Pedal interaction with amps

[yurka]

Iv been wondering what makes an amp circuit more pedal friendly. I built a DR based amp with no vibrato that sounds great, the dynamics and sensitivity is everything i want out of the amp. My only caveat is that it absolutely hates every single dirt/fuzz box i place in front of it.

Amp is based on the schematic below, with a few minor changes.

http://home.comcast.net/%7Edaniel.soude ... tic_V3.pdf

[Firestorm]

Hypothetical, but I have to believe it's gain related. I have a 66 DR that loves single coils in the #1 input, but will take humbuckers only in #2. Haven't thought to try P90s, but I'm sure it hates them even more.

[Andy Le Blanc]

That's Impedance matching, Alot of pedals have a relatively low to med impedance of around 1k.
Tubes in general like to see a med to high input impedance.

[10thTx]

I am wondering if you changed each of the gain stage 22uf cathode caps to 4.7uf, IF the amp would take pedals better?

I like the way my amps sound with a modded zenith drive and another OD pedal I have using 4.7uf and even 2.2uf caps.

I quit using 22uf caps for cathode caps some yrs ago. I feel like they add way too much grit to the tone I am looking for.

Just a thought.

With respect, 10thtx

[vibratoking]

Deleted

[iknowjohnny]

What you could do if you want to really know is ask what amps take pedals extraordinarily well and which are bad, then look for a common difference. That will likely tell you more than anyone can.

[Tillydog]

Iv been wondering what makes an amp circuit more pedal friendly...

I think it has a lot to do with where the first triode is biased. The schematic has 1.5k cathode resistor / 100k load resistor on the 1st triode which at a rough guess gives a roughly central operating point (unless you have some outlandish preamp voltages). No problem for guitar, but if you hit it with a boost, it sounds bad (based on my experience using the same values).

I've formed the opinion that the warm bias (typically 820R cathode resistor on a 12AX7 with 100k anode resistor) is key to 'pedal friendliness'. I dont believe it makes any difference for the guitar directly into the amp (EMGs, etc. probably excepted). The trouble is, it just looks to everyone like you copied Jim Marshall!

It's easy enough to try - just tack a 1.8k resistor across the 1.5k cathode resistor on that first stage.

[HeeBGB]

I know I am not a fan of most dirt boxes. These days I use a Geek triple B for dirt but most of the stuff I play (basic blues and classic rock) I cam get what I need using the guitar volume a clean boost and a cranked 15-18 watt amp. Sometimes it is too loud and I use an airbrake to attenuate.

Have you tried a clean boost?

[yurka]

The amp does well when it is being pushed with clean boosts. Surprisingly enough, the DR takes the Zoom G5/G3 multi-effects amp sims really really well. I have tried throwing OCDs, Timmys, skreddy fuzz pedals, and most of them sound up sounding raspy, nasaly, over processed.

[Cliff Schecht]

You are probably hearing the amp start to fart out if you followed that schematic verbatim. The biggest problem I see right away are large coupling caps after the reverb recovery stage (0.1uF) and after the PI (again 0.1uF). You can try reducing these (start with 0.022uF-ish as a start) and see if reducing the bass will make the amp more high gain pedal friendly. When you throw too much distorted bass at these amps they really crap out (literally getting farty) unless they are designed to handle this.

[pdf64]

The biggest problem I see right away are large coupling caps after ... the PI (again 0.1uF)

Yes, 0.1uF feeding 220k at the power tube grids seems too great a time constant.
I prefer 0.047uF and reducing the grid leaks to 100k.
That has the additional benefit of reducing the max voltage swing from the phase splitter, thereby reducing blocking distortion from another angle.
A Deluxe Reverb, using 6V6 tubes, will be more prone to this than 6L6 amps, due to the lower bias voltage (because the power tubes will enter grid conduction at lower signal levels).
Pete

[yurka]

Sounds great. Time to whip out the soldering iron. Thanks gentlemen.

[pdf64]

The other key thing is the speaker being used - what model is it?
The C12K that Fender fit as stock is probably one of the best choices as it has a smooth top end, lots of bass and has high sensitivity.
Pete

[Firestorm]

The biggest problem I see right away are large coupling caps after ... the PI (again 0.1uF)

Yes, 0.1uF feeding 220k at the power tube grids seems too great a time constant.
I prefer 0.047uF and reducing the grid leaks to 100k.
That has the additional benefit of reducing the max voltage swing from the phase splitter, thereby reducing blocking distortion from another angle.
A Deluxe Reverb, using 6V6 tubes, will be more prone to this than 6L6 amps, due to the lower bias voltage (because the power tubes will enter grid conduction at lower signal levels).
Pete

I like this idea (provided the undesirable effects are power amp related). But maybe tweaking the grid loads/leaks/return resistors AND the coupling caps at the same time is "unscientific." The time constant of a 220k with a 0.047 cap is the same as with a 100k and a 0.1 cap so maybe fiddle with one value first.

[yurka]

Iv tried a wide range of speakers. Iv had a cannabis rex for a while, and currently switched to a Tone Tubby Alnico.