I finally understand bass spec amps for guitar.

[DesolationBlvd]

Years ago, I bought an Electro-Harmonix MIG-50 for bass use with a bass cabinet. The stock circuit has the bass tone stack and no bright cap on the bright channel volume, but smaller PI coupling caps and split cathode like lead spec. I made the changes and snipped the mixer cap so the channels would be even. I would plug my bass into one input and my Minitaur into the other. I also use a 12AY7 so the bottom of the volume range doesn't go from zero to blasting me out of the room so quickly. With my Ampeg 1x15 cabinet, I get a rich and vintage sounding bass tone for the living room and studio.

Then I was studying the evolution of the 5F6A into the JTM45 and then the JMP 1987 / 1986. The mixer cap was what set Marshall apart from the 5F6A. As the price of a new SV20H in the USA is insane, I went through with finishing the mod on my MIG-50 rather than buying an extra SV20H for this project. The chassis already had a hole, I just drilled the front panel to insert the toggle to switch the mixer cap in and out.

The mixer cap is quite a subtle change, but maybe it's because of the 270k mixer resistors instead of the 470k normally used on JMPs. I still prefer it off for bass use most of the time, but with the Rick-O-Sound outputs on my 4001, the cap helps the bridge pickup sound even brighter, to compensate for the darker sound of flats. But this thread is about bass spec amps for guitar use, so I got my ES-335 out and ran into the 1966A with Greenbacks (just the regular 25w 75hz kind). First off, I got fat cleans, and all three settings (no mixer, and the bright and normal channels with the mixer) gave me tone sculpting options beyond just the tone knobs. Lead spec amps like my SV20H are a little tricky to get breakup (not roaring distortion) tones dialed in, balancing the input volumes while jumpered because of the huge bright cap, but bass spec amps get there much more easily. Higher gain settings without the mixer cap revealed the Fender Tweed heritage of the circuit, and with the mixer cap were still recognizably Marshall, just more laid-back. Then I tried all knobs full-out, treble booster into the normal channel, and found a transparent, yet gritty and honking, sound for sleazy glam riffs and responsive blues leads alike.

I haven't tried a 12AX7 yet. Maybe the tone falls apart at the highest gain settings with them? It's also worth noting the MIG-50 is for 5881 / 6L6, not EL34, use.

With an amp that can do both guitar and (vintage recording) bass, I'd love to have a speaker cabinet that can handle bass while still sounding good for guitar.

 

[FleshOnGear]

Very cool! Personally, my lead-spec amps do exactly what I want them to do. But I can see how being able to switch between the two voicings could be useful.

 

[Gene Ballzz]

Years ago, I bought an Electro-Harmonix MIG-50 for bass use with a bass cabinet. The stock circuit has the bass tone stack and no bright cap on the bright channel volume, but smaller PI coupling caps and split cathode like lead spec. I made the changes and snipped the mixer cap so the channels would be even. I would plug my bass into one input and my Minitaur into the other. I also use a 12AY7 so the bottom of the volume range doesn't go from zero to blasting me out of the room so quickly. With my Ampeg 1x15 cabinet, I get a rich and vintage sounding bass tone for the living room and studio.

Then I was studying the evolution of the 5F6A into the JTM45 and then the JMP 1987 / 1986. The mixer cap was what set Marshall apart from the 5F6A. As the price of a new SV20H in the USA is insane, I went through with finishing the mod on my MIG-50 rather than buying an extra SV20H for this project. The chassis already had a hole, I just drilled the front panel to insert the toggle to switch the mixer cap in and out.

The mixer cap is quite a subtle change, but maybe it's because of the 270k mixer resistors instead of the 470k normally used on JMPs. I still prefer it off for bass use most of the time, but with the Rick-O-Sound outputs on my 4001, the cap helps the bridge pickup sound even brighter, to compensate for the darker sound of flats. But this thread is about bass spec amps for guitar use, so I got my ES-335 out and ran into the 1966A with Greenbacks (just the regular 25w 75hz kind). First off, I got fat cleans, and all three settings (no mixer, and the bright and normal channels with the mixer) gave me tone sculpting options beyond just the tone knobs. Lead spec amps like my SV20H are a little tricky to get breakup (not roaring distortion) tones dialed in, balancing the input volumes while jumpered because of the huge bright cap, but bass spec amps get there much more easily. Higher gain settings without the mixer cap revealed the Fender Tweed heritage of the circuit, and with the mixer cap were still recognizably Marshall, just more laid-back. Then I tried all knobs full-out, treble booster into the normal channel, and found a transparent, yet gritty and honking, sound for sleazy glam riffs and responsive blues leads alike.

