Marantz 8b feedback circuit

[mr2racer]

Hey All,

Anyone out there familiar with the 8b circuit? I'm now using Dynaco ST70 transformers on my 8b clone. I have two questions. First, is the 100ohm resistor between the EL34 grid and the ultralinear tap neccessary? The 8b used 44% taps where the ST70 used 33% taps. Can it be ommitted?

Second, there is local feedback between the EL34 output and the opposite side of the phase splitter input. Does anyone know the purpose of this feedback?

Kevin

[Eli Duttman]

The 8B has a dedicated, tertiary, winding for NFB. Also, you can go to the bank that Sol Marantz used better "iron", than that found in a Dyna ST70. IMO, all bets are off.

[dcgillespie]

I would most definitely leave the 100 screen stability resistors in place -- they will help maintain stability in the output stage, and minimize any chances of output tube arcing.

The cross-coupled feedback between the plates of the output tubes and the grids of the phase inverter work in conjunction with the other HF tailoring networks present to shape the ultimate response of the amplifier to suit the high frequency characteristics of the 8B's OPTs. Dynaco transformers are generally very tolerant of cross-coupled feedback. I would at least give the connection a shot, and then modify the values as necessary for best square wave performance and HF stability.

Eli's point is well taken though. The 8B's global NFB loop is taken from tertiary windings on authentic 8B OPT. You will need to develop your own loop from the secondary of the A-470 for best performance. If memory serves me correctly, the Dynaco catalog has a similar (but less elegant) type topology design shown (using a different Dynaco transformer however) that you could at least reference to help get you started. Good luck with it!

Dave

[mr2racer]

Thanks All,
I originally talked to "Doc" Hoyer about building some 8b OPT's. He said they would only be $600 each. He also told me that most of the NFB comes not from the tertiary windings but from the 4 ohm winding on the main secondary. He also said that what Sol was trying to do was limit high frequency ringing more than anything else. And that the tertiary windings weren't necessary. (Doc described Sol as a monkey at a typewriter with the 8b design.) Kevin Hayes at VAC told me that Doc had wound the OPT's for the recent Marantz reissues that VAC built for Marantz. When I told him I planned on using the Acrosound TO300s he suggested the Dynaco transformers would work much better.
As always Eli I value your opinion. I'm not trying to build an exact copy of the Marantz. But I think it will still be an interesting amp to build and listen to. And I have a friend in Toledo that owns two 8b's so I will be able to a-b them in the end.
And, DC your point is well taken. Now that I know what "Cross coupled feedback is called I can do some more research.

Kevin

[smoking-amp]

Looked at an 8B schematic and a 9 schematic and I only see the cross coupled capacitors on the 9? Looks like they are there to transition from global feedback to local feedback at high freq., where local would be more stable.

[ronaldw441]

Perhaps there were different versions of the 8b. The schematic at:

http://homepage.mac.com/rhuegli/Reso...ice_Manual.pdf

shows the cross couple caps.

[SY]

Those caps extend the open loop response. That may or may not be needed in a new circuit, depending on the output transformer ($600 each seems a bit rich- you can buy some pretty excellent COTS for considerably less). I would agree with dc, keep the screen stoppers.

Basically, it's a Mullard 5-20. You can modernize it a bit by using matched plate resistors in the phase splitter, then a CCS in the cathode circuit to ensure balance. The tertiary winding is nice, but you can do just as well with a more conventional feedback arrangement- I'd run the feedback from the tap where your speaker is connected.

[beetel]

Marantz 8B Replica by Satoru Kobayashi
702kobayashi1817

http://www.triodeel.com/dynamull.gif

[mr2racer]

Thanks All,
The plan is to keep the grid stoppers. Keep the cross coupled local feedback. And start with the original feedback design where it takes off from the 4 ohm tap. I'll see what that sounds like and go from there.
Kevin

[mr2racer]

Hey All,
I was hoping to finish this amp tonight and I find I only have two .22uf caps for the bypass on the cathodes of the EL34's. I do have four .1uf caps. I checked Morgan and his section on cathode bypass seems more aimed at very low frequency. Where these tiny caps are more about passing high frequencies. Do I have this right? And would it be alright to use the .1ufs instead? Or should I wait for .22ufs?
Kevin