Dual 800/plexi 20W complete

[dennyg]

This is basically Sluckey’s Dual 50 JCM800/Plexi pre-amp circuit into a 20W PP (6V6).  Amp build #6 set several new personal records 1) completed in less than 40hrs including layout design. I’ll add another 8hrs for cabinet build yet to be done 2) worked without any issues on first boot up (although I did test each stage of the build).   also my first fixed-bias PA.  A few notes of potential interest:
•   Used term strips vs board layout
•   Built all PS components on one board: tucked away at the far end from the preamp: AC heaters, DC heaters (preamp only), star ground connect, B+1/2/3, bias supply, 5V relay supply
•   Experimented with OT placement at opposite end from PT for weight balance and less jumbled wiring at the one end with PT and PS board.  Ran the primaries across the back chassis for shielding and amp is hum free. Yay!
•   Cap multiplier on C2 as I’ve done on last 3 builds; nearly eliminates ripple right out of the gate thereby enabling small 10uf decoupling caps for the pre-amp PS chain and yields dead quiet, hum-free amp
•   Footswitchable channels (plexi/800) and added a footswitchable lead MV, very useful for playing live
•   Variable NFB with presence control
•   Preamp mods relative to Sluckey’s dual 50 schematic
o   220K Ra stage 1 of 800 and 33K Rk stage 2 – copped from Freedman’s 800 mods to goose the distortion (although he used 39K I believe – but I only had 33k handy)
o   Removed the high-pass circuit between 800 stage 1&2; I found original 800’s to be a bit bright – makes for much more mid-focused, fat tone IMO
o   Added Vg1-k boot-up protection for the cath follower (note that I do not use standby switches – I took Merlin’s side of that debate)
o   Use .047uf grid bypass on PI second grid (reduces chance of blocking distortion)
o   Use 100pf snubber cap across PI outputs (vs 47pf) – smooths the 800 highs a bit more at higher volumes
o   Add 100k between MV and PI – picked that up from AX84 designs – supposedly smooths the edge for higher gain (e.g. 800) inputs
o   Use 25K pot in parallel with 4.7K DC bypass for presence – put the .1uf cap between pot and ground lead to take DC off the pot
•   The only thing I would do differently is move the channel switch relay next to the MV/Lead relay – when dimed the plexi (first channel nearest input) has slight buzz due to the 5V supply which annoys my OCD self but is not audible once a guitar is plugged, particularly at that level.  Otherwise I’m taking this layout as-is to my next build (a 50W Uber High Gain) 

Finally my two favorite classic amps in a dual-channel 20W version that just kills with old-school 70’s/80’s Marshall tone with added benefit of getting power tube distortion w/o blowing the roof off your house. 

[sluckey]

That really looks nice! Congrats on a successful build.

[VMS]

Looks great!


I might take some pointers for my next build. Little bit hybrid design too, one preamp channel on pcb-board and the other channel on turret board, rest of the circuit probably on terminal strips.

[adamG]

Hi,

I built my copy of this amp, as well.
The whole project came out pretty well and neat.
But, I've got a problem with high frequency hum on Plexi channel engaged.
It's not annoying very much, but its level is to high IMO.
I am afraid I do not quite follow what you mentioned;)
So, could you extend below of yours, please?

•   Cap multiplier on C2 as I’ve done on last 3 builds; nearly eliminates ripple right out of the gate thereby enabling small 10uf decoupling caps for the pre-   amp PS chain and yields dead quiet, hum-free amp
•   Variable NFB with presence control
o   220K Ra stage 1 of 800 and 33K Rk stage 2 – copped from Freedman’s 800 mods to goose the distortion (although he used 39K I believe – but I only had 33k handy)
o   Removed the high-pass circuit between 800 stage 1&2; I found original 800’s to be a bit bright – makes for much more mid-focused, fat tone IMO
o   Added Vg1-k boot-up protection for the cath follower (note that I do not use standby switches – I took Merlin’s side of that debate)
o   Add 100k between MV and PI – picked that up from AX84 designs – supposedly smooths the edge for higher gain (e.g. 800) inputs
o   Use 25K pot in parallel with 4.7K DC bypass for presence – put the .1uf cap between pot and ground lead to take DC off the pot
•   The only thing I would do differently is move the channel switch relay next to the MV/Lead relay – when dimed the plexi (first channel nearest input) has slight buzz due to the 5V supply which annoys my OCD self but is not audible once a guitar is plugged, particularly at that level.

Main issue is HF Hum in my build, but other ones sounds interesting to me, as well.
I wonder whether FS jack and channel swithing relay near to input jack is a reason of hum?!
I realize this might be too brief for such complicated issue, but for the beginning it could be;)

Thanks in advance.

Regards,

Adam   

[dennyg]

Adam - the 5V relay power supply is quite dirty/noisy and if the relay is anywhere near your input jack or first stage, you'll hear some buzzy hum.
However, you can knock that down a lot by connecting a larger (1 to 10uf) cap to the positive side of your 5V power supply, and the other end of the cap to a circuit ground connection near to the power amp.  That will smooth the DC and should reduce that buzz down to an acceptable level.  The only way to eliminate it is to move the relay away from the input side of the amp. 
While you're under the hood, take a pic of your layout and post it.
Hope that helps!
Denny 

[bnwitt]

DennyG,
Wow!  Nice project.  Well thought out and great implementation.

