Has anyone ever seen a AA764 actual fender champs voltages or have them

[catnine]

 Listed using todays line voltages? I was just curious to see what they find in a working amp . I do realize that the two dropping resisters work in a seriers relationship and if the OT is drawing down the plate more than the 1k is drawing down the screen VDC and since this is the case there is more VDC feeding the 12ax7 plates causing them to be 30 volts higher in relation to the plate on the 6V6 tube.

 As I said before I tried the lower HT secondary 300-0-300 taps instead of the 330-0-330 HT taps how ever the end result was the same the voltage dropped about 30 VDC all the way down to the 12ax7 plates yet the 12ax7 plates were still higher in relation to the plate of the 6V6. Not that the 12ax7 has a higher plate voltage than the 6V6 .

 The 30 VDC drop required a lower value cathode bias resister but the end result was just a thinner sounding amp.

 There seems to be one solution and that is to raise the 1k and 10k dropping resisters to a higher value as has been suggested by a few members here. The 1k only drops a bit over 3 VDC and the 10k drops 21.5 VDC. As it is now the 6V6 screen draws 1.9mA which does not seem like much .

 I was looking at schematics for older tweed champs and princetons using a choke in place of the 1st dropping resister and they have the screen on one side of the OT and the plate after the OT winding then a 22K resister feeding the preamp and what seems to be a much lower HT VDC PT.

[John]

This is sort of related to your question, so I'll throw it in here.

I just built the HoSo56 amp, but of course not exactly the way Geezer and Tubenit have done it.  I used a PT that gave me higher B+ than I wanted on the EL84 plates (just under 400) so I swapped the SS rectifier for a 5y3. It lowered my power tube plates to ~300, but the voltages down at the preamp end are still just a bit higher than Geezer's schem shows. 

I *think* part of my problem is that I did not change that 47uf reservoir filter to a 22uf, which I plan on doing tomorrow. My theory is that it's taking everything the 5y3 has to keep the 47u filter cap full, (not to mention it's gonna burn out that rect. tube) and so my initial voltage is lowered much more than I expected. Then the following caps, being smaller, are more easily filled and so my voltages down the line are closer to where they should be. I do know with the SS rectifier, I had way high voltages on all tubes.

So anyway, I would think about what is drawing your initial voltage down, and yet letting your preamp voltages come up. It's what I've been cogitatin' on, anyway.

And of course, it's quite probable that my theory will get one of these  from the more experienced guys here. But I sure am enjoying my education!

[catnine]

 John I had a 47uf for my reservoir cap with a NOS 5Y3 and then remove the 47uf and installed a 30 uf , didn't have any affect on the voltages at all. Then I installed a 22 uf , still no change . All it did was increase the highs so i put the 47uf back in. The only concern is the 5Y3 arking, because of the surge when the cap is discharged and then charged off the 5Y3 . I was told to install a 47 ohm 3 watt resister in series between the pin 8 of the 5Y3 and the first cap . I have not had a problem with the 5Y3 . I had a 73 champ that can stock with a 5Y3 and a 40 uf first cap in the cap can and the others were 20 uf . It still had the same old 5Y3 .

 I still feel what is drawing my voltages down is the OT before the 1k and 10k . It drops 14 VDC before the 1k , the 1k only drops 3 VDC .

[John]

Well then, I don't know what to tell ya. 

[HotBluePlates]

Third post on the same issue with the same amp. I'd argue it is a non-issue, but that's my opinion.

I do realize that the two dropping resisters work in a seriers relationship and if the OT is drawing down the plate more than the 1k is drawing down the screen VDC...

Chip answered once in a way that pointed to a wrong cause for the 6V6 plate voltage being lower than the screen voltage.

When you posted your voltages in another thread, you had 367v B+, 364v at the other side of the 1k (the node feeding the 6V6 screen), and 344v on the other side of the 10k resistor feeding the preamp. That means the preamp draws about 364-344 = 20v/10k = 2mA. This jives with the voltages you posted across the preamp cathodes resistors. Assume each is exactly 1500 ohms. You posted 1.61v and 1.74v, which amounts to 1.61v/1500 = ~1mA and 1.74v/1500 = 1.16mA. Together, that's about 2mA (you'll get less resolution on the higher voltage scale).

Across the 1k resistor, we see 367v-364v = 3v/1k = 3mA. Since the preamp is drawing 2mA, the remaining 1mA is idle screen current. You could verify by measuring the voltage across the screen resistor and dividing by the actual measured screen resistor value. In other threads, you mention doing this and getting ~2mA (and that the drop across the 1k indicated 4mA, which still checks against what I wrote above).

Now what about the 6V6 plate? In other thread, your math pointed to the 6V6 drawing about 35.5mA, and assuming you did that by dividing the voltage across the cathode resistor by its resistance, then 2mA of that is screen current, leaving 33.5mA plate current.

You were concerned about the plate being at ~354v, and the screen being at ~363v or so. First, this is no problem. But forget the "primary impedance" of the OT... that full impedance only exists with respect to an a.c. signal. D.C. sees the OIT primary as a coil of wire, with only the resistance of that wire; a.c. sees that "d.c. resistance" plus the inductive reactance presented by the coil. The total is then the primary impedance.

You can measure the d.c. resistance of your OT primary (with the amp off); we'll calculate and guess it's 367v-354v = 13v/.0335A = 388 ohms. That sounds high to me, but within the realm of possibility. The reason there is more voltage drop from the power supply to the 6V6 plate than there is through the (higher resistance) path to the screen is that there is almost 17 times the current flowing through the path to the plate.

And the tube don't care anyway.

So what about those pesky 12AX7 stages? Well, the 30v difference is only 15% high.

