Power Transformers and Voltage

[cminor9]

So, I recently built a 6G3. I bought a hammond 290bx for the task, 330v and 120ma on the secondaries. It puts out a bit more than 330v on the secondaries, more like 345v. The schematic calls for 333v on the secondaries, and 365 on the plates of the 6V6s.

But after the 5AR4 I was getting voltages of 435 on the plates. Why?

I zenered the voltages down, and they are spot on perfect. I know the schematic has a tolerance of 20% and technically the higher voltages are within spec...but it's a totally different amp with those voltages.

So why would a transformer that puts out slightly higher voltages (+15v per side) on the secondaries produce an amp that runs +60 on the plates? That seems a little disproportionate. Does this relate to the current on the secondaries?

[Enzo]

The transformer has no idea what it is connected to, all it does it put out the secondary voltage and waits for a load to draw current.

The output of a transformer is rated at full current output. Your transformer would put out the 330v only when you are also drawing 120ma from it. When you draw very small or no current, then the voltage rises.

Even at 330v, that rectifies to 466VDC, less the rectifier tube losses. believe me they were not counting on losing 100v in the rectifier. Your 30v drop sounds reasonable to me.

Schematics are not gospel. The 6G3 says 333VAC on the rectifier plates, and 375VDC after the rectifier. But if you look ahead to the AB763 version Deluxe, you will find the transformer specified at 325VAC, with the B+ 420VDC after the rectifier. That corresponds pretty well with what you have.

[txstrat]

Schematics are not gospel. The 6G3 says 333VAC on the rectifier plates, and 375VDC after the rectifier. But if you look ahead to the AB763 version Deluxe, you will find the transformer specified at 325VAC, with the B+ 420VDC after the rectifier. That corresponds pretty well with what you have.

I found the same in my 6G3 clone.
Enzo, are you saying they might simply put the wrong voltages in the 6G3s schematic?

[Enzo]

Certainly possible, the ones on there don;t work, in my mind. Using the same rectifier tube, the two versions have substantially different B+ levels with not much variation in PT AC.

[txstrat]

Certainly possible, the ones on there don;t work, in my mind.

Yeah.
The bias voltage on there seems to be wrong too (am I allowed to say that?) cause when I first used the bias voltage off the schematic the 6V6s ran at 43ma each (1 ohm cathode resistor measured).
Even when I subtract the screen current, the remaining plate current was way to high for the given voltages.
I biased them to 32ma. Still high but the amp runs (and sounds) fine.

[cminor9]

ok, interesting info here. Thanks guys.

txstrat, did you try zenering that voltage down? I did and liked it better that way. BTW and offtopic, what did you think of your 6G3? I like it, but am a bit surprised at how loud and clean it is even with the "correct" voltages.

Enzo, you said something I didn't know about transformers: if you don't use up all the current and it will put out more voltage. I never knew! So, when planning a project how should one go about getting the right transformer? I look at a tube datasheet and see that 6V6s running in PP AB1 have Ia of 70 and Ig2 of 4, with no signal applied (TDSL Tube data [6V6-GT]). I interpret this as the tubes wanting to see at most 74mA each from the PT (not counting the heaters.) I figured that a transformer that put out 120mA would suffice. Obviously, it's a bit too hot for the voltages I want (cause I am dropping roughly 60V with zeners.) So how do I figure out the correct PT for a given set of tubes using the datasheet?

[txstrat]

ok, interesting info here. Thanks guys.
txstrat, did you try zenering that voltage down? I did and liked it better that way. BTW and offtopic, what did you think of your 6G3? I like it, but am a bit surprised at how loud and clean it is even with the "correct" voltages.

I left the voltages as they were. You did hear the soundclips of my 6G3.
I'd say it's perfect for my taste, but I agree, quite loud before breaking up.

I believe what Enzo considered. The voltages in the AB763 seem more likely the "correct voltages" for the 6G3 as well. When you follow the calculation for a rectifier = voltage times 1.41 (roughly) your 345vac end up at 485vdc under no load. Subtract the load and the drop of the GZ34 and you get your 435vdc. That's not far from what my amp is running at. (423vdc).

365vdc after the rectifier (with a secondary voltage of 330v) really seem quite unlikely. The same applies to a 6G11 where the vdc is also stated as 365v. Who knows what the guys in the early 60s measured the voltages at - maybe the amp was full up and being played. I don't know if the voltage can drop that far down in this case.

[octal]

TXstrat-

It seems like you think that that Fender's stated AC voltages on the 6g3 are correct and the DC voltages are wrong (i.e. too low.)

I'm wondering if it might possibly be the other way around- the AC voltages are wrong, but the DC voltages shown are right. There's only one unit of measurement of DC voltage, but AC voltage can be peak, RMS, etc. I think that if there was an error in the schematic voltages it's more likely to be on the AC end.

I also wonder if the voltages were arrived at theoretically (i.e. "well, it's a 660VCT transformer, so write 330 V on each plate of the rectifier on the schematic") or actually measured from a working prototype, and if they were actually measured- was the meter an accurately calibrated true RMS one?

It's an interesting question, and I'm not claiming to know the answer- it's just a theory. It'd be great if someone with a real brown deluxe with an original power tranny could settle this.

Nathan

[txstrat]

Your theory is quite interesting. Have a look at it from the other side.
For instance: all Hammond replacement power transformers (I claim they somehow know what to do) run at 325 -330 volts secondary, depending on the model. From Champ over Deluxe to Vibrolux, Concert, Bassman, Twin etc. (Hammond Mfg. - REPLACEMENT - Tube Guitar Amplifier Transformers & Chokes)
I think it would make less sense if there was one or two models which didn't run at these voltages. See the 6G3 or even better the 6G11. The schematic claims 365vdc. That would mean (if you subtract the load of the tubes) roughly 280vac.
I believe if there was a transformer with more than 40 volts less than the others, there would be a replacement.
But anyway, we would know for sure, if there was someone with a genuine 6G3 or 6G11 who could clarify this.

