Bottlehead Forum

Bottlehead Kits => BeePre => Topic started by: grosen on January 27, 2015, 04:19:20 AM

Title: Beepre singing (on its own) [solved]
Post by: grosen on January 27, 2015, 04:19:20 AM

Hi everyone,

I just got my Beepre up and running and it sounds great ... when it's not singing on its own.  Apart from the usual microphonics — pings and boings — I'm getting a pure high pitched feedback-like tone (second E-flat above middle C) that persists *even when I turn the Beepre volume to zero*.  What's that about?  (I don't have the lead donuts yet.  I'm thinking they may help, but I'd like to understand what's going on anyway.)

The build was easier than expected, by the way.  More steps than the other BH kits I've built (Seduction, Eros) but easier thanks to the extra space under the hood.


-- Gideon

Title: Re: Beepre singing (on its own)
Post by: Paul Birkeland on January 27, 2015, 08:13:53 AM

How are the voltages inside the amplifier?  It sounds like you have a loose connecting, is the feedback present on both channels?

Title: Re: Beepre singing (on its own)
Post by: grosen on January 27, 2015, 08:40:39 AM

The voltages were spot on last time I checked, but I'll check again.  Yes, it's in both channels.  I should say that it's not always there.  I can go an hour without hearing it.  My sense is that it's triggered by microphonic noise and then builds up a bit in a feedback loop.  The weird bit is that it persists with the volume off.

G

Title: Re: Beepre singing (on its own)
Post by: Grainger49 on January 27, 2015, 08:48:46 AM

The volume and balance are both before the tube.  So if the feedback is in the tube the volume will not help.  Try wrapping your hand in cloth and grabbing a tube next time it happens.  That should dampen the feedback and tell you if it is microphonics.

Title: Re: Beepre singing (on its own)
Post by: grosen on January 27, 2015, 09:26:21 AM

Ah, thanks.  I didn't realize the volume was before the tube; that's what I was missing.  I'll try the damping the tube by hand tonight.

Title: Re: Beepre singing (on its own)
Post by: Doc B. on January 27, 2015, 09:27:59 AM

A steady tone is oscillation, which is sometimes caused by contaminated tube grid pins making bad contact with the socket. You might try cleaning the pins and making sure the socket contacts are gripping the pins well.

Title: Re: Beepre singing (on its own) (solved)
Post by: grosen on January 28, 2015, 11:00:51 AM

Well, that worked. The 300B pins were pristine, but I cleaned them anyway and the oscillation is gone. (The EL84 pins were gunky, but I'm guessing they can't cause audible oscillation.) Anyway, all's well.  Thanks as always,

G