Fisher X-101B dim bulb procedure question

Hi all,

Quick question (kind of) about a Fisher x-101b… I’d like to power this unit up with a dim bulb tester that I just built. I’ve found some very helpful links so far that cover the “general” procedure for using a DBT, but I’m still unsure of a couple of things specific to this unit.

So far what I’ve done:

1.) Powered the unit up with NO TUBES using a DBT (40 watt bulb): Bulb glowed dim – ok so far I think.
2.) Powered the unit up with RECTIFIER TUBE ONLY using DBT (60 then 75 watt bulb): again bulb glowed dim each time; and rectifier tube also began to light).

The above tests were done with volume set to minimum, speaker switch set to OFF, bias pot set just a tick forward of full counter clockwise, and ac balance pots set at midpoint.

I’m assuming my next step is to insert ALL tubes and test with a 100 watt bulb…

My confusion is regarding the proper settings and load on the amp when powering up:

1.) Should the speaker switch be set to ON or OFF?
2.) Should I be conducting the DBT with a load on the amp (cheap speakers; non-inductive resistors)?
3.) Where should the volume be set?
4.) Is the position of the bias pot important for this test (e.g. should bias be set full CCW or as close as possible to its proper position?) - FYI it is a cathode biased amp, 44v at pin 5.
5.) Would it make sense to insert the ouput (7591) and small signal (12ax7) tubes in one at a time to better spot a potential short?

I just want to make sure I get this amp up safely before I plug into full ac and start probing for proper dc voltages.

I totally appreciate any input!
Adam

Put all the tubes in, connect speakers, hook it up to the DBT, and turn it on.

So answers to questions.
1. Either or.
2. Yes absolutely! Never run a tube amp with out a load on it.
3. Minimum
4. If it hasn't been touched leave it. You're NOT going to adjust it while on the DBT. But you ARE going to check to see if it's BELOW 44V at this point.
5. ALL IN AT ONCE due to the topology of the circuit. You need V1-2-7-8 in the bias circuit to complete it.


Why did you mess with the AC Balance and the Bias pots. Tubes are a lot more forgiving than SS gear when it comes to this. And procedures for SS gear aren't always applicable to tube gear. Generally the Bias & AC Balance doesn't get messed with tho unless there is a hum or testing shows a gross imbalance or bias is off/redplating tube.

If your DBT glows dim for 30 seconds on the unit, then turn it off, and plug in to wall AC. Make sure your Voltmeter is connected to the bias test ports or however you check bias and watch the bias voltage as it warms up. Be ready to adjust the voltage immediately to keep it below 44V. Let it warm up for at least 15 minutes then recheck and adjust as necessary. Adjust the AC balance per the manual, or put it 1/2 way if you don't have the complete manual. It's actually phase inversion balance. It requires a scope to set correctly per FISHER. Putting it 1/2 way is usually close enough to get it running.

Unless there is something grossly wrong with it, it should come right up.

Larry...thanks! I really appreciate the info.

I should have given a little background on the amp. It's been in the family since new (my father...now me). I wanted to deoxit the amp, which is why i had to move the bias pots through their range a few times. For the main dc bias pot, I figured I would leave it in CCW position until I was sure I could safely power it up and adjust accordingly (44v).

I have some new caps (coupling and bias) which should arrive tomorrow. Once I get those in, I'll use your helpful advice to get this puppy up a little safer with a DBT. I'll update soon...

Thanks again!
Adam

Best way is to check the actual ohms reading on the pot, then mark it. When done DeOxit, and exercise, move to mark and check the oHms reading again. It should be very close. Get it to within an ohm or so. Fire up and final adjust.

Next Best way is to mark the pot location on the board and pot shaft (like marking a distributor when you take it out...if you are too young to remember this disregard) with a Sharpie. DeOxit the pot and exercise it, then put back to original spot.
Fire up and do final adjust. Wait 15 minutes and check again, adjust as necessary.

LArry