Volume Loss on one channel

[fredpg]

Hi, I own a Bluespearl bandmaster amp (great boutique amps, the company disappeared a few years ago).
According the the maker it s a BF bandmaster clone. The amp is the size of a super reverb with only 3 10' speakers and a 3 knob reverb.

The amp has 2 channels, like all fender BF amps.

Problem I get is volume losses on one of the 2 channels. Volume can be ok for a few minutes, then it goes down and comes back again and so on. The other channel works just fine.
Any suggestion where to look? I would think a cap has gone bad somewhere.

Thanks in advance,

Fred

[Chuck H]

Hello Fred and welcome.

Problem I get is volume losses on one of the 2 channels. Volume can be ok for a few minutes, then it goes down and comes back again and so on. The other channel works just fine.

Which channel, 1 or 2?

Any suggestion where to look?

First freamp tube. Oterwise, need more info.

I would think a cap has gone bad somewhere.

Why?

Intermittant problems are tough because invariably the amp won't give you any trouble when you want to find the problem. Can you reliably induce the problem? Is there any other info you can give? Tonal changes, etc.

You mention that one channel loses volume. Which one? Are you using both channels at the same time? Are you A/B ing them? Is your A/B box battery powered or mechanical? Do you use pedals (volume pedal, tuner, distortion box, etc.) If you do, is the problem always on the channel with a particular pedal? Or does the problem also happen with the guitar plugged straight in, no pedals at all. You get where I'm going.

[fredpg]

Hi Chuck, Thanks for your reply.


Which channel, 1 or 2?

Channel 2



First freamp tube. Oterwise, need more info.

Sorry I don t get what you mean here




No tonal change , just important volume loss. Volume is usually normal when I start using the amp, then it drops quite a lot. Sometimes it comes back to nrmal and comes bacl up again etc... No other problem apart from that

You mention that one channel loses volume. Which one? Are you using both channels at the same time? Are you A/B ing them? Is your A/B box battery powered or mechanical? Do you use pedals (volume pedal, tuner, distortion box, etc.) If you do, is the problem always on the channel with a particular pedal? Or does the problem also happen with the guitar plugged straight in, no pedals at all. You get where I'm going.[/QUOTE]
I don t A/B, I don t use pedal and I do nt use both channels at the same time.

[Chuck H]

Do you use the vibrato? Try pulling the third preamp tube and turning the intensity pot all the way down. Does the amp still fade in and out? I'm thinking maybe a bad preamp tube, vibrato tube or a bad vibrato roach. If the problem still happens put the third tube back in and try a known good tube for the second preamp tube.

[fredpg]

There s no vibrato on this amp. The last 3 knobs at the right of the panel are for reverb, just like on a fender reverb unit (dwell, tone, level reverb knobs).
The reverb also works on channel 1 and everything is fine.

I did try already to change tubes, but no luck...

[J M Fahey]

Inject a steady 100mV 1KHz signal, set a reasonably loud volume, controls where you like them, measure AC voltage at different points from input to output and write them down.
BEWARE YOU'LL HAVE 150 TO 250 V DC ON PLATES.
You'll need a reasonably good multimeter, typically having a wide range of AC volts, at least from 200mV AC to , say, 500V AC.
The cheap ones with only 200 and 750V AC scales are not suitable for this.
Let it on (sorry, neighbors).
When you lose volume, remeasure along the path to see where you lose signal.
Post it.
PS: also post the schematic, even if hand drawn.

[fredpg]

Inject a steady 100mV 1KHz signal, set a reasonably loud volume, controls where you like them, measure AC voltage at different points from input to output and write them down.
BEWARE YOU'LL HAVE 150 TO 250 V DC ON PLATES.
You'll need a reasonably good multimeter, typically having a wide range of AC volts, at least from 200mV AC to , say, 500V AC.
The cheap ones with only 200 and 750V AC scales are not suitable for this.
Let it on (sorry, neighbors).
When you lose volume, remeasure along the path to see where you lose signal.
Post it.
PS: also post the schematic, even if hand drawn.

Many thanks for your reply. I don t have much knowledge but I think I can learn to do this. What kind of signal generator do I need?

Also I don t know which points to take the measurements from...

Would that help if I posted pictures of the circuit?

Thanks

[J M Fahey]

Dear fredpg.
I am *very* sorry but this particular kind of problem is a pain in the 4ss, even for experienced technicians, because usually when you test something, that part works perfectly, only to fail while you look somewhere else. Argh !!!
The worst scenario to learn; even more so by the lack of a proper schematic.
Try to get some tubehead friend or friend of a friend to have a look at it.
*If* it were a regular Bandmaster or whatever, there's much onformation floating around, such as schematics, layouts, they are studied in Forums, etc. ; but a Boutique amp, by definition, is "Boutique", the maker certainly did at least some things his own special way.
Not to forget you have lethal (as in "electric chair", no kidding) voltages there.
Bad scenario.
The *only* suggestion I can make, and with reserves, is that with the chassis open, held safely on some wood blocks or books, you turn it on, obviously connected to the speaker, apply some music at the input (an MP3 or CD will do fine) and carefully whip it in different places, move *slightly* wires, lightly tap on switches and pot cases, the tubes themselves, to see if something makes the sound die or reappear.
Use an insulating stick, such as chinese sticks or an only plastic ballpen and never touch the amp itself.
Good luck.