6BG6GA IT coupled

[scott17]

Here's my latest creation. "Super" 6BG6GA, fixed bias push pull, Lundahl LL1660S interstage transformer splitter, 6N23P paralleled driver, separate 6BY5 hybrid rectifer per channel.

I'm now a big fan of fixed bias and interstage coupling. Wow, sounds absolutely amazing.

[Geek]

Very industrial retro-Telefunken-pro-stuff. I like it!

I assume you have panels for visiting kids or kitties?

[Shannon Parks]

Very nice concept and implementation, scott17. Can you link to what that tubing/girder is? How rigid is it?

Shannon

[WA4SWJ]

You don't see that kind of construction every day. Very nice work!!! I like it a lot.

[scott17]

Hi all,

Thanks. I had plans to use some sort of lexan panels or such. Haven't figured that part out yet. No young fingers or domesticated wildlife around here anymore. At this point I'm just glad to be listening to it.

The rail system is manufactured by Microrax http://www.microrax.com.

It's pretty strong stuff, and this amp weighs 37lbs. From the website, it is designed for projects 18" cubed or smaller. My amp is 12.25"W x12.25"D x9.5"H.

Scott

[TerrySmith]

Very nice! This reminds me of most of the machinery cages where I work (I work for a major automotive supplier).

Is there a schematic online?

[scott17]

I have attached the schematic. The bias circuit is a different than shown as my plan to use the 70V tap on the Antek transformer would not work. If I had though about it more I should have realized that since the 70V tap is not a separate winding that it does not actually deliver 70V except for half of the AC wave. For the other half wave it gives 330V (400-70) and there is no way to obtain a ground reference since I'm using the full winding for a full wave bridge. Anyway, I ended up converting the bias rectifier to a full wave bridge and I'm using a small flatpack 5W, 12V - 230V transformer with the 12V side connected to the 6.3V filament supply. This only gives me -30V bias. So right now the power tubes are running at 60mA instead of my planned 50mA @ ~ -45V bias. The additional power tube current has brought my B+ down to 400V. I have ordered a 6.3V & an 8V transformer to replace it, not sure which one will be right. When I get that issue resolved, I'll post a revised schematic. That being said, it still sounds great.

Edit: One more change, I ended up using 50R series resistors for the 6BY5 instead of 62R, and 50uF for the second PS capacitor instead of 100uF.

[Kyle K]

Have you tried setting the output tube bias through the usual cathode bias resistor and grounding the center tap of the interstage transformer secondary? I asked Kevin Carter years ago about running the bias voltage through the LL1660S secondary as you did and he said it would work, but sound different. Just curious. He didn't elaborate on the differences in sound.

[scott17]

Thanks, I have not tried that, but then it would be a cathode bias amp. My design goal was a fixed bias amp. I've made many cathode biased amplifiers.

[Quad]

Great Scott!! That's so clean and nice.

[Brik]

Very nice construction.
I like the professional, precision look.

Thank you for sharing!

[scott17]

Here is the revised schematic. Now the output tubes are bias at 49mA. I'm very happy with the way this amp sounds.

[Cubdriver]

Scott, that's pretty slick framing material - it's like mini 8020 stuff. http://8020.net/

Thanks for the pointer.

LOL - more stuff to buy.....

-Pat

Hi all,

Thanks. I had plans to use some sort of lexan panels or such. Haven't figured that part out yet. No young fingers or domesticated wildlife around here anymore. At this point I'm just glad to be listening to it.

The rail system is manufactured by Microrax http://www.microrax.com.

It's pretty strong stuff, and this amp weighs 37lbs. From the website, it is designed for projects 18" cubed or smaller. My amp is 12.25"W x12.25"D x9.5"H.

Scott

[leadtower]

Scott; Can I ask what drove you to consider this circuit? I mean did you hear an IT amp and like the sound? Or were you intrigued by the idea of using that topology? I'm curious what motivated you to try this. Hope you don't mind me asking. Great job.

[scott17]

Hi, thanks. I had not heard an IT amp before this and had not built a fixed bias amp prior to this. I have done much research on fixed bias and IT coupling, and yes I was intrigued by the topology. I had the tubes and most of the parts in stock. I came to discover that paralleling the 6N23P-EV sections and running at this operating point would (should) work well with the Lundahl LL1660S. I had the transformers custom made by Heyboer, as I wanted a 6 ohm seondary to match my speakers. I am very pleased with the way it turned out. Very dynamic. Deep clean bass, clear midrange, sparkling highs. The IT splitter works very well. The stereo imaging is excellent. I measured 10W RMS in triode and 25W RMS in UL, before clipping. I will build more of these, but probably in a conventional chassis and not the framwork system. It's not the easiest thing to work on after it's built, although it can be done.

There is one mistake on the last schematic I posted. I did not tie the cathodes of the 6BY5 rectifiers to the B+. At first I was concerned about possibly exceeding the heater-negative-to-cathode rating of 450V. Since the heater winding is not referenced to ground there's no problem. Also, tieing the cathodes to B+ effectively puts the two rectifiers in parallel, which I did not want.

Scott