6BX7

[vetmed]

I have been using this tube in place of 6as7 as I have quite a few left over from a project that did not quite materialize. Seems to work very well in my Crack with Speedball, been in almost daily use for the past couple of weeks. I'm also using 6sn7 with an adaptor in place of the 12au7. Now the pinout for the 6bx7 is the same as the 6sn7 so I wonder if it would work as the driver tube as well as the cathode follower. Like drives like? I've already determined that running two of these is not going to exceed the heater current so I'm wondering what might need to be done to make it work. I apologize for not doing a little more homework but since this is my most recently completed Bottlehead project I of course can't find either manual! Thanks

Robert Lees

 

[Paul Joppa]

It might work with Speedball (bad idea without though), but the current and voltage are way low on the driver, throwing all the stress onto the output tube and making the distortion situation well off optimum. I'd run 10-15mA and 5v bias to get the plate voltage up to 75v. That would require a heat sink on the driver C4S.

 

[vetmed]

I would like to try it if someone can help me figure out what needs to be changed. As I said I can't seem to find the Speedball manual so I would only have a very vague idea how to proceed. Thanks for the help.


Robert Lees

 

[vetmed]

I've found the speedball manual! I also read the thread about using the 5687. Does there exist a diode with a 5 volt drop that might be used with the 6bx7? Or could I use resistor and bypass cap on the cathode, say 5V/.015 A ~ 333 ohms suitably bypassed? Perhaps a battery on the cathode? Perhaps a negative battery supply for the grid? Whatever gets used it seems that a more robust transistor than the mje350 would be needed, perhaps the one used in the 5687 mod, suitably heatsunk? I believe R1 would have to be changed but have no idea to what value, only that it would be what was necessary to get ~75 volts on the plate. I'm open to direction, suggestions, guidance. TIA



Robert Lees

 

[vetmed]

From a copy of the c4s manual I believe the following changes should allow me to use a 6bx7 at 10 ma. First change R1 to 100 ohms. Second change R2 to 85Kohm. Third a clip-on heatsink for the MJE350 transistor should be fitted. To bias at 10 ma and PJ's suggested value of 5 volts requires a 500 ohm resistor suitably bypassed. I'll try this when time allows and I've collected the necessary parts. If there are any errors here please let me know. And if anyone perhaps crazier than I am tries this I'd love to hear how it works out!


Robert Lees

 

[Paul Joppa]

With the current production HLMP-6000, R1 would be closer to 86 ohms for 10mA - the ols C4S manual used a different version of the LED, which is no longer available.

The 500 ohm resistor is not a bad idea; you may even want to use a 1K variable resistor to set the plate voltage exactly. The only problem is that some people hear sonic degradation from the bypass capacitor. I say, give it a try and if it seems promising then you can look into better bias schemes.

Among those possibly-better bias schemes, you can stack LEDs in series and/or use other diodes - the high-voltage SiC Schottkys are currently popular on the forums - I have not tried them. I've heard of problems with battery bias (exploding batteries being the worst) so I'm cautious about that. In the Paramount "soft-start" board we use a 431 chip as an adjustable zener-equivalent.

 

[vetmed]

Thanks for setting me straight about that resistor value. IIRC the voltage drop for the red LED used to bias the driver stage is ~1.3 volts. From the thread on subbing the 5687 I see that the bright blue LED drops about 3.2 volts. Any trouble using the 2 in series to get close to the desired 5 volts? Thanks


Robert Lees

 

[Grainger49]

Paul,

I just bought a hand full of HLMP-6000s.  I offered some to Xavier for his project.  How do I determine whether they are old or new versions of the LED?  Forward drop or current with a specific voltage across them?

Thanks!

 

[2wo]

Robert,

You can string LED's in series to get the voltage you need. When I do it I use 2 of the same LED. I don't know if using different ones together is good or bad. If you look thru the catalogs you can find LED's with a wide variation in drops...John     

 

[vetmed]

I've found 5V drop LEDs at Digi Key, red, green, and yellow. Apart from aesthetic considerations what other physical parameters are relevant. Most of the ones they stock are 20ma and are of varying light intensity. Several even look like the ones used in BH products.


Robert Lees

 

[vetmed]

Well I ordered some parts from Digikey and installed them. I put the 2 resistors on the side opposite that of the transistor so that the heatsink wasn't shorting anything (its anodized so I don't know if it is conductive or not). Installing the 5V LEDs was a bit of a pain, they're quite a bit larger than the old ones. Checked voltages and everything was OK except plate voltage was 75V on one and 105 volts on the other! Since everything else checked out OK I tried another tube and now plate voltages are both around 75 volts. Must have been a bad tube, not sure how that works. Thanks to everyone that helped, especially PJ.


Robert Lees