Voltages - other than expected

[tedweber]

Hi TW

I Also I'm quite unsure about the ma bias setting for the tubes and if the weber calculator states the right setting for the specified voltage.

Yep, it's correct. If you look at the source and pour through the java script, you can see that I just used the published data for the tubes and worked backwards with it to arrive at the currents, etc. It's pretty simple math, really. It's also straight forward and linear.. no kinks in it.


Ted

[txstrat]

Thanks Ted,

that would mean I was right with my settings at around 52ma with 397 volts at the plates (presuming I go for 70%).
I always used your calculator for my builds and everything was fine as yet.
This would leave me with only one problem: the grids are running at higher voltages than the plates for whichever reason.
Could that be the reason for the harsh sound? If so, is a bigger value grid resistor the only way to get the grids down?
Since I copied the channel of a Vibrolux schematic I assumed everything would turn out fine while using these values.

Bob
Now we've come to an almost full circle with my troubleshooting.
I was also thinking about tube recto or emulation with diodes (remember the thread?)

Matt

[Bruce / Mission Amps]

Matt, did you not read or understand what Enzo is saying?
The actual DC resistance of your 90ma choke is so low that the smaller voltage drop across it (due to the low, combined currents of the power tube screens and the following number of preamp tube's plate currents) .... are
not dropping the B+ vdc at the screens as much as the effect of the larger amount of the power tube's higher idle current, through the DC resistance of the OT windings, ... which remember, is only feeding the power tube's plates.

As you increase the idle current through the power tubes you are dropping more voltage through the OT from the center tap to the outside legs.
If you want a better plate to screen balance you merely add a 5-10 watt 500-1000 ohm resistor in series with the wire from the end of the choke to the screen node... you don't change the 470 to 1000 ohm resistors on the power tube sockets.

This is very simple Ohm's Law where a voltage (B+) is dropped across a resistance (the DC resistance of the OT compared to the choke) and the vdc drop is more with more current (huge difference of power tube idle current vs very low current of the screens and the preamp tubes) through those resistances and of course, there will be less drop with less current (change the power tube idle currents).

Sorry for the redundancy Enzo.

[txstrat]

Bruce / Enzo

Thank you. At last the penny dropped.
I know how to use Ohms law but didn't bare the different currents in mind (although there's a huge difference).
Gonna change the idle currents of the power tubes.

Thanks again

Matt

[Enzo]

REdund all you want Bruce. The more ways we say something, the more likely it is to communicate. I often restate what others have said, just to get it in different words.

[Bruce / Mission Amps]

Pitiful boring blather again....

Bruce / Enzo

Thank you. At last the penny dropped.
I know how to use Ohms law but didn't bare the different currents in mind (although there's a huge difference).
Gonna change the idle currents of the power tubes.

Thanks again

Matt

Yes that's it.
The 5-10 watt power resistor I mentioned would go at the end of the choke (screen and preamp tubes side), from the junction of the first dropping resistor .... straight towards the power tube's sockets, ....steering current to each socket's, mounted screen resistors.
That new resistor would carry only screen current to the tube sockets and you leave the standard 470 ohm to 1000 ohm screen resistors on the sockets as they are.
As an example... EL34s draw way more screen current then 6L6s.
So, if you had some well used and soft EL34s in there, where a nasty rockin' dog, screamin' monkey amp ....would allow the screens carry an absurd amount of current... say 50ma each, then the total current would be 100ma through the new B+ lowering resistor.

*** By the way, each 470 ohm or 1000 ohm screen resistor, soldered right on the sockets, have to carry the 50ma of screen current and that is why EL34 amps typically have 5 watt screen resistors where 6L6s amps can get smaller 2 watters.***

Since you have an understanding of Ohm's Law.... using the formula for POWER, power = current squared, times resistance.... or, P=I^2*R
Try 1000 ohm steering resistor and that absurdly high 100ma.

I can tell you 100ma through that resistor would heat it up pretty good:

.1a^2*1000 ohms = 10 watts.
Now try Ohm's Law to solve for voltage (pressure) needed to push 100ma (energy) through a 1000 ohm resistor and you'll see that the voltage drop across that resistor would be:
I*R=E
.1a*1000 ohms = 100v

That's a huge voltage drop across that new resistor and except for some EL34 amps (which are notorious for blowing screen resistors) I doubt you'd ever see that.

Now do the same thing for your choke... but use it's actual, measured DC resistance.
Remember what others have said previously... the choke supplies all the current to every preamp tube and the power tube's screens.

This time use a more realistic screen current though (just a few ma of current for each 6L6GC screen) and about 2ma to 3ma of current for each preamp tube... (a 12AT7 used as reverb driver is higher).
Maybe the total current for a non reverb amp would be 10ma, 12ma or 14ma (at idle and no signal) for all of it including the screen nodes.

