Cool old tube based band pass filter

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lancebug
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Cool old tube based band pass filter

Post by lancebug » Fri Apr 27, 2012 8:26 pm

Has anyone ever run into one of these? Its a Krohn-Hite 330BR Band-Pass Filter. Picked this up today at the state surplus warehouse, so I am thinking it just came out of some laboratory closet. Haven't tested it out yet, but it looks to be in good, clean condition and it is full of tubes. If it is working, I am thinking about trying to adjust the cutoff points, as the current ones both run a little low.

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telepathy
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Post by telepathy » Fri Apr 27, 2012 11:57 pm

damn! Cambridge, Mass, getting it done!


let me know if you want to flip that thing. I'd buy it just for the fonts.
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Post by Scodiddly » Sat Apr 28, 2012 5:18 am

Electrolytic capacitors probably dried out, meaning it may still work but not very well. Tubes probably not the weak link at this point, unless that's why they shelved it.

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Post by vvv » Sat Apr 28, 2012 7:30 am

Score! 8)

I just wanna know, who is "Davenport"?
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Post by reedblack » Sun Apr 29, 2012 10:00 am

Beautiful looking piece! What are the EQ cutoff points you can choose from?

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lancebug
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Post by lancebug » Sun Apr 29, 2012 5:55 pm

The filters are continuously variable, with switches to select multipliers from x0.1 to x100. I am about to post some internal pics.

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Post by reedblack » Sun Apr 29, 2012 5:56 pm

And then plug that shit in and tell us how it sounds! :D

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lancebug
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Post by lancebug » Sun Apr 29, 2012 6:11 pm

Well, looks like there is not much inside besides a bunch of cans of spam and some weird old glass bottles from overseas. Does anyone know what the spam cans are? There are about 15 tubes in all, eight of which are 12ax7's two are 12au7's, one is a 12 ay7 and the other three are odd sized ones (in the front around the xfmr). The 12 series ones are almost all Telefunkens and the rest are RCA's. The 12's look pretty clear without any signs of blackening or wear. The big ones have some darkening at the top. Haven't powered it up yet though.
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Post by creaturesleeper » Tue May 01, 2012 5:07 pm

The tube in the back there by the large tranny and cap can looks like a 6ar4...halfwave rectifier tube...

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Post by Scodiddly » Wed May 02, 2012 4:34 am

The spam tins look like they're meant to pop right open. Maybe more tubes, or air capacitors. Or something full of nasty old chemicals.

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Post by emrr » Wed May 02, 2012 6:49 am

Spam are paper/oil caps. They will be fine. These use an electrolytic cap after a cathode follower for output, so it may have large DC leakage on the output. Measure it before you connect to something else. I'm not sure you can move the range easily. These are a 4 section tuned circuit. Very cool for active eq sweeps in a mix, if pots are clean. Range switches: don't switch them with speakers up, many times these send a huge thump. If you paid $20, you paid 10-20x under market.
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lancebug
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Post by lancebug » Thu May 03, 2012 5:15 am

Sounds like the "spam cans" are almost as toxic as actual spam. I once got paid to do spam sculture at a public event. I surprised at how it burned my hands.

Still havent passed audio through it yet. Will do it this weekend. Good idea to meter it though, before connecting it to valuable gear. So is the consensus that the paper-oil caps are still ok in a 40-50 year old unit?

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Post by emrr » Thu May 03, 2012 7:15 am

95% likely still good, and within spec.

Turn the knobs full throw 30-40 times before firing it up for initial cleaning. Flip switches a dozen times.
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