Custom Amp Build Tweaking Help

[Megachunk]

Hi Guys.

Been tweaking and tweaking my amp build that Tubenit helped me with. I realize now I like the tone of my JCM800 as I went back to it repeatedly. I used it on the new recording and it sounded great. Now I have the tone stack as a JCM 2204, but left most of the rest the same; I only have 2 preamp tubes and I kinda wired it to bypass, or not include V2. I realize that the gain will be less in this, and have been told to add a second preamp tube. Space is a big limitation, or I'd do it in a snap. I was wondering if there's somthing I can do to increase gain without adding another tube. Would it be easier, since it's around 30-35 watts, make a Bluesbreaker type preamp? There is no rect tube on this amp.

Let me know if you need any more info. I dont have a schematic or layout, but could make one in a few days. It's a JCM800 as close as it can get with only 2 preamp tubes.

Mike G.

[Geezer]

I think we need more info on what you've got....look thru the schematics here on the site ( http://www.el34world.com/charts/Schematics2.php ) & see which JCM800/2204 is closest to yours.

I know that one version had a "low" input that bypassed the 1st triode, then had 2x gain stages, then a cathode follower into the tonestack. The "high" input added another gain stage in front.

Do you have the V1 of that circuit (2x gainstages) going directly into the tonestack (deleting the 3rd gainstage AND the cathode follower)??

What are the values of the V1A and V1B cathode resistors & bypass caps? adjusting those could possibly get you a bit more gain, depending on what you actually have in there already......

[sluckey]

The high gain 2204 only uses 2 preamp tubes. Are you saying you need even more gain? Or,,, are you saying you only have 2 little tubes, one for the preamp and one for the PI?

Either way, there are a couple things to do to get some more gain. You can use smaller cathode resistors. Also, use a smaller value for the first resistor in the voltage dividers between stages. I can't really get specific without seeing a specific schematic.

[Megachunk]

Thanks guys.

I pretty much figured I'd need to make something. I do have 2 12AX7's, one for preamp and one for PI. I used this 50 watt JCM layout I had from when I bought a board a while ago to wire everything up without the second preamp tube. PI is still there. Not using a choke either. The bias is like a fender-style, and the 68K on the input is a 33K. I can try to make a layout or schematic, but it'll take me a while. i did include pix of the guts and board.

Are you talking the cathode resistors on V1? Trying to get the amp to "open up" a bit more. Sounds s little choked. Sounds good, but I think it could open up a bit more. I do notice when the treble/mid/bass knobs are down all the way, it goes quiet, like turining down the volume. Kinda like the tone stack is acting like a volume control too.

I admit my knuckle-headed-ness. Still just trying to find my sound.

Thanks,
Mega.

[Geezer]

I see that there is not a bypass cap across the 2nd gain stage cathode resistor.....try clipping a 0.68uf to 10uf cap across that resistor (see attached pic) with the negative to ground (if you use an electrolytic cap). That should give you a pretty good boost.....

G

[Geezer]

Also, that gain stage/triode is biased way cold (10k Rk resistor) which is part of the Marshall 2204 sound, but only when it has 2x triodes in front of it, and you only have 1x.
If the bypass cap (as I suggested above) doesn't get you what you want, try lowering the value of that 10k (to say 4.7k, or 3.3k, or 2.7k)......whatever gets the sound you're looking for, but that's the area I would start messing with. Try different resistor (lower) and bypass cap values to change the gain structure there a bit.....use alligator clip leads to sub different values in until you find your spot.

$0.02

G

[sluckey]

Looks like Geezer and me are on the same wavelength. In addition, remove that resistor on the volume pot. See pic...

[Megachunk]

Cool.

I have another .68uF I can tag in there. The fat black cap with the resistor tied to it to the right of that 10K is a .68 too. I'll try all that and let you know what I hear.

Thanks guys.

Mega.

