[quote author="Gus"]I think we should be looking for 3 gigs[/quote]I got some at Newark about a year ago that were 1/2W and either 2.2 or 2.2Gs, forgot which. Seems like they were cheap, too. Maybe around $2 each. If I find teh part number, I'll post it.
Do you know how hard it is to get resistors over
100 Meg. Lucky for us in the USA there is Victoreen
(They also make real good Geiger counters if the mushroom clouds come)
It seems Ohmite bought their resistor product line.
These thick film resistor datasheet is here: http://www.ohmite.com/cgi-bin/showpage.cgi?product=v_maximox
In the USA since DigiKey should stock them they are our best bet.
Why do you need 3G?
This resistor only effects the low freq. pole of the mic?
Why do you need 3G?
The higher you go above what you really need is inviting trouble.
From surface leakage. And grid currents in tubes.
[quote author="AudioJunkie"]On the topic of the 1Gig resistors. Digikey has been out of stock of all 1Gig, 1%, .25W resistors for over a month...[/quote]Digi-Key does a run on these every few months. Trick is to place your order now and you'll be on the list when they come in.
Tim why the hell are you using a 1% use the 5%
DigiKey part # MOX200J-1000M-ND
They are $3.68USD and they have 4065 in stock!
Using 1% for the capsule bias resistor is crazy and expensive.
You do not care about absolute value here so 5% is
great I am supprised it is not 20% but resistors
have gotten a lot better so 5% is easy to produce today.
Remeber the resistor and the capsule capacitance sets your
low frequency corner 63 pf capsule and 1G is less than 3 Hz.
That should be fine unless you are recording the space shuttle takeoff.
[quote author="adrianh"]Tim why the hell are you using a 1% use the 5%
DigiKey part # MOX200J-1000M-ND...[/quote]Opps, my bad - I need to read more carefully! I WAS referring to the 5% ones, as they were always out of stock when I used to buy them. I see now he was referring to the 1% ones. No, don't use those!
The noise seems to be lower with higher than 1 gig. This does depend on the tube. Its not the roll off as much as the better noise and "sound".
On my list of many things to do is to break open tube and check the grid to cathode distance and the type of grid construction. I think alot has to do with the electron cloud size and shape. If I understand what I have read the size is affected by the heating of the cathode this might be part of the reason underheating works. The shape by the grid wire wind distance and distance from grid to cathode.
Also some grids are gold plated this changes how the electrons scatter off thegrid If I am understanding correctly.
High gm tubes tend to have close grid to cathode spacing this might not be good for tube microphones. Then there is grid alignment etc.....................................
I wish someone made ac701s again. I have not used one but that tube specs look to be the best for tube microphones.
So much is going on in a good tube microphone circuit. I need to find more old tube books.
Then there is the noise from the random emission from the oxide. Some oxides are better than others for tube microphones.
Oliver A. pointed out at another forum that the vf14 u47 cathode is held very stiffly at 1.1V via the bias used. This was done for a few reasons one was that the work function of the oxide emitter of the vf14 at the heating voltage of the u47 is about 1eV. This somehow controls the emission by the cathode V being just over the work function. I am still tring to understand all of this.
I'm having serious problems tracking these resistors in Europe.
Rscomponents doesn't want to sell to ??? (I can't find the word, but I mean people that don't buy for a company and don't want to buy a hundred of them)
Even the 33M resistors are hard to find.
[quote author="ward"]Rscomponents doesn't want to sell to ??? (I can't find the word, but I mean people that don't buy for a company and don't want to buy a hundred of them)
Anyone has some tips ?[/quote]
Yes. I (and probably others here) order almost weekly from RS and/or Farnell. So just tell me what you want, and I'll order it for you.
Slightly off the G7 topic, but it's worth noting here that there are 2x 1gig resistors in MXL2001, Nady and similar mics - so if you're modding one of those or scrapping one for the capule make sure you don't throw them away.
I found Newark?s part # for the 2G ohm (not 2.2G-2.5G like I first thought), 10% Vishay resistor. Newark?s part number is 19C8063, they?re $2.64 ea and in stock. It?s made by Vishay - part # RNX-3/8 2G0 K M.
It?s about the size of a 1/2W resistor, but it?s rated 1W @ 25 degrees C and 1/2W at 125 degrees C. I was able to fit two of them in an Oktava MK012, but I had to solder them onto the back of the board.