Tube Amps / Music Electronics
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|4/25/2002 3:31 PM|
||pin 1 - 6V6GT - OK for tie point|
Is it OK to use pin 1 on a 6V6GT for a tie point ? I was thinking I could put a screen resistor from 1 over to 4 and just tie the screen wire to 1.
Right or wrong ?
|4/26/2002 5:45 AM|
|Bruce /Mission Amps
The Fender way is, screen resistor from 6 to 4 and 1 to 5 for the grid stopper resistor.
|4/26/2002 7:29 PM|
Bruce: Ok.. looking at the Tweedy Deluxe layout - you're putting the grid stopper between 5 and 6 and feeding 6. Ok.. so it might be better 'plumbing wise' to use pin one to feed the grid stopper going to pin 5. That would leave more unobstructed space for the 'big' screen resistor between 4 and 6. Ya. You don't specify screen resistors on your Mission Tweedy Deluxe. Why is that ? Is it because the 40v voltage drop across the 27k resistor on the rail provides enough of a cushion that a screen resistor is unnecessary ? I see Hoffman does put a 450 ohm screen resistor in his version. I got them in my choke input power supply ver. of the 5E3..which shows a high bias until a note is struck, at which point the screen resistor drops the screen voltage and reduces the bias...
|4/27/2002 4:02 PM|
|Bruce /Mission Amps
There is a 5K1 resistor from the plate filter cap to the screen filter cap.
All current flowing in the two 6V6's screens, the 12AX7 and the 12AY7 must flow through that single resistor. Assume it's about 6ma to 8ma at idle.
That would loose about 30vdc to 40vdc.
That's reason there is a much lower voltage on the screens of a tweed amp with no choke, then in a Fender amp using a choke.
Because of the lower screen voltage and if everything else is equal, the tube is not running quite as hard and will bias up at a lower current then if the screen voltage were raised higher.
The screen voltage changes with signal drive as a function of losses through the power supply and current flow through the singular 5K1 resistor or choke.
Simple Ohm's Law stuff where I x R = E and that is voltage drop through the resistance. That's how I derived the 30vdc to 40vdc drop above.
The preamp tubes are running class A and the average current through them will remain constant while the current through the 6V6 plate and screen will vary all over the place.
So, yes, the screen voltage will drop and rise. As it lowers, the tube will draw less DC current.
Keep in mind, a choke has a finite DC resistance too, as a resistor does, but it much much lower then the 5K1 resistor so the drop is not as severe. Possibly a 250 ohm choke with 6ma to 8ma flowing or 1.5vdc to 2vdc drop, but we aren't so interested in the DC currents as we are the AC current!
In order to make the tube work for us we want to talk about the AC currents through the tube and subsequently, across the primary of the OT.
The variable AC fields in the OT is what we are trying to couple to the speaker.
The same amp but built with a choke will sound different because of all that.
However, if a 6V6 amp builder were to use a choke between the first two caps and live with the screens being just a few volts less then the plates, then I would solder a 1K 2W screen current limiting resistor on each socket, not 470 ohms. That should take care of protecting the screens from excessive current with high voltage on them.
As far as a more stock power supply set up for the 5E3, there is nothing wrong with using the 470 screen resistors on the tube sockets, but IMHO they are unneeded because of the current limiting effect of the 5K1 resistor in the main supply.
Again, you should use some screen resistors if you use the choke set up.
The dropping/isolation resistor after the 5K1 is 22K, not 27K.
Only the current flowing in the 12AX7 and 12AY7 has to go through that one, so the effect of the resistor is less, hence, the resistance value has to be made larger to get the voltage drop you want.
IN your case becasue you did not drop the screen voltage, I'd expect to see something larger then 22K.... like maybe 30K to 33K.
You'd have to experiment a bit or maybe you have and that's why you mentioned the 27K.
|4/27/2002 5:14 PM|
Bruce: MANY thanks for taking the time to lay that out. It seems that with the screens up very close to plate voltage you're sort of running as a triode. A NOS 6V6GT wouldn't like that ! I'm using the EH 6V6 and it has handled the very excessive screen voltage so far... but how long can that continue ?
One of my engineer buddies suggested putting the 5K1 in the screen line itself. As I understand things (not much !)... the 5K1 would NOT reduce voltage to the 22K and the preamp tubes would continue to run too hot.
In a previous thread on this same subject it was suggested I put the 5K1 in series with the output of the choke.... sort of a 'best of all possible worlds approach'. That's probably what I SHOULD do.
I'll be conducting a 'shoot out' between the choke and the resistor Deluxes soon and I'm hoping my customer likes the resistor better.
Thanks for your recommendation to go to 1K screen resistors if I stay with the choke and contine to leave the 5K1 out. Also thanks for the reminder to adjust the 22K upwards to deliver the correct voltage to the preamp tubes...
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