From: Shea (stmoxon@earthlink.net) Date: 12/27/2000 11:02 PM Subject: dual pot for preamp gain Here's an idea - take a high-gain preamp, like a 2204, replace the gain pot with a dual-ganged 1 meg pot, and connect each of the 1st two gain stages to one of the pots. Why do this? Preamp distortion sounds smoothest when the gain stages reach the point of compression together. I've discovered this in my amps that have a Western Electric circuit, in which I've installed a cascading gain "hot switch." The hot switch mode always sounds best when both channel volumes are on the same setting, from clean up to saturation. A dual-ganged pot could be used to achieve the same effect in a high-gain amp. No more harsh crackling, and then a JCM 800 would sound better at less than full gain. Shea From: Steve A. (steve_ahola@yahoo.com) Date: 12/28/2000 10:18 AM Subject: Re: dual pot for preamp gain Shea: So you are suggesting adding a second volume pot right on the input of the first stage which would be controlled simultaneously with the "factory" volume control going to the grid of the second stage? Just making sure that I understood you correctly... I think that such a dual control might "wimp out" a BF preamp, but with a higher gain 2204, it could be just what the doctor ordered... BTW the 2204 preamp is very similar to the Pignose G40V, the Soldano Atomic 16 and the OD channel of the Electroplex Rocket 50 so this suggestion could be applied to those amps as well (the clean sounds leave much to be desired in at least two of those amps and I can see how you suggestion could help). --Thanks! Steve Ahola From: Jay F. Date: 12/28/2000 3:27 PM Subject: Re: dual pot for preamp gain I was wondering why they did that... Check out the fender Custom Shop Tone Master. It uses the very same setup that you're talking about. A very sweet and smooth sounding amp. From: Shea (stmoxon@earthlink.net) Date: 12/28/2000 6:36 PM Subject: Re: dual pot for preamp gain Steve, I was talking about putting the pots on the output of each of the first two gain stages, not the grid. Or, from a different perspective, putting one pot on the grid of the second stage, and one on the grid of the cathode follower. Yeah, putting a pot right on the signal from the input jacks would be wimpy. Shea From: Steve A. (steve_ahola@yahoo.com) Date: 12/29/2000 2:37 AM Subject: Re: dual pot for preamp gain Shea: Maybe you can spell out exactly how you wired it up... I'm looking at the 2204 schematic from the Schematic Guy right now and between the 2nd and 3rd stage there is a 0.022uF coupling cap which goes to a 470k/470pF RC couplet which then goes both to the grid and to a 470k grid load resistor (to ground, of course). So you run the 470k/470pF couplet to the "top" of the 1M audio taper pot section, with the "bottom" going to ground and the wiper going right to the grid of the third stage? You would get more gain and distortion with the pot maxed, but you say that it cleans up nicely as you back off the pot? BTW if there is plenty of gain in your preamp, it sometimes works great to add a grid stopper after the wiper; for a 1M pot I will usually try a 100k resistor. I believe that it was Kevin O'Connor who strongly recommended the grid stopper after a pre-PI MV, and Bruce Collins mentioned that it does help preserve some of the bass response at lower settings of the MV (he confirmed that with a scope, IIRC). In my Blues Express amp, I found that a 100k grid stopper after the BF-style 1M volume control worked nicely to cut the highs (which had been a bit excessive). It is amazing what a cheap resistor can do... ;) --Thanks! Steve Ahola From: Shea (stmoxon@earthlink.net) Date: 12/29/2000 4:07 AM Subject: Re: dual pot for preamp gain I think you have the right idea. I haven't had a chance to apply this to a 2204 specifically, and I probably won't, since I don't fix amps for a living. I am pretty confident that it could be made to sound good based on my projects with other amps, like I mentioned in my original post. Once you put the pot in after the 2nd stage, you likely wouldn't need a resistor referencing the grid of the 3rd stage to ground. The pot will do that. But, you might want the grid stopper and the pot to be of the same value, to get the same level of attenuation that you get with the 2204's stock arrangement. My general theory is to bring both of the 1st two gain stages up to the compression point & clipping point at the same time, rather than to push the 1st gain stage to clipping while the 2nd stage might still be clean. You have the know-how to work out the specifics for arranging pots, resistors, and couplings, but I think what you proposed would put you on the right track. I have likewise found a grid stopper to be helpful in the preamp of one or two of my projects. In one of them, it was essential to keep the bass & gain from loading up the 2nd gain stage and gettin' all flubby. Shea From: Shea (stmoxon@earthlink.net) Date: 12/29/2000 4:14 AM Subject: Re: dual pot for preamp gain To clarify a little better, I'm thinking of connecting the top of the extra pot to the.022 coupling cap after the 2nd stage, the bottom to ground, and the wiper to the 470k / 500p pair on the grid to the 3rd stage. Take out the resistor connecting the grid of the 3rd stage to ground. It may be necessary to use a 500k pot or to change the 470k grid stopper to 1 meg, so that the pot and the resistor are of the same value. The pot & grid stopper between the 1st & 2nd stages might have to be of the same value as the ones between the 2nd & 3rd stages. Shea From: Rebel420 (giorfida@drifteramps.com) Date: 12/29/2000 5:31 AM Subject: Re: dual pot for preamp gain Similar to this: http://www.fna.muohio.edu/ax84/media/ax84_m39.pdf ? That is the schematics for the AX84 project, Hi Octane ... which is basically a single ended el84 power amp driven by a JCM800 preamp ... From: Shea (stmoxon@earthlink.net) Date: 12/29/2000 9:01 PM Subject: Now you've got it That is just what I'm talking about. On further reflection, what I'm thinking of would involve a 470k resistor to ground before the grid of the 2nd stage after all. Also, if you put a 1.5k resistor and 220 or 330uf cap in the cathode of the 1st stage, you get a nice, deep foundation to your tone, but it doesn't get muddy. The subsequent stages brighten it up and keep the mud away. If you take this schematic but replace the cathode on the 1st stage with the 1.5k / 220uf, replace the EL84 with an EL34, and use solid state rectification, then it would be very close to something I did to a red-knobbed Champ 12. It's a Marshallized Champ now, and it's one badass little practice amp. Shea From: Shea (stmoxon@earthlink.net) Date: 12/29/2000 9:07 PM Subject: & the next step I've been thinking about building an amp that has a preamp like the one we're talking about for a rhythm channel, and a blackface/silverface circuit channel for leads. I like the brightness & compression of the blackface preamp for leads, because it adds sustain. On the other hand, Marshalls & tweeds are great for rhythm, and they fill up the room. I would put a pre-pi master on the rhythm channel, but no master on the lead channel. I'm starting to think that a silverface Bassman would have the right layout of preamp tubes & knobs to do this without any cutting, and without using any mislabeled knobs. Can anyone tell me whether both channels on the SF bassmen had midrange controls, and whether they had master volumes? Shea From: steve. (scowell@fundsxpress.com) Date: 1/4/2001 1:57 AM Subject: Re: dual pot for preamp gain No bf/sf bassman head had either master volume or mid controls, to my knowledge. From: Steve A. (steve_ahola@yahoo.com) Date: 1/4/2001 2:56 AM Subject: Re: dual pot for preamp gain Steve: No bf/sf bassman head had either master volume or mid controls, to my knowledge. The later SF Bassman heads did have a master volume, but I'm not sure about the mid controls... (The Fender Field Guide site is great for settling bets.) Steve Ahola