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deicide67 5th December 2012 02:41 AM

ST-70 mono makeover
I have a big pair of amps I am working on from a few Bogen MO-200s, but tubing the project is going to cost a fortune.

So, I decided I want to try a similar concept, but on a smaller scale. I have a pair of big Baldwin organ power transformers, and a quad of A-470 output transformers.

I'm going to parallel the secondaries, but drive both pairs from a single driver and phase inverter. The only thing I am really stumped about is how to handle the NFB. Once paralleled, there will be an 8,4, and 2ohm output.

Should I just tap off the 8 ohm to feed the feedback back into the driver stage. Will this cause any issues between the output stages, etc.?

Here is one channel chassis:

deicide67 6th December 2012 09:11 PM

Like this? Ignore the front end. It is representative for now.

Link if that does not work:



deicide67 10th December 2012 03:46 AM

Lots of views, no replies. Here's a few images anyway:

llwhtt 11th December 2012 07:57 PM

Since you mentioned the Bogen MO-200 I suppose you the schematic, now go find the schematic for the MO-100. Compare the feedback network for the both of them. You'll see the the paralleled MO-200 output xfmrs have either half or double the resistance and capacitance, I don't remember which. Anyway it should help you to see what's going on when parallel output xfmrs.


deicide67 11th December 2012 08:01 PM


Yes, but this is a different beast. I'm driving both PP pairs from a single VA/CF vs. two completely separate drivers.

It sounds very nice so far, but I have not installed any form of feedback.

bear 11th December 2012 10:00 PM

For feedback to work, you need "excess gain" or else your overall gain will drop kinda low... there is no requirement for gnfb (global negative feedback) or loop feedback to be used. I am not sure you have enough gain with just the 12AX7 in the front end, but that's ok.

Of course you do have gain in the second stage... getting that stage to be balanced, and the output to be balanced usually will require the use of a balance adjustment or more sophisticated means of forcing balance in the LTP...

But your question is simple to answer. If you have the secondaries in parallel then the feedback comes from the same place as if you had one single transformer. If you have the secondaries in series, then the feedback comes from the spot that is somewhere between the mid point of the two transfomers and the top end of it.

Unless, of course, you use feedback from the secondary like was done by Audio Research. Look up an early Audio Research tube amp schematic - it has been discussed here i am pretty sure.

Another option is to use lower level gnfb and use a loop feedback from the output tubes back to the LTP or farther back. Or even use a "balanced" feedback path to each tube of the LTP if you do a connection like Audio Research at the output iron...

I'd make a provision for triode strapping the outputs, I think it sounds better... watch the screen grid dissipation!

I think that 10 ohms on the cathodes makes for a bit of a "soupy" bass, but that is up to your ears to decide.

I think I recall reading that the 12AU7 is not a very linear audio tube... you might want to read up here on what people have measured for these various driver/front end tubes and maybe check that...

Btw, if you really have that B+ there, you're running the tubes pretty hard... watch the screen dissipation. I think I said that already.

Hope some of these comments might be useful...


deicide67 12th December 2012 02:11 AM

Hi Bear,

First, thanks for your commentary!

The schematic shown is representative when I was asking about how to configure the output transformer secondaries.

I started with a 6H30 VA and 6CG7 Phase Inverter. The 6H30 was nasty with oscillations and noisy as heck to boot.

I swapped it out for another 6CG7, and things are much better. I get approximately .35VAC input at 1Khz to clipping, so there is room for NFB.

I am not sure where I really want to inject it though. I am debating putting it back at the VA stage cathode like I see on traditional amplifier topologies.

I will try to draw out a final working schematic tomorrow or soon.

Regarding triode, I may do that on the faceplate with a switch.

The comment about the voltage. It is the same B+ as the Dynaco ST-70 for UL. I would never run the EL34 in triode with over 400V B+.

Thanks again!


bear 12th December 2012 03:52 AM

Today's tubes don't handle the B+ that the old ones do, and the screens definitely don't handle the same current. Not without shorter tube life.

There are lots of ways of doing tube amps. Pick one, and go for it.


deicide67 12th December 2012 04:00 AM

:) these are NOS RFT, so they should be good. I'm anxious to get the other channel built so I can hear it all in stereo. I will probably wait until I can get over to my friend's house to use his distortion analyzer to tweak the feedback.



deicide67 12th December 2012 04:03 PM


Here is an actual working schematic of what I have done:

I still do not like what the feedback does to the amp. I understand the illusion of less bass due to freq extension, but I am playing a song that has a few solid 30hz notes at the very beginning, and with no feedback, it's there. With it, it sounds extremely distorted and "out of breath". I injected it at the cathode of the VA stage before i swapped to LEDs. I'm also wondering about adding a few hundred uF of filter after the choke to "stiffen" the PS.

How do my voltages look? I just added the LED string this AM. A little more gain, but it sounds nice.


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