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arnoldc 2nd March 2004 04:46 AM

I want to make a simple UL PP amp
 
2 Attachment(s)
hi all,

my goals are:

1. use the least number of tubes
2. have enough gain to drive 89dB speakers with no preamp
3. get 30W something from it
4. understand all of these :)

in order to achieve #1 and #2, i chose to make use of 6SL7. half of which will be the driver and half is the phase splitter.

i used tubecad to come up a textbook grounded cathode driver for 28.6dB gain with an unbypassed cathode resistor. the bias will be 1.5V and Vout is +45/-50 V.

now the phase splitter, with one triode i can only do a split load.

in a split load, does the Ra computation matter? normally it would matter to determine the gain of the circuit, right? however, since a split load does not have gain, how do i compute this correctly?

the 27K in the circuit was just a "guess" to find out what gain i will get which turned out to be -0.455 dB

what else should i look for? any additional readings on split load? (incidentally, there is a very good article about differential phase splitters in the Aiken Amplification website)

cheers!

ps.

B+ is 420V and B++ is 260V

Sch3mat1c 2nd March 2004 05:38 AM

To estimate gain you have to figure half of the output voltage in series with the input, cathode follower-style NFB but half. This is current feedback so the Zo at the plate increases by as much as distortion and gain are reduced, while all three figures are reduced at the cathode. Important to know when it comes to HF response, which means some sort of buffer (CF or plate follower; the latter would make it a Williamson driver topology) is preferable. You're driving pentodes so I'll hesitatingly say it won't make much difference. But for 30W output I'd rather have something like a 12AU7 triode there instead.

Um back to the cathodyne... equivalent AC circuit places the EL34 grid leaks in series, paralleled with the plate and cathode resistors in series, for the plate load.

In general you can assume you will get about as much voltage out as in, so 30V input at the grid will produce 30V at the cathode and -30V at the plate. Obviously it'll be a bit less, but I wouldn't expect more than 3/4 loss unless you really screwed up your resistor values. ;)

If you want to maximize power output, you'll have to drop near class B. More efficiency allows more power output for a given plate dissipation. (Mind the ideal load resistance in this situation!) Supposedly starts sounding like SS though...

Tim

arnoldc 2nd March 2004 07:27 AM

hi tim,

is your choice of 12AU7 has got to do with the transconductance, plate resistance, or both? is a higher transconductance, lower plate resistance also beneficial for a split load?

> In general you can assume you will get about as much voltage out as in, so 30V input at the grid will produce 30V at the cathode and -30V at the plate. Obviously it'll be a bit less, but I wouldn't expect more than 3/4 loss unless you really screwed up your resistor values.

ok, i am shooting then for the same Vout as my driver stage without having too high a B+ for me to worry about the cathode-heater issue.

fscarpa58 2nd March 2004 11:13 AM

2 Attachment(s)
hi

I think you can avoid one res and one bypass cap
at the cathode of the output tubes as follows

federico

SY 2nd March 2004 11:27 AM

At 27K, you're running the 6SL7 pretty hard, and with such a small value, the swing will be limited. I'm not optimistic about this circuit. If you want to use a split-load, you might consider one of the dual triodes made for vertical oscillation/deflection in TVs; they usually have a high mu/low current triode paired with a low mu/high current triode. Use the former as the input stage, the latter as the split-load.

Have you considered a constant current diff amp instead? With a 6SL7 and 100K plate loads, you should be able to get a gain of roughly 30-35, and the current source can be a pretty simple fet or bipolar circuit. This would be even better if you could bend to accept one more tube to interpose as a driver diff amp, then take some local feedback, but I don't want to restart THAT war.

There are a bunch of details to clean up, too (you'd want input resistors on the output stage, possibly screen resistors, too, and you only need one cathode resistor if you're bypassing, etc, etc), but let's get the basic topology nailed down first.

EDIT: Looks like fscarpa nailed down that last point already while I was getting coffee :)

arnoldc 2nd March 2004 11:49 AM

Quote:

Originally posted by fscarpa58
I think you can avoid one res and one bypass cap
at the cathode of the output tubes as follows
federico
that would be great! i'd have to double the wattage of the cathode resistor though, right?


Quote:

Originally posted by SY
At 27K, you're running the 6SL7 pretty hard, and with such a small value, the swing will be limited. I'm not optimistic about this circuit.
hi sy, i think tubecad confirms what you said as my Vout is now lower than the input stage and i need to ramp up B+ to catch up.
Quote:

Originally posted by SY
If you want to use a split-load, you might consider one of the dual triodes made for vertical oscillation/deflection in TVs; they usually have a high mu/low current triode paired with a low mu/high current triode. Use the former as the input stage, the latter as the split-load.
like a 6EM7 perhaps? or how about a 12AU7 as my split load? i'll keep the 6SL7 as driver?
Quote:

Originally posted by SY
Have you considered a constant current diff amp instead? With a 6SL7 and 100K plate loads, you should be able to get a gain of roughly 30-35, and the current source can be a pretty simple fet or bipolar circuit. This would be even better if you could bend to accept one more tube to interpose as a driver diff amp, then take some local feedback, but I don't want to restart THAT war.
yes, i did. that will be may last option due to the CCS which needs a visit to Gary Pimm's website. :) i have nothing against feedback so that' s not a problem with me :)
Quote:

Originally posted by SY
There are a bunch of details to clean up, too (you'd want input resistors on the output stage, possibly screen resistors, too, and you only need one cathode resistor if you're bypassing, etc, etc), but let's get the basic topology nailed down first.
oh ok, how about 1K grid stoppers to the EL34 grid?

many thanks to both of you!

