Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

[kmcguitars]

Now that I put a Weber 10" Cali series in my Princeton reverb, It seems to
have unlocked the tonal posibiltles of the amp. Now I can crank it without
the speaker farting out. It sounds great with single coils on 7 or 8 But
with humbuckers (PG, 59, and CC) at that volume I get a aweful raspy
distorion. The tubes are all good ( JAN 6V6's all NOS preamp tubes) Even
got a 5751 in the VI socket. When I first got the amp 10 years ago I
did the torres Perfect prince mod. If I remember correctly I just changed
the two P.I. .1 caps with balanced orange drops (same value) and the
grid resistors to 470 ohms. I just found two new PI mods to increase
the headroom. Before I try them I was hoping to get some opinions from
the guys in the Know?
Stokes PI mod
Paul C. Mod
Layout w/t torres mod
I was also thinking about changeing the filter cap. Does anyone know where
I can get the Exact can( Not a retro fit) besides Torres?
EDIT : I just opened up the amp to see what I did using the torres mod
The only thing I see not stock was a 1k resistor wired in series w/t the stock
56k ? I guess this is supose to balance the circuit? and the two 470 resistors
across the tube sockets ( better protection i guess?) and the orange drops.

 

[Curly]

Re: Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

as far as the cap can, I think I got one at antique electronic supply

not sure if they still have those or not

 

[VoodooAmplification]

Re: Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

Now that I put a Weber 10" Cali series in my Princeton reverb, It seems to
have unlocked the tonal posibiltles of the amp. Now I can crank it without
the speaker farting out. It sounds great with single coils on 7 or 8 But
with humbuckers (PG, 59, and CC) at that volume I get a aweful raspy
distorion.

When you say Raspy distortion do you mean harsh or is it like a smeary/muddy type of bottom end that lacks definition?

The tubes are all good ( JAN 6V6's all NOS preamp tubes) Even
got a 5751 in the VI socket. When I first got the amp 10 years ago I
did the torres Perfect prince mod. If I remember correctly I just changed
the two P.I. .1 caps with balanced orange drops (same value) and the
grid resistors to 470 ohms.

The 470ohms would be for the screen grids

I just found two new PI mods to increase
the headroom. Before I try them I was hoping to get some opinions from
the guys in the Know?
Stokes PI mod
Paul C. Mod
Layout w/t torres mod

You would have to try both mods to see if it's something you like (both sound and feel). I'm not sure exactly what it is you are trying to over come, change or alter sonically so it's hard to recommend things that will work for where you want to go sonically.

I was also thinking about changeing the filter cap. Does anyone know where
I can get the Exact can( Not a retro fit) besides Torres?

Is it a can cap in your amp or the regular axiel filter caps?

EDIT : I just opened up the amp to see what I did using the torres mod
The only thing I see not stock was a 1k resistor wired in series w/t the stock
56k ?

Essentially two resistors in series with one another will add together.

I guess this is supose to balance the circuit? and the two 470 resistors
across the tube sockets ( better protection i guess?) and the orange drops.

It's good to have screen grid resistors if you are cranking the amp. It helps to save on tube life but it also tends to give better tone when you crank the amp to the point of the phase inverter and power tubes breaking up.

You may want to put some 1.5K's on the grid pins (pin #5) and that will help elevate some of the raspyness for you, assuming your amp does not already have those resistors. If you need to roll off some top end (if you find it's too harsh) you can increase the input resistors (2.2K, 3.3K, 4.7K, 5.6K, etc.) until you find the right value that works best for you.


Hope it helps
Trace

 

[Curly]

Re: Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

FWIW, the PR I had was a BF - '66 I think, and if I still had it, it would not have any mods done.

these are great little amps - one guy I used to buy parts from called them "the most recorded amp"

they can really crank, and sound larger than you'd think, but IMO, the trick is in using the right speaker and getting it biased well. In other words, just get the darned thing set up well - that's all you should need.

 

[kmcguitars]

Re: Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

The best way i can discribe the distortion ( using HB's) is it sounds
like there's a bottleneck bettween the preamp and power section?
That is... A bunch of buzzy preamp dist. with little power tube break up?
( This doesn't happen w/t single coils?) last night I had the amp open
soldering iron hot. When I discharged the filter cap(yes, it's the can style)
20-20-20-20mfd @475 volts. I noticed I got NO SPARKS? this could be
playing into the problem maybe? It's the original cap,Last time I discharged
it I remember a Lil' pyro technices. When I saw the extra resistor in series
with the original 56k I decided it would be unwise to do a mod on top of
another mod without consulting someone more knowledgeable. Now by
input resistors... You mean the 68k's by the input jacks? it's been mentioned
that ceaser Diaz incresed SRV amps to 100k. I guess I could play with the
value on the #2 jack for HB's and keep #1 stock for SC's? Thanks Trace
and thanks Curly, they still have the orignal style can caps $29 and U.S. made!

 

[VoodooAmplification]

Re: Amp P.I. Mods opinions- Jeff S.,Kent, Lew, anyone!

The best way i can discribe the distortion ( using HB's) is it sounds
like there's a bottleneck bettween the preamp and power section?
That is... A bunch of buzzy preamp dist. with little power tube break up?
( This doesn't happen w/t single coils?)

If there are no input resistors on the power tubes then I would recommend installing some. It will help with what you are describing.

last night I had the amp open
soldering iron hot. When I discharged the filter cap(yes, it's the can style)
20-20-20-20mfd @475 volts. I noticed I got NO SPARKS? this could be
playing into the problem maybe?

Discharging caps and sparks have nothing to do with you tone my good sir. If you do not want sparks then install some bleeder resistors, which will drain the caps for you each time you turn the amp off.

Now by input resistors... You mean the 68k's by the input jacks?

No, re-read my prior post and you will see that is says pin #5 of the power tubes, not the preamp tubes. Look at other schematics of Fender amps and you will see where they have 1.5K's installed on the power tube sockets.


it's been mentioned that ceaser Diaz incresed SRV amps to 100k.

There were several other mods Diaz did on SRV's amps but the 100K I'm not familiar with. On the input jack, it would alter frequency response and limit signal to a degree.


Hope it helps
Trace

 

[kmcguitars]

Re: Amp P.I. Mods opinions-

Re: Amp P.I. Mods opinions-

Did the Stokes mod... Mission Acomplised !! The amp is now Humbucker
friendly. :dance: Thanks Trace and Curly for your input
Maybe a filter cap will help to. I really gotta stop doing mods to this Amp !!
But I can't help it. :smack: I think I need to join modaholic's Anonomus. :laugh2: