Line level to power tube section of amp

[Paul Lebow]

Hi all. -

As an experiment I sent the line level output of a guitar amp plug-in in Logic to my 100 W Seymour Duncan Convertible amp. The amp's FX loop returns to the beginning of the pre-amp stage so it adds all the effects of that stage, distortion, EQ. I didn't want that. Instead I located the input to the power tube section on the circuit board and tapped into that. Ran the headphone output from my Focusrite Scarlett interface. Seems to be the most transparent approach. I can run a Dumble amp sim right into the power section of the Seymour Duncan amp. Is there any downside to this?

Thx!

Paul

[R.G.]

All I can think of is that the line level input may be too small to drive the output section to full power, plus any grounding issues.

The input to the PI is after all where the FX loop usually returns.

[Roe]

Normally you need a gainstage to drive the power amp

[R.G.]

I was confused by the OP's post too. I initially thought that he was trying to drive the output tubes directly Then I realized that without a PI, this would not work. So I guessed that he was using the PI grid(s) for the input. A differential PI will have gain, just maybe not enough.

I dunno...

[martin manning]

This amp is weird. The preamp runs into the PI in the usual way, but then there is a 12AU7 gain stage coupled to each output phase. Following each of those there is a 12AX7 attenuator, called a "power control," just before the power tube grids. It appears that the 12AX7 triodes are used to bleed signal from each phase to ground, basically a PPIMV.

[Paul Lebow]

Yes, the Seymour Duncan amp is unconventional with its 12AU7 attenuator stage after the 12AX7 inverter stage. I injected a line level input into the inverter stage, such as a mono headphone output. It seemed to work fine. Can inject the output of amp and otherplug-ins from the audio interface output. It seems that if the voltages are comparable to the outputs from the previous pre-amp stage it should be OK.

[Paul Lebow]

Here is the Seymour Duncan power amp section

[lonote]

This amp is weird. The preamp runs into the PI in the usual way, but then there is a 12AU7 gain stage coupled to each output phase. Following each of those there is a 12AX7 attenuator, called a "power control," just before the power tube grids. It appears that the 12AX7 triodes are used to bleed signal from each phase to ground, basically a PPIMV.

That really is pretty different. I was wondering about the jack in that section, & as it turns out, you could plug a volume pedal in there for remote adjustment of the attenuator.


https://www.seymourduncan.com/blog/late ... guitar-amp

[LOUDthud]

There are a number of mistakes or SNAFUs on that schematic. Some are corrected in this thread:

https://music-electronics-forum.com/for ... air/46089-