SLO-Nakid_6L6_Layout help/question about how to tame one power tube (V5)

[Canezila]

Hi everyone!

I am going to list items involved and then my question.  I will post pictures. 

Parts:
PT: Marshall Style 50W Power Transformer - APD-9023H Laydown version of APD-8023H w/100V tap
https://shop.amppartsdirect.com/collections/apd-custom-by-heyboer/products/marshall-style-50w-output-transformer-apd-8023h-by-heyboer-transformers

OT: Marshall JTM45 Style 45W Output 4/8/16 Transformer - APD-8039H
https://shop.amppartsdirect.com/products/copy-of-marshall-style-100w-output-4-8-16-transformer-apd-8026h-by-heyboer-transformers-1

Choke: https://www.hammfg.com/part/193G

@Robrob layout:  https://robrobinette.com/images/Guitar/Soldano/SLO-Nakid/SLO-Nakid_6L6_Layout.png

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I run this through a variac and then my lightbulb limiter.  Once turned on no light illuminates.  When the tubes warm up and start providing plate current it starts to illuminate.  I get sound.  The tone circuit works well.  There is no noise, oscillating, or bad things like that (thankfully). 
I am testing my bias with Eurotubes bias probes.  Plate voltage and plate current are provided. 
I am running lower mains through my variac.  Around 100v
V6 runs ~70   but V5 runs ~150

1.  Do I need to look at my resistors within the bias control?  I made a new ground lug and grinded around the ground to make sure it was a very solid connection to ground.

2.  Where would I look to that would help lower V5?  V5 grid leak?  220k  This is basically where I am at right now.  I need to get them closer to a happy number that wont destroy my tubes in short session.
I look here:
 https://robrobinette.com/Tube_Bias_Calculator.htm
but I am not positive I am putting in correct numbers so I am not positive that the results are correct because of user error.  Besides, I am using a variac and that changes things.

** I used a 270k instead of 300k for V3B grid leak. It's all I had.
________________________________________________________________________________________________________________

[mresistor]

Are you on the LB limiter this whole time?


The power tube grid leak resistors are not responsible for current through the tube.  They are for noise suppression. The bias circuit places a voltage potential on the grid of the power tube to set quiescent current level or the base current level of the power tubes. Changing the grid leak value isn't going to have any effect on tube current.


We don't adjust bias while on the LB limiter.  Its just used to make sure there are no catastrophic problems in the amplifier and that it is safe to run normally.
Adjust the bias to max negative voltage first with no power tubes installed to ensure you don't redplate a tube once you fire up the amp off of the limiter or normally.


Sounds like you may have a very mismatched set of power tubes. What are the votages reading on the plates of each tube?

[Canezila]

I briefly tried bypassing the lightbulb limiter and nothing smoked or burned up but the V5 red-plated so I pulled the plug.

[mresistor]

OK  so pull the power tubes    then turn the amp on normally again set the bias voltage to max negative  and measure Plate V on the power tube sockets

[Canezila]

V5, pin 5= 5.2vdc
V6, pin 5= -31.8 through -3.81 controlled by 25k lin bias and 50k lin micro bias to ground adjustment.  V5, pin5 doesn't react to bias at all and its positive vdc. 

[mresistor]

Well there's your problem(s)     the grids of the power tubes have to have negative voltage to keep the power tubes in quiescent conduction state, then when signal is applied to the grid the tube won't redplate.


Troubleshoot your bias supply.   Is the capacitor installed backwards?

[mresistor]

I see the bias cap orientation looks ok   


If you could take a picture directly over the top of the Power Supply area with the bias supply we might be able to see what the problem is.


Also, why did you leave the OT and choke mounted with what appears to be screws missing?

[Canezila]

About the missing mounting hardware, I ran out of nuts.  I need to replenish that soon.

I am attaching a picture of the bias control and V5.  Hope it helps.  Thanks for looking.

[mresistor]

right off the bat it looks like V5 pin five the resistor isn't soldered to the socket pin.


did you take the power tubes out?  looks like one is still in V5 socket.

[Canezila]

It does look like it is not soldered to pin 5.  The socket has three holes on each solder pin.  I used the bottom pin and it is, in fact, connected there.

The Eurotubes probe is connected but the power tubes are not connected to the eurotubes probe.

I will unsolder pin 5 and use one of the other, higher holes on the pin tab. 

[Deric]

The middle lug of the bias balance pot should not be grounded. 


Remove that gound and make sure the balance pot is centered.  Then - with the tubes removed - recheck voltage at pin 5 of both output tubes.

