Inverted OTL with 6SN7/6AS7

What people are working on at the moment
Wolfgang
Old Hand
Posts: 229
Joined: Mon May 07, 2018 3:08 pm

#121 Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang »

Yes, live and learn! I suspected that a DC choke was not ideal in this position. But it’s really not so easy with this amp to decide what is untamed joy of playing music and almost infinite dynamic power and what’s already a little too much sometimes with some music styles.
The InvOTL sounds overall very different than typical OTLs so there was no real comparison possible. For example the InvOTL creates such a solid and deep and wide bass foundation which can carry and support a lot of action in the treble that it’s not obvious whether it is sometimes too much energy in the treble or over the top dynamic.
Only the direct A/B comparison with a lot of different recording material showed over time what is what. So now this amp has got another quality on top of the rest: equipoise.
Wolfgang
Old Hand
Posts: 229
Joined: Mon May 07, 2018 3:08 pm

#122 Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang »

I finished my InvOTL some time ago and thought the result is worth a picture.
InvOTL.jpg
DC offset can be easily readjusted with the two pots in the back. The mV meters can be turned ON/OFF for checking and getting it down to 0mV.

I mounted all the tubes on a separate aluminum board (would have been too many screws showing otherwise) which can be adjusted in height and tension/damping with the 3 screws in the middle. I found out later that it also improved the SQ a lot. It's amazing how much the 6AS7 respond to vibrations coming from the amp housing.

The chassis is packed to the rafters with circuits/psus/Vishay resitors for the SLB but all the AC is in a separate housing. Not even the slightest audible hum.

After drilling almost 50 holes with a drill press and WD 40 as a lubricant I decided to get a CNC router (2x2 feet) for future projects. Great toy, didn't regret buying it. Vector drawing is after all not that complicated!

The only problem I couldn't solve so far is the effect of the DC protection circuit on SQ. If it's connected (I keep it ON right now only until the amp has completely stabilized which is for about 7min) I can hear a little degradation. I used a separate 12V psu for the circuit. Maybe I should use the SLB for driving it (it has only about 50mA current draw)?
User avatar
Nick
Site Admin
Posts: 15707
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#123 Re: Inverted OTL with 6SN7/6AS7

Post by Nick »

I decided to get a CNC router (2x2 feet) for future projects. Great toy, didn't regret buying it. Vector drawing is after all not that complicated!
Metalwork is one of my pet hates, what did you get and how are you finding it?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Ray P
No idea why I do this anymore
Posts: 6294
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

#124 Re: Inverted OTL with 6SN7/6AS7

Post by Ray P »

Wolfgang wrote: Tue May 12, 2020 4:13 pm I finished my InvOTL some time ago and thought the result is worth a picture.
That's a nice looking build Wolfgang. I think some more pictures would be in order, maybe under the bonnet, or, as you would say, hood.

It's a shame the Owston get together has had to be cancelled, a casualty of Covid-19, as it would have been nice if you could have hopped on a plane and brought it over to demo it. :lol:
Last edited by Ray P on Wed May 13, 2020 6:36 am, edited 1 time in total.
Sorry, I couldn't resist!
Wolfgang
Old Hand
Posts: 229
Joined: Mon May 07, 2018 3:08 pm

#125 Re: Inverted OTL with 6SN7/6AS7

Post by Wolfgang »

Metalwork is one of my pet hates, what did you get and how are you finding it?
I got the E4. Now it has been replaced by the “Evolution 4”. Basically the same but with the one piece spoilboard and a stronger router mount, important for precision which was actually the only weak point IMO with the E4.
https://www.bobscnc.com/products/evolut ... router-kit

Together with Vectric software it's nice to work with. The precision of the cuts is about +/- 0.5mm (maybe that is no longer a topic with the improved router mount?). This one also needs a PC /laptop. More expensive CNC routers come already with a controller. The plywood construction is deceiving as it looks kind of flimsy at first but it’s solid like a rock. Just takes some hrs to put everything together.
That's a nice looking build Wolfgang. I think some more pictures would be in order, maybe under the bonnet, or, as you would say, hood.

It's a shame the Owston get together has had to be cancelled, a casualty of Covid-19, as it would have been nice if you could have hoped on a plane and brought it over to demo it.
The “under the hood” picture is rather disappointing regarding aesthetics but there was simply no other way to fit both channels in the biggest amp housing I could get. But it’s 100% clean regarding electrical influence between AC for the 6AS7 heaters and psu/control boards and signal paths.
InvOTL inside1 .jpg
To configure the amp as two monoblocks would have been the right decision and more aesthetically pleasing but that would have doubled the Maida reg. & control board. So it is what it is. Sound is all that matters. I could remove all the wooden parts as they are only used for centering the sub-chassis with the tube sockets until the top cover has been screwed on and holds it in place. But I left them in place in case I want to adjust/change something.

Yes, it would have been fun to demonstrate this amp at the Owston meeting. I am sure there would have been many positive reactions as the sound quality is top notch and has the speed, clarity and airiness of OTLs, the lower mids timbre and mellowness of very good SE tube amps with OPT (the one thing I really always miss with TS OTLs), and solid state punch and speed in the bass.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#126 Re: Inverted OTL with 6SN7/6AS7

Post by Paul Barker »

The description u give sounds a bit like the experience of walking into Bruce Edgar’s room at VSAC.

Also not dissimilar to my experience when I breadboardes the Glass Audio version of Rosenblit’s early adaptation of Futterman.

I liked that and haven’t heard an otl I like as much as that early Futterman inspired Rosenblit.

At last though you’re report sounds bang on the money. I’m just gutted I never got given a round tuit at the time. But I suppose no one to blame but myself. I spent my round tuit’s working too hard for my employers and lining their pockets with too much of my time.

Good old Stephie for giving the world the inverted operation in an OTL. Stephie is the best!
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply