Phase inverter plate driven ?

[Colas LeGrippa]

Buenos días amigos !


Since a couple of days, I have been trying to design an amplifier for hard rock, with 4 stages of 12ax7 in cascade, the plate of the last stage driving the phase inverter, but unsuccessfully. I am a bit confused with the circuit  and I don't know if it is beacause of the lead fumes or if my mind is getting crazy looking at my wife's legs straight as 2X4's.  Though I'm not a newbie in amps building, I can't understand why it doesn't work to my taste (sorry for my english a bit weak...hablo mucho español ahora y he olvidado mucho inglés que habia aprendido). I haven't found any schem of plate driven phase inverters here or on the net. All I can see is CF driving PI. All the voltages of my f....bad ''new'' design are good, so are the solders/connections/tubes. (I don't smoke, drink nor take drugs or pills, not even Tylenols. I have a university diploma, had a good education and know how to hold correctly my fork and knife...and soldering tip). Yes, I could rewire the second tube as a CF but then I'd loose one stage of gain.


What can I do know ?  (I am not talking about my wife, so please....)


Colas LeGrippa

[sluckey]

All the PI circuits I'm familiar with have a high input impedance and can be 'driven' easily from the plate of a preceding stage. Most all Fender amps except the 5F6A drive the PI from a previous plate. You don't need a cathode follower to drive a PI. Using a cathode follower in an amp is usually all about driving the tone stack. It's not about driving the PI at all.

Show us your schematic. Maybe we can see something that needs to be changed in your circuit. With 4 stages of gain it will easily make any power amp squeal and howl and just misbehave badly. A lot of precautions need to be taken with lead dress and padding the signal between stages.

[Colas LeGrippa]

OK. Too much gain then. I haven't drawn a schem yet , I'll do it when the amp works.....I will do what's necessary to reduce gain or pad the stages. It's howling like a million wolves (maybe more... ).


I started to have problems with the beast not because I have removed the CF, but by replacing it by another stage of gain. I was not quite sure at that moment but now I know. I will use the second 12 ax7 as a single gain stage, by paralleling the two halves , and see whait will happen. Thanks.


Colas

[Colas LeGrippa]

Do you remember the Johnny Winter's song ''One step at a time'' ? I was singing that song while I was tweaking my amp that didn't want to work. All of a sudden, I got an idea: ''One stage at a time''.......
So I hooked up the first half of the first 12ax7 right onto the long tail pair: check ok, beautiful tone. Hooked up a 1 meg pot, returned the signal from the first half to the second half and right onto the PI: check ok, more gain sounds perfect. So I kept on doing this, one stage at a time, till the 4 th stage. Works perfect. I had to do it that way to succeed. Everytime it's the same for me: If I make too many changes from one shot, it never works. The man is happy now . (What went wrong the first time ? God only knows).


thanks for help


Colas

[DummyLoad]

if you were playing this; you'd have got it right the first time!   


https://www.youtube.com/watch?v=Iq-bfXz3eaQ


--pete

[Colas LeGrippa]

right on !

[kagliostro]

May be this old job of Steve can give you some inspiration

http://www.mediafire.com/file/op0ulq9u8f7xhig/Aro_Project.pdf

Franco


p.s.: Unfortunately, here too the PI is preceded by a Tone control (and a CF)

[Colas LeGrippa]

i 've already seen it but thanks anyway


Jack

[tubeswell]

'Plate driven inverter'?


Do you mean a DC-coupled LTP?


(http://www.valvewizard.co.uk/dcltp.html)

[sluckey]

Not dc coupled. He means the PI input signal originates from a previous tube's plate, not it's cathode.

[SolidStateCircuitReject]

Everytime it's the same for me: If I make too many changes from one shot, it never works.

Yea, I can relate to that.... always important to check after one change, especially when experimenting... even if it is so much of a pain in the arse.

May be this old job of Steve can give you some inspiration

http://www.mediafire.com/file/op0ulq9u8f7xhig/Aro_Project.pdf

Franco

p.s.: Unfortunately, here too the PI is preceded by a Tone control (and a CF)

Interesting, what do 2 bypass caps on V1a do??

[Colas LeGrippa]

I was asking me the same question, why two cathode bypass caps ? I guess it is our choice. At this point, let's install a rotary switch with 4 or 5 values to have more than 2 options....

[sluckey]

Interesting, what do 2 bypass caps on V1a do??

Gives you a total capacitance of 1.68µF. Why??? Who knows. That's the way it was on the original schematic. Not my work. I just did the layout for forum member Aro.

[drgonzonm]

Buenos días amigos !
..... my mind is getting crazy looking at my wife's legs ..., (I don't smoke, drink nor take drugs or pills, ... know how to hold correctly my fork and knife...). Colas LeGrippa

You have two of the three tenets of life (Rock and Roll, sex, and wine), ok, no wine, and good health, good mind (as shown by the questions you ask) ,  and you are complaining?  To me, you have the good life, 

[SolidStateCircuitReject]

Interesting, what do 2 bypass caps on V1a do??

Gives you a total capacitance of 1.68µF. Why??? Who knows. That's the way it was on the original schematic. Not my work. I just did the layout for forum member Aro.

That's it... why am I so stupid sometimes?

[sluckey]

Hey, I questioned it too. Why not just use a single 2.2µF? Who knows. Probably some tweaker temporarily tacked a cap across the existing bypass cap and either said I like it or maybe just forgot to remove it. Anyhow, it was on the schematic so I put it on the board layout. I never heard back from Aro so I don't know if he ever built this or not.