Critique my schematic, please

[Jag]

Bruce, what about if he uses a choke filtered supply like you showed me a year or so back? Will that take things too low?

[daz]

It is going to be a choke. a 20h 100ma to be exact.

[Paul P]

If you're going with a choke filter (choke comes before any caps) it'd better
be rated for the 250 ma of your PT, or whatever maximum current you're
planning on.

If it's a capacitor filter with a choke after (which I believe is the case) you
should be ok.

Paul P

[daz]

I was told to get a 100, and that even a 75 would be ok. Heres the schematic below, is that ok the way it is?

http://img3.freeimagehosting.net/ima...c69dba0f75.jpg

[Jag]

What I was asking about was a choke input where the choke is in series before any caps, which is what Paul was talking about in his first line. That method brings down B+ quite a bit.

Like this http://music-electronics-forum.com/a...4&d=1180230529. Notice that the choke is in series before any filter caps.

I agree he'd need a higher rating than I used for my single ended build.

[daz]

Ok, then i'm fine as is. By the way, does anyone know what value the choke in the matchless chieftain is? The schematic is illegible.

[Bruce / Mission Amps]

Bruce, what about if he uses a choke filtered supply like you showed me a year or so back? Will that take things too low?

A decent rule of thumb for a choke input filter is to use .9 x the measured AC secondary voltage while under load... that's the hard part... under load.
I have a test jig with 100 watt resistors to come up with this crap but a home builder is kinda at the mercy of one at a time experimentation.

A choke input is probably not needed nor wanted in this amp though.
That will bring the B+ down pretty low.
Actually, I wouldn't be over concerned with too high of B+ even if it ended up being 420-440v with cathode biased EL34s.
If you want more sag and lower B+ use a softer rectifier tube, like a 5U4GB.

Daz if you were using 250 ohms to 300 ohms as your cathode biasing resistor, the idle current, per EL34, with 30vdc on the cathodes would be 50ma to 60ma each.
If your plate to cathode voltage was then also say, 420v -30v = 390v then the power tubes would be idling at around <20 to 23 watts.
That is a bit hot for modern EL34s but because of the lower plate to cathode voltage, wouldn't really hurt them.

A decent way to add a little sag and still use the PT with some fat idle current would be to limit the B+ a little.
What I would do is put some series resistance on the high voltage AC to the rectifier tube.

Maybe a 120 to 150ohms at 5 to 10 watts on each leg.
With the tubes drawing about 80ma, there would be about a 10 drop at idle... before the rectifier and filter cap. But with the amp really cranked up the current through those resistors will be much higher and so will the voltage drop.
If the power stage pulled 150ma to 200ma at high volume, then the drop would be 18vac to 24vac... and that would be some real sag.
Just an idea.

[daz]

Thanks Bruce. Of course as usual some of this is over my head enough to generate more questions.

A choke input is probably not need in this amp though

So i shouldn't have bought it? I was going on the advice of others, so i'm curious why they told me this. But if you really believe this is the case, should i use a R in it's place, and if so what value?

If you want more sag and lower B+ use a softer rectifier tube, like a 5U4GB

Is this a plug and plat tweek? In other words, can i just plug it into the socket with no changing of pin wiring?

Daz if you were using 250 ohms to 300 ohms as your cathode biasing resistor, the idle current, per EL34, with 30vdc on the cathodes would be 50ma to 60ma each.
If your plate to cathode voltage was then also say, 420v -30v = 390v then the power tubes would be idling at around <20 to 23 watts.
That is a bit hot for modern EL34s but because of the lower plate to cathode voltage, wouldn't really hurt them.

What value should i use in order to bring the bias to a happier medium?

As to the sag idea, i may try that if i feel it needs more, but maybe start with lesser values and work up to where it feels right.

Thanks

[Paul P]

A choke input is probably not need in this amp though.

So i shouldn't have bought it? I was going on the advice of others,
so i'm curious why they told me this. But if you really believe this
is the case, should i use a R in it's place, and if so what value?

There are two forms of power supply filters commonly used with tube amps.
The "capacitor input to filter" which has a capacitor directly after the
rectifier, and the "choke input to filter" which has the choke directly after
the rectifier. If you look in the RCA Receiving Tube Manual under 5AR4/GZ34
you'll see that for an example power transformer the "capacitor input" filter
will provide 475vdc whereas the "choke input" filter will provide 275v. The
two filters have different advantages and disadvantages.

Your schematic shows a capacitor directly after the PT so you have a
"capacitor input" first filter, and will get the higher voltage.

Is this a plug and plat tweek? In other words, can i just plug it into
the socket with no changing of pin wiring?

You can just swap the tubes but you should count on having to play with
the power supply dropping resistors to get the voltages you want for the
different stages. Having a range of resistor values on hand is nice when
you get to this point so you don't have to wait a week or more between
each trial.

Paul P

[daz]

Thanks Paul. thats some stuff i really needed to hear and understand. I had no idea the choke would drop the v so much when it's placed first. Only thing that puzzles me is, when you use it as i am isn't it then going to drop the PI and preamp stages an awful lot?

[Paul P]

Only thing that puzzles me is, when you use it as i am isn't it then going to drop the PI and preamp stages an awful lot?