[bluesbear]

The rule of thumb is:
1. Lower the cathode resistor value for more gain. I generally use 1.5k but I've used 820 ohm, 1.8k, or 2.2k, depending on where it is and what it's doing.
2. Use a bypass cathode cap for more gain. Marshall uses a lot of .68mf caps. Fender usually uses 22mf. Both do the job but the 22mf's are a bit warmer sounding in my opinion. On single ended amps, I put the first stage bypass cap on a miniswitch. I call it a boost but actually, you'd probably call it a cut. Either way, it works.
3. Raise the value of the resistors in the B+ rail. I know this seems counterintiative, the tubes will get dirtier with less voltage. Some Allen amps have a "brown" switch that raises or lowers the voltage to the preamp. It makes the amp sound more 50's. It's subtle but it works.
Dave

[Megachunk]

The 820R/.68uF combo made it sound much better. More gain, too. The OD isn't exactly like a JCM, but nice enough. EQ sounds OK, but a little odd to my ears. Could be my test cab. I am going to replace the generic OT to a Hammond or better soon.

If I were to use a choke, where would I connect it and what value choke should I use?

Thanks again, guys,
MikeG.
Megachunk

[Geezer]

EQ sounds OK, but a little odd to my ears.

Megachunk

The original circuit drives the tonestack from a cathode follower (low imp).....you are driving the same stack from the plate of the preceeding triode (hi imp). That is why the EQ is funky.

You could change the TS values to those more suited to being plate driven, maybe Fender Blackface values(?)

[tubenit]

Megachunk,

How close is this schematic to what you currently have now?   NOTE on this tone stack that there is a PAB switch
that you could install. Use a mini-toggle with spdt. You can experiment to see if you like this by simply disconnecting the wire between the treble and bass pot.  This will give you a HUGE boost in gain. You could use a relay and make this footswitchable.

With respect, Tubenit

[tubenit]

Megachunk,

IF you added a mosfet CF like I just did on my Carolina Songbird amp, then you'd have the Marshall topology (of sorts) and the tone stack would work more effectively for you.  This should be a reasonably simple mod that you could do with a terminal strip inside the chassis by the V1b section.  You would not have to add another tube.

Maybe Sluckey or Geezer can comment on this idea.

With respect, Tubenit

[Geezer]

Megachunk,

IF you added a mosfet CF like I just did on my Carolina Songbird amp, then you'd have the Marshall topology (of sorts) and the tone stack would work more effectively for you.  This should be a reasonably simple mod that you could do with a terminal strip inside the chassis by the V1b section.  You would not have to add another tube.

Maybe Sluckey or Geezer can comment on this idea.

With respect, Tubenit

I had the same thought....it would be fairly easy to add the mosfet CF.

G

[Megachunk]

Hi guys.

Thanks for chiming in, Tubenit. The schematic is pretty off now. I'll have to redraw it.

I think I got it where I want it gain-wise, going to try it at the gig this Friday. I use the Full-Tone Full-Drive 2 on mosfet and this helps get me where I need to go. It sounds really great now. If it doesn't grab me this weekend, I'll try the mosfet CF, if you can draw me a little layout, I have a strip or 2 I can add. Switching that in would be very cool.

Having a combo cab made for it - Closed back, but open head. I'm just loving tweaking this amp! Fun stuff!

Thanks all.
Mike G
Megachunk

[tubenit]

Mike,

Please post how it sounds at the gig. I will be interested to hear.  And let me know IF you are wanting to actually try the mosfet CF approach.  IF so,  I'd be happy to draw you up a layout using a terminal strip etc.....

Best regards, Tubenit

[Megachunk]

Well I played it at the gig. It sounded great at times. Now I don't know if it's just my ears, but the amp sounded kind of spongy or flabby at high gain settings. Not a very tight overdrive sound. It almost sounds tube rectified. The bass control did not seem to do much and the bottom and could be tighter. The presence control works great! When it's gonna clean setting it sounds okay, kind of sender, but seems to have some vids missingI would like to tighten things up and maybe get a little bit tighter overdrive. Any thoughts or suggestions are welcome.

Megachunk

[jojokeo]

Mega, if your amp is built using the same values as the layout picture, I'd suggest you replace the two coupling .022 caps closest to the right side of the board connecting to your 100k plate resistors. Early in the circuit these are allowing too much bass through and being re-amplified. I've used as low as .002's there or start w/ .005's and see how it cleans & sharpens up your tone. Then you can clip another .005 or whatever across one or both of these on the fly and see how increasing the capacitance changes things for better or worse. You'll be able to dial it in much better and faster this way.

