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Post subject: Re: Fender Princeton reverb biasing question
Posted: Wed Oct 09, 2013 10:09 am
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Wiki's bit on diodes:

http://en.wikipedia.org/wiki/Silicon_diode

PS: site only allow two URL's per post


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Wed Oct 09, 2013 3:57 pm
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Yeah, that URL thing is weird. In fact the forum has been freaky lately. The length of the post, number of URLs, special symbols, and other reasons that I haven't figured out, all affect whether or not a post goes through. :?

But, as far as diodes, I know all of the technical details, and yes it can be confusing, let's keep it simple though, and look at diodes as rectifiers (which is what we're dealing with here). We're not dealing with current flow, whether you follow Franklin or conventional, the "arrow" is either forward or backward to the current flow, that really doesn't matter. Using a digital multimeter and this Princeton Reverb circuit, put one meter lead on the bias diode Anode (opposite end from the stripe), and the other meter lead to the chassis, read a negative voltage. Flip that diode around, put the meter lead on the Cathode (striped end), now you have a positive voltage. Or simply look at any schematic/layout and note that the Anode is marked "-" (negative).

I always remember that with a rectifier, whether a single diode, a half-wave, or a full wave bride, the "arrows" (Cathodes) always point to the positive. Looking at a full wave bridge, four corners, the arrows always point to the positive corner, the opposite corner of the bridge is therefore negative. That leaves the other two corners as the AC inputs. When you apply AC to a rectifier, the output polarity is really only dependent on the reference. If you ground the negative out of a full wave bridge, you get a positive voltage out in reference to ground, so if you ground the positive out, you get a negative voltage in reference to ground.

Simple! :D

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 11:54 am
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ok so i tried some things:
changed 100k bias resistor (38) to

120k = 42.5
86k = 21

I like the 86k sounds better but I am wondering if i should go buy a 90 or 95k to try and get to 25? is that necessary so it wont be to cold?? thanks j


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 12:24 pm
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shimmilou wrote:
Yeah, that URL thing is weird. In fact the forum has been freaky lately. The length of the post, number of URLs, special symbols, and other reasons that I haven't figured out, all affect whether or not a post goes through. :?


surely an attempt to control the SPAM problem. A lot of the SPAM in the past was just a message with 50 different URLs.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 1:21 pm
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What you could try is a 92k-ohm for somewhere between 20-25mA (I assume that this is what you mean by 21 and 46.5). Depends on how tough your 6V6GTs are. If they are NOS tubes --- no problem. If they are RI Tungsol 6V6GT error on the side of conservative or colder settings --- for tube life.

For modern made 6V6GTs try to keep the tubes below 12 watts per tube (voltage times current, in amps). And you should be fine. For good NOS 6V6GTA or 6V6GTY you can push 14 watts. Be sure to check for red plating or overt overheating. And too early distortion.

Good luck! Keep us informed. :D


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 1:40 pm
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thanks..how do I determine the watts? would 12 watts be around 20ma?

that's right where it is now with the 86k resistor

Marsh Amplification sent me JJ 6v6 S tubes? are those correct? schematic says 6v6gt
also the rectifier tube they sent me is a GZ34-S the schematic says 5AR4?


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 1:48 pm
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also with the new 20ma setting the vibrato became audible..also the vibrating stopped


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 2:39 pm
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jimitrader wrote:
ok so i tried some things:
changed 100k bias resistor (38) to

120k = 42.5
86k = 21

I like the 86k sounds better but I am wondering if i should go buy a 90 or 95k to try and get to 25? is that necessary so it wont be to cold?? thanks j


That's odd. :?

You ended up lowering the value of the resistor, yet the current went from approx 11 mA for one tube and 12 mA for the other tube, up to 21 mA per tube? Are you sure that your initial readings were correct? Something isn't quite right here.

jimitrader wrote:
thanks..how do I determine the watts?...


Multiply the Plate voltage (VDC on pin 3 of the output tube) times the Plate current (your measured current in mA), this calculation is per tube.

jimitrader wrote:
Marsh Amplification sent me JJ 6v6 S tubes? are those correct? schematic says 6v6gt
also the rectifier tube they sent me is a GZ34-S the schematic says 5AR4?


The JJ 6V6 will work just fine, albeit closer to a 6L6 sound than a classic 6V6 sound, kind of in-between. 5AR4 and GZ34 are the same/interchangeable.

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Last edited by shimmilou on Thu Oct 10, 2013 2:59 pm, edited 2 times in total.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 2:46 pm
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shimmilou wrote:
Something isn't quite right here.


+1

I feel a great disturbance in "the force"......

:shock:

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 2:57 pm
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Yes.

Maybe the new readings are with the new tubes? According to the second readings, after changing the resistor value (and the tubes?), the 100K might be just about right and give somewhere close to 25 mA per tube. :idea:

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 3:45 pm
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25mA is way hotter than I'd dare run a pair of 6V6's.

Arjay

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 5:22 pm
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Same here, I should have done the math. If 440 Plate volts, maybe 21 is just fine (about 9 watts, plenty hot), leave well enough alone. :idea:

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 8:03 pm
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Sorry the first readings where wrong...my meter was messin up anyway the inital readings where:

with the 100k 37.9 and 36.4 ma
someone said to try a 120
that came in at: 120k = 42.5 ma
then i put in a 86k = 21 ma
then a 96k = 27 ma

should i go back to the 86k = 21ma ? its at 96 =27ma? some replies said that's maybe to hot?

I appreciate all the advice..zeroing in on the tone!!!


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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 8:47 pm
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At 27mA I'm surprised that the output bottles aren't red-plating.

:shock:

21mA is well within the "comfort zone" for a pair of 6V6's in class AB operation.

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Post subject: Re: Fender Princeton reverb biasing question
Posted: Thu Oct 10, 2013 9:35 pm
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thanks ..i changed it to 21 ma ... sounds real good too..i will post a couple pix of the tubes in the dark glowing so you can see..do you know a good article on adding a bias pot to a Princeton Rev?


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