Fender 400 PS bias

[pontiacpete]

This amp had a tube short, according to the owner. There was a flash in one of the power tubes, a fuse blew and he smelled smoke. He pulled the 6550 that shorted and the one next in line on the far right next to the 6L6(you can see the tube layout if you scroll down on this link The Fender 400-PS Updates & Mods). He has this idea that the output tubes work in consecutive order in pairs. Which seems strange. The primary on this output transformer looks normal, a winding and a CT. The secondary is unusual with the way the taps are drawn. If anyone wants to explain this, that would be appreciated.
So now the amp is here. I started going thru the 'Test Procedure' on that link. I set the bias voltage to -39v on both of the pots.
I find that the amp draws 1.5A without a load. I put an AC ammeter across the fuse holder with the fuse removed.
Added a 4 ohm load and do not find the primary current is at 2.1 - 2.5A. With the 4 ohm load in all the output jacks there is little change in the amount of primary current drawn. This is where I stopped
Does anyone have any opinions about this 'test procedure'? Would you follow it through?
Would you use a conventional method of biasing the tubes?
Thanks for any ideas or suggestions

[Justin Thomas]

Can't help with the bias, but I've read that the output tubes do indeed work in pairs - The only way to get the full power output is to use three cabinets. One pair each for ~135W, supposedly. One cab gets you 1/3 power, two gets 2/3, etc. As far as WHICH particular pair of tubes does what, exactly, would depend on how they are physically wired. But it looks to me like those are not "taps," but actual separate secondary windings. The schem is a bit fuzzy.

Be careful with that thing, and take lots of pictures for us!

Justin

[Leo_Gnardo]

Everything you'd ever want to know about the 400-PS here:

The Fender 400 PS Amplifier Web Pages, by Rich Koerner

no crappy tubes need apply! this is a way for-serious amp.

[Jazz P Bass]

This article states explicitly that the current draw method is THE WAY that you bias the amplifier.
The Fender 400-PS Amp Designed By Ed Jahns

Output balance adjustment for minimum current draw is also critical.

If you do not get an increase in current when a load is applied, then I would be looking at the tubes, the cathode resistors or (shudder) the OT.

[bob p]

Everything you'd ever want to know about the 400-PS here:

The Fender 400 PS Amplifier Web Pages, by Rich Koerner

no crappy tubes need apply! this is a way for-serious amp.

That site has a good picture of what happens when you pack those tubes in too close together.

Part of the problem with amps like that is that the designers claim to be working in close collaboration with the guys who developed the tubes, but then they don't follow the mounting instructions on the tube's data sheet. Nobody does. But when you're designing a beast of an amp, failure to obey the mounting instructions results in failures of the tubes.

I seem to remember reading an MOV data sheet that required that the tubes be mounted 4" OC, with adequate space for air cooling, paying special attention to plate orientation so that heat is directed out away from the amp.

Nobody, and I mean NOBODY likes to honor that 4" OC rule. As you can see in the 400-PS, they don't like to honor the free air requirement either. They snug those tubes into a tight spot and then put a metal heat shield close by instead of mounting them far apart in free air or putting a fan on them. Is it any wonder that this happens when they ignore the rules?

[Leo_Gnardo]

They snug those tubes into a tight spot and then put a metal heat shield close by instead of mounting them far apart in free air or putting a fan on them. Is it any wonder that this happens when they ignore the rules?

It's also amusing, simultaneously annoying, those amps that rig a bunch of output tubes in a line, put a fan at one end and that's supposed to fix things. The tube nearest the fan runs cool, and the furthest runs hot. Even if you went to the trouble of installing a matched output tube set, thermal differences inevitably disturb the balance. Mesa: you're first in line for this one!

[pontiacpete]

The good thing is that the owner supplied me with several NOS 6550a's matched in pairs.
So is the bias procedure presented at the TE site what you would use when installing a pair of 6550a's'?

Here are some pictures:


Any reason why my current readings are not near the 2.1A mark that they say should be the case?
Should I be concerned and just move on?

[pontiacpete]

This article states explicitly that the current draw method is THE WAY that you bias the amplifier.
The Fender 400-PS Amp Designed By Ed Jahns

Output balance adjustment for minimum current draw is also critical.

If you do not get an increase in current when a load is applied, then I would be looking at the tubes, the cathode resistors or (shudder) the OT.

JP - I didn't see this post before responding last.

I do see an increase of current draw when applying a load. One load into any of the output connections increases the current draw from 1.50A to ~1.54 and when added another load 1.62A and a third ~1.72A

Are the tubes weak possibly?
Should I increase the bias voltage?

I'm not sure I understand this step:
"6 -Insert the three (3) speaker load plugs and adjust the "Output Tubes Matching" control for MINIMIM primary current. There will be a drop of approx. 0.2A from maximum, when the control is set properly"

[Jazz P Bass]

You can check each tube pair by inserting the load one at a time.
A, B & C.

In fact you can check each tube one at a time.

Remember that the load jack connects the cathode resistor.
Have you measured the voltage across them?

And don't forget the screen resistors.
This amp is using a way lowish voltage on the screens (ala Univox).

The last step does what it says.
Set the control for the least current draw. (with good tubes.)

[pontiacpete]

Thanks for pointing out the cathode connection to the output!!
Then according to the schematic the outer tubes V8 and V13 are on the same output and V9 and V12 are together and so on...
Am I right about this?

The owner pull V8 and V9, saying that they are working together on the same output jack.

[pontiacpete]

OK, looks like some interesting voltages on the cathodes. Not sure what they mean.

V8 - 0
V9 - 0
V10 - .225vdc
V11 - 38vdc
V12 - .24vdc
v13 - 24vdc

V11 and V13 are very different from the others. Any thoughts?

[Jazz P Bass]

Thanks for pointing out the cathode connection to the output!!
Then according to the schematic the outer tubes V8 and V13 are on the same output and V9 and V12 are together and so on...
Am I right about this?

The owner pull V8 and V9, saying that they are working together on the same output jack.

With V8 & V9 pulled, the owner was left with one option: Jack A.

[Jazz P Bass]

OK, looks like some interesting voltages on the cathodes. Not sure what they mean.

V8 - 0
V9 - 0
V10 - .225vdc
V11 - 38vdc
V12 - .24vdc
v13 - 24vdc

V11 and V13 are very different from the others. Any thoughts?

These voltages are with the proper load in the respective jacks?
Jack A:V10 & V11.
Jack B: V9 & V12
Jack C: V8 & V13.

Check the 10 ohm/ 2 watt cathode resistors.
V11 & V13 are the only tubes conducting.
Also look for any open screen resistors.

You could use either of those sockets for now & test each tube individually.
V11 use Jack A
V13 use Jack C.

[pontiacpete]

These voltages are with the proper load in the respective jacks?
Jack A:V10 & V11.
Jack B: V9 & V12
Jack C: V8 & V13.

yes, these are voltages across the 10R cathode resistors with a 4 ohm load on each.

The cathode resistors are opened on V11 and V13. Duh, didn't check those the first time around.
I will replace with 5watt 10R , happen to have some of those.

[Jazz P Bass]

Well, you got me on that one.
I would have thought with 20 some volts across V11 V13 cathodes that they Where conducting.
And especially V8 & V9 at Zero volts.
Something is odd to say the least.