Does Feedback Need to Change When Pulling Two Output Tubes?

[65greg]

Hi All,

I'm playing around with a "6V6 Plexi" style build and currently I have 4x6V6 in a Super Lead / Bassman style circuit.

I'm planning on running it with two of the tubes pulled and I started thinking about the negative feedback as no one seems to mention this when discussing pulling two tubes to reduce power.

So, if you pull two tubes you roughly cut the power in half.

This would mean you reduce the open loop gain (gain of the PI and output) by -3dB

If we reduce the open loop gain by -3dB and leave the feedback resistor ratio the same, the amount of feedback "in dB" is different after you pull the two tubes.

Following the examples on Aiken's website "Designing for Global Negative Feedback":

Gain in his example is 41 and the feedback factor is 0.048
so ACL (in his example) is 41/(1+41*0.048) = 13.8 or 20 log (41/13.80) = -9.5 dB

If we reduce the gain of 41 by - 3 dB the gain would be 29

if we leave the feedback factor / resistors the same - we get 29/(1+29*0.048) = 12.12 or 20 log (29/12.12) = -7.6 dB

Am I correct in my thinking here?

[martin manning]

Just move the impedance selector down one click (or connect a speaker with twice the impedance). Everything else stays the same. You get half power by doubling the load at the same voltage.

[Stevem]

If your yanking out 2 tubes to get less volume your not going to get a full 50% reduction.

Also now your going to have 50% more power supply current available with 2 less output tubes before the power supply starts to sag and your V+ voltage will be higher so be sure to check that your caps in the first two filter stages have the needed voltage rating especially if the amp uses a SS recto, in which case you want a safety voltage margin of 10%.

Also be sure to check that with the two tubes out that the amps heater voltage does not rise above 6.7 volts. As not only will that shorten up tube life it will very much change the gain structure of the preamp section!

If it where me I would leave the 4 tubes in the amp and then on two of them install a switch in there pin 8 wire that either shorts pin 8 to ground as is needed in a fixed bias circuit or allows a 470k 2 watt resistor to be in series which will stop the tube from conducting.

[roberto]

Totally agree: guitar speakers are often in the range of 100 dB/Wm (even more, considering a 4x12 with Celestion V30s: https://celestion.com/productpdf.php?id=1 ), so you need to go way below 1W to play at home.

[65greg]

Thanks for the replies.

I guess I'm really just trying to understand if the math above is correct. If it is, I guess it would mean that feedback resistors do need to be adjusted to maintain the same amount of feedback in dB and actually get the full dB reduction expected. Anyway it would be only around 2 dB of feedback so probably not so noticeable.

I understand all about the power supply current, changing voltages, and bias stuff when removing the tubes and thanks for the reminders.

Measuring voltages at the speaker terminal while playing a sine wave I do get a difference of -2.2 dB when I yank the two tubes and move the speaker from the 8 Ohm tap to the 4 Ohm tap. A little reduction but not all that noticeable. So again, maybe not worth it.

The goal of my current tinkering is not to play at bedroom/home practice levels but to find the right combination of amplifier power and speaker efficiency so the amp can be "in the sweet spot" at band practice / gig levels without lugging around my old Scholz power soak or having a dirt pedal always on.

This amp is 4x6V6, tube rectified, running 315V, cathode Bias, and into my 2x12 (one creamback H and one cream M) is a bit too loud to be "in the zone" at the level by band plays.

I think I'll give it a try with the two tubes pulled into a 1x12 which should give me around -6dB reduction and see how I like it. Then, if it works out I guess I'll just have to build a 2x6V6 version!

[pdf64]

As Martin alluded, the (voltage) gain wouldn’t be much affected.

[tubeswell]

If it is, I guess it would mean that feedback resistors do need to be adjusted to maintain the same amount of feedback in dB and actually get the full dB reduction expected. Anyway it would be only around 2 dB of feedback so probably not so noticeable.

Technically yes, but the FB loop resistors make a voltage divider (so its the Vac-in:Vac-out ratio that's important here) and the voltage source at the OT secondary won't really shift that much, so the output vac from the FB voltage divider won't shift all that much, so you probably won't notice much/any difference in a guitar amp. With a multi-tap Ot secondary, I'd calculate the FB from the highest impedance tap and just keep the FB source from there. (no matter what impedance speaker you decide to plug in. If you're going to muck around pulling out tubes and changing speakers you're going to get some - but not much - difference in sound anyway. YMMV)

[Stevem]

Add a transformer driven 8 ohm input reverb circuit to the amp like a normal Fender set up and then you have a 1 watt 8 ohm impeadance amp for home usage right there, lol!

[65greg]

So I tried it last night at practice with the two tubes pulled (still using the 2x12 cab) and indeed it does make a noticeable difference.
I was not playing any quieter than before (with the 4 tubes in) but I did have to raise the volume control up more to achieve the same “playing with band” level which let the amp get higher int the “zone” of nice breakup. Almost just right.

Next I’ll lug over my 1x12 to the space. I think I might find the amp will then be in the sweet spot but I wont have enough headroom left to get it “louder” when I hit boost pedal for a solo