The Unofficial THD Flexi, Univalve and Bivalve Thread.

Stu Blue

3,167 , the unofficial home for THD fans and owners seems to have died, which is a great shame since they were an unusually helpful, friendly, and civilized lot on that forum. So i'm starting this thread for those of them that pop in here too.

I myself run a Bivalve, which is a class A, parallel single ended amp, which was designed to produce power amp dirt, so it doesn't really do real fender cleans or Marshall hard bright either (I also have a vintage JTM45). What it does do is produce very musical distortion, notes with character rather than sheets of white noise. It's one of those amps that come to life on stage and naturally fits in to a band's sound, adding warmth and class... I've taken it to open jams and never heard it sound unpleasant, no matter who and how they'd dialed it up.

The ability to run any octal valve from 6G6 to KT88 is interesting, but you can't get it to go from fender through vox to marshall just by swapping valves. In the tone stakes circuits rule. Valves add cork sniffing flavours not fundamental character.

Enough from me, over to you guys.

EDIT Jan 2012 New THD forum here
Last edited:


I've got a Flexi 50 coming in next Monday or Tuesday. Really excited to spend some quality time with one. The only time I've played one was at fairly low volumes for around 10 minutes. I've got a few jazz gigs coming up in January and I'll be using it extensively in a 9 piece.


I had a BiValve. Wish I still did. It sounded like a wide-open Plexi. I got the best tone out of it when using 1 6L6 with 1 EL-34. I am probably going to buy another one. It's perfect for band practices because you can get some really nice output tube distortion at a reasonable volume.

When I was using this at brand practices I took the Hot Plate out of the circuit, dimed the volume, and just controlled the tone with my guitar's volume knob. At home I was running it was with 1 6V6 and it was perfect even though it sounded nothing like it does when it's loud.

If you're into 4 input Marshalls but have struggled with them being too loud, give this amp a look. It's really that good. But if you're into high gain, it's not going to work for you.


Senior Member
I'll sign in,,,, Stu, glad your here, always liked your posts about running the BiValve in live situations. Too bad about the Univalve net, I really enjoyed it.
OK,,,,so us THD users can have our small part of the action, unofficialy of course.
Use my Univalve 'most every day.


Gold Supporting Member
I had a Bi-Valve that really helped me figure out what types of tubes produce what tones, etc. I had hours and hours of fun swapping and experimenting with it. I kick myself for getting rid of it being as flexible as it is. Plus you can't beat having a built in attenuator.
Thank you Stu Blue for starting this thread .....

I also was a regular at the other forum .... and have learned to trust and respect many over there. Hey everyone ....:)

As some of you know THD makes my favorite amps .... at this time I own a Series One Plexi, Bivalve and Flexi and a few other things THD has made. For me they very much make tools I use. Be back soon .....


Hi there ,
I am a BiValve user ,
Proud to own one , great amp . I have tons of tubes for trying.

I would like to share some info here .

Best Regards ,


Agree, to bad that is shut down.

I had a Univalve that i sold a couple of weeks ago, now i have a Flexi.
Now have got my self some tubes so it´s time to expriment a bit.

I play a Eric Johnson strat and have some pedals of course :)
The pedals are:
TIM- Keeley katana - Zenkudo - Clay jones clone - MXR Phase 90 - T-rex reverb and a boss chorus.

Merry Christmas to you all :JAM


Silver Supporting Member
I used to own a Flexi-50, and would like to get another one soon. Some of the sweetest tones!

Stu Blue

For those of you who own uni/bivalves and didn't visit, here are the tone settings that Andy Marshall (who designed the amp) uses himself.... (strat and often slide as I understand it) ...

Roll (less) vol and all tone at max (edit corrected, now right)
or Rock (more) vol noon
Treble and bass at 3 o'clock
Attitude (high end gain) at Max

It's a fine high lead tone with a strat, but being an extremist by nature, I tend to run mine all tones at Max with rock volume backed down a bit to 11 o'clock. I mention this because the tone circuit (Baxandahl) seems to add more gain as you turn up the controls than most other amps I've used. Also getting a wide range of good tone from your guitar volume control requires careful setting of the amp's gain/volume control... Class A amps don't behave the same as push pull AB1 ones... the dirt comes on more gradually.

I had a little trouble coming to terms with the bivalve when I first got it and Andy was extremely helpful in sorting me out. Once I used it "live" all my doubts about the amp disappeared. It's chalk and cheese different to my JTM45 but I'm quite happy to take either of them out.....
Last edited:


I have had a Flexi for about 5 years and love it. An ASAT special into Flexi has been my main gigging rig for almost 3 years in a modern country band. The gritty pushed clean sound that can be sweet or mean, works really well in a band context. Took me a while to get the amp dialed and get my chops up to the level they needed.


Flexi owner here. Never in my life have I received as many compliments as I have with my Flexi.

After becoming a complete tube snob because of this thing, I've settled on a pair of Tung-Sol 5881 Reissues and couldn't be happier.

Killer amp. I can't see me ever getting rid of it.



Had a Univalve when they first came out. Cool amp but not quite what I was into at the time and sold it a number of years back. Nice though.

Last December I picked up a Flexi since I knew about the THD quality already and that is an absolute stellar amp! I use mine for heavier classic rock to big band jazz and everything in between. Killer

Joe Porto

Currently running:

UniValve (NOS 5881) into Palmer speaker simulator for direct recording.

BiValve (NOS Mullard EL34 and RCA 6L6) into Tone Tubby AlNiCo 2x12 for clean guitar recording.

Flexi 50 (GE 6550s) into Eden 2x10 for bass guitar recording.

I've thought about selling them, as they are not used as my main amps, but I always find a use for them in the studio, so they stay.


Silver Supporting Member
Bivalve here, Rocking 6v6's and mullards in the preamp stage.

Lovely little amp that is built like a tank.




Signing in as well. Miss the old, and would be happy to find a home for it if the old owner/moderators are interested.

I use a Bivalve for recording. Pretty much Strat->couple of pedals->BiV. Since I record into Logic Pro, i add a touch of something or the other once i've tracked the pure tone to disk.

It's currently got 1x6v6 and i have a 6G6G on standby, ready to try out once i'm done with my current recording project. My pre tubes are various 12AX7/a's from Mullard, Telefunken, RCA, etc.

At some point I will get a boutique combo like a Reeves, Emery Sound or Tungsten - but for now i couldn't be happier with my sound.

Happy Holidays all
BiValve 30
GK250ml mk2
bunch of pedals - esp. Fulltones

Custom cabs w/Jensen, Weber, Celestion


Silver Supporting Member
I've got a Uni and a Bi. The Uni I use in wet/dry stereo setup, with the lineout feeding a 100% wet delay feed into a Tech21 Power Engine for ambience. Amazingly touch sensitive. A lower gain preamp tube was the key to getting it to be something other that Rock City all the time, although it does a superb Rock City.

Trending Topics