Variac a Super Lead instead of attenuating

[dirtycooter]

Shoulda tried the low sensitivity input while I was in there-just thought of it

[jnew]

Glad you're not taking offense. I don't know the circuit of those things but if it's laid out like a 12 series, then no reason you couldn't get it from being in the zip code to perhaps the same block with some component and/value swaps. Or perhaps if it all original and you don't want to make changes.

[dirtycooter]

I am not against making changes.. in fact I may have you eyeball some things and see if there may be something you see thats definite quick easy no brainer upgradeable as you and rob got your shit dialed in to a T, it was just there was the all the mods and then ralle then mod 5 then now everyone is back to square one stock non cascaded spec again. The ppimv does mess with the negative feedback and presence and thats part of the proper equation in the tone chase. While there isn't spot on vh'ness to my clip there absolutely are elements I hear more now not in MV methods and cascades.
I wanna see how long tubes last and how this holds up with the variac business. Its workin great so far for volume control while not breakin the chain of events inside the amp. Thats really whats cool to me about the variac. All that is unimpeded as a whole picture.
Then if she holds up I think some values and parts could be changed around, different tranny, nothing drastic and keep it stockish or maybe even mod it to true DF spec. Whatever moves it in the right direction.
I don't wanna use attenuators so much either as thats a tonal detour sometimes. I wanna be able to get just the head alone to run like Ed did it. IF thats what he did and didn't reamp. So far theres no evidence of any loadbox yet we know of back in the day pre 5150. I think it really was just his variac. Maybe a rodeo keepin tubes in it all those years for him. Who knows. But I guess I will find out and if it breaks it breaks. Total road test report to test the myths or facts-whichever they may be. Cum bleed or blister its a road unexplored really by many so what the hell

[jnew]

I don't know if this is amp is your only platform to work with but some years back and through all 2203/4 front ends, PPIMV's, attenuation, Mod5's, etc, it hit me one day to just use what Ed used and do what he did. My first and only re-build (at the time) went to the simple 4 input, 2 stage plexi circuit (which by now everyone should have no doubt that this is what Ed's amp was) It all started to workout from there. The variac was next. Big realization there. Then came the speaker and cab phases where I finally made my choices. We talked about that. Then came the Echo Plex pre-amp circuit which about knocked it out of the park for me. So there's the 7 year pilgrimage I made only to have a simple 4 hole, NMV, 2 stage plexi circuit with a killer 4X12 and a home bre Echo-Plex pre amp in front. Cheap, common carbon film resistors, and easy to get, cheap ass Mallory caps. Once a stock stock JTM-45 issue to Hardly a step above a radio shack plexi. Everything on 10 except Vol I. Only on 6, and yet it's all that's needed. I have an old vintage MXR 6 band but only use that for the few certain VH I tracks that have all that. I'm the one, On fire, etc. only like 4 or 5 of the songs. So if you decide to streamline that Mojave amp, my recommendation would be the known and documented simple 2 stager. It just works. That cheap ass amp is the amp for the Jamie's Cryin, Dance the Night Away, and Somebody Get Me a Doctor clips. Pretty darn simple.

[dirtycooter]

We all look in here for secrets but more and more it seems there really weren't any
I totally agree-use what Ed used and figure from there. Several times I been ready to mod this thing into some cascaded MV thing but now..... like you said about the variac, it kinda unwinds now some for me too right there.

Now its like I wanna DE-MOD what I have as its a little tricked right from Mojave. Mostly with the extra paralleled preamp tube and inputs. But I like its basic core tone and amount of gain it has right now even though I even think its too much-never thought I could get that much out of it until now. Its opened my eyes some to maybe how the real deal was probably.

