300b SET by J.C. Morrison

[Blutwurst]

My first diy 300b SET - found the schematic online and started to order parts...funny project and fantastic outcome.

5u4GB as rectifier, 6SN7 as driver and 300BXLS from emissions LAB - tested output with my HP8903 and managed to get 15W pure tube craziness


signal cable

thought if I build an amp than I could do the signal cables as well....ordered pure silver wire and all the other things I needed to build those thingys

300b

mono block - step by step

300b

mono left side

300b

right mono

300b

PSU

300b

all3 united

300b

close up

300b

close up

300b

PSU close up

300b

front view

300b

rear view...multicore connection for PSU and WBT speaker terminals

300b tubes

JJ Tesla, some guadong tube ( survived one evening ) and the 300BXLS

my playground

my little relaxing area...if stress at work I take a few hours and build something....not so important what as long as I'm busy

The final picture

Completed and hooked up....300BXLS and my sexy horns

 

[VB]

Well done! What is the operating point of your 300BXLS'es? I've made something similar many years ago, but that was with standard 300B tubes.

 

[Blutwurst]

Hi VB,
Thanks a lot !!!
The 300BXLS'es can handle max 550V...I run them at 466V @ 80mA...as far as I remember was the plate curve quite linear around that voltage

 

[VB]

And B+ or bias voltage if I may ask? (sorry for this "interrogation". Just trying to learn as much as possible).

 

[Blutwurst]

I think the grid voltage is about -100VDC, B+ @ about 460VDC and the plate current is about 80mA...if you like I can send you a copy of the schematics

 

[Dyret]

Nice! Can you just post the schematics here?

 

[VB]

If the operating point of the output tubes is 466V/80mA and bias is about 100V, I would guess that B+ is 566V with so called "auto" bias.

 

[Blutwurst]

sure

I made a few modifications on the way and changed R8 and C7....R8 to reach higher voltages C7 for the eyes. Thought they would look better so they are just for the optics
The large silver cylinders on the PSU are the C7 caps. Solen 650VDC 100uf. Had to polish them for a week or two.

 

[Blutwurst]

If the operating point of the output tubes is 466V/80mA and bias is about 100V, I would guess that B+ is 566V with so called "auto" bias.

You know what I'm kinda keen to find that out....have vacation now and try to find time to hook up a handful of multimeters and measure once more...its been a while that I completed the build. So I could be wrong...better save than sorry

 

[VB]

Yes, get back to us. But be careful and don't electrocute yourself in the process. I hate to be the one responsible for that.

 

[Blutwurst]

Yes, get back to us. But be careful and don't electrocute yourself in the process. I hate to be the one responsible for that.

I make sure that I have one hand behind my back while measuring

 

[jane]

The 300B needs a good driver that delivers enough clean voltage into the grid. But you will pay a price when the driver DC connects to the input stage. The price is that it burns away more than 70 volts in the cathode resistor (R3), voltage that the driver cannot exploit. If you look at the schematic, the max positive voltage swing from the driver stage before it runs into cutoff is equal to the voltage drop over R4 which is 290v-215v = 75v. That can limit the ability to drive the 300B into full tilt, due to the design of the driver.