Bogen MO-200A conversion

[Blair]

Hi Mark,

I have. I was in hopes to get it all on one chassis, but it isn't looking like it will all fit.

That would free up space for a more conventional layout, etc.

It's also a bit scary to use an umbilical with almost 700V riding on it.

Blair

[Blair]

How bout this? You think the chokes are too close to the output tubes?



Blair

[Blair]

Still looking at the PS in the original bogen amp, and I see no filtering besides the two 100uF caps in the doubler. I sim'd this in PSD II, and I get a secondary of about 325V under a 250mA load to get the 680V B+ in the original amp design.

So, I asked the other day if the caps in the doubler need to be full B+ voltage because Bogen used 450V caps. It was recommended that I use caps that will handle full B+. the issue is finding sizable caps that will handle 800v.

A few things while playing in PSD II.

If I use poly caps in the doubler (20uF), it drops my B+ to around 520V. Is there an issue with 20UF caps there? That is a much better B+ to buy caps for!

Another option, assuming the secondary is really 325V, is just a FWB for a B+ of around 410V. At 410V, is there a possibility that I will not get 100W out of a quad of tubes?

If dropping the doubler caps to "tune" the B+ is not harmful, it seems like a good option.

Thanks!

Blair

[kheper]

So, I asked the other day if the caps in the doubler need to be full B+ voltage because Bogen used 450V caps. It was recommended that I use caps that will handle full B+. the issue is finding sizable caps that will handle 800v.

Look at C9 and C10. They are (2) 450V caps in series, providing for a max V of 900. Look at the voltage coming off of C9. To me, it looks like 640 V (or 680 V).

Another option, assuming the secondary is really 325V, is just a FWB for a B+ of around 410V. At 410V, is there a possibility that I will not get 100W out of a quad of tubes?

Blair

I suspect your secondary is only 230-250 V.

[Blair]

Thanks Joe,

Sorry, I've never really done anything with doublers. It makes perfect sense to me because the lower leg of the transformer is at 1/2 B+, and the top cap sits at 660v, but since it is in series it can handle the full voltage.

This helps a bunch!

Blair

[kheper]

Still looking at the PS in the original bogen amp, and I see no filtering besides the two 100uF caps in the doubler

http://tubes.nekhbet.com/#fil

The minimum PS filter capacitance advised for an amp with that much power and high B+ is ~460uF.

You mentioned adding a choke without additional filtering. That Bogen model was a PA. To convert it into a hi-fi, a choke and more filter caps in the supply is recommended. See the doubler/choke in the 211 schemo:

http://www.bonavolta.ch/hobby/en/audio/211_1.htm

[Blair]

Thanks Joe!

Now that I saw your post, I will be able to add quite tha amount of capacitance and choke.

Will this work as a grid choke to feed all 8 G2 screen grids?

http://item.mobileweb.ebay.com/viewitem ... 0460308248

It says 70mA, so I would think so.

Thanks!

Blair

[Blair]

I confirmed with someone regarding the size of the Bogen transformers, so I tinkered with the layout a bit. How does this look? Are the chokes too close to the output tubes? Do I need a steel divider wall?



Thanks!

Blair

[kheper]

Thanks Joe!

Now that I saw your post, I will be able to add quite tha amount of capacitance and choke.

Will this work as a grid choke to feed all 8 G2 screen grids?

http://item.mobileweb.ebay.com/viewitem ... 0460308248

It says 70mA, so I would think so.

Thanks!

Blair

Looking at the RCA handbook for El34 PP amps, they state ~10ma current draw per screen grid. Some datasheets are higher. 70ma is a little shy. I'd hold out for a 150ma choke.

[Blair]

Finally got my donor amp. Anyone see anything wrong with this layout?



Thanks!

Blair

[battradio]

I hope your using 4X8 for a base so it doesn't fall through the floor .

[Blair]

Hadn't thought much about that yet:)

The scary part is that I'm punching a second top plate in case I find another one on a good deal.

Blair

[Blair]

I tested the output trannies last night, and using the color codes from the original MO-100 schematic, it looks like the primaries are 4.5K.

However, I ran test on each winding to do the math to see what my impedance would be into various loads, and I think I have a pretty decent plan.

The yellow winding into 4 ohms is around 2.9K

The Green into 8 ohms is around 2.3K

The Orange into 1 ohm is around 3.1K (Thinking about possibilities for ribbon speakers)

So, my personal speakers are 4 ohm nom. 3K is where I should aim because the green winding into 8 ohms is 2.3k so it would just run out of steam a little quicker right? At 3K it is really a 10 ohm winding.

What tubes in Pentode would work well into a 3K load? The Eal34 does look best from what I can see.

Also, assuming I may actually run the mono at some point, when you parallel the output transformers, it does not change the primary? (What the tubes see)?

I see there are no changes to the ST-70, etc. to run in mono.

Thanks!

Blair

[kheper]

What tubes in Pentode would work well into a 3K load? The Eal34 does look best from what I can see.

Blair

Have you thought of 6L6GCs? They are 30W tubes, and some of the datasheets show that they like lowish primary input impedance ~3.5K in PP pentode mode. [I don't not think the primary impedance is all that important. You will be running in PPP pentode mode.]

http://www.nj7p.info/Common/Tube/SQL/Tu ... Type=6L6GC

http://tdsl.duncanamps.com/link.php?target=5525C52D

[Blair]

Hi Joe,

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

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.

Of course, then I need anothe MO-200, but hey....

Thanks!

Blair