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Romy the Cat

Boston, MA
Posts 9,579
Joined on 05-28-2004

Post #: 161
Post ID: 25278
Reply to: 25274
It is all good.
 anthony wrote:
First was the issue with the 6e6p  for the HF Channel not being able to draw enough current.  I checked the bias and it was fine and the potentiometer allowed plenty of adjustment so I changed nothing in the circuit.  A new tube was swapped in and bingo! the current is back and all is good. 

Hm, I did not understand that you did not start with it. It is always one tube has less current them others. In my estimation 1 our 10 tubes of this class are not useable. The irony that some (!) of them still sound fine despite lower current. The 6e5p has higher ration of not usable tubes, I would say 1 our 7 tubes. They are not defective tubes, it is just the bias setting not wide enough to cover them. You might put a bias attenuator in play, I did experiment with it and in my final design I did not do it intestinally. I discover that in most of the cases the low current tubes with lower current do sound worse. I think it has to do with cathode potency and then we drop bias too much to help the tubes. To evaluate each tube and to see if it sounds good at lower current is too much pain and it much easier to get a new tube. I paid for my 6e6p under $3 per tube, so why bother…
 anthony wrote:
All that I can think is that all the time that I ran that tube without pin 8 grounded has worn it out and it may be near failure.  When I first left the amp on for several hours a couple of weeks ago now the current from that rube was fine and it slowly dropped over time.  However, the three 6c33c channels were varying as well so I did not think too much of it.  The new tube pulls the required current with the pot somewhere near the middle of its range and I will leave the amp going all day today with the hope that the current will not fall as it did with the previous tube.

Be prepared to trash all tubes that you use during the design stage. I did some very barbarian this to them during my expedients and during my act of building stupidity. So, as you have all your operation points locked  and the amp function properly then juts put the set of the “design phase” tubes in a separate box and use all new tubes.

 anthony wrote:
My second problem was not being able to set the DC Offset to zero.  It sat at about a minimum of 230mV plus or minus 15mV regardless of the gas tubes I randomly swapped in there.  I tried half a dozen all up and decided that the issue was elsewhere.  What I did in the end was replace a 1k resistor that leads into the 100k potentiometer for trimming DC on the positive side with a 100k resistor, so 100 times larger.  The offset now trims to zero with the pot close to halfway through its travel.  Romy, I don't know if that is an error on the schematic or if I have some other issue, but the offset seems stable. 

 Nope, there is no error on the schematic, you are just the recipient of a very good design. :-) I’m not kidding, let me to explain. You can put in there 10M resistor and get rid of 420K resistor and will be able to adjust anything. The resolution of the agistment will be very low however. The 420K resistor acts as buffer that slightly decuples the rheostat from the signal. The 100K variable resistor has this value because a good feeling between the resistor turns of the voltage if balances. And the 1k resistor is a wild card joker that set all thing in place. The pression of the positive supplely is very had to estimate accurately. It balances 6 channels and the voltage of your positive supply transformer is not identical to mine. It should not be. The 1K resistor before the potentiometer is not necessary from voltage perspective but is necessary as independent adjustment tool. Your experience is a perfect illustration why the 1K resistor is there. As you can see I use the same 1K wild-card resistors earthenware.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
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