Mixing 5F6A/Plexi channel with Blackface channel

[Sonny ReVerb]

I'd like to resurrect a thread that wasn't really solved, mainly because I am planning a build that is virtually the same and I'd like to understand the theory involved.

http://music-electronics-forum.com/s...ead.php?t=9235

The concept is replacing the normal channel of a Blackface style amp with a 5F6A/Plexi channel and mixing them prior to the PI. Hasserl came up with a work-around for his application, but was unable to find a solution to the original problem.

I'm not modding a Bassman, but starting from scratch. My design uses a 5F6A/Plexi channel, a Blackface channel with reverb but no trem, and a 5F6A style PI and Power section. Like this:



It seems that Hasserl's design may have had loading issues with the TS directly feeding the mixing resistors resulting in loss of gain.

From another forum:

Typical tone control circuits like to see a fairly high impedance at the input of the following stage.

and from R. Kuehnel:

The tone stack is driven by a 12AX7 cathode follower circuit with a very low output impedance capable of providing a source voltage that isn't easily dragged down by a varying load. It drives a long-tailed-pair phase inverter with a very high input impedance. These two factors, a low impedance source and a high-impedance load, make the tone stack's frequency response relatively independent of the preceding and following stages.

Normally, the 5F6A/Plexi TS directly feeds the PI which has a high impedance input. Would adding a MV 1M-A pot between the TS and the mixing resisitor 'unload' the TS? Example:

http://www.lungcookierecords.com/ran..._Schematic.pdf

I thought about adding a blocking cap between the TS and the mixing resistor, but there's no DC to block, just AC signal. It seemed to work for Chocopower though:

http://www.thegearpage.net/board/sho...07&postcount=9

Another thought was to mix the channels just prior to the reverb recovery. The added benefit of that is the signals would be in phase enabling bridging of the channels if desired.

This type of design seems very popular. Any thoughts or ideas would be greatly appreciated.

[Don Moose]

Note that the Chocopower design has the gain-recovery triode after the BF stack - which yours lacks.

If you add that second triode after the BF volume, the two should be pretty well isolated - better still of you make those mixer resistors 1M - that should minimize the CF master's ability to interact with the BF's output.

After that, you may want to raise the values of the PI plate resistors to make up a bit of the gain lost in the mixer.

**Half-remembered idea - IIRC, the two PI grids can both be considered inputs to the PI - you could send the BF preamp to one and the CF preamp to the other ... if having the NFB connection there doesn't foul that up; and if I remember that right.

Hope this helps!

[Sonny ReVerb]

Hi Don,

Thanks for the reply. I see you're also from Indianapolis!

Note that the Chocopower design has the gain-recovery triode after the BF stack - which yours lacks.

My design has a gain recovery stage after the stack on the BF channel. It goes GS>TS>GS>Mixer just like an AB763 Vibrato channel.



It doesn't have the 'extra' gain stage that the AA270 Bassman has, though. The problem is with the 5F6A/Plexi channel not having a recovery stage after the TS.



A recovery stage would certainly be a solution, but at the cost of another 12AX7. I could probably use the other half for an Overdrive boost, but I was trying to stay somewhat true to the spirit of each original channel.

As far as the inputs to the PI, it is possible to run one channel on each 'leg' but I was trying to stick with a somewhat proven design

[loudthud]

Why not feed the 5F6 signal to the grid of the tube that receives the signal from the reverb control? Use another 3.3Meg resistor to provide isolation.

[Sonny ReVerb]

I've been thinking about that quite a bit. That is where I'd like to bring in the 5F6A channel, but the reverb return also ties into that point. I thought that might be an issue.

I have seen the 'reverb on both channels' mod before.

http://music-electronics-forum.com/s...ead.php?t=3199

http://www.silvertoneamps.com/micKfe...-Channels.html

I think that might be worth a little experimentation.

[Don Moose]

Sorry - missed the recovery stage.
I like 'Thud's idea.

[hasserl]

Sonny, you've read my thread and so you know what I went through with that amp. In the end it actually worked out well with both channels going into the 3rd (recovery?) stage driving the voltage back up after the 250k mixing resistors. Now I still don't understand why that was necessary. I have modified another Bassman amp turning the Bass channel into a "Marshall" or 5F6A channel, and it worked just fine; I had no issues with voltage loss across the 250k mixing resistors. And I built another amp that has a 5F6A channel and an AB763 channel into a 5F6A power amp. Again I mixed the two channels with 250k mixing resistors following a straight 5F6A circuit and a pretty straight forward AB763 channel minus the reverb but still using 3 gain stages. I just had to attenuate the voltage between the 2nd and 3rd stages. But I needed no recovery stage after the mixing resistors for it to drive the PI properly. See attached schematic.

So, I don't know why I had a problem with the one Basman mod, and I've built very similiar circuits without that problem, and of course I'm not the first one to build a 5F6A channel into a Bassman head Bass chanel, it's actually fairly common mod, and I don't think others had the same problem. So I'm still at a loss as to why I had that problem, but I don't think you need to be overy concerned about it. If I was in your shoes I'd go ahead with the build and expect to not have any problems. Well, with low voltage like I did. You might have other problems, like with any build.