Adam,
I hope you get rid of that hum.  The amplifier looks fantastic!
Barry

[adamG]

While you're under the hood, take a pic of your layout and post it.
Hope that helps!

Thank you Denny.

Here are two pics of interior. I followed Sluckey's project...
As you can see, the relay is close to input jack, as well as FS jack is just below of the input one.
Actually, I had such suspicion of its placement too close to input jack, but I got that risk, anyway. Silly me;)
Relay is ground circuit isolated of 12VDC.
So, the 1-10u cap should be placed on relay board(+) and (-) connected via wire to first star ground near to PT?
I think, I'll need to move the relay to more proper place.

Adam 

[adamG]

Adam,
I hope you get rid of that hum.  The amplifier looks fantastic!

Barry,

Thank you very much, but the topic belongs to Denny. I am only his guest...

Regards,

Adam

[dennyg]

Adam - these threads belong to the whole forum and you are totally on topic with your posts.  That's an awesome job on your amp!  Yes, try the cap as you described - the larger the better - I used a 22uf 100V cap just because i had a spare and it knocked the noise right out.  I placed the relay same spot on my previous build (20W TOS) and used a .1uf cap, but on this build I had to go larger to kill the noise.  If it works as well as it did for me, you won't need to move the f/s jack nor relay.  Besides, you don't want to leave a hole in that beautiful faceplate. 

[bnwitt]

Gentlemen,
I would love to hear some sound clips when the amps are dialed in.

[tubenit]

I would love to hear some sound clips when the amps are dialed in

I would also!

[dennyg]

I'll do some clips this weekend.  I'm  taking this bad boy to band practice and will do clips with the band too.  Looking forward to trying out the footswitchable lead feature which now I want to add to my TOS.  I'd be interested if anyone else has done a 20W version - it KILLS for mid-gain 800 tone on full-throttle that would collapse your aural infrastructure with 50W.  The compression and power tube distortion are so pronounced because it's only a moderate gain amp. 

[adamG]

These threads belong to the whole forum and you are totally on topic with your posts.  That's an awesome job on your amp!

Denny,

Thank you very much for assistance and words of praise.
I'll try this trick for sure;)
Would you be so nice and extend these issues, a well?

-Removed the high-pass circuit between 800 stage 1&2; I found original 800’s to be a bit bright – makes for much more mid-focused, fat tone IMO"

Do you mean bright 470pcap removal or both 470p and 1n?
If so, then I must agree with 470p removed as a good idea. I also disconnected it because 800 channel was too bright with it to me. I mean #1 and #2 mods

-Added Vg1-k boot-up protection for the cath follower (note that I do not use standby switches – I took Merlin’s side of that debate)

What boot-up protection?!

Thank you in advance.

Regards,

Adam

[adamG]

Gentlemen,
I would love to hear some sound clips when the amps are dialed in.

I'm not a musician but the....chicken
Another story is that my workshop is too small for recording 50W amps.

Regards,

Adam

[dennyg]

Adam - I removed both 470pf and 1nf caps in mine just because it sounded better to my ears.  The boot-up protection is simply adding a 10K resistor in series with an N4007 diode between the grid and cathode of the cathode follower - cathode-side of diode attached to the triode cathode.  This prevents possible arcing between grid and cathode at boot-up.  At startup, the CF grid gets full HT from the previous stage anode while the CF cathode is at ground until the CF warms up, which could arc and damage the grid and/or cathode.  Adding the protection allows the cathode to  be at same potential as the grid, and once the tubes warm up, the diode allows the bias to settle in slightly positive relative to the grid.

[adamG]

Adam - I removed both 470pf and 1nf caps in mine just because it sounded better to my ears. 

I only removed 470p on first step of 800. Hights are quite comparable on both channels to my ears.

The boot-up protection is simply adding a 10K resistor in series with an N4007 diode between the grid and cathode of the cathode follower - cathode-side of diode attached to the triode cathode.

If I am right, that solution is provided in ax84' SE amps.

Thank you Denny, very much.

Regards,

Adam

[dennyg]

Yeah man, AX84 was ground zero for my amp hobby (obsession?).  Speaking of the AX84 SEL (also my first amp), I realized over the weekend that the SEL pre-amp is nearly identical to a JCM800, except for stage 2 (and a few variables in the stage coupling filters).  So I'm adding a simple switch between the 10K bias on an 800, and the bypassed 1.2K from the SEL.  I actually use a .1uf bypass on this new amp vs 1uf on the SEL as i found the bass to be a a bit much on my Dual 20 - although it sounds great on the SEL - so must be something to do with the power amp response between an SE and a PP.  Anyway, the gain (distortion/sustain) of the SEL is good bit more than with the cold bias 10K (or even the 33K i used), so now I can use both within the same amp.  Just thought I'd pass that idea along. 

