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PT/OT Question


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3/11/1998 4:55 AM
Bob K.
PT/OT Question
Background info: Fender amp, 60 watts, two 6L6's, 473v on the plates.  
 
Here's the question: Can I remove the existing PT, put in a PT from (say) a Deluxe Reverb, keep the same OT, and swap out the 6L6's for 6V6's to bring the output on the amp down? If this can be done, what will the new output be....50w,40w, less?  
 
Thanks in advance.....  
 
Bob
 
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3/11/1998 7:38 AM
Whit

Bob,  
 
Sounds like a viable plan to me...  
 
The 6V6's will likely start "melting"  
at around 420V, so you're correct in  
replacing the PT (tube rectifier? about  
325-0-325 secondary PT is in the ballpark)  
 
OT primary impedance for PP 6L6 or 6V6  
is similar, so the existing OT should work  
fine.  
 
Power? -> less, probably around 25 Watts.  
 
Sounds like fun... what are you doin' with  
the old PT ? ;-)  
 
... Whit
 
3/11/1998 9:42 AM
Bob K.

Thanks Whit.  
 
The amp is an 80's Fender Concert (Rivera design?), the one with reverb, channel switching, FX loop, etc., essentially a 60w version of the Twin II. I'm a basement playerr and 60w is a tad too loud for me. My intention is to replace the PT, make the the B+ compatible for 6V6's and rebuild the front-end to a more "standard" design maybe one channel to AB763 specs and the other to 5F6 specs.......who knows. Take out the fx loop, and use the 12AX7 that's freed-up to put the vibrato back in. Big plans.  
 
I will have the old PT left over. Let me know if you need it. If you've got a 6V6 compatible PT hanging around let me know, maybe we could swap.  
 
Thanks again for the help.  
 
Bob
 
3/11/1998 11:12 AM
Whit

Bob,  
 
Sorry, I'd trade if I had one, but I don't.  
 
Sounds like a noble project. Although a  
new PT is the best way to go, if you're  
tight for cash, there are ways of dropping  
B+ with your existing one.  
 
I don't have enough experience on this  
topic to attempt a description, but others  
have mentioned the use of Zener diodes on  
the centre tap of the PT secondaries, or  
just big power resistors in the supply  
line to drop the voltage.  
 
It should even be possible to switch  
between B+ values, so you can go with  
6V6 or 6L6 tubes depending on the size  
of the "basement" you're in.  
 
Perhaps a question about dropping B+  
to the DIY board will get you a more  
descriptive answer from someone who  
has done this ...  
 
Enjoy...  
 
... Whit
 
3/11/1998 12:38 PM
Bruce

There are other options here Bob.  
RG has a nice piece on his page about installing an additional small transformer in with the one you have in a voltage bucking scheme to lower your "overall" voltage.  
But I think I would buy a big 50 to 60 volt 50 Watt Zener diode, a 150 ohm 10 watt resistor and insert them in series with the center tap of the power transformer...  
if it has a center tap tranny, and use the 6V6s.  
If it isn't a center tap tranny, we can get around it a different way.  
This will drop your overall high voltage dowe by at least 40 to 50 volts or more.  
Rebias for the 6V6s at around 25ma to 30ma, + or - tone and and plate glow. yeow, watch that!  
Put a nice 1K to 1K5 5 watt resistor in each of the screen voltage lines to the 6V6s and it should run OK that way.  
I was running a twin with 6V6s like this for a while and I never blew anything up. And they were NOT NOS 6V6s. They were Ruby 6V6GTBCs.  
Or how about connecting the screens to the plate through a resistor in triode mode with the 6L6s or the 6V6s? That will drop the power and volume a little more too.  
 
By the way, the power will be under 25 watts with 6V6s and it still will be loud as hell.  
6L6s in triode mode will still be pretty loud too..  
 
Save the stock parts as much as you can so you can sell the amp later.  
I think a smaller power amp is in your future.  
Like the rest of us, you'll decide 20 watts is plenty powerfull for home use.  
 
Bruce
 
3/11/1998 1:37 PM
Bob K.

Bruce....  
 
thanks for the info. Dropping the voltage via a zener would certainly be a cheaper way to go. It is a center tapped transformer so there should be no problem. This amp has a zillion filter caps in it. I plan to take some of the 'extra' ones out and see if by doing so I get rid of some of the bass response. This amp has way too much of it for my tates. BTW, I have heard of zener option before but not the 150 ohm 10 watt resistor in series with it. What's the purpose of the resistor?  
 
Do you what B+ voltage I should be aiming for when running 6V6's? I'd like to check it before I plug anything in.  
 
I have a Twin II and have toyed with the idea of dropping the voltage down in it low enough to take a pair of 6V6's and a pair of 6L6's in it at the same time and working out an arrangement to switch so that I'm running the V's, the L's, or both (ala O'Conner's book). I know that to do this I will have to be able to adjust the bias to each pair of tubes independently. It would be nice to have the option though.  
 
Again, thanks for the input.  
 
Bob
 
3/11/1998 2:25 PM
art
To reduce the bass response you might want to  
reduce the size of the first cathode bypass  
cap (which is 25uF?) rather than reducing the  
power supply filtering. Try values around  
.47uF-1uF...  
 
art
 

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