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 71 
 on: July 19, 2019, 10:55:40 am 
Started by LLL - Last post by LLL
Hi All,

Had a conversation with a collegue earlier, regarding testing the head amp of his U87.
The method he was going to use, was to connect a sig gen direct to the gate of the FET. ( with head removed)

After looking at the circuit, I would go for possibly injecting across R6 ( with head removed) ?

But, would the correct way be to remove the capsule head, and inject signal through a small cap ( say 22-47pF ??) into pin 4 on the head socket ?

Could we then do a frequency sweep to plot response of the amp, as well as test the gain of the head amp ?

Or, would it better off using the calibration input for this ?




 72 
 on: July 19, 2019, 10:41:47 am 
Started by LLL - Last post by LLL
You are not looking at the right schematics.
BOTH C28 types use B+ fo 120V (do not confuse this with plate voltages!)
However, heater max for the Nuvistor C28C is 5.0 VDC, and 6.3VDC for the 6072 glass tube versions.

Thanks for clarifying Klaus.
Would it be better if I de-rate the heaters by 5% ? just so the tubes are not stressed ?

 73 
 on: July 18, 2019, 12:34:01 pm 
Started by jomo1234 - Last post by klaus
Helvetica may be the case on pre-1986 models. Later versions (U87Ai) changed the font/size several times.

 74 
 on: July 18, 2019, 12:30:45 pm 
Started by LLL - Last post by klaus
You are not looking at the right schematics.
BOTH C28 types use B+ fo 120V (do not confuse this with plate voltages!)
However, heater max for the Nuvistor C28C is 5.0 VDC, and 6.3VDC for the 6072 glass tube versions.

 75 
 on: July 18, 2019, 10:00:28 am 
Started by LLL - Last post by LLL
Hi All,

Ive got a quick question regarding the PSU requirements of both the C28A (6072)and the C28C (Nuvistor 7586)

looking at the schematics for both, it looks like the N12 and N28A PSU's would be good with both mics ?
B+ 120V (pin 5 N12 / N28A)
H+ 6.3V (pin 8 N12 / pin 4 N28A psu side - pin 4 mic side)

I always thought that the Nuvistor 7586 needed around B+100V. After just checking the datasheet for the 7586, it quotes maximum plate voltage as 110V.

The reason for asking is, that Ive got to build a power supply for each of the C28's !


 76 
 on: July 18, 2019, 09:34:31 am 
Started by jomo1234 - Last post by gtoledo3
I am looking at a couple different eras, and see total style consistency with Helvetica Bold, and some that appear to simply be Helvetica. Characters and default kerning appear to be a match for Helvetica. I have only examined a couple different era labels on U87s.

Keep in mind that in order to obtain the final line weight, character size, etc,  that it is not necessarily uncommon to commission someone to make a customized version of a font.

 77 
 on: July 17, 2019, 01:42:46 am 
Started by jomo1234 - Last post by klaus
A special price was just awarded to you, for asking the most esoteric question ever on this forum. The price money will arrive in a neutral envelope, to protect our privacy.

Fonts on Neumann's anodized i.d. band have changed through the years. It would help to post a close-up of a band from the era you have in mind.
 

 78 
 on: July 16, 2019, 12:18:07 pm 
Started by jomo1234 - Last post by jomo1234
Can anyone tell me which font is used in the black banding on the bottom bell of U87/67/269 microphones?

Thank you!

 79 
 on: July 14, 2019, 08:31:14 pm 
Started by afterlifestudios - Last post by afterlifestudios
Thank you Klaus and Uwe. (So sorry Uwe!)

I was getting 40VDC at the plate when b+ was 102 last week, so I think Iím all set.  Most importantly, they sound incredible.

Unless any problems arise, I will check B+ at the PSU every 2 months just to keep an eye on things.

Aside: While I was searching for photos of insides of NG supplies online, I found this one.  Seems like red and brown PT leads are reversed from schematic?  I think itís ok because it is strapped for 220VAC so they are still in series.  But if this thing got strapped for 110VAC, it would go poof, right?


 80 
 on: July 14, 2019, 08:20:40 pm 
Started by afterlifestudios - Last post by klaus
You are getting the light to illuminate, so your connections are correct. No need to fuss with it further. Uwe (!) explained the mathematical calculation of the slow rise to nominal voltage. I concur from my experience.

Let's not focus on B+ or the heater supply alone (by the way, 107VDC will give you almost exactly 36V on the heater in many cases of NG + U47 + VF14 I have observed): a good portion of the health of the tube can be deduced from the plate voltage generated. Anything above 32VDC is fine, anything much below this will be audible as tube noise. Observe* that voltage 5 minutes after start-up, then again after a few hours.  After a long time on the shelf, sometimes a lower than nominal plate voltage of  VF14 will come back up, once the cathode is freed of deposits.

* Don't do it if you are not familiar with the mic's circuit. The voltage inside the mic can be deadly. Even experienced technicians will keep one hand behind their back when measuring tube mic voltages!

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