Kid "safer" PP 6550 amp

[Blair]

I am still working over some big sweep monoblocks, but I'm going nuts with my SS amp and no tube amp. This chassis is made from 12ga steel, and lots of venting.





I think I can pull this off with all the tubes inside the chassis. I'm planning on running UL at ~450VDC B+. The 6550 datasheet says at max signal, two 6550 tubes needs 275mA for 100W output. I mostly see 6550 and KT88 amps rated at 70W output. Do I really need 275mA per pair of tubes?

See anything wrong with this layout?

Also, I always use big 2W pots for bias. I know it's way overkill. What wattage is acceptable for these pots? I'm trying up keep the circuitry small.

Thanks!

Blair

[kheper]

Below - in the data, the idle bias is 50ma per 6550 tube. That's a breezy 22W idle for a 35W tube. Perhaps, 70W audio output is a bit optimistic. What does the 275ma figure refer to? Max anode current?

AB1 U/L 40%
Ea: 453
Vg1: -59.0
Ia: 2x50
Z: 4000
Pout: 70
THD: 2.0

If you look at the VTA board, it uses 4 trimmers for biasing the output tubes. The old, Dynaco pots are removed from the amp. I do not know what trimmer is used, but its max wattage should be around 1/2W (Do not hold me to that.).

[Blair]

Right,

I always see zero signal plate current and max signal plate current. Such as above:

B+ 450V zero signal plate current is 150mA for two tubes
Max signal plate current 265mA for two tubes at 70W

http://www.drtube.com/datasheets/6550a-ge1972.pdf

Am I reading it wrong?

Thanks!

[kheper]

From Mouser, here is a really nice 1/2W chassis mount pot.

http://www.mouser.com/ProductDetail/Vis ... vZyBLvcQ==

[kheper]

Right,

I always see zero signal plate current and max signal plate current. Such as above:

B+ 450V zero signal plate current is 150mA for two tubes
Max signal plate current 265mA for two tubes at 70W

http://www.drtube.com/datasheets/6550a-ge1972.pdf

Am I reading it wrong?

Thanks!

Well, the idle bias per tube is 75ma. 34W plate dissipation per 6550. Acceptable to some, but I'd drop it to prolong tube life. Why not start at 50ma, and raise it until you hear a "sweet spot"? I would not go over 70ma.

[Blair]

That's an awesome looking pot! I like the knob!

I would run it at ~50-55mA idle myself. I am just wondering about that max signal spec. If that's the case, you would need 450V at 600mA to have a stiff enough supply.

I received my tubes today for this project, but I was sent the wrong tubes. Being honest, I sent the NOS Tung Sols back to the guy for the cheapies I won on eBay:(

Sucks being honest sometimes.

[kheper]

I would run it at ~50-55mA idle myself. I am just wondering about that max signal spec. If that's the case, you would need 450V at 600mA to have a stiff enough supply.

A 600ma supply would be ideal (bass and transient wise) for that amp, but I do not think the amp will draw a max of 600ma. I'm guessing a bit over 400ma? The specs are all over the place from 300-600ma.

[Blair]

Definitely agreed!! This has been the most allusive spec for every output tube I've seen. I don't want to skimp, but also don't want to try to wedge a beast of a supply in an amp if it's unnecessary.

Thanks!

[kheper]

http://www.antekinc.com/details.php?p=710

As an SS FW cap input, this transformer should be good for 450Vdc @ 630ma. You'll need to series the windings and ground the center, however. 5.1" d x 3.1" h might fit into the chassis.

[Blair]

Yep,

That's the one I'm looking at. It will fit just fine.

[kheper]

Or this Antek. It has the same dimensions, but a 360-0-360. As an SS FW cap input, it will produce a little more than 450Vdc @ 550ma.

http://www.antekinc.com/details.php?p=711

[Blair]

I'm going to steer this project in a different direction. I love the idea of the "safe" chassis, but it leaves me buying, yet, another piece of iron.



I'm going to do a pair of Poseidin type monoblocks on steroids. The 6CB5 is there for reference. I have everything else to build these already for the most part.

[Blair]

http://www.turneraudio.com.au/audiofilt ... page2.html

I am looking at this PS site from Patrick Turner (the traditional choke input), and noticed that the 6K bleeders are 30W! Dow these really have to be 30W?

I think this is the PS I will use on my amps.

The other question is that the second choke I have is only rated at 715 WVDC. My supply transformer is 1100VCT. Would a current limiter help the startup spikes? Is it OK to use the 715V choke in the second position?

Thanks!

Blair

[battradio]

Hi Blair ,

With snubber caps and using flyback diodes between the chokes , it will kill most of the spikes , and you will be fine , the Chinese 50 watt metal resistors are cheep use them for bleeders , I use them for cathode resistors . A little over kill never hurts .

Mark

[Blair]

Hi Mark,

How do you choose the fly back diode value and the resistor value?



This is what you are talking about right?

Thanks!

Blair