Bogen MO-200A conversion

[Blair]

Hi Dave and Mark,

I plan to test the output transformer's primary into a 4 ohm load. (my speakers are almost perfectly resistive at 4 ohms) before deciding on output tubes.

It will be interesting when complete. I have looked at various Bogen PA amps over the years, and the MO-100 and 200 have claimed specs that are closer to hifi than PA. Back to something I alluded to early in this thread, would the 1.5 ohm tap be better for ribbon speakers that dip to .8-1.5 ohms? Seems like a good solution for ribbon owners.

I think since I'm transplanting this amp that I will feed the driver, PI, and grids from a separate supply and just leave the big monster PT for the power section and filaments.

Thanks!

Blair

[Blair]

Assuming this amp is going to run an 8 pack of EL34s for two 100W channels, what current choke should I buy if I want a choke filter on the B+ supply?

http://www.tubesandmore.com/products/P-T193Q

Something like this? Or is this too many Henries and too little current handling? I'd prefer to only have one choke for space purposes.

Thanks!

Blair

[battradio]

THat should work , don't think you will find a biger , choke unless it's custom wound .

[Geek]

http://www.tubesandmore.com/products/P-T193Q

To give you an idea of the size of that beast

http://www.classicvalve.ca/images/845_front.jpg

[Blair]

I know it is huge, but not any bigger than the bogen transformers.

So, one will handle the current of all 8 EL34s?

Thanks!

Blair

[battradio]

It might drop to 8 Henys doing it but it will work

[Blair]

OK,

I bought two of the 193Q chokes. I did see the size, but not the weight! 21lbs! The Bogen had a shipping weight of 65lbs.,

I am not sure what I need to do now. Hopefully, I can figure out the layout when the new top plates and the amp arrive.

Blair

[battradio]

I have two of those chokes on the self down stairs waiting for a project , was going to use them with a pair toroid power transformers and a pair of Antek output toroids with PPP 6550s , but it got side tracked with the PPP 6B4G amps im going to build with UTC -LS58 outputs .

The outputs i got from you are on a pair of baldwin amps , the torids i have now are the Ultralinear tapped ones , May use the latter when i find the right tubes .

[Blair]

I drew up a top plate the size of what I ordered on paper last night, and I'm not sure I could get away with two chokes.

I did order two top plates, so I could go mono, but the goal was to make this a single bridge able amplifier. You know, in case I find another one and decide I want a second 200W amp;)

It seemed like a good idea to add a second choke to ensure they will handle the current at full swing, but I'm just not sure the second will physically fit.

Blair

[Blair]

I have only seen this on the OddBlocks, but why don't more amps use LM317HV or similar on power tubes as a CCS for bias?

It looks like a nice option. Something I am considering for this build.

http://diyaudioprojects.com/Tubes/KT88- ... -Tube-Amp/

Thanks!

Blair

[Blair]

I changed the title so I did not have to post another thread.

I should receive the MO-200A early next week.

Rather than doing this the way most people do it, and probably for good reason, I'm transplanting to a new chassis and modifying the PS.

A few design questions:

Regarding the power supply, the original design used a doubler. I see that the caps used were 450V. When you use a doubler, the initial caps used in the doubler do not have to handle the full voltage potential?

I'm adding a choke. Do you have to add a cap after the choke if the caps in the doubler are sufficient?

I'm adding a separate transformer for the screen Grids. I always hear that choke filters are best, but is there anything wrong with CRC filtering for this assuming that I only provide voltage for the Grids and the driver tubes plates?

Last:

I read somewhere that these are 5k primary output iron. I am only going off online information, so I will have to test these myself.

If that is the case, and I hook up a 4 ohm load to the 6.25 ohm tap, then the primary will look like a P-P loak of 3.2K, or if I hook an 8 ohm load to the 16 ohm tap, it would be a P-P load of 2.5K.

The EL34 looks to be a good choice for PPP into these loads. Is there any way to address the 700 ohm difference in loads between using the 16ohm tap and the 6.25 ohm tap?

Sorry if this does not make sense. I'm trying mostly to figure out whether to use a single tap, or put a tap to use with 8 ohms speakers and 4 ohm speakers even though the primary impedance will be different.

Thanks!

Blair

[battradio]

Most people only regulate the bias when the regulate the B+ , because if the B+ increases the bias doesn't when its regulated the tubes can draw too much current if the AC line goes higher .

For a voltage doubler i would use both caps rated at the full voltage , this is over kill but i hate cleaning up exploded cap .

After the 10 henry choke put the largest filter cap you can afford .

A choke in the screen suppy coud be several hundred ohms and not mater with th low current drawn , a output transformer from an AA5 might work real nice and would be cheap

A voltage regulator for the screens would be good also .

http://www.antiquewireless.org/otb/resto0504.htm

[kheper]

A few design questions:

Regarding the power supply, the original design used a doubler. I see that the caps used were 450V. When you use a doubler, the initial caps used in the doubler do not have to handle the full voltage potential?

In the doubler schemo below (like yours), the initial caps "see" the full voltage. Half of the voltage can be tapped off where the initial caps and the other leg of the secondary meet.

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

I'm adding a choke. Do you have to add a cap after the choke if the caps in the doubler are sufficient?

I'm adding a separate transformer for the screen Grids. I always hear that choke filters are best, but is there anything wrong with CRC filtering for this assuming that I only provide voltage for the Grids and the driver tubes plates?

For better filtering, I would go with a cap after the choke.

A separate screen transformer is superior to tapping off the B+. The current draw of the screens will be low (about 5ma per tube), so you could use a small-sized choke.

[Blair]

Thanks guys! This is helpful. I will play around with PSD II a bit and see what I get. I've issue that is most apparent is real estate! I may need to build this fella a bit taller than I anticipated.

[battradio]

Have you considered a seperate chassis for the power supply