Well, the PT specs seem to be Fender's, and Tony's layout is fine. Wonder if you could take and post pictures of your power section? There might be something improperly wired somehwere.
In the perfect world both tubes would be perfectly matched and if the delta relationship between the tubes current draw follows the tube, then it would lead me to suspect the tubes. If not, ohms law doesn't lie and I would suspect the OT. However, I wouldn't rule out oscillations as cause of the red plating issues as well. I do have a few questions though.
1. Do you have a AT7 in the PI position?
2. Does both power tubes red plate or just one?
Like Gil, i've never used a 27K at the bias tail, mainly 15-22K.
What's weird is that, as I understand it, two sets of Wing C's will glow red when the idling current is in the mid 30 mA's. That seems off, as well as the fact that it should take a more negative voltage to bias a pair of 6L6s than what's reported. However, I remember I stopped using Winged Cs many years ago when I bought a quartet that required -60 VDC to bias right...
Gil
ToneMerc wrote:In the perfect world both tubes would be perfectly matched and if the delta relationship between the tubes current draw follows the tube, then it would lead me to suspect the tubes. If not, ohms law doesn't lie and I would suspect the OT. However, I wouldn't rule out oscillations as cause of the red plating issues as well. I do have a few questions though.
1. Do you have a AT7 in the PI position?
2. Does both power tubes red plate or just one?
Like Gil, i've never used a 27K at the bias tail, mainly 15-22K.
I went as low as an 18K for the tail.
Yesterday I went all the back to the 27K and started working my way back down again. I think I'm at 20K now. It got too late to work.
Both tubes redplate. I can set the bias so they don't, but it seems like the bias voltage should be more negative given the current, and the amp is kinda lifeless.
V5 redplates sooner than V4, as the current is higher on that side. I've tried several different sets of 6L6s, including some Mesas, JJs. and another set of Winged Cs.
I'll mess with the "front end" (try the 2K2) and see what that yields. Basically it seems like I need to make the bias more negative for a current in the 32mA-36mA range. I've been trying to do that with the tail resistor, but that seems to be the wrong way to do it.
telentubes wrote:I'll mess with the "front end" (try the 2K2) and see what that yields. Basically it seems like I need to make the bias more negative for a current in the 32mA-36mA range. I've been trying to do that with the tail resistor, but that seems to be the wrong way to do it.
I don't think the adjusting the "front end" is moving towards solving the issue. When things are normal it really doesn't matter where you fine tune the bias range whether it's at the bias circuit front end or at the safety tail. This situation appears outside the norm.
Now all I get for bias is -72, with almost zero current, no matter what tail resistor is used or pot setting. Before that I would get a range of values when adjusting the pot, mixed with intermittent -72s, till finally the pot would have no affect at all, and I get -72V for all settings. Maybe I cooked the pot with too much soldering.
I gotta take a break and come back to this later. I think I'll rebuild the whole system from scratch. I also need to get back to work.
I changed the PI, before this started to happen, and the current on V4 and V5 were better balanced.
I've replaced everything up to the tubes, with the exception of the 100uf filter cap, because I didn't have one. This includes the 1 Ohm cathode resistors.
I replaced the 10K bias pot with a new one.
I put in new tubes, including the PI.
I tried about 16 combinations of "front end " resistors, and tail resistors.
This includes "front end" values of 2K2, 3K3, 4K7, 8K2, and 10K.
And tail values of 15K, 20K, 22K, 33K, and 47K.
I'm shooting for a bias of -46ish, and a current of 32mA-ish. I can get theses readings, but never both at the same time. If I get -46, the current is way low. If I go for a 30mV setting, the bias is too positive.
The bias voltage does not matter! just get the idle current adjusted for reasonable plate dissipation (Pa). The 6L6 is 30W max Pa, so setting 70% of that using cathode current (as sensed by your 1R resistors) will put you at 67-68% of max plate dissipation (some of the sensed current is going to the screen) which should be safe. So, 30 x 0.7 = 21W. Divide that by plate voltage and that is the cathode current to shoot for. You'll have to iterate since changing current changes the plate voltage. For e.g. 438V, 21/438 = 48 mA, so you would be looking for 48 mV at the bias test point. Many people run at lower Pa, 60% say. I am running =C= 6L6's in my 124-ish amp, and I'm setting 40 mA with 420V on the plates-
O.K. cool. Gil said the same thing a while back, but somehow I got myself railed into thinking it mattered because folks were always listing the current AND the bias together. Guess I'll go back in, find something that works, and play the dang thing. You folks are patient with slow learners like me. I appreciate the help.
