Imperial PI operation

[Bieworm]

Hey guys..
I keep looking at the PI circuit of my imperial clone. It keeps puzzling me.
I can't find a similar PI anywhere but the ancient  of which I forgot the name. Can anybody explain to me what's going on there?

[vampwizzard]

do you have the schematic for reference? Is it the same as https://www.thegearpage.net/board/index.php?threads/schematic-for-a-1993-tone-king-imperial.2122563/

I thought the imperials used LTP and cathode biased 6v6 to make their magic

[Bieworm]

Here are the ones I used

[vampwizzard]

that certainly looks like a LTP to me.

Edit: it is not a LTP.

[sluckey]

PI = paraphase

[Bieworm]

I get that, but the LPT here has the plate of triode 2 connected to the grid of triode 1. What's that about?

[vampwizzard]

PI = paraphase

I shouldve read Merlin's page closer. Good call.

[sluckey]

...the plate of triode 2 connected to the grid of triode 1. What's that about?

That's how all paraphase inverters work. So quit calling it a LTP because it ain't. 
 

[Bieworm]

Thanks Steve
Somehow I missed that on merlin's pages. I only found old docs that lead me to the Williamson design. Makes much more sense now. Sadly I can't try the paul ruby mod on this type of PI. I already added a snubber cap per schematic,  but it still has some sizzle riding on the OD notes. I already tried everything I know. (Tube rolling; larger grid blokkers and on every grid; voltage devider before the PI.. which now apparently only acts on 1 side of the PI  due to the paraphase situation; chopsticking lead dress; reflowing joints; ...)
It's probably not poweramp related because it is already there on lower volumes. It goes hand in hand with the dial on the midbite pot. In fact, the more drive, the more sizzle.
In the end I settled for a 5157 on V1, the 10k grid stoppers on preamp tubes, 68k grid stopper on the PI and the 1M voltage divider with 1M to ground. It's all about compromising  because I have to settle for less OD too.
Any point in trying A500k pots on the volume and tone knobs? That channel is very high mid focussed anyway.. it wouldn't hurt shaving some of that off.

[sluckey]

voltage devider before the PI.. which now apparently only acts on 1 side of the PI  due to the paraphase situation;

There's no reason you can't put a divider before V5 pin 7.

[Bieworm]

There is on my build already a divider on that spot. It's the other side I'm speaking of.. grid pin 2. Ain't I unbalancing the PI even more with that divider on pin 7?
How about a vox cut control on the grids of the 6V6's?  That would be the last tone shaping point
I'm going to try that with alligator clips. Maybe it gives good results and maybe there is even a sweet spot so I can use a fixed resistor in stead of a pot...

[sluckey]

Don't mess with pin 2! It already has the necessary divider (R74 and R75) so pin 1 output matches pin 6 output.

BTW, the paraphase is old technology. If you want to see some Fender amps that use the paraphase inverter, browse through the early schematics. The 5A?, 5B?, and 5C? all used the paraphase. They all look the same. When the 5E? series came out Fender had switched to the cathodyne.

[Bieworm]

How about a vox cut control on the grids of the 6V6's?  That would be the last tone shaping point
I'm going to try that with alligator clips. Maybe it gives good results and maybe there is even a sweet spot so I can use a fixed resistor in stead of a pot...

[sluckey]

The cut will definitely change the tone.

[Bieworm]

No harm in trying I guess

[sluckey]

Put a 100pF cap between pin 1 and pin 6 like Fender did. Easy to do right on the V5 socket. That should reduce some sizzle.

[Bieworm]

Here's a sound clip. First on the drive channel and then on the clean channel. The fizz kicks in when there's overdrive.
It's not v2 ,3 or 4. With the reverb off there is still the same fizz. The only shared parts of the circuit are V1A,  V5 (PI) and the power tubes. I have 10k grid stoppers on the power tubes.  68k on the PI and 1.5k on V1. There's a 1000pf snubber cap across the plates of the power tubes.
I looked for lead dress issues, soldering joints,.. nothing..

https://www.dropbox.com/s/syt7n4ovmvocy8a/Imperial%20fizz.m4a?dl=0

[Bieworm]

 No ideas anybody?

