Guiguang 300B for 300Beast.....

[Alastair E]

Ive just got two matched pairs of 300B from fleagay....

These are for a new project I'm doing which would be very loosely based on Tubelab's '300Beast' (I say Very loosely as I only have his description of the 300Beast....)

--I'll be winding the OPT myself based on a fairly large EI120 size core, and have a nice Parmeko oil-filled mains Tx for PSU, just need to find or maybe wind a choke for the PSU and get a nice chassis to build it on...

These 'GuiGuang' 300B seem to be pretty well made, heavy quite thick well formed bulbs, and the internal construction is 'solid' looking, and there is very little if any variation in the shape/construction of the internal parts or mica positions between all the four examples I have etc.
--Unfortunately, I dont have another 300B of similar type to compare to, only a single Shuguang mesh-plate 'globe-shape'--Which is quite different.....

I have No idea as to their reliability or what they sound like, as Ive not built the amp yet!
--Just thought I would let you lot know that they may be a fairly reasonable alternative to some of the mega priced 300B out there, and maybe worth a try for you 'tube-rollers'....

--These cost me 100 bux a pair plus shipping....

[tubelab.com]

These are for a new project I'm doing which would be very loosely based on Tubelab's '300Beast' (I say Very loosely as I only have his description of the 300Beast....)

The 300Beast was based very loosely on a 300B push pull amp published by Kevin Kennedy in VTV about 10 years ago. I morphed his design into soimething I could make with parts on hand. He used to have the schematic on his web site but it is not there any more. I know that I posted the schematic for the driver in a thread about it a while back. Sadly the 300Beast died about 2 years ago and I never got around to fixing it. I liked that amp a lot, and will not dismantle it until I make something new that sounds better. I can still make the Beast play if I connect it to an external power supply since only the supply has blown (shorted cap and fried diodes).

My tube time has been very limited lately so progress on the new one has gone very slow. After trying at least a dozen driver circuits the first choice if I were to build it today has to be the driver design developed live on this forum in the 6L6GC in AB2 thread. I have decided that there is no better choice if triode tubes are used. It also bears a striking similarity to Kevins original design. I will experiment with pentode based LTP driver circuits a bit before deciding what goes in the 300Beast MKII.

[tubelab.com]

The 300Beast driver board was done in 1999 using an old DOS version of Eagle. It will not translate into the new Windows version. I made a PDF of it which is rather poor. The input tube is a 5751 and the SRPP stages are 6FQ7. I believe I hacked up the PCB a bit after this schematic was made to make a "LTP" out of the SRPP stages by tying the cathodes together and using a common resistor to the negative supply.

I also have a PDF of the new driver which I believe works far better. The input tube can be a 6SN7 or a 6SL7 depending on the gain needed. You need a 6SL7 with a 300B. The output tube is a 6SN7. The mosfet followers allow a bit of A2 and eliminate blocking distortion, but thay may be removed if sand offends you. This design is fully differential and can be used to make a fully differential amp. Triode output tubes are needed, and they are wired in LTP with a suitable CCS in the tail. Grid current is a no-no here, so I bypassed the mosfets. I used 6L6GC's with an LM317HV CCS. The regular LM317 will blow up.

[Alastair E]

Hi George!

Thanks for the input...

What I was thinking up until your reply was summit like a 6SL7 LTP, phase-split with a CCS 6BH6 in its tail, followed by 6AM6 voltage-amp, then the mosfet-follower and finally the 300B pair...

The valve choice is what I have around, and no access to the 10M45 in the UK easily!

[kevinkr]

I selfishly decided some time ago that I needed to monetize my designs in order to help defray the cost of the site and my hobby activities. The original VTV article can be found here on: Tube Audio Project and Information Page - Lulu.com

Here is the most recent incarnation of this amp, built by a fellow in the UK:
Kevin Kennedy's push-pull 300B power amplifier

[JLH]

Originally Posted by tubelab.com

After trying at least a dozen driver circuits the first choice if I were to build it today has to be the driver design developed live on this forum in the 6L6GC in AB2 thread. I have decided that there is no better choice if triode tubes are used.

I'd like to hear what you think about the differences in the 6L6GC AB2 driver and the one used in the "Opus 5.0 Modern Mullard" driver as found in the links below.

The Opus schematic
OPUS 5.0 A Modern Mullard


The 6L6GC AB2 schematic
6L6GC AB2 Amp


The Pentode driver schematic
6L6GC AB2 Amp

How big of a deal is the variable DC bias on the first stage for the fully differential circuits? Obviously the Opus is not fully differential, but what merits do you see in each of the three driver types above?

Just a couple observations on my behalf - I would put a CCS in the Power Drive FET source follower like it is in the Opus. In the Opus I would ditch the 12BH7 in the Opus in favor of a triode wired 5879. Is fully differential all that much better?

Rgs, JLH