High gain amp questions.

[erock0138]

Hello all,

I recently built a 100 watt 4x6L6 amp, with a high gain preamp section. Very similar to a slo or dual Rec.
My plate voltages are all in spec, and it works and sounds great! Couldn't be happier. But my preamp voltage at my master volume after the tone stack is getting about 8 vAc and is sending too much to the phase inverter and making the volume control very sensitive. I then added a cathode follower after the tone stack, with a voltage divider to send to the fx loop, and another very low gain recovery stage for the return. I used a 1k resistor at the bottom of the 100k of the c.f. To get my voltage I need to the power amp. Is there anything wrong with this arrangement? I know I lose current through the divider, but was just curious if I should worry about anything else.


Thanks

[loudthud]

1K seem a little low, like maybe you might not get enough signal to the power amp to drive it to clipping. But it's hard to say without puttin a scope on it. Is it possible to get a clean signal out of the preamp?

[erock0138]

I had a 2.2k, but was still too high. It has plenty of volume, and alot of headroom. I'll take some exact measurements tomorrow, but there is still quite a bit of signal present even with a 1k. The input sensitivity for the phase inverter is roughly 2.5v (If i remember my calculations correctly)

Would it be a bad thing to add a 100k pot for the master volume immediately after the tone stack to lose some more signal there? I have a 500k now, but the control is just so damn touchy. I can't get it about 4 without it being very loud.

In regards to the dual rec schematic, would it be better to do a send like that 82k/22k send, rather than the 100k to 1k stock I have now? I haven't seen such an arrangement as theirs, and was curious of the differences.

[Gregg]

There's nothing wrong with your arrangement. Depending on your preamp you need a 22:1 to 50:1 voltage divider in order to get the signal down to 0 or -10dB. You can put this divider either in the CF cathode or use one after the CF so 22k/1k to 47k/1k divider is OK. This way your output impedance will be very low unlike when you use a 100k send pot for example. If you need a Send pot just replace the 1k resistor with a 1k pot.
You can also wire your return stage as an inverting feedback amplifier (virtual earth mixer) and thus lower it's gain .

[erock0138]

Thank you for your reply, Greg, I have been out and about for a while and am just returning to this.

I had the effects loop return wired as just a simple 12ax7 stage, 100k plate, 2.2k cathode, and B+ from the beginning preamp stages (around 330Vdc) so it is more reduced. It changed the sound of the amp a little more than to my liking, but I haven't tried changing it around a whole lot. I'm not very familiar with a inverting feedback amplifier, though I read valve wizards page on them. Seems like an interesting concept to try, but would there be any harm in just having the return a regular gain stage? My since my send is from a C.F. with a voltage divider, I know I'm losing some current driving ability there. But configuring a stage with just minimal gain for the return be such a bad thing to try?


Thanks again,

Eric

[Gregg]

There's no harm using a regular return gain stage. Concerning the inverting feedback amplifier stage get an AFD100 schematic and you'll see how it was implemented there. Also Merlin's book has some nice examples of FX loops using the IFA. Also you can always use a lower gain tube like 12AU/AT7 for your FX loop.