My inclination is that there's more likely something wrong in the PSU, but without seeing other M49c reference voltages, I'd be guessing and going in blind.

Thanks,
Josh
Bias and B+ stay the same regardless of pattern. Only voltage change is on rear capsule:

Omni  0V
Cardioid  57-60V
Fig8  full B+

To narrow down your problem, unplug mic and cable, and just power up PSU. Measure B+ at appropriate pin on connector. Start changing patterns and see if your B+ remains stable. Then measure output of pattern switch at connector to see if you get the numbers listed above in each position.

If all measures good, problem is in the mic or cable. If B+ rises or drops when changing patterns, or you don't get those numbers from the pattern switch, problem is in PSU.

(if PSU is passive you'll want to load first to get accurate readings)


Bias and B+ stay the same regardless of pattern. Only voltage change is on rear capsule:

Omni  0V
Cardioid  57-60V
Fig8  full B+

To narrow down your problem, unplug mic and cable, and just power up PSU. Measure B+ at appropriate pin on connector. Start changing patterns and see if your B+ remains stable. Then measure output of pattern switch at connector to see if you get the numbers listed above in each position.

If all measures good, problem is in the mic or cable. If B+ rises or drops when changing patterns, or you don't get those numbers from the pattern switch, problem is in PSU.

(if PSU is passive you'll want to load first to get accurate readings)
Wisdom has Spoken +1 :)
Assuming the mesurement is HZ compensated :)

Shouldn't it be stable no matter which pattern you choose? (Really honest question, I've to learn a lot about tube circuits)
Cathode on circa 1,3-1,6v and Anode at 44-46v (I think yours also fine).

I assumed it should be about the same, but it isn't.

Are you saying that you have the M49c, and the voltages are the same for each pattern? Or have you not tested in different patterns?

Thanks,
Josh

TillM

Yes I've built M49c and M49b.
But.
I would say that the Cathode and Anode voltage is stable, like Banzai said.

I've use a potentiometer for the pattern.
So I can turn it from 0 to 116v.
Measure with your DMM on pin 6 for the pattern voltage.


Bias and B+ stay the same regardless of pattern. Only voltage change is on rear capsule:

Omni  0V
Cardioid  57-60V
Fig8  full B+

To narrow down your problem, unplug mic and cable, and just power up PSU. Measure B+ at appropriate pin on connector. Start changing patterns and see if your B+ remains stable. Then measure output of pattern switch at connector to see if you get the numbers listed above in each position.

If all measures good, problem is in the mic or cable. If B+ rises or drops when changing patterns, or you don't get those numbers from the pattern switch, problem is in PSU.

(if PSU is passive you'll want to load first to get accurate readings)

I finally got around to getting a 47 ohm resistor to test the PSU. B+ was stable, and Pin 6 reads as you stated: Omni 0, Cardioid 50%, and Fig8 100% of B+.

How can I rule out the cable before throwing the mic at the wall? Continuity is correct for each pin relationship, and resistance/capacitance is very low.

Thanks,
Josh

I finally got around to getting a 47 ohm resistor to test the PSU. B+ was stable, and Pin 6 reads as you stated: Omni 0, Cardioid 50%, and Fig8 100% of B+.
Sounds like you loaded the heater instead of plate:

Strap a 150K or thereabouts resistor between plate and ground pins on the connector. With dummy load installed and pattern switch set to omni, set B+ to 120V. Then carry out same measurements as before on B+ and Pin 6.

When you change patterns B+ should briefly fluctuate but quickly settle back to 120V. In all patterns.

Sounds like you loaded the heater instead of plate:

Strap a 150K or thereabouts resistor between plate and ground pins on the connector. With dummy load installed and pattern switch set to omni, set B+ to 120V. Then carry out same measurements as before on B+ and Pin 6.

When you change patterns B+ should briefly fluctuate but quickly settle back to 120V. In all patterns.

Do you mean pin 1 and ground? The anode connects through the transformer to pin 1 in the mic, so I'm assuming that's what you mean?

Sure. Whichever pin your B+ is on, strap to ground with 150K resistor. You want to simulate plate load of ~ 0.7mA. Doesn't need to be exact, ballpark 0.7-1mA load is fine:

150K resistor and 120V B+ simulates a 0.8mA load. Good enough.

Sure. Whichever pin your B+ is on, strap to ground with 150K resistor. You want to simulate plate load of ~ 0.7mA. Doesn't need to be exact, ballpark 0.7-1mA load is fine:

150K resistor and 120V B+ simulates a 0.8mA load. Good enough.

Oh, you meant connect the resistor to B+; that's pin 5 per the schematic. Thanks, I'll try that out.

EDIT: Side question: is there any reason I wouldn't want to dummy load both the heater and B+?

Thanks,
Josh
« Last Edit: July 26, 2019, 01:27:52 PM by JMPGuitars »

Can do both together, no problem.

Just make sure your resistor ratings are sufficient. Heater current is 100-200x higher than plate. 1W is cutting a bit close. 2W is much safer.

For plate load, 0.25-0.5W rating is fine


Can do both together, no problem.

Just make sure your resistor ratings are sufficient. Heater current is 100-200x higher than plate. 1W is cutting a bit close. 2W is much safer.

