# How can a single pre amp gain stage provide enough signal voltage to the power amp?

#### peteb

##### Poster Extraordinaire
5C1, 5C2, 5D2 and the 5C3 all somehow get by with a single pre amp gain stage. The 5C3 is followed by a paraphrase inverter which I believe has little or no gain.

some of these amps get by with a pre amp tube with a gain of only 70.

I consider 0.03 VAC as an adequate if not standard input signal for full clean power output testing.

0.03 VAC X a gain of 100 is 3 VAC of signal.

0.03 VAC X a gain of 70 is only 2.1 VAC of signal.

can 2-3 volts of signal really wake up a 6V6 power tube?

why do my 6V6 power tubes require much more signal to get moderate or full volume?

it has to work somehow but I just don’t get how.

and, a tube with a gain of 70 is only going to have a gain of half that when placed in a circuit, and there will be other losses.

how could a single volt of signal drive a 6V6 power tube?

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#### Lynxtrap

##### Tele-Afflicted
Perhaps this can shed some light on it?

#### Paul G.

##### Friend of Leo's
There is no tone stack in those amps, just a simple treble rolloff. The tone stack tosses 80-90% of the gain of the first stage, so you need a recovery stage to compensate. A treble rolloff only tosses the high frequencies, so you're losing little or no gain at all. One stage of a 12AX7 or 12AU7 is plenty to drive the phase inverter. In fact, the cathodyne PI in most of those amps is easily overdriven, and it's not pretty.

#### bebopbrain

##### Tele-Holic
The Fender '59 Bassman reissue schematic shows AC RMS voltages at each stage, which is a helpful reference. They start with an input of 3mV and eventually have 31.7 V AC on the 6L6 grids (and 257V AC on the plates). Driving 6V6 tubes would be similar.

> The 5C3 is followed by a paraphrase inverter which I believe has little or no gain.

The 5C3 has two gain stages before the power tubes.
The second stage uses the first triode of the second 6SC7.
The no-gain phase inverter uses the second triode of the second 6SC7.
If we plug in our most generous numbers:
0.03V * 70 * 70 = 147V
which is plenty of drive.

If we use a 3mV input and lower gain stages, the grid drive will be slightly anemic.

#### Lowerleftcoast

##### Poster Extraordinaire
5C1 has a pentode as V1 which is more gain than a triode.
5C2 has two gain stages and little tone stack loss.
5D2 has two gain stages and little TS loss.
5C3 has a Floating Paraphase Inverter. The first triode of the PI has gain. The second triode of the PI does not. This has similar (a little less) gain on the 6V6 when compared to a 5E3.

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#### peteb

##### Poster Extraordinaire
Perhaps this can shed some light on it?

the 5E1 has two pre amp gain stages, like the 5F1 and pretty much all fenders.

that is the point. Take that one volt and run it thru a second gain stage and then one can easily boost it to 20-30 volts of signal. A BF/SF champ can easily put 15-20 signal volts on the grid of the power tube.

More amps with one pre amp gain stage are the 5C4 and 5C5 which use the identical pre amp as the 5C3 to power a pair of 6L6 power tubes.

interestingly, the 5C8 twin has the same pre amp but they slipped a 6J5 tube stage in there to drive the new treble and bass controls.

#### peteb

##### Poster Extraordinaire
There is no tone stack in those amps, just a simple treble rolloff.

the 5E3 also has only a tone control, just like the 5C3, but the 5E3 has an added second gain stage, similar to all amps from that point on.

#### Peegoo

##### Doctor of Teleocity
Gain is not a voltage; it's a ratio of input-to-output levels, and it compounds logarithmically.

This means as more gain is incrementally added, the volume achieved is not proportional to the voltage increase through the previous gain stage. It compounds, like interest on a credit card

#### peteb

##### Poster Extraordinaire
The Fender '59 Bassman reissue schematic shows AC RMS voltages at each stage, which is a helpful reference. They start with an input of 3mV and eventually have 31.7 V AC on the 6L6 grids (and 257V AC on the plates). Driving 6V6 tubes would be similar.

I agree that the modern fender schematics are a great resource for knowing what the signal voltage level is supposed to be. Personally, I investigated signal level on the different tubes in different amps with a guitar and a DMM to understand the pattern.

I see the modern 5F6-A schematic calls out 3.85 mV of signal input to the amp. I question how 3-4 mV can work. I would expect 30-40 mV of signal.

what if 4 mV were input into an amp with a single pre amp gain stage? Let’s give the single pre amp stage the benefit of the doubt and say it has a gain of 100.

0.004 VAC times 100 is only 0.4 VAC of signal for the power tube to work on, on the 5C1 champ or 5C2 Princeton.

I really don’t see how they can test with 4 mV of signal.

