Question on 12AT7

[Alan0354]

Hi all.

My question is the max plate voltage of 12AT7. I read data sheet, the typical MAX plate voltage specified is 350V. But when I look at the Vintage Fender reverb circuit. The driver of the tank is basically a single ended "power" stage using both elements of the 12AT7 in parallel. The plate drive one side of the primary of an output transformer, the other end of the output transformer just goes to +400V.

This means the idle plate voltage is about +400V. And being at transformer input, the peak voltage can even goes up to double the idle voltage. So the maximum plate instantaneous plate voltage can just to close to 800V!!! Is that true? Is that safe for the 12AT7?

Also, the plate resistance of one 12AT7 is spec at about 10K, so the reverb output transformer has to have retio to match the input impedance of the tank to about 5K(two 12AT7 in parallel)?

Thanks

[Enzo]

I'd be very surprisaed if the reverb circuit of a Fender put 800v anywhere, but maybe.

But let's think about the question a moment. Fender has been using this circuit now for 50 years, maybe close to 60 years. I see a tube failure once in a while there, but not all that often. SO I see no need to worry about the 12AT7s now, they got this far.

And as to reverb transformer impedance, remember, this is a guitar amp, not a NASA space probe. Nothing about it is a precision circuit. Not only that, the freq response of the reverb drive is purposely limited, and driving a long spring, fidelity is not important.


And a note about the data sheet. RCA or whoever published specs for the tubes, and the data sheet max voltages are for designers using the tube. They intent was for long life and reliable operation in consumer goods - your grand dad's table radio. The last thing they had in mind was pushing the tubes to any limits and certainly not ever overdriving them. Overdrive and breakup were never on Leo's mind. Even so, fender and others routinely exceeded those specs in the tube manual.

[Alan0354]

Thanks for the reply. I am just curious when I studied the schematic. The 800V I referred to is because if the other leg of the primary of the transformer is going to +400V, the plate of the 12AT7 is therefore idling at about +400V.

So if you apply a signal so the output is swinging a peak to peak of say 200V, the plate of the 12AT7 is going to swing from +300V to +500V.

With the same logic, if you put enough input, the plate of the 12AT7 can swing say 600V peak to peak. This means that the plate of the 12AT7 can swing from +100V to +700V. If you look at the circuit, the reverb is driven by the second stage of the vibrato channel, the signal can get very high if the volume is cranked up.


I understand this would not happen in the other preamp tubes, the maximum voltage is the only up to the +B when the tube is off. But if the tube is driving a transformer, it will go above the +B voltage just like the primary of an output transformer.

[Enzo]

It is a class A , single ended, circuit, where do those volts above B+ come from? In a push pull amp, the plate of one end goes up and the primary winding acts as an autoformer, but this is a single winding.

[Alan0354]

It is a class A , single ended, circuit, where do those volts above B+ come from? In a push pull amp, the plate of one end goes up and the primary winding acts as an autoformer, but this is a single winding.

Yes, it is a single ended design, it is my understanding that when you reduce the plate current, the output transformer should fly up in response to the decrease in current. So if you input a sine wave, you'll see a sine wave at the plate center at +400V. So it will swing as I said in the last post. Or you think I am wrong?

I thought even for single ended amp, it is like half of the push pull amp that if the center tap is at 400V, each leg of the primary is going to swing 400V+/- the AC voltage. That it can swing from +100 to +700V.


I guess the essence of my question is does the plate of a single ended amp swing beyond the +B just like the push pull amp.

[Enzo]

What center tap? What "each leg"? It is a single winding, not a push pull split primary. There is no other half of the winding to act like an autoformer.

[Alan0354]

What center tap? What "each leg"? It is a single winding, not a push pull split primary. There is no other half of the winding to act like an autoformer.

No, I said it's like half of the circuit of the push pull.

I think even the single end configuration, the plate will swing up as high as it swing low from the 400V.

[Alan0354]

I posted the question of plate voltage swing in the Theory section and confirm that the plate do swing above the +B. This leads back to my original question. If the +B for the reverb circuit is +400V. The plate voltage of the 12AT7 can swing beyond +600V if the peak to peak signal is 400V. This is possible as the input of the 12AT7 is from the second preamp stage of the the vibrato channel and the input voltage swing is high if you turn the volume up. So I would expect the plate of the 12AT7 can easily reach +600V. That is way way over the design limit of 12AT7 that spec for +350V!!!

[Enzo]

So many issues.

But WHERE are you getting all these volts? There is no 600v p-p here. The signal at the plate of the tube is more like 40-60v RMS full out. How could you drive that stage hard enough to get that large a signal? Won't happen in a guitar amp. If you don't believe me, look at any number of Fender schematics that include voltage readings of signal.

The voltages in the data sheet are DC voltages, not signal voltages.

Refer back to post #2. WHy would this turn into a worry now after 50-60 years of it working successfully and reliably?

[Gingertube]

In a push pull amp where one tube cuts off that side of of the push pull (while the tube is OFF) can go to 2 x B+ voltage due to voltage induced from current in the other push pull half.

In a single ended amp (like the parallel 12AT7 sections driving the reverb transformer) this is NOT the case. The triode sections never cut off and the anodes can never go above B+. Think of it as simply changing the current through the primary to induce a voltage into the secondary. This current varies either side of the zero signal idle current.

Cheers,
Ian

[g1]

As far as a single ended amp swinging above V+, up to double B+ is theoretically possible.
nickb was kind enough to send me some links explaining it, thanks nick!
Electronics 26
Transformer coupled class a power amplifier - Electronic Circuits and Diagram-Electronics Projects and Design