100w Bluesmaster- I've made a start

[martin manning]

I've removed the HT fuse today and fitted a DPDT on/on switch in it's place.

Now I'm having a bit of difficulty understanding the half power switching.

1: does the LED matter apart form a visual clue?

It adds a couple of volts to the cathode "lift" (see under 3). You could leave it out.

2: the 1R resistors I have to ground from pin 8 for bias measurement are not shown in the schematic/Layouts I've seen. Do they just go in line straight to the switch?

Yes, they go to the poles (center lugs) of the switch.

3: looking at the schematic it seems whichever way the switch is the signal always goes to ground , it's either direct to ground or via the 10k resistor. How does this shut off 1 pair of valves?

When current flows through the 10k's the voltage rises in the amount of Ik*10k. The LED in series with the 10k's is biased on, and therefore has voltage at its anode is Vd higher than its cathode. In half-power mode the voltage at the power valve cathodes is above ground by Ik*10k + Vd. The power valve grids are held at the fixed bias supply voltage, so the actual bias voltage becomes Vb - (Ik*10k + Vd), which puts those two power valves in cut-off, actually at some equilibrium point at very low current.

[norburybrook]

thanks Martin................I understood the first two sentences


I've re read the last paragraph and there's been some stirring in the grey matter.


regardless of fully understanding the technical explanation, I've fitted the switch this morning.

thanks again everyone for your help.

marcus.


in hindsight I should have put the LED over the other side of the switch so it's actually above the 50w label.

[Structo]

How much volume reduction do you get on the 50w setting?

[norburybrook]

How much volume reduction do you get on the 50w setting?

bugger all





Marcus

[norburybrook]

Another question

What's the difference sonically between 6L6's and EL34's?

I have 6l6's in both my amps and just wondered about trying a set of EL34's in the BM as it's a Marshall based amp. Obviously if it's not going to change things very much I won't bother, but as I'm tweaking at the moment I was just curious as to things to try for different flavours.



Marcus

[Structo]

If you try EL34's, make sure pins 1 & 8 are jumpered.

Pin 1 on a 6L6 is not connected to anything and is used as a tie point on some amps.

Pin 1 on th EL34 is a suppressor grid and must be connected to pin 8 the cathode.

It doesn't hurt anything to have pins 1 & 8 jumpered, just allows more flexibility.

As far as the sonic difference?

Dunno

[norburybrook]

Tom

I tend to have jumpers 1 and 8 connected anyway like you say.

What would you recommend as a good bias voltage for EL 43's if I've got 465 v Dc on my plates? I'm currently running abound 38mV on my 6L6's


Marcus

[martin manning]

EL34 is a 25W Pd max tube, and 70% of that is a good place to start: 25W*0.7/465V = 31mA. When you set the bias you are measuring cathode current with your 1R resistors from cathode to ground. The screen current is included in that, so the actual anode dissipation is ~65%, which is fine.

[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.

Just for anyone in the future who might have a similar issue. I narrowed this down yesterday once and for all. For some reason it retuned again the other day.....

It was......................................................


The send return jacks.... the power of this amp with the EVM 12 had just rattled them loose and was causing break up at certain frequencies , obviously the ones that resonated with the chassis/jacks.


I took the precaution of re soldering the connections and then tightened them up with a wrench this time and the amp is now as clean as a whistle.


The clean on this amp is lovely, really chimey but not harsh. The OD is lovely as well. I think it's my number 1 go to amp now.

Only thing it doesn't do is that slight beak up you get when pushing a fender amp.

Has anyone tried using a low powered speaker with one of these amps to get a slight crunch? I don't have any low powered speakers to try this with.


Marcus

P.s. the ported 1x12 'Larry Carlton' cab with the EVM is louder and bigger sounding than the 2x12 G12 65 Dumble cab..... go figure...... it's just a big sound.

[Structo]

Wow, good you found that.

Grounding has to be one of the most important things in electronics, especially tube amps since we are dealing with high power. (E*I=P)

Making sure all fasteners are tight with good grounding if required.

Should be sounding better now.

Turn the preamp up and the master down a bit to get the clean a bit hotter.