• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The ultimate 300B

Jump to Latest
tubelectron

tubelectron

Member
Joined 2008
guilhemamplification.jimdofree.com/
#1
The ULTIMATE 300B

Well, I know that the name is somewhat pretentious... No importance.

I have chosen this name just because it is the last version of my serie of prototypes that I experimented with VT25/10Y, 50, 2A3 and ultimately 300B. It was on Feb. 6th. 2008... But the final model is now on construction... Since a few days...

This amplifier is built in 2 monaural identical "mono blocks" that I wanted to built as compact as possible : 275mm x 175mm x 65mm for the chassis, with a vintage electronic instrument look.

It has a bi-power switching : LO 4Wrms / HI 8Wrms nominal power.

Here are the performances measured on the 2008 prototype :

THD for 1KHz Sinus @ 8ohm resistive load, no NFB :
HI 10W = 5.32% (at the threshold of clipping)
HI 8W = 3.75% (at the nominal power)
HI 4W = 1.69%
HI 1W = 0.7%
LO 5W = 4.96% (at the threshold of clipping)
LO 4W = 3.50% (at the nominal power)
LO 1W = 0.85%

Bandwith @-3dB/8Wrms = 8Hz-25khz

The 2008 prototype, still alive of course :
An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


Tracing the chassis :
An externally hosted image should be here but it was not working when we last tested it.


Starting the drilling :
An externally hosted image should be here but it was not working when we last tested it.


Drilling completed :
An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


Wait and see...

A+!
 
#5
I need some custom engraved plates, me too... can you recommend a company?
Click to expand...

Unfortunately, no...

I used to go in a multi-service shop where the boss had a laser-engraving machine which was compatibe with .jpeg fichiers. His machine was not enough powerful for engraving metal but gave excellent results on plexiglas and dilophane plastic materials.

The shop closed its doors last year for lack of business 🙁 , but the boss accepts to still engrave me custom plates as a "part-time" job 😱.

I think that in Belgium you would certainly be able to find some specialists in that field, if you are willing to pay the price for a custom job : I will have to pay 100E for the 6 different custom-engraved plates of my ULTIMATE 300B. I will display them here when I will have it.

A+!
 
#7
#9
Hi,

Well done.

Can you tell us more about the driver stage and how it all sounds?

Ciao,
Click to expand...

Yes, thanks ! I can only speak about the results of the protoype, since the 2 definitive amps are not wired yet...

1 - this amp has only one driver stage, so a total of 2 stages including the 300B power stage. The driver is a // 12AX7 SRPP with a voltage gain of 64, RC coupled to the 300B. Power output is 8W rms/8 ohms @ 0.776V rms input.

2 - The performances are much better than the 6SN7 direct coupling 2 preamp stage configuration found everywhere (that I also tested on my prototype). More power, more headroom and linearity : the 300B is suitably driven, giving 8.0W rms @ 3.75% THD with no NFB, and the harmonic gradation is regular, H2 predominant.

A+!
 
#10
Hi,

The driver is a // 12AX7 SRPP with a voltage gain of 64, RC coupled to the 300B. Power output is 8W rms/8 ohms @ 0.776V rms input.
Click to expand...

That's what I figured seeing the pair of 12AX7 on the chassis.
Personally I'm not a fan of using //'ed triodes as it tends to create a somewhat "diffuse" sound. This is (and it's just a guess) probably due to small electrical differences within the twin triodes.

Either way, in order to obtain similar gain and IMHO markedly better drive characteristics using a pair of EC91 may be a viable option?

OTOH: see your signature.😉

Ciao, 😉
 
#15

Attachments

  • DSC_9725.jpg
    DSC_9725.jpg
    397.2 KB · Views: 799
#16
SRPP

My understanding is that the SRPP is really just a stacked cathode follower, which is a very viable way to wire up a triode driver stage for the 300B tube. However, cathode followers have their drawbacks too. The one way to "fix" a cathode follower, according to the late, and very brilliant, Allen Wright, is to fix the current through them. Thus, are you using, or have you considered using a constant current device above the SRPP tube?

Also, is the feed to the 300B being tacken off the plate of the lower tube, or the cathode of the upper tube in your SRPP setup?

Retsel
 
#18
Thank you, Guys !

using a pair of EC91 may be a viable option?
Click to expand...

Yes, probably ! But I just wanted to use "all-around" tubes, in fact...

Which speakers you are using with it? Have you tried full range Fostex?
Click to expand...

I have several speakers to use with my amps : crossover-modified KLIPSCH HERESY 1 HBR, Magnepan SMGb, and several DIY-design enclosures from FOSTEX FE-103Sigma, TRIANGLE T17FL2, Bookshelf models to 3-way horn+15" KLIPSCH CORNWALL-like. No multi-amplification.

where did you get the capacitor holders?
Click to expand...

I had them from RadioSpares, these are black plastic made.

The one way to "fix" a cathode follower, according to the late, and very brilliant, Allen Wright, is to fix the current through them.
Click to expand...

I use another method (a balancing bridge) but my circuit is still the conventional SRPP circuit, except for the // triodes. In fact, the SRPP has some variation issues only when you power it on and off : the balancing bridge null these.

A+!
 
#19
Unearthing the topic :yawn:...

I finished the wiring of my ULTIMATE 300B today : it's never too late 😀...

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


Next step : checking and powering... In the days to come... I hope...

A+!
 
#20
The amps are finished, tested and meet their expected specs.

Sine waves at 4 and 8 Watts Rms output :

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


The complete schematic of the instrument :

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


You've got it all now...

A+!
 
Last edited:
You must log in or register to reply here.
Top Bottom