Cathode resistor value for SE 6L6/EL34/6550

[tubeswell]

Hi Steve

What are the DC voltages throughout the amp (including at the B+, and at the different B+ nodes in the R-C supply 'dropping string', and at the plates, and screen, including on either side of R22, and cathodes)?

If we could see all the information together if might make things immediately clear. Also if you can measure the exact DC resistances of the cathode resistor and of R22, that would help (just to make sure you haven't accidentally got the wring value part in somewhere).

[Steve A.]

Hi Steve

What are the DC voltages throughout the amp (including at the B+, and at the different B+ nodes in the R-C supply 'dropping string', and at the plates, and screen, including on either side of R22, and cathodes)?

If we could see all the information together if might make things immediately clear. Also if you can measure the exact DC resistances of the cathode resistor and of R22, that would help (just to make sure you haven't accidentally got the wring value part in somewhere).

I have measured the cathode and screen resistors several times and they are what they are supposed to be. I traced out the audio signal path before doing the PicoRocket mod, assuming that the power supply was OK but I need to trace out of the power supply too. This amp had sat on the floor at GC for many months so there is no telling how it might have been abused (I think that a LOT of amp heads on the floor at GC get plugged in without the proper speaker load- or any speaker load at all). You don't have that problem in the small Mom'n'Pop music stores because they watch you like a hawk and make sure that you don't screw up their equipment. Damn- I didn't bother to ask if they had a PicoValve still in the box because I thought it was a special clearance deal just for that head...

Steve

P.S. I will measure the power supply voltages. Should I do it with and without an output tube since that is the big load in an amp which could point to the problem?

[tubeswell]

P.S. I will measure the power supply voltages. Should I do it with and without an output tube since that is the big load in an amp which could point to the problem?

Should be able to tell with the output tube in. Also, the way the screen resistor switches between the plate supply node and the 'screen' supply node, you should get a difference in screen grid current (and screen voltage) depending on which way the switch is toggled.

[Steve A.]

Should be able to tell with the output tube in. Also, the way the screen resistor switches between the plate supply node and the 'screen' supply node, you should get a difference in screen grid current (and screen voltage) depending on which way the switch is toggled.

Sorry for the ambiguity- what I meant was measure the voltages both with the tube in and with the tube out, to get an idea of what kind of voltage drop we are getting with it in. I bypassed the pentode/triode switch (5W/2W) since the triode mode sounded like crap. Hmmm... I've never heard a pentode/triode switch sound so crappy so I have a hunch that there is a problem in that area.

I need to investigate further and will post my measurements. I have my "brand new" 1994 Mesa Boogie Maverick on my work bench/dining room table, tracing out the circuit because I want to "blackface" the Clean channel. Damn- they sure squeeze a lot of components onto their PCB! When my eyes get tired I will dig into the PicoRocket.

Update: I got the Rhythm channel on my Maverick sketched out and I figured out the safest and sanest way to blackface it. Completely reversible so I can undo any or all of it. Yeah- it surprised the hell out of me, too!

Steve

[tubeswell]

So in the chart you uploaded:
I've never heard of the 6P1 or 6P3S tubes- is there an alternate number for them?

Data for 6P3S and 6P3Se is here:

6L6 = 6L6GT = 6P3S tube. Output tetrode - Vacuum radio tubes, capacitors, socket for tubes, nixie, ferrites from military Russia. Datasheets tubes and help. Worldwide shipping

More blurb here:

6P3 (6

After trying 6P3S above 400V they didn't last very long, but run good up to about 380V.

However the 6P3Se can comfortably handle 450V and totally rocks

[kleuck]

6P3S are close to 6L6GT and 6P3S-E to 5881.

[jmaf]

6P3S are close to 6L6GT

Also very close to 6L6GB.

I put a pair of 63PS in place of 5881 after I bought some via eBay, last year I think. It almost fried in seconds. Then I learned about the 6P3S-E, the hard way...

[Steve A.]

Completed Maverick -> Blackface Super mod... hot damn!