#### 2L man

##### Friend of Leo's
If that 4mV is RMS then it help when power tube drive signal need is Vpp

#### peteb

##### Poster Extraordinaire
The 5C3 has two gain stages before the power tubes.
The second stage uses the first triode of the second 6SC7.
The no-gain phase inverter uses the second triode of the second 6SC7.
If we plug in our most generous numbers:
0.03V * 70 * 70 = 147V
which is plenty of drive.

I agree that the 5C3 looks like two gain stages feeding one tube. My research of the para phase inverter got as far as Merlin saying it has no gain.

I see one gain stage followed by the para phase inverter.

for simplicity sake can we focus on the single ended champ and Princeton circuits with a single gain stage and no PI straight into the power tube?

#### peteb

##### Poster Extraordinaire
5C1 has a pentode as V1 which is more gain than a triode.
5C2 has two gain stages and little tone stack loss.
5D2 has two gain stages and little TS loss.
5C3 has a Floating Paraphase Inverter. The first triode of the PI has gain. The second triode of the PI does not. This has similar (a little less) gain on the 6V6 when compared to a 5E3.

I concede that I am unfamiliar with the para phase inverter. Merlin says it has no gain. I agree the upper triode appears to have gain. Merlin does comment on this point, as he is saying no gain.

I agree that the 5C2 sends the signal twice through the first triode. I missed that.
5C1 has a pentode as V1 which is more gain than a triode.

it looks like the early champ is the only Fender with only one gain stage.

how much gain is there in a 6SJ7 pentode?

#### printer2

##### Poster Extraordinaire
I agree that the modern fender schematics are a great resource for knowing what the signal voltage level is supposed to be. Personally, I investigated signal level on the different tubes in different amps with a guitar and a DMM to understand the pattern.

I see the modern 5F6-A schematic calls out 3.85 mV of signal input to the amp. I question how 3-4 mV can work. I would expect 30-40 mV of signal.

what if 4 mV were input into an amp with a single pre amp gain stage? Let’s give the single pre amp stage the benefit of the doubt and say it has a gain of 100.

0.004 VAC times 100 is only 0.4 VAC of signal for the power tube to work on, on the 5C1 champ or 5C2 Princeton.

I really don’t see how they can test with 4 mV of signal.
0.213 / .00385 = 55X of gain. Pretty normal.

40 mV x 55 = 2.2V Works for testing the stage.

But, the 4 mV should allow the amp to develop full output. With your 40 mV rather than 4 mV the PI voltage would be 317V rather than 31.7V. Obviously that is not possible.

You want to test out a Champ type circuit, pull up a Champ schematic rather than a Bassman.

Input signal of 20 mV. More to your liking? The Bassman is a more sensitive amp.

#### printer2

##### Poster Extraordinaire
I concede that I am unfamiliar with the para phase inverter. Merlin says it has no gain. I agree the upper triode appears to have gain. Merlin does comment on this point, as he is saying no gain.

how much gain is there in a 6SJ7 pentode?
The top triode has gain, the bottom one is to have the same amplitude, just reversed. So the signal into the tube gets reduce by the amount of gain in the lower triode.

The gain and output of the 6SJ7? Look up some datasheets or figure it out.

edit : Sorry, grid lines are bunched up too much to figure it out easily. I'll be back.

OK, some guesstimating going on here.

V+ - 250V
Vq - 100V
Screen - 30V

Grid bias ends up at 0.84, let us make it easy on us and get it to 1.0V. Change Vq to 140V.

Now we go down to the Output headroom and we increase the voltage until we see the distortion make a steep rise. (we are assuming a 1V signal will pass with the 1V bias)

10V - 0.85% THD
20V - 1.67%
30V - 2.47%
40V - 2.24%
50V - 4.0%
60V - 4.7%
70V - 5.43%
80V - 6.11%
90V - 6.75%
100V - 7.3%
110V - 7.9%
120V - 8.9%
130V - 13.3%
140V - 25.8%

So say we have 1V in and a gain of 130-140X for the pentode. Compare that with two triodes in series, 50 x 50 = 2500X.

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#### John_doe_blc

##### TDPRI Member
A single preamp gain stage can provide enough signal voltage to the power amp because the gain of the preamp stage is multiplied by the gain of the power amp stage. This is how amplifiers with a single preamp gain stage achieve the necessary signal voltage to drive the power tubes.

Regarding the specific amps you mentioned (5C1, 5C2, 5D2, and 5C3), these amps are designed to work with a single preamp gain stage and typically have a high gain power amplifier stage that can multiply the small signals from the preamp into a usable output.

A preamp tube with a gain of 70, even when placed in a circuit with other losses, can still provide enough signal voltage to the power amp, although it may require a higher input signal to reach the desired output level.

The required input signal for full clean power output can vary depending on the design of the amplifier and the specific power tubes used. For example, a 6V6 power tube may require more signal to reach moderate or full volume compared to other power tube types.

