SE EL34 & Medium-Gain 6N2P schematic review request

[boodle]

I am putting together a SE EL34 amp based on Merlin's Medium-Gain preamp.  Hoping some would review the draft schematic for any glaring issues.

What I am hoping for input on:
1. Inter-stage blocking cap values after V2a.
2. Anyone with experience on either the Bone Ray or Voigt tone stacks.  Considering both as they are both supposed to be flexible (and my mother's maiden name is Voigt!)  I plan on breadboarding both a lower voltage to get a flavor, but knowledgeable experience is welcomed.
3. Does the Trim control look like it will serve well before an effects loop.  I may decide to not implement the effects loop.  Not sure at the moment.
4. Does V2b look well designed to either be a recovery stage for the effects loop or just the tone stack? If no effects loop, will I be driving the EL34 too hard at low MV settings without a R Div resistor shown in the schematic?

I'm sure I am missing something, so please have at it.

Thanks for any and all help!

[kagliostro]

I don't know wich will be the right values to be used to have a Voight or Bone Ray Tone Control on the Merlin's Medium Gain Preamp but our friend Dwinstonwood modified the values of a standard AB763 tonestack to adapt it to Merlin's Preamp

https://el34world.com/Forum/index.php?topic=30681.msg338118#msg338118



BTW at DIYItalia a guy is modding a Geloso G1 1020 A to use that preamp and I've draw a Layout for it

if you are interested to the (Merlin's Middle Gain) preamp section Layout, here it is

(Use an insulated female Cliff Input Jack - Note that we use 12ax7 so if you plan to use 6N2P you must modify the wire connections to the tube pin)



Franco

[boodle]

Kagliostro,
Once again, you are the man!  Thanks for your input.  I will consider that tone stack.  The layout looks like about 1/2 the board shown would be applicable to me?  I have found another layout that you did for Merlin's medium gain that I am referencing.

Appreciate the help!

[kagliostro]

The preamp Layout stops at the input of the Tone Control so your interest is till that point (in the Layout the Tone Control is the two knob version planned by Mr Merlin)

Do you want the .vss .vsd file ?

Franco

p.s.: which other Layout (by me) are you referring ?

[dwinstonwood]

Hey boodle, cool project! This is a really nice preamp. As kagliostro stated, I built it with a scaled Fender TMB. I mated mine to a cathode biased PP6V6 power amp with about 360VDC on the plates.
FWIW, here is a PDF of my schematic and layout.
Keep us updated.

[boodle]

dwinstonwood,
Yes, your schematic is in my info folder already!  Very nice project!

kagliostro,
The layout I was referring to is attached.  I believe it is yours.  I have been using DIYLayoutCreator for my layouts.  Would the .vss upload to that program?

Also, to anyone, am I overdoing the pi filters on the B+?  I want a silent SE amp, but happy to have fewer pi's if that will get me there.

[kagliostro]

No, not my layout

The .vsd file is a Visio file

Franco

[boodle]

kagliostro,
Got it.  I will use both layouts as I go forward.
Tim

[kagliostro]

My layout is based on a 40mm tall Board because it is to be used on the Geloso 1020 in vertical

but if you want you can use it in the usual way

Franco

[pullshocks]

I had the Merlin Medium-gain Preamp wired up going into a SE 6V6 power amp for a while.  The 3 triodes were plenty to drive the 6V6.  I did not use the 4th triode.


I liked the sound but I thought the "Bandmaster" tone stack acted a little weird with the bass above 6-7.  Tone Stack Calculator In the Web seems to confirm what my ears were telling me, with drastic changes of the response curve at higher tone settings.


So I think you are right to try other tone stack options.  I look forward to seeing/hearing your results.

[boodle]

pullshocks,
I was reading some other "Merlin Medium-gain" threads and saw that someone wasn't a fan of the Bandmaster stack.  [Likely you were that someone!]  It was those posts that had me looking in Merlin's book for other options. 
I saw a tubeswell post that said the Bone Ray was very good, so I will likely start with that one.

Since you have built that pre with SE PA, I would love your thoughts on whether I am over doing it with the power supply filtering.

Tim

[pullshocks]

Yes, I did post about my experience with the preamp.  Updating my 5 watt SE amp with Merlin's medium gain preamp (el34world.com)


As far as your power supply, I am by no means an expert.  My main comment is you have the plate voltage coming from node A.  To my knowledge, noise can be reduced by inserting an extra node before the plate voltage take off.  All the schematics below do this.


I used the AX84 5 Watt power amp choke filter option, but with a tube rectifier.  Many say there is no point in using a tube rectifier with SE, but I wanted the option to use different 5y3, 5v4, 5ar4 to get different voltages.  Because of the tube rectifier, I have the first filter cap as 33uF.


With SS rectifier first filter cap can be higher, as on Dummy Load's 6L6GC SE amp see attached schematic.  Edit--all the attached  schematics are ss rectified and use higher value first filter caps.


I see you have the heaters elevated via your node F.  The AX 84 uses a similar approach at an earlier node.  Others that know more about this may comment on the merits of which node to use.  The Ax 84 5W schematic works for me.  However the AX84 SEL Single Ended Lead (KT88) elevates the heaters via the power amp cathode

[boodle]

Great stuff!  Thanks, pullshocks!

[boodle]

Update...input welcomed!

I have modified the schematic as follows:
1. Will start with Bone Ray stack
2. Will have FX loop with V2b for recovery
3. Moved volume control after V2b and right at the EL34 grid...this seems to be a move that carries some doubt on the forum, but this will allow the Trim control to also function as a 2nd Gain control and allow V2b to distort.  Please advice if this is foolhardy.
4. Modified the PS filtering to have an extra Pi before going to OT and EL34 plate.
5. Moved the PS tap for the heater faux CT because it was an easier location in my proposed layout.

This should all fit fairly comfortably in a 10X5X3 Bud chassis that fits nicely in a 1-10 pine cab that has a tweed-style top mount control layout.  The control panel layout is full but still comfortable.

Attached: new schematic, draft layout, draft control panel
Note-on the control panel, the large circle around the gain and volume controls are the diameter of full sized pots, the square is a lighted power switch, the input circle is the diameter of the plastic nut of the input jack.
Name of the amp is still under consideration: Full Gainer, Thunder Girl, and Beastie are currently in the running.  Suggestions welcome.

[I fixed a couple of grounding issues on the layout since the initial post.  pdf replaced with corrected version.]