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Gain of Long Tail PI?

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  • Gain of Long Tail PI?


    How much is the gain of a typical Marshall /and many more/ style long tail PI?

  • #2
    I couldn't find a formula for this either. It's tricky with it generally being part of a global nfb loop, but if there's no global nfb, and the unused ltp grid is ac grounded, then measurements I made indicate that the gain between the input and anodes is roughly the anode resistor/cathode resistance, so giving a fender bf ltp a gain of about 4.5, vox about 2. So it's well worth putting a master vol after the ltp, plus the overdrive from an ltp tends to sound good. Peter
    My band:-


    • #3
      I did some measurements today and here's what I got.
      I injected a clean 1kHz/100mV signal via typical 2 gain stage /with a TS between dialed for flat response/ Fender style clean channel to the PI. With 1V at PI input the signal at plates was ~25V, 2V - 47.7V and 3V - 61.1V. It looks to me like a very hot PI with a gain of 20-25. PI's plates voltages were 265V and 256V. Tube was 12AX7EH. No wonder why with higher input signals oscillated like crazy.The output power measured with a resistive load of 7.5 Ohm was ~22W /13V/, 50W/20V/ and 64W /22V/. Tubes /6L6EH/ were biased at 44ma /across 1Ohm cathode rsistor, bias -43.2V/ , plate voltage was 440V. All measurements were taken with a DMM. Unfortunately I don't have a scope and can't comment on the signal shape.
      Last edited by GainFreak; 04-03-2008, 05:24 PM.


      • #4
        Using the unbypassed common cathode amp gain equation, this is what I got with a BFDR schematic, but this is without any NFB -

        Av = (mu*Rp)/(Rp+ra')
        Av = (60*59735)/(59735+106500)
        Av = 22


        • #5
          A 1 volt signal at the first triode corresponding to a 25V PP swing at the output tube grid does not sound atypical to me (from what I have read).

          I don't think it is necessarily anything to worry about. The voltage-swing measured at the output tube grid will be the result of the voltages right through the pre-amp from the first voltage amplifer stage through to driver and inverter. The PI won't be putting out 25VPP on its own. My understanding is that it will be feeding off the voltage coming from the previous gain stage(s) to get that output.

          Interestingly, both the LTP type inverters and Paraphrase type inverters are combined driver/inverter stages. Examples of these two different driver/inverter types are in the 5E9A and the 5G9. Both use a parallel single Voltage Amp stage (i.e. the 'front end' of a 5E3 deluxe) feeding into a combined driver/inverter. The two amps are biased differently, which would also account for some difference in power. The 5G9 (which uses a LTP) reportedly puts out 15-18 Watts. I don't know what the output of the 5E9A is - I would be curious to see a comparison between these two types of inverters.
          Last edited by tubeswell; 04-04-2008, 05:55 AM. Reason: bad spelling
          Building a better world (one tube amp at a time)

          "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo


          • #6
            I was curious about that because I noticed that some high gain amps schematically don't "allow" more than 5-6V at PI's input while some allow the full signal after the TS /which can go up to 30V/ to pass to the PI. With gain of ~20 3Volts are enough for overdriving the amp. I noticed when up to ~20V clean signal is applied to the PI you get even more distortion but it becomes very harsh and unpleasant.


            • #7
              Originally posted by GainFreak View Post
              I noticed when up to ~20V clean signal is applied to the PI you get even more distortion but it becomes very harsh and unpleasant.
              a lot of that is due to the inability of the splitter to contend with output stage grid conduction, which will happen any time Vgk approaches zero in the power tube(s).


              • #8
                a lot of that is due to the inability of the splitter to contend with output stage grid conduction, which will happen any time Vgk approaches zero in the power tube(s).
                Which means don't just blindly follow the schematics but listen to your ears in the first place.


                • #9
                  comparing gain of Spitfire vs 5e3

                  Some simple calculations (ignoring the tone stack):

                  A Spitfire has a single 12AX7 preamp and LTP, which would give a gain of:
                  100 x 25 = 2500

                  A 5e3 has a single 12AY7 preamp, 12AT7 preamp and cathodyne PI
                  40 x 60 x 1 = 2400

                  Substituting a 12AX7 dramatically increases the gain
                  100 x 60 x 1 = 6000
                  100 x 100 x 1 = 10000

                  Is this correct?

                  The next question would be the distortion characteristics.

                  In a 5e3 the top of the wave is clipped when it reaches B+, and the bottom when the tube clips.

                  I believe the LTP clips symmetrically - both at the B+.

                  Does this sound right?
                  See the birth of a 2-watt tube guitar amp - the "Dyno Tweed"


                  • #10
                    close, but not entirely correct.

                    the mu of a tube is the theoretical maximum voltage gain. you basically have to jump through LOTS of hurdles to even think about approaching that number, and even then it's probably not going to quite get there.

                    the operating point and load impedances that the tube is working against are what's going to dictate what the net voltage gain is going to be.

                    further complicating matters is the fact that different tubes have different characteristics such as mu, rp, and gm, and are therefore going to "prefer" different load impedances and operating points to work optimally.

                    thankfully musical instrument amplifiers have slightly relaxed criteria--if it SOUNDS good, it IS good!

                    (you might want to add a slight caveat to that--it also has to not melt down or blow up... lol)


                    • #11
                      As well, the gain number really only holds true at some reference frequency. If you do some cyferin' with a Champ BASS/TREB passive EQ, you can get a 1st stage gain anywhere from ~50 to ~10, depending on the frequency applied to the amp input.

                      The numbers, and equation I posted above show that a 12AX7 in a LTP PI has a gain of ~20, even thought the AF of the tube is 100. The main contributor to common cathode amp gain is the load on the plate, which includes the plate load resistor, and everything else hanging off of it .


                      • #12
                        My basic thought was comparing the 5e3 to the Spitfire in terms of gain and distortion.

                        From my calculations, the stock 5e3 and Spitfire has about the same gain, but you can increase the 5e3 gain substantially by swapping preamp tubes.

                        My other question involves adding another gain stage to the Spitfire, which has an unused 1/2 12AX7. Will the only result be moving the onset of distortion down a few numbers on the volume dial? This would happen because the amp clips at the same voltage in the LTP.

                        My design does use a TB tone stack, which has more insertion loss than the single knob tone stank in the Spitfire. Most likely that's the reason my amp doesn't get as much distortion as a stock Spitfire, which is why I'm looking at adding another gain stage.
                        See the birth of a 2-watt tube guitar amp - the "Dyno Tweed"


                        • #13
                          Essentially, if you can get the output tube Vgk to 0V via the PI plate signal(s), then the amp is maxed out.

                          If this is true in your design as is, then adding another gain stage pre-PI will pretty much just add distortion...which may be a good thing.

                          Another thing you can do, is jack up the value of the mid-slope resistor in the tone stack. Some guys even use a switch to 'lift' that resistor.....removing the tone stack...and get a kinda boost feature.