Switching Plate Load

[tbryanh]

You can dig up formulas for stage gain, but in a 12AX7, the most it can be is 100. Typically it is probably about 50.

The hottest guitar pickup probably puts out about a 1/10 of a volt. I don't know if this is peak or RMS. To be on the safe side, lets use 2/10 of a volt.

2/10 X 50 = 10 v

If you use a pedal that boosts the signal to overdrive the amp, and if the pedal has a gain of 10:

10 X 10 = 100v

The pedal might have a gain of more than 10.

It is always safe to err on the side of overestimating than underestimating.

It might be best to make sure the switching device can handle the full B+, and to have some breathing room, you might add an additional 20% - 50% on top of that.

[tbryanh]

If you had an inductive load, say in the output stage, the flyback voltage can be an issue, and you might get double or tripple the B+.

This should not be an issue in the preamp stages.

[Colonel_Sanders]

Tbryanh, so even though the switch is in parrallel with the permanant plate load, there is a chance it the full B+ might develop across the 2nd plate load resistor?

I do realise the JFET On/Off situation, just get a bit confused at times :-). Thanks for the 105 recomendation, gonna check em' out now.

Enzo, I'm not sure if it will make much of a difference either, but I plan to overkill first, then whittle the design down once I have it going.

Thanks heaps for all the replies!
C_S

[d95err]

Enzo, thanks for the info. Makes the ratings heaps clearer.

I am planning this switch, and a myriad of others in the amp, for live use, so its entirely possible that the parrallel Ra will get switched in while the tube is in saturation. That means I WILL need a relay to potentially stop all the B+? Eep, confusing.

If you use a switch like this, there will be a huge BANG when you flip the switch, because you are creating a sudden DC shift. Never ever switch (high) DC voltages in the signal path. To be safe, you should ensure the amp is in Standby before flipping such a switch.

I wouldn't bother with this kind of switch. If you want to change the gain or output level of the stage there are lots of other options (e.g. switching the cathode bypass cap in/out).


EDIT: Since you say you're a newbie - this design looks way too complex. It seem like you want to make an amp that can do absolutely everything. My advice is:

1) Stick to the "KISS" principle (Keep It Simple Stupid!). Build an amp that can do one thing really well, rather than one that can do lots of things badly (or not at all).

2) Stick to a well known amp design (e.g. a "Plexi").

3) Once you get one simple amp working - experiment and mod it to death!

4) Once you've modded everything you possibly can in that amp - start designing your dream amp

(Hope you don't take this the wrong way, this is just some friendly advice based on my own experiences)

[Colonel_Sanders]

I know where you're coming from. Last year I didn't know what a capacitor was! I do want to design my own amp for my first build, but if there are simpler ways to achieve what I'm trying to do I'm all ears. Can you point me to a site or book that can explain how I can calculate the gain of a 12ax7 stage, and with and w/o the bypass cap? Not quite sure what to look for...

Thanks,
C_S

[tbryanh]

Yeah, if you use a mechanical switch, your probably going to get a big pop during switching.

If you use a jfet or mosfet, you can probably use an RC time contant to gradually make the transistion and avoid the big pop.

I haven't seen any circuits that have switches in the plate section, so I am a little curious about it.

As d95err asked, what is it your trying to accomplish?

Changing plate resistors probably changes the characteristic curve of the stage and therefore the distortion effect; i.e. different plate resistors probably provide different distortion effects.

Some amps mix it up. The first stage uses a 100K, the second stage uses a 220K, the third stage uses a 100K, etc.

I heard that mixing it up give a fuller body distortion.

[Colonel_Sanders]

I'm trying to design an amp that can mimic 3 different amp's, using the same expensive components (trannies, tubes etc.) in each design. Needs to be switchable for live use.

Seems like I'll have to go with an alternative:
What about an LDR in parrallel with the plate load?

I like the idea of a fet and ramp, but the switch would still need to be rated at 300V+, yes? Is it possible to parrallel mosfets for higher voltage/current ratings?

Thanks,
C_S

[Enzo]

WHy not take an existing amp, tack a resistor to the B+ and a switch between the other end and the plate and flip the switch back and forth. What happens? We have a lot of rationalizing going on here, but why not take five minutes and just find out?

Switching the parallel resistor in and out would not in my view create a large DC shift. Unlike a cap that would have to charge, the resistor is already at B+ potential before the switch is flipped. The only difference I see would be the small change in plate voltage due to the current through the tube. The old "IR drop."

[Colonel_Sanders]

Great idea, why didn't I think of that... I'll pop one my bassman tomorrow and post back with results.

Thanks,
C_S

[Colonel_Sanders]

Sorry about the delay there. Anyway, I put a 240VAC DPDT switch with a 820K resistor in parallel with the 100K anode load of my Bassman Re-issue. Definitely 'pops' when I hit the switch. I couldn't really hear a difference, but there 'might' have been a small decrease in gain. It was only a 100K/90K parallel difference though, not the 220K/100K I'm going for.

So, any ideas on how to tame the pop? And do you guys have any advice for getting a measurement of the max voltage swing while operating? I tried with my multimeter in AC mode but it just jumped around like crazy.

[Enzo]

Meter jumps around? Are you measuring with a steady sine wave test tone? No reading will be stable unless the signal is.

Did you wire the extra resistor to one end and switch only the remaining end? Or are you switching both ends of the resistor in and out? One end should be soldered on, ther is no reason to switch both ends.

If you want 100k/220k, there is no resistor you can parallel with a 100k to make it larger. STart with the 220k, and switch something parallel to it to lower the total to 100k. Just a second 220k - a standard value - would result in 110k total, which ought to be close enough.

[Colonel_Sanders]

I am switching both ends, good point. Also, I didn't think of the steady tone, was using my ipod lol. I'm going to try a 1kHz tone out of my laptop and rewire the switch now. Cheers for the tips.

The 220K will be the stock load in the amp I'm building (like the schematic).

C_S

[Enzo]

The one end has a better chance of less pop, but I am interested in your results. I have never switched a plate load resistor. But in switching cathode bypass caps in and out, maintaining a DC charge on the cap is important.

it may be difficult to eliminate the current shift and resultant noise. One thing comes to mind would be using something like a Vactrol instead of an actual switch.

[Colonel_Sanders]

Its still gives a good pop when I switch it in. I've got the B+ end connected and the anode being switched. Max voltage swing I could get from my laptop was 17VAC measured from the anode of plate load to the anode of the tube. Is this the correct place to measure?

If I can't get rid of the pop I guess I'll look into Vactrols. Are they pretty easy to control?

BTW, my filter caps drain themselves. Is that normal? (still short the caps for safety though)

Thanks again,
C_S

[Enzo]

I don't know that a Vactrol would handle the voltage, you'd have to check the data sheets. If they would be suited, they are easy to control, they are used for channel switching in lots of amps. I just meant it conceptually - as in a solid state relay sort of thing.

The DC voltage across your switch should be of course zero with the switch closed, and whatever is across the permanent plate resistor when open.

Signal level would be with respect to ground, but I am not sure what you are up to at the moment.