SY 2nd March 2004 12:51 PM

A 6EM7 will work, as will a 6DR7- and many more, too. A conventional choice would be a 7247.

IME, you don't have to get fancy with a diff amp current source. A single bipolar circuit will work very well and save you the hassle of making a B- rail. One transistor, two resistors, and a diode.

Between 1 and 2.2 kohm on the EL34 grids will work fine.

Sch3mat1c 2nd March 2004 04:50 PM

7247 is half and half 12AX7 and 12AU7 combined :)

Yeah, cathodynes don't like low volts...you need as much headroom as you can get. It'd be a good idea to run the 6SL7 from +400V and bias it at 1/4B+. Here you'll have maximum possible output from the splitter (because it's cathode will be at 1/4B+ (100V) and plate at 3/4B+ (300V) making for 200V p-k voltage). Sy's right, an LTP would be another good choice here. If you prefer tubes ;) you can use a pentode just as well instead of a transistor CCS. You'll have a bit less headroom, but then, you still have that 400V supply..

In answer to your second post, 12AU7 is just an all-around good compromise. In this case a 6C4 would be better since you only need half (unless you want to parallel both halves).. It has reasonable current levels (um 12mA max at 100V?) and <10k Rp making it good for drivers. Low mu, and likewise Gm (rounds out the triangle: mu = Gm * Rp) but what are 'ya gonna do.
You can use a 6SN7 or equivalent (um single is 6C5 isn't it?) just as well.
You're just driving EL34s so I wouldn't bother spending the current for anything larger like a 6BX7 or vertical driver tube (6FD7 etc.).

UL still has an output impedance so I'd want more gain than a 6SL7 or 12AX7 for the first stage, so you can add some global NFB. I'd recommend a pentode front end like on my Frankenhouse.

Tim

Jaap 2nd March 2004 06:14 PM

I wonder
 
if the Allen Wright design of a PP-amp would be a good choice. What I read about it says that it is the best: state of the art"".
What tubes does he use ?
I suppose that his design is not more difficult to build or much more expensive. So why not go straight for the best solution ?

Jaap

arnoldc 2nd March 2004 11:31 PM

Re: I wonder
 
2 Attachment(s)
Quote:

Originally posted by SY
A 6EM7 will work, as will a 6DR7- and many more, too. A conventional choice would be a 7247.
now i'm not sure if i can source them locally.
Quote:

Originally posted by SY
IME, you don't have to get fancy with a diff amp current source. A single bipolar circuit will work very well and save you the hassle of making a B- rail. One transistor, two resistors, and a diode.
Ok, or a tube :) but that will violate my goal, he he.
Quote:

Originally posted by SY
Between 1 and 2.2 kohm on the EL34 grids will work fine. [/B]
Ok and I chose 1K.


Quote:

Originally posted by Sch3mat1c
[B]7247 is half and half 12AX7 and 12AU7 combined :)

Yeah, cathodynes don't like low volts...you need as much headroom as you can get. It'd be a good idea to run the 6SL7 from +400V and bias it at 1/4B+. Here you'll have maximum possible output from the splitter (because it's cathode will be at 1/4B+ (100V) and plate at 3/4B+ (300V) making for 200V p-k voltage). Sy's right, an LTP would be another good choice here. If you prefer tubes ;) you can use a pentode just as well instead of a transistor CCS. You'll have a bit less headroom, but then, you still have that 400V supply..
and because of this, i am bailing out of the split load for now, until i source some tubes more appropriate as sy and yourself has recommended.
Quote:

Originally posted by Sch3mat1c
UL still has an output impedance so I'd want more gain than a 6SL7 or 12AX7 for the first stage, so you can add some global NFB. I'd recommend a pentode front end like on my Frankenhouse.
in my circuit below, is this still an issue?

Quote:

Originally posted by Jaap
if the Allen Wright design of a PP-amp would be a good choice. What I read about it says that it is the best: state of the art"".
What tubes does he use ?
I suppose that his design is not more difficult to build or much more expensive. So why not go straight for the best solution ?
Jaap

hi jaap, as much as i'd like to but allen's circuit is triode strapped and i want more power :)

---

because of what have transpired so far, i am ditching the split load in favor of a long tail pair. i'd like to give the 6SL7 another try, as shown in the diagram below. i thought the -1.5V tail would be nice for a battery source?

i'd appreciate further comments on this circuit gentlement.

cheers!


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