Sherwood S5500 II w/7868s- take two

[gadget73]

Changing the grid resistors on the tube won't really change the amount of grid voltage the tube gets. There is virtually zero current draw, so almost no voltage drops across that resistor. It will also affect the filter formed by the coupling capacitor and the grid resistor. Its basically a high pass filter, making it different per-side might cause some odd performance.

If you put a pot at the output of the bias supply, with the wiper going to the bias level pot and the two outer legs of the extra pot going to the two grid resistors that will give you a balance. Won't be fail safe, so if the pot fails the tubes probably will too, but its a quick and easy way of adding that function with a minimum of parts.

Maybe play musical tubes to see if you can get 2 that behave reasonably together?

 

[oldman55]

I matched up some tubes and they are now all are about 45ma cathode current at abou -17vdc design bias.
As this amp doesnt seem to be as loud as my other 7868 amps, I was going to increase the current a little bit but when looking at the RCA chart for the tube, I confused with what the max current should be set at idle.

Does this mean that they should be set at a max of 32 ma without a signal? So basically 12.8 watts?
Or am I misreading this table?

 

[gadget73]

30-35 ma each seems pretty typical. Also about 13w of plate dissipation seems fairly typical as well based on what Fisher amps seem to usually run at. I think I have mine set at 35ma each.

 

[oldman55]

When matching up tubes, I had a couple of Sherwood branded tubes that were 68 ma. I only knew because I installed the resistors. Had I just put them in at the specified -17 v, They would have been doubling that.

So do you call it good or adjust the bias to get them down to a lower output?

 

[gadget73]

I'd probably set it around 32-35 and call it good.

 

[oldman55]

Put this on the bench as sound wasnt right.
Powered up, heard the sound of two resistors blowing.
Without power tubes, measured bias in line at -17v but voltages were all 60-100v high along the B+.
Put in power tubes, powered up to 90v on variac, and heard a loud pop accompanied by a flash. Testing of the tubes showed one pegged on meter for grid leakage (ouch, just installed NOS tube)
Two of the 10ohm/1W cathode resistors had popped. One on each channel. Bias remained per spec; -17v.
Dont know what caused the high voltages but guessing that is what overloaded the resistors and tube?

 

[gadget73]

shorted tube probably. The high current will take out the cathode resistors.

 

[oldman55]

So the higher voltage could have caused the screen to flame out?
Sencore shows grid leakage but no shorts on that tube. Other tube on other blown resistor tests good. It had a voltage of 6.0 across the resistor before I powered down.

Would a 1/4w screen resistor have popped first and saved the tube? And what could have caused the HV to jump up (and stay up)?

(Edited for further questions.)

 

[gadget73]

what was the current set to? The bias voltage doesn't matter much, but if the current was 68ma like you mentioned earlier it probably just failed from extreme overheating.

B+ will be high with no tubes installed, you'd have to measure things with all the tubes in and biased at a normal level to know what its doing.

 

[oldman55]

Current was set to 35ma w -17v bias..current must be higher than that w 6.0v across 10 ohm w maybe 430v plate/screen.

Bias remained steady at -17 regardless of b+

 

[gadget73]

6 volts across the 10 ohm is far more current than the tube is able to support. If its doing that, you either have no bias or a major internal failure on that tube.

 

[oldman55]

I should have made it clearer, it was across the fried resistor which is now x.xMohm. But the two resistors blew prior to the tube flashing out. Trying to see if high voltage is/was the cause of the the circuit overload or if it is another issue.
Hesitant to put good tubes back in for testing.

 

[gadget73]

cathode voltage goes high when the resistor opens, but the current drops to nearly nothing. The B+ also goes high with no load.

 

[oldman55]

So chalk it up as bad tube? But what about the resistor that blew on the other channel and the tube testing good?

 

[gadget73]

honestly I don't know. Sometimes tubes test good but do not work right in the circuit. Testers don't provide the same operating voltages as the amp. I'd probably replace the resistors with 1/2 watt parts and try it again to see what you get. With some luck the resistor acted as a fuse and saved the tube, but I wouldn't count on it for sure. The resistor might fry again, if it does I'd say the tube is probably gone. Once those things arc internally they seem to be prone to doing it again.

This does have a resistor on each tube between the screen supply and the screen terminal doesn't it?

 

[oldman55]

I didnt add screen resistors if that is what you are referencing. Its a tight space in the pocket under the tube shield (photo with shield removed).

But after replacing the cathode resistors and swapping tubes, they are all running and set to about 13.8 watts at idle. One of the Sherwood tubes I removed was running at over 40w with the same -16v bias. It may have caused the other resistor to blow.

 

[gadget73]

Cool. Trying to remember how these are set up, but I want to say there is a resistor between the screen supply and the capacitor anyway so individual resistors probably aren't needed. So many schematics for these things and so many variations though. Mine is set up that way, but mine uses different tubes so who knows.

Big imbalances between tubes can make dumb things happen. I'm fooling with something now and trying to come up with a reasonably matched set to sort out stupid problems. I have 8 of them and have managed to make up 2 pairs, but ideally it should be a matched quad.

 

[oldman55]

Was taking from shelf to listening room.

The bias adj pots give a range of -15v to -19v where design is -17v.
However, when I put in my better 7868s, they draw 60-90 ma even when the pots are down all the way to -19v.
I have had to put in questionable tubes to get them in the 35 ma range (after 5 or more minutes of warming up). I would think that these will not produce the highest quality sound.

Earlier, I went through 4 tubes with good emissions but though difficult to test, they had grid leakage. Dont know if the tubes came to me that way or if something on the unit caused this to happen.

Thoughts would be appreciated.

Oldman55

 

[oldman55]

edit: double posted