[OT] MOSFET B+ dropper in tube amps

[Mattoverminder]

After all of the stompboxes I've built, thanks in part to this wonderful community, I've decided to come out of my lurkerdom with an amp question...  

I decided to try out the MOSFET based B+ dropper, that can be found on GEO in the MOSFET follies article, and while it did drop the voltage by the expected amount, there was an unexpected side effect...

The 6V6 tubes would not bias above 1.5ma, even with the bias pot turned to max (they're supposed to be around 23ma). If I removed the voltage dropper, the amp returned to it's normal function. The only thing I can figure is that the MOSFET circuit is somehow blocking current flow as well.

I'm using an IRF820 MOSFET, 1A zeners, and 1/2W metal-films for the resistors, and triple-checked to make sure that I didn't reverse any pinouts on the FET. I even built a second unit with fresh parts to make sure that I didn't fry something the first time.

Do any of you have any ideas as to why this is happening? I'll admit that this sort of wizardry is a little beyond my current understanding of electronic theory. In the meantime I've gone with a string of 5W 10V zeners to acheve the desired effect, but the MOSFET dropper seems like such an elegant solution, and would likely handle heat better, since I had it sinked...

Thanks in advance!

[RedHouse]

Maybe you need different resistors in your BIAS circuit, look into that area.

Often it is the case when using different tubes in like a Marshall where the bias for EL34's use a 220k resistor in the circuit but to BIAS for 6550's or KT66's often requires a 150k resistor in the bias circuit.

[R.G.]

In general, the MOSFET dropper doesn't affect bias voltages, but it depends on your amp circuit. Can you post the schematic? I've used that circuit myself, and it's pretty close to being a purely amplified zener.

Measure the raw bias voltage and the bias voltage to each output tube with and without the MOSFET dropper in circuit. how does the bias voltage change? From your description, it sounds like the bias voltage at the tubes gets more negative.

If that's the case, it's possible that your bias voltage circuit is referenced to the centertap of the B+ winding and not to signal ground. Putting in a zener in the centertap lead separates those two, where they were connected before. If you left the bias voltage reference on the CT and didn't move it to the signal ground side of the zener, that would add the MOSFET/zener voltage to the bias voltage and bias the tubes 'way cold.

[Mattoverminder]

Sorry, I meant to mention the amp model in the first post. It's a '68 Fender Deluxe Reverb, that's otherwise identical to the AB763 (blackface) schematic, save for a higher voltage PT that was designed for a 5u4 rectifier, instead of the GZ34 of the earlier models. In mentioning that, the B+ is still too high with the 5u4 due to the higher voltage mains in these modern times. The schematic calls for 415V on the plates. With a GZ34, I'm getting 465V, and the 5u4 gets around 440V.

The PT does have a separate bias tap, and the center tap was originally grounded. Also, the 5W zeners worked like a charm, which is why I find it strange that the MOSFET solution does not.  :?


Here's the schematic from the Fender Field Guide.

Thanks again.

[R.G.]

The PT does have a separate bias tap, and the center tap was originally grounded. Also, the 5W zeners worked like a charm, which is why I find it strange that the MOSFET solution does not.

So do I. The MOSFET/zener combo should act exactly like a zener, but with better dissipation.

Mother Nature must be trying to tell us something subtle.

(1) Can you measure the bias voltage and B+ with zeners, then with the MOSFET/zener?
(2) with the bias generated from a B+ tap, the bias voltage positive end is irretrievably tied to the CT, so any bias voltage generated by the power supply is automatically 50V higher (for a 50V zener), which would explain the inability to get more current in the output tubes.

Look at it this way- the raw bias voltage is referenced to the CT of the power transformer because it's generated by the same winding and CT that makes B+.  If you insert zeners in the CT wire, you have just moved the entire amplifier *up* from the CT by the amount of the zener voltage. So you have caused a more-negative bias on the output tubes because the bias generator is generating the same voltage it did before, it's just that the amplifier ground has moved up. By that reasoning, the zener string should have caused the same problem.

That all makes sense to me. What doesn't make sense is why the zeners worked. Either  (a) they're hooked up differently from the MOSFET zener or (b) the MOSFET/zener is doing something odd like oscillating.

[Mattoverminder]

Ya know, it just occurred to me that I used a lot less total voltage drop with the zeners than I did with the MOSFET/zener. I'm sorry that I'm leaving things out, as I'm at work now and doing this from memory.

I was using about 30V total in zeners vs. 50V with the MOSFET combo. That may explain the difference between one way working and the other one not. Also, the voltage dropped from the CT did not directly reflect on the B+. It was about 60-75% (30V worth of zeners came out to roughly a 20V drop on the B+). Now that I think about it, I bet that 50V dropped crossed a threashold of some sort, which sent the output tubes way off bias. Anyway, up until now, I've been adding the zeners inline with the B+ for a more direct approach. I just did this experiment to try something different. Would the MOSFET/zener trick work inline with the B+ current, or would the DC cause it not to work?

In the meantime, I'm going to hold off on this until the weekend, where I'll have my amp in front of me, as to not cause any further confusion.

I really appreciate your help so far, and I'll make an effort to be more accurate with my data in the future.

[R.G.]

That's it! Mother Nature was trying to tell me that you had the zeners in the B+ line, not in line with the CT. In the CT, the zeners ought to do the same thing (if they dropped the same voltage, that is.) They were hooked up differently.

In one case you're subtracting voltage from B+ only (zeners) and leaving ground and bias alone. In the other (MOSFET) case, you're changing the relationship of bias and signal ground, and hence the available voltage to bias the tubes.

The MOSFET/zener will work fine floating in the B+ line. It's just more dangerous to you to work on and any service personnel, because then the whole mess is up at B+ voltage.

Any change in the voltage location of the CT in an amp with a bias tap will cause a corresponding change in the bias voltage, because it changes the relation of bias voltage to signal ground by the amount of offset of the voltage.

Sure, put the MOSFET/zener in the B+ line and then BE CAREFUL CAUSE IT'S HOT TO B+.

[Mattoverminder]

Hey thanks!

Like I said, I did try the zeners in the CT too, but it was a smaller dose of zeners and there was probably a line that got crossed with the full 50V...

Anyway, I might try the MOSFET in the B+ trick. I'll be careful with the B+ rail... I'm the only one who'll be working inside the amp, since I'm my own tech. I've been working on and restoring old tube amps for a few years now, and even built one from scratch not too long ago (with another one in the works). My technique and safety are good, but my theory's somewhere around an intermediate level. I'm always striving to learn and improve though!

I'm very greatful for the help, and I'll keep you informed if I find out anything else that's interesting regarding this subject.  

[dorrisant]

Old thread yes... but here:

http://ampgarage.com/forum/viewtopic.php?p=332271&sid=d277fd752f8a4c896eb3ea8dd27cf3af