What will be coolest for me is if used sensibly and not too low [70v is plenty low volume for me and could actually raise it some] is that tube life isn't drastically decreased and as some say they have experienced good longevity here. That's what I want to know mostly.
Because that will mean my volume control issues are done if thats the case.
To me this is a huge piece of the puzzle-the volume management and not toasting fragile greenbacks.
I am certain Ed was doin what I am when he first got his variac. "Gee that's cool I can even get my volume down when I go lower." I doubt he was afraid to go that low as no one or nothing was said about variacing amps until after he did it. He wasn't probably worried too awful much about the bias either at first. I imagine that came later.
But the pilot light does go completely out at a certain point for about everyone here-I believe he was lower than 90 more than we think. Check his out here as its one of the few if not only one showing him play with 12301 in action. Is it a prop? Or is it really the amp running live on vid?? I see no echoplex or anything on the floor as they don't show that far down. But here he has one head sitting on what looks to be his cab stripped tolex and all. I find this very intriguing as the light is out

[youtube] https://www.youtube.com/watch?v=HYLmngZjEqE [/youtube]

Next up is messing with a set low voltage and twiddle the bias some there for me. Maybe try that low sensitivity input and pull 2 tubes and see what comes out. I am gonna continue to get more gain than normal though until the stock Mojave extra tube/input stage fandango probably gets taken out of the equation.

[dirtycooter]

Went to check bias this morning. Set it up and one of the inside pair is out and dunno if its its my probe interferring or if the tube is actually dead yet. Was in a hurry so I shut it down. My other probe was reading 11mA at about 71/72 volts which is really really cold. When I get back I wanna see if that tube lights back up without the probe in there-I am sure it was workin fri night when I made the clip or at least I thought it was.
Its possible it may have said see ya but the other three look fine. I hope I don't have a bum probe either

[dirtycooter]

For the record my tube did not die just bad connection with probe in between for future references to this thread. All is good so far.

How are you guys measuring your plate current/voltage to calculate from.... what method of access?

[Krinkle]

My 12000 chassis has been out of the head case for weeks. Measuring with a DMM and I have 1 Ohm resistors on all 4 power tube cathodes.

[Krinkle]

Round 2

With the variac set at 63.5VAC I just measured my voltage and bias current. Voltage was at 250VDC, bias current was 7.5mA!

Calculated bias current for 250VDC = 70mA, I'm not even close. BTW, max cathode current for a 6CA7 in Class AB1 is 150mA.

I originally had a 47k bias resistor in there, stock. When I tried to bias to 50mA at 90VAC I couldn't get high enough so I put another 47k in parallel.

With that resistor configuration I tried getting more than 7.5mA, I got to 12.1mA, nowhere near 70mA. I'm not sure what the current capability of the PT is so I have to look that up before I start reducing the bias resistor more.

Okay, so I just noticed that the power dissipation for a 6CA7 is 25W. I've been calculating using the PD for an EL34, 17.5W, including the calculation above.

Given that, the new bias current at 250VDC should be 100mA!

What I'm doing here is surely poor man's Power Scaling, having bought all of Kevin's books and having many long discussions with him.

[vh junkie]

No... 6ca7s and EL34s are the same...
http://robrobinette.com/Tube_Bias_Calculator.htm
How the hell are you getting any sound at all out of an amp running at B+ of 250vdc? Not being a dick here...but maybe change the batteries in your meter?

[jnew]

too funny. I just posted about this in the other thread.
I agree about 6CA7's being the same as EL-34's.

[Krinkle]

No... 6ca7s and EL34s are the same...
http://robrobinette.com/Tube_Bias_Calculator.htm
How the hell are you getting any sound at all out of an amp running at B+ of 250vdc? Not being a dick here...but maybe change the batteries in your meter?

My batteries are fine. Why wouldn't I get any sound at 250VDC?

[jnew]

My amp is loud as shit even at 250V's on the plates.

[dirtycooter]

This must be counter productive to a degree. Bias UP to compensate for low voltage and it is back too loud again?
Thinkin about what this must mean.
Did Ed bias at 90v right up to close max then drop even further just to maintain decent bias further down?
My clean isn't quite so good and could be a bit better so maybe not much more bias would cure that up some while not goin to extremes.
Certainly an aspect to ponder here some on.

[Krinkle]

My amp is loud as shit even at 250V's on the plates.

Yeah mine is certainly loud enough. This is not rocket science, I'm sure I'm missing something very simple here. I've got some ideas, too much to write down right now. I have to get the scope out this weekend and check some things out.