any mods for Top Boost?

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withmittens

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I have a modded Top Boost by someone out there. It has a top boost switch and an EQ switch, bright switch. Problem is that while backing off the bass and treble to get some mids, it starts to this out a bit. is there a bass boost and treble boost mod out there? hopeful to compensate a but while dialing in the mids?
 
How "modified" is your module? Basically, did the modder implement the proper (but quirky) AC30 tone controls?

From my cursory review of the circuit on my un-modded Top Boost, the tone stack looks like the standard Marshall arrangement but with AC30-like sounding values (it doesn't look like Randall duplicated the classic AC30TB circuit by grounding the bass pot like the AC30 (Gibson copy) mistake).

If your tone control circuit is fairly stock, then you can probably tweak the mids with some C14 / R31 mods. What values do you have in there at the moment?
 
C14 is removed and wired into a 2 way switch. This is the mid shift switch. This switch has 120 +/- 5%-------47 +/- 5% positions

R31 is a carbon 7, 0, 4, 5% not sure what that is. violet, black, yellow, gold

the mid pot is a cut control
 
If you haven't got a copy yet, I'd highly recommend Duncan's tone stack calculator - http://www.duncanamps.com/tsc/

The "Marshall" tone stack is probably closest to what Randall implemented on the Top Boost module. R1 and C1 in the Marshall is the equivalent of R31 / C14 on the Top Boost module.

Are you sure R31 is violet, black, yellow (700k Ohm) and not brown, black, yellow (100k Ohm)? Sometimes the brown can look violet. Typical R31 values are between 33k and 100k for the Marshall type slope resistors.

Anyway, to give you an idea how your tone controls are currently operating, here is a TSC plot of the differences between the 47p (green line) / 120p (blue line) options.

TSC47-120p.PNG


If your mid pot is now a cut control, then maybe the modder has implemented the Vox Bass/Treble control in which case you can use the TSC Vox tab to see what values might give you what you need. To tweak the mids (which is fixed on a Vox tone circuit) you need to find the equivalent of Vox R2 on your modded module. Take a look at the Vox circuit in TSC and it should be fairly easy to find. Upping the value of R2 will give you more mids without removing bass/treble, but remember your ears will make it seem like you've got less bass/treble, even though on the maths the amount will be pretty much unchanged.
 
hey, thanks for that info. I'll have to think about this for a while to understand completely. Are you saying that R2 should be the one to change or I'll need to find the equivalent of R2 on my board?

It seems the mid shift switch does not radically change the curve. I could up the 120 p side a bit, you think?

R31 could be brown but it looks more like pink so I thought pale violet.....

I'll try and upload an image....maybe it will show?
https://drive.google.com/open?id=0B-vdnF3V4bbXejl0Um9NY2JiWkk

and, actually, looking at the cut knob i don;t see any direct mods to it. Was this not an actual cut circuit originally?
 
Looking at the pics, it looks like the tone controls are the standard Randall "Top Boost" circuit so the modder has not cloned the AC30 tone circuit.

With that in mind, use the Marshall option in TSC to see the effect of C14 / R31 changes.

As to the value of R31, its a carbon comp and because they are brown the brown colour is often represented in the way your resistors look. It seems you have the same value resistor in R7. I'm pretty sure that R7 and R31 are 100k Ohm resistors. You can measure R7 using a multimeter. Put your boost switch to the boost position (which isolates R7 from the rest of the circuit) and measure across the resistor.

Reducing R31 (say to 56k or even 33k) will give you a boost to bass and treble frequencies. Upping the value of C14 will shift the mid dip higher in the frequency range.
 
Also, if you want some more sonic options, then replace your SPDT "boost" switch with a DPDT switch wired as I have done here - Switchable Top Boost mod. You'll need to install C7 and C13 (22uF 50V electrolytic caps). This way you get a "boost" switch that gives you the original Top Boost circuit, plus the Vox circuit that you can switch between.

IMHO you need the C7 and C13 coupling caps to get the full "Boost" effect of removing the 100k resistor from R7.

By way of background, C7/R17 and C13/R29 are part of the module's common cathode circuit. Each pair form a cathode bias resistor and cathode bypass cap for one gain stage. The resistor creates a local negative feedback effect which lowers the tube gain's stage. This works at all frequencies. The bypass cap then forms an alternate route for bypass of certain frequencies, dependent on the value of the cap. Lower the value, more treble. Higher the value, more bass.

Once again, there are programs that can help you visualise changes in values - https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

There is also a fantastic video here that explains all about common cathode circuits - https://www.youtube.com/watch?v=HrkYUPmEUhk
 
Thanks for that great info. I'll digest this and will probably give it a try, seems easy enough for me. you would recommend the mid shift switch remain the same? I reckon I'll revisit the tone calculator to see how cap value changes on that switch may effect the curve......

strangely the Mid knob does sound like it function as a Cut control as it does not add or subtract mids, it simply cuts or boosts high frequency
 
withmittens said:
...Was this not an actual cut circuit originally?

We'd really need to trace out the tone control part of the circuit to know for sure (or get a circuit diagram from the Top Boost).

Anyone got a circuit diagram?
 
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