Changing Electra-Print DRD45

[eeyore]

Hi, I was building the standard DRD 45 as shown on the EP website . I was also wanting to change the 6AN4 to a D3a triode. Can anyone advise if anything needs to be changed in terms of the cathode resistor/cap or the grid choke?

From what I understand it seems the 6AN4 is biased around 3V, but the D3a seems to be happier at around 1.25V to 1.5V, would that mean I need to change the cathode resistor to say 300R or something to achieve 1.5V?

Thanks again!

 

[eeyore]

Actually, looking at the 6AN4 datasheet, it seems that the bias should be more like 1.5V thus the mu and bias is very similar to the D3a. Also, the 6AN4 table shows lines for positive grid voltage, which confuses me even more...

 

[dsavitsk]

It looks like you could change the driver's resistor to 120 ohms to get the same plate voltage (~160V) at the same current (~15mA). But, if you can't do that calculation, a DRD might not be the amp for you. It is worth the time to step back and trace all the currents and voltages to understand what is going on here as it can be a little counter intuitive. Getting it wrong could easily blow up your power tubes.

Edit: Jack's numbers don't add up for me -- not necessarilly surprising as he is probably using experimentally found operating points for the 6AN4 which are unlikely to match the datasheet very closely.

 

[eeyore]

Yeah, the grid choke is what confuses me in trying to work out the currents and operating points. Though a simple looking circuit, it is actually pretty complex. I looked at an article on Single Driver Website, which has the operating points, but for the life of me can't seem to work out how 9.4 mA is determined.

 

[dsavitsk]

Yeah, the grid choke is what confuses me in trying to work out the currents and operating points.

The DC resistance of the plate choke is ~1500 ohms IIRC.