Mod'ing organ amp for guitar:

[Rafael]

Here's a schematic for an organ amp I'm mod'ing for use as uber-sweet guitar amp. I hacked out the existing verb send/receive section then hacked the return section into the amp [it's the second 12AX7 section from left to right] and threw in a 1M volume control where you see it.

The thing sounds great, but needs a little polishing. First though, does anyone see any obvious errors in values from the hack? When turned past 10oclock the volume control gets a little flaky: add a fixed resistor in series with the wiper? The tone looses highs as the volume goes up: any ideas? I'd like to add a tone control: again, tips?

Thanks! This is my first hack of an amp and I must say, I ___love_ working with tubes!

EDIT ALERT: Note I have upgraded the image file for the schematic.

[Rafael]

There should be a 100k resistor to +275 at the inverter plate. Sorry for the error.

[PRNDL]

Originally Posted by Rafael

When turned past 10oclock the volume control gets a little flakyThe tone looses highs as the volume goes up: any ideas?

The volume is interacting with the cap for the following stage that connects to the feedback.
The easiest mod is to move the volume back one stage.

The simplest idea would be to follow a well-known circuit, such as the 5e3 tweed deluxe or Trainwreck Blues Express. Both use the cathodyne and a similar circuit, although they don't use negative feedback.

I'd try clipping the cap "C" and the .0012 from the grid to cathode of the second two gain stages. That will open up the high end, which might cause oscillation problems. Most likely the amp will sound more alive.

Also change the 470K input resistor to a 1M/68K combo that is more suited for guitar pickups.

You could also simplify the cathode biasing of the second stage, but I'd start with those.

[cbarrow7625]

I agree with moving the volume control ack one stage. I would clip out and/or tweak the values of all of those caps that are in place to bleed of highs (the ones pointed out before & the plate cap around the 3rd stage). If the highs are too much start adding some back. You might want to look at removing the .001 / 470k combo across the output tubes. Some people (and some amps) like this, its not usually my cup of tea. Try it and see what happens, it won't hurt anything.

Where is the grid resistor for the second stage? I see a series 1K resistor but no grid resistor to ground.

Chris

[Rafael]

Originally Posted by cbarrow7625

I agree with moving the volume control ack one stage. I would clip out and/or tweak the values of all of those caps that are in place to bleed of highs (the ones pointed out before & the plate cap around the 3rd stage). If the highs are too much start adding some back. You might want to look at removing the .001 / 470k combo across the output tubes. Some people (and some amps) like this, its not usually my cup of tea. Try it and see what happens, it won't hurt anything.

Where is the grid resistor for the second stage? I see a series 1K resistor but no grid resistor to ground.

Chris

Let's work backwards from your post: I left the grid resistor off the schematic [I like how I can edit the attachment of my post on this forum: Kudos to the hosts!] it should have been a 47k between the .02 cap and the 1k resistor.

I'm figuring I'd replace that resistor with a pot to move the volume up to this gain stage?

Losing the grid bypass caps [do I have that nomenclature correct?] should be an easy enough mod. This thing came from an organ so I'm assuming stability is king from their point of view. Me, I want raunchy gain...As for the output cap/bleeder combo: w/ such a small value, is this for hypersonic cutoff?

Thanks for the info....

[Rafael]

Originally Posted by PRNDL

The simplest idea would be to follow a well-known circuit, such as the 5e3 tweed deluxe or Trainwreck Blues Express. Both use the cathodyne and a similar circuit, although they don't use negative feedback.

%< snip!

You could also simplify the cathode biasing of the second stage, but I'd start with those.

I'll pull out my copy of The Tube Amp Book 4.1th Edition and check into the 5e3 deluxe.

I'm assuming the biasing of the second stage is due to it's original use as the verb amp? The driver was extra-special quiet design as well [the only b+ w/ a choke, etc.].

But first, I'll throw a pot between the 1st two stages and try that along w/ losing the bypass caps. Thanks!

[dobsont]

What did this thing start as? I've got a reverb/power chassis out of an a-100. Looks like fun -- there's not enough good info out there on these conversions -- so good job!

[Rafael]

Originally Posted by dobsont

What did this thing start as? I've got a reverb/power chassis out of an a-100. Looks like fun -- there's not enough good info out there on these conversions -- so good job!

