100w Bluesmaster- I've made a start

[norburybrook]

Martin, I've stuck the lead on the R18 resistor without disconnecting the coax, and it's the same signal loss. Would disconnecting the coax there make any difference regarding testing? don't really want to take the pre amp board out if possible obviously


Marcus

[martin manning]

I see your point. If the signal is getting grounded out, then yes it will make all the difference. Take the center conductor off of the input to the drive pot then, and make sure that the resistance from that lug to ground is 250k (regardless of the wiper position), and that the resistance from the center conductor of the coax to ground is infinite. You can check for V2A signal on the center conductor, which should go directly to R18.

[norburybrook]

took the coax center off the input to the drive pot. Tested pot resistance 250k. tested coax resistance- inf.


first thing I noticed when I switched on the amp was a loud buzz/hum, this was without the lead in the return jack, it also was still buzzing when switching to the clean channel.


this didn't seem to change when I was probing the test points(now including the coax center).
I couldn't get a clean signal on any point now without buzz


Marcus

[norburybrook]

came back to this after a break. Reflowed all connections from the V2a plate all the way to the OD pot.

switched it on.............................................................................


WOW...it sprang into life on the OD channel



what a sound too..........raucous in the best possible sense


I've got all my HRM trim pots at 12 o'clock and it sounds amazing with my strat.

I'm going to finish my ported Larry Carlton 1x12 EVM 12 cab tonight/tomorrow and see how it sounds with that before I put it in it's cab.



Big thanks again to everyone here on this forum for the help, especially Martin again.


I'll post finished pics soon.



Marcus

[martin manning]

Great! Bad solder connection in there somewhere I guess; signal tracing helped to close in on it.

[norburybrook]

finished the ported 1x 12 cab today, it's a copy of Larry Carltons cab with a 1x12 EVM12 s 300w in it....yes the EV is upside down.........

put matching cloth on the grill to match the head cab.


going to let things settle in and do the tweaking before I house the amp but thought I'd show some pics anyway.



Marcus


p.s. one more thing to do......make a footswitch and cable........I hate this bit for some reason..

[norburybrook]

interesting update. I went down the studio today to play the amp a little and I realise it's the first time I've played it in it's head cab.

What seems to be happening is; as the head is sat on the cab and it's 100w and loud certain notes are causing vibrations from the cab to interfere with the amp and cause nasty crackling sounds. If I place the amp anywhere but on top of the cab it's fine.

is this something in my build that's at fault here? I swapped out a JJ V1 for a Chinese one and it certainly helped a little. Is it dry joints? cables vibrating?

???? any ideas


thanks


Marcus

[martin manning]

Yes something like that. You have corrected one bad solder joint issue, so perhaps there is another somewhere. If certain notes are triggering it that would seem to indicate a resonance, with the mechanical movement created putting some stress on the bad connection. If replacing V1 had some effect, then the connections around that socket would be a good place to start. Chop-sticking leads and joints while listening for the crackle is a good way to search.

[Aaron]

Nice build Marcus.
I had the same problem with my ODS combo and it was a faulty volume pot. With the amp on try tapping the pots to check.

Aaron

[norburybrook]

Thanks guys,


Just when I thought I was safe


Right back out of the cabinet then.


I'm away all next week so it will have to be put on hold for a week.



Onwards and sideways.


Marcus

[Structo]

Intermittent problems are the worst.

I don't have to tell you that.

Since you discovered soldering problems, you should reflow every connection in the amp.
Especially the grounds.

I can't count how many times folks have trouble with newly built amps that have either poor soldering techniques or faulty grounds.
Sometimes even commercially sold amps have the same problems.

Of course check out the tubes first (golden rule)

Usually on newly constructed amps it is a wiring mistake.
Second is solder joints.
If they aren't neat, clean and shiny they are suspect.
But it could be just as well a new, faulty component.
I've had bad pots.

Good luck on your new amp.


They are great when they operate correctly.

[norburybrook]

so' I'm back after a few days away, so I thought as I'd a spare hour or so I'd re-flow the joints on V1,V3 (i'd already done V2) and the boards/pots.

no joy yet, still crackles with heavy vibration.


the journey continues.....


Marcus

[martin manning]

So you are saying that none of the 12AX7 heaters are not glowing? High voltage on the plates indicates that there is little or no current flowing, which would be consistent with cold cathodes. Where and how are you measuring the filament voltage on the preamp tubes? Are all of the preamp tube plates showing the high voltage?

Eh? Did your "no sound" problem disappear?

[norburybrook]

ha, well it went yes, I was testing voltages again with the guitar plugged into the return and I notices the plate voltages were back to 200.


plugged back in and was back to normal!

anyway, the noise is still there, if I play a 'C' on the A string it farts and fizzles.

I've a feeling the two things are probably connected somehow, I/e the loss of all sound then back again.

I've been poking around and I can't fins anywhere that makes it stop or get worse.

As I said if I put the amp on the floor it's fine.


????????????

marcus

[martin manning]

Were the preamp filaments out? I'm willing to bet that problem will return.