For plate load, 0.25-0.5W rating is fine

I modified my dummy load. It had a 2W 47Ω resistor for the heaters, and I added a 150KΩ 1/4W resistor for the B+ load.

I still get the expected results. B+ stays at 120V (dips slightly and then comes back to 120V when switched). Polarization voltages also as expected: 100% (120V) in F8, 50% (60V) in Cardioid, and 0% in Omni.

Thanks,
Josh

Perfect. That's PSU confirmed good.

Now keep going by elimination. Plug in cable, and measure on female side to check everything still measures correctly, on the right pins. If so, cable is good too.

Then onto the mic. Starting at the connector, what you're looking for is shorts, cold joints, or wiring mistakes.

Check that each pin in the mic corresponds to the correct pin in PSU. Reflow anything suspicious and double check everything, including tube wires (might be a simple error).

Perfect. That's PSU confirmed good.

Now keep going by elimination. Plug in cable, and measure on female side to check everything still measures correctly, on the right pins. If so, cable is good too.

Then onto the mic. Starting at the connector, what you're looking for is shorts, cold joints, or wiring mistakes.

Check that each pin in the mic corresponds to the correct pin in PSU. Reflow anything suspicious and double check everything, including tube wires (might be a simple error).

Thanks. I ruled out the cable. The unloaded voltages through the cable match that of the PSU in all positions.

On to the mic. This will probably be tedious. I highlighter verify everything as I build on the schematic, so whatever is causing the issue will not likely be that obvious. Though it would be nice if it is. ;)

Thanks,
Josh

Here's my voltages taken at the mic for each position (and where the voltages were taken from).

Mic Connected:

B+ (Pin 5 on mic PCB):
O 120
C 117
8 116

Polarization (Pin 6 on mic PCB):
O 0
C 56
8 114

Cathode (K tube turret):
O 1.54
C 1.93
8 1.94

Anode (A tube turret):
O 55
C 34.9
8 33.5

Heater dropped to 5.2 (taken at H+ and HG turrets). I didn't bother raising the heater voltage yet, it was previously set to 6.29 on the mic before retweaking the voltages today.

It appears my voltages are only happy in Omni. Where should I start? Does the voltage being good in Omni rule out the tube?

Thanks,
Josh

TillM

The Polarisation voltages are really good.
Also B+ is totally fine.

In my opinion, everything is fine. Did you do a test with your mic?
Do you get Sound? Or sound it thin ?

The Polarisation voltages are really good.
Also B+ is totally fine.

In my opinion, everything is fine. Did you do a test with your mic?
Do you get Sound? Or sound it thin ?

I'll post a test so you can hear it.  What I said before is "Omni is much louder and sounds best. Cardioid sounds like it has a bass roll-off or something. F8 sounds lethargic."

Shouldn't the anode/cathode voltages stay closer to what they are in Omni?

Thanks,
Josh

Mic Connected, 5840W heater set to 6.3V:

Cathode:
O 1.6
C 1.93
8 1.93

Anode:
O 52.4
C 34.7
8 33

M49c tested directly through Studio Capture into Studio One Pro. Input sens set at 38:
https://soundcloud.com/user-275398202/m49c-test/s-ckv0Z

With the anode voltage dropping like that, I would expect output to drop, and it clearly does.

Thanks,
Josh

My bet is still on a short.

No choice but to go over every single joint and connection one by one. Focus on everything connected to pin5 and 6.

Maybe share some pictures. Can sometimes be easier.

My bet is still on a short.

No choice but to go over every single joint and connection one by one. Focus on everything connected to pin5 and 6.

Maybe share some pictures. Can sometimes be easier.

Thanks, but I don't agree with the "every single joint" idea - that can easily cause more problems. After reviewing the schematic, I'm going to start by following the path of pin 6 to S2a/b. If that doesn't do it, I'll move on to pin 5 as you suggested.

It makes more sense to start at pin 6 since at least in Omni, pin 5 appears to do what it should. It isn't until voltage is applied to pin 6 that the issues begin, so that's where I'll start.

Unfortunately I need to wait until tomorrow for a temperature sensor to arrive so I can calibrate my soldering station. Stupid maintenance. ;)

Thanks,
Josh

EDIT: C1 was defective. I'm waiting on a replacement to arrive so I can install and test again.
« Last Edit: July 31, 2019, 09:53:23 AM by JMPGuitars »

I'm happy to report that replacing the defective C1 did the trick! The mic sounds great now, and all voltages are as expected.

https://soundcloud.com/user-275398202/m49c-bad-capacitor-replaced/s-Pu9zc

Thanks,
Josh

PS. I wasn't very scientific with that demo. Handheld mic, not careful about distance, etc... But obviously it works. ;)


 

Related Topics

  Subject / Started by Replies Last post
338 Replies
83971 Views
Last post August 07, 2019, 03:20:39 PM
by Rlucas41
10 Replies
1886 Views
Last post June 01, 2018, 06:15:23 PM
by hop.sing
16 Replies
2314 Views
Last post July 11, 2018, 01:32:20 PM
by aazaa
12 Replies
959 Views
Last post March 07, 2019, 07:08:13 AM
by alhe