[Canezila]

I fixed/adjusted the pin5 lug.

Then Deric was right on.  I was so excited to cut the ground connected to the center lug. 

Unfortunately,  it still is not right.  I forgot to plug it into the lightbulb limiter and the V6 plate current was over 100 and V5 plate current was over 250.  I am shocked it wasn't red-plating.  Despite the crazy numbers, it still sounded ok.  I quickly disconnected as soon as I realized.

** I didn't have a 50v line from my PT.  Instead, I used a 220k from one of the HT lines before they are connected to the rectifier diodes.  Should I look at trying different values on that resistor? 

[mresistor]

i'll say this one more time     take the power tubes OUT!!   get them away from the amp     don't touch them again until you get your bias straightened out.


Put the darn limiter away  all the voltages in your amp are not going to read right on a limiter.   It's done it's job now put it away. 


If it were me I'd ditch the variac as well.  What is your wall voltage ?
 



What kind of meter are you using to measure the voltage? What is the brand and model?does it have fresh batteries in it?   Something isn't right.

[Canezila]

I understand you.... Now.  Tubes out. Light bulb limiter out.

I think I had a bad solder on pin 6.  I cleaned pin 6 and measured pin 5. Finally! Negative numbers! The bias control works for both v5 and v6.

I believe next step is to slowly bring it up in the limiter and then, if OK, go into the mains. Set the bias control to 85%.  Is this the next steps?

[mresistor]

If your tubes have redplated they might not be any good anymore. If the amp passes the limiter test without the power tubes in and you have around -35 to -45 vdc on pin 5 of each tube then the amp can be taken off the limiter. I would adjust the negative bias voltage to maximum negative.  I think it would be wise to measure the voltages of each pin of the 6L6 sockets while the power tubes are out and post them here for us to see prior to putting the tubes back in and firing it up


Those values of negative voltage you show on your meter are low.  Both should go higher, say up to -50vdc.  This is why we want to get this ironed out prior to putting the tubes back in and firing it up.
You may have to change that 3w 220 ohm resistor, the bias range resistor, in order to get more negative voltage on the grids. We need Both 6l6 pin 5s to have maximum negative voltage to keep the tubes safely cut off when we fire up the amp with power tubes installed.


If all the voltages look ok then you can put the power tubes back in   no limiter and be ready to shut it down if the tube current runs away..  remember  maxiumum negative on the bias. 


Lets hope the tubes are not blown. Would be best to have another new 6L6 duet handy if they are.


Hope you understand.

[mresistor]

Robinettes layout calls for -35vdc bias voltage and you only have -6 on one and -28 on the other..  adjust them both as far negative as they will go. Report back.

[Canezila]

I have - 35 on v5 pin 5. I am going to get numbers on the rest of the pins first.  I will be back in a few with numbers.

[mresistor]

What is the negative voltage on pin 5 of V6?


Please tell me you are not on the limiter..    put it away..   

[Canezila]

V5
pin1 - 35.6 vdc
pin2   77.54 vdc
pin3.   488.56
pin4.    487.56
pin5 - 35.6 vdc
pin6.   485.74
pin7.    78.63
pin8.     1.76 vac  0.0005 vdc.

V6
pin1.  - 32.74 vdc
pin2.   78.35 vdc
pin3.    491.67 vdc
pin4.     493.15 vdc
pin5.      - 4.11 then adjusted to - 17.42
pin6.      486.72 vdc
pin7.     78.36 vdc
pin8.     0.00001

[Canezila]

Haha! No limiter.  Not yet.

** I drained the caps each time before touching anything inside the amp.

[Esquirefreak]

Do you have the correct value grid stopper on v6? Looks like the voltage is dropping across the resistor.

/Max

[Canezila]

I pulled one leg and checked. It should be 220k.  Came back as 217k

[Esquirefreak]

Grid stopper, not grid leak

[Canezila]

Oh yeah. My mistake.
I was talking about the grid stopper pin1 to pin5 but referenced the grid leak. Senior moment.

It was to be 2.2k but measured 2.17k.

I measured the screen resistor and it wasn't close so I changed that while I was checking things.  Pin4 to pin6.

I will remeasure each pin ASAP but it will probably be in the am.

[mresistor]

might have to increase the bias range resistor.

[AlNewman]

Looks to me like having your balance pot grounded biased either one or both of your tubes so hot it started taking out components, IE your screen grid and likely (judging by voltages), your grid stopper.

