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PostPosted: 25 Nov 2012, 10:05 
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Joined: 26 Feb 2012, 13:13
Posts: 23
Location: Virginia
Geek wrote:
Awesome work, Nodric! :o :D

But I have just one picky.... I see those 3.3K resistors just standing on the terminal strip. They have a free-air rating of about 3W or so and are only 25W rated when heat sinked to the chassis.

Cheers!


Thanks for the encouragement :)

I ran down to the workshop, amp in arms, whipped off the bottom, and quickly tested the circuit due to your concerns :)

Here's the math.

The plate voltage is 160v, with a volts drop across the resistors of 83v. This gives 25.15ma current (divide 83 by 3300). This is also proven by my own real measurements during build, with each side of the triode drawing around 25ma, and the total for the amp is 52ma.

Wattage dissipated by the resistors = 83x0.025 = 2.075w. I checked and the free air rating is 2.25w, so we are good to go, just! I also noted that they are actually 30w resistors, and not 25w as I earlier stated.

I've also measured the resistor temps using my Fluke infrared laser thermometer (I got it cheap) and it reads max temp of 110f/43c. The resistors are good to 150c albeit 100% de-rated at that temp. At 43c they are supposed to be good for 12w. I will investigate a heat sink, but wonder if chassis mounting is needed, or just a good heat shunt? Now I need to read up on heat sink calculations :( Any pointers on this?

The triodes are dissipating about 1.9w ea, so quite a powerful little amp :) The data sheet states they can be pushed to 125ma and 13w! I must read more about using these in Class A SE amps :)

The whole chassis gets real warm, not skin burning warm, but nice and toasty to the touch. I can lay the back of my hand anywhere without any sense of 'ouch' or burn for extended periods. The rectifier is a 10w heater, and the 6SA7 is about 16w, so we have quite some radiance there before we add in all those WW resistors (all over spec'd), and the transformer (which also gets nice and warm). I also notice the tube sockets get real warm on those big tubes.

The glass of the triode is close to 200f/93c in places!

I now see why commercial stuff has great big wire cages over them!

EDIT

I did some quick reading, and I think these would be more than good enough?

http://www.mouser.com/Search/ProductDetail.aspx?R=WA-T220-101Evirtualkey58810000virtualkey588-WA-T220-101E


Last edited by Nodric on 25 Nov 2012, 10:29, edited 1 time in total.

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PostPosted: 25 Nov 2012, 10:08 
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Location: Virginia
EXILE wrote:
This is my current head amp. It is basically the same but with the 6as7 paralleled


Apart from the need of a HUGE tranny for heaters :) How does the extra tube improve gain/sound/performance? What about load impedance for the phones?

What's your rectifier setup?

Schematic?

:)


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PostPosted: 25 Nov 2012, 14:40 
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Nodric wrote:
Wattage dissipated by the resistors = 83x0.025 = 2.075w. I checked and the free air rating is 2.25w, so we are good to go, just!

....

Now I need to read up on heat sink calculations :( Any pointers on this?


Oh, OK! I thought they were dissipating a lot more than that :hot:

In a chassis and using IR thermometer, I've found resistors need to be derated as much as 10:1 w/o ventillation or 3:1 with. I bet if you just took a spare piece of aluminum and drilled holes on the back to mount them with a little heatsink goop, you'd be OK :D

Cheers!

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PostPosted: 25 Nov 2012, 16:14 
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@EXILE: I suppose you paralleled the tubes to get the plate impedance down. Lucky the 6AS7 has low plate impedance anyhow. When I built my first 6AS7 it was my first SET. I was impressed with the sound of it (all three watts) . Now leaning back to the SE UL pentodes.

Could a simple headphone amp be made with an EL34?

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PostPosted: 25 Nov 2012, 19:10 
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Very nice looking on the top and underneath. :thumbsup:

Any chance you'd be willing to share your PS design?

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PostPosted: 26 Nov 2012, 15:13 
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booangler wrote:
Very nice looking on the top and underneath. :thumbsup:

Any chance you'd be willing to share your PS design?


