Fender Dual Showman bias/balancing

[ian1739]

Hello, I've never retubed before and have a couple questions for the experts!
I bought new tubes (tungsol 6l6gc str, tungsol 12ax7, electro harmonix 12at7) for my red knob fender dual showman, and when I went to set the bias/balance I had way to much voltage on the balance. The back of the amp says it should be 0v and I'm at 9.7v. Here are some pictures of the amp and what I was doing:








The tubes read Ip: 39 and Gm: 4,300 for all 4, is this too high for this amp?
Having 9.7v difference for the balance, will that hurt my amp? Also I'm wondering about the impedance of my cabinet there, the amp is set at 4ohms right now, how can I verify that is the correct setting?

Any advice would be great! Thanks
-Ian

[ian1739]

bump...

I really don't want to damage my amp so i'm afraid to turn it on... any help?
thanks

[52 Bill]

bump...

I really don't want to damage my amp so i'm afraid to turn it on... any help?
thanks

Did you start by adjusting the bias voltage control to .04 volts?

If you turn the balance control the balance voltage will not go lower than 9 volts?

Check your cabinet with your ohmmeter to see what the output switch should be set to.

[ian1739]

Hi, thanks a lot for the reply.
Yeah I was able to get the bias to .04, but regardless of where the bias is set the balance will not come closer than 9.7v. I turned the balance clockwise and there was easily more than 20v difference, at which point I quickly turned back the other way. 9.7v is the lowest.
I'm afraid to turn the amp on, I have no problem burning out a couple tubes but not about to rebuild my amp. I am decent at electronics and could handle changing the pot or replacing some caps to get the 9.7v within range. The amp is old and original, time to replace some caps? or could it just be a couple bad tubes?
As for the impedance of the speaker, using a dmm to measure resistance won't give me impedance for a speaker (coil), inductive reactance isn't resistance... or is there some dc resistance ratio for ac xL?
thanks

[52 Bill]

...The amp is old and original, time to replace some caps? or could it just be a couple bad tubes?
As for the impedance of the speaker, using a dmm to measure resistance won't give me impedance for a speaker (coil), inductive reactance isn't resistance... or is there some dc resistance ratio for ac xL?
thanks

Try swapping pairs of tubes (switch the two right with the two left) and see what happens to the balance voltage. If the voltage problem stays the same it's probably the amp, if it changes it may be the tubes.

And yes, technically the dc resistance will not be the same as the impedance of the speaker load, but it will indicate how the cabinet has been wired. The resistance will always be slightly lower than the impedance, so if it reads somewhere near 4 ohms or near 8 ohms or near 16 ohms, you can tell what the load impedance should be set to.

[ian1739]

Hi Bill,
I borrowed a more sensitive dmm, and set my impedance to 8ohm as the resistance was 7.4ohm, and tried swapping tubes around in different combinations. The bias I can get to 35mv, just shy of the 40mv it should be at. the balance is still 9volts out, regardless of which tubes are where. I even tried completely removing tubes 2 and 3 and tried running the amp at half power (30/60wrms instead of 50/100) and still the balance is out.

so it's gotta be my amp, right?
can I change the balance pot from (if it were 10kohm) up to say 100kohm, and balance it that way?
Does anybody out there have a schematic for this pig so I can start checking resistance values and testing caps?

As for the sound its a really fat, muddy distortion with NO clarity or fidelity to the higher strings, say I hit an open low E string and try to play a note on the high E string, the high E sound is basically non existent coming through the amp. sounds good for a couple songs but not how a fender clean should sound.

any help to get this amp rocking again would be greatly appreciated.

[Enzo]

There are three test points. One for each side output tube, and the third is ground. The pots are labelled bias and balance, but really all it is is a bias for each side. If you set one than measure against the other, it will balance at zero when both are sitting at the same voltage.

SO instead of trying all this balance nonsense, do this:
Measure resistance to ground with power off from each test point. One will be ground and the other two will be 1 ohm. COnnect your black probe to the ground one. Power on. Now set the left one for whatever voltage it wants, then set the right one for whatever voltage it wants. When both are the same, measuring between them should show a "balance" of zero.

I suspect you are not using the right test points for your balance test.

[ian1739]

Hi Enzo thanks for the help. Still a bit fuzy on what i need to do here... As I guess you know, this is what i'm working with:


The left pin(1) is 0 ohms to ground, middle pin(2) is 1.1 ohms to ground, and the right pin(3) no connection to ground.

From pin 1 to pin 2: 12.8mV to 36mV by adjusting bias pot. balance no effect.
From pin 1 to pin 3: 8.8v to 13.2v by adjusting balance pot. bias no effect.
From pin 2 to pin 3: 8.8v to 13.2v by adjusting balance pot. bias no effect.

I don't quite follow what I am supposed to do now...
Here's the readouts I was getting. Thanks


With the bias pot fully counter-clockwise, or what would be LOW on a volume knob.

[Enzo]

I'd say there's your problem, you have an open 1 ohm resistor from the cathodes to ground on the one side. probably burned out from a tube failure.

Pin 8 to ground should be a 1 ohm resistor on each side tubes.

Since that resistor is open - or the connection to it broken - the cathode floats up to some voltage, so you can never bias that side right, so of course it never "balances" either.

[ian1739]

Awesome, thanks so much enzo. I'll crack this thing open soon and replace resistors as necessary.
You say the tube went first, which caused the resistor to open? and not the other way around?
Is there other components I should be replacing when I have the amp open and the soldering iron fired up?
thanks again

[ian1739]

Thanks

Just wanted to say thanks again Enzo. Finally got a chance to crack open the amp and sure enough the 1/2watt 1ohm resistor was blown. Easy fix. Took a pic of where the resistor is for reference if anybody else has this problem.


Also there is a weird black burn mark on the power supply pcb, shown here:





Any thoughts? the amp sounds fantastic since that resistor fix, so i'm wondering if it ain't broke don't fix it, or should I be desoldering caps and testing? Thanks!