In summary, while it may seem counterintuitive, a single preamp gain stage can provide enough signal voltage to the power amp when the amplifier is designed with the right combination of preamp and power amplifier gain.

#### peteb

##### Poster Extraordinaire
Gain is not a voltage; it's a ratio of input-to-output levels, and it compounds logarithmically.

This means as more gain is incrementally added, the volume achieved is not proportional to the voltage increase through the previous gain stage. It compounds, like interest on a credit card

Gain is not a voltage it is a ratio of voltage levels.

I believe we are saying the same thing.

the 12AX7 pre amp tube has a theoretical gain of 100 which means in practice that the gain will be 50 or less.

an example of how it works is a 0.030 VAC signal is input to V1.

0.03 X 45 = 1.35 VAC

the signal on the plate will be 45 times the signal input to the grid.

the blackface tone stack passes a quarter of the signal at best to V2.

1.35 VAC divided by 4 is 0.34 VAC.

V2 also has a gain of 45.

0.34 VAC times 45 is 15.3 VAC.

15.3 VAC would be a very usable maximum or near maximum signal voltage to input to the power tube.

This means as more gain is incrementally added, the volume achieved is not proportional to the voltage increase through the previous gain stage. It compounds, like interest on a credit card

I agree that voltage and volume are not going to track as the same. I will say that as the volume goes from 1 to 10 on a blackface amp, the signal voltage input to the power tube does increase in a near linear fashion, not the volume, but the voltage does. With the use of audio taper volume pots, the idea is that the volume will rise in a near linear manner. That is why audio taper pots are used. If a linear pot were used the result would be non-linear.

#### Peegoo

##### Doctor of Teleocity
0.03 X 45 = 1.35 VAC

the signal on the plate will be 45 times the signal input to the grid.

the blackface tone stack passes a quarter of the signal at best to V2.

1.35 VAC divided by 4 is 0.34 VAC.

V2 also has a gain of 45.

0.34 VAC times 45 is 15.3 VAC.

15.3 VAC would be a very usable maximum or near maximum signal voltage to input to the power tube.

I agree; the math does not add up because we're talking about magical pixies here

#### peteb

##### Poster Extraordinaire
If that 4mV is RMS then it help when power tube drive signal need is Vpp

do you think 4mV will work as a test signal?

I would expect 30-50 mV.

#### peteb

##### Poster Extraordinaire

I misread what the valve wizard wrote.

he wrote that a trim pot cuts the signal before the para phase inverter, and then the para phase inverter boosts it back up to unity:

A very simple option is to use a potential divider or trimpot to attenuate the signal before it is sent to the paraphase inverter, which then amplifies it back up again (and inverts it). This also avoids overdriving the paraphase valve. The divider needs to attenuate the signal by the same proportion that the paraphase amplifies it, thereby giving overall unity gain. The cathode bypass capacitor (Ck) could be left out to give the paraphase valve less gain (more headroom).
In a hi-fi amp we would probably use a trimpot so the inverted signal can be critically adjusted to match the amplitude of the non-inverted signal. But in a guitar amp a fixed attenuator would do. Guitar amps don't need precise balance, so minor changes in gain as the valve ages or is replaced are not too troublesome. If it's a cathode-biased output stage then the divider can do double-duty as the grid leak for one of the power valves.

#### peteb

##### Poster Extraordinaire
A single preamp gain stage can provide enough signal voltage to the power amp because the gain of the preamp stage is multiplied by the gain of the power amp stage. This is how amplifiers with a single preamp gain stage achieve the necessary signal voltage to drive the power tubes.

the power amp has less gain (around ten) than the typical pre amp gain stage. The 12AX7 has a gain of 100 but only 50 or less in a circuit.
A preamp tube with a gain of 70, even when placed in a circuit with other losses, can still provide enough signal voltage to the power amp, although it may require a higher input signal to reach the desired output level.

this is a good example.
can still provide enough signal voltage to the power amp

it is good to know how much a power tube needs.

In a single ended circuit the 6V6 needs at least 10 volts of signal. Probably more like 15 volts if not 20 for cranked volume.

the math is not going to work.

0.03 - 0.05 volts is a typical test signal. Let’s choose the larger. Heck, we can go 0.06 VAC.

A preamp tube with a gain of 70, even when placed in a circuit with other losses, can still provide enough signal voltage to the power amp

0.06 X 70 = 4.2 volts. Not enough in my estimation or my experience in measuring signal voltages or looking at signal voltages on schematics.

although it may require a higher input signal to reach the desired output level.

I don’t think this is an option.

I have measured the signal that my Tele puts out. I believe full output is around 100 mV. That is heavy chording and not a clean signal. I have read that humbuckers can output 200 or more mV, not clean.

I have reason to believe that for a reasonable clean output test, the signal should be significantly less than 100 mV.

I see that you are relatively new here.