Do the same for the OT's measured DC resistance (from the center tap to one outside leg) and each power tube's idle current you have measured and things will pop right into view as to what is happening.

And for those that already know all this... sorry, but keep in mind, this is an open forum where there are lots and lots of younger, new, upcoming builders who are just learning all this "practical knowledge"... ha ha...

To experiment with values go to this link:
http://www.the12volt.com/ohm/page2.asp#13

[txstrat]

Just an update.
Took the amp home yesterday and biased it with different current values.
Even with the bias at 32ma the grids were higher than the plates (408V / 407V). The overall sound of the amp didn't change as much as I expected (still kinda harsh). Think I'm gonna get through it thoroughly to see if I can find something wrong.

Bruce
I keep your proposal of the power resistor in mind. Thanks for the lesson.
Somehow I think it should also work without that resistor, shouldn't it? I mean in the original schematic there is none either.

[Bruce / Mission Amps]

The expected differences of screen to plate voltage... i.e., "screen voltages must be lower the the plate voltages"... is GROSSLY over stated all over the Net.
Yours is so close I wouldn't even think anymore about it and instead concentrate on the tuning-tweaking aspect to get a more suitable tone you desire.
Without me going backwards and reading the whole thread again.. is this a homebrew 6G11... using what type of parts?

[txstrat]

OK understand, voltage differences not bad.
I'm using a Hammond 35w OT carbon film resistors and German WIMA caps (they sound great in some other amps I own). Used orange drops before but the sound with them was even more harsh and brittle.
It's actually a twin channel amp sort of the Deluxe Reverb II with the clean channel being a copy of the 6G11 and the drive channel of the DR II.
The power amp is sort of a mixture and I think that's where the failure might be.
I looked over the schematics again and found that the voltages in the 6G11 are not far from what I have. NFB is too small (too high a resistance) right now and I' gonna change the whole power amp tp 6G11 specs to see what happens.
That would be only a couple resistors and shouldn't be much of a burden.

[txstrat]

Changed the long tail PI to 6G11 specs.
Cathode resistor from 680 ohms to 820 ohms. Grid resistors still 1M.
I don't know what to call them but the resistor downwards from the cathode resistor is still 6.8 k and the next resistor 1.5 k to ground (before 4.7 k). NFB is 56k now (100k before).
What are the purposes of all these resistors? (except the NFB - that I know)
I'm asking cause the changed values altered the sound completely. This is an absolutely great sounding amp now - soft and dynamic but it it can also growl when pushed.

Biased it to around 43 ma. According to the schematics my bias voltage runs in between a 6G9 and a 6G11 (it's -39v).
The plates run at 389v now and the grids at 388v - perfect I'd say.
I'm really happy with this one now.

[Robert M. Martinelli]

I don't know what to call them but the resistor downwards from the cathode resistor is still 6.8 k and the next resistor 1.5 k to ground (before 4.7 k). NFB is 56k now (100k before).
What are the purposes of all these resistors? (except the NFB - that I know)
I'm asking cause the changed values altered the sound completely. This is an absolutely great sounding amp now - soft and dynamic but it it can also growl when pushed.

Hi Matt,
seems like your amp was a little too "hi-fi" in its previous form; those resistors you mentioned set the amount of "tail" of the LTPI, and, the higher the voltage ( longer tail ) are the more the PI behaves closer to an ideal one ( but you also get lesser gain ) so, changing those resistors you reduced the tail length, introduced a little unbalance between the two triodes ( which enhances even harmonics, the "sweet" ones I mentioned before ) and you also made the PI stage gain higher ( and this is the reason why the amp now is capable of "growling" when cranked IMHO ).

Hope this helps

Best regards

Bob

[txstrat]

Well, never ending story.
As I stated before it's a double channel amp and I put a 1M master volume pot right before the PI. That way I can level both channels even to each other and set the output volume with the master (so far my theory).
I found that the amp isn't that satisfying when I turn down the master volume. If I turn the master full up and set the headroom with the preamps volume pot the sound is absolutely great.
Would it make any difference if I use a 100K master volume pot?

Thanks for your patience guys.

[tubeswell]

Well, never ending story.
As I stated before it's a double channel amp and I put a 1M master volume pot right before the PI. That way I can level both channels even to each other and set the output volume with the master (so far my theory).
I found that the amp isn't that satisfying when I turn down the master volume. If I turn the master full up and set the headroom with the preamps volume pot the sound is absolutely great.
Would it make any difference if I use a 100K master volume pot?

Thanks for your patience guys.

If the MV pot is a voltage divider (as most vol pots are), lowering it to 100k from 1M would attenuate more signal when the pot was set at max vol (i.e.; so you would be grounding off more of the grid VAC of the following stage more more with a 100k pot than with a 1M pot when it was set to max vol). So based on this logic I don't think it would make a positive difference.