Demo of my cathode-biased tube-rectified 6V6 Plexi

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Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

https://www.youtube.com/watch?v=tF9a8yhygy4

I did a complete video log of the build if anyone is curious. It's built entirely from scratch:
https://www.youtube.com/playlist?list=P ... uDdyoHHHWm

There's a schematic and description on my website of what it WILL be when it's complete:
http://www.snicksound.com/projects/6v6ultimateplexi/

But basically: it will get a Cascade switch + a Cold Bias switch (to simulate the JCM800 sound), a Superlead switch for the Bright channel (to go from JTM45 shared cathode to Superlead split cathode to even more aggressive shared cathode + 2.2nF coupling cap) and a Bright switch to active a bigger treble cap on the Bright Volume.

It already has a tube buffered loop as demonstrated in the video. And it has a trick half-power scheme that disengages the bypass capacitors on the power amp cathode bias resistors (each tube has its own) but I'm not digging it so the switch will be used as a NFB defeat instead (there's already a very cool tone stack defeat)
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
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Littlewyan
Posts: 1911
Joined: Thu Sep 12, 2013 6:50 pm
Location: UK

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Littlewyan »

Nice amp man :). Good tone.

I've never seen a low power switch like that before, thats pretty cool! Maybe try changing it to a triode/pentode switch so it keeps the distortion at a lower volume. It will change the tone but you might like it.

With regards to the Effects Loop, try setting it up how Soldano does it in the SLO100. I've attached a snip of the schematic. Maybe use an AC Cathode Follower? You might lose less signal that way. Hopefully others will chime in as I don't really do effects loops :shock:

http://valvewizard.co.uk/accf.html
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Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Littlewyan wrote:Nice amp man :). Good tone.

I've never seen a low power switch like that before, thats pretty cool! Maybe try changing it to a triode/pentode switch so it keeps the distortion at a lower volume. It will change the tone but you might like it.

With regards to the Effects Loop, try setting it up how Soldano does it in the SLO100. I've attached a snip of the schematic. Maybe use an AC Cathode Follower? You might lose less signal that way. Hopefully others will chime in as I don't really do effects loops :shock:

http://valvewizard.co.uk/accf.html
I read about that "half-power" scheme on... I don't really remember what board, either AX84 or Ampage. Either way, I wanted to try it (plus having dual bias resistor means an imbalanced pair of power tubes will still work well), but it's not for me. Not a fan of Triode mode either, for similar reasons (changes output impedance and makes the power amp for "stiff").

This amp does have an easy way of actually getting half-power: replace the GZ34 with a 5Y3, and the 6V6 with 6K6. I will get to that test.

As for the effects loop, I'm aware of the SLO one, but it's fairly colored (it's part of the SLO sound) and placed before the tone stack. I considered an AC coupled cathode follower design, à la Mesa Dual Rectifier, but eventually the circuit I used is from Merlin's book. It's not a cathode follower stage, but it's still fed off the cathode. Merlin says that he likes that idea because the output can only be so many volts, you don't have to worry about the voltage rating of that output cap, or about sending a bit dose of voltage down your pedal board while the cap charges.

Ultimately, the culprit is the voltage divider before the loop (to take it down to pedal level). I took a shot on the values (470k and 8k2 IIRC) based on how much gain I assumed the recovery stage would provide, apparently it's not enough, so I'll increase that 8k2 bottom leg.

The actual schematic:
http://www.snicksound.com/wp-content/up ... kSound.gif
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Changing the voltage divider to 470k/22k pretty much fixed the loop's volume drop. Also, swapped a smaller coupling cap in the lead channel (0.0033uF), and experimented with a 12AY7 for more clean range

https://www.youtube.com/watch?v=Jn6mz9ROHDY
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
User avatar
Littlewyan
Posts: 1911
Joined: Thu Sep 12, 2013 6:50 pm
Location: UK

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Littlewyan »

Ah cool glad you sorted it.

Does the presence control work well? If not you might want to consider changing the NFB resistor to 82K as at the moment you don't have much feedback with that 100K.

Sounds good though :). I struggle to get a clean tone with my 20W Master Volume amp when the gain and master volume are up high. The best clean tone that I can get is when using my JTM50.
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Littlewyan wrote:Ah cool glad you sorted it.

Does the presence control work well? If not you might want to consider changing the NFB resistor to 82K as at the moment you don't have much feedback with that 100K.

Sounds good though :). I struggle to get a clean tone with my 20W Master Volume amp when the gain and master volume are up high. The best clean tone that I can get is when using my JTM50.
I tend to prefer low NFB, last 6V6 Marshall build I did was a 2204 and I put a 3-way switch from NFB which gave the options of 56k, 156k and open circuit (no NFB), it pretty much always stayed in 156k. I believe I was tapping off the 16 ohm output, so 156k would have been close to 100k on the 8 ohm tap (technically 110k). But you're right that more NFB would help with clean tones.

Then again, I never play my amps clean, I leave it in crunchy territory, then use my guitar's volume or an OD pedal to go one way or the other.

Still, this being the "Ultimate" iteration of a 6V6 Plexi (or trying to), making the NFB switch a 3-way could be a good idea.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Littlewyan wrote:Ah cool glad you sorted it.

Does the presence control work well? If not you might want to consider changing the NFB resistor to 82K as at the moment you don't have much feedback with that 100K.

Sounds good though :). I struggle to get a clean tone with my 20W Master Volume amp when the gain and master volume are up high. The best clean tone that I can get is when using my JTM50.
So I experimented with the NFB, and I eventually settled on a 56k (à la JTM45). Wasn't expecting this, but with THIS amp I'm liking it.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Newer demo with more tweaks (including the smaller NFB resistor)

https://www.youtube.com/watch?v=3bWiXUqG_5A&t=361s

Starting to sound pretty good IMHO
Having never owned an amp that made less than 50W before (save for my 8W JCM8 which is more of a toy), it takes some adaptation
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
User avatar
Littlewyan
Posts: 1911
Joined: Thu Sep 12, 2013 6:50 pm
Location: UK

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Littlewyan »

Is that 56K on the 16Ohm tap?

The amp sounds good!
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M Fowler
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Location: Walcott ND

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by M Fowler »

Reverb should go through some kind of effects loop, I use an interrupt type passive loop to drive reverb and delay.

Sounds good.
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

Littlewyan wrote:Is that 56K on the 16Ohm tap?

The amp sounds good!
56k on the 8 ohm tap

Thanks
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: Demo of my cathode-biased tube-rectified 6V6 Plexi

Post by Badside »

M Fowler wrote:Reverb should go through some kind of effects loop, I use an interrupt type passive loop to drive reverb and delay.

Sounds good.
It is going through a loop (a slightly modified version of the one in Merlin's book).

Reverb sounds perfect, but 15W means not a lot of headroom so reverb gets compressed at higher volumes (and without a high volume, there isn't a lot of distortion available).
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
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