Taming low end response - cathode bypass caps?

[Martin Beer]

I have a Carlsbro CS100/8 PA amp which I use for bass. It's an EL34 based 100 watter - the schematic can be seen here
Carlsbro CS 100/8 P.A. Amp Schematic
One channel has had the James tone stack replaced with a standard Fender type stack (with fixed 10K mid resistor), but I tend to use either channel depending on which sounds better in the room. I'm happy with the sound of the amp at low volume, but when it is turned up to where it's still fairly clean but compressing a little, the low end becomes overwhelming. I end up running it with the bass control backed most of the way off just to get a balanced tone.
I'd like to reduce the response of the preamp below 100hz or so. I think I could do this by changing the 64uF cathode bypass capacitors for a lower value, possibly as low as 2.2 or 5uF. Really I'm just looking for a quick sanity check to see if this is my best option or if there are other things I should try.

[Mike Burgundy]

I'd say it's the easiest and cheapest way to experiment (these caps don't have a really high voltage rating). A little harder is coupling caps (voltage!), harder still is making global feedback frequency dependant. I think you might be surprised how low a value you need. I'd expect a PA amp to be more or less fullrange, if it's really dark there *might* be something off, depending on the rest of your rig, taste, etc.

[pdf64]

It seems strange. I would replace the power supply caps, try a new set of EL34s in there, and check the the bias was suitable before considering mods; sorry, there goes maybe £80.
Pete.

[Martin Beer]

It seems strange. I would replace the power supply caps, try a new set of EL34s in there, and check the the bias was suitable before considering mods; sorry, there goes maybe £80.
Pete.

A sensible suggestion, but these are things which have all been done quite recently on this amp, along with weeding out some out of spec resistors, so I'm sure I have a healthy platform to tweak from. I use a passive bass with flatwounds, so I suspect I have a fairly bassy signal going in and I just need a little different tone shaping than is available to me at the moment. The amp doesn't sound bad at the moment, it would be a great sound if I were playing reggae for instance, so it's more a matter of voicing to taste.

[Jazz P Bass]

I would try lowering the first stage bypass cap.
Marshall got it's 'thinness' by using a .1 uf.
And a 25uf on most vintage Fender amps will make it fart.
See where this is leading to?

[Martin Beer]

64uF does seem unusually high. One advantage of this being a PA amp with four separate input stages is that I could change the value on one stage and compare it side by side with one of the stock input stages. I may try that and report how it turns out, though I can't turn the amp up outside of a gig. It's a shame that the ampbooks.com calculator seems to be down at the moment, as that would have been useful.

[pdf64]

OK, then the most significant potential weird thing I can spot is that the typical BF Fender phase splitter input coupling cap is 1nF or 500pF , whereas that is shown as 0.22uF, >200x bigger, so maybe try reducing that.
Was the Fender tone stack channel's vol control changed to 1M? If the 100k pot was left in, the turnover frequencies may get shifted.
Pete.

[Martin Beer]

Thanks, this is useful stuff which I may not have spotted!

[Chuck H]

There may still be many more things that you OR we can't notice. For example... Did you move the volume control when you changed TS type, or are you running the TS without a volume control and leaving the volume control after the TS recovery stage??? In other words, posting a schematic of the amp you altered doesn't give us much information WRT questions about the alterations.

[pdf64]

Regards the phase splitter input cap value, a quick check seems to indicate that 500pF was used for heads (eg for use with closed back or tone ring speaker cabs), 1nF used with open back combos.
Bassman amps from AB165 onwards used a different global feedback arrangement (virtual earth), which makes the coupling cap values not directly comparible.
Pete

[Martin Beer]

There may still be many more things that you OR we can't notice. For example... Did you move the volume control when you changed TS type, or are you running the TS without a volume control and leaving the volume control after the TS recovery stage??? In other words, posting a schematic of the amp you altered doesn't give us much information WRT questions about the alterations.

Most of the work on the amp has just been replacing tired parts (filter caps, power valves etc) with like-for-like values and setting the bias. The only real modification of the amp to date is the Fender type tonestack on the first input channel. This was simply inserted in place of one of the original James stacks, after the 0.1uF V1a coupling cap and followed by the original 100k channel volume control and 220k mixing resistor. Looking at the Duncan Tone Stack Calculator, the 100k load loses a few dB of signal compared the 1M pot usually used after a Fender TS, but does not alter the frequency response of the stack significantly. If I plug into one of the other three input channels, the amp is entirely as per the stock values shown in the schematic, and the excess low end I mentioned in the first post is evident whichever channel I use.
My plan of attack at the moment is to experiment with the value of the first stage cathode bypass caps, and then look at the coupling caps if I am not happy with the results.

[Chuck H]

IMHO altering the first stage bypass cap in BF/Marshall/Vox type stacks can greatly alter the adjustment and ratio of the bass and even mid controls. Again, JMHO, but "I" would sooner change the second stage bypass cap and/or decrease the value of a later coupling cap and/or bypass cap and/or decrease the value of the PI input cap.

[Martin Beer]

IMHO altering the first stage bypass cap in BF/Marshall/Vox type stacks can greatly alter the adjustment and ratio of the bass and even mid controls. Again, JMHO, but "I" would sooner change the second stage bypass cap and/or decrease the value of a later coupling cap and/or bypass cap and/or decrease the value of the PI input cap.

This morning I changed the second stage bypass cap for 2.2uF, which does sound like it has cut some of the low stuff without going too thin. The second stage is shared for all channels, so this affects the stock James stack channels and the modded Fender stack channel.
Unfortunately, I can't turn the amp up to stage volume where I live so I'll have to wait until I gig the amp before I know if this change has done enough, but I'm hopeful it will have helped.