This was originally the amp from a Hammond Organ [I'm not sure of the model] but the amp, the verb tank and a couple of other parts was salvaged by a friend of mine who recently turned me on to some of the clutter in his back storage area.

Unlike some other setups I've seen this one started as a pretty clean preamp, verb send/receive and power amp on one chassis. The power xformer is probably oversized due to the need to drive the tone modules etc. but everything else is pretty conducive to guitar amp conversion. With very little knowledge I was able to quickly patch in an additional gain stage and make this thing a little screamer. Now I'm working on tweaks as the platform is 90% there out of the box. I should take pictures and do a how-to when I'm done.

[dobsont]

Does it look like this one (in the middle, on the floor -- the only pic of it I have handy).

Sounds a lot like mine -- and my plan for mine. Did you keep the reverb on your modded version?
I've been driving mine with a Rangemaster clone but I'd like to add another gain stage -- keep it relatively clean, and keep the reverb because it is absolutely wild. I'm hoping that with the sparkly el84s and monster reverb section I'll be able to build a cool little Surf and Twang amp that's under 20w.

[Rafael]

Originally Posted by dobsont

Does it look like this one (in the middle, on the floor -- the only pic of it I have handy).

Sounds a lot like mine -- and my plan for mine. Did you keep the reverb on your modded version?
I've been driving mine with a Rangemaster clone but I'd like to add another gain stage -- keep it relatively clean, and keep the reverb because it is absolutely wild. I'm hoping that with the sparkly el84s and monster reverb section I'll be able to build a cool little Surf and Twang amp that's under 20w.

Yep, mine looks identical to yours. I don't ever use reverb on guitar and wanted to simplify things so I re-used some of the verb circuitry to add another gain stage. I'm considering rewiring the left-most 12BH7 socket to be a fire-fly style cascode boost stage.

So far I dumped some bypass caps and changed the input to a fender style 1m/68k and it sounds great: brighter than stock and more gain.

[dobsont]

Dang. I was hoping you'd kept the reverb...

I'd like to add another tube - like both sides of a 12ay7 or something right in front of everything, and then leave the reverb as is. I'm pretty sure that PT can handle it -- particularly since it also fed a preamp chassis in its original configuration (thats IIRC, though). I've got a proper schematic for the whole organ around here somewhere.

[Rafael]

Originally Posted by dobsont

Dang. I was hoping you'd kept the reverb...

I'd like to add another tube - like both sides of a 12ay7 or something right in front of everything, and then leave the reverb as is. I'm pretty sure that PT can handle it -- particularly since it also fed a preamp chassis in its original configuration (thats IIRC, though). I've got a proper schematic for the whole organ around here somewhere.

I don't see any reason why you couldn't keep the verb...check out the firefly amp over at the AX84 web site for a switchable boost stage using both sides of a twin pentode. The chassis is getting a little tight as it is so I decided against adding additional circuitry to the unit, but you are right: the power xformer is plenty big enough to handle anything you want to throw at it as it did originally power the entire organ. That's the one thing about this conversion: the PT is HUGE in comparison to what you really need. Good luck.

[Rafael]

Here is the latest version of the amp schematic. Not sure why but adding the first level control seems to have further reduced highs? What can I do to kick up the highs?

[ajchance]

Newbie here - I just finished my first amp build with the AX84 High Output schematic using an old Hammond power amp as my starting point. I ran across this thread searching for ideas for the L-100 reverb amp I got in the mail yesterday. (This is an addictive hobby!) I had ideas of gutting and starting from scratch with a Marshall 18 watt clone, but your tinkering has me interested. I'll stay posted! Here's a website I found with a Hammond L-100 manual if it'll help:
http://dl.analoghell.com/index.cfm?id=1

[TimothyLee]

I was given one yesterday. Mine came from an early Hammond M100. Photos and a schematic are on my DIY amp blog accessible from my web site. I will likely modify this amp instead of gutting it. I will post back when I do this, but it could be some time. I am first trying to build a Fender Champ 5E1.

http://home.fuse.net/phoeflich/music

[Gtr_tech]

Originally Posted by Rafael

This was originally the amp from a Hammond Organ [I'm not sure of the model] but the amp, the verb tank and a couple of other parts was salvaged by a friend of mine who recently turned me on to some of the clutter in his back storage area.

Thats the amp chassis from an L-100 Hammond or similar. I have several of those in my junkpile.