[dennyg]

Here's a quick demo of all three basic tones of this awesome amp.   I added a "high/low gain" switch to the 800 channel.
1) plexi channel with the 'normal' and 'bright' pots set on around 2:00 so has bit of edge on it
2) jcm800 - standard (low gain engaged); note that when i decided to add a high-gain option, i dropped the 220K Ra back to 100K and 33K Rk (second stage) back to 10K so it's basically stock 800 circuit less the 470pf hi-pass cap between stages 1&2.  Preamp level dimed. 
3) high-gain setting: added panel switch to swap the 10K Rk with a bypassed (1uf) 1.2K Rk.  Preamp level 3:00 (beyond that gets a bit too saturated on the iphone recording). 


https://www.soundclick.com/html5/v3/player.cfm?type=single&songid=13683289&q=hi&newref=1

[dennyg]

The finished product - faceplate is laser cut by Mason at TMI Amplification

[SILVERGUN]

Looks great denny!

Makin Philly proud...again

[dennyg]

good gosh Dave - that pic captures my two passions!! can't wait to send to my buddy Rob DiAntonio who has taken me under his wing as an apprentice but whom I also forgive for being a Giants fan, his only character flaw.  BTW, did you make it down to the parade?  I was there with my two boys and it was the experience of a lifetime.  Hopefully Doug won't bounce my post for being off topic :o

[SILVERGUN]

No parade for me, although I now feel like I missed out.

And Doug wont bounce you off your own thread. You can go as off topic as you want, just look at sluckeys "final amp" thread. I think we are all still there just waiting for someone to pull the plug, but sluckey's too nice. He'll let us go on forever. 'specially Ed.

Back to your amp....I like the sound but I wonder if the recording really does it justice. I always wonder that when I can hear the sound of the pick hitting the strings mixed in with the recording. If the amp was in full meltdown (where it sounds the best) we wouldn't be able to hear you strumming.
Come on down when you get a chance and I can try to get you a louder demo. I recently got set up with some mics to do some better demos myself.
The longer you wait the more amps you'll have so unless you have a minivan I suggest sooner than later. 

[sluckey]

Doug's not the bouncer. That's what he hired me for!   

[SILVERGUN]

Doug's not the bouncer. That's what he hired me for!   

Yeah, watch for him in his next role as "THE THREADKILLER" where he'll ask questions like:
- Any progress on this?
- I gave you a list of things to test, how's it going?
- Have you tried reversing the OT wires?

 

[dennyg]

well if sluckey's the bouncer then i'd think he'd go easy on me since it was his design i built! 
i'd love to drive down with a few amps - give me a few weeks to finish my next project because i think you'll really dig it being into high gainers. 
if you're curious, it's a 50W version of the Uber Gain circuit from ax84. I built an SE KT88 version that the other guitarist in my band fell in love with and so I sold it to him. 
i'll shoot you a message when done - will take you up on your offer to do some quality demos too
denny

[SILVERGUN]

Alright, I'll look forward to it

[dennyg]

Thought I'd post a live clip of this amp with my band - it's an original band but we cover an AC/DC song that you guys would know - I'm using the high gain mod setting so it's hotter than what Angus uses but we do a breakdown in the middle where i use the clean plexi channel, and use the lead boost for the lead. No pedals used on this song so it's all from the amp. 

https://www.facebook.com/LocalMusicLives/videos/441516876304384/UzpfSTg3MjI0NTkwOTU0Mzg1NzoxMzM4Mjg3MDAyOTM5NzQz/

[macula56]

That really looks nice! Congrats on a successful build.

Is this schematic still around here somewhere? I couldn't find it. I want to build another one.

[dennyg]

below is Sluckey's stock schematic - I built 20W PA and did a few minor mods to it detailed above - would highly recommend the hi gain mod and second mv for lead if you plan to play it on stage. 

[macula56]

below is Sluckey's stock schematic - I built 20W PA and did a few minor mods to it detailed above - would highly recommend the hi gain mod and second mv for lead if you plan to play it on stage.

Many thanks Sir. I have a copy somewhere but can not seem to locate it. I built a version of it a few years back and I really liked it.

[Ed_Chambley]

No parade for me, although I now feel like I missed out.

And Doug wont bounce you off your own thread. You can go as off topic as you want, just look at sluckeys "final amp" thread. I think we are all still there just waiting for someone to pull the plug, but sluckey's too nice. He'll let us go on forever. 'specially Ed.

Back to your amp....I like the sound but I wonder if the recording really does it justice. I always wonder that when I can hear the sound of the pick hitting the strings mixed in with the recording. If the amp was in full meltdown (where it sounds the best) we wouldn't be able to hear you strumming.
Come on down when you get a chance and I can try to get you a louder demo. I recently got set up with some mics to do some better demos myself.
The longer you wait the more amps you'll have so unless you have a minivan I suggest sooner than later. 

Ed who?


dennyg, great job.  Watch out for Silverpanties tho, he will appear to befriend you and right when you think he is you buddy, he will call you out for Hi Jacking and he is the worst HiJacker on the forum, well next to me that is.


Now realize some of us old folk ramble, but I remember as Jojokeo says I am a KOOK.