything unusual happen before your issue developed.

I bought it a few years ago as "restored" or "rebuilt" from BT. It had the imbalance then, but it's gotten worse; probably doubly so. Since I bought it, I've replaced all of the can caps with Hayseed Hamfest equivalents, because it had some humming. I rebuilt the bias supply. I replaced the output sockets because they were intermittent. I replaced the flimsy phenolic speaker terminal cardboard-like strips with sturdy screw terminal strips. I changed the coupling caps to V-Q and later Westcap. Other than that, it's as i received it.

 

[primosounds]

I bought it a few years ago as "restored" or "rebuilt" from BT. It had the imbalance then, but it's gotten worse; probably doubly so. Since I bought it, I've replaced all of the can caps with Hayseed Hamfest equivalents, because it had some humming. I rebuilt the bias supply. I replaced the output sockets because they were intermittent. I changed the coupling caps to V-Q and later Westcap. Other than that, it's as i received it.

If the issue seems to be getting worse then i would suspect the pot. Unfortunately, finding a replacement pot is very difficult. I hope you have been looking. NOS pot may be impossible to find. In the meantime you can try cleaning with deoxit. Also try cleaning your connections, rca jacks, speaker posts, and tube sockets.

 

[gadget73]

almost sounds like an open feedback connection on the loud channel. Double check the resistor that runs between the speaker termials and the phase inverter. If those are open or way off value, the gain in the power amp is affected.

Quick and dirty check, take a jumper wire and clip the two center terminals of the volume control together. If the imbalance remains, the problem is in the power amp section. If it goes away, its somewhere between input and volume control.

 

[GuyNoir]

I just started looking. Pfft...good luck. I thought bypassing the balance control would be a simple matter, but from what I can gather, it's not.

 

[GuyNoir]

almost sounds like an open feedback connection on the loud channel. Double check the resistor that runs between the speaker termials and the phase inverter. If those are open or way off value, the gain in the power amp is affected.

Quick and dirty check, take a jumper wire and clip the two center terminals of the volume control together. If the imbalance remains, the problem is in the power amp section. If it goes away, its somewhere between input and volume control.

Thanks Gadget! I'll check both areas.

 

[GuyNoir]

almost sounds like an open feedback connection on the loud channel. Double check the resistor that runs between the speaker termials and the phase inverter. If those are open or way off value, the gain in the power amp is affected

I'm such a friggin' DOLT. There are NO resistors, thanks to my ignorance. When I updated the speaker terminal strips to withstand heavier speaker cables, I omitted the .47 ohm resistors, thinking it was part of the switchable damping circuit. I was defeating the damper anyway, so I didn't see the need to transfer the resistors.

So after you had me check the resistor between speaker terminals and phase inverter, the only wire that went to the 7199s was a white wire from each common terminal, soldered directly to the "center pin" of each 7199. The schematic shows the resistor up near the 7199, but old photos show it to be soldered near or around the speaker common terminals. First pic is the default arrangement, second is my butchery, third is schematic.

One thing the amp does that I didn't mention: From a cold start, the output tubes are reasonably balanced and quiet. After a number of hours, the left channel starts to hum moderately, and I find that the tubes are no longer able to be balanced....with the balance switch on, there's just a loud buzz no matter where the wipers are. You can hear the null points, but there is still a buzz superimposed where there should be silence. Does this fit the omission of the resistors?

 

[tonyg88]

I know you checked output tubes but preamp tubes are just as important

 

[GuyNoir]

I know you checked output tubes but preamp tubes are just as important

Yep, I went through a rolling phase...have had at least four trios of 12AX7s in the preamp stage, along with a myriad of others. And always swapped them around. t's always the same.

 

[GuyNoir]

I found a good picture of the resistors. It seems if you have the terminal jumper across the three common terminals, the resistors are out of the circuit anyway. So Mr. Gadget - which resistors from speaker terminals to phase splitters should I be looking at? Thanks.

 

[gadget73]

The feedback resistors are a 15K off the 16 ohm terminal. The thing is, those 0.47 ohm also need to be there otherwise it doesn't properly complete the path to ground on the 7199 tubes. If you just replace them with a jumper wire, that will make the circuit work like any typical feedback amplifier without the adjustable damping whatnot. Basically you'd be taking R152 (and the equivalent on the other channel) direct to ground. That gives a voltage divider for the feedback, and a proper path to ground so the 7199 is biased correctly. With the 7199 being biased wrong, it will also upset the phase inverter balance since its direct coupled, and that will reduce output power and increase distortion considerably.


and the warm hum could very well be the 7199 tubes. Swap them and see if the hum moves. H-K leakage is a problem with those, and it usually shows up as a 60 hz hum.

 

[GuyNoir]

As it turns out, in my effort to update resistors to 1% metalized, I had already replaced those 15K resistors a few days ago. I started from the output tubes north, to just to the east and west of the 7199s, but not the clusters of resistors attached to the 7199s themselves. That's all the progress I've made so far (pics later tonight). The original 15k resistors were in spec, but I'm hoping to eliminate noise by replacing as many as I can get to. In fact all of the carbon resistors that I've tested are or were, more or less at rated tolerance. There were no appreciable changes in the amp's behavior after I went on the resistor changing spree.

The hum on the left channel remains after PS swapping and I have a number to choose from: Pairs of 7199, also adapters plus numerous pairs of 6u8, 6gh8. and one pair I'm particularly fond of, E80CF. Thank you everyone, for your time and efforts.

 

[GuyNoir]

Another thing: that humming in the left channel? It's particularly sensitive to higher voltage. If I plug the unit into the wall socket cold (125v), the hum is significantly louder. And once it starts acting up hours later, it's quite pronounced. I normally run the amp on 115v. thru variac.

 

[GuyNoir]

The 7199 sockets are such a mess - I'm going to order the ceramic caps in C0G and redo them.

 

[gadget73]

They're probably fine honestly.

Did you replace or bypass the 0.47 resistors? If the jumpers are still in place and connected, there is a bypass in place.

 

[GuyNoir]

I'll have to order some up. My 'Joe Knows Electronics' assortment only goes as low a 1 ohm. I think I understand what you're saying, but if I jumper across the common terminals, isn't that the same as having the .47 ohm resistors in place but having the factory jumper across all three com terminals, thus effectively bypasings the resistors? To quote, "All Sherwood amplifier now provide selectable damping factors of 10/1 (normal) or 2/1 (for use with today's popular compact speaker systems)." As I understood it, 'normal' is with the jumper in place.