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Post subject: Bias on non-adjustable BF and SF Princeton and PR
Posted: Thu Dec 29, 2011 9:29 pm
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If you change out the diode sitting on pin 6 of the GZ34 or 5U4GB rectifier (with a UF4003 or UF4004 diode)--- you need to drop the 100K-ohm (5%) resistance between the rectifier and the diode.

82k-ohms = about 28-32mA (I recommend only NOS 6V6GTY, GTA, or JJ of this setting)

75k-ohms = 22-25mA. It's what a tested a new TAD, NOS RCA 6V6GT, Syvania VT-107B with. Works and sounds fine.

68k-ohms = 15-18mA. Kinda cold sounding.

Of course, how much current your tubes pass depends on their age, make, specific sub-type. Etc. These tests were all done at 415-425 VDC (using a BF Princeton and a SF Princeton reverb). Try to use a 1% resistor, as these make for a rock steady bias voltage.



http://www.webphix.com/schematic%20heav ... aa1164.pdf


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 3:52 am
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Do you mean that you adjust amp bias by changing the bias diode rectifier ? Or you said in case we must replace this diode by a new model we should check the 100 k resistor ?

In bias adjustment, the most used method is to replace the 27 k resistor.


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 8:40 am
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I don't understand what affect that changing the diode will have. :? The voltage would be nearly identical for just about any diode that you put there, it can't be much different from one diode to the next. Using an UF diode isn't going to change the voltage more than maybe a few tenths of a volt, if that.

Of course, you should disable the "vibrato" during biasing or for diode comparisons, otherwise the "vibrato" could be affecting your reading for the bias current, being slightly different when powering off then back on later. Even if the pots are all the way down, the bias can be a little different each time depending on how much resistance is still in the circuit from the pot. This might account for any difference that you noted.

I agree, change the 27K for bias. The 100K is needed for current limiting.

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 10:28 am
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Diode types have a big effect on voltage. That's why you don't use UF4007. As its PIV dictates the voltage. And 5AR4 or 5U4GB can supply quite a bit more voltage if demanded.

The 100k-ohm resistor is a voltage dropping resistor and not a current limiter, as far as I know. It's like the voltage dropping resistors in a PS supply for gain and PI stage tubes.

I've tried altering the 27k-ohm resistor to ground. It'll work, but you have less range of resistance to work with. And it seems to have more of an effect on the vibrato/tremolo section, than the 100-k ohm prior to the diode.

Try it out... and see if you get similar results.

Thanks!


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 12:04 pm
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I don't think so. The PIV (peak inverse voltage) has nothing to do with how much voltage a diode "puts out", it is the rated amount of reverse voltage that it can handle, the higher the number the more stout the diode. A diode is a rectifier, not a regulator. Any similar diode in that circuit will drop 7/10 of a volt and the amount of DC delivered is determined by the amount of AC applied to it, regardless of PIV rating. Any of the IN400x, or UF400x diodes will give the same voltage as each other, the higher PIV ratings are for circuits that have a higher collapsing voltages when switched off. Using a higher rated PIV diode is OK for a replacement of any lower PIV rated diode, and will have no affect on voltage out.

Tapping off of the AC supply to the rectifier tube to feed the bias diode, not from the rectifier tube. The rectifier tube doesn't supply the voltage to the bias circuit. The bias supply is the same AC supply to the rectifier tube. That diode is the rectifier for the bias supply, the bias supply is not from the tube. A different rectifier tube might have the affect of lowering/raising the AC voltage supply to the bias circuit (different current draw for tubes), but a different diode will not have an affect on the voltage.

You are correct, the 100K resistor drops the voltage to the desired level, but it also limits the current in the circuit. If you lower the 100K, you also increase the amount of current allowed to flow through the circuit. Changing the 27K only drops the voltage, changing the amount of current through the tubes, and doesn't allow more available current to flow through the circuit than was available to begin with.

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Last edited by shimmilou on Fri Dec 30, 2011 12:24 pm, edited 1 time in total.

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 12:24 pm
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+1000 shimmilou


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Fri Dec 30, 2011 6:56 pm
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Shimmy and Strat, i have to disagree. A 4003 will put out much less voltage than a 4007. It's why ppl sometimes get into trouble using a 4007 to replace this diode. The Fender tranny will easily supply the voltage, as there is very little current demand in a Bias Power supply.

http://pdf1.alldatasheet.com/datasheet- ... F4004.html

Click onto: "If you want to see date sheet." And then, "All Datasheet" then, "If you Can't See Datasheet." Whew! :mrgreen:


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sat Dec 31, 2011 1:32 am
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You must be thinking of a "Zener" diode, which is used as a regulator. The Zener acts like a normal diode when forward biased, but when reverse biased it will conduct at it's rated voltage, so it can actually regulate the voltage to not more than it's rating. The Zener voltage rating is completely independent of PIV, as PIV is not the voltage output rating of the diode, it is the amount of reverse voltage that the diode can handle, such as when the AC supply to the diode is switched off, the voltage collapses and can reach certain peaks briefly, but high enough to ruin a diode with a low PIV rating. A diode with a high enough PIV rating is needed to handle the collapsing voltage peaks.

The diode in the bias circuit in a tube amp is not a Zener, it is just a regular silicon diode (rectifier). All of the 1N400x and UF400x (regular diodes) are the same as far as output voltage, they can not regulate voltage and their output is solely determined by the amount of AC applied. The PIV rating is the amount of reverse voltage that the regular diode can handle, it is not a rating of output. The numbers on the regular diodes are current and PIV ratings, higher numbers are higher ratings, there is no such thing as voltage output ratings on regular diodes.

