custom circuit -- proofreading needed --

[mort]

This project is basically a blackface super reverb without the verb and vibe. The normal channel is left as a clean channel and the other channel has an EF86 in front. Someone helped me a little with implementing the EF86 into the circuit and now I'm ready to start assembling the actual amp. Before getting started just wanted to run this drawing by the forum and see if anything can be picked up as a mistake or oversight. Thanks in advance for any help.

[Malcolm Irving]

The square box with 'A' in it seems odd, unless I am missing another 'A' somewhere in the diagram. Should be a resistor to ground (possibly with a bypass cap in parallel)?

[mort]

I was just noticing that as I'm going over and planning my layout. I forgot to add the connection to a cathode in the drive channel. Here's an updated pic

[Mike Sulzer]

I was just noticing that as I'm going over and planning my layout. I forgot to add the connection to a cathode in the drive channel. Here's an updated pic

Why are you tying those two cathodes together rather than using separate bias resistors and bypass capacitors?

[mort]

Why are you tying those two cathodes together rather than using separate bias resistors and bypass capacitors?

That's actually just carried over from the original Super Reverb drawing.

http://www.thevintagesound.com/ffg/s...a763_schem.gif

[MarkusBass]

EF86 used in such a configuration may be microphonic. What is the proposed layout of the amp? If you run into trouble with the tube, I may have a solution for you .

Mark

[mort]

It's going to be a head, situated similar to a marshall. The front controls will be inverse from the standard blackface super(normal channel all the way to the right of the chassis when looking at it from the front). The EF86 tube socket will layout very close to its connections on the eyelet board. Was planning on using shielded input wire for that channel (or for both really).

[Gingertube]

Fidling at teh edges BUT since it is a new build you may like to consider adding a few things which are sometimes needed during set to work to fix parasitic oscillations, fine tune the tone etc.

I would add at least 4K7 grid stops on every 12AX7 and 12AT7 triode grid, including the cathode followers and the phase splitter.

Definitely use separate bias for V1B and V3B. V3B being in the dirty channel may need smaller bypass cap to trim some bass to avoid muddyness. Maybe 1K8 bypassed by 1uF for that and 1K5 bypassed by the 25uF (or 4u7) on V1b

For reliability of GZ34 add 1N4007 in line with each anode feed (SS diode anodes to the transformer leads and cathodes to the GZ34 anodes).

10nF 2kV ceramic across the standby switch.

6L6GC grid stops increase from 1K5 to 4K7.

Just spme ideas to consider or ignore as you choose.

Cheers,
Ian

[Dave H]

I'd ditch the standby switch.

[Tom Phillips]

I can't tell by your schematic symbol exactly what is going on at your speaker impedance switch but you should not switch the feedback connection. Given the component values shown in your feedback circuit I recommend that you connect the feedback line to the 4 Ohm secondary tap and leave it there. It is OK that the 4 Ohm tap remains connected to the feedback circuit when your speakers are connected to one of the other taps. In fact, it is required to maintain the correct amount of voltage feedback.

If you would like to simplify the input circuitry consider if you really want the #2 attenuated input on each channel. If you never use the #2 then you could go with a single input jack for each channel.

+1 on separate preamp cathode components and ditching the standby switch.

Cheers,
Tom

[tedmich]

Many say an EF86 sounds best at B+ ~250v feeding into a cathode follower. Jukka at Ax84 had some ideas you may find interesting depending on how dirty you want it.

[Richard]

Why is ditching the standby switch desirable?

[tedmich]

Why is ditching the standby switch desirable?

I don't know Dave's reasoning but many people think that standby switches are more bother than they are worth, Merlin Blencowe for one:
The Valve Wizard

[mort]

In this case that stand by switch is already dyed in the wool

[Tom Phillips]

In this case that stand by switch is already dyed in the wool

Ah...But "stfu" can be accomplished other ways than the traditional standby circuit which interrupts the high voltage power rail.