Ceriatone Chupacabra tweaks?

Hi guys,

Was bored the other day so picked up a Chupa out of curiosity. Getting some ok sounds. but a few questions for those with some experience and a few mins:

1. Without either of the bright switches this thing is really dark, thick and compressed. Yeah I know the bright switches are there for a reason but damn, it's wooly. I thought this was a bright amp. Is this normal? Using bright switches solves some problems but creates others.

2. Anyone modded one? I had a quick look at the schematic and there a few things that don't 'look' great but I'm no expert.
- Plate resistor on first stage is huge (470k) - maybe adding to the compression and mushy feel?
- Output coupling caps are .1uF - maybe too much low end passing through?
- 820ohm/.68uF RC combo on cathode of 1st three stages. This seems like a lot of gain. Maybe changing to 2.7k on the first two would help by reducing some gain?

3. Has the loop liked many of your pedals? Hit and miss for me.

Tried a variety of guitars and cabs, and it's all pretty consistent (dark, thick). I want this thing to cut, sting and terrify small children!

Thanks.

Just my opinions

The .1uF output couplers are fine imo.
The 820//0.68uF combo doesn't have much more gain than 2k7 but does have more low end (potentially flub) and feels different. I usually hate 820ohm on that first cathode but like it fine on the last one.
470k plate is quite high and could be contributing, for me 330k is usually about right but varies person-to-person and amp-to-amp.

Maybe check for snubbers in the circuit. People often overdo it on snubbers imo.

Also check the NFB, that has a big effect on the brightness.

Change the first stage to 100k plate, 2k || 0.68uf which is a bit hotter than 2203

Second stage should be normal Marshall territory, 100k plate with 10k cathode

Third should be 100k with 820 0hm. Bypass it with a 0.68uf

Change the first coupling cap to 0.0022uF to back down the flub

The other coupling caps should be fine.

Try that and see what you think.

SpiderWars":1de37exd said:

Just my opinions

The .1uF output couplers are fine imo.
The 820//0.68uF combo doesn't have much more gain than 2k7 but does have more low end (potentially flub) and feels different. I usually hate 820ohm on that first cathode but like it fine on the last one.
470k plate is quite high and could be contributing, for me 330k is usually about right but varies person-to-person and amp-to-amp.

Maybe check for snubbers in the circuit. People often overdo it on snubbers imo.

Also check the NFB, that has a big effect on the brightness.

Click to expand...

This^^^

A bypassed 802R cathode resistor does not lead to much difference between bypassed and unbypassed gain. I like 3k3 for the first gain stage in a Jose type amp (often with 470n bypass cap). Try it.

psychodave":1v9085vb said:

SpiderWars":1v9085vb said:

Just my opinions

The .1uF output couplers are fine imo.
The 820//0.68uF combo doesn't have much more gain than 2k7 but does have more low end (potentially flub) and feels different. I usually hate 820ohm on that first cathode but like it fine on the last one.
470k plate is quite high and could be contributing, for me 330k is usually about right but varies person-to-person and amp-to-amp.

Maybe check for snubbers in the circuit. People often overdo it on snubbers imo.

Also check the NFB, that has a big effect on the brightness.

Click to expand...

This^^^

Click to expand...

Yup, change the first 2 cathode resistors to 2.7k, and drop the first plate down to 330k.
There's only 1 snubber, a .001uf to ground, so that should be OK.
The NFB is 39k off the 8ohm tap, and that could be an issue. Change the 2 cathode resistors and the plate load first, then look at raising the value of that 39k. I would also move the NFB wire to the 4 ohm tap. Just moving the wire might lower the NFB enough to get you in the ballpark. If not, then start raising the value of that resistor. My Jose mod 1987x runs 47k / 82k switchable off the 4 ohm tap.

Thanks guys, great info as always.

First up I think I had a dodgy JJ 12AX7, so after a tube swap things were sounding better already. I changed the 1st cathode resistor to 2.7k and the plate down to 330k and I'm really happy, she rips!

I suggest before getting too carried away with a lot of changes, bump the 39k NFB resister up in value. In series with this type of depth control, the value of the resistor affects the amount of edge and clarity the amp has. Try a few higher values between 68k and 100k. It will make the amp breathe. IMHO, a 39k is a huge blanket for this design.

I agree. From the 4ohm tap it's about as low as I'd go (I always use 4ohm tap). My Jose Depth pot is push-pull and goes from 39k to somewhere between 68k-82k (can't remember). I've considered increasing the 39k to 47k.

SpiderWars":ygbaxhha said:

I agree. From the 4ohm tap it's about as low as I'd go (I always use 4ohm tap). My Jose Depth pot is push-pull and goes from 39k to somewhere between 68k-82k (can't remember). I've considered increasing the 39k to 47k.

Click to expand...

On the Brad era CCVs, the 3 way dark switch let you choose between 33k/66k/99k. On the Marks, they varied, but were often higher between 68k to 125k

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