I would definitely systematically go through every component in your bias and output section before even turning the amp back on.

[Canezila]

V5
pin1 - 35.6 vdc
pin2   77.54 vdc
pin3.   488.56
pin4.    487.56
pin5 - 35.6 vdc
pin6.   485.74
pin7.    78.63
pin8.     1.76 vac  0.0005 vdc.

V6. reading 7/19                                       V6 reading 7/20
pin1.  - 32.74 vdc                                        - 35.4 vdc
pin2.   78.35 vdc                                          79.2 vdc
pin3.    491.67 vdc.                                      489.71 vdc
pin4.     493.15 vdc.                                     489 vdc
pin5.      - 4.11 then adjusted to - 17.42.     - 0.8 vdc
pin6.      486.72 vdc.                                    488.7 vdc
pin7.     78.36 vdc.                                        79.3 vdc
pin8.     0.00001.                                           0.0003 vdc

I measure the one leg of HT that I am feeding the bias circuit.  It measures 360 vac as expected.  After the 220k bias resistor it reads 151.6 vdc.  Earlier it read 199 vdc at this point.  After the bias Rectifier diode it's - 11.774 vdc
** is the HT bias resistor (220k) trying to get to 50v?  **
25k linear bias pot
Left lug - 37.419 vdc
Middle lug before resistor - 32.877 vdc after 6.8k resistor, - 29.344 vdc
Right lug - 36.894 vdc

[mresistor]

Measure the resistance of the 2.2k grid stopper on V6. What is it? 


If you need -35vdc for bias  then it would be good to have more negative voltage to adjust bias with,  so I'd probably start with a 270k bias range resistor.


Id probably want a range or -15 to -45/50vdc. Not all tubes are going to bias up at the exact same voltage. 


As AlNewman mentioned it would be prudent to measure all the components in the bias circuit and your screen grid resistors.

[mresistor]

I got a PM from Canezila   and he said he has fixed the amp and got it working.  Didn't say what fixed it but said he'd post back to tell us here.

[AlNewman]

I got a PM from Canezila   and he said he has fixed the amp and got it working.  Didn't say what fixed it but said he'd post back to tell us here.

I bet you had it pegged with the grid stopper.  That was a good catch.

[Canezila]

Very good news!  I am thankful for those who helped me narrow things down.

1.  Mresistor was spot on. I needed to lower my negative dc so that I could control things as expected in the bias control.  He mentioned changing the 220k value and that is what needed to happen.  At first, I increased the resistance to 300k, then 360k, and I wasn't getting the response I needed. I was feeling like Rodney Dangerfield. Turns out I was going the wrong way. I put in lower numbers and ended up with 100k that is perfect for my situation. Now I can easily navigate under and over - 35vdc.  Huge hurdle for me.

2.  My bleeder /divider was too high.  The solution was to fix my mistakenly placed cap. Should have been 10uF.  I but a 1uF by mistake. Can't believe I missed something that obvious.... Oh, actually, I can believe that!

3.  My PT was 25v too high.  B+ was 475 and I used a zener 1N5360BRLG right after the bridge Rectifier and before the first cap.  Now I was at 456.

4.  This didn't need to happen, but in the meantime while I was troubleshooting, I put in a rotery switch for the OT.  I ran the negative feedback off the 4ohm like in the Soldano SLO 50 schematic.

5.  Last thing was to install the Metropoulos active fx loop.  It works.  Weird thing is that I have one pedal that it doesn't work with and I don't know why.  My JHS spring tank just didn't work. I was going through all these steps to figure out what was going on but, after a while, I used my TC time machine and it worked!  Tried a bunch of other pedals and they all worked..... Just not the spring tank.  I bought it used.  Guessing it takes traditional 9v adapter. It does work if I plug it into the front of the amp.

One more thing to fix : I am pretty sure I am fighting a bad connection with the high input.  IMO, the shielded cable is tough. The wire is so small and fragile. I would really prefer to not use it but I know it helps to keep the signal clean so I will figure out if that needs work.  I can crank the master and the gain.... You would not know it's on until you play it's that quiet.  Normally that's great. This is the SLO 50. I need more! I will look at robrob's suggestions for increasing gain if my connections are solid (TBD)

[Esquirefreak]

Just a shot in the dark, but maybe something is up with the impedance of the JHS tank?

/Max

[mresistor]

Sorry Canezila  I always get the bias range adjustment backwards.   Here's info https://el34world.com/charts/Biascircuits.htm


Glad you got it worked out.