No, but I'd have to pull out my books to remember why. On my amp the
voltage is hardly dropped at all. I measured 391v before the choke and 389v
after the choke, and this is with a larger than usual current draw from my
preamp.

Paul P

[Bruce / Mission Amps]

Thanks Paul. thats some stuff i really needed to hear and understand. I had no idea the choke would drop the v so much when it's placed first. Only thing that puzzles me is, when you use it as i am isn't it then going to drop the PI and preamp stages an awful lot?

The choke after a first filter cap will only drop the B+ rail by the DC resistance of the coil in the choke and that's pretty much all.
The first filter cap will charge up to the peak DC voltage from your rectifier.
Remember that 300vac from the PT? Well it won't be 300v but let's ignore that for a minute.
That is RMS voltage not peak voltage.
The peak voltage form your PT is actually 1.414 x the rms or .707 of the 300v on each side of the PT with respect to the center tap, which is 600v.
As a matter of fact, that PT is actually a 600vct tranny. That means 600v with a center tap!

600v x .707 = 424v.
1.414 x 300 = 424v.
The only thing in between those numbers is the rectifier and what the impedance of that device is.
However all these Hammond PTs make about 6% to 12% more voltage then they are rated at... especially when lightly loaded and then run at normal line voltages, which around here are 122v-126vac.
Most of those Hammond trannys were spec'd out with primary windings seeing 115vac to 117vac. That makes a difference.

So the actual peak voltage could be as high as:
(1.06 x 300v) x 1.414 = 450vdc to, (1.12 x 300) x 1.414 = 475vdc when rectified under no load and then filtered with a high voltage cap.

That 6% to 12% still applies when you use a choke input filter instead of a capacitor input filter because it is before the choke or cap.

But, the choke input will limit the peak AC voltage from the PT because it impedes the peak voltage to something less then the RMS voltage.
In inductors, AC voltage is bucked while current passes rather unimpeded.
That is why I said to roughly use .9 times the 300v rms number.
.9 x 300v = 270v.
Now take the 300v and use the rule of 6% more, or x 1.06.
1.06 x 300v = 318v
I think you will see at least that from the PT and probably more.
318 x .9 = 286vdc... that too low for this amp.
All the current needed to run all the high voltages in the amp has to pass through this choke and needs to be BIG, probably at least as high of current rating as the PT.
That is why I said don't use a choke input filter with this PT unless you want really low DC voltages for EL34s... and that means through out the entire amp.
It is fine with a pair or quad of EL84s though.

The normally seen screen choke after the plate supply, (and it is MUCH smaller, supplying only screen current to the EL34s or what ever and the tiny load from the preamp tubes), has nothing to do with dropping the B+ rail in these numbers. That choke is exposed to B+ that has already been filtered and smoothed by a big capacitor that has been charged up to the peak DC so there is very little AC component left.
I doubt you'll see more then a few volts drop with a choke used after the first B+ node to the screens and the other preamp tubes.

OK
This is why you need to read up on all this stuff before wearing me out! ha ha

[daz]

Ok well as long as thats the case you don't have to explain it to me .....i'd never understand it anyways

[Bruce / Mission Amps]

Thanks Bruce. Of course as usual some of this is over my head enough to generate more questions.



So i shouldn't have bought it? I was going on the advice of others, so i'm curious why they told me this. But if you really believe this is the case, should i use a R in it's place, and if so what value?



Is this a plug and plat tweek? In other words, can i just plug it into the socket with no changing of pin wiring?



What value should i use in order to bring the bias to a happier medium?

As to the sag idea, i may try that if i feel it needs more, but maybe start with lesser values and work up to where it feels right.

Thanks

I guess I don't know what choke you bought.. what are the specs?
And I am not clear if this is just a small screen node choke or a big input filter choke.
You have to experiment with the cathode biasing resistor based on the B+... you can't just pull it out of your ah, hat... it doesn't work that way.
Very low voltages will use a smaller one.. 220ohms maybe,,, high B+ will need a larger one.. 330ohms maybe. etc.
Yes with that PT you can use just about any octal socket rectifier I can think of.... well the ones used in any guitar amps. You know, GZ34, GZ30, 5U4GB, 5U4, 5V4GA, 5Y3GT etc etc.

[daz]

Hammond P-T193C

Specifications
Rating: 20 Henries, 100 mA
Max. VDC: 600
Resistance: 181 ohms
Mounting centers: 2" x 2.1875 vertical mount
Weight: 3.3 lbs.

Heres the schematic showing where i was going to put the choke.....

http://img3.freeimagehosting.net/ima...c69dba0f75.jpg

As for choosing a cathode R, how then do i go about it? I suppose i should use a safe value like a 330 ohm so i can start the amp then calculate what i need by the voltages? if so, is there a layman's tutorial on how to go about this that you or anyone knows of ?
I really tried hard to get every tiny bit of info i could before i ordered the parts. I asked every question i could think of at many forums, and once i had everything i thought i needed to order the parts and build the amp, the answers i got elsewhere are then shown to be apparently flawed. I was told by several people that 250 ohms would be fine even tho they knew exactly what i was doing and how the voltages were not yet known. Now i hear this and unless i can find out how to do this i'm kinda screwed. It never ends.