Volume imbalance between channels mid '80's .50 Caliber

Pre Recto days. Boogie modified Fenders, Mark I-IV, Dual Calibers, etc

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thunders
Bottle Rocket
Posts: 3
Joined: Tue Jan 16, 2018 7:26 am

Volume imbalance between channels mid '80's .50 Caliber

Post by thunders » Fri Jan 26, 2018 11:09 am

So, my first MB...problems. I could tell that there was a serious volume drop between then two channels when trying out the amp, also the lead channel sounded really fizzy, too overdriven, not like any MB I've ever heard before.

Buy it anyway I thought, it's a good price, surely I can get it fixed.

I just got it back from the amp tech after a good service, he's added a bit more breakup to the rhythm channel, and added a separate gain control for the lead channel, but when I'm playing along nicely on the rhythm channel and hit the footswitch to solo, the volume drops by half...now surely no-one has ever designed a two channel amp where the lead channel is half the volume of the rhythm channel.

Can anyone shed any light on this at all ?
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thunders
Bottle Rocket
Posts: 3
Joined: Tue Jan 16, 2018 7:26 am

Re: Volume imbalance between channels mid '80's .50 Caliber

Post by thunders » Fri Jan 26, 2018 8:17 pm

Has no-one else ever had this type of problem ?

darkbluemurder
Mark II
Posts: 104
Joined: Fri Jul 28, 2006 5:58 pm
Location: Europe

Re: Volume imbalance between channels mid '80's .50 Caliber

Post by darkbluemurder » Mon Jan 29, 2018 4:11 am

I have not personally had this problem but I would guess that this is not normal. Looking at the schematic there is only a few components in the lead mode which are not active in rhythm mode.

The first thing you can do is replace V1 with a known good 12AX7 (aka ECC83 or 7025). If the volume does not go up with a known good tube, next thing to check would be the cathode cap on V1. If it's shorted lead mode would not work. If it's otherwise bad, gain would be lower. However, that should make the tone less distorted but not necessarily less loud.

Next, if the coupling cap off V1 (it's C5 on the schematic) is leaky it would throw off the operating point on the second stage. In the rhythm mode you would not notice it since it's out of the circuit in rhythm mode. If it is bad it must be replaced.

Lastly, the optocoupler LDR2 could be bad and not correctly close the circuit in lead mode. Ideally when in lead mode it should have a resistance of zero. A small resistance (say up to 1k) would not be a problem but if it is much more, it could negatively affect volume and tone.

The only other component which is added for the lead mode is a 22k resistor to ground in parallel to the master volume. It's purpose is to compensate for the higher gain and level of the lead mode. I would check that it is really a 22k (and not a 2.2k). This resistor could be increased to 27k or 33k to provide a better balance but I would pursue that as a last tweaking measure if everything else is working.

Schematic is here: https://schematicheaven.net/boogieamps/boogie_50cal.pdf.

If you are not familiar with tube amps, best to take it to a qualified technician, apart from the V1 tube change. I would do that first, and I would not be surprised if the amp then works correctly.

Good luck,
Stephan

thunders
Bottle Rocket
Posts: 3
Joined: Tue Jan 16, 2018 7:26 am

Re: Volume imbalance between channels mid '80's .50 Caliber

Post by thunders » Mon Jan 29, 2018 7:35 am

Thank you very much Stephan for your detailed reply.
I'm not well acquainted with tube amps myself, but on discovering this problem, I did take the amp to a local highly recommended amp tech, but to no avail.
I'll pass your reply on to him and see what he makes of it.

Many Thanks indeed.

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