Hot Sherwood S5000

[gadget73]

Running 117v off of variac, the HV+ rail readings are close to spec (curious as to why voltage is the same each side of the 33 ohm resistor coming off the rectifier).

That doesn't really make sense. What do you measure directly across the resistor for voltage drop?

Voltages at the 12ax7s are close to spec (though it gets angry when I test pin 2)

Sensible. Pin 2 is a grid, so high impedance. They tend to hum or possibly do other stupid things when you connects stuff to that point. Shouldn't be much voltage there anyway in most circuits. It usually ties to ground through a resistor of some value. If that resistor is open it can do dumb things though.

But, at pin 3 of the 7189s I only have .23v on one pair and .15v on other vs spec of .5v

Looks like its a 12 ohm 1 watt resistor on each pair of tubes. 0.5 volts across that would be 41.6 ma per pair. If the voltages at the plate and cathode are right, but the voltage at the cathode is low, it means you've got low current to the tubes. Either the bias supply is overly negative or the tubes are weak. I see no means of adjusting the bias supply, so you may want to consider modifying that if the tubes aren't done for. Putting a 5K trim pot in series with the 4.7K resistor in the bias supply would give you an adjuster, but it would change all 4 tubes at once. If 5K ends up being too touchy, a 2.5k may be a better choice.

constant hum which increases with volume
volume is low

Sounds like issues in the preamp area. Try feeding input through the tape monitor connections to see if anything changes. That eliminates most of the switching and 1/2 of a 12ax7 stage as a possible trouble area. Also, if you haven't done so, make sure the selector switch and the tape monitor switch are clean along with the rest of the controls and switches.

The warm can cap may be related to this. Usually that indicates internal leakage, which usually goes along with bad filtering and low voltage. Or maybe its just close to stuff that gets hot.

 

[oldman55]

I'm still scratching my head on the 33 ohm resistor. With a fresh battery, my vom measures the resistor at 32.9ohm. Voltages drop as follow (at 117v power supply). Dont think I have a good chassis ground on the can cap but dont see how that would matter.

Bias supply:


I've tried three set of tubes, two quads that work in other units. Will do some more cleaning and checking and try again. There is an original 50uf ecap at V1 which I'll replace. Couple of the power tubes are still a bit loose in the socket but apparently making connection.
Would taking voltage reading at some of the pre amp tubes shed some light on what is going on?

Thanks for taking the time to respond.
old

 

[gadget73]

If the cap doesn't have a good ground, its not in the circuit.

The bias supply is overly negative. That -20v is important. If its at -24v, the tubes are under-biased. You're drawing very little current through that thing. If the numbers are accurate, the total B+ is about 30ma when it should be 100ma according to the schematic. Going back a post, total current through the output tubes is about 30ma when it should be right around 84ma.

Try adding some resistance between the output of the diode and the 2.2K resistor to ground. Looks like around 10 ohms will get it done. If the -20v source is still too negative, bump the value of the 4.7K resistor or add a trim pot after it so you can adjust it.

 

[oldman55]

Adding 10ohms in series with a 2.2kohm doesnt seem like a very big change.
Also, when I rebuilt the can, I have a chassic ground post and a ground buss post on the terminal strip I added. They are separated from each other and only the caps go to chassis ground, others go to the ground buss. It appeared that was way it was wired when it was a "positive common" can.

 

[gadget73]

The 2.2K is to ground. The tube heater load will pull through the 10 ohm resistor though, if my math is right you should see 1.5 volts worth of drop across it. The resistor needs to be between the point where the two diodes come together and where they connect to the 2.2K to ground. I'd use a 1 watt part for this. If you have 1/2 watt on hand, that will do for testing. It should be dissipating just under 1/4w.

Wasn't sure what can you were talking about. The body doesn't need to be grounded as long as the common terminal is connected properly. Many of them just happen to use the can as the common point.

