Bogen J-330 Tubes hot, bias and voltage questions

[Glassweights]

Hi Gang!

Ok, so I was reading another thread about biasing and started thinking about my glowing 6L6GC tubes in my Bogen J330.

I have one mondo tough old tube that just laughs and a newer style tube with a red streak on the plate. (well, I guess it is a 'plate', it is a sheet of metal in the tube for sure!) Swapped socket position, issue stayed with the tube.

I stuck in some new tubes and they both had the streak, So I think my one tube is just Old stock and can hang.

So, I am of the opinion that either my plate voltages are too high or my bias is off, or both being even more likely.

And I am apparently clueless on how to bias this configuration.

I tried measuring the 47 ohm resister's parameters and inserting pots to adjust and I got the voltage down from 31.95/32.65 to 27.60 on both legs. I know, you are thinking what is the current? well the stupid pots were both at 0 when that happened and I decided that I in fact had no clue what I was doing

Before doing the adjusting I measured the pins on one of the sockets, I found:

Pin 3 449
Pin 4 369
Pin 8 32.65
Pin 5 very close to zero

The junction between the two 47 ohm resisters was 28.56

So I calculated out my voltage drops 32.65-28.56=4.04 and 31.95-28.56=3.46

And, hell, those are all rises? Should that confront me?

So I got 4.04/51.2=78.9 ma and 3.46/62.8=55.1 ma

So I figured they were mismatched and started the potentiometer fun listed above.

What is the correct way to bias this? or is it self biasing and I should go read the tube book until the stupid is slapped off me?

I tried to do searches and find where this was probably already asked/answered and I found some really good threads on setting up test points and even a nice adjustment pot next to it. I would like to do that.

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Ok, so here is an amusing story about my brilliant bench techniques. The guy who introduced me to working on tubes showed me about draining the caps on Fender amps, and leave a jumper in to counteract the charge building back up.

So when I start gathering all the info on this amp I am being all careful and forget the jumper in there. I fire it up and the nice RCA 5u4GB I bought starts flashing over. I had seen it do that a tiny bit on this tube, so I was like, it will stabilize.
Well it doesn't and I finally get smart and turn it off and find my all black jumper. (note to self, get bright colored jumper wires) Tried again, 5u4 still flashing brightly.

Stuck in a good 5u4gb with broken positioning pin, it flashes too!

So I pulled the tube, fired it up and checked the transformer outs, 20 volts?? I am like, dang, I fried it! I am bummed. So I check the fuse. Someone has stuck a 30 amp fuse in there!! WTH!! So I start looking on epay for a cheap bogen with the same kinda transformer, when I realize, I was measuring the Output transformer not the power transformer! ZOMGosh.

Long story short I had jumpered across the second power cap and only blew up the first 8uf cap. I had a spare and what is the deal with tubes with broken locators? Are they like special that they never die? hah, it still works.

So I am having a lot of fun working on tube amps aren't I? It goes about the same working on cars, stuff breaks but I don't let it confront me none, I can buy more parts!

Anyway, any hints on what I am doing wrong? btw, I do not have any matched pairs that are not in amps that I am really happy with. This amp is transitional, when I get it running properly I plan to take two of the mic inputs and make them into a guitar front end, leaving one low impedence mic input and the RCA input. . . dang but it is mono, maybe I need to convert one mic to a second RCA and just use a single stage for the guitar. Just for fun, it could do it all! maybe not great but certainly loud enough.

Dan

 

[WopOnTour]

Nice Bogen
Would be killer to find another for stereo pair (I'm a sucker for 6L6GC amps)
To me all your voltages appear proportionatley higher likely due to higher applied line voltage. (a common problem) An easy fix is to use a variac as a regulated power supply and adjust until your plate voltage at pin 3 is 420V and the rest will likely fall in line. Are all the tubes GC? or are some of the tubes just 6L6, "G", or "GB"??

If it were me I would get rid of all of those old caps and replace both of the output tubes with matched pair of SED 6L6GC ($49.50 from Jim Mcshane)
WOT

 

[Glassweights]

Bogen J330 voltages and Bias

I recapped it and replaced a few resistors.

I am really a sucker for this old clunky look. And the amps with cages on the top also. The mic inputs being male instead of female is kinda interesting. I will have to rig a gender bender or swap ends on one of my cords.

A lot of the resistors are pushing +10% and a few are higher. I need to order some up for replacing them all.

I could maybe lower the value of the first capacitor to try to bring the voltages down?

I did just get a lil variac that I could try out and see if that helps. I have to make an enclosure of some sort for it . . .it is very basic!

Dan

 

[cademan]

Dan, good job on those replacement capacitors. That usually remedies the red plating problem, although, every now and a great while, I'll come across a 6L6 tube that redplates no matter what I do in replacement parts or adjustments. It is just a bad tube with a short in the grid windings.

 

[GordonW]

I'm thinking that the best solution, is probably just to reduce the screen voltage a bit.

What you could do- is to add, maybe, 330 ohms or 470 ohms (possibly higher), in series with the input of the 3K side of the big center-tapped resistor (R33 on the schematic, if I'm reading it right). You want to reduce that 369v on pin 4 of the 6L6, down to maybe 340V or so (a bit lower than stock, to compensate for the slightly higher-than-stock plate voltage). This looks like it'll require adding a terminal strip somewhere nearby, transferring all those leads from the far-end connection of the double-tapped resistor to the terminal strip connector, and connecting the new power resistor from that terminal strip contact, to the now-empty contact on the double-tapped resistor...

This should result in minimal extra heat or wasted power... and should get that 6L6 back down to more normal current demands.

If that's not quite enough, I'd then try replacing that 200 ohm cathode resistor with a 220 ohm. You will naturally, then need to scale your bogey bias voltage (at pin 8) up about 10% from stock numbers... where they say 25v, you'll wind up with maybe 28v or so, once the resistor is changed...

Regards,
Gordon.

 

[Jim McShane]

Dan, good job on those replacement capacitors. That usually remedies the red plating problem, although, every now and a great while, I'll come across a 6L6 tube that redplates no matter what I do in replacement parts or adjustments. It is just a bad tube with a short in the grid windings.

If it was shorted to the control grid it would likely run away. More probable is a misaligned screen causing local overheating of the plate.