Pre-amp distortion, which gives more 'grainy' sound?

[tubeswell]

having twiddled with my amp ad-nauseum for the last week, I have a burning question I would like some opinions on.

At the suggestion of a certain well-known amp guru, I dumped the Cathode Follower mod in V2 of my Classic 30, in the quest for more grainy/growly distortion. (like the 69 zep sort of sound, withut too much sustain. Not a metallic sound but - more classic rock/bluesy sound I'm after)

Setup at the moment is

(V2A - 1.5k grid stopper (this isn't in the stock circuit), 1.6k Cathode Resistor, 22uF Cathode Bypass Cap, 150k Plate Resistor, .001 Coupling Cap, 470k attentuator to V2B with 470k Grid leak making the voltage divider; V2B 910 Ohm Cathode Resistor, 100k Plate Resistor, 1M attentuator and ".022 uF" coupling cap into VR3) gives a real smooth full-on high-gain/ sustainy sounding distortion when the gain pot is let loose, even when I put a 12AT7 in V2. I've decided its too much, and I want more grainy/growly harsher sound.

I realise its a bit of a complicated equation, but for arguments sake, assuming I'm only changing one part at a time, if I wanted less smoothness, and more grainyness, which bits of the circuit have generally which effect?

For example - would lowering or removing the Grid Stopper to V2A make it less smooth/more grainy? (i.e. does increasing the attentuation here typically make the distortion in V2 even smoother?); or

would a lower V2A Plate resistor (Say to 100k) make the distortion more grainy?; or

increasing the V2A Cathode Resistor to say 2.2k?

or dumping the V2A cathode bypass cap?

OR

Do I have to play around similarly with V2B?

OR

is it more to do with the relationship of the following gain stage (i.e. the padding between the plate of V2A and the grid of V2B) that impacts on the smoothness or otherwise of the distortion?

That is to say, which part of which gain stage would have more influence over the smoothness versus the graininess/hardness of the distortion?

Any conflab welcome please (Before I open my amp up again)

[Satamax]

69 zep? Say sound city!

[tubeswell]

Well, thank you Satamax. I'm not exactly getting alot of replies. Maybe I'll have to talk to myself some more...

It occurs to me after much slow and ponderous thinking that maybe it is in the relationship between gain stages. So if I made the earlier stage really gainy, and the second stage noticeably less so, would I get more 'grainy' gain, (or is that too much of a brainy painy)?

[Satamax]

Try putting one to three megs to ground on input instead of 470K! May be get rid of the caps too, thta makes a frequency filter. And change the preamp's 1K5 cathode resistors for 820 ohms.

[tubeswell]

Try putting one to three megs to ground on input instead of 470K!

Out of curiousity, what would be the maximum safe value for this 'grid leak resistor' (even though the 470k is technically part of a voltage divder) - I typically swap between a 12AT7, 5751 and 12AX7 in the V2 position.

May be get rid of the caps too, thta makes a frequency filter.

Which caps? Don't I still need the coupling caps (to block DC entering the grid of the next gain stage)?

And change the preamp's 1K5 cathode resistors for 820 ohms.

Both V2A and V2B, or just V2A? I was wondering whether holding V2B at a higher value (say 4.7k) would help get a more grainy sound?

[Amp Kat]

I think if you used the Soldano an SLO 100 Values for the Cathode bypass and resistor network mainly the first stage you will get more growl. The first stage pretty much sets the voicing. Like a Fender is set to get a clean tone and a Marshall gets that bright tone so the Soldano IMO has a grainier first stage which gives it that growl. Of course doing different networks to the following stages may have an impact as well but I think you really get that vibe from the first stage change. The Topology is different also on an SLO which also has a lot to do with it.

Cajun

[tubeswell]

Thanks Cajun

I see the SLO100 has a 470k grid stop to V2A, I take it your suggestion involves removing R 9 and C11 from the C30 circuit?

I see also the voltage divider going to the grid V2B has a 1M to ground after the 470k attentuator, and then VR1 (Is that a tone control?) going to ground, also before the grid of V2B. Whereas the C30 has the tone of this part of the signal modfied by C1 (and of course C2). How should I compensate for the tone here (if i can't put in a VR - whatever that is)?

[cbarrow7625]

try this

If i was to change only one component in that setup it would be the 910 Ohm cathode resistor. Increase it. 1.5K-10K Ohms, no bypass.

Second, change the coupling cap from .001 to something slightly larger, maybe .0047. More low frequencies will usually help reduce the 'sustainy' hi-gain sound to something a little crunchier. I don't think that circuit will be very happy with too large a coupling cap, it will get farty.

Next, I would change the input cathode resistor to something like 2K-3.3K Ohms. Less gain & more grain. You may want to reduce the size of the cathode cap here as well to complement the increased coupling cap in #2 above. in the neighborhood of 5-10uF, maybe less if that is your taste.

The last thing I might change, depending on the results above, would be to change the 150k plate resistor to 100k. However, that may just plain reduce gain too much. That's more of a 'season-to-taste' kind of change.

Just one guy's thought progression.

