I‘m currently planning to build a Mk4 preamp.
It’s basically implementing the channel switching of Mk4 on the Mk2C circuit.
There are some questions remained while organizing the circuits.
Older Mark series(before 5) used LDRs for switching, mostly VTL5C1, and partially used VTL5C4 for its slower decay. (lifting master volume pots/GEQ filters)
VTL5C1 : rise 2.5ms, decay 35ms
VTL5C4 : rise 6ms, decay “1.5sec”
decay time is to leach over 100K
Mark 2C
Mark 4
I don’t still understand why the slower LDR(VTL5C4) is needed for lifting the MV pot.
I’v read some reports that if slower LDR is not used on that, very loud volume is leaked for a moment right after switching- lead channel to clean.
Why is this happening?
The only changes in the circuit is just disengaging the OD stage in the middle of it.. just like Dumble ODS.
Is it taking noticeable time to block the signal of OD stage completely with LDR? I’v never seen like that including SLO..
If so,
I have a plan of replacing the LDRs to relays for OD stage in/out.. Then is it OK to use faster ldrs on MV..? or even relays.
The question is same on GEQ bypass LDR. Why is it needed to be slow?
Moreover, I don’t understand why the relatively small ‘anti pop’ resistor is there, even it’s switched by LDR.
I suppose.. maybe the circuit is very sensitive with abrupt shifting? Or it’s problematic with the circuit is lifted over very high resistance..?
I saw someone used a SSR here without the resistor.
It's interesting that SSR is more advantageous than LDR for popping, even without anti-pop resistor, and I don't know why..
Questions about ‘slower’ LDRs on Mesa Mark series
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Questions about ‘slower’ LDRs on Mesa Mark series
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Re: Questions about ‘slower’ LDRs on Mesa Mark series
All those capacitors in parallel create leakage current that would build up a substantial voltage across the dark LDR.Moreover, I don’t understand why the relatively small ‘anti pop’ resistor is there, even it’s switched by LDR.
The resistor isn't so much, "anti-pop" as it is anti-whump. The rise time is 6.5ms, which 1/6.5ms is 166hz, quite within the audio bandwidth of the amp.
The 68k keeps all those nodes at ground potential, so when the LDR is activated it doesn't whump.
You can do a similar analysis for any kind of switch circuit.
Re: Questions about ‘slower’ LDRs on Mesa Mark series
Thanks for your reply, nuke! 
I couldn't understand it perfectly though, I'm starting to get the concept.
the keyword ‘anti-whump’ was a good hint.
Again, thank you!
I couldn't understand it perfectly though, I'm starting to get the concept.
the keyword ‘anti-whump’ was a good hint.
Again, thank you!
Re: Questions about ‘slower’ LDRs on Mesa Mark series
To think it through, one really has to go back to the original Perkin-Elmer datasheets and compare the parameters and graph curves, because that's what Mesa would have been looking at long ago.
Today, vactrols aren't what they used to be, since they're all reproductions and they're kinda, well, whatever they are.
The EQ circuit is one thing. The bypass resistor would bleed off any leakage voltage from all those capacitors. In the other places in the amp, I'm not certain why one would pick one vactrol over the other. It might be for technical reasons, or budgetary reasons, availability, or just no reason in the choice. Just copy and paste from the last circuit BOM -- something I've seen in real life engineering of products.
An aside about the EQ module, that's kind of old-school way to build such a thing, with actual inductors and what not. A stompbox type based on gyrators is easier and cheaper. The BA3812L chip is discontinued, but there are still some out there to scrounge up. It's an 18-pin single inline chip that implements a 5 band eq with a minimum of external parts and no inductors. Just resistors and capacitors.
This looks like an interesting build...
https://dirtboxlayouts.blogspot.com/201 ... or-eq.html
Today, vactrols aren't what they used to be, since they're all reproductions and they're kinda, well, whatever they are.
The EQ circuit is one thing. The bypass resistor would bleed off any leakage voltage from all those capacitors. In the other places in the amp, I'm not certain why one would pick one vactrol over the other. It might be for technical reasons, or budgetary reasons, availability, or just no reason in the choice. Just copy and paste from the last circuit BOM -- something I've seen in real life engineering of products.
An aside about the EQ module, that's kind of old-school way to build such a thing, with actual inductors and what not. A stompbox type based on gyrators is easier and cheaper. The BA3812L chip is discontinued, but there are still some out there to scrounge up. It's an 18-pin single inline chip that implements a 5 band eq with a minimum of external parts and no inductors. Just resistors and capacitors.
This looks like an interesting build...
https://dirtboxlayouts.blogspot.com/201 ... or-eq.html
Re: Questions about ‘slower’ LDRs on Mesa Mark series
I’v collected various Mesa EQ circuits converted to gyrators such as OPAMP or TRs,
But since I already bought the set of inductors from the official Mesa parts shop without thinking when I first planned this build(they are quite cheap).. and this is my first Mesa build, I am going to implement the original old school way for fun
Vactrol for simple switching is actually easy to source.
I got a sufficient amout of CDS that has dark resistance over 40M / 200~600R on resistance, and very fast response.
I’v rolled it myself and they are working seamlessly in my SLO build.
The problem is that ‘slow speed’ of the VTL5C4.. cannot guarantee the performance with other LDRs.
What I am currently looking forward to is;
I’ll replace the LDRs to relays for switching the OD stage IN/OUT as I mentioned above, and I assume(hope) that this will eliminate the need for the ‘slow’ Vactrols on master volumes..
But since I already bought the set of inductors from the official Mesa parts shop without thinking when I first planned this build(they are quite cheap).. and this is my first Mesa build, I am going to implement the original old school way for fun
Vactrol for simple switching is actually easy to source.
I got a sufficient amout of CDS that has dark resistance over 40M / 200~600R on resistance, and very fast response.
I’v rolled it myself and they are working seamlessly in my SLO build.
The problem is that ‘slow speed’ of the VTL5C4.. cannot guarantee the performance with other LDRs.
What I am currently looking forward to is;
I’ll replace the LDRs to relays for switching the OD stage IN/OUT as I mentioned above, and I assume(hope) that this will eliminate the need for the ‘slow’ Vactrols on master volumes..