Single Ended 6V6 Power Amp Too Much Plate Current

[mwelch55]

I have built a Single Ended amp that has OT switchable between 8K and 5K.  I originally installed a 5881 and used the 5K Tap.  That worked great and my plate current was within limits (around 60 ma).  I wanted to see what a 6V6 would sound like (using the 5K) and I thought that it would work, but the plate current was too high (around 100 ma).  Then I realized I probably should use the 8K Tap for 6V6, so I repeated the experiment using the 8K tap.  It was better, but I thought that was too high also (51 ma).  I was hoping for somewhere around 30-35 ma.  I have seen cathode values of 470-500 ohms in single ended 6V6 power amps, so I thought my circuit would work OK.  It looks like I might have to put a switchable 250 ohm resistor in series with the 500 ohm resistor to be able to bias the 6V6.

Why is my plate current so high in this circuit using 6V6?

[shooter]

Why is my plate current so high

ohms law says it has to be 
with your values 378 plate, 500 ohms Rk, .051A Ik;
I get 25.5vk - 378Vp = 352.5v * .051A = 17.9W (call it 17W after G2 gets it's cut)
which probably won't sustain tube life very well 

[ECV02]

I think it's probably a number of small "issues" that is causing this. Your plate voltage seems a bit high to me; The 6v6 datasheet quotes a max plate voltage of 315v (JJ quotes 500, but in my experience the reliability of those tubes are questionable at such high voltages). Most people run them a bit above their max rated value (I usually see 340). Keep in mind that as the plate voltage increases, the cathode resistor must also increase to keep the tube within its limits. Another factor is the 6v6 itself. You may have a tube that simply pulls more plate current than others. If you have another 6v6 on hand, try subbing that and test again. A final thing to consider is your screen voltage/dissipation. Make sure that you are deducting the screen current from the final measured current; a 1 ohm shunt is a perfect way to measure this. Also, bear in mind that the screen voltage controls the total plate current. Your screen voltage is very close to your plate voltage. This is going to allow more plate current to flow at idle. I usually keep my screen voltage at ~90% of my plate voltage (a few people have told me this is too low, a few people say too high, but it's a nice rule of thumb for me). 

Hope this helps,

E

[sluckey]

Why is my plate current so high in this circuit using 6V6?

Because of the high B+ and the cathode resistor value. Lower the B+ or increase the cat resistor to decrease the plate current.

[PRR]

> use the 8K Tap for 6V6

The load only matters at MAX output power.

The idle is all about the B+ and Rk.

You want to be a lot closer to 300V of B+ with 6V6.

[mwelch55]

I wanted to build a power amp that can take some of the common octal power tubes (6V6, 6L6, 5881, el34).

I will increase the value of the cathode resistor to decrease plate current.   I tried adding a 250 ohm in series with the 500 ohm cathode resistor and I got 39 ma at 386v on the plate.  It's still a little high, but moving in the right direction.  I will probably try 330 ohms in series with the 500 ohm cathode resistor to see if that is within specifications.

The 6V6GT datasheet says you can have a maximum of 315v on the plates, but I have seen that specification exceeded may times without problems.  I checked the 65 Princeton Reverb circuit and it has 440v on the plate.  I have also seen other commercial amps that have over 400v on the plates that seem to work OK.  The tube I am using is a Chinese 6V6 from AES.  That tube sounds great in this circuit.

[Colas LeGrippa]

Why don't you plug in a pair of el34 ( or only one if only one tube) ? You won t have to worry about too high a voltage and to swap the k resistors. Keep the 6V6 for a project where the only B+ available will be 300v or so.
An el34 works better at higher voltages and a 6v6 at lower voltages. Just a suggestion based on my preference for el34 ( 6ca7 in fact but they re almost the same)
Jack