I haven't tried a 12AX7 yet. Maybe the tone falls apart at the highest gain settings with them? It's also worth noting the MIG-50 is for 5881 / 6L6, not EL34, use.

With an amp that can do both guitar and (vintage recording) bass, I'd love to have a speaker cabinet that can handle bass while still sounding good for guitar.

I'm not sure I understand which cap you are referring to as the "mixer cap?" Please explain and/or show it on a schematic?

Thanks,
Gene

 

[DesolationBlvd]

I'm not sure I understand which cap you are referring to as the "mixer cap?" Please explain and/or show it on a schematic?

Thanks,
Gene

The mixer resistors come after the volume pots on 5F6A-descended amps. The mixer cap is highlighted in blue on the 1986 schematic - it is in parallel with the mixer resistor for the bright channel. Compare to the 5F6A schematic, which delivers a bright channel by putting the bright capacitor over the channel's volume pot.

 

[Gene Ballzz]

The mixer resistors come after the volume pots on 5F6A-descended amps. The mixer cap is highlighted in blue on the 1986 schematic - it is in parallel with the mixer resistor for the bright channel. Compare to the 5F6A schematic, which delivers a bright channel by putting the bright capacitor over the channel's volume pot.

Thanks for that clarification. That's kinda what I expected. There are also some designs that include caps in both locations (1987X/1959SLP and others)), leaving the question of which is actually considered the "bright" cap, as they both add brightness? As you've discovered, little tweaks can make big differences!

Simply Discussin'
Gene

 

[FleshOnGear]

Thanks for that clarification. That's kinda what I expected. There are also some designs that include caps in both locations (1987X/1959SLP and others)), leaving the question of which is actually considered the "bright" cap, as they both add brightness? As you've discovered, little tweaks can make big differences!

Simply Discussin'
Gene

I tend to call the cap across the volume the bright cap and the other the mixer cap. I don’t really know the preferred nomenclature.

 

[GregM]

I bought a bass guitar speaker cab that rocks my lil jcm800 ( studio classic )
It's an Australian company , is a 4x10 cab , called lorantz . Basically reverse engineered off a greenback , they have graphs and stuff that mean nothing to me , but probably do to someone smarter .

 

[chocol8]

I tend to call the cap across the volume the bright cap and the other the mixer cap. I don’t really know the preferred nomenclature.

Same. I have always used the term "bright cap" to be a cap across a volume pot that addresses the loss of high end when turning a volume pot down.

The mixer bypass cap (or mixer cap) is serving a similar function, but it is a little different in that the resistances are fixed and it impacts the other channel.

 

[XTRXTR]

The mixer resistors come after the volume pots on 5F6A-descended amps. The mixer cap is highlighted in blue on the 1986 schematic - it is in parallel with the mixer resistor for the bright channel. Compare to the 5F6A schematic, which delivers a bright channel by putting the bright capacitor over the channel's volume pot.

The 5F6-A 1959 Bassman was made at a time when Bass playing was super clean and the pups were still weak - in general. The gain level between 12AY7 and 12AX7 required a different attenuation circuit and impedance setup for the next gain stage which is the likely difference between the 270k//100pF cap and the 470k//500pF cap versions. This is just speculation on my part.

Billboard Year-End Hot 100 singles of 1959 - Wikipedia

en.wikipedia.org

Just to make it clear for future readers, the two inputs Bright and Normal are reversed between the two drawings. One is clearly marked the other has the Bright input on the bottom. 👍

 

[DesolationBlvd]

Another surprise: With the shared cathode, the normal channel is actually usable alone. The normal channel on lead amps is almost useless by itself and really is meant to add back lows to round out lower-gain settings.

I also just learned today that having neither a bright cap nor a mixer / peaking cap yields a PA spec. I guess with the mixer cap off, the amp does sound pretty good with full-range synths.

 

[chocol8]

That's because using a mixer bypass cap allows the highs from the normal channel to pass back through around the mixer as well. This reduces channel separation and highs on the normal channel, which would be undesirable on the PA amps.