MK2 Vox AC50 Bias issues..,

[Jalmeida]

Hello,

I have a 1964 tube rectified AC50 MK2 that I am out of bias range adjustments on the bias pot. This version has a single 1m bias pot for both tubes. The schematic shows -32v to -40v from the bias pot wiper. The closest I can get is -46v all the way CCW. My thoughts were to try a higher value on either the 100k resistor after the diode or a lower value resistor on the grounded end of the bias pot.

For the bias circuit, all the components are per the schematic except for the 800R resistor(changed to 2 1500r resistors for total of 750R)before the 100r screen resistor.

Have any thoughts?

[SEL49]

Changing the 1M pot to the correct size of 250K will probably get a better adjustment range. If still not happy, see pic...

[Jalmeida]

Changing the 1M pot to the correct size of 250K will probably get a better adjustment range. If still not happy, see pic...

Great ideas! For some reason I didn’t think that 1M pot versus 250K would cause this specific problem, but it is easy enough to swap out and verify that’s for sure. Thanks for the recommendations.

[Jalmeida]

Okay, so my pot may not actually be a 1M. I will have to disconnect one end so I am not ringing through other circuit components. No visible markings. I believe the 1M I read was ringing back through the circuit. Will doublecheck tonight. Although I still am not sure this is the problem.

Another issue arose while working away on the amp last night. One old erie 100K resistors that bshunt buas voltage to ground on one end of the pot was reading 120k as opposed to the 100K. I replaced this and instead of aiming for the -32v to -40v I decided to use a bias probe to physically check the bias current. The -32v range is not the best bias methodology given I am not using original Mullards. Somewhere along the line the HT fuse popped. All I had were regular 3a fuses and not the time delay slow blow. So I replaced the fuse with a standard and it blew again. In the short interim before blowing again I was able to get the amp bias close to 30ma. However I powered down, pulled out the OT tubes and went to double check HT voltage to reconfirm my bias current and when powering on again my fuse popped. So today I will get some slow blo fuses and check out the circuit.

A few things I checked.

1. Verified bias diode is good
2. Checked the other resistors and capacitors in the bias circuit
3.  Checked for shorts in various parts of bias circuit

All of the above looked good. I did have speaker load applied. I am curious if the HT is too high being 120v versus 115 and if that could cause this?

Also, there are two components in the circuit that are not per the scehmatic that I want to rule out. Two resistors. On the attached schematic, the green circled items are checked good. The blue is replaced. The red items are different from the schematic.

[pdf64]

Repeated fault current incidents seems something to avoid subjecting 60 years transformers to.
Hence a light bulb limiter would be very beneficial when initially powering up after messing with the bias supply.

[shooter]

since you didn't post yours, here's one so we can all see the WHOLE amp


did you VERIFY there was bias voltage at the PA tubes grid pins WITHOUT the tubes installed?????

[Jalmeida]

since you didn't post yours, here's one so we can all see the WHOLE amp


did you VERIFY there was bias voltage at the PA tubes grid pins WITHOUT the tubes installed?????

The PDF 2nd attachment to my 1st at the top post is the factory schematic.

Here is is again of my original attachment didn’t work for others. 

[Jalmeida]

did you VERIFY there was bias voltage at the PA tubes grid pins WITHOUT the tubes installed?????

I verified the bias voltage was present at the junction of the two 220K resistors, but not at the output tubes. I did see bias change with the bias pot with the bias probe inline with the output tubes which tells me it is getting some bias voltage, although does not tell me it is correct.

[tubeswell]

did you VERIFY there was bias voltage at the PA tubes grid pins WITHOUT the tubes installed?????

I verified the bias voltage was present at the junction of the two 220K resistors, but not at the output tubes. I did see bias change with the bias pot with the bias probe inline with the output tubes which tells me it is getting some bias voltage, although does not tell me it is correct.

Lesson for next time. Each output tube grid pin is where the bias action is critical. (What shows up on the pot might not be making it to the grid pin.)


Also, you need to do this check with the output tubes removed (to verify it’s not one or more bad output tubes causing the Mains Fuse to pop).


Also, ^what pdf64 said^ about using a current limiter to diagnose potential faults.

[Jalmeida]

did you VERIFY there was bias voltage at the PA tubes grid pins WITHOUT the tubes installed?????

I verified the bias voltage was present at the junction of the two 220K resistors, but not at the output tubes. I did see bias change with the bias pot with the bias probe inline with the output tubes which tells me it is getting some bias voltage, although does not tell me it is correct.

Lesson for next time. Each output tube grid pin is where the bias action is critical. (What shows up on the pot might not be making it to the grid pin.)


Also, you need to do this check with the output tubes removed (to verify it’s not one or more bad output tubes causing the Mains Fuse to pop).


Also, ^what pdf64 said^ about using a current limiter to diagnose potential faults.

I did check also power on with the output tubes out. Pops the fuse still.

The second attempt to bias was with a Eurotubes Bias check which reads milliamps at the output tubes. I did not however check for the -21v at each tube with the output tubes out.


We are 100% on the same page about bulb limiter. Great advise always!

[Jalmeida]

Got it resolved. Had a rectifier tube taking out the HT fuse. Got the bias at 34ma(25/512x.7). I am a but concerned at the 512v at the plates. Gonna check my heater voltages to see off they are. Thinking I may end up running an amprx brownie or similar inout voltage controller to get closer to 115vac.