Trying to set bias on Bluesbreaker - crackling and popping

[McGoogle McDougal]

Ok, for anyone who remembers, I got one of George's Bluesbreaker board kits and a Heyboer JTM45 OT for my amp many months ago and ran into a problem with a loud howling feedback sound when I switched it off standby. Well, this turned out to be resolved by swapping the leads from the OT to the output tube sockets. In the process of plowing through the schematic I also caught a few other errors and corrected them. Unfortunately I then lost a safe and appropriate workspace for a while, so it sat for some time.

I recently got back at it and swapped a few bias resistors to get myself in what seemed like the right range to try setting the bias. I'm now getting a range on pin 5 of around -38VDC to -55VDC when I turn the bias pot with no output tubes installed. I've also got about 450 volts on pin 3. Seems ok right?

Here's my problem: when I install all the tubes, connect a speaker, and power it up to check the bias I hear quite a bit of fairly loud crackling and popping, so I turned it off. When I flip the power switch off I get very loud pop. This is loud enough that I worry about blowing a speaker. Clearly something is wrong and I'm looking for help in putting together a strategy for tracking it down. I'm thinking possibly I've got some bad solder joints
somewhere or maybe something is shorting under the board somewhere?


Obviously I should try pulling the board off the mounts and inspecting the underside to see if anything is shorting underneath. I know that you can check for sold solder joints with the chopstick method, but I don't think that's safe in this case since I may blow a speaker or worse. Should I power it up without the output tubes and check voltages throughout the board?


I have already tried a known good alternate pair of output tubes, but not preamp tubes (didn't have any spares, but could pull some from one of my other amps)...

thanks in advance!
Eamon

[VelvetGeorge]

Definately power the amp up without output tubes. If there is voltage going somewhere it's not sypposed to, we should be able to see that.

If you can, post voltage readings from the tube sockets.

George

[McGoogle McDougal]

Thanks George, I did record the voltage from every socket without the output tubes in, but unfortunately I just realized didn't bring the chart home with me. I'll get it later today and post the voltage from every tube socket.

Eamon

[McGoogle McDougal]

Ok here's my current voltage for all sockets except V6 (tremolo tube), where I forgot to measure anything but the heaters.

I don't have a JTM45/Bluesbreaker chart, but they seem somewhat higher, but at least in the range of a 50 watter. The obvious thing that stands out, however, is the 52 volts showing up on pin 2 of V3. I shouldn't see voltage on pin 2 of any of the preamp tubes, right?

thanks,
Eamon

[Flames1950]

I would have thought that voltage like that on an input grid meant a leaky coupling cap. In another thread one of the guys said the voltage chart fror 100 watt amps on George's CD shows 20-some volts there. I don't have any voltage chart that claims any DC voltage on an input grid, that's part of what coupling caps are there for.

[McGoogle McDougal]

Thanks for the reply Flames. Do you mean the output coupling caps or the 1st to 2nd stage coupling caps? I would think the output ones since they connect to V3... What's the best way to test this?

thanks,
Eamon

[VelvetGeorge]

You will find some sympathetic DC on the PI grids. But 50 seems a little high. Let me take a look at your chart again. It's not showing up in the reply page.

Flames, a CD is in the mail to you.

George

[VelvetGeorge]

Let's not blow any speakers, but can you try something?

Power up the amp with the tubes in, except for V1. If you still have the popping, pull V2. Same with V3. This will sometimes tell you which stage is the source of the noise. Obviously if it goes away when you pull V1, you know to start there.

George

[McGoogle McDougal]

Thanks George, that sounds like an excellent test to try. I won't be able to try it until tomorrow, but I'll post my results as soon as I know.

thanks,
Eamon

[Flames1950]

You will find some sympathetic DC on the PI grids. But 50 seems a little high.

Thanks for clearing that one up, it's the second thread I've seen that in and I couldn't find a chart that showed it. Never tried taking a measurement there on my own amps, I might do it for fun as an experiment next time they're cracked open!!

[McGoogle McDougal]

Ok, I finally got a chance to do the pulling preamp tubes test. When I pull any of the three preamp tubes out the snap crackle pop sounds go away, as does the loud pop when flipping the power off. Any idea what this tells me? Does this mean that the problem might be in or before the first preamp tube?

thanks,
Eamon

[VelvetGeorge]

So, no matter which one you pull the crackling goes away? That makes a pretty vague diagnosis. We need to zero in more.

What if you only pull V1? Let's start there.

George

[Flames1950]

Hey Eamon, do you have any fresh chassis pics of that build you could throw up here? The only ones I could find were a few months old, and you said you'd found some errors and corrected them in an earlier thread. Just wanted a look at the current state of things if possible.

[McGoogle McDougal]

Hi guys, thanks for the replies. It definitely went away when I pulled just V1... I'll try to get the digital camera down there and take some fresh chassis shots too.

thanks,
Eamon

[McGoogle McDougal]

Ok, so I finally played through my Bluesbreaker!!! I did 2 things before powering it up - I loosened the bracket for the 16x16 cap and pulled it farther away from the board (in case it was making contact with something on the underside of the board) and I disconected the tremolo circuit (unsoldered the wire from the mixer resistors and pulled it's tube). After doing that powered the amp up and... like magic, no crazy sounds (in fact dead quiet). So I biased the KT66's to between 39-40 ma and plugged my strat in...

It's dead quiet with the volumes down and nothing but normal single coil hum with the amp volume up (on both channels). It makes a sweet clean sound and high up the dial starts to crunch and sing (although a strat will never push it too hard without some help). It also no longer makes a pop of any sort when I switch it off or into standby.

The one thing that's not working right is the treble control. I get my most treble with the knob off and then it disappears completely by halfway up the dial. Obviously I need to investigate but I just played it as is for a while - it sounded pretty nice with the knob in the zero position!

My next step is to test re-introducing the tremolo circuit to verify if it was indeed the cause of the snap, crackle, pop sounds (and if so, start retracing the circuit). I was going to take some pictures, but the damn battery went dead in the digital camera

Anyways, I feel like the finish line is in sight now - I can actually play through it and have it sound good. Much thanks to George and everyone who has helped me so far!

Eamon