In a half wave rectifier like the bias circuit with one diode, the DC output is calculated: (1.41 x VAC - 0.7) So, 1.41 x 30VAC - .7 = 41.6 VDC, meaning that in order to get a 41.6 volts from the bias circuit, you must apply 30VAC to the diode. The 30 VAC would be what you read with a meter, or the "RMS" voltage.
The formula is the same regardless of a 1N4002, 1N4004, UF4003, UF4006, it does not matter which diode used, the results are the same.

The 0.7 is the most voltage that any regular diode will drop, so it's output is completely determined by the amount of AC applied. All of the 1N400x and UF400x will drop 0.7 volts, and output voltage is according to what's applied. Some diodes will only drop 0.3, but the ones used in guitar amps drop 0.7 volts. Changing the diode in the bias circuit can not, and does not change the bias voltage. There is no difference in output voltage between the diodes listed, ie 1N and UF and all other similar regular diodes, they all follow the same laws of electronics.

Note that the datasheet that you linked to shows PIV (reverse voltage) ratings, and the difference in the ratings for each ending number on the diodes, but there is no rating for "output" voltage, as there is no such thing for these type diodes (regular).

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sat Dec 31, 2011 3:58 am
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BMW 2002Ti, I do not doubt you've seen a difference in voltage bias with the use of two types of diodes adjustments. But I doubt whether the diode which is the cause. I looked at the specification sheet and I see nothing. I am not a specialist in diodes.

All I can assume is that the diode would have a different internal resistance of the diode of the original amp and the resistance of 100 K in the amp, would be valued too high and or 27 K is too low. The bias of the circuit, amplifier would "miscalculated" This combination of diode / resistor confine your current (voltage) of bias.

Note ; Translate with the help of google


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 2:56 am
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You guys are absolutely correct about the bias voltage--- whether one uses a UF4003, 4004, or 4007. And the vibrato section of the Princeton Reverb, which runs off the Bias Power Supply "appears" to be a tad bit stronger, with the UF4007. Could be a slight increase in current at same negative voltage. Or my imagination.

I tried a UF4007 and got simialr results with the resistor values before the diode (resistor between pin #6/ tranny connection and the diode), as I did with a UF4004. I have not played with the 22 or 27K-ohm resistor tied to ground parallel to the filter cap.

Anyone have the correct values of both resistors and UF4007 replacement diode? And getting about 22-26mA current per tube at idle with a AA1164 PR?

BTW... I use a 25mfd/50VDC for the filter cap in this amp's bias supply.

Much thanks!

http://www.webphix.com/schematic%20heav ... aa1164.pdf


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 3:36 am
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Any standard diode rectifier like 1N4007 or UF4007 or other will do the job for bias . This is not a problem there. I' ll use always 1N4007.

For proper value of the 100K an/ or the 27K, you have to try some resistors to find the bias you want with you own power tubes.
There are not really a standard value if you want a very specific bias. 100 k and 27 k are "standard" value , but does'nt work for you.

About your 50 volt bias capacitor, is it enough 50 volts ? How much voltage you read ? 75 to 100 volts should be better to be sure you have a safety range.

I wish this answer to your questions.

Happy new year BMW2002Ti


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 9:07 am
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Wow, it's been awhile. Since, what, last year. :lol:

That's a tough one BMW2002Ti.

It seems that the bias modulated tremolo would limit your options somewhat. As you pointed out, changing the 27K will also affect the tremolo action. Maybe you're just limited to that small range of adjustment with this particular amp, while still having the tremolo. Maybe increase the value of the 100K a little, and then change the 27K if needed from there. Just a suggestion that might keep the current in check and still give some range. Sounds like a good idea anyway.

Yes, happy new year everyone. (well, except for all of us miserable BJr owners TIC) :lol:

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 9:30 am
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Shimmy, yeah. And the odd thing is that amp will sound good at nearly any bias point. Whether NOS tubes or new TAD tubes. It only likes to fry RI TungSol 6V6GT's. All the Princeton and PR amps now run around 22-26mA at 415 to 425 VDC on the plates. Which is fine. 18-22 watts output.

You know, those TAD will still give off a blue light show every now and then. Quite a bit of contamination still being burnt off. But, they sound good. And keep a steady bias. AND don't go red plate.

I haven't tried the TAD in any of my Champs. They idle around 30mA at 305 VDC on the plates. With the cathode bias prolly only put out around 6-8 watts. But, not sure about TAD handling that current flow at idle (which is around 85% max dissipation-- the way I figure it).


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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 10:18 am
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I wonder what affect a lower gain tube for the tremolo would have? Of course only half is used for the tremolo, and half is used for the reverb recovery, so the reverb would be affected also. I have never tried a lower gain tube for tremolo. :idea:

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Post subject: Re: Bias on non-adjustable BF and SF Princeton and PR
Posted: Sun Jan 01, 2012 3:12 pm
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Shimmy, my amp is even weirder. One-half of a 12AX7 operates the tremolo/vibrato and the other half is the phase inverter. The reverb seems to be recovered by one-half of the 12AX7. Whose other half is a part of the cathodyne inverter stage(?)




http://www.webphix.com/schematic%20heav ... aa1164.pdf


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