Increase the .022 bass cap for more bass - look at the wire leaving pin 8 of V2 that connects to the 33k & 250(?)pF cap - immediately to the left is this cap. It goes to lug 3 of the treble pot and lug 2 of the bass pot. (the schematic you posted doesn't show this cap connecting to anything from the board)

[firemedic]

This may be too little too late but you could add a triode to your preamp if you went to a split-load phase splitter. That's what I would do, anyway.

[sluckey]

All that would do is move the gain stage around. You would still have the same number of gain stages. Split load splitter has a gain of about 0.8 to 0.9.

[Megachunk]

Thanks guys! You give me so much and I can give back so little due to my lack of any real electrical engineering knowledge. I am forever indebted. I also apologize for that last post, as it was from my Android and I was voice texting, and we all know how that can end up!

I am using a LOT of the same values as the layout. The amp is only 30-35 watts and I've always wondered if the values in this amp and the values on the layout (which is like my main amp - the one I use for 99.9% of all my gigs, and is one I built as well) should be different or are OK the same, even with the different wattage. The big caps in the B+ are not the same - They're only like 20uF instead of 50uF. The first is one in the chain is 40uF, though.

The overdrive/gain is sooooo close! I'll try those smaller value caps and larger bass cap first. If that doesn't help, I'd try the mosfet setup. It won't send me into metal-shredsville will it? I like crunchy and chunky, but not saturation/scoop. I play primarily rhythm but hit a few lead lines here and there. I love the volume, it's perfect for anywhere I play.

Thanks again guys. Much respect.

Mike G.
Megachunk

[tubenit]

I would not worry about metal/shredsville with the mosfet. The mosfet simply "acts" like a 12A_7 triode in that CF position.  A mosfet in your OD pedal is wired up differently and with a different purpose in mind.

With respect, Tubenit

[sluckey]

The overdrive/gain is sooooo close!

Referring to the pic I posted... Did you connect that brown wire directly to the volume pot? Doing so will give you more gain.

Using 50uf filter caps will tighten up the low end.

[Megachunk]

Hey guys.

Sluckey, I did take the resistor and cap off that brown wire and connect it directly to the pot. I'll try changing out the first 2 caps on the right side of the board (two .022's) and see what that does. I'd like to try the mosfet though. Just knowing how to make it and apply it would be very helpful to me. If you wouldn't mind drawing a little layout up, that'd be great. As far as the caps in the rail, would you go 50uF all around? Mine start with the 40uF at point A and then all after that are 20uF. There's a 20uF terminal that's not being used. I could double up on that for a larger value, but would know where to put it. Point B?

Sounds like the mosfet and these other things may get me there. Another gig on Saturday with the big band. A guitar builder out here is making me a custom guitar and it should be done by this weekend. I'll try the tweaks and play it on Sat.

Thanks guys.
Mike
Megachunk

[jojokeo]

All 50uF's will help but depending on your layout they won't help on the first couple 12ax7's it only affects tone there slightly. Most sag/flabbiness/sponginess feeling especailly playing power chords at higher volumes is caused from the power section not being able to "keep up" w/ those low cap values you've used. As you hit your chords a large voltage drop occurs and if they don't have the reserve to supply the current needed then that's what you're feeling.

*Another thing I'd do if I was you and/or try: If you've used a 68k on the input jack? sub in a 33k there. You'll get a bit more brightness and slightly stronger signal that you're probably going to like and give you a sense of more gain just from that little change. You're effectively playing throught the "low input jack" on a normal amp by using the 68k there.

[Megachunk]

I do have a 33k right off the pin on V1. It's the one thing I kept. I'm gonna try 40's on the first 2 spots on the B+.

Thanks man.
Mike G
Megachunk

[tubenit]

You can probably drop down to a 22k on the input jack if you wanted.  Some Dumble amps did that.

With respect, Tubenit

[Megachunk]

OK, let's see.

 I now have 40uF on the first 2 spots on the B+ rail. A bit more bottom. I also put in two .008uF caps in the first 2 spots on the board, going to the treble and mid. Sounds good, a bit different. Nice feel. Still a little sproingy in the drive tone, feels like it needs to be tighter, but not necessarily brighter.

I'll try it at the gig this Saturday and see how it sounds with the band at real volume. Steve Bartek is playing with us this time and he's a wild guitar player, not to mention an amazing orchestrator. Will let you know how it sounds.

Thanks,
Mega

[tubeswell]

If I were to use a choke, where would I connect it and what value choke should I use?

