Hoffman 6v6 Plexi

[Big_Mike]

So I have completed my EL34 Plexi single-ended project, and while I like it, I think I realize that the "magic" of the plexi circuit is the mixing and matching of the two channels and the phase inverter distortion.  Obviously, you need a push-pull amp to get the phase inverter distortion.  So now I am thinking of going for the 6v6 plexi based on the Hoffman circuit.  I have a couple of questions about the 6v6 plexi:

1.  From the clips I have heard on Youtube, it seems to be very close to an EL34 sound.  How much difference in sound is there between a 6v6 plexi and a regular 50 watt?  I have not heard a 6v6 plexi in person.

2.  I would probably add a PPIMV master volume to the circuit, using the Rich Mod II form that is used extensively over at the Metro forum.  I think I can figure out how to mod the Hoffman layout for that, but I might need some help.

3.  For lower volume playing, would there be any benefit to having a pot to vary the NFB resistor?  I have heard that can help with the lack of NFB at lower levels. 

4.  As far as transformers, just use the 18 watt PT and OT, along with the choke that is on the 6v6 build page?

Thanks!

[Big_Mike]

Anyone compare a 6v6 plexi with a 50 watt EL34 Plexi?  I assume you can get power tube crunch at lower volumes.

[pdf64]

If you think that it's a key part of the tone, why not add an LTP to your SE amp?
It should work just fine if only one output is used and would enable the NFB loop / presence control to be implemented exactly the same as with the 50W amp.

[Big_Mike]

I just have an itch to build a 6v6 Plexi.  Hoffman is out of 18 watt power transformers, what could I use in place of it?  Would a Tweed deluxe PT work fine?  Also, would a deluxe OT work instead of an 18 watt OT?

[sluckey]

Deluxe Reverb PT and OT would be my first choice. Might not fit Hoffman's little chassis though.

[DummyLoad]

I just have an itch to build a 6v6 Plexi.  Hoffman is out of 18 watt power transformers, what could I use in place of it?  Would a Tweed deluxe PT work fine?  Also, would a deluxe OT work instead of an 18 watt OT?

http://www.mojotone.com/amp-parts/Transformers_1/British-Style-18-Watt-Output-Transformer


--pete

[Big_Mike]

If I were to use one of the Deluxe Power Transformers, those put out in the range of 360-380 volts.  The 18 watt puts out 290 volts.  Would I have to make any modifications to the Hoffman 6v6 plexi circuit because of the higher output voltage from the deluxe power transformers?  I know that you need about 400 volts at the plates for the 6v6s.

[PRR]

> I know that you need about 400 volts at the plates for the 6v6s.

6V6 original rating was 315V.

A million good-sounding 6V6 audio amps run around 300V.

Fender's "hot" 6V6 amps do run around 400V. Like Tommy Ivo tipping Nitro into his Buick until it blew-up more often than it finished the 1/4 mile. Buicks and 6V6es are a little tougher than the makers admit.

But if you want a "hot" amp, why stop at 6V6 abuse? EL34 or 6L6 at full Plexi voltage will rock a small stadium. If you are not rocking stadiums, 6V6 at 300V will annoy the neighbors with ease.

[dude]

Take the chassis you want to use, if it wants a laydown PT, get one that fits that chassis without cutting, as clean fit. Is it a head, maybe you want a stand up PT, bottom line it all depends on the chassis you're going to use. Once you figure that out, you want around 400v B+ (just because most have that voltage on there 6V6 plexi's), like PPR says that's a lot of voltage for 6V6 data. Basically, anything between 290-0-290 (about 360v B+ depending on rectifier used, might get more with diodes) or 325-0-325 (about 400v B+ depending on rectifier used). The Tweed deluxe you have if it fits your chassis, use it. I'm working with 340-0-340 PT from a Hammond organ, a little high but I like hot rodding my Buick, probably around 325-0-325 with tubes. The tweed deluxe OT, IMO is too small, you'd want at least 20/25 watts but that's me, I don't like OT distortion, rather get it from my power tubes or preamp. If Hoffman doesn't have it, AE Supply has PT's and OT's that would work for you. Use the Deluxe PT if it fits, but think about the OT if it's small, now a Deluxe Reverb PT and OT would work well. Don't think the Deluxe and Tweed are the same, maybe I'm wrong.


al

[Big_Mike]

The 18 watt PT fits the bill and fits the chassis. I might go with a beefier OT like one for the Deluxe Reverb.

[shooter]

I might go with a beefier OT

I used EL34's in my plexi, PP self biased, so 20ish W audio, I used an UL OT meant for an 80W Marshall
I believe that was a big part of the tone
anyway, fwiw

[Big_Mike]

Has anyone installed a PPIMV instead of the pre-phase inverter master volume on the Hoffman 6v6 layout?  I would like to add that, was wondering what the tonal differences are.

[dude]

Big Mike, forget all questions, just build the amp as Sluckey or Hoffman has it, exactly the same. Use the PT and OT they suggest, you should have 400v B+, 18 watt PT would work but the voltage would be too low, IMO.


Forget the PPIMV, Sluckeys MV works great, amp is loud, sounds fantastic, very clean to crunch. Perfect for gigging. You can use 6L6s, for more bottom fullness and headroom if you have the right PT. An 18 watt will do 6V6 but might not do 6L6s, safety and justice.


I'm sure you read the link about the AO43 conversion I did, only suggestion I'd make is change the NFB R to 22K and the first stage cathode, 220uf or 330uf to 22uf. That's it, if you like a little floppy bass keep the 220/330uf.


You will not be dissatisfied. 


dude