Dumb question: How do I increase Dp on this SET?

[SHiFTY]

I am building the fi primer 2a3 SET from here which has a Dp of 15W by my calculations.

I am using the Sovtek 6B4s, which are basically a downrated 300B, and can apparently handle Dp of 25W or so with no problems. (NOS ones are rated at 15W) I would like to squeeze out another precious watt or two!!

So anyway, I am guessing that I would just decrease the value of the top cathode resistor to something like 400 ohms or so, so I dont affect the 99V of the 6SL7 cathode.

(Or the raised heater of the 6SL7, but that doesnt matter so much)

Would this work? Thx

[bournville]

Do these Sovtek 6B4s have a Dp of 25w at the same anode voltage as the 2A3? (I don't know the specs of these Sovtek tubes - all my tube info states that a 6B4 has the same characteristics as a 2A3 apart from heater voltage/current).

If they do, then they would be drawing about 100ma rather than the 60ma or so of the 2A3. Will your power supply stand this? Even if it does, the extra current drain could cause the HT voltage to drop, which might upset the operating conditions of the 6SL7.

[Joel]

Shifty,
Adding another watt will not give you any gain in headroom. You have to nearly double the power to get it.

[fdegrove]

Hi,

Even if it does, the extra current drain could cause the HT voltage to drop, which might upset the operating conditions of the 6SL7.

Especially with regard to the direct coupling of the 6SL7 to the 6B4G.

Also keep in mind that the 6B4G will be far more sensitive to hum than their 2A3 brethren.

Cheers,

[SHiFTY]

Do these Sovtek 6B4s have a Dp of 25w at the same anode voltage as the 2A3?

The specifications for them state 15W, but they use the same (or very similar) internal construction as a Sovtek 300B which has 40W dissipation, and it is widely rumoured on usenet that they can be used safely up to 25 and even 30W dissipation.

Adding another watt will not give you any gain in headroom. You have to nearly double the power to get it.

Yes but when you are going from 3W to 5W it IS nearly doubling the power!!

So anyone got any ideas? I guess I just try changing the cathode resistors around to keep all other voltages equal. My power supply is pretty beefy, should be able to handle a smidge more current.

[bournville]

Shifty,

If the Sovtek 6B4s take about 100ma and you change the top cathode resistor from 750ohms to 400ohms, the voltage at the cathode where the the 33k resistor from the 6SL7 joins will increase from 144 volts to over 200 volts, and if the demands on the power supply lower the HT, the voltage at the 6SL7's cathode (which currently is directly connected to the grid of the 2A3) will also change from 99 volts.

You might find that you are going to have to re-evaluate virtually all the resistor values in the amp to keep balance if you to convert to your Sovtek 6B4s. If the voltage drop at the cathode is still going to be greater than 144 volts then you might have to consider a higher HT voltage also...

[SHiFTY]

Shameless Bump

I am still keen to try boosting the plate dissipation for this amp, I think if I could sqeeze another couple of watts out of it, it could be pretty darn great. I have already boosted the B+ to 410V for a little more headroom. The sovteks are hardly even getting warm at 16W diss. My power transformer is large and doesnt even get warm with this arrangement.

So if I replaced the output cathode resistors with slightly smaller values, and replaced a few other resistors here and there to keep all other voltages the same, it should be alright? I would be looking for around 70-75mA for the plate current.


Please!?

Any suggestions? I suck at the maths, but am keen to try any ideas put forth by the hive mind!