I just got done rewiring the Rhythm channel of my MB Maverick to Blackface specs and it sounds incredible! The stock Mav Rhythm channel was almost identical to the Subway Blues, which puts a 1M5 resistor in parallel with an 180pF cap between the input jack and the grid of the 1st stage (the grid load was 1M but bypassed with a 250pF cap. How can the pure signal from your guitar get through that crap? It's like using a condom made of 1/16" neoprene!) Enough on that!

I will be getting back to my PicoRocket on Sunday (I had to brag to SOMEONE about my latest mod!) My grasp of electronics leaves much to be desired but I am great at stealing other people's designs! <--- That is Leo turning over in his grave

Steve Ahola

[Bruce / Mission Amps]

If you have DC voltage on the signal/control grid (g1) then the coupling cap feeding the grid is probably leaky, and this would be contributing grid current (via the coupling cap and the previous stage's plate supply) which could well manifest itself as higher cathode current on the output tube, thus affecting the bias (since the current being 'stolen' from the previous stage's plate would be flowing through the coupling cap and through the output stage's cathode). Lift the the 'grid-end' of the coupling cap from the board and re-check the cap-to-ground DC voltage. It shouldn't have any DCV on it.

Just adding to this ... another way you can say the coupling cap is leaking is to take the tube out and measure for any real DC voltage found on the tube socket's "signal grid lug". Make sure it has a high DC resistance connected to ground, like the commonly found grid load resistor.
A bad tube(s) is commonly one that leaks DC voltage to it's signal grid, causing all kinds of mind numbing problems that go away when the tube is out of the socket.

[kleuck]

Yes, the more i think about it, the more i think there is some DC on the grid Steve.

[jmaf]

A bad tube(s) is commonly one that leaks DC voltage to it's signal grid, causing all kinds of mind numbing problems that go away when the tube is out of the socket.

The tube leaks a positive voltage to the grid internally? It's gotta be physically shorted, otherwise all the space charges in there are negative. Correct?

[Gingertube]

IF the battery was bad in your multimeter then that measured 54V across the 470 Ohms cathode resistor will need to be remeasured.

Big warning for casual amp fixer up'er'ers - I have been caught this way a couple of times, adjusting bias etc - only to find the multimeters flat battery meant that it was telling me fibs. Generally meters read HIGH when the battery is low (because the meters internal reference voltage against which the measurement is made, drops).

Approx 2V DC on the grid1 (control grid) could be due to grid current through too high a grid leak resistor (the resistor from grid1 to 0V which includes any grid stop resistor) or could be leaky coupling cap as stated above. Pull the tube and see if it goes away - if so it is not the coupling cap.

Cheers,
Ian

[Steve A.]

I changed the battery on my Fluke 16 since it was getting erratic voltage readings that I did not post here (I was using my Bel Merit meter until I got my Fluke working again). I like to have at least 2 meters handy to confirm readings. Most of my cheaper meters do not measure B+ accurately- I always figured it was because they expect a more filtered DC voltage (like a computer power supply).

I got DC on the grid with 2 different GT 6L6GC's and 1 NNOS*** 6V6, but none at all with the Mesa EL84 or with no tube at all. I already replaced the new .022uF coupling cap ahead of the grid stopper with a new .047 cap (this is after the tone stack which theoretically should filter out the DC).

I spent several hours today on a rooftop working on a packaged heating/cooling unit at a friend's house- something I have not been able to do for several years (that was the first time I've climbed a ladder to a roof since I retired on disability). I'd been too sick to do anything like that for a long time so I'm on the road to better health. My doctor says that I am as healthy as any man twice my age!

I'll be rechecking the PicoValve tomorrow evening and will post what I find. I've dug up some of my old test equipment so I'll be able to measure all sorts of things (like AC current going into the amp and DC current practically anywhere).

Thanks for all of your suggestions!

Steve Ahola

*** NNOS = Not New Old Stock
I figure I needed a category for the used but tested old stock tubes I've bought on the internet.

[kleuck]

I'd been too sick to do anything like that for a long time so I'm on the road to better health. My doctor says that I am as healthy as any man twice my age!

That's a great new !!!!

[jmaf]

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please help me to choose.

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