Hi Mike

The choke in Marshall amps goes in the power rail between the Output tube screen supply node and the OT centre tap supply node (in place of that 1st supply resistor between the 1st two cap cans). This has the effect of boosting the screen voltage, which will increase the gain of the output stage. The only thing you have to then watch is that you add chunky resistors at the output tube screen grid pins, so that you don't end up melting the screens with too much current. For EL34s I'd use 10W 470R - 1k screen grid resistors.

I see you also reported squishiness at higher volumes. The answer to that is probably more filtering at the reservoir cap node (which won't hurt if you have SS rectification). Do you actually have 2 x 50uF cans at that 1st node as per that layout you posted, or some lesser capacitance? Even if you do, you cloud tack on an extra 47uF or 100uF filter cap in parallel with that (but make sure the polarity of the additional filter cap is the correct way around).

A choke will also boost all the voltages in the power rail slightly. If you want to hit the output tubes with a bigger signal you're going to have to increase the voltage swing from the PI. The higher supply voltage will in part do this. But you could get it even bigger by lowering the PI tail resistor from 10k to 6k8. Now you have more ability to drive the output tube grids into clipping.

But there's probably one more thing you'd need to do, and that is boost the gain in the 1st pre-amp stages. In this regard, I see you have tried lower cathode resistors in the pre-amp tubes. You could also try increasing the plate resistors from 100k to 150k or 220k. Try V1a first. You may get a bit more hiss unless you use metal film resistors.

JM2CW
Pete

[Rev D]

 Watching this with interest Megachunk, cool to hear how it sounds in a gig environment. I'm trying to get started on a jcm800 with 6v6's and slightly lower voltages, something around 22 watts or so (actual wattage not an important aspect, looking for deluxe reverb sorta power but sound like a Marshall, small club amp), sorta like a jcm800 version of Mark Huss's 6v6 plexi, so all these jcm800 post are helpful. Good luck with your amp! And asking these sort of questions IS giving back as it helps a lot of us guys like yourself who aren't electronic engineers! Thanks!

Regards,

Don

[jojokeo]

Rev, I had some of the same feelings and did just that: http://www.el34world.com/Forum/index.php?action=media;sa=search;search=Sheriff+-+jojokeo+Marshall+JCM800;sch_kw

[Megachunk]

Hi All.

I played it this weekend with the big band. Sounded good. Needed to really push it this time, everything close to all the way up. Maybe because I was using a new guitar with passive pups. I want to get rid of the bandmaster/vibrolux 4 ohm trannie. But as far as the caps, I used 40uF for the first node, and jumped two 20uF posts for 40uF on the second node. With the new values on the treble and mid, the amp sounds more focused. I like that aspect. bass control still seemd to do nothing. Seemed to lack a bit of power this time, though. Maybe the tubes got jostled. Load-in was tough.

I would like to have a bit more headroom than the 6V6 or EL84 gives. Still would like to make an attempt to clean it up somewhat if needed. Pedal can get me to where I need. Just want the amp and it's sound to be strong and tight. When I can put video on YouTube, I'll post it. The guitar player I was next to, Steve Bartek, is a great player and he plays an early Matchless. It will at least gvie you an idea as far as how it fits in the mix. That's been the consistent thing; it does that very well.

Megachunk

[tubenit]

Look forward to the Utoob video.  Thanks for sharing.    Getting it "just right" is part of the challenge and the fun!

With respect, Tubenit

[Megachunk]

Here's a song from the show. I'm on left playing red Farnell. Guy with Strat is Steve. Steve also used to be in the Strawberry Alarm Clock.

Don't know how much you can tell how the amp sounds, but it was a fun night.


Thanks,
Megachunk

Vatos Boingo Dance Party with Steve Bartek - Nothing To Fear

[John]

Pretty cool putting faces to names. Thanks!

[sluckey]

Incense and peppermints, the color of time.

 

[jerrydyer]

mmmmmmmmmmmmmmmmmm Solens 

[jojokeo]

Very nice Mega. I've always loved Oingo/Danny E especially when they'd play Halloween here locally back in the day w/ "It's dead man's party" theme. Now it's all tribute bands for so many groups these days. The amp sounded great. Love the brass too, you're a lucky man to get the opportunity to play in such a group. They're fewer and farther between nowadays but I appreciate the musicianship that goes along w/ all that also.