• 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.

300B Bias ?

Hi , at all.
I finished an amp, with 390v at plate and -68v in 300B Electro Harmonix cathode with 1K resistor.
Only have 68mA current.
If i lower the 1K resistor to 880ohms , to increase current , but low the voltage to 64V and 72mA.
Could i have 80mA or more current with the 400V ?

Santiago.
 

Attachments

  • 5.jpg
    5.jpg
    68.2 KB · Views: 319
Anode to cathode voltage

You only have 325 volts at the plate, not 390, you need to subtract your cathode voltage:) Not impressed with your output transformer that drops 15 volts, which means the primary has an approximate resistance of 230 volts. Not, in my experience, a very good indicator of quality:(
 
What's the 45 tube doing that the D3A can't do by itself as a driver for the 300b? There a few circuits out there using just this 2 stage combination. D3a in triode has a gain of around 70. Quite enough for the amp. I've never used it but from what I read you have to take care to stop it oscillating. Are you able to check that?


.
 
Last edited:
Member
Joined 2004
Paid Member
LL1664 each primary has 74R DCR, so series connection results 148R, which is acceptable.

If you use it in this amp, must to use LL1664/100mA version (the other SE version only capable 50mA) ... but it has only 17H primary inductance, which calculated impedance is only -about- 2k at 20Hz.
 
LL1664 each primary has 74R DCR, so series connection results 148R, which is acceptable. If you use it in this amp, must to use LL1664/100mA version (the other SE version only capable 50mA) ... but it has only 17H primary inductance, which calculated impedance is only -about- 2k at 20Hz.

I've used LL1664/70mA with my 300b amp but it sounded a lot less good than LL1682/50mA which is 5K into 5R. My speakers are nominally 8 ohms Alpair 10M. I don't know whether to attribute the LL1664's inferior sound to less inductance or a lower Primary. Or could be both. But if inductance is the issue, 100mA would be worse still.
 
What's the 45 tube doing that the D3A can't do by itself as a driver for the 300b? There a few circuits out there using just this 2 stage combination. D3a in triode has a gain of around 70. Quite enough for the amp. I've never used it but from what I read you have to take care to stop it oscillating. Are you able to check that?


.

Andy, how are you.
I have had 2 previous amps with D3a 300B, following Andrea Ciuffoli's diagram.
This is just a test.
With respect to oscillation, a channel is oscillating.
Sound wise I like it and it doesn't sound like 7 or 8 watts like the old amps.
It seems like there were many more.
 
With 450v in plate and -77v at cathode with 1K resistor , would be 373V x 77mA , 28.7W of dissipation for the 300B ?

For a real 300B (on average) yes. And if the tube is good (linear) it should give something like 8W with some 3% THD into 3K.
With so many variants in the market and untested tubes sold at lower price (but not cheap by any standard) some might be disappointing.
 
32W is hot but possible, even for the real 300B by WE.
36W is in practice an absolute max but tube life I don't think is long. 30W dissipation is in my experience the best compromise between performance and tube life with 300B's (solid plate and near perfect replica of the original). All other variants with mesh plate or punched plate...better to stay at 24-25W max.
The only 300Bs that can run at 40W or more are those on steroids made by EM and KR and called XLS etc...
 
After 1000-1500 hours you will surely see a clear drop in performance. Not worth. Better to go for a PSE where tubes run at 30W. The pair in the PSE will last much longer than 2 tubes (one after the other of course) in the SE running at 40W, you will get 50% more power with the PSE at the same distortion or even better...
In other words tube life drops rapidly when you approach max plate dissipation.

Some tubes are rated conservatively, especially the older ones or the sweep tubes, so you can run then at max rated power and get long life. For the 300B, the original and copies, 40W is more an absolute max than a design max. Even 36W is too hot....
 
Last edited: