I could go to a guitar amp forum and get an earful of mojo mummbo jumbo. I come here for the scientific perspective :>)

I sold the 1960 Symphonic single ended amp -12AX&, 50L6, 35Z5 - amp to a guitar player who loves it. But, he wants lees gain, can he substitute a 12AT7 for the 12AX7? The 12AT7 draws a lot more current, from what I have read. He may not like the result.

He wants to play the amp with the volume at 7 and his guitar pickups on full volume. Don't ask me why, but this is what they call the sweet spot. He is afraid this is too hard on the little 6" Jensen speaker.

To lower the gain on the 12AX7, I can increase the cathode resistance, correct? Remove the bypass capacitor?

To reduce the power output to reduce volume and speaker stress, should I look at dropping the plate voltage on the 50L6? Any ideas here?

Doing any of that is no substitute for just turning the volume control down. The term “ sweet spot “ is where the little amplifier is doing its best with minimum distortion.

Are both sections of the 12AX7 being used? If they are, you might be able to make a switchable bypass so that the signal goes thru only 1 section of the 12AX7, reducing gain....the switch will allow him to choose a gain level. Or,if cathode caps are used, put a bypass switch on any of the cathode caps used to help him dial in the gain. Or, put a switch on the cathode resistors, using 2 different valuse, allowing him to put different resitance values in play. BTW--Most guitarists (I know, I've been one for many years) are rarely happy with their 'tone' for any length of time. Do the best you can to appease him, but he'll probably get tired of this amp in a few weeks/months and move on....

Warren, you are right of course. I could put a resistor in series with the volume pot, but then the amp wouldn't be loud enough turned up to 10. :>)

Thanks, Brian, all good ideas. I think I will give him a 12AU7 to try. That is as far as I want to go with this at this time. Like you say, he may change his mind in a little while and learn to like it as is.

If you friend can't understand the purpose of a volume control and thinks there is something special about setting it to 7, it is unlikely you will be able to please him for the simple reason that he is an idiot.

:Warren, you are right of course. I could put a resistor in series with the volume pot, but then the amp wouldn't be loud enough turned up to 10. :>)
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:Thanks, Brian, all good ideas. I think I will give him a 12AU7 to try. That is as far as I want to go with this at this time. Like you say, he may change his mind in a little while and learn to like it as is.
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Some electric guitar enthusiasts will pay top dollar for tubes that have a D shaped getter ring, thinking that it somehow affects performance.

At any rate, sometimes there is a tonal difference at different parts of a volume control.

I tend to find that, if anything, there is a lot more brute-force mid-range as the control is turned all the way up. This might be due to possible capacitive coupling at lower volumes, causing the music to be brighter and more well rounded, and less mid-rangy. Series resistance between the wiper and the high end also tends to attenuate treble, so any stray capacitance would couple back in the highest frequencies. However, since there is also reduced resistance between the wiper and the low end, bass notes tend to be attenuated, and so the general result with an ideal volume control is that the over-all frequency response remains unchanged--treble is cut by the high resistance between the wiper and the high end, and bass is cut by the low resistance between the wiper and the low end.

...But not all volume controls are ideal, and so linear response is not always the case.

It might also be wise to inspect the actual volume control circuit to see if it is a plain potentiometer, which offers the most linear tonal response, or if it has extra bass or treble enhancing/muting circuitry built around it.

12AU7 would be an excellent choice for gain reduction. I was thinking of that myself. There are, of course, plenty of other options. Cutting the screen current to the output tube would reduce output and offer a bit of 'grunge' to the sound.

Thank you, Thomas.
I know there is something to his madness, at least to his ears, and your analysis is probably as close as we will come to a solution.
I must confess I may have caused the condition myself. The amp volume control was very noisy, and I saw that, per the schematic, it was on the input, ahead of the first preamp stage. To reduce noise, I connected the input jack directly to the grid of the 12AX7 with a reasonable value resistor. The volume pot was then connected between the plate of the second preamp and the original tone control. This works better to my ears but may have lead to an imbalance that renders the amp louder than it was originally. I'm hoping the 12AU7 will be a satisfactory solution, because it is certainly the simplest!

:Some electric guitar enthusiasts will pay top dollar for tubes that have a D shaped getter ring, thinking that it somehow affects performance.
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:At any rate, sometimes there is a tonal difference at different parts of a volume control.
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:I tend to find that, if anything, there is a lot more brute-force mid-range as the control is turned all the way up. This might be due to possible capacitive coupling at lower volumes, causing the music to be brighter and more well rounded, and less mid-rangy. Series resistance between the wiper and the high end also tends to attenuate treble, so any stray capacitance would couple back in the highest frequencies. However, since there is also reduced resistance between the wiper and the low end, bass notes tend to be attenuated, and so the general result with an ideal volume control is that the over-all frequency response remains unchanged--treble is cut by the high resistance between the wiper and the high end, and bass is cut by the low resistance between the wiper and the low end.
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:...But not all volume controls are ideal, and so linear response is not always the case.
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:It might also be wise to inspect the actual volume control circuit to see if it is a plain potentiometer, which offers the most linear tonal response, or if it has extra bass or treble enhancing/muting circuitry built around it.
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:12AU7 would be an excellent choice for gain reduction. I was thinking of that myself. There are, of course, plenty of other options. Cutting the screen current to the output tube would reduce output and offer a bit of 'grunge' to the sound.
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This can cause hum difficulty with some isolated chassis, and can really play havoc with other types of circuitry, such as the vertical hold control on my old DeWald BT-100, IF the control is 1 Meg or higher, but spraying the control with WD-40 often cures the noise. ...Not always.

Perhaps use a fixed potentiometer on the input, or a simple voltage divider network that, as a whole, comes to the complete value of the original volume control. Try not to use series resistance to reduce input, as this also cuts treble, unless that is what you are looking for.

It is best to have the master volume control on the input, or else the input can be over-driven even if the control of the output is turned down low. Having two controls allows for even more versatility. In that way you can either control the volume at the input and get full power without distortion, or turn down the volume between stages and drive the input at full volume for some grunge. For this you need a high gain input tube such as the 12AT7.

OK. I think I have a plan for a solution in the event that a simple tube swap isn't satisfactory. I will wait for feedback first (pun). But if he brings the amp back, will explore altering the input voltage divider for more treble and possibly lowering the screen current as you mentioned.

I felt the volume on the input was a mirror image of the volume on the guitar, which is essentially at the same point in the signal chain. Of course, he keeps the guitar volume cranked up full. He says he loses treble when he turns the guitar down. That makes sense if my modification is reducing the treble already.

Thanks a bunch. This is stuff we don't need to deal with in radio very much. Lucky if the radio has a tone control at all :>)

If his guitar has a volume control, then it is a duplication at the front end. However, if the cable is noisy, the volume control at the front end can reduce this, where-as the control at the guitar can't. Many guitar amplifiers do not have a control at the front end, though, and if all is well, the control on the guitar should suffice.

If treble is dropping off as the control on his guitar is turned down, observe wiring of the control. If it is wired in conventional potentiometer form, then cable capacitance might be muting treble on the output of the potentiometer as the control is turned down. At full volume, the low impedance of the pickup coils would likely overcome this capacitance.

One might analyze the capacitance of the cable with a capacitor analyzer, and then connect a similar capacitance between the center terminal and high terminal of the volume potentiometer to overcome this loss. Adjustments might be necessary in capacitance to achieve the results you want. You might even consider a series resistance if things get too bright at low volumes.

Another option might be to use a cable with thicker insulation between the wires to reduce capacitive loss.

...All sorts of possibilities and options.

You are wasting your time but if your idiot friend believes whatever kind of nonsense you can make up to sound highly technical (like that posted by someone who truly believes himself to be one of the smartest people on earth) then go for it. He might even have some idiot friends that will pay you to "fix" their systems.

Harold, I'm half deaf, so whatever he says sounds good to me. The differences are subtle, but I know they are there. I got out of amplifier repair because there is too much mystical mojo for me.

I have cruised the archives here for information going back a decade or so and have a good idea as to who knows what they are talking about.

Mr. Know it all strikes again.

:Mr. Know it all strikes again.
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I do know it all. Please bow down and worship me. :D

::Mr. Know it all strikes again.
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:I do know it all. Please bow down and worship me. :D
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Those of you who think they everything are extremely annoying to those of us who do.
Lewis

Hey Thomas.... great to see that you haven't fallen off the edge of the Earth.
Nice to see you here again.

:Hey Thomas.... great to see that you haven't fallen off the edge of the Earth.
:Nice to see you here again.
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:D Nice to see you, too! I see that the petty inferiority complex bickering hasn't ceased. Now I remember why I haven't been on here for so long.

Well good to see that AM radio is still a viable hobby. Too bad in my area it SUX major sux. There is NO music to listen to, at least that I'd like to listen to. ...And that digital sideband crap sounds terrible. No treble what-so-ever. Old AM radio is actually somewhat impressive in comparison.

Hey, and what happened to the grill cloth manufacturer(s). WOW! Was gonna look for some grill cloth, and they're out of business!!!!!!!???!! Must be sucky AM radio and the great recession combined. Boo. I'm sad.

:Hey, and what happened to the grill cloth manufacturer(s). WOW! Was gonna look for some grill cloth, and they're out of business!!!!!!!???!! Must be sucky AM radio and the great recession combined. Boo. I'm sad.
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Hi Thom:

I buy some very nice stuff at fabric stores... especially the upholstery material... they still seem to have a lot of patterns out there that look a lot like the grille cloth of old.

I'll check it out. Kinda liked the authentic repros they had out there, but I'm sure I can find something interesting at a fabric store.

T.

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::Hey, and what happened to the grill cloth manufacturer(s). WOW! Was gonna look for some grill cloth, and they're out of business!!!!!!!???!! Must be sucky AM radio and the great recession combined. Boo. I'm sad.
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:Hi Thom:
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:I buy some very nice stuff at fabric stores... especially the upholstery material... they still seem to have a lot of patterns out there that look a lot like the grille cloth of old.
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