100w Bluesmaster- I've made a start

[norburybrook]

thanks Martin, I do try and keep to the method you suggest when soldering, I have a Chinese temperature controlled solder station that seems to be holding up well.


While I have your attention;

I'm thinking of putting a half power switch on the amp. What type of physical switch would you recommend? I'm probably going to use the method used on the #124 layout unless you have any other suggestions.


Marcus

[martin manning]

I'd go with lifting the cathodes a la #124 for half-power. You'll have to integrate the 1R current sensing resistors with the #124 layout, and you should bump the impedance selector down one click when you run half-power.

[norburybrook]

do you mean the bias resistors as I call them? I presume they'd go in line with the switch, and would need to be in full power mode from then on when checking bias voltage.

I was wondering what actual type of switch to use, DPDT, on /off, etc?


so on an 8 ohm cab drop the impedance down to 4ohms?




Marcus

[martin manning]

Yes on the bias resistors, if you mean the 1-ohm's through which the cathodes are grounded. And yes you would have to be in 100W mode for those valves to be conducting current you can measure to set the bias. So that's where you found the bad solder joint, eh? I wasn't quite sure what you meant above.

A DPDT on-on will do it, as shown in the 124 schematic. I'll have a look later and think about a layout.

When you take two of the output valves off-line, you need to increase the load impedance by a factor of two to keep the load line in the same place for the remaining valves. By connecting the speaker to a secondary tap that is half of what it should be, the load reflected on the primary will double; exactly what you want.

[norburybrook]

damn!!

the static's back, the 'dry joint' must have been a coincidence last time, didn't think my soldering had suddenly gone bad.

happens on and around low 'A' and tends to come in on the decay of the notes.

here's a sound file in case anyone recognises the sound.


chop sticking around doesn't have any effect on this, so I'm at a loss where to start looking.

to recap;

I've checked all valves.

rewired heaters.

reflowed all visible eyelets and pot connections.


changed all output valve pair configurations with just 2 valves.



???????

stumped


MArcus


p.s it's now disappeared again like yesterday, this time it didn't time itself with me doing anything, so that's a start.


p.p.s thinking about this, the same thing happened yesterday, i.e. after the amp has been on for a few hours it seems to stop. So I presume we an say it's something that's thermally affected, however ...what?

I'm stopping for the day now, tomorrow when it's cold and misbehaving, I'll try taking the send out to another amp to check the pre amp section and then do the reverse and take my 102 pre amp into the return of the BM see if that narrows it down.

[ayan]

I'm afraid I don't know what to say, it sounds like this could be a number of different things and you've apparently had some problem with bad solder joints in this build. The one thing I wonder is if you have a scope so that you can try to trace the bugger and see at what point it appears in the signal path. Otherwise, it could be anything starting with a bad instrument cord, all the way to a bad speaker cord at the other end, which makes it a pain in the neck to try to fix.

Gil

[norburybrook]

Thanks Gil, I realise at times like this a scope is absolutely necessary in helping trace problems.


I have a friend with a scope so if I don't sort it I'll pay him a visit


Marcus

[martin manning]

That noise sounds like perhaps an oscillation is breaking out when the harmonic content of the signal is just right. Is it still the case that it only happens when the amp is stacked on top of the cab?

[norburybrook]

No Martin, It's sat on top of another speaker out of it's cab.


The weird thing is it seems to stop after the amp has been on for a few hours.


Unless that's a coincidence as well.



Marcus

[norburybrook]

well came to the studio today and I can't for the life of me get it to misbehave today. My thought's are mixed now as it's either sorted itself out, i.e. a faulty(ish)component has heated up to the point it's now working , OR this is going to return at the most inappropriate time.

as I can't get it to misbehave I can't carry on trouble shooting.

So, after playing it for a couple of days my thoughts;

It's a very versatile amp with the right guitar/s

the sealed, ported EV 12 speaker cab is unbelievably loud/clear, it needs driving though and just puts out an incredibly loud unbroken sound, you could really damage your hearing with those as they just don't seem to break up. the 1 x 12 Dumble open backed celestion cab sounds better at lower volumes.

the OD channel is too loud compared to the clean.

I have 2 valves pulled and it's absolutely deafening through the 1x12 EV cab even with the OD volume down to 1 1/2.

Is there anyway to adjust the OD volume so it's more controllable? everything happens between 0 and 2 on the volume control.


Marcus

[ayan]

What comes the mind is that something can't be right in the wiring or part values. I appreciate the fact that being a Blueamaster with only one master for the OD, as opposed to cascaded masters in non HRM amps, it will get louder quickly. But i remember the masters in my now gone BM, for clean and OD, would end up being set similarly. Maybe something's off with the second tone stack?

Gil

[norburybrook]

Gil, the HRM board you mean? would that have much effect on the level? I suppose thinking about it that's what the boost does and that makes a difference.

I did find that there wasn't a lot of noticeable difference tweaking the HRM trim pots, should they have a 'big' effect?

Marcus


edit; just tested my 1M Od volume pot and it's only measuring 335K


so that would do it....if it's a knackered pot then that also could have been the problem I had earlier as well!!

right time to get a new pot in there.

[Duble]

the HRM board you mean? would that have much effect on the level?

The HRM tonestack certainly pads down the overdrive, lifting it will give a significant Boost in signal. Adjusting each trimmer should give a noticible frequency change, if adjusting the trimmers arent making a noticible change something is up

[norburybrook]

the HRM board you mean? would that have much effect on the level?

The HRM tonestack certainly pads down the overdrive, lifting it will give a significant Boost in signal. Adjusting each trimmer should give a noticible frequency change, if adjusting the trimmers arent making a noticible change something is up

Ok I've turned then all the way up and down now and yes, they do have a marked effect

I think the main problem is the faulty/bad OD pot


Marcus

[ayan]

The HRM tone stack will make a huge difference in signal level, as you've already found out. How did you measure your OD master volume pot, you disconnected say the ground connection from the circuit and measured it? If you didn't do something along those lines, your measurement would be off.

Gil

Gil, the HRM board you mean? would that have much effect on the level? I suppose thinking about it that's what the boost does and that makes a difference.

I did find that there wasn't a lot of noticeable difference tweaking the HRM trim pots, should they have a 'big' effect?

Marcus


edit; just tested my 1M Od volume pot and it's only measuring 335K


so that would do it....if it's a knackered pot then that also could have been the problem I had earlier as well!!

right time to get a new pot in there.