PLEXI 6V6 PLUS

[TIMBO]

Hi guys, The Marshall Popular was a bit TOO noisy for my liking and although it sounded dam good I'm going to shelve it for the moment.

So I have this great looking cab to fit another amp to.

My SPIDY senses thought the popular may not work, so a second chassis was bent. :yep
I have been wanting a low watt Marshally amp for a while and the PLEXI 6V6 sounds awesome.
 

From the schematic


The plexi circuit is fairly standard.
And the added triode is from the Bogner Ecstasy Blue channel.
I have had success with circuit in a Bogner clone I did some time ago.
http://el34world.com/Forum/index.php?topic=15795.msg152841#msg152841

[Ed_Chambley]

Again Timbo, great work

[shooter]

to shelve it

from your shop pics, it looks like you have to build a shelf just to shelf it 

[silverfox]

So I'm a little confused. Is the video of the amp you built according to the schematic posted? It sounds great! Good range of tone.
silverfox.

[TIMBO]

Hey silverfox, The video is not of my amp and I am assuming that the plexi circuit will sound similar.

[purpletele]

Hey silverfox, The video is not of my amp and I am assuming that the plexi circuit will sound similar.

Timbo,

I have not seen the Blue Gain circuit before, what does the Centre Switch do to the tone.
Does that Blue Gain knob go on the front faceplate.


Curious, looking forward to the real clip.


BV

[davidwpack]

Found this the other day and thought it might interest you.

[davidwpack]

There's sound and video clips on his page. http://chasingtone.com/dvnator/the-cesspool/

[TIMBO]

Hi purpletele, I'm not exactly sure what the switch does but I sure it will affect the tone.
The blue gain is the volume on the front panel
 

[rake]

Timbo,
The switching on V1B? (Boost, Structure)

Is that one DPST switch or two SPST switches?

Looks good!

[TIMBO]

Hi rake, The boost/structure are on separate SPST switches.
Relays can be easily added for a foot switching.

[rake]

Thanks Tim!
This layout looks very versatile tone wise.
When doing covers in clubs, parties, weddings, etc.
you need to cover a wide spectrum of tone.

It's hard to get over when you have to play Daddy's Little Girl
and your axe and amp only give you a sound like Angus Young....   

[TIMBO]

Hey rake, With the extra gain stage added to the plexi circuit just adds a little more drive that is not too over the top.

Progress.....


 

[TIMBO]

Hi guys, this is how this one has turned out.

Just need to finish the cab and this one is a keeper.
 

[pdf64]

Looks and sounds great
Just to note that with a 'hot switching' type of standby arrangement, it's best not to use standby when in the 'vintage' (GZ34 rectifier) mode 

[pullshocks]

Great sounds, and nicely structured  demo video showing the effects of pick attack

[dude]

Wouldn’t adding an e-cap to the S/B, take care of the hot switching?                                     al

[Paul1453]

I built the Hoffman 6V6 Plexi as posted here with a lower B+ around +320Vdc, because that is what the PT I had would do.

The single input with normal and bright gain pots allow you to tailor your gain tone and sounds outstanding to me!

Especially with a premium Mullard 12AX7 in V1!  The MV allows you to get those great tones at bedroom/apartment volume levels!

I was very satisfied with the Hoffman 6V6 Plexi design!   

[pdf64]

Wouldn’t adding an e-cap to the S/B, take care of the hot switching?                                     al

Scroll up to the schematic in post #1; the standby is in the ac circuit, so a regular ecap there would likely explode

[dude]

Wouldn’t adding an e-cap to the S/B, take care of the hot switching?                                     al

Scroll up to the schematic in post #1; the standby is in the ac circuit, so a regular ecap there would likely explode

My mistake, put the cap off rectifier tube pin 8 or on the vintage side of the Modern/Vintage switch. Wouldn't that work?

[pdf64]

...My mistake, put the cap off rectifier tube pin 8 or on the vintage side of the Modern/Vintage switch. Wouldn't that work?

I don't think that would help, as the cap would discharge in standby mode, and hence (along with the main reservoir cap), would require charging when standby was switched to 'operate' mode, thereby drawing surge current via the rectifier.

[Big_Mike]

I built the Hoffman 6V6 Plexi as posted here with a lower B+ around +320Vdc, because that is what the PT I had would do.

