Tube pairings of the Sunn 2000s

[pbman1953]

Last year I was helped , from the forum, with adding a bias pot in order to have 2 pots controling a pair of tubes. If I understand right the outer tubes are paired and the inner 2 are paired. if that's wrong please correct me.

The reason that I'm writing is that I thought I have the pairs set up right with the respective pot. But turning the pot effects tubes that shouldn't change.
I sent pictures and please let me know if the picture angles are proper enough to understand the wiring



Plexi please feel feel to chime in

Thanks

[pbman1953]

pic 2

[pbman1953]

pic 4

[sluckey]

Sounds like the mod is wired wrong. It should be easy to verify which tubes are paired by comparing the schematic to the wiring. If not obvious then connect one lead of your ohm meter directly to pin 5 on the left most (outside) tube in your pics. Now take the other meter lead and, in turn, touch pin 5 of all three of the other tubes. One of them (hopefully the right most outside tube) will read approx. 2KΩ. These two tubes are a pair. Tubes that are not paired will read approx 200KΩ.

[pbman1953]

The 2 outer tubes from pin to pin 5 measure 1.999k. The 2 inner tubes measure 1.999k. There are no other measurements with any other combination

[sluckey]

I looked thru all your posts but can't find any discussion about adding a bias pot to any amp. Do you have a schematic or layout of what you did to the 2000S?

[pbman1953]

second bias addition

[sluckey]

Your schematic looks good. The circuit will work when it's wired accordingly. I can't tell what's wrong from the pics. I've annotated your schematic just to be sure which tubes are paired. V4 and V6 make one pair. V3 and V? make the other pair.

[pbman1953]

I re-checked all my connections and it seems that I did the right thing. The stock pot controls the inners and additional pot controls the outers.

For the second pot I took power from the original pot and soldered to the right leg. It's cap positive is grounded and negative is soldered to the middle leg. Then I sent a wire , from the second pot output to E1

There was a conecting wire, when it was totally stock, that had to be taken out to separate E1 & E2. The weird thing is the turning one pot seems to effect a non-connecting pair. Very strange

[FYL]

The weird thing is the turning one pot seems to effect a non-connecting pair. Very strange

Not so strange: one leg loads the other and the pots are thus somewhat interactive. Using a level/balance approach eliminates this small problem.

[sluckey]

I did this exact mod to my Sceptre. The pots are not interactive. Keep looking for your error. You may have to cut the lacing cord to separate the wires so you can see better what's going on.

[pbman1953]

I tend to agree , once that link is cut to tie E1& E2 together, there should be 2 independant circuits.

I'll check

Thanks

[sluckey]

I wonder if there is just some confusion about which tubes should be controlled by each pot? This amp is not set up like a Fender Twin Reverb. IOW, if you wanted to just run with two tubes instead of a quad, you would remove the two right hand side tubes in your pics. Then you would basically have one pot controlling one tube and the other pot would control the other tube. Look at the attached pic to see how the pots should operate. Then tell us exactly what happens to the negative voltage on pins 5 of each tube as you vary a pot. One pot should only change the voltage on two tubes. If one pot changes voltage on more than two tubes, or changes voltage on the wrong two tubes, it's not wired correctly.

I can post some pics of my mod tomorrow, but I'm not sure if they will help.

[pbman1953]

I haven't done what you said yet but if you look at the picture you sent me:

The pot with the blue radial electrolitic is the stock pot. It's output wire is going to E2. The additional pot ( grey radial electrolitic) has its yellow wire going from the the pot o E1

[sluckey]

I've looked very closely at your pics and I can make out just about everything. But, if you could clip some of the lacing on the wire bundles between those two parallel terminal strips and the tube sockets/bias pots and take some more pics I think I can verify all your wiring. I haven't actually spotted anything wrong but I need to see the wires separated, especially those violet wires. One thing that was throwing me off was that 15K and 3K in series to make up the 18K off the leg of your added pot.

Pull the power tubes to do these bias voltage checks. Just so we're in sync, E2 (yellow wire) is the second terminal on the right terminal strip? And E1 (stock wire maybe green?) is the second terminal on the left terminal strip? That's how it is in my amp.

Your new pot should vary the bias voltage on pins 5 of the outside tubes. Verify that it does. Also verify that the new pot does not vary the bias voltage on the two inside tubes.

Then verify that the original pot varies the bias voltage on pins 5 of the inside tubes, but not on the outside tubes.

If these simple checks pass, then it's wired correctly. Set both bias pots for max negative voltage on pins 5 of all tubes.

Now you're ready to actually set the bias for operating conditions. That'll be the next step.

[pbman1953]

I should mention that I have a Weber Bias Right with the volt meter built in. Can I do these tests with the Bias Rite?

Plus you're saying to conduct these tests without any power tubes installed , correct?

[sluckey]

I should mention that I have a Weber Bias Right with the volt meter built in. Can I do these tests with the Bias Rite?

NO.

Plus you're saying to conduct these tests without any power tubes installed , correct?

Correct, and that's why you can't use the Bias Rite.

Your mod deals with providing separate bias pots for each side of the push/pull amp. The bias pots simply adjust the bias VOLTAGE fed to the output tube grids (pins 5). You MUST get this circuit working correctly before moving on to the next step (which will be using the bias rite unless you have installed 1 ohm resistors on all 4 cathodes).

The negative bias voltage on pins 5 control the amount of current your bias rite will measure. I think the bias pots may already be working properly. Do the checks above with your voltmeter and report the results.

[pbman1953]

 
Pull the power tubes to do these bias voltage checks. Just so we're in sync, E2 (yellow wire) is the second terminal on the right terminal strip? Yes


And E1 (stock wire maybe green?) is the second terminal on the left terminal strip? Yes


Ok, I made the test and the stock pot controls the insides and additional controls the outside . No interaction between inny's and outy's.

[sluckey]

OK, your mod was working properly all the time. Set the bias voltage on all pins 5 to max negative voltage. Now you can put the tubes back in and proceed to check the bias current.

Does this bias rite check only one tube at a time?

[pbman1953]

all 4

[sluckey]

I just looked on weber's site. The bias rite I saw only checks 2 tubes. But you have one that checks 4 tubes at once, ie, has 4 sockets?

[pbman1953]

yes, 4 leads

[pbman1953]

FYI

After calculations and taking into consideration of 60-70% of the disipation. Almost all 4 tubes meaured at around 26

That's  voltage  x  cathod current

I'm told the range would be 25.2- 29.4

[sluckey]

Sounds like you're all done. Here are the pics of my dual bias mod...

[pbman1953]

Very nice

Are those 6550's?

[sluckey]

yes