[DummyLoad]

that paraphase is a waste of parts for a cathode biased amp. also, AC load characteristics for the inverting phase triode is lopsided. could be part of the tone experience of the amp?

i suspect a slightly modified fender 5C5 paraphase would sound much better. just my opinion... :D



--pete

[Bieworm]

I was thinking maybe a PI like a 5E3 or even an 18 watt. If there is fizz on that one there's always the paul ruby mod to get rid of that.

What do you mean by slightly modifying the 5C5 PI ? Which values need to be changed?

[Bieworm]

is this a correct use of the 5C5 PI ?

[Bieworm]

Is that really goong to sound that much better?
After reading a lot about them paraphase phase inverters people tend to have the same problem. Seems to help when the inverting triode had a higher voltage than the other triode. Makes sense?
What puzzles me too is that it's a mark bartel design.. should be top notch?

[sluckey]

Is that really goong to sound that much better?

I doubt it.

[Bieworm]

This is my voltage chart:

I have a 12au7 (NOS jan GE) .. to try that, do I have to tweak some other components for biasing and such?

[66Strat]

This is my voltage chart:

I have a 12au7 (NOS jan GE) .. to try that, do I have to tweak some other components for biasing and such?

You could use a 12AU7, but the resistor values in the voltage divider would need to be adjusted for it to balance properly.
 
Attached is an excerpt from the RCA Receiving Tube Manual RC19 that explains the operation of the paraphase PI and how to calculate the voltage divider resistance values. The chart that is referenced is contained within the manual. It lists the operating points and resulting voltage gain for a variety of tubes.

Or you could copy the 5D3 phase inverter. It is self balancing and delivers near perfect AC balance over the life of a tube. It is the Ron Popeil version of phase inverters. Just set it and forget it.

[Bieworm]

I have been looking into the 6V6 plexi PI at sluckey 's page

https://www.google.be/url?sa=t&source=web&rct=j&url=http://sluckeyamps.com/6v6plexi/6v6plexi.htm&ved=2ahUKEwjn38PD6tLtAhWH7KQKHePsA7sQFjAKegQIEhAB&usg=AOvVaw0aSCcSGqFRzVezgwahXhKJ

Is that something to consider? I read a lot of problems about the paraphase phase inverters...

[DummyLoad]

of all the PI variations, the cathodyne is the nastiest sounding when pushed into overdrive. ampeg uses a paraphase, so does the fender pro, the supro thunderbolt and all three sound great. the smoothest overall when overdriven is the long-tail. each type has a unique character.   


--pete

[Bieworm]

Well, the paraphase PI in my imperial clone is about the nastiest I've ever had. I just can't seem to tame it. Would the LTP be a good match with this very mid focussed OD?

[DummyLoad]

no clue, as i have no experience with that amp. what you are hearing could be be artifacts from another source(s) in the system. if that amp is not what you expected, perhaps selling to someone who does like the tone would be the best path, rather than hacking away on it?

--pete 

[Bieworm]

no clue, as i have no experience with that amp. what you are hearing could be be artifacts from another source(s) in the system. if that amp is not what you expected, perhaps selling to someone who does like the tone would be the best path, rather than hacking away on it?

--pete

I get that. But it's more fun to experiment I guess... keeps me busy and learn stuff. I wouldn't call it hacking.. I built the amp myself. During the quest for answers I can only conclude it's the PI.  If I can't source the problem I will change the drive channel into a superlite TMB,  with a fixed mid.

[DummyLoad]

ah! ok, a homebrew! sorry, misunderstood. hack away. try a long tail...what have you got to lose?

the 5D3 paraphase circuit values are selected to work with a 6N7 tube. use the 5C5 paraphase plan, if you're headed down that path.


