another Express type build

[norburybrook]

Day#3. The RTV silicone now's cured looks like it will be good to hold the caps securely.

I've more or less finished. I need to wait for an Audio taper master dual pot i ordered a linear one by mistake and also 2x 2M2 resistors before i can finish up.

I'm going to start on the cabinet while i wait.

I think I'm going to get the glue gun out and glue the bright caps together and a couple of the other caps that might be better being solid. I've not Added any grid stoppers on this build so I'm looking forward to seeing if it behaves.I also lowered the value of the second cap on the fast slow switch. i think 0.1uF kill way too much bass compared to the 0.002uF of the standard express so i've used to 0.02 uF on the fast setting this time hopefully that will make less of a tonal difference. I've also added a 4M7 between the caps so hopefully it won't pop when switched.

Express 3.jpg

TBC.

[norburybrook]

I woke up thinking about the caps on the fast slow switch this morning and have I got it the wrong way round in regards to bass.

The original express has a 0.002uF from the bottom of the plate resistor to the 150k. My last buid with the split plate I had a 0.002uF and a 0.01uF I'm not sure which one was wired to which part of the switch .

Will the 0.002 or the 0.01uF give more bottom end in that position? I know on the cathodes a 4.7uF cuts more bass than a 25uF so is that a general thing everywhere?



M

[martin manning]

In R-C filters, larger C moves the knee down in frequency. If it's a high-pass like a coupling cap followed by a grid leak or a pot to ground, that means more bass. The 0.01u should give more bass, and if you kept the original 0.002u at the plate, I'd think the 0.01 must be at the junction of the two plate resistors. What values make up the split load?

[norburybrook]

In R-C filters, larger C moves the knee down in frequency. If it's a high-pass like a coupling cap followed by a grid leak or a pot to ground, that means more bass. The 0.01u should give more bass, and if you kept the original 0.002u at the plate, I'd think the 0.01 must be at the junction of the two plate resistors. What values make up the split load?

I have 22K and 88K for the split load and I had the original 0.002 on one and originally 0,01 on the other, I can't remember which way round though. The sonic effect was that the slow switch on the first amp had a fuller sound. I was wanting to keep that fuller sound and have it on the fast switch as well, or close too.


I'd always thought the express traditionally had a lot of bottom end hence me getting things the wrong way round it would seem.



M

[martin manning]

I have 22K and 88K for the split load and I had the original 0.002 on one and originally 0.01 on the other, I can't remember which way round though. The sonic effect was that the slow switch on the first amp had a fuller sound. I was wanting to keep that fuller sound and have it on the fast switch as well, or close too.

22k on the plate and 88k from the supply? With the 0.01u connected above the 22k (slow) and the 0.002u on the plate (fast), slow will have about 2dB less gain and the -3dB corner point a few hundred Hz lower (fuller sound). You could always increase the 0.002u a bit.

[norburybrook]

Thanks Martin.

I'm away today but will have a think about this over the weekend.

Interesting that the standard express is 0.002 and not 0.1.


M

[Roe]

yes, but some amps used 4nf instead of 2nf (or 4.4 instead of 2.2). I much prefer the 2.2 myself. even 3.3 is too much for me

[martin manning]

That's the spot where Trudy and Ingrid had another cap paralelled, so yes there was some variation.

[norburybrook]

That's the spot where Trudy and Ingrid had another cap paralelled, so yes there was some variation.

Interesting, I think the Komet 60 has a 0.1uF there. I've put a 0.1 uF on the 'Slow' and a 0.05uF on the 'Fast' and I'll see how that sounds. My friend always loved the Slow setting as it was fuller ( he mostly plays a one pickup Firebird) so I'm hoping he'll have a similar tone on the fast now. until i fire it up i wont know. At least i'm now aware of the filtering effects at that spot so i can fine tune to taste with him in the room if he doesn't like it.

Any thought on grid stoppers? is it worth trying without or shall i just put them on? and final qusetion.

What's the most elegant way of wiring a cut control and the LARMAr V2 master volume? they obviously both could parallel off the PI nodes but I'm sure there's a way to do it with the least/shortest wires.
I may want to have a bypass switch for the cut as well.

M

[norburybrook]

I've just found this.

Drawing of LaMar PPIMV.jpg

I like the shielding idea, but it also says LINEAR pots, I htought it was Audio taper /Log pots that were used.. Are they the safety resistors Martin was talking about?

Also the point where it looks like the shields are attached and then sent from the dual pots to the board, isn't that where the BIAS voltage comes in? it looks like they're saying it's a ground or have I misunderstood this?




M

[sluckey]

The 2.2M on the pots are the safety resistors.

Nothing on that drawing connects to chassis ground. The blue wires are connected to the bias voltage point on the board. The bias caps provide an AC ground for the signal.

[norburybrook]

The 2.2M on the pots are the safety resistors.

Nothing on that drawing connects to chassis ground. The blue wires are connected to the bias voltage point on the board. The bias caps provide an AC ground for the signal.

thanks Steve, so why does it say 'shielding Grounded here'? is that a poor choice of words

also that looks like he's using mic cable, would that be able to handle the voltage/current? it would be an elegant way of wiring if so.

M

[sluckey]

so why does it say 'shielding Grounded here'? is that a poor choice of words

IMO that is a poor choice of words. I would have said "shielding connected here". But then, someone would say "I thought shielding always had to be connected to ground." Of course they are thinking chassis and dirt are the only real grounds. The only way to get your head around it is to realize the fact that the bias caps provide a good AC ground for 50/60Hz AC ripple ***AND*** audio AC signals. Same applies to B+ filter caps.

also that looks like he's using mic cable, would that be able to handle the voltage/current?

I don't know that is mic cable. All I can tell is it is two conductor shielded cable. The only current flowing through the cable is the signal current. Not much, so mic cable would be sufficient. However, mic cable usually has a thick protective rubber jacket that may not look very pretty inside an amp. I would use 22AWG STP (shielded twisted pair) just because I have access to miles of it. You may have some friends that work at an air traffic control tower?

[norburybrook]

so why does it say 'shielding Grounded here'? is that a poor choice of words

IMO that is a poor choice of words. I would have said "shielding connected here". But then, someone would say "I thought shielding always had to be connected to ground." Of course they are thinking chassis and dirt are the only real grounds. The only way to get your head around it is to realize the fact that the bias caps provide a good AC ground for 50/60Hz AC ripple ***AND*** audio AC signals. Same applies to B+ filter caps.

also that looks like he's using mic cable, would that be able to handle the voltage/current?

I don't know that is mic cable. All I can tell is it is two conductor shielded cable. The only current flowing through the cable is the signal current. Not much, so mic cable would be sufficient. However, mic cable usually has a thick protective rubber jacket that may not look very pretty inside an amp. I would use 22AWG STP (shielded twisted pair) just because I have access to miles of it. You may have some friends that work at an air traffic control tower?

thanks Steve, that makes sense and I was kind of thinking along those lines. I have some thin shielded patch cable or some inner multicore cable that I've used on my front mounted ,footswitchable FET's on my Dumble builds. Both are very thin hence me asking about voltage/current limits. I have no friends in air traffic control

as i have a linear pot in there I'm going to wire it up tomorrow. I always htought master volumes should be Log but thinking about it a Master volume usually is working in reverse i.e. it's on full and worked backwards to taste. I can always swap it out later


cheers

M

[martin manning]

I like doing the PPIMV like this. My express have a cut control, and it is useful for adjusting treble at low PPIMV settings. See in the link I posted above. It's typical Vox type with 0.002u cap and 250k pot.