Limiting the grid current of PowerDrive

[tomchr]

Folks,

I'm playing with a circuit that's inspired by George's PowerDrive circuit. Like he, I'm using a source follower to drive a 300B. In my simulations, I have noticed that the grid current of the 300B gets rather high when the tube is driven hard into A2. Basically, with the source follower there is as much grid current available as the power supply can deliver.

But how much grid current can the 300B handle and would it make sense to limit the current to a safe&sane value by inserting a resistor in series with the drain of the source follower?

Thanks,

~Tom

[tomchr]

Just to clarify (and maybe get a response): The PowerDrive circuit is what's used in the TubelabSE (5842 + source follower).

Am I the only one who's a little worried about driving a 300B to the point where it saturates the OPT? OPT saturation produces rude sounds in the speaker that aren't on the original program material. Nasty...

~Tom

[rknize]

It seems like it is something one would have to try to see what happens. Wouldn't it depend on how much headroom the OPT in use has? So your question comes down to which is worse: clipping and blocking distortion due to insufficient grid drive or distortion due to oversaturation of the transformer?

[tomchr]

The root of my question is this: When the source follower brings the output tube grid positive with respect to the cathode, grid current will flow in the output tube (300B in my case). The source follower is capable of supplying, basically, unlimited current (amps!). Is this safe/healthy for the output tube? If not, what would be a reasonable limit on the grid current?

Originally Posted by rknize

It seems like it is something one would have to try to see what happens.

Well... I "tried" in PSpice and noticed that the grid current of the 300B exceeded 70 mA. This made me wonder if this was safe (healthy for the tube) in the long run. Hence my original question.

Originally Posted by rknize

Wouldn't it depend on how much headroom the OPT in use has?

It would depend on the air gap in the transformer, which is rarely specified. In my case, I'm using an Edcor CXSE25-8-5k. Edcor doesn't specify the max current or B field.

Originally Posted by rknize

So your question comes down to which is worse: clipping and blocking distortion due to insufficient grid drive or distortion due to oversaturation of the transformer?

Blocking distortion sounds pretty bad, but I don't think it'll damage the tube or speakers. Clipping (running the output tube or driver stages into shut-off) doesn't sound good either, but isn't likely to damage the tube or speakers either. One could argue that if you leave the amp running in clipping for hours, the added power in the high-order harmonics would eventually make the tweeter fry, but I'll assume the user has turned down the volume by then due to crappy sound quality. All this I'm comfortable with. I am, however, NOT comfortable with the woofer of my speaker hitting the excursion limit when the output tube is driven so hard that it pulls 100's of mA through the OTP, thus, causing OPT saturation. Remember that when an inductor (or transformer) saturates, the inductor will basically act as if it had no core at all. So the inductance drops like a rock and the current shoots up. When the primary current shoots skywards so does the secondary current. This causes the woofer on my KRK R6 speaker to hit the excursion limit (I think it has shorting rings, otherwise it would have fried by now!) Not only does this make rude sounds not present on the CD, it also makes me worry for the overall safety and well-being of my speakers.

I should note that the circuit that is capable of driving the OPT into saturation has higher gain than George's original PowerDrive circuit. It's a 6J5 followed by a 6BX7 and is capable of driving at least a good 100~125 Vpeak before running out of gas. The 300B enters grid current once the drive voltage exceeds about 75 Vpeak...

Thanks,

~Tom

[Ian444]

The powerdrive cct has no gain last time I looked at it. Maybe the gain you have added causes the woofer to slam to its limits. How much gain is too much gain?

Ian

[tubelab.com]

Am I the only one who's a little worried about driving a 300B to the point where it saturates the OPT?

If you are indeed hitting a 300B hard enough to saturate a CXSE25-8-5K then there may be cause for alarm. I have been known to push things as far as they will go, and I haven't seen this yet. The CXSE is a pretty big OPT and I haven't seen it saturate even when fed with glowing sweep tubes.

I enclosed two pictures of an extreme test of a pair of crusty old 6BQ6GA's that were found in a $5 box of tubes at a hamfest. These tubes were running at about 150 mA idle current and the amp was making 20 WPC at 3% distortion. No saturation was seen. I hooked this thing up to my speakers and it rocked.....for a short while. My CXSE's are the early versions that were painted "pimp my ride" metallic blue.

The PowerDrive circuit (as concieved) can indeed feed the grid of a tube as much current as it can eat. Under normal music conditions transitions into the positive grid current regions should occur only on transient peaks, unless you run your amp into clipping a lot. These conditions should not cause excessive G1 dissipation since the time in the grid current region is very low.

