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Post subject: Fender Princeton reverb biasing question
Posted: Mon Oct 07, 2013 10:49 am
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I have made a Marsh/Mojotone Fender Princeton Reverb amp Kit and I am wondering if the resistors and capacitor values shown in photo of schematic are right? they seem different than some specs i have seen online/? I am getting a reading of 10.8 and 12.3 MA on power tubes (seems low to me) it sounds good to me but the vibrato is useless..any help would be greatly appreciated :-) thanks Jimba


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Mon Oct 07, 2013 11:47 am
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Are your diode and biasing cap correctly polarized?

That measured bias current does seem mysteriously low for a pair of 6V6GTA's in class AB configuration.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Mon Oct 07, 2013 11:56 am
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thanks for the response...I believe since i been playing it for awhile the cap would have exploded if it where backwards? right? but the diode is a little more confusing to me is the side with the "stripe or line" the pos or neg side? if it was backward would the amp still work? thanks


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Mon Oct 07, 2013 12:19 pm
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The backward cap would blow up like a freaking stick of dynamite.


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Mon Oct 07, 2013 1:56 pm
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Since the bias is not adjustable, the tubes will pull current dependent on their rating. To adjust the bias for each particular set of output tubes, you must change the value of the bias resistor. In this case you would need to change the value of the 100 K resistor, maybe to 120 K or so. It might take a little experimenting to find the correct value.

The end of the diode with the stripe would be considered positive. :idea:

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 1:29 am
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shimmilou wrote:
Since the bias is not adjustable, the tubes will pull current dependent on their rating. To adjust the bias for each particular set of output tubes, you must change the value of the bias resistor. In this case you would need to change the value of the 100 K resistor, maybe to 120 K or so. It might take a little experimenting to find the correct value.

The end of the diode with the stripe would be considered positive. :idea:



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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 1:31 am
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Jimtrader , you may have lot of noise with these very long wires in you amp , don't you ?


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 8:28 am
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stratele52 wrote:
Jimtrader , you may have lot of noise with these very long wires in you amp , don't you ?


+1

Indeed, a prime candidate for unwanted noise and/or parasitic oscillations. Those leads should be re-dressed to eliminate that rat's nest. Use photos of a stock blackface AA1164 chassis as a guide.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 8:38 am
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Retroverbial wrote:
[


Indeed, a prime candidate for unwanted noise and/or parasitic oscillations. Those leads should be re-dressed to eliminate that rat's nest. Use photos of a stock blackface AA1164 chassis as a guide.

Arjay


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 5:49 pm
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You need to play with the resistance value of that 22k-ohm, line-to-ground resistor, parallel to the filter cap, to get the proper bias voltage and the proper voltage to the vibrato section. Remember everytime you change 6V6GTs, you will need to play with this resistor's value to get the proper idle bias.

You try try adding a variable pot inline. Say a 10k-pot and a 18k-ohm set resistor in place of the 22k-ohm one. But, it really isn't that convenient --- unless you mount the pot so that it can be adjusted without removing the chassis.

The higher the resistance = the more negative voltage = colder bias. And vice-versa.

Good luck! :D


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 6:25 pm
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BMW2002Ti wrote:
You try try adding a variable pot inline. Say a 10k-pot and a 18k-ohm set resistor in place of the 22k-ohm one. But, it really isn't that convenient --- unless you mount the pot so that it can be adjusted without removing the chassis.


+1

Here's a great illustrated tutorial on how to correctly add an adjustable bias pot to a Princeton Reverb......

http://www.thefenderforum.com/forum/sho ... hp?t=48209

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Tue Oct 08, 2013 8:13 pm
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jimitrader wrote:
... but the diode is a little more confusing to me is the side with the "stripe or line" the pos or neg side? if it was backward would the amp still work? thanks


It's hard to tell in the photo but the diode appears to be polarized correctly. The stripe on the diode corresponds to the line on the diode symbol in the schematic.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Wed Oct 09, 2013 12:14 am
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Striped side = cathode or negative side. Your diode is soldered correctly with the cathode flowing negative voltage toward the 6V6GT tubes.

http://www.fordscorpio.co.uk/images/doorsmal/diode.bmp


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Wed Oct 09, 2013 4:59 am
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BMW2002Ti wrote:
Striped side = cathode or negative side...


:?

You have that backwards, that picture is wrong. :wink:

I have always changed the 100 K for the bias, it seems a little easier to me.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Wed Oct 09, 2013 9:40 am
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Shimmy, the layout is correct. It's just that the diode is placed "backwards" in a bias supply. The anode side to the PT --- not the cathode, like in the main FWBr rectifier. Really, (+) and (-) symbols are a bit confusing with diodes. Diodes should be thought of as gates, allowing passage of charge, whether "positive" or "negative" depending on orientation.

I think of it this way, the symbol shows the direction of electrons during top part of sine wave. But, flip the diode and you get only the bottom half of sine wave.

From anode to cathode like the arrow of the schematic is pointing --- for positive voltage. In the bias supply, what the single diode facing in the "opposite" direction is doing is only allowing the bottom portion (or negative AC) through. In half-wave form, such that the top half of AC sine wave not conducting.

Whereas a full-wave bridge "flips" the negative AC or bottom half of the sine wave. So the sine wave now is only positive. With both top & bottom conducting.


Half wave (positive voltage):

http://www.duncansonelectric.com/blog/w ... 24x368.gif


Full wave:

http://www.duncansonelectric.com/blog/w ... 24x368.gif


Last edited by BMW2002Ti on Wed Oct 09, 2013 10:16 am, edited 3 times in total.

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