The single input with normal and bright gain pots allow you to tailor your gain tone and sounds outstanding to me!

Especially with a premium Mullard 12AX7 in V1!  The MV allows you to get those great tones at bedroom/apartment volume levels!

I was very satisfied with the Hoffman 6V6 Plexi design!   

With regard to the master volume, I am debating between going with a pre-phase inverter like in the schematic, or trying for a post-phase inverter master volume.  For the stock circuit, how is the sound at low volume (bedroom or apartment) levels compared to when the master volume is opened up?  I need to be able to dial down the volume for night use and want to preserve the tone.  Thanks!

[TIMBO]

Hi Big_Mike, Having the two master volumes are a great combo.
Really lets you dial in all the qualities of the amp at a usable level.

[Big_Mike]

Unfortunately I only have space for one master... Which one would you pick?

[TIMBO]

Are you able to mount the post MV on the back of the amp?
IF not, post MV.

[Big_Mike]

I think what I might do is put a PPIMV in the amp and use the a rear slot for a variable negative feedback pot.  That way I can dial in more NFB when the amp volume is turned down.

[dude]

I have both Pre and Post MV on my 6V6 Plexi.  I agree with Timbo, they do indeed blend well together. My PPIMV is not the LaMar or Rich Mod MV but another trainwreck version as posted above. I tired the Rich II mod (with the pre-MV) but didn't really like it, not it it didn't work well but 3/4 of the effect were in the first quarter turn, just a hair produced a change. I'd have to fiddle with the knob to get where I wanted to be. The Allen Trainwreck version worked better, IMO, because the the range was much better than the Rich II MV. And I didn't have to fiddle with the knob. When I turned down the PPIMV about a quarter turn, I'd turn up the Pre MV a bit, to get a little more crunch at a lower volume. Blending the two MV's along with the other two volumes gives a very versatile amp. Now, I'm not playing bedroom levels, I'm talking gigging levels. Again IMO, if you want bedroom levels I would go with the Sluckey pre-MV on the front, much more of an effect, in fact every slight turn lowers the volume. You might not get power tube crunch, use a pedal. Just my two sense.


al

[dude]

...My mistake, put the cap off rectifier tube pin 8 or on the vintage side of the Modern/Vintage switch. Wouldn't that work?

I don't think that would help, as the cap would discharge in standby mode, and hence (along with the main reservoir cap), would require charging when standby was switched to 'operate' mode, thereby drawing surge current via the rectifier.

Not on my amp, the caps stays charged but it's unloaded and voltage is higher. I would think as long as that cap off the tube rectifier (pin 8) is rated high enough, it won't blow, you'd eliminate hot switching. My Plexi on SB reads 440vdc (it's a 500v cap) "constantly", flip SB off the cap reads 403vdc constantly. Am I wrong here...?


al

[pdf64]

...I would think as long as that cap off the tube rectifier (pin 8) is rated high enough, it won't blow, you'd eliminate hot switching. My Plexi on SB reads 440vdc (it's a 500v cap) "constantly", flip SB off the cap reads 403vdc constantly. Am I wrong here...?

I think you may be wrong, as the schematic shown in post #1 shows a DPST standby switch in the ac HT circuits.
Hence it makes no difference that a reservoir cap is connected to pin 8 cathode, there will be surge current to charge the cap (via the rectifier) when standby is switched to 'operate'.

[dude]

OK, I was referring to the usual standby wiring off dc, like most Plexi's, I see the a/c cut, turns off the a/c power supply each time.

[Big_Mike]

Regarding master volumes, Metropolous on his Metro-plex uses a pre-phase inverter master volume with some tweaks.  The master works really well, and the benefit is that it does not mess up the negative feedback that is so important to Marshall circuits.  I am going to try and figure out what his "magic sauce" is that makes his pre-phase inverter master volume work so well.  Maybe we can add it to the Plexi 6v6 circuit.

[Big_Mike]

George Metropolous responded to a post I placed on another board regarding a mod to the standard Fisher type 4 master volume, as used on the Metro-Plex and on Doug's Plexi 6V6.  He said to add in a 220K resistor in series between the treble pot and the master pot, and then add a 5-10 pf cap across the master volume pot wipers.  This he says preserves bass and treble frequencies as the master volume is lowered.