Tube suggestions

[Blair]

Hi guys,

This is a spin off from my Bogen thread, but my ST-70 mono project got me really thinking about this build.

I have been thinking about the 680V B+ the doubler produces, and I'm not too intimidated by it, but it looks like I could have a fairly stiff 350V supply off today's wall voltage with the same transformer using a FWB.

The first tube that comes to mind is the 6L6, and the filament voltage will allow for a six pack per output transformer.

Despite what several people have said, I test the output iron at almost all frequencies at 4.7-5.2K, so lets call it a 5K.

How would the six 6L6 tubes do into 5K?

If not, I am still thinking about the PP tetrode KT88 at 680V B+.

Thanks!

Blair

[DeathRex]

If you want to use the 680 B+, you can try a 1625, 6BG6, or GU50. But they will need about 7K to 8K ohm output transformer. They will also need a seperate screen voltage of about 250 - 300VDC.

A single pair of 6L6s can use a 4200 to 5K ohm output transformer. Six of them will need about a third of that, or 1400 to 1600 ohm transformer.

[Blair]

Right. Same with 6146. I just don't have the primary Z on my output transformers for them.

Here is something I have been trying to figure out how to safely do. Is there any way to safely shave 100V off the supply for 580V vs. 680V?

Doing that would put me in a much better frame of mind when applying it to modern KT88s.

Blair

[DeathRex]

If you use a 5U4 instead of diodes you can kill 20-40 volts. If you have a CLC filter, replacing the first C with a lower value (15 in place of a 47).
Plug in your transformer and see what the filament is loaded. If it's too high, a bucking transformer, or using a filament secondary that isn't being used works wonders. If your 6.3 volts is 7 volts, then a 12V bucking transfomer will lower the voltage about 10%, your filaments will be 6.3, the B+ will also decrease 10% or goto 612. A 6.3 volt bucking transfomer will lower the voltage about 5%. A CL-70 will lower your input AC about 2-3 volts.
And a bigger choke, that has a higher DCR will lower the voltage.

[Blair]

Those are great suggestions for dropping the primary. I've bucked using 5V windings before with success. The CL will help too.

I do not plan on tube rectifiers though. I'll check my filament windings and see what I get.

Thanks!

[kheper]

6146Bs run at 680V. With a doubler, you could tap off the positive lead "between the electro-caps" for the screen voltage. However, you need a power transformer with lots of current capacity for a doubler, and I would use a separate power transformer for the screens. Some guy on another forum modded the MK IIIs for 6146 tubes; He says they sound quite good.

In the datasheet, the primary impedance for PP 6146Bs is 4.2k-7.2k. 5K is fine.

http://tdsl.duncanamps.com/link.php?target=3B259E5D

[20to20]

I have been thinking about the 680V B+ the doubler produces, and I'm not too intimidated by it, but it looks like I could have a fairly stiff 350V supply off today's wall voltage with the same transformer using a FWB.

The first tube that comes to mind is the 6L6, and the filament voltage will allow for a six pack per output transformer.

How would the six 6L6 tubes do into 5K?

If not, I am still thinking about the PP tetrode KT88 at 680V B+.

Thanks!

Blair

When you designed the doubler for the voltage you needed did you also 1/2 the available current you could get through the doubler? You can't get double the power through a doubler. You have to cut the current, too.

[Blair]

Hi ,

I considered the 6146, but I keep researching and finding threads about quality issues with them.

Yes, the transformer is rated, from what I would figure for 1-1.5A. It used a doubler and ran 8 8417s.

After bucking it using a single 6.3V winding, I get 240V secondary voltage unloaded. That's good for about 680vdc unloaded.

My experience is that loaded, it will give me about 70-80% of the 680. I'd be really happy if it dropped the 30% from 680V.

Thanks!

Blair

[20to20]

Hi ,

I
Yes, the transformer is rated, from what I would figure for 1-1.5A. It used a doubler and ran 8 8417s.


Thanks!

Blair

Holy Frijoles!

[Blair]

:) and was rated for a 200W amp at 100% constant duty cycle, so it is beefy.

[kheper]

The Bogen MA-200 schematic does not give the specs for the power transformer. However, the Harman Kardon Citation II used a 180V@1.7A power transformer with a doubler, and the Marantz 8b used a 1.2A AC secondary to get 330ma DC with a doubler. I suspect the Bogen is over 2A.

[Blair]

Even better:)

That's been the real holdup on that project. I'm still debating the 6146, but it has a really low screen voltage max.

I also worry about resale if I don't want to keep them. Audiophools know the KTs and ELs, but not the 6146.

Blair

[kheper]

807s like a high primary impedance (10k-12k at high voltages), but they have a lowish screen max of 300V. 6 x 807 should be about right at 5k. In the link, look at the primary impedances for 600-750V.

http://www.nj7p.info/Common/Tube/SQL/Tu ... p?Type=807

[Blair]

How big are the sockets? I am already drilled for octals.

It looks like, if I could drop my voltage on my B+ a bit to 600V, a quad would be about perfect.

[kheper]

You should be able to find a socket with dimensions close to what you've bored.

http://angela.com/chinapremiumfivepince ... ckets.aspx

http://angela.com/5pinceramicsilvertopm ... china.aspx

http://angela.com/5pintubesocketceramicwafergold.aspx

http://angela.com/5pinwhiteceramicgoldt ... tubes.aspx