diytube.com

And it's only the third 300B I like the sound of

The AIST (Active Inter Stage Transformer) works quite well buffering the otherwise-unable-to-drive 6SL7 mu-follower.

I couldn't Google worthwhile info, but 300B grids aren't as delicate as people's opinions lead us to think. I've had her pretty hard into A2 and the distortion on bass isn't that objectionable and not near as nasty as the transformer saturating. Couldn't even see a stress on the grid even through my camera on macro, which is pretty IR sensitive.

I'm still playing with the cathode resistor. Got a pair of el-cheapo Valve Art tubes in there now, but when dropping in Sophia mesh plate, they are so hot that a typical 880R cathode resistor will make the mesh plates redplate with 400Va-k.

The pic shows a 5U4 in place of the GZ34... that's because we only had one in the store and the boss snitched it to demo the Golden Tube 300B up front

A big complaint people have is a 300B with ZNFB has no bottom end... well, the CFB makes this a "fat bottomed girl"

Cheers!

Do play with that rectifier... this amp is extremely sensitive to it.

After a listening session in the store, it was determined that between a GZ34 (2 types), NOS 5U4 and some el-cheapo Chinese 5Z3, the 5Z3 was everyone's favourite and the GZ34 everyone's least favourite.

Cheers!

Looks like that 300B has *plenty* of drive! Indeed I've heard great things about Tubelab George's Power Drive drive 300Bs. I like your circuit better, Gregg. Seems pretty bulletproof.

Shannon

So George did do that.... I Googled and Googled for design references and nothing came up in English, French or German
.....
DERP! I even have that page in my bookmarks

I originally designed for combination bias and have a knob to tweak it for each tube like George shows. But the customer insisted, quote, "I want drop in and go. Maybe a hum balance is OK. But no mussing around like this other one you guys have for sale; adjust this, adjust that, it's ridiculous!"

We had a larger audition for the amp yesterday. The response was the most encouraging I've ever had ... and it looks like I may be busy for a while if the deposits are made

Cheers!

The power supply would be a lot easier to build with the Vishay 50uf 900 volt caps and other value caps http://www.digikey.com/product-search/e ... =search_go

Do you think the output stage is that voltage sensitive that 40 volts less would make that much difference , or it is the fet being affected by B+ change .
Is the wiring all point to point , or part of it on a circuit board

Unknown.

On the equipment, the frequency response remained the same, as did the transient response. The only difference was a slightly reduced power output. Being class-A, supply impedance should be moot

She's pretty tight in the chassis (upskirt pics soon) for larger caps.

Cheers!

Here's the underchassis shot:

http://geek.scorpiorising.ca/contrib/Ge ... sis_sm.jpg

She wasn't fully zap-strapped yet, so excuse the mess

The driving MOSFETS are mounted on the chassis stiffening L-bracket outer edges. The black block near them is a clamp-on choke on the gate lead to help the beads do their job better.

The circuit board on the left is the regulators and the one on the right is the cap board.

The power resistor near the cap board has heat sink goop on it to transfer heat to the bracket and after even an hour running, the resistor is still cool enough to touch for 5 seconds.

The chokes and rectifier supply transformer are on a U-bracket, mounted via the power transformer bolts, all rubber grommetted for mechanical hum isolation. There is an electrical ground safety lead to the mechanically isolated components.

The capacitors epoxied to the chassis behind the stiffener are the cathode bypass caps wired across the 1K/20W resistor, which is barely visible, mounted to the stiffener.

The bottom plate has vent holes only directly under the vertical power resistors of the regulator board and the cathode resistor. There is enough convection so that after an hour running, they are within a temp range to keep them at 70% of their rated dissipation. Given that I've overdesigned the power components with at least a 2:1 margin (closer to 3:1 with many of the components), it's good to go for summer use

Cheers!

Back a year or so ago, Shannon posted an ebay link with cheap Chinese 300Bs, I think for $50. Did anyone try them, and how do the Valve Art tubes sound? Are they worth $100?

I didn't get the 300B's at the time because I thought I'd never make a 300B amp

I dunno what the Valve Art's are worth without lookup because they're pulls from a Golden Tube we got in from a Craigslist purchase. The boss dropped in his Sophias and handed me the VA's. Worth $100? You betcha!

I've listened to both the Sophia's in this amp as well as the VA's.... the amp is less sensitive to the 300B than the rectifier. In other words, the little improvement with the Sophia's didn't justify the extra value they had.

The VA's sound WAY better in this build than the Golden Tube or the Canary we had on loan (which I shouldn't compare since the Canary was a PP).

May not be a tube rollers delight, but is kind of a trademark of my builds... fair consistency with rolling.

I wish I could describe the sound, it's a 300B that doesn't sound like a 300B - bass and mids are balanced with extra sparkle on top.

Cheers!

DeathRex wrote:Back a year or so ago, Shannon posted an ebay link with cheap Chinese 300Bs, I think for $50. Did anyone try them, and how do the Valve Art tubes sound? Are they worth $100?

My cheapo 300Bs have few hours on them, but sound A-OK and test like true 300Bs - super linear curves. But the bases had stickers on them instead of silkscreen. Guess I can't complain.

Thanks for the under-the-skirt pic, Gregg!

Shannon

Hi Shannon,

...super linear curves...

Are you using some sort of curve tracer, or are you constructing the curves the painful way? If you have a curve tracer, what can you tell us about it?

*edit*
Schematic removed - decided to keep it proprietary after discussions with those close to the project.

Those that have it, please do not repost it elsewhere, thanks!