Bogen MXM Direct out Mod (attached schms in reply posts)

[Rompstomp89]

I just noticed you have a 620 ohm resistor across the grid of the EF86. The probably explains your low gain. Do you have the T157 input transformer fitted?? If you do then it needs loading with a much higher resistance, like 150K or more.

Cheers

Ian

I ended up changing that the other day because I had suspicions that it would drop gain (and it did).  The schematic shows a 1M and I think I originally put the 620R in there to match with  the other channels gain wise.  I put a 1M back in there and it got better.

 

[Rompstomp89]

There's a table at the bottom of this page that specifies the various part values of an EF86 RC coupled amp gain stage. The Vout/Vin column is the gain you get with the values specified on that line. I think you were running B+ at about 350V, so look at the row with Vb at 350. Check your part values against those. Pay attention to Rg, which is what the grid leak resistor of the following stage must be for proper loading.

Meant to post this yesterday but time got away from me.

A few days got away from me as well. Thank you for posting!  That handbooks looks to have ALOT of great info.  Whats the section under the first section on the page you posted?  I see that there are 2 350v options for me to go off of.  My current comp choice seems to match with the second 350V column. Thanks for dealing with my newbie questions!

Romp

 

[Rompstomp89]

I think I recall that he mentioned the T-155 rather than the 157, Ian. Same transformer with a 200 ohm primary rather than 50.  Loading requirements should be the same. I've used the 155 on another (non-Bogen) pre I built and loaded it with 256k. Performance is good with that load. I'm pretty impressed by all those silver-can Bogen inputs. Not so much the later black ones.

BT

Yeah, I have the 155s.  I have already mentioned this in a later reply but I ended up changing it back to a 1m.  I put a 1M pot and swept across the resistance and from basically from 100K to 1m there was no difference in gain or freq from what I could see.  The 620R I had in there was defiantly was a bad decision. 

Romp

 

[rackmonkey]

A few days got away from me as well. Thank you for posting!  That handbooks looks to have ALOT of great info.  Whats the section under the first section on the page you posted?  I see that there are 2 350v options for me to go off of.  My current comp choice seems to match with the second 350V column. Thanks for dealing with my newbie questions!

The two sections are two different tables that tell you the gain you end up with at a given B+ voltage with the part values listed in that row. The first/top table shows configurations that yield lower gains, and the second/bottom table shows configurations that yield higher gains. If you are after more gain, try part values closer to the ones in the row for 350V Vb (B+) in the second table.

 

[johannpolzer]

Hi there! Pushing this Topic back to life!

I have a Bogen MXM and in need for T155 Transformers for the Mic Inputs.
Is there anybody out there that could help me out?

Best Regards
Johann