Plate voltage preamp

[Jonathan Wilder]

Exactly. If I would've compensated the cathode resistor, I would have had increased headroom while maintaining symmetry. But leaving it at 820R not only makes the stage clip sooner, but also reduces the symmetry as well (i.e. positive clipping before negative on the input while negative clips before positive on the output).

 

[RickyLee]

Another question regarding that load chart with the 100K plate and 820R cathode and applying an AC signal voltage to the grid of let's say 3V peak to peak. That would put the plate voltage rise/fall at 250V to 85V, and be quite close to max of what the valve could tolerate.

And if we went anywhere over an AC signal voltage to the grid of 3.7V peak to peak, the valve would go into saturation?

 

[Jonathan Wilder]

Another question regarding that load chart with the 100K plate and 820R cathode and applying an AC signal voltage to the grid of let's say 3V peak to peak. That would put the plate voltage rise/fall at 250V to 85V, and be quite close to max of what the valve could tolerate.

And if we went anywhere over an AC signal voltage to the grid of 3.7V peak to peak, the valve would go into saturation?

The valve would already be in saturation at 2Vpk-pk...but only on the positive 1/2 cycle of the input. This would cause the output signal to clip on the negative 1/2 cycle of the output (in and out are 180* out of phase) but be clean on the positive swing of the output/negative swing of the input. This accounts for the asymmetrical clipping aspect of valves.

Both 1/2's of the sine wave would be clipped at 3.7Vpk-pk, but the negative cycle of the output will be clipped more than the positive cycle of the output would be.

 

[RickyLee]

OK Jon, one more question . . . . for now anyway.

This would be regarding the total resistance of the circuit from the feed into the 100K plate resistor to ground/Common reference. And using the 270V B+ and 820R cathode resistor. And I am assuming the resistance will change value between operatiing conditions/heat and circuit/valve/voltage OFF.

I am wanting to figure out the resistance of the valve/plate itself under the operating conditions given. We know this is a series circuit and current remains the same. We know the applied voltage and we know we have a current of 1.2mA as well. We have a voltage drop of 120V across the plate resistor and 1V across the cathode res leaving us with a balance of 149V across the valve plate.

149V / 1.2mA = 124,167 Ohms of resistance on the valve plate.

100,000 + 124,167 + 820 = 224,987 or 225K of total branch resistance.

Am I figuring this correctly?

 

[Jonathan Wilder]

OK Jon, one more question . . . . for now anyway.

This would be regarding the total resistance of the circuit from the feed into the 100K plate resistor to ground/Common reference. And using the 270V B+ and 820R cathode resistor. And I am assuming the resistance will change value between operatiing conditions/heat and circuit/valve/voltage OFF.

I am wanting to figure out the resistance of the valve/plate itself under the operating conditions given. We know this is a series circuit and current remains the same. We know the applied voltage and we know we have a current of 1.2mA as well. We have a voltage drop of 120V across the plate resistor and 1V across the cathode res leaving us with a balance of 149V across the valve plate.

149V / 1.2mA = 124,167 Ohms of resistance on the valve plate.

100,000 + 124,167 + 820 = 224,987 or 225K of total branch resistance.

Am I figuring this correctly?

That would be correct. And if you take the total branch resistance and multiply it by the total branch current -

225K x 1.2mA = 270V

You'll see that the math adds up to the total supply voltage.

Of course...the total branch resistance is only 225K when there is no input signal present on the grid. Branch resistance goes to near infinite when the grid signal pushes grid voltage down to -3.7V and down to about 148,043 ohms (148K) when the grid is driven to 0V relative to the cathode.

 

[Les Moore]

Nudge, your invitation to Sweden just got nullified
On the subject. I´m missing something fundamental here. I follow the reasoning after the Rp and I can draw the load line if I´m alowed to think for a while. But I´m stuck on that B+. I look at my chosen Calssictone PT. 320V. I follow the B+ rail and end up at V1. Would I then have 320V instead of your 270V? Be nice and bear with me. I have a feeling I´m close, but I could be wrong. Cool of you to chip in Ricky.

 

[Jonathan Wilder]

No, there's a bit of math you would need to go through in order to arrive at that value. A bit much to explain at this point but I'm working on typing up the explanation.

I will leave you with this. When a transformer AC output is rated, it is rated in RMS AC. RMS (Root Mean Square) is 70.7% of the peak AC voltage (i.e. the voltage at the peaks of the AC sine wave). The Peak AC is 1.414x greater than the RMS AC.

When AC is rectified and filtered, the filter caps charge up to the peak AC value. Thus the DC value ends up at 1.414x the RMS AC value. For 370VAC RMS, your rectified/filtered DC value would be -

370VAC x 1.414 = 523VDC

Now this is the voltage output of the supply when it is unloaded. Since the supply is unregulated, this voltage will drop by about 25-30V when the valves are in and idling. This DC value is the value you start with.

I'll leave you with that to give you time to digest. Once I finish typing up how to calculate the remainder of the voltages throughout the amp, I'll post it.