-Chris Barrow

[mykey]

For example - would lowering or removing the Grid Stopper to V2A make it less smooth/more grainy? (i.e. does increasing the attentuation here typically make the distortion in V2 even smoother?); or

no, the grid stopper wont make the difference

would a lower V2A Plate resistor (Say to 100k) make the distortion more grainy?

no, lowering plate resistor= less gain

increasing the V2A Cathode Resistor to say 2.2k?

no, decrease= more gain, try 680 ohm.

or dumping the V2A cathode bypass cap?

no, don't dump- bypass cap = more gain and removing = less gain

Do I have to play around similarly with V2B?

yes

is it more to do with the relationship of the following gain stage (i.e. the padding between the plate of V2A and the grid of V2B) that impacts on the smoothness or otherwise of the distortion?

That is to say, which part of which gain stage would have more influence over the smoothness versus the graininess/hardness of the distortion?

the cathode resistor and adding bypass cap
the type of tubes you are using

Any conflab welcome please (Before I open my amp up again)

change all preamp tubes to Chinese 12AX7 /AT7 Sino brand also sold as 12AX7CH by groove tubes, you will notice instant improvement. More gain, like night and day difference. enjoy grinding.

[Rob Mercure]

Tubeswell,

This probably won't help your immediate issues - I have trouble visualizing your mods w/o a schematic in front of me - but the LZ studio stuff was mostly recorded on some sort of little funky amp like a Supro or such. I can't remember which but the information is widely available on the net. And this explains much about how very different the guitar sound is on the live recordings - crunchy but lacking that smoothness. Even in those "primitive" days of the late '60s much of the studio sounds were "manufactured" and not really possible live. Find a few live recordings of some of the numbers you like and compare your sound to them - you may have already achieved your goals.

Rob

[mykey]

Tubeswell,

This probably won't help your immediate issues - I have trouble visualizing your mods w/o a schematic in front of me - but the LZ studio stuff was mostly recorded on some sort of little funky amp like a Supro or such. I can't remember which but the information is widely available on the net. And this explains much about how very different the guitar sound is on the live recordings - crunchy but lacking that smoothness. Even in those "primitive" days of the late '60s much of the studio sounds were "manufactured" and not really possible live. Find a few live recordings of some of the numbers you like and compare your sound to them - you may have already achieved your goals.

Rob

most Zeppelin was recorded on a fender champ, or according to Paige speaking to interviewer from Guitar Player magazine, also used an echo-plex.
I believe it, there is nothing on those albums that sounds like any more than that. As usual, its the talent of the guitar player and not the gear that makes the sound. I have oft said that you can buy any effect pedal and spend any amount of money, and it won't make you sound any better, or make you a better player. Of course this has pissed off some pedal manufacturers, lol!
PS it helps if you have a Les Paul, but Paige used a Dan Electro like the ones sold by Sears.
I gave you some tips here because I've modded amps for 34 years, and you want to keep the gain on the high side to get the sound you are seeking.
and it helps a lot to use the higher gain (Chinese) tubes too.

[tubeswell]

I think if you used the Soldano an SLO 100 Values for the Cathode bypass and resistor network mainly the first stage you will get more growl.
Cajun

FWIW, I did the SLO100 mods to the O/D channel of my C30 last weekend and its definitely got more grind/grain. I replaced C4 with about .002uF in parallel with a 470k resistor. I dumped C7 and R9 and put a 470k grid stopper at the grid pin of V2A. I changed the Cathode resistor on V2A to 1.8k bypassed with a 1uF cap, and changed the cathode resistor to V2b with a 39K resistor (unbypassed). I dumped C1 and put the V2A and V2B plate resistors at 220k and raised the B+ to about 300V I think (I put a 1W6.8k supply resistor in R60), and put R59 back to 1W10k. I might try playing around with the B+ a bit more - perhaps lowering it and raising the B++ a bit (maybe with 8.2K in each respective supply resistor). I have left a 10k in R13 , but in series with a 47pF cap in place of C16, but I might dump these bits still.

I initially changed the cathode resistor and bypass cap at V1A to 1.8k and 1uF respectively, and this works well with the OD channel engaged, but didn't sound as good as the 1.5k/22uF combo did on the clean channel (whereas the latter made the O/D channel too muddy). So I put in a switchable 1uF/22uF pair of bypass caps grounded at the -ve end via a 2.2M resistor (to get the fender clean sound when I want it), and left the 1.8k cathode resistor in there. Overall I was pretty happy with it at practice tonite, but I might try a couple of different tubes in V2 now - the 12AX7 is just a bit too powerful.

Thanks for the suggestion and advice Cajun.

[Amp Kat]

Yes that was the only thing I was kinda worried about. You may try a 5751,12AT7 or we may have to go in and change the 220k plate resistors back to 100k's.

[dbtk44]

Page used a Supro amp ( nobody seems to remember exactly which model ) and a Telecaster for LZ I, plus i THINK a Tonebender.

LZ II was Les Paul and Telecaster mostly thru a Marshall, think the Danelectro on a song, and maybe the Tonebender thrown in somewhere

IMHO I think a lot of the grainy stuff came from the Tonebender and less so from the amps he used.