They are very simple. I believe simple is best, and I wanted to try and avoid solid state attempting to be a purist to the designs of the 50s. I almost got away with it, but AC heaters proved to be too noisy for the 6922. I did get hum to almost nothing with AC, but I could still here it, so after 2 days of fighting I gave in and went to DC. Sometimes it's just not worth the battle.

The B+ Supply
Image

The B+ supply is simply a 3 stage RC filter, with a small input capacitor for the rectifier tube. You can use any 5v rectifier, or for that matter any tube that will give you the ma you require. I didn't want any sag so chose one that would give at least 100ma. The 5V4 was chosen for looks as much as anything, so such a large tube is not needed. 5Y3, 5U4, 5R4 etc...

The 6.8uf was chosen as it will suit 90% of tubes, and there is no need for it to be big. If you exceed the tube ratings for the input capacitor you'll simply smoke the tube. It is a misconception it needs to be as big as the tube will take. Any small cap will do. Then I used 2x100uf and 2x330R 1% 1W Metal Film resistors in the first two RC filters. The last stage of filtering was done using a 1.25K 5W WW resistor just to get the voltage down. It needs to be around 150-170v, so depending on your transformer this will change. The final cap was 470uf, but probably could have also been 100uf, but I had the big one so used it anyway :) Ripple is <1.5mv!

All Caps are 450v rated, except the 470 which is 400v.

The Heater Supply
Image

The heater supply works like a charm, but the secondary of the transformer should be rated at 1.4 times the heater load. My transformer was 5amps so I was good to go. The resistors drop about 0.5v so dissipate about 1.4w, but I used 5w to keep em cool! The diodes are SB5 5 amp Schottky as they are very fast switching and have a low volts drop. The large caps are needed to smooth a massive amount of ripple! Use high ripple varieties so as not to end their life prematurely. The transformer secondary does not need to be higher than 6.3v, as rectification increases the voltage to about 9v with no load. With load it drops back to around 6.5v DC.

Yes you could us a voltage regulator, but they get hot as hell, and have to be chassis mounted. I found this method worked just fine, but a regulator is also a fine solution.

The end result is a wonderfully inaudible power supply. There is nothing to hear. It's so quiet you end up touching the signal leads with your finger to make sure the thing is working :D

Happy Building!


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PostPosted: 26 Nov 2012, 18:34 
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@Nodric: Nice and simple PS. I have built dozens like it myself. Even my latest "Heavy Metal" 6N7 preamp uses a similar PS. The only thing I would change is the heater PS. I would have gone for a higher voltage tranni and used bigger resistors in the CRC cct. something like 100ohm would be better. But as you say the HP amp is quiet so what the heck?? I have found that you don't need over filtering of the heater voltage if the cathode is electrically isolated from the heaters e.g. 12AX7.

Good work.

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PostPosted: 26 Nov 2012, 18:40 
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Nodric wrote:
The 6.8uf was chosen as it will suit 90% of tubes, and there is no need for it to be big.


Oh yes, THIS! :thumbsup:

I have seen SO MANY designs with thousands of uF after a GZ34 then a few posts later, the builder wondering why he eats tubes and/or transformers :eek:

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 26 Nov 2012, 18:48 
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I know for bigger rec. tubes e.g. 5U4, 20uf is considered max. A couple of 100ohm Rs from the tranni sec. to the plate connections help stacks. It's just the first cap which is critical.

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PostPosted: 26 Nov 2012, 20:00 
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Joined: 26 Feb 2012, 13:13
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Location: Virginia
mwhouston wrote:
@Nodric: I would have gone for a higher voltage tranni and used bigger resistors in the CRC cct. something like 100ohm would be better.


I agree and would have liked more volts, but I had what I had. i.e. The tranni was NOS and save be $80 :) It also had lots of amps :)

The benefit of bigger resistors I guess would be improved smoothing. I'm not an expert on RC filter design so I could be wrong :D


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