PLEXI GAIN 20 LOADS OF FUN

[TIMBO]

Hi guys, Had a great afternoon (rained at work as so i had to go home early  ) put my new SUB BOX to use and saw able to do some great tweaking  But, there is questions.......

I have circled three 2.2m resistors. These were on the original schematic, i have not seen this done on other amp designs,is someone able to tell me what these do and would deleting/changing have any effect on tone/sound?

Just for fun i added the IRF820 to see what it would do.The mosfet follies explains it as an enhancement and thats pretty much what it does but in this case it starts to really wigout (over distort) when upping the drive level but that was to be expected.
Would altering the the two 470k resistors help smooth this section out, i played with the 10k cath resistor on V3b but this did not do much.

I am amazed how much you can tweak with these things and i makes i hard to settle on a final component value.Thanks 

[tubenit]

I will offer an uneducated guess ............

My guess is that the first 2.2M resistors (looking left to right) are supposedly to help with switching noise. Look at the Dumble schematic mid switch where they use a 4.7M supposedly for that purpose.

In reality, they are paralleling the outer lugs of the respective volume pots which are changing the original taper. It would make a 1M pot to be 660k paralleling a 2.2M resistor for example.

I personally would try lifting the ground of all three of those 2.2M resistors and then use insulated alligator clipped wires to A/B the sound/tone/volume difference. Hopefully there will be no added noise when switching.

Regarding the the 470k/470k resistors. You can try a 220k/220k or even lower the pot going into that CF section to maybe 250k?

In fact, I would eliminate the 470k/470k, change the drive level pot to 250k and lower the cathode resistor to 1.2k.


IF you try that, please report back your findings.    With respect, Tubenit

[Tone Junkie]

Im pretty shure there there for switching noise I was told to always use 10m its so far up there it doesnt change the frequency any i dont know why they used the 2.2m resisters.
I used them there to in my build without questioning them then chasnged the whole thing to mimic the vht deliverance lead channel as you switch through the 3 channels its really kicks ass.
Bill

[TIMBO]

Hi guys, Had some success. The 2.2M resistors have no effect on sound/tone and switching of any kind so i left them in. Changed the 470k/470k to 220k/220k and at this point has no real change (left the 220ks in).Changed the 10k cathode resistor to 1.2k and this really smoothed out the overdrive  The 1M drive level pot remains the same (too hard to get too) paralleled a 220k resistor and had no major change.The + is the IRF820 CF has added a great effect (nice distortion) and sustain (i think the note is still ringing) to the already overdriven signal and being able to alter the sound/tone/overdrive with both controls.

The tone controls don't seem to be very responsive treble and bass not much effect and mid seems to be average, checked the wireing can't see any miss wiring  would like the use the tone stack calculator but i don't know how to use it.

As usual i have some buzz,thought i might still be coming from the DC circuit so i increased the 51r resistor to 330r but no change.
The buzz is on two levels the loudest goes away when touching the guitar strings but there is still an under lying buzz that increases when the vol is increased 
The preamp circuit is on this board which has the filter caps and assoc. tube grounds, tone stack grounds and input ground with it all grounded at the bolt in the bottom left corner and PI and power amp grounds at PT bolt  I can't see what more i can do to fix this???? Thanks

[TIMBO]

OOPS wrong pic

[tubenit]

the loudest goes away when touching the guitar strings

I would presume that to be a grounding issue. 

with respect, 10thtx

[Tone Junkie]

Timbo I would also ask is it buzz or when all 3 gain stages are in is it the common high gain hiss from so many gain stages . If its the gain stages i have something that will help.
 If its buzz, let the hunt begin . Why do the good ones always have to be so hard.  to trace down.
Bill

[TIMBO]

Hi guys, this one is a real battle, Tone there is some white noise/ hiss when all three stages are on but at this time it is bearable.I spent alot time trying to locate the source of the buzz and of course it in the first two tubes. I'm getting buzz when on guitar is plugged in,same when first tube is removed, same when first is replaced and second removed and no buzz when both are removed. Checked all joints in grounding,input socket grounded by itself and used clip leads to try different grounding locations for preamp with no change, bypassed the active FX loop and disconnected the DC heaters and connected to elevated heaters no change 

I did find the plate voltage on V1 was only 50v and thinking that this was low i replaced the last 10k dropping resistor to 1k this then gave me about 55v on plate and 2.2v on the cathode  The voltage @ node F was 230v. Thanks

[tubenit]

55v on plate

That doesn't sound right. For a 12AX7, I'd want about 145-165v. Check your cathode and plate resistor values thru out the amp.

A 12AY7 will have lower voltage, but put a 12AX7 in that position and see what the voltages are and post that, please.

With respect, Tubenit

[TIMBO]

WOW THIS IS FREAKY  Took these voltages after some minor changes one being the last dropping resistor for node F from 10K to 1K just to get the volts up a bit on the tail.Some interesting numbers as i did not think that it worked this way, as the cathode resistor is increased the plate voltage also increases (very interesting)
NODE A 399v
        B 399v
        C 360v
        D 318v
        E 239v
        F 234V
So far all good!!
WITH 100K plate resistor and 12AX7 tube (voltages taken @ pin1 and pin8)      120K plate resistor
1k2 - 57v - 2.1v                                                                                     6k8 - 130v - 6.6v
1k5 - 63v - 2.5v
1k8 - 69v - 2.9v
2k2 - 76v - 3.4v
6k8 - 117v - 6.8v

WITH 100k plate resistor and 12AY7                                                          120K plate resistor
1k2 - 101v - 1.6v                                                                                    93v - 1.4v
1k5 - 110v - 1.9v                                                                                   101v - 1.6v                                                                           
1k8 - 119v - 2.1v                                                                                   109v - 1.8v
2k2 - 128v - 2.4v                                                                                   117v - 2.1v
3k3 - 134v - 2.6v                                                                            2k7 - 125v - 2.4v
                                                                                                    3k3 - 134v - 2.6v
To be able to get either tube to a good plate voltage a large cathode resistor is required and yet the sound produce by the 12ax7 with a plate voltage of between 57v - 76v sounds good but i am leaning towards the 12ay7 with plate of 128v. Good learning stuff 

[tubenit]

Somethings not right?  A 12AY7 should have less plate voltage than the 12AX7. At least they do on all 3 of my amps.

Again, I "typically" like 145-165v on a 12AX7.

with respect, Tubenit

[sluckey]

as the cathode resistor is increased the plate voltage also increases (very interesting)

That's the way it should be. Increasing the cathode resistor lowers the current thru the tube. This lower current also flows thru the plate load resistor and results in less voltage drop across the plate load resistor. This leaves more of the B+ to reach the plate, so the plate voltage increases.

If you increase the cathode resistor to infinity (open circuit) no current flows and the plate voltage would increase to be the same as the B+ supply.