I was looking over a few things on my 6G2 build and have a question .

[catnine]

I see that the 6G2 fender princeton uses a 5Y3 and a 1k for the first dropper and a 1k for the second dropper. When I started thsi build I set it up like a 5E3 as far as the dropping resister values are concerned , this was the beginning of this build so I have a 5 K 1st dropper and a 22K second . I used an aleen amps PT a TP25 which still has the 325-0-325 VDC secondary yet instead of the mA rating on 70mA of a stock 6G2 PT 125P1B at least that what I think the rating is the later SF princeton uses a 125P1B . Mine has the same vdc as the 125P1B also used in SF champs rated at 70mA mine is rated at 150mA .

I was trying to calculate the mA draw of my build using one triode of a 12ay7 , both triodes of a 12ax7 second gain and PI and 2 6V6GTA JJ tubes. I did replace the 22K second dropper with a 33K to lower the rail to the preamp and second gain and PI from 315 VDC to 290 VDC more what the fender schem calls for and it does sound better. How do I calculate the voltage drop if I were to use the stock 1K first dropper in place of the 5K . right now the B+ can be as high as 415 VDC the plates are 413 VDC and the screen after the 5K is 389 so my screens are 24VDC lower than the plates on the fender schem the screens seem to be 2 vdc lower than the plates. I don't think the screens should be 24 VDC lower than the plates . I could parallel some resisters just to see what 1K will offer and I know it will also raise the preamp rail VDC if I go down to 1K . So since the 6G2 seems to use a lower power PT and mine is more in line with the later SF princetons like a AA964 using a GZ34 rect tube. I could add the 1K then like the AA794 go with the 2 18K droppers adding one more filter cap as per the AA794 . I just wanted to know if this is worth the trouble and how I can calculate what dropper values to use after the 1K to get it close to the 290 VDC it now is. I can run a GZ34 if need be I tried it and it only raises the B+ about 7 VDC above the sovtek 5Y3 I now use in this build .

[Enzo]

I am looking at 6G2, and I see a 1k as the frist drop and a 10k as the second, not another 1k.

Your plates are a hundred volts higher than the 6G2 drawing and you are concerned over a 24v difference on the screens? Why do you think the screens should not be 24v lower than the plates? I;'d be happy as a clam to find that on my amp.

As to calculating draw, your 6V6s are not drawing any current through the dropping resistors.

Look at the 6G2 schematic, mine is a little fuzzy, so correct my numbers if need be. My rule of thumb is roughly 1ma per triode in the small tubes. I see from the screen node of 312v we drop to 280v at the node for the triodes. That is across 10k. Ohm's Law tells us then that 32v dropped across 10k ohms must have 3.2ma flowing through it. For the three triodes, that is pretty darn close to my 1ma each. You can play with the B+ voltage, but overall it is not going to change the current through those preamp tubes a lot. The plate node seems to drop from 315 to 312 across that 1k. 3v across 1k sounds like 3ma dead on to me. Of course that current includes the original 3.2ma PLUS the screen currents. So the schematic itself is suspect as to voltages.

So you want to know currents? Use whateve you have in there now, and use Ohm's Law. Measure the drop across your second dropper, and calculate from that the sum current of the three triodes. Now go meausre the drop across the first dropper to calculate the current through it. SUbtract from that the triodes' current, and the remainder will be the screens current. If you want different voltages, use those currents and ohms law to change the resistances.

[catnine]

Enzo your right I said 1k when it is 10K . I was just trying to make use of the higher plate and B+ I have. See my Plate and B+ are close to the AA964 the only reason I have a lower screen is because of the 5K first dropper. I just thought there was a reason fender used the 1K 1st dropper on the AA964 princeton and used an extra filter cap and two 18K droppers to get the rail down to 290VDC for the preamps. I don't have the trem so I don't have the other the extra triode current draw.

I guess my question is does it matter how you get the screen and preamp voltages where they should be ? ie right now with the 5K and 33K droppers and three filter caps I am real close to the AA964 other than my screen is lower so sound wise what I have now compared to the AA964 princeton using one extra filter cap and 2 18K droppers and a 1K screen is there really any difference in the two sound wise not counting the difference in the tone stacks AA964 being vol. treb and bass and mine vol and tone.

I just thought it mattered how you got the voltages . I have no idea why fender used a 5K dropper in their 5E3 unless they wanted to drop the rail down and screen to make it breakup sooner. I don't know what the 5E3 had as far as plate and screen and preamp voltages . When I got these tranny's my goal was more headroom far more than a 5E3 offers and I got there but do feel I can get a bit more but am not sure how to do it. I was after a higher powered princeton with more headroom . Mine as it is now starts to break up at 5 when I have the NFB switched out and the 25 uf second gain bypass cap in if I flip the switch I added and take out the bypass cap and add in the 56K NFB like the 6G2 has then it does not break up until 7 and it's tighter sounding but in reality all it seems to do is cause me to raise the vol up from 5 to 7 to have the same volume level all I gain is a tighter sound at the same volume level, ie with no NFB the amp at 10 is louder than with the NFB .

My OT is much larger than the AA964 too it is a beefed up one used for a BF deluxe reverb made by Allen Amps like the PT is.

[Enzo]

When Fender designed the 5E3, they were not looking for breakup. That is a latter day thing.