 

[FlaCharlie]

We discussed the addition of the 10 ohm resistor previously. I posted pics of how I installed it in both the slant (preamp) tube version and the vertical (preamp) tube version (like yours) back on page 4 of the thread.

 

[oldman55]

The 2.2K is to ground.

Maybe I need to look a little closer to how I built the bias cap. This point in the circuit is isolated from ground and goes to the neutral buss.
In fact, possibly all of the cap positives should possibly go there in lieu of chassis ground.

The common positive of the caps doesnt look like it go to chassis ground as I have done. I wired it like the crude sketch on page 4 but with all the positives going to a new chassis ground.

Will take a closer look.

 

[gadget73]

Doesn't really matter if it goes to a buss or to chassis, circuit-wise its the same. Basically my point was to identify where the 10 ohm resistor needs to go. The 2.2K is not relevant, its just a point of reference on the schematic.

 

[oldman55]

Added the 10ohm resistor and my readings are now:

This unit had/has a 47ohm 7 watt in lieu of the design of 33ohm.

 

[gadget73]

What do you read at pin 3 on the output tubes now? Should be higher I expect. B+ also is likely lower.

Heaters are a teensy bit low, but shouldn't be low enough to bother anythting as long as they aren't weak. Pretty sure those are just the phono tubes anyway.

 

[oldman55]

.20v and .42v

 

[gadget73]

Better. Could still do with an adjuster to get that closer to right, and its possible the tubes are a little weak on one side. Me being me, I'd probably modify the bias circuit to be adjustable on each channel but one adjuster and a matched quad would get it done too.

 

[oldman55]

Ill replace the tube with a couple matched pair. Have some Russians in for testing but also tried a good quad of GEs.

On the 7199s, I get the following readings:
Pin1 (260v spec); 226v and 244v
Pin2 (90v spec): 112v and 95v
Pin3 (60v spec); 52.3v and 51.4v
Pin7 ( 1.75v spec ); no readable voltage
Pin 8 ( 95v spec); 119v and 104v

Shouldnt the grid have some voltage on it even at idle?

Hoping by posting these readings that my problems will be evident to someone that knows a lot more than I do.

I was thinking about swapping out some of the preamp tubes while the unit is running to rule out a bad tube. Would this do any harm to other components/tubes if I lower the voltage?

old and scratching my head.

On the bias supply where the voltages are low to V1 and V2, should I change the 47ohm back to a 33 ohm to pick up the voltage?

 

[FlaCharlie]

On the bias supply where the voltages are low to V1 and V2, should I change the 47ohm back to a 33 ohm to pick up the voltage?

That section supplies the DC heater voltage to the phono section tubes, not the bias to the outputs. I wouldn't worry about it being a little low, especially if you are measuring these voltages using a variac at 117v. When you plug the amp into the wall, those voltages will go up some.

 

[oldman55]

As far as bias then, why dont I just up the value of the 4700 ohm resistor to get the voltage down to 20v when running from the wall outlet?

 

[gadget73]

You can do that, but personally I'd stick a trimmer after the 4.7K so you can adjust it to where it needs to be. Adjust it till the voltage at the cathode of the output tubes is where it belongs.

 

[oldman55]

I wasnt able to find something like a shaftless 5w 5 ohm trimmer to fit under the chassis.
What is the most common cause of loud hum? Gotta get that under control.

 

[gadget73]

Bias trimmer can be 1/2 watt or so. You want about 5K, not 5 ohm. Its not carrying much load at all.

have you done anything with the balance pots? If those are way off it will make things hum more than they should.

Does the hum change with volume control or no?

 

[oldman55]

The hum balance works to some degree, but not much. Hum is fairly loud without volume and increases with volume. I have done a complete swap out of the 12ax7s from a working set and the noise is still there.

 

[gadget73]

ok, so the hum problem is in the power amp section.

Does it stop if you pull the 7199 tubes?