Fred Cameron U87 Schematic?

I have a Fred Cameron U87 tube modded mic & power supply on my bench, burnt up a bit. Does anyone have a schematic for this mod?

If it is a tube microphone it should be easy to trace.

Does it use the u87 transformer if so I would guess a plate out circuit.

What tube number?

Cathode or fixed bias.

A couple pictures would help.

The problem seems to be in the power supply, where there are totally burnt resistors. As I do not know the suply voltages, it is a challenge to figure out the correct values.

I understand about burned resistors.

I think I can make a good guess at the power supply voltage from the tube circuit.

Most tube microphones are textbook simple plate out triode or pentode as triode gain stages to a transformer. No big deal(except they are for very HIGH Z input).

Whats the tube used and what is the plate resistor value?

Once we have the tube circuit we can guess the B+ wanted then it is simple ohms law to cal the resistor needed to get the voltage drop in the RCs in the power supply.

At one level tube microphone circuits are very simple things. It is all the little things that matter.

[quote author="nielsk"]The problem seems to be in the power supply, where there are totally burnt resistors. As I do not know the suply voltages, it is a challenge to figure out the correct values.[/quote]

It seems logical to contact Fred Cameron himself.
Hm, there is something wrong with the mic, as PSU resistors in mics don't burn that easy.

On the other hand, it is possible to calculate the B+ from the resistor divider for capsule bias.

Neils,

Tough one... I know that Fred can't answer unless you're pretty handy with the ouija board...

However, if we know the circuit in the mic, I'm sure that a couple of the tube-savvier here can have a good guess at what voltages it's going to be happy at. -I presume that it's going to be a couple of wires for heater voltage, one or two capsule polarisation lines, a pair for signal return and -perhaps just a single B+???

-Since I don't know of anyone continuing the modifications over at Cameron Labs, I don't think that there'd be any problem posting the schematics here: after all, these sort of things are going to need service and repair in the future... I did hear of another Cameron mod mic out there which was cracking, some time ago... -Perhaps if we could get the two together, we could find out what we need to fix 'em both?

-Keith

Anything?

I'm sensing a spirit in the room, but the glass isn't moving yet.... :wink:

Keef

a bit difficult to contact Fred unless you are trained medium. He passed away about 2-3 years ago. you might try his old e-mail address. I think his son may still be answering.

Wil

wilebee

a 5751. It is using what looks like a Jensen transformer (# sanded off) and seems to have a single B+ and heater rail sharing a common. The B+ is fed through a pot forming the pattern control.
Someone has to have docs on this out there.......I'm sure Fred would want to have his mods kept alive, even if he....well, you know
anyway, this is reputed to be a great sounding mod, it would be nice to see for myself!

I think I read SP used a 5751. Does it have a 100K plate? If it is cathode biased the cathode R will give a good hint. Look at an C28 AKG or the church microphone schematic.

Like Marik posted the resistor divider network will be a big help. MOST of the time a real 87,67 capsule is run at 48 to 60Vdc. In this case I would make my first guess at 60V

110 to 230 V B+ can be a good range for a tube like a 12at7,ay7 and maybe a 5751(I have not used one).

If there are two equal value resistors for part of the capsule voltage I would guess 120V B+ and >5.7VDC <6.3VDC for the heater.

A GUESS 100K plate, 1.5K cathode with a cap bypass a 1uf out cap into a 8:1 to 12:1 transformer. 200 meg grid to ground.

I wonder what was done for the high end rise of a 87/67 capsule did he just let it run?