The resistors are there to serve the circuit, not the other way around. You start with the power supply you want for your plates, then you decide what sort of voltage you want at the nose, then you note the current each node will use, and Ohm's Law tells you what resistor to put there. It isn't like Fender or anyone says to themselves, I think we ought to make a 5k amp, they think I want the preamp to run on 300 volts - or 280 or 220 or whatever - and knowing the triodes will draw 3ma or whatever, figure out the resistor needed to go from where you started to where you want to be.

[catnine]

When Fender designed the 5E3, they were not looking for breakup. That is a latter day thing.


The resistors are there to serve the circuit, not the other way around. You start with the power supply you want for your plates, then you decide what sort of voltage you want at the nose, then you note the current each node will use, and Ohm's Law tells you what resistor to put there. It isn't like Fender or anyone says to themselves, I think we ought to make a 5k amp, they think I want the preamp to run on 300 volts - or 280 or 220 or whatever - and knowing the triodes will draw 3ma or whatever, figure out the resistor needed to go from where you started to where you want to be.

I know that . I was only wondering why fender went with the extra filter cap on the AA964 and why they settled on a GZ34 instread of a 5Y3 . I can see why they added the two 18K droppers simply because they were needed to bring the rail voltage down for the preamp section. I would imagine if I did the same with my build it would end up very close voltage wise. I know fender was after a clean tone and may have upped the B+ to get there. What I don't know is if I made these changes if my build would sound any different than it does now. Voltage is voltage and there are many ways to get there. Seems as time went on fender added more filtering in the PS . wondered why this was, is all , quiter amps , stiffer PS = tighter sound and more headroom , higher plate = more headroom based on plate VDC against 6V6 mA draw. this sort of thing. I know the fender schems are not right on and end all . I may find that if I change to the same setup in the PS as the AA964 that the screens may end up higher than the plates .

[catnine]

Ok I have a build that uses 2 JJ6V6's . my plate is @ 407 VDC since I have a 5K dropper for the screens the screens are 385 VDC so there is a 22 VDC lower screen than plate. I read this at Aiken amps "The screen has much more control over the plate current in a pentode than does the plate voltage. However, it does not have as much control as the grid voltage does. When biasing, what counts is *plate* dissipation, which is equal to plate current multiplied by plate voltage. However, you must take into account the duty cycle factor of the class of operation. For most guitar amplifiers, the screen voltage is the same as the plate voltage, and the amplifier is biased in a moderately hot class AB. As a side note, the screen grid has dissipation limits as well, and you must insure they are not exceeded. If you see the screens glowing brightly when you apply a signal, you are likely exceeding their dissipation ratings. A measurement of the voltage drop across the screen grid resistor under signal conditions will allow you to calculate the screen dissipation.
In some amplifiers, the plate voltage is made very high, higher than the maximum allowable screen voltage, so the screen must be run at at lower voltage in order to avoid over-dissipation of the screen grid element, or internal arcing between elements.

In this case, the amplifier is usually biased in class B, or a very cold AB, in order to take advantage of the greater power output capability provided by the higher plate voltage.

At first glance, you might assume that as long as you don't exceed the plate dissipation at idle, you can bias the tube at a current equal to the plate dissipation divided by the plate voltage. However, this will only work in a true class A amplifier, because in a class AB or class B amplifier, the average plate current *increases* above the idle value when amplifying a signal, so the plate dissipation is higher during some point in the output range than it is at idle. The maximum dissipation point does not necessarily occur at full power, typically it is closer to midscale. For further details, see this article - Idle Current Biasing - Why 70 percent?

This is where the "70%" factor comes in when bias calculations are given. In a typical guitar amplifier running at moderate class AB duty cycles, the difference between idle and max power dissipation amounts to about a 30% increase, so as long as you don't set the idle current to a level that results in more than 70% of the maximum plate dissipation, you're okay.

As you can see, the screen voltage can affect the maximum allowable bias current, but it is dependent on the plate voltage and the class of operation. If you take a "standard" guitar amp and lower the screen voltage, you can raise the grid voltage (less negative) and bring the bias current right back up to where it was at the original screen voltage without overdissipating the plates at idle or under signal. In this case, since it takes a smaller negative grid voltage to attain the same plate current (because of the lower screen voltage), you will lose headroom. If you don't raise the grid voltage, you will simply have the amp biased more towards class B, with the resultant increase in crossover distortion." Is he saying I need to raise the raise the grid voltage since my screens are lower than the plates?

I use JJ 6V6s tubes. Limiting values 500 VDC plates , 450VDC screen Ug2. My screens are 385 VDC and the plates are near 410 VDC. It varies if the plates are 415 the screens are 392. It depends based on the line VAC which never exceeds 120VAC thus the 415 plate. Since my screens are at most 25 VDC less than the plates. I am sort of lost what might be the best bias I like to stay at 60% . Most fender amps run the plates on some at 410 to 420 VDC and the screens are 5 to 10 Volts lower than the plates.

Based on a RCA 6V6GTA the max plate is 350 plate , 315 grid 2. This is a P-P adj/fixed bias build. I know fender pushed things since BFDR runs 415 on the plates and close to that on the screens and these amps lived a long time.

I am not certain since my screens are 25VDC less than the plates do I need to set the bias hotter according to aiken or does it matter that much?