 

[Les Moore]

You´re a gem Jon, really, I mean it. I got the rms and 1.414 part so far. It was part of the fundamentals in the Navy thing I mentioned earlier. Reading right now a post I found where a guy asked the exact same question I did here. It´s about adding the tubes current draw and calculating voltage drops over resistors on the B+ rail. I´ll read that for fun and digest your posts until you post again. Feels like I have bits and pieces... Need to get it together. Not planning on designing amps, that is way too complicated.
Take your time Jon. I´ll wait eagerly I wont give this up.
Many thanks again
PS You too are invited to stay at my house if you ever come to Sweden. Damn. I think I have invited half MF already. We´ll have a Marshall party!!

 

[RickyLee]

Hey Les. That 320V you are referring to on your power transformer, is that the secondary HT voltage? That sounds real close to the HT voltage I have on this old Drake power transformer I just picked up. This one I just got came out of an '82 2204. I just tested it earlier today using my VariAC. What I do is hook up the VariAC to the primary side just like if you were plugging it in while installed in an amp, except that I set the VariAC at 12V. The reason for the 12V is more for safety as I usually have all the secondary winding/taps wires exposed for me to get the voltage readings. I multiply my readings by 10 as my wall voltage would be 120VAC - that's where the 12VAC come in. If my MAINS was 240VAC, i would set the VariAC at 24V. I also crank the VariAC up to 120V just to double check the power transformer. But as you know, the real test of the power transformer will be when it is installed and under load.



My HT tap measured 300VAC across both legs, 150VAC from each leg to the center tap. Then, as Jon explained earlier I multiply the 300V times 1.414 to give me my unloaded B+ voltage I will have. I should see aprox. 424V unloaded for my B+, which I am going to use this PT for my first project amp - either a 50W Plexi or 2204. I do not own a Plexi, so I am leaning that way.

 

[Les Moore]

Sorry Ricky. It´s 350V secondary HT. It was in the middle of the night, early morning here now and I´m going of my shift soon. 320? Where did I get that from I´m reading and doing too much calculations now... Can´t stop.
I don´t have the tranny yet but it´s been suggested to me by a guy at Classic Tone as the one to go with for my project.

 

[Les Moore]

That thing with the variac was smart. Wouldn´t that be a very good thing to do in a testing phase, before powering up? Taking measures all over the circuit?

 

[RickyLee]

I am sure you will dig the ClassicTone PT. I used their transformers in my 2555 Jubilee Clone. Well built products and decently priced as well.

 

[Les Moore]

Everybody´s talking well about them. In fact I haven´t read anywhere that they would suck in any way, shape or form.

 

[Les Moore]

Crap. My calculations are all over the place. Time to go to bed and dream about Jons next post. Read you later guys

 

[RickyLee]

That thing with the variac was smart. Wouldn´t that be a very good thing to do in a testing phase, before powering up? Taking measures all over the circuit?

Yes, with all valves out of the amp. You don't want to run the amp at extremely low voltages with the valves installed as that is not good for the heater filaments - at least that is how I understand it. If I am wrong on that, hopefully someone will chime in on that.

 

[chadjwil]

The heater elements themselves couldn't care less, it's the fact that without the proper heat coming from the elements the valve acts totally different. When you hear people talking about "boiling off electrons" that's what the heaters do, and without the proper electron potential, the entire circuit around it is thrown way out of whack. As for causing damage to the valve itself...that I do not know, but I would imagine it can't be good for it.

 

[RickyLee]

The heater elements themselves couldn't care less, it's the fact that without the proper heat coming from the elements the valve acts totally different. When you hear people talking about "boiling off electrons" that's what the heaters do, and without the proper electron potential, the entire circuit around it is thrown way out of whack. As for causing damage to the valve itself...that I do not know, but I would imagine it can't be good for it.

That sounds about right. I should not have used the words I wrote, as I meant that the low voltage is bad for the valves.

 

[Jonathan Wilder]

Low voltage is not bad for the valves.

Where the Variac issue comes into play is that most people who've used one or at least wanted to do not have a clue as to what is going on in the amp voltage or bias-wise as they're doing it. It gives them a place to adjust available energy where they should not have a place to adjust it.

The jury is still out as to whether the cathode-stripping argument even holds water. Not enough conclusive evidence to determine whether it is a factor or not.

Variances in AC mains voltage cause much greater variances in B+ voltage throughout the amp as well as the bias reference voltage. The variac itself is not the culprit. It is the people who use them not having the slightest clue as to what is going on inside the amp. Not something you want to recommend to just anyone to do as it's one of those things you don't do unless you truly know how an amp works, how the variac affects things in the amp electrically...basically only if you truly know what you're doing when using one.

Truth be known there are much better and safer methods for accomplishing the same tonal results as a variac would. Power scaling is one of the known methods.

 

[RickyLee]

I was referring to using a VariAC at a very minimal setting for troubleshooting the amp. I had all the valves pulled out the time I was powering up the amp at 0 to just a few volts. I had figured it was not good to run the valves at something like 12V MAINS. I initially had set the VariAC at 12V to test a power transformer that was out/not installed in a guitar amp.

 

[RickyLee]

So what is the final say on what I was doing with the very low or minimal voltage running the guitar amp?

Are you guys saying that I do not have to pull out the valves if I am testing the amp with something like 12VAC for the MAINS instead of the 120VAC?