Stand alone reverb project - dry signal attenuation issue

[casey73]

I put together a stand alone reverb with a twist on tradition. Wanting to try something different and avoid the use of a 6V6, I lifted the 12AT7 reverb driver used in most Fender amps, and married that up with the split/mix arrangement of a 6G15 stand alone reverb. Surprisingly, it works better than I expected. I need to do some fine tuning, but there is plenty of dripping wet reverb available. The dwell, tone and mix controls work pretty well.

The issue I'm having is the dry signal drops quite a bit just going through the unit. It taps off the input of the first 12AX7, goes to a cathode follower and to the output via the mixer pot. All the research I've done says this is a common issue the the Fender 6G15 also.

The single triode section of the first 12AX7 provides enough gain to drive the dual 12AT7 sections. I read up on cathode followers and the most I one can get is unity gain (actually a tad less). I was thinking about using the extra triode section to bump up the dry signal a little before it hits the cathode follower to compensate for the loss of dry signal overall. Before I go to the trouble of adding that section in the chain I'd like to know if there is a better solution to this. I've read that the advantage of a cathode follower is the low impedance output, so I'm thinking it would be best to put the extra gain stage in front of the cathode follower. Am I missing anything obvious here?

[loudthud]

The cathode followers' low output impedance really isn't taken full advantage of in the 6G15. The main function of that stage is to prevent any of the wet signal from re-circulating back to the input. The reduction in the dry signal is mostly due to the mix control on the output. An easy way to add a little gain would be to convert the cathode follower to something that resembles a concertina phase inverter but instead of a pair of 56K resistors has a 33K in the cathode and a 68K in the plate. That will give you a gain of about 2. You can use the standard 1.5K resistor to establish cathode bias (like the 5E3) or use a fixed bias arrangement like the Paul C mod with something like a 2.2Meg to ground and a 10Meg to B+.

[casey73]

Thanks! I'll give it a try.

[casey73]

I set up the dry signal pre-mix triode in a cathodyne arrangement as you suggested. And it did provide a gain of about 2 as you stated. However, the signal drop is still very pronounced. It appears as you stated, most of the signal loss is at the passive 250k pot mixer before the output. I've also noticed that even with the mix control full counter clockwise for all dry and no reverb, I'm still getting reverb. I'm going to try a 500k pot for the mixer to see if the reduces dry signal loss and helps isolate the wet signal from coming through on the dry side. I also thought I might use the extra triode section as an active mixer on at the output. Any thoughts on that?

[loudthud]

Hard to implement the Mix control unless you want to use a dual pot, but an active mixer stage should give more isolation of the dry signal.

[casey73]

The dual pot idea is a good one. Wire them opposite each other, correct? The 500K pot did nothing the help. So I'll try a little mixer with my left over triode section.