[loudthud]

Nobody has mentioned that Hammond transformers are spec'd with 115VAC or 117VAC primarys. It should be noted that rectifiers have lower drop and power tubes bias hotter when the heater voltage is on the high side.

[octal]

Your theory is quite interesting. Have a look at it from the other side.
For instance: all Hammond replacement power transformers (I claim they somehow know what to do) run at 325 -330 volts secondary, depending on the model. From Champ over Deluxe to Vibrolux, Concert, Bassman, Twin etc. (Hammond Mfg. - REPLACEMENT - Tube Guitar Amplifier Transformers & Chokes)
I think it would make less sense if there was one or two models which didn't run at these voltages. See the 6G3 or even better the 6G11. The schematic claims 365vdc. That would mean (if you subtract the load of the tubes) roughly 280vac.
I believe if there was a transformer with more than 40 volts less than the others, there would be a replacement.
But anyway, we would know for sure, if there was someone with a genuine 6G3 or 6G11 who could clarify this.

Just one more thought here; the original PT in the 6g3 was p/n 125P2A. From what I've found on Fender transformer charts, this was a unique PT to the 6G3 and nothing in the current Hammond line cross references to that part number, so I think you might be comparing apples to oranges by looking at the range of secondary voltages of transformers not designed for this application. (I think what you were getting at here is that "many Fenders use AC secondary voltages in the 325-330V range, so why should the 6g3 be an exception?")

It's speculation on both our parts without having an actual 6g3 to measure or at least the specs for an exact replacement transformer for the 6g3.

Certainly, a 375V B+ for a 6V6 amp is not ridiculously low; if anything, I'd say that the 420 B+ on the BF Deluxes is pushing it moderately hard- so it's at least plausible that the brown deluxe MIGHT have had a lower B+, though I'll admit that it just as easily could have not!

[txstrat]

so it's at least plausible that the brown deluxe MIGHT have had a lower B+, though I'll admit that it just as easily could have not!

I see what you mean. Your thoughts are very plausible especially because there is no direct replacement for the 125P2A.
What puts me on the contrary side is the differences in some other schematics. Compare the schematic of the 6G11 (brown Vibrolux) and the 6G9 (Tremolux). They both have the same PT (125P6A) and in the 6G11 schem the B+ is stated as 365vdc while in the 6G9 schem it's 400vdc. Same rectifier. Only the power tubes are different and I doubt that would make for a voltage drop of 35 volts.
I think they might didn't take it as particular in those days as they should have (no offense) cause it wasn't a matter of necessity.

[cminor9]

I left the voltages as they were. You did hear the soundclips of my 6G3.
I'd say it's perfect for my taste, but I agree, quite loud before breaking up.

Yeah, all preference I guess. My 5E3 is running higher voltages. I like it better that way. I prefer my 6G3's response with the lower voltages. But yours sounded really nice in those clips you posted, so much so that it got me really excited to finish my build.

Man, you must play with a loud band! The 5E3 is too much for my group; when I crank it people are ready to throw stuff at me! I am afraid the 6G3 will be way too much. I have been thinking about a 5F4 down the road, but I am ever more afraid I'll never touch the nicest parts on the volume knob!

Then again, I play mostly Jazz, Blues, R&B, and some classic rock with older guys. Just like I would strangle the 21 yr old version of myself, when I ran a Hot Rod Deville dimed, they would go nuts if I ran a 5F4 hard. Of course, I'll still build one

[cminor9]

well, the schematic does have a tolerance of +/- 20%. So you both could be right. 20% of 375 is 75, so that would allow for a HUGE range of voltages. There are no components (at least that I used) that couldn't safely take 450v. When I was having some problems with the built at first, I had 475v on the plates. I actually arced the indestructible pair of JJ 6V6 tubes I have (and yes, they still work) when I hit the strings hard and the amp was cranked (should not have done that, something told me.) But nothing burned up and the thing works now with all the same components.

I think Hammond touts the 290bx as the replacement simply because a 300-0-300 transformer might not get the voltages high enough (zenered down, my PT puts out 318 on the secondaries), whereas a 330-0-330 will go high but not dangerously high. This allows them to produce a transformer for a Blackface deluxe, and then re-use that tranny for a brown Deluxe (it has a bias tap that I just ignored for my purposes.) Browns aren't as popular as the tweeds or blackfaces. I doubt it'd be cost effective to manufacture trannys specifically for browns, esp when you can just as easily substitute in something very popular (parts for a BF) and thus cheaper to manufacture.

All of the voltages on the schematic are in line with the lower voltages. I zenered the transformer down, so that I am getting 375 on the first filter cap. All other voltages on the schematic just fell right in line with the schem within a couple of volts.

Primary voltage would certainly factor in. I measured mine at 117-122 depending on the time of the day. I know that accounted for a little bit of extra voltage.

Thank goodness for Zeners. We can all tweak to taste. Me, I just don't need the volume that comes with higher voltages.

[txstrat]

I am ever more afraid I'll never touch the nicest parts on the volume knob!

What about a stereo pot master volume (don't hate me for the suggestion ) in between the PI and the power tubes a la Fender Tonemaster.
That way you get all the breakup from the preamp including PI, less some power tube saturation (which you can dial in whenever you want). Might be worth a try.