What to do with 50 watt 6L6/EL34 build

[Beerzgood]

Hey folks, my 1st post on this forum. I have built 3 amps, with the last about 10 years ago. 2 SE amps based on AX84 designs, and an old Carvin X100B chassis modded to a 50 watt JCM800 based design. I lost a good job and for awhile I was doing tube amp work for side money. That was great because I was able to play a lot of really nice commercial amps. Then I damaged my left hand and kind of went on a guitar hiatus for some years. But I'm back and have learned to play fairly well again. Mostly acoustic, but I still play electric, mostly with a headphone rig. I have 2 of my amps, and my son has the other one. I have 1 working amp, the 50 watt, but it needs work. So I am planning a rebuild and looking for ideas.

The build as I mentioned is JCM800 based with a lot of tone mods, switches etc. The OT was from a 6L6 100 watt which isn't good for pairs of 6L6 or EL34s. So I'll be replacing the OT. Going to use a 4.2K Classic Tone transformer. That alone may make a big improvement. One result of the mismatch is NFB controls don't work as well as they should. Like some builders tend to do I have too much going on with it. For instance, I have a pre PI master, a post PI master, and I installed power scaling. I know it's ridiculous, but other than the new OT, the rest of the power section will remain the same for now.

Looking to rebuild for a lower gain preamp that cleans up well but can still make some dirt. I play old school rock and blues. I like to hear the strings. Occasionally I'll play metal but I'll be using pedals to get there. The JCM800 circuit doesn't clean up too well. That said I have only the high gain input and haven't tried adding in the low. Any suggestions on a preamp? Dumble or Trainwreck designs? Or just stick with the proven Fender based preamp? Back in the day I was happy to say my only effect was the cable. I'm past that, now I want all of the great sounds you can get from pedals so I need an amp for that. The amp doesn't sound bad at all, just more dirty than I would like.

I have attached a schematic of the amp. It's close but I was sometimes making daily changes just trying stuff. I have some mods in the amp not shown but those are just voicing mods.

Thanks,

Steve K from Sparks NV

[sluckey]

You could convert the 2204 preamp to a 1987 Plexi input. Or, here's a simple switch mod you could do on your amp to allow you to select high gain or low gain, just like the real 2204...

[xtian]

Welcome to TAG!

I've modded three Carvin X100 amps. They have great USA transformers. There is NO reason you can't use two 6L6GCs for ~50 watts with that OT, and have it sound just as good. You will just match the load. For example, plug an 8-ohm cab into the 4-ohm tap. Here's an article about this: https://www.premierguitar.com/articles/ ... duce-power

[Beerzgood]

Thanks for the suggestions guys. The output tap labeled 8 ohm on my schematic is the original 4 ohm tap. Really my only beef with the OT is the presence not working well. It works but it's far more subtle than most amps I have played that have presence controls. I figure I'm never going to make this into a 100 watt so may as well put a better match in there. Not sure of the quality of the original X100B transformers. Mine is pretty old, bought it new in 1982. It is possible I will just be wasting money.

I have been considering the Plexi circuit. One thing I have thought about is a 3 channel Plexi, with low, mid, and treble gains to mix into the CF stage, with a MV of some kind. I tried a Plexi in one of my SE amps as a 2nd channel. Haven't tried it in the 50 watt. All the amp needs right now is a new gain pot and less gain. I have a Fulltone OCD for overdrive. I am intrigued by the Dumbles that look to be using a Fender based stack off the 1st stage and a later stack more like the Marshall. Just looking. I have been out of it for a long while so I'm getting refreshed. I see AX84 is gone, I used to be fairly active on that forum. I liked the Hi Octane, my other amps are based off that circuit with KT88 SE outputs. After the 50 watt, I will be converting my dual channel to a PP 6V6 with one channel setup like a Deluxe reverb and the other a higher gain circuit for preamp distortion.

Steve

[AgAuthority]

Some real quick and dirty ideas without changing the circuit completely

--Gain pot from 1M to 500K(Did this on a 100W build of mine years ago and liked it--took a little fizz out but with amp cranked, cleaned up nice rolling the guitar volume pot but still had some balls) Can solder a 1M across the outside lugs of the pot as a quick test

--V1B cathode back to 10K and go up from there--15K is a number heard suggested before

--Drop the value of the 470K to ground before V2A

--Want to get real crazy could try some split-loading on either one of V1A or V1B

--Could just eliminate V1B and try plexi circuit

--Low input isn't a horrible plan either

Ag

[MakerDP]

You mentioned you have built some AX84 designs and are considering something Plexi-ish... have you considered the AX84 October with it's adjustable NFB pot? Use the switch to go between JTM/Plexi and you have all that you are after I think. Just throw the AX84 Crunch Preamp on there with the above mentioned switch and add the NFB pot from the AX84 Stage schematic and you are there!

