Getting more mids from a JCM-800 preamp?

[Mark]

Hi

I have a few clues on how to get more mids. Firstly, increase the 470pF cap in the tone control circuit, increase the capacitance on the first cathode bypass stage, use a cap and resistor in series with the 10K cold clipper to get a mid boost.

What are your thoughts on getting a mid boost?

The desired sound is a switchable bluesy tone.

[Roe]

does the presence control help?

are you looking for low or hi mids?

[cdemike]

Hi

I have a few clues on how to get more mids. Firstly, increase the 470pF cap in the tone control circuit, increase the capacitance on the first cathode bypass stage, use a cap and resistor in series with the 10K cold clipper to get a mid boost.

What are your thoughts on getting a mid boost?

The desired sound is a switchable bluesy tone.

Increasing the value of the mids pot works towards bypassing the tone stack altogether, which would result in a relative mids boost. Van Halen reportedly used 50k. Depending on the magnitude of the desired effect, you could also essentially implement a "raw" switch, i.e., eliminate the tone stack by taking the concept further and significantly increasing the tone stack's resistance to ground. That'd be easy enough to implement via a push-pull pot or toggle switch on the amp where the switch would short a large resistor in series between the mids pot and ground. If you want that to be on a footswitch, a relay or optocoupler might be your best bet from a noise perspective, IMO.

[Roe]

try a .1uf or smaller bypass cap across the 10k cathode reistor or a .47uf across the 820/1k cathode resistor

[Mark]

does the presence control help?

are you looking for low or hi mids?

Thanks for your reply.
Low mids primarily, the idea is to get it to work with a Strat.

[Mark]

try a .1uf or smaller bypass cap across the 10k cathode reistor or a .47uf across the 820/1k cathode resistor

I already have a 22uF across the 820 ohm cathode resistor.

[Roe]

does the presence control help?

are you looking for low or hi mids?

Thanks for your reply.
Low mids primarily, the idea is to get it to work with a Strat.

see if you like a 2n2 across the 470k resistor, instead of the 470pf

also consider a different bright cap than the 1nf (which works with higher gain settings)

[Mark]

Thanks for your assistance.

[GlideOn]

Try reducing the Phase Invertor tail from 10k to 6.8k and use a .68uf cap for the Presence Control.

I did same to my Marshall circuit recently and it's a nice linear mid boost without being bloated like it can at the V1 or V2 cathodes. It's dirtier overall but in a good way. IMHO, it's a pretty big part of why the Fender Bassman sound so good.

I put it in a push/pull 25k pot to parallel both the tail and the cap at same time. Works well.

Also, more mids or gain means the amp has to either cut bass or tighten up. If you're leaning in that direction, consider stouter filtering (heavy 100uf + for mains) or reduce NFB resistor a couple of K. Probably the later first as a way to experiment.

[Mark]

Try reducing the Phase Invertor tail from 10k to 6.8k and use a .68uf cap for the Presence Control.

I did same to my Marshall circuit recently and it's a nice linear mid boost without being bloated like it can at the V1 or V2 cathodes. It's dirtier overall but in a good way. IMHO, it's a pretty big part of why the Fender Bassman sound so good.

I put it in a push/pull 25k pot to parallel both the tail and the cap at same time. Works well.

Also, more mids or gain means the amp has to either cut bass or tighten up. If you're leaning in that direction, consider stouter filtering (heavy 100uf + for mains) or reduce NFB resistor a couple of K. Probably the later first as a way to experiment.

Thanks for your advice.

[pdf64]

try a .1uf or smaller bypass cap across the 10k cathode reistor or a .47uf across the 820/1k cathode resistor

I already have a 22uF across the 820 ohm cathode resistor.

22uF is taking the boost down into sub bass.
http://bmamps.com/CapCal.html

[Roe]

try a .1uf or smaller bypass cap across the 10k cathode reistor or a .47uf across the 820/1k cathode resistor

I already have a 22uF across the 820 ohm cathode resistor.

22uF is taking the boost down into sub bass.
http://bmamps.com/CapCal.html

yes try a .47uf instead

[pdf64]

does the presence control help?

are you looking for low or hi mids?

Thanks for your reply.
Low mids primarily, the idea is to get it to work with a Strat.

see if you like a 2n2 across the 470k resistor, instead of the 470pf

I've been using that for a few months now on neikeel's recommendation, it's a noticeable improvement.

also consider a different bright cap than the 1nf (which works with higher gain settings)

I like 4n7F, from anode coupling cap to gain pot wiper.
It's not subtle but gets the job done

[pdf64]

does the presence control help?

are you looking for low or hi mids?

Thanks for your reply.
Low mids primarily, the idea is to get it to work with a Strat.

The most significant change is to get a tone control on to the bridge pickup, as per Rory Gallagher.

[Roe]

a curly coil also helps lowering the treble-peak into the mids, making for a fatter sound