Bogen MO-200A conversion

[kheper]

Hi Joe,

I did, but I'm not putting almost 700v on the plate of a 6l6.

My error. I forgot to mention that you would need a new power transformer for 6L6GCs. They have a max plate V of 500. However, do you know what V your secondary runs at? You said earlier 325. I estimated 230-250.

If 325, you could run a FWB for 455Vdc, which would give you extra current to run in PPP (more current available from the FWB than from the doubler).

I'm reconsidering a few things here. A) PPP, while it is nice, it will require twice the current as PP. The "correct" primary impedance of the output iron is 4.5K, so of I just wire it like it was designed with the output in parallel, I could run 6550 or KT88 in PP and it is a perfect Pentode load for them and the high b+ is similar to their datasheet yielding 100W at 3% using 300V on sg2.

Any thoughts on why that would be a bad idea? Then I could use a single choke as well.

Blair

I was thinking PPP KT88s in triode mode. I do not think you will like pentode mode.

Another possibility for PP pentode mode, the 6146B.

http://www.nj7p.info/Common/Tube/SQL/Tu ... Type=6146B

Plate Voltage 600 V
Grid No. 2 Voltage 200 V
Load Resistance 5.6K Ω
Power Output (approx) 96 W

[Blair]

Hi Joe,

That's an interesting little guy! Wish it didn't have top plate lugs!

What is it that makes Pentide mode undesirable on the 6550/kt88 family?

Oh, and you were correct. I think I got around 280v unloaded on the PT.


Thanks!

Blair

[kheper]

Hi Joe,

That's an interesting little guy! Wish it didn't have top plate lugs!

6146Bs are a lot cheaper than KT88s, too.

What is it that makes Pentide mode undesirable on the 6550/kt88 family?

They invented UL mode to correct the distortion and other nastiness (weird bass frequency response) of Pentode mode. Maybe with some good regulation, you will be satisfied.

Oh, and you were correct. I think I got around 280v unloaded on the PT.


Thanks!

Blair

Too bad. 394Vdc is the most you can get out of that power transformer with a FWB. I don't know what a quad of 6L6GCs will do at 394Vdc. I'm thinking 75-80W.

[Blair]

Hi Joe,

That is my intention. To use a heavily filtered separate supply for the screens to achieve better results.

I keep looking at that little pentode though:)

Blair

[Blair]

Hi guys,

If I understand correctly, the higher the plate voltage you go, the lower the bias point. So, with 650-680V on the plates, I figure it will probably be between 25-35mA per tube.

I do not know how to calculate the max current draw from each tube at full swing. If it is 60-70mA, then a single 500mA choke should work even though it may be pushing saturation at full tilt.

The thing is that I have 4.5K primaries, so I'm trying to decide on a quad of KT88s or and octet of EL34 before I start punching holes. I think both tubes will work just fine into the load, but the choke is the limiting factor. It will easily handle a quad of KT88 or KT120s, but the 8 X EL34 seems like it may be pushing it.

Any ideas before I bust out the punch on this 3mm thick piece of aluminum?

Thanks!

Blair

[kheper]

Hi guys,

If I understand correctly, the higher the plate voltage you go, the lower the bias point. So, with 650-680V on the plates, I figure it will probably be between 25-35mA per tube.

I do not know how to calculate the max current draw from each tube at full swing. If it is 60-70mA, then a single 500mA choke should work even though it may be pushing saturation at full tilt.

680V * .060A = 41W= GULP IF EL34s ARE USED

The thing is that I have 4.5K primaries, so I'm trying to decide on a quad of KT88s or and octet of EL34 before I start punching holes. I think both tubes will work just fine into the load, but the choke is the limiting factor. It will easily handle a quad of KT88 or KT120s, but the 8 X EL34 seems like it may be pushing it.

Any ideas before I bust out the punch on this 3mm thick piece of aluminum?

Thanks!

Blair

I'd go with KT88s for their 42W PD, rather than worrying about saturating the choke.

[Blair]

Thanks Joe,

That's where I am leaning. A) simpler, B) same power out C) less current hassle.

The 500 mA of current handling of the 193Q is just barely under max plate current for the KT88 or 6550, and I will never ask that of the amp.

Plus, it frees up top plate real estate.

Blair

[Blair]

OK,

I could think about it for months, but I decided to cut today. Here is one channel. The space in front of the choke and the orientation of the choke are because there will be another one for the screen supply in front of the 193Q. Thanks Mark!!

Anyway, it looks like this will be a nice build. I hope the proximity to the 193Q to the closest tube does not introduce noise.

Now to decide on the front end.



Blair

[Blair]

Here is the day's work before work:)







Looks like I still need a piece of angle or two.

Blair