[JD0x0]

I could be wrong, but I've always felt, IME, amps with cold clipper stages don't clean up quite as well as amps using more 'typical' cathode bias values. The one's I've played always seemed to have a bit of 'hair' on the sound. Yes, a hotter bias will technically increase gain a bit, but if you're moving the load line more towards the center of the tube's operating point, you can increase headroom, and you could always adjust gain with voltage dividers. I'd try 1k-2.7k on the second triode. (1k is similar to what Hiwatt did on the 'Lead' series amps. 2.7k seems to be Friedman's choice)

I feel like IME, a higher B+ makes amps clean up a bit better, even with lots of gain. Maybe reduce the B+ rail dropping resistors a bit. Personally, I've found a B+ around 300VDC-400VDC on the preamp can give a ton of dynamics and help clean up with backing off on picking dynamics or the guitar's volume. I also think this will help with 'hearing the strings' typically, I've found as B+ gets lower the sound tends to get more 'Brown', compressed, 'warm' and mushy, while increasing B+ tends to sound more dynamic, with more punch and open and articulate highs, with less 'blur'.

I see you already have Mosfet driven 'VVR' for power scaling, you could also run the same thing on the Preamp B+ rail to control the amp's behavior. Scale back the preamp voltage for a more 'brown' sound with more preamp clipping, or leave the VVR maxed out for more headroom and dynamics.

[Beerzgood]

Hey guys I've been working so kind of got pulled away from amp obsession. But I'm back. I think you're right on the cold clipper not cleaning up as well as a typical cathode bias values. I ran my amp with a switch for awhile that gave me the standard 10K and something else. I tried a lot of values but settled on 6.8K. The distortion sounded better for my ears. Funny how Soldano went all the way to 39k. Tried that too. And then Mesa and Peavey stole the circuit. Kind of rotten but that's business.

I have considered using the power scaling for preamp voltage control. It's a great idea. I tried several preamp drops and settled on what's in there now. I agree that higher preamp voltage is more clean and clear. Lower voltage definitely gets more brown as you put it. Maybe something to have on a switch high and lower voltage setting. It's subtle but there's a difference. I didn't do this myself but some people scale the whole amp.

I'll have another look at the October. I don't have much experience with the Plexi circuit. I played Mojave Amps Coyote. Think it was a Coyote, but it's been awhile. As far as I know it was basically a jumpered Plexi with the 10k pot in the PI bias. I did try the 10K pot in the PI, think it was a reverse log taper. Didn't much like it but it's all subjective.

Having fun thinking about this stuff again. Just have to dig out my parts and get it on the bench. Probably will be soon. Thanks for the ideas!

[dorrisant]

I built a JCM800 50 watter a while back... turned out fantastic! I'm gonna build it again with 6V6s in a Hammond AO-43 chassis.

My build log was here: https://ampgarage.com/forum/viewtopic.php?f=9&t=30633

Here is a link to Dave Os original post: https://ampgarage.com/forum/viewtopic.p ... sc&start=0

I strongly suggest reading that last post, there is a ton of good info there!

Welcome to the forum Beerz!!

Btw, nice to see the scaling in your schematic. I have been wondering about that one from London for a while.

[Beerzgood]

I'll check it out. Most likely will be trying a different circuit, but I like reading this stuff. When I built the amp I was doing a lot of reading, particularly Tone Lizards JCM800 mods and info.

That's an old version of the London Power scaling. From the dates on my drawings this amp was built in 2008. I looked at their page recently, the current version has more transistors/mosfets it appears. But be doing basically the same thing. I like it and I don't. All the way up seems to be transparent. Dialing it way down makes it easy to overdrive the power amp. But I don't much like it. Usually set about 75% where the preamp distortion seems to work better with the power section. Might be an entirely different thing with a clean preamp. Had the opportunity to play a Suhr amp at the time and the scaling worked great on that. Better than on mine.

[Beerzgood]

I got this amp on the bench yesterday. Tried fattening it up with larger bypass cap 1st stage from .68 to 2.2uf. Also soldered a 1M across the volume pot to drop overall gain. And warmed up bias on 1st 2 stages. Still too much gain. So next step is convert to the 1987 circuit. Do most of you running this circuit jumper the channels? Guess I'll put that on a switch and see for myself. Most likely today as I'm temporarily unemployed.

I would be interested in hearing about any tone shaping mods to this circuit.

For the OT, my original drawing for the donor chassis (Carvin X100B circa 1982) didn't show OT wiring colors. Found a more recent drawing showing 4 ohm tap as green and 8 as yellow. I had the yellow hooked up thinking it was 8 ohms. Switched to the green which ohms slightly lower than the yellow, and I have my presence control working like gangbusters now. So, probably won't buy a new OT.

[Beerzgood]

Well I turned the preamp into the 1987 circuit. What a beast! I kept my preamp master. Not sure how you can run these without it. Loud and clean but will break up when pushed for sure.

So if anyone has proven mods for taming down the bright channel I want to hear them. Serious ice pick. That .005uf bright cap is maybe too much.

Best wishes in these troubled times!

[thetragichero]

read this on the Marshall forum and tried it on a build: change the 470pf call on the treble peaker to 2.2nf. of my calculations are correct that lets everything but frequencies below 154hz pass instead of the stock 740hz
easy enough to test out too. just tack solder a 2n2 cap on place and see how you like it

[Beerzgood]

I'm new to this circuit so still feeling it out. Since the amp already had some switches installed, pull switches on pots etc, I have both the bright cap and peaker on switches right now. The bright cap dominates especially at the low volume settings I'm running. Since I am looking to run clean or slightly pushed, I'm thinking of adding 100k or better in series with the bright to reduce the bypass effect. I'll give the larger peaker a try. Thanks man!