You noted a preamp supply voltage of 364v. There's ~230v on the plate of each preamp stage. If the resistors were exactly 100k, there would be 134v/100k = 1.34mA flowing through each triode. We found numbers lower than that when we assumed 1500 ohms exactly for the cathode resistors. So it doesn't add up.

Newsflash: the numbers don't add up on the Fender schematic either. If they did, then judging by the plate voltage drop for the first triode, the current should be 1.3mA, and that would give 1.95v across a 1500 ohm cathode resistor. The second stage numbers imply the cathode voltage should be ~1.93v. And the currents of the 2 stages add to about 2.6mA, which should give 26v drop across the 10k resistor, but a 20v drop is shown.

So you could have plate load resistors a bit less than 100k. Or cathode resistors a bit above 1500 ohms. Or your 12AX7 could be drawing less current than the imaginary one in the schematic (which can happen if you have a tube with a transconductance different than "spec").

But my final argument is that while you might hear a difference in tone with a 100v plate voltage change on a preamp tube, you most likely cannot hear a difference with a 30v change. If it bothers you, change the 10k resistor. With a 2mA draw, it drops 20v, and you want ~50v drop. 50v/2mA = 25k, but the nearest standard value is 27k. Preamp plate current may drop a hair with the slightly lower supply voltage, which would offset the changed resistor value, so 27k might hit your target better than 25k anyway.

[catnine]

HotBluePlates:

 I agree it IS a NONE issue. As far as all my posts I learned something . I was simply trying to understand and get a grasp on how things worked. I never bothered to measure preamp tube plate voltages on this amp but decided I would just to make sure everything was at least close to what they should be and not causing the highs I experienced. The 100K and 1.5k plate and cathode resisters are both just a bit higher than stated , more like 103K and 1.53K not a big deal. I have tried 5 different 12ax7's all different brands and all read the same.

 Part of the mA draw on the 6V6 on the screen well I tried a new JJ6V6 and then an old RCA 6V6 GTA. I got different readings on the RCA I got a voltage drop across the actual 462 ohm resister of .86 VDC 1.86 mA and the screen voltage was 7VDC higher than the plate . On the JJ there was .51 VDC drop across the 462 ohm screen resister or 1.1 mA and the screen was 10 VDC , this is not to add a question .

 I think mainly I was all over the internet before I came here and was loaded with all sorts of info and or opinions .

 This site seems to sort things through much better .

 This last post was an attempt to see if anyone had actual AA764 voltages just as a comparision .

 I think in the post I said and which one it was I can't recall there are so many , I said I will try another speaker and live with the amp as is.

 I am sorry I have gone too far. At the same time thanks HBP's for bearing with it and taking the time to lay it out so well.

[HotBluePlates]

I am sorry I have gone too far. At the same time thanks HBP's for bearing with it and taking the time to lay it out so well.

Don't think that. If you learned something, then it's worth it. And if you learned something, 12 other lurkers did, too.

Somewhere is storage, in another state, I have a binder of info from amps I repaired. All kinds of actual in-circuit parts values (useful for a 40 year-old amp, when you're figuring out what drifted where, and why it sounds so good) and voltage measurements. Unfortunately, I don't have it here.

[catnine]

I am sorry I have gone too far. At the same time thanks HBP's for bearing with it and taking the time to lay it out so well.

Don't think that. If you learned something, then it's worth it. And if you learned something, 12 other lurkers did, too.

Somewhere is storage, in another state, I have a binder of info from amps I repaired. All kinds of actual in-circuit parts values (useful for a 40 year-old amp, when you're figuring out what drifted where, and why it sounds so good) and voltage measurements. Unfortunately, I don't have it here.

 

 I was looking at the AB764 bronco schem and the voltasges are almost the same as my AA764 build. I notice one other thing. Like my 73 SG champ it has the 40 uf 1st filter cap and the 330pf cap across the 6V6 pin 8 and 5 whcih my 73 SF had and I never removed not that that makes any difference .

[HotBluePlates]

There's a reason for that.

The "Bronco amp" (with reddish letter, instead of the normal silverface turquoise) was a Champ amp that was paired to be sold as a set with the Fender Bronco guitar.

The 40uF first cap stage might be due to using the same cap can as found in the Princeton (but not using a 20uF section). I really don't know. The 330pF cap from the 6V6 grid to the cathode might as well be to ground. It was a silverface tactic to reduce highs and kill any oscillation. Other silverface models had the same cap, but maybe in a different value.

[catnine]

There's a reason for that.

The "Bronco amp" (with reddish letter, instead of the normal silverface turquoise) was a Champ amp that was paired to be sold as a set with the Fender Bronco guitar.

The 40uF first cap stage might be due to using the same cap can as found in the Princeton (but not using a 20uF section). I really don't know. The 330pF cap from the 6V6 grid to the cathode might as well be to ground. It was a silverface tactic to reduce highs and kill any oscillation. Other silverface models had the same cap, but maybe in a different value.

 All I know is the 73 SF Champ I had came stock and it had a three terminal cap can marked with a square and a triangle and a half moon same as the one shown in the bronco schem and I replaced that old cap can with the same values and brand and it also had the same markings on the cap can. , the half moon tern marking was the 40uf . I put it in the same as the original cap can. also on the bronco schem they go as far as to show the cans symbles on the layout  of the can itself as well as on the schematic. CE Manufacturing made the original and still makes the same cap can 40/20/20 uf @ 500 volts , the orig was 475 volt. I have seen the same cap can with 4 terminals on later princetons , Who knows , all I was trying to do is use the values my 73 champ had out from the factory so when  schem was different I chose what fender actually installed. Fender schems do not state the year and probably because of all sorts of changes so they just offer the basic schem .  Different people draw up the basic schem as you know than the people who put the amps together so I would suspect they used what they had to keep producing amps .