How to add EF86 channel to Cathodyne PI

[Shamadan]

HI. I am in a process of making another amp. I am trying to combine Tweedle dee with EF86 plus 1 tube reverb (+ some other mods)
Sorry for such a noob question but what is the best way to make it work

Thanks in advance for suggestions
Will this work?

[menger66]

I think Weber has ef86 add on kit there might be a layout on his site

[Shamadan]

Weber changed website design and I can't find this add-on. I have seen lots of schematics with ef86 but none with 2 channels and cathodyne.

[roberto]

It will no more be out of phase with the other channel, but you can simply use a passive mixer made by resistors like Marshall did on his 1959 to mix the two channels, just before the PI (use two 220k or 470k to taste).

[sluckey]

Most EF86 preamps I've seen connect to a LTP PI which has gain. Your cathodyne has a gain of slightly less than 1. You 'may' need to mix the EF86 preamp with the other preamp at the same place the other volume controls connect in order to get the AC-15 (or similar) sound. This would give you some extra gain, but, the 5E3 tone/volume controls will interact with the EF86 channel. I don't see a simple way around that. Maybe the EF86 preamp will have enough gain for you even going straight into the cathodyne.

[tubeswell]

What Steve said - I'd also be more inclined to go with an LTP inverter. (You can steal the triode from the 'tone control recovery' stage.) That way your amp's tweed channel will resemble a 5G9 preamp - (which is cool). The EF86 can simply connect to the other LTP grid (in the same way that a Vox AC30 LTP is used to mix the channels).

[Shamadan]

Wow thanks for great ideas. I will try to connect channels to one triode and minimize vol and tone interactions using pots with spst and spdt switches. I would like to try 5G9 idea but than reverb won't work on all channels.

[sluckey]

You need a coupling cap just to the right of the junction of R2 and that 150K resistor to prevent your added circuit from affecting the grid bias of that cathodyne. The cap will also keep dc voltage off the reverb and volume pots.

[Shamadan]

Thanks Sluckey. Schematic updated. I connected all chanells to one triode. Do I still need mixing resistors?

[tubeswell]

Thanks Sluckey. Schematic updated. I connected all chanells to one triode. Do I still need mixing resistors?

You don't 'need' them, but without them the AC load on the prior gain stages will be higher (which means that you will attenuate more signal), and when any of the volume controls is cut to 'zero', then everything will be cut to zero.

[Shamadan]

Thanks. Looking at this problem there is no easy wayout.
1.If I use mixing resistors than I may lose some high end due to the input capacitance of the following stage (see the Miller effect) - http://www.valvewizard.co.uk/resistive.html
2.If I don't use them gain would be lower and would have volume interactions. (I want to wire them as 5E3 so if I have volume on 0 on one channel it wouldn't cut signal on other channels)

So If I use 2M Volume Pots will this fix load problem?

[jazbo8]

1.If I use mixing resistors than I may lose some high end due to the input capacitance of the following stage (see the Miller effect)

Not sure if C_Miller is a big concern here... How about the following resistor mixer? It should provide enough isolation between the various channels, no?

https://dl.dropboxusercontent.com/u/132 ... 20edit.JPG

[Shamadan]

Thanks. I would try it as well. I would finish this amp in a few weeks and will let You know about final results. Maybe some audio samples.

[roberto]

If you go straight to the cathodine, being its gain less than unitary, you have no issues with Miller effect.

[Shamadan]

Cool I will try it as well.