--pete

[Bieworm]

ah! ok, a homebrew! sorry, misunderstood. hack away. try a long tail...what have you got to lose?

the 5D3 paraphase circuit values are selected to work with a 6N7 tube. use the 5C5 paraphase plan, if you're headed down that path.


--pete

Going to try the LTP from the 6g3 first. I don't have a 6n7... I do have 12au7 and 12at7 tubes. I don't wanna tame it too much.. just that spacey fizzle needs to go. The od sounds almost like a nice drive with a layer of cheap solid state distortion on top. I want crunch without the splat 😃

[66Strat]

The 5D3 uses a 12AX7 for the PI.

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5d3.pdf

[Bieworm]

Applied the 6G3 LPT.  Still fizzy, but I like it better. More control over the volume...still good amount of OD.  I think I'm better off buying a scope and start from the input
I did preserve the 2x100k Rp instead of the 82k + 100k. This could hardly cause the fizz I think.  The 100k are CC. Anybody happen to know the target voltages for this PI?

[DummyLoad]

The 5D3 uses a 12AX7 for the PI.

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5d3.pdf

apologies. i was looking at the model 26 schema.


--pete

[ac427v]

I suspect that the first stage allows too much bass to hit the following stages, causing them to distort at harmonic frequencies. C6 is .047-a very large value for  the first stage of a distortion channel. You could test that theory by turning the tone control to maximum treble-minimum bass setting. I know you don't like that tone but does the fizziness go away? If it does then try replacing C6 with a lower value cap like .01 and adjusting the tone control setting to that new reality.

[Bieworm]

that's a possibility. I added a cathode cap at the first stage, hoping it would get rid of the nasty high freq fizz.. so it was already there before the cap. I settled with a 15uf/100V. But now the LTP is installed the amp seems to have a fair amount of more low end.. was planning to revisit that cathode cap anyway.. but thx for the tip... maybe I will have to deal with C6 too.

I can't help from going paranoid now that the previous steps didn't help. I'm starting to think of all other possibilities like:
- the 1/4th power switch. Maybe something wrong with those resistors thus compromizing the output stage? on full blast the fizz seems less (4/4th swichted)
- the power tubes
-...

[Bieworm]

I removed the cathode bypass cap on V1. Less fizz, but less thick too. So I put in a 1uf 63V cap. Better thickness!
The fizz is still there but not that prominent.
Should I raise the grid blockers on the 6V6? I have 10k there now...
Or put a grid blocker on the input of the PI tube with the enhancement cap across it? 

[Bieworm]

Bumping for advice😀

[PRR]

Bumping for advice😀

Post pictures. It is almost certainly oscillation but on a home-made amp we can't make smart suggestions on an amp we have never seen.

[Bieworm]

Need details of certain parts?

[PRR]

Thanks. Lots of shielded, which is normally good. Trying to clear snow here today so not able to stare at that. Maybe someone else sees a sneak-path from high-level stage back to low-level stage? In this front-to-back layout the tone controls are liable to bleed supersonics.

[Bieworm]

Thanks. Lots of shielded, which is normally good. Trying to clear snow here today so not able to stare at that. Maybe someone else sees a sneak-path from high-level stage back to low-level stage? In this front-to-back layout the tone controls are liable to bleed supersonics.

I don't quite follow.. I paid good attention to the rules of layout. But I'm no expert.. maybe there is someone more capable that can point things out?

[mresistor]

That's a good looking ground buss.   

[Bieworm]

Bumping for advice😀

Post pictures. It is almost certainly oscillation but on a home-made amp we can't make smart suggestions on an amp we have never seen.

It's gotta be oscillation.  I tried 100k grid stoppers on the 6V6's.. same deal. It does the same when clean channel is cranked. So the common parts of the circuit are V1, PI and powertubes..