new design/build im calling "5F2-Ac15-RVT"

[tony321owen]

In the past couple years, with much research and help from the nice folks on here, I designed/ built a few amps that I really love to this day! One was based off the 5f2A tweed Princeton but with tremolo and a few other modifications/ customizations. Another was an Ac-15 top boost circuit with reverb and tremolo built into Princeton reverb style chassis and cabinet for a friend of mine. I loved it so much that I didn't want to let it go! However I was also happy that I came through for my friend and that he likes it enough to be taking it on the road with him. To this day I don't own an amp with EL-84s and so I have decided on using one of my older builds as a donor to convert into a new design. I am always thinking up ways to blend classic circuits to satisfy my hypothetical tone chasing dreams and sometimes when I actualize them they become cherished instruments!  Here is my design in progress which blends the 5f2-a preamp section with an AC-15 inspired power amp section. It also uses a one tube reverb circuit that I found on the forums here as well as power tube bias vary tremolo.

Next is to design my layout which brings up a few questions for who ever wants to chime in!

1.Is this layout logical for a 4 tube preamp section or should I reverse the tremolo and reverb?
components would also be laid out on turret board in this order.

V1:1st and 2nd gain stage
V2:one tube reverb
V3:tremolo
V4:LTP phase inverter

2. Is there a reason not to put the Dwell control on the front panel next to the reverb knob?

3. Feel free to add your two cents about gain staging or any design tips/opinions

[PRR]

> add your two cents

Putting the reverb INside the NFB loop seems wrong somehow. NFB will try to reject the reverb because it is not in the input to the loop. But time and causality mean this won't happen. I dunno what will really happen. Odd little distortions? Back to the future? Let us know if you try it.

I don't think the 2nd stage needs to be inside the NFB loop. In fact three stages under the loop is difficult to stabilize.

[echuta13]

I personally would deep-six the NFB.  It would be a bit more Voxy that way.   

[tony321owen]

Thanks PRR! Thats the kind of thing some one at my level will tend to overlook. Super helpful! I thought variable NFB could be a nice way to tame the highs and give me the option to tighten things up a bit like it does on my tweed princeton type amp i built but now it seems to add complication to the design... I don't think ive ever seen NFB injected into a LTP phase inverter often in any of the schematics I have read.  I may just cut it out like echuta13 said and add a Vox cut control instead. That vox chime is pretty sweet just gotta have a way to tame it a bit in my opinion! Almost finished my layout today. will post soon with a revised second draft schematic as well. I was pretty careful but I think theres a couple of slight errors in the first one I posted!

[DummyLoad]

attach NFB to your LTPI as a presence control and add in/out switching - get the best of two worlds, variable NFB or no NFB.


--pete

[PRR]

....I don't think ive ever seen NFB injected into a LTP phase inverter often in any of the schematics I have read....

5F6a runs NFB to the LTP. It does it in an odd way, to the bottom of the tail at the offside grid cap. The "right" way, to the offside grid, is seen in most tube op-amps and a few hi-fi power amps.

[tony321owen]

The presence control sounds intriguing! Would the values be the same as the 5F6 for an amp like this that only runs at 15 watts or so?

It seems logical that negative feedback loops need to be tweaked to match output levels at the speaker jack.

Also if I am not mistaken the presence cap could potentially bleed off different feedback signal frequencies depending on its value...
Bigger the value, the more lows along with highs to ground and the more  it acts as a normal variable feedback.
    On the contrary if the cap is made smaller only highs go to ground so it would only cancel out highs in the output leaving lows and Mids untouched...

Does it work cleanly in the circuit where with presence at 10 no feedback is applied or is a switch really necessary to get the no feedback sound?

Im running out of panel space on the front and back in my little princeton reverb chassis so the extra switch may be hard to pull off...

Anyone have a recommendation on circuit values for this particular build?

 
 

[shooter]

need to be tweaked

Wire up Version 1 with the intent of at least 3 mods, what floats my boat might sink yours, tinker til you got a pema-grin 

[thetragichero]

i like nfb on a switch for vox-y type things. my main amp i play at church is pentode preamp (12au6) long tail pair (12ax7) into push/pull 12v6gt (12v heater version of 6v6... ebay-find torroid power transformer with 12v heater winding originally intended for a record player apparently). just volume, vox cut control, and loose/smooth (nfb) switch. can be vox-y, can be fender-y. definitely would suggest giving it a try

[tony321owen]

first draft layout..

[Willabe]

I haven't look at your draft, but, you might need this at some point with a new build.

Grounding.

http://www.valvewizard.co.uk/Grounding.html

[tony321owen]

Thank you for this Willabe, I bought merlins book a few years ago and it has been a huge help in my builds/designs! However on my last build I was struggling with implementing his grounding scheme.. If I remember correctly he grounds everything including the power amp to one point by the input jacks.. I feel like in the past I have used his scheme successfully but when I was getting noise on my last build someone here gave me the advice to try hoffman's  grounding scheme with separate preamp and power amp grounds. I followed it religiously and it worked well! I guess with merlins method I always wondered whether I would be better off with short ground leads off the pots that connect to the bus bar right next to each pot or whether a longer ground lead that terminates at its local stages cathode cap would be better. Seems like theres a compromise between short leads and grounding locally to the stage that the controls are part of?

[Willabe]

Read that link. 

[tony321owen]

Good stuff here! It's so easy to overlook the resources that are right there the whole time. Hoffman has such neat layouts, filter cap placement options, and just good general amp building knowledge!