KB300 (1983 model) Pull Gain?

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dak
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KB300 (1983 model) Pull Gain?

Post by dak » Sat Jan 15, 2022 7:23 am

Hi,

I am trying to find usefully faithful EQ settings for my KB300 (Black Widow version) and it's a bit irritating. For one thing, it has a significant tendency to sound muffled and/or tinny. Now an amp from 1986 does not have the freshest speakers. However, according to what I can find regarding speaker wisdom on the net is that piezo speakers don't have much more than complete failure as deterioration mode and both of the tweeters in the horn measure and sound the same. And a bass speaker that is wearing out its suspension is supposed to gain on the low end rather than the middle. And if I measure the frequency response, that suggests more or less setting the controls this way:
DSC06592.JPG
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Namely, keep the bass, dial the mids down about one notch, dial the highs up by a bit more than one notch. It's possible that the default setup with a Scorpion speaker is more balanced by default: I think that the Black Widows are supposed to be a bit more efficient and that could imply a stronger magnet that would help more with the mids than the lows.

Now what is irritating is that using the pull gain appears to do part of the job regarding the highs: I'd have thought that pull gain is, well, for gain. However, the circuit diagram corroborates that the pull gain is not neutral:
pull.png
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So that "pull gain" starts adding some non-trivial additional gain above about 720Hz. Is that some heuristic to compensate for, say, the way the comparatively low input impedance works on guitar pickups, possibly the main target for the "pull gain" switch? Or is there some other rationale I just don't get? I haven't actually found a manual for the 1983 version: the one available on the Peavey page is already for the model with XLR input (the 1986 model I think) that does not have the "pull gain" I think.

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Enzo
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Re: KB300 (1983 model) Pull Gain?

Post by Enzo » Sat Jan 15, 2022 2:29 pm

Piezo drivers can fail open, they can fail cracked and distorted, they can get buzzy (moreso than the natural buzziness of piezos), they can get intermittent. They should measure like a cap.

I have seen failed Black Widows. No one makes a speaker you cannot destroy. But I can't say I ever heard a worn out BW. If you think the speaker may have a problem, connect the amp chassis to a different speaker and find out.

Pull gain switch is there to add enough to help cut through the mix for solos and such. Just my opinion.

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dak
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Re: KB300 (1983 model) Pull Gain?

Post by dak » Sat Jan 15, 2022 3:50 pm

Enzo wrote:
Sat Jan 15, 2022 2:29 pm
Piezo drivers can fail open, they can fail cracked and distorted, they can get buzzy (moreso than the natural buzziness of piezos), they can get intermittent. They should measure like a cap.

I have seen failed Black Widows. No one makes a speaker you cannot destroy. But I can't say I ever heard a worn out BW. If you think the speaker may have a problem, connect the amp chassis to a different speaker and find out.
Ok, that corroborates more or less the net wisdom and would mean that my speakers are likely fine. The piezos do measure like caps (and whine equally when seeing the faradmeter). They just need a bit of EQ. The double piezo horn would not even be an expensive replacement part — if I were living in the U.S. As it is, I'd have to pay triple to get one because of postage, so it would make more sense to get some arbitrary(?) piezo speakers with the same kind of 3-screw drivers and hope for the best. But Motorola speakers are no longer around, CTS doesn't seem to be available here, and it is not clear what of the mostly unbranded lot (you can get two small rectangular piezo horns that look like having the right driver for $10 including postage) is actually useful or is recycling the fallout from the actual manufacturer. The "most branded" version are probably Monacor speakers for about double that price.

But since the sound is fine after using the EQ on the amp (and my volume needs are not such that I'd overload them easily)...

Connecting the amp chassis to a different speaker is actually a bit more involved than with later models since the amp does not have an external speaker connector (and no headphone connector, and only a single line out).
Pull gain switch is there to add enough to help cut through the mix for solos and such. Just my opinion.
Ah, that would make sense. Not for adapting to different sources as much as different purposes. It would also explain the difference in gain not being Earth-shattering, merely significant. So for some purposes you'd not need to fiddle with gain and EQ as much as just pull the gain and reset it afterwards.

Thanks!

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Re: KB300 (1983 model) Pull Gain?

Post by Enzo » Sat Jan 15, 2022 5:02 pm

Instead of buying whole new dual horn piezos, I just get new singles and swap the motors onto the horn assembly. Nothing wrong with that.

Connecting the chassis to a different speaker does require taking something apart yes. You can pull the BW and either stick a different 15 in there, or use clip wires on the dangling wires. OR... pull the chassis and unplug the speaker wires from the board and use clips to the bared posts.

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