It should be noted that it might be possible to damage a tube by long term sine wave testing at or above the clipping level since the tube will see grid current on every cycle. I have not seen this happen on conventional G1 drive but I have melted a couple of screen grids using PowerDrive for screen drive. There are reports that excessive grid current might cause gold migration in tubes with gold plated grid wires.

You are reporting OPT saturation. This is a phenomenon where all of the OPT's magnetic headroom is used up. It is related to the DC current, the audio power level, the frequency of the audio signal, and the actual speaker impedance at that frequency. With my small speakers it is impossible to saturate a CXSE without blowing the output tube into 27 pieces, since the impedance of the speakers is in the 10 to 20 ohm range below 70 Hz. If your speakers have resopnse in the 40 Hz and below region, and the impedance is low enough, it might be possible to get some saturation in the low bass region. It will occur only on strong low bass notes, and does sound pretty nasty. On a scope OPT saturation will look like a divot taken out of the leading edge of the wave form just before the peak. If you are hearing nasty distortion when there are no strong bass signals present it is not saturation.

You mention a circuit using a 6J5 and a 6BX7 driving a 300B. Are you using the 6BX7 as a cathode follower to implement the PowerDrive circuit, or are you using two stages of gain before the mosfet follower?

Is the nasty sound you are hearing present on all frequencies, but only as the volume is turned up loud?

If the nasties are present without strong bass, and you are using a mosfet, I would bet there are short bursts of oscillation occurring just as the tube enters (or leaves) the grid current region. A scope may reveal this, but sometimes the oscillation will not occur with the scope probe attached to the circuit. Small valued (100 to 500 ohm) grid stoppers, ferrite beads on the grid and gate leads, and careful layout are your best defenses here.

[Ty_Bower]

Originally Posted by tubelab.com

On a scope OPT saturation will look like a divot taken out of the leading edge of the wave form just before the peak.

Does it look anything like this? This is from a push/pull amp, but I'd imagine the manifestation should be similar. If I turn the volume down just a bit, the trace snaps back to a nice clean 20 Hz sine wave.


http://i69.photobucket.com/albums/i4...e/P1080401.jpg

[tubelab.com]

Does it look anything like this?

YES. That is what saturation looks like in a P-P amp except for the little spike on the negative half cycle. In a SE amp the distortion is usually seen only on one half cycle since it is coincident with peak current through the OPT. Depending on your scope and the absolute polarity through the OPT it could be on the top or bottom.

If I turn the volume down just a bit, the trace snaps back to a nice clean 20 Hz sine wave.

If you were to leave the volume in the same spot and increase the frequency the distortion should disappear. It would also be reduced if the load resistance was increased since the peak current through the OPT drops even though the voltage goes up.

[Ty_Bower]

Originally Posted by tubelab.com

If you were to leave the volume in the same spot and increase the frequency the distortion should disappear.

Something like this? This second trace is at 40 Hz. You can see in the photos the time division and voltage knobs are the same. I must have turned the volume up a little, since the 40 Hz photo looks like it's almost 24 volts P-P, while the 20 Hz trace is less than 20. If anyone cares, the transformer under test is a Hammond 1620. The amp circuit driving it is approximately a Dynaco Stereo 35. The clever reader will notice that Hammond rates this transformer at 30 Hz. Perhaps my 'scope shows the reason why.

Is it safe to assume that gobs of negative feedback will encourage an output transformer to saturate early?


http://i69.photobucket.com/albums/i4...e/P1080396.jpg

[tubelab.com]

I must have turned the volume up a little, since the 40 Hz photo looks like it's almost 24 volts P-P, while the 20 Hz trace is less than 20.

It is more likely that the amp begins to roll off the output below 30 Hz to avoid OPT saturation. In a tube amp record warp can saturate OPT's. Many turntables of the time transmitted some of the motor vibration to the cartridge (rumble). Low frequency rolloff helped with both issues. Extreme LF response makes stabilizing the NFB loop more difficult too.

Years ago (like about 35) I got this wild idea to build an amp and preamp (solid state) with frequency response from 5 Hz to about 40 KHz (good in 1975). This proved to be a dumb idea. A warped record (CD's weren't invented yet) would make the speaker cones move in an out resulting in distortion at high volume, and accidentally dropping the tone arm resulted in a blown speaker.

Is it safe to assume that gobs of negative feedback will encourage an output transformer to saturate early?

Yes, NFB can encourage OPT saturation, and is possibly the reason for the funny artifacts seen on the negative half cycle in your 20Hz photo.