Ken Fischer PT mods

[Ivabiggun]

I was reading something I downloaded concerning a conversation with Ken Fischer. It was about modding metal panel's to sound more like plexi's. Ken said that on the PT primary, if you disconnect the common, and use red, (120V), in it's place. Then put the purple, (240V) in place of the red wire, you will lower the voltage because you are using a different length of the primary. This will work like a variac and make the amp sound more like an old plexi. There were other suggestions, but I wanted to ask about this one. Has anyone tried it? Is this similar to what George's PT does with the lower voltage tap? I assume you would have to change the bias feed resistor too.

Iva

[rockstah]

i don't know much about this but it seems if u want to get a metal panel to sound like a plexi, put plexi specs in it.
lower filtering as well as a few cap and resistor changes away.

[novosibir]

Ken said that on the PT primary, if you disconnect the common, and use red, (120V), in it's place. Then put the purple, (240V) in place of the red wire, you will lower the voltage because you are using a different length of the primary. This will work like a variac and make the amp sound more like an old plexi.

I can't and won't believe, that Ken this ever has said! That's obviously a story, falsified by hearsay to a rumour!

Keep in mind, that an amp operating at 120V wall pulls double the current than at 240 wall voltage. Hence the primaries from the 120V tap upstreams are thinner lead, that the 0-120V lead.

Now attach your wall voltage of 120V across the thinner PT's primary lead from 120V tap to 240V tap and play the amp dimed... ... and wait for your nice smell

Impossible, that Ken Fischer ever has recommended such a Cardinal's mistake! A guy with his knowledge and experiance? Never! Let him RIP!

OTOH the 120-240 primary has exactly the same numbers of turn, than the 0-120 primary - only more voltage drop across the 120-240 occurs, hence it's thinner and hence has more copper resistance.

Larry

[awangotango]

Iva, I've done (or something very similar) this to my '72 . Steve (SDM) turned me on to if i recall? It's been while so I can't verify if it's the exact same instructions as fisher, but it sounds very familiar. It lowered the voltage from 525 or so down to 450. Heaters came down along-with, and are now at 2.8 (just checked it). In any case, it worked, pure brown tone, simple. (I also seem to recall the fact that only some metal panel PT's can do this re-wire.) FYI, those metal panel amps with HIGH B+ are some of the most killer marshalls out there. But unforunately require high quality old tubes to handle it. And they are killer loud of course. these 2 factors is why I lowered it.



http://img187.imageshack.us/img187/6240/img2763qp0.jpg

[novosibir]

First:

I'm curious, how long your PT will survive this, fed through the much thinner 120-240V primary lead now, which has a higher copper resistance, than the 0-120 primary. The reason, why your B+ is lower now, is the enhanced voltage drop across the 120-240 primary - a voltage drop of probably 15V - what tells me, that your PT is only seeing 105V supply voltage now, and additional tells me, that the primary lead becomes horrible hot inside the PT.

Keep us updated, when your PT went to the ancestry, covered in smoke

Second:

Your heater voltage is down now @ 5.6V across the entire secondary (your measurement was from ground to one end). Hence you've reduced the tube's life to about 1/3 of the regular estimated lifetime due to under heating the cathodes, what lets the cathode coating wear out much quicker now. Take a look onto the graph below!

Hope, you don't be wasting valuable NOS glass in this amp!

Larry

[rockstah]

interesting Larry, i have heard to pay no attention to the "graphs" when it comes to heaters as stated. there are lots of people that run their plexi's at 90v for years and have no problems with cathode stripping. so around 4.9v is still nothing to worry about.

[awangotango]

LArry, it's been almost 2 years and counting.....I use the amp a couple times a week with Mullards. I'll keep an eye out for the PT getting hot to the touch

[awangotango]

here's the quote I believe we'all are referring to....

Ken's recommendations

The primary of the power supply has the following color codes:

orange = common red = 120

-> disconnect them, use red as common and violet in place of red as 120 (you're using a different length of the primary winding in this case)

this drops the supply voltage to 460volts which achieves same results as EVH's variac method except you don't have to worry about blowing variac - verify that this voltage is correct in your amp if you do this. This makes metal face marshalls sound more like plexi;

[novosibir]

If that's true and definitely a recommendation from Ken Fisher, then - for my sake say, its blasphemy - but then slowly are arising some doubts up into my head, concerning the ingenuity of the 'Mystery Man'

- here a very hot running PT's primary with the side effect of cathode underheating

- in most Trainwreck's not twisted OT's primaries and only barely twisted grid wires to the output tubes (one twist)

What else?

I'd be curious to hear the opinions of some other cracks like i.e. Randall Aiken or Kevin O'Connor to this concern...

Sorry - w/o being impiously, but in my eyes his effulgence slowly is flaking

Larry