Blackstar Series One 100--Squeals like a Stuck Pig

[nevetslab]

This Blackstar Series One Model 100 Guitar Head was sent over from inventory with High Pitched Squealing. Only does it under drive (using burst pink noise), and, does it quite easily in the OverDrive 1, OverDrive 2 and Crunch Channels with the Super Crunch button engaged. In all three cases, you need the Gain cranked to near max, Volume cranked to near max, midrange anywhere from moderate to full on, Treble from mid to full on, Presence from mid to full on and Master Volume from mid to full on. I can make these three channels behave at far less aggressive settings, but, not being a guitarist (only a Bass Player), I'm just finding WHAT sets this amp off into nastiness.

My last go-around with a Blackstar Series One I was able to cure by swapping out the full-width densly-populated front panel Preamp PCB. That was the Model 200, though the PCB & controls on it look like the same thing here. I used the only spare board I had that came out of a stage prop, and was lucky to have that board working. These are built in the UK, Lead-Free solder throughout, and poor quality PCB at that. I had no luck in repairing that previous PCB, getting traces lifting, solder refusing to come out of plate-thru holes, etc.

I've only gone as far as pulling the chassis from the cabinet, one by one swapping out the five preamp / driver tubes, with no change in this behavior.

Now it turns out we have three more of these heads in our inventory. None of them have been here for service or scrutiny. So, I'm only guessing this is a function of having so much gain in the circuits that you can dial up a setting that will squeal badly. The noise you end up with on such settings is obnoxiously high. Not that much different than playing with an EVH 5150, a Peavey 6505 or the Mesa Dual & Triple Rectifier Solo heads, all having enough gain to get you into trouble.

Am I just flogging a dead horse here, thinking this is just the way they are, or is there really something wrong in three of the channels (or a common circuit to all four channels in the post-Volume circuits? I do NOT have schematics for this amp.

[Chuck H]

I don't imagine there's an instability issue with the three successively gainier channels. Otherwise the two higher gain channels are redundant to the design, right? With your report I'm thinking there may be a ground fault in an area that affects all three channels. Yay lead free solder

[nevetslab]

I don't imagine there's an instability issue with the three successively gainier channels. Otherwise the two higher gain channels are redundant to the design, right? With your report I'm thinking there may be a ground fault in an area that affects all three channels. Yay lead free solder

I certainly won't deny that being a possibility. I have the two reject front panel PCB's from a model 200 in front of me, which looks to be the same physically as what's in this Model 100 amp. Thruout the board, the excess leads everywhere are folded over on the solder side of the board, many being dangerously close to shorting to other surfaces, though theoretically the solder mask would save that from happening.

I'll admit diving into these Blackstar amps is not high on my list of where I'd spend labor hours that I won't be able to bill for, not having the schematics for it, not to mention they're really designed for manufacturing, NOT for service.

I think the first thing I need to do is have our Guitar Dept send over another one to see how it behaves. If it works correctly, then at least I have something to make comparisons to.

Blackstar has a forum, so I should sign up to that and see what else I can find out. My impulse, though is to pass on the bloody amp.

[nevetslab]

A second Blackstar Series One Model 100 arrived, per my request from our inventory. I stacked the two up on the bench, dialed up the first one that reliably squeals on the three channels, then switched over to the one that came over late yesterday. The onset of squealing on this second ones' settings are VERY similar to the first one, though not identical. Close enough though, and the character of both sounds the same. I'd now call this a design feature, and NOT a defect per se, though I too would reject the bloody amp if that arrived on stage sight-unseen and gave that sort of behavior after dialing up the sounds I want.

These are manufactured in Korea....probably means the PCB's are made there, and shipped back to the UK for final assembly? Not sure.

I'm still waiting for the conformation email from the Blackstaramp forum, but I did scan thru all of their posts..very little in the technical domain we indulge in here, while I found only five references out of ALL of their models spanning from 2012 thru present describing anything to do with Squealing.....most were pointing to pickup issues. Not so in this case on these two Model 100's. Signal source is burst Pink Noise (GenRad 1382 Noise Gen, Krohn-Hite 3550 BP Filter, GenRad 1396B Tone Burst Generator, HP 353A Isolation Xfmr/Step Attenuator), so 600 ohm source impedance.

Nice to find the same behavioral characteristics. I really DIDN'T want to go digging for a cause in one of these amps!

[Leo_Gnardo]

The onset of squealing on this second ones' settings are VERY similar to the first one, though not identical. Close enough though, and the character of both sounds the same. I'd now call this a design feature, and NOT a defect per se, though I too would reject the bloody amp if that arrived on stage sight-unseen and gave that sort of behavior after dialing up the sounds I want.

I'll admit diving into these Blackstar amps is not high on my list of where I'd spend labor hours that I won't be able to bill for, not having the schematics for it, not to mention they're really designed for manufacturing, NOT for service.

Good that you were able to make a 1:1 comparison. And the differences in knob settings before onset of squeeks & squeals - can have everything to do with manufacturing tolerances of the pots, and slight gain differences between their tubes.

Thanks for the heads up nevets! I'll make sure to give any of these Blackstars a big miss. You've spent your time in hell, now onto better things...

[vintagekiki]

Blackstar Amplification

https://www.blackstarinsider.co.uk/forum/viewforum.php?f=24
Blackstar Amplification Official Forum/ Blackstar Products/ Series One (210 topics)

http://dealers.korgusa.com/svcfiles/Blackstar%20Bias%20Procedure.pdf
Bias Procedure for Blackstar

[nevetslab]

Good that you were able to make a 1:1 comparison. And the differences in knob settings before onset of squeeks & squeals - can have everything to do with manufacturing tolerances of the pots, and slight gain differences between their tubes.

Thanks for the heads up nevets! I'll make sure to give any of these Blackstars a big miss. You've spent your time in hell, now onto better things...

My curiosity factor may lead me to making some simple overall gain measurements of the four channels, while I'd also be tempted to set up the bode plotting system to run frequency response curves to see what their peak boosting curves look like. To really see the nasty details, gain/phase plots would reveal where these get into trouble, but..........to what purpose? Just how many Blackstar amps are in our inventory, anyway? Six in the Artisan Combo Amp series, 3 more Artisan Heads, different wattages, one HT Metal 100W head, in the Series One family, a 50W, four of these 100W and a couple 200W, and a couple Blackfire 200W heads (all guitar amps), nothing in Bass amps (assuming they make those too). A small amount compared to our Marshall inventory. I don't have direct access to client usage of them vs other brands.