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Bogen MO-200A transplant /conversion

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Anyway, I used a variac to determine the OPT primaries based on the original loads, and they are around 4.5K each, so fully parallel, they would be 2.25k which would be perfect for PPP KT88 or 6550s.

Careful with that. If you measured each transformer to be 4.5k to 8R, then putting both primaries and secondaries in parallel results in a 2.25k to 4R transformer. See Broskie's writeup, read the paragraph "Design Example- two ST-70s"
 
Yeah,

I used the original Bogen secondary impedances to get my #s.

An externally hosted image should be here but it was not working when we last tested it.



Here is an image of the various primary impedances based on different loads.

The ones highlighted look like they would work OK for PPP KT88/120 or 6550. I figure, I could just aim for the average of 2.5K. The way I see it, speaker loads vary anyway throughout the frequency range, so this is a decent approach.

I'm still a bit worried about the B+ being 650+V. I think my screen supply will dial right in at 300V or so and I can always add CRC to tame it down a touch. It will be choke input, so it should be relatively well regulated.

What do you think based on the #s in my chart? The highlighted numbers are with the output transformer primaries and secondaries in parallel.

Thanks!

Blair
 
The highlighted numbers are with the output transformer primaries and secondaries in parallel.

That statement gives me concern that you may not understand how this works. What you measured is a voltage or turns ratio. That doesn't change regardless of how many transformers you have in parallel.

The actual impedance seen by the tubes DOES change when you put transformers in parallel and a real load on the secondaries. Try reading Broskie's article carefully. I can't explain it any better than he did.
 
Hi,

I have read it several times, and I interpreted to state that, if I parallel the secondaries, the result would be 1/2. Same for the primaries.

Like this:?

D22D8116-2B4E-4076-9368-1D92D713DC4E-399-000000615B53B97C.jpg


Quote from Broskie:

"Design example: two Dynaco ST-70s
What if you only own two Dynaco ST-70s? This time the desired reflected impedance is 2150 ohms and the solution is fairly straightforward. We start by placing two output transformers in parallel, both the primaries and secondaries. Next, we re-label the secondary taps; the 16-ohm tap becomes the 8-ohm tap; the 8-ohm tap, the 4-ohm tap; and the 4-ohm tap, the 2-ohm tap. Now each impedance load will see 1.414 times the voltage that a single ST-70 would deliver. (What I like about this arrangement is that none of the secondary is left flapping in the wind when an 8-ohm load is used.)"

Right?




Blair
 
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are you measuring impedance or DCR? If you are measuring impedance , what type of signal injection does the impedance tester use? some use DC, which is useless for audio signal circuits. some use mhz which again is useless for audio signal circuts. some use frequencies in the 400hz, 1000hz, 10000hz range which is a good portfolio of signals to use for power amp circuits.
 
Pictures are good, eliminates ambiguity.

You're proposing to operate it aggressively. Don't know what tubes you're going to use, but let's say for the sake of argument that the proposed tubes like to see impedances roughly on the same order as the 8417.

So you're proposing to operate the same OPTs in the same configuration but with half the tubes. You're operating the tubes at half the Raa that Bogen does.

Now the Bogen operating point is relatively conservative at 50W a pair. You're proposing to operate at 100w/pair at half impedance, which depending on the tubes may or may not be conservative at all. And your distortion may be higher.
 
Thanks for understanding the drawings.

I plan to use either KT88 or KT120s. I am not bent on 200W either. I would be happy with 160W or so which is much more reasonable from a PPP KT88 amp in Pentode.

I think with the 120s, 180-200W is pretty feasible.

The 2.5K load is about right for these tubes, and it stresses the PS less, so that's why I killed the 8 X power tube idea.

Thanks!

Blair
 
As I am striping the second amp and popping the end bells off for paint, I noticed something again that I just ignored on the first one. Can anyone explain why the output transformers have a different part number from each other on each amp? They are different also in that the laminates are slightly shorter on one than the other.

Will this cause any issues when p parallel the two transformers?

It is just strange.

Thanks!

Blair
 
according to the schematic they are different part numbers and the schematic does show a difference in the laminate representation.....will it hurt your project...no becuase you are paralleling them and thats how they are on the MO-200A amp. I would leave them mixed matched ....there might be an inductive orientation based reason for the differences due to how close they were installed...
 

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Thanks!

I kind of figured. It just seemed a little odd to me.

That is a really clear schematic! Better than the one I have. Any chance you could email it to me?

Thanks!

Blair

pm me your email addiress...uploading to the forum shrinks the resolution. It went from 2K to 300kb
 

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Hahaha, no, that's pretty funny though.

The schematic symbols have nothing to do with the physical attributes of ANY electronic device. Just like on a capacitor, if you draw the two lines further apart it doesn't mean the plates of the actual capacitor are further apart.
 
Hahaha, no, that's pretty funny though.

The schematic symbols have nothing to do with the physical attributes of ANY electronic device. Just like on a capacitor, if you draw the two lines further apart it doesn't mean the plates of the actual capacitor are further apart.

makes sense....they used different part numbers and different laminate spacing on the schematic for no reason. I'll measure their inductance during the week and report back.
 
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