AX84 P1-triode mode

[isaac_teller]

Hello. I built a P1 exactly as per the attached schematic(point-to-point). It works perfectly and sounds great. The only catch is when in triode mode there is a 100 Hz moderate hum. In pentode mode it is dead quiet. I used UF4007 diodes in the power supply. What can remedy this situation? Thanx.


https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/ax84_p1_091125_quitar_tube_amplifier.pdf_2.png

[kagliostro]

To me a reason can be that connecting G2 to the Plate you are connecting the G2 to a less filtered node

what happen if you add an e-cap in parallel to C2 or add an in series, after the first node one other 100R resistor with one other 47uF cap to form one other filter and feed plate from that node ?

BTW are you sure you want the Pentode / Triode Switch ?

The most people I've hear don't like much the triode connection

Franco

[zircontweezer]

Sorry I've built the P1 (still got it) but I don't remember a pentode triode schematic and I don't think that link shows a switch on the EL84. It looks like the original schematic.
edit: sorry I do remember the triple output tube schematic (Octal, EL84 or 6NP?) which used a dual triode in parallel. I tried it and found the EL84 better. I've still got the amp and it's surprising what happens with a 6V6 considering the simple gain structure and the transformers.

[isaac_teller]

I'll try adding an electrolytic at C2 and seeing what happens. Yes, I know that there is no pentode/triode switch in the original but since it was an easy mod I thought I'd tryit.

[kagliostro]

As a simple mod you can try to add a resistor in series with the G2 resistor, say 2.7k as a start point, put a switch in parallel to the added resistor, when the switch is closed you have the standard circuit, when the switch is open you have a compression effect

Remember that 2.7K is a value that you must determine in sperimental way

Franco

[acheld]

Within the schematic notes, there is a comment that states that a (grid stopper) at the first stage may reduce power supply noise, as well as extraneous input noise.   I've always used grid stoppers in this position, so have no experience without, but the schematic itself does not show one.    Was one employed?

[kagliostro]

The grid stopper on V1 input purpose is to avoid the grid to be joined by noise like RF noise that will be amplified by the first gain stage

If you have nothing there and want to avoid such problems you can put a 33K grid stopper (always connect the grid stopper directly to the grid pin, not to the input female jack)

A better way is to add a 10K resistor in junction with a .... 400pf cap, the cap will be connected to the tube pin and to ground

Something to be remembered is that the bigger is the resistor the less noisy it is by himself, remember that a bigger in W resistor is less noisy than a resistor with the same resistence that is rated only for a 1/4 W and also metal film resistors are less noisy than CarbonComp resistors

Mesa Boogie adopted a ferrite bead in order to avoid the use of a grid stopper in V1 but I don't know which are the spec of that ferrite bead

Franco

[isaac_teller]

I did add a grid stopper(47K). The additional E-cap at C2 reduced the hum a little but not significantly. I'll try kagliostros G2 resistor mod.

[sluckey]

The only catch is when in triode mode there is a 100 Hz moderate hum. In pentode mode it is dead quiet.

I'm assuming you have the switch wired correctly. I don't know if there is a fix for this, other than don't operate in triode mode.

[isaac_teller]

I quadruple checked everything. It's wired correctly. I guess I'll have to manage without triode mode. I can manage just fine with 5 watts and a master volume.

[kagliostro]

You can reduce the grid stopper value from 47k you used to 33k, less impact on the signal and always good RF noise rejection

I suggest you give a try to the 10K plus capacitor solution

Franco

[PRR]

.