New Standel 25L15-ish Tube amp with compact chassis

[kingplank]

This is an unusual project. After using a DIY Dark Terror amp and not happy with the tone, I have decided to chanhe it into a compact Standel 25L15-ish amp; replace the preamp mainly and retain the power amp and using as much as the power supply as possible.
The Dark Terror actually does operate quite well; runs very cool, very loud; its just that the tone is not aggressive as Triple Rec or 5150, ENGL; etc.
I did build 2 Standels before, quite happy with ethe results. So why not the third one with EL84s?

I would like to hear your comments with my schematic and layout.
(HBP with probably cringe at this idea.)

[Williamblake]

Is the tone control working by aplying negative feedback through the cathode resistor?

[HotBluePlates]

This is an unusual project. After using a DIY Dark Terror amp and not happy with the tone, I have decided to chanhe it into a compact Standel 25L15-ish amp; replace the preamp mainly and retain the power amp and using as much as the power supply as possible. ... (HBP with probably cringe at this idea.)

How can I cringe if I don't see it? 

Your power supply lifts the original's plan of providing a lower voltage to the screen; why would you want to do this with an EL84?  At a minimum, it will lessen plate current which means the existing cathode resistor might not be the right value to bias the EL84s now.  I'd recommend a good bit of thinking on whether to do that.

The Mid control (P8) needs to be in series between C4 and ground to work properly, rather than in parallel with C4.

Is the tone control working by aplying negative feedback through the cathode resistor?

The Treble & Bass controls sit within a feedback loop from a plate output of the 12AU7 to one side of the 12AT7.  This feedback signal is mixed with signal in the other half of the 12AT7 at the shared (unbypassed) cathode resistor.

[PRR]

> Is the tone control working by aplying negative feedback through the cathode resistor?

[kingplank]

Thanks HBP for your valuable input.


Reverting the power supply back to the Dark Terror original .
Question with choke: the Dark Terror uses a 150R resistor; and the Jim Root version uses a choke instead. Is it work the trouble? since space is very limited(see picture)
Thanks

[HotBluePlates]

... Question with choke: the Dark Terror uses a 150R resistor; and the Jim Root version uses a choke instead. Is it worth the trouble? ...

That's a judgment call; power supply filtering is usually better with a choke.

There's some space taken up by the perfboard in the photo that could be used for a choke if things were mounted differently.  But your schematic also shows 2 parallel filter caps right after the rectifier... If hum was an issue, inserting a resistor (maybe 150Ω) between the 2 caps will help.

Separately, the 25L15 doesn't distort even when the Volume control is wide open.  I haven't investigated it in detail, but I figure that could be because the phase inverter won't throw a peak output as-big or bigger than the bias on the output tubes.  That may not still be the case when using EL84 (their required bias voltage & drive signal is much smaller than 807/6L6).  This may matter (or not).

[tubenit]

Why not try the same thing with JJ 6V6's instead of EL84's?

With respect, Tubenit

[kingplank]

I have already build a Standel with 6V6 about 6 years ago.
I was built from a Hammond M3 chassis, complete with working tubes and transformers.
All other parts are new.
I use this amp regularly, alternated with my other Standel with 6L6.
Cant wait to get the  EL84 version working

[kingplank]

Updated schematic, with power supply consistent with Jim Root Dark Terror.
I would really like to use a choke, and the most convienient place is on to of the upright style)chassis on top od the preamp section. Is it defeating the purpose here being close to the preamp?

[HotBluePlates]

... I would really like to use a choke, and the most convienient place is on to of the upright style)chassis on top od the preamp section. ...

That perfboard in the photo takes up a lot of space.  It would occupy less if it was on L-brackets so it stood up vertically in the chassis instead of horizontally.

I have no sense of the space you need for a choke, or how much room is over on the side of chassis with the perfboard.  But you could mount the choke inside the chassis on the side-wall over there by the power wiring, especially if that perfboard was vertical & out of the way.   

[kingplank]

Wondering if it is possible to add a passive effects loop, before the inverter? Will there be enough gain on the return side?

[PRR]

> enough gain on the return side?

No.

[HotBluePlates]

Wondering if it is possible to add a passive effects loop, before the inverter? ...

Why do you need the loop?  There shouldn't be any significant distortion in the preamp, so there shouldn't be any difference placing time-based/reverb effects before the input jack or before the phase inverter.

If you get any distortion at all, it will be due to overdriving the EL84s, and a loop won't help you out with that.  You'd need some kind of load/re-amping device to place effects after the output transformer.

[kingplank]

Sounds like passive effects loop is not a good idea here; no problem.


Coupling caps: I want to reuse some of those expensive (in Canada) Solen caps, but some of them not exact match. Other than changing bass response, will there be any other ill effects?

[shooter]

any other ill effects?

only if they're leaky, got damaged solder/unsolder/resolder

[kingplank]

Hi folks,
This project is a success beyond expectations.


Beautiful sounding amp; and as expected, it sounds a little brighter that my other 2 Standel copies with 6L6 and 6V6.


Connected to the Celestion V30: Good all round tone, but not too Standel-ish


Connected to JBL D130: definitely got the Standel vibe, more bright and chime than the 6L6 version. Very surprised that it can push the bass down to A1 (55hz) on a baritone guitar


Connected to JBL D123: this is probably the best suited for this amp, got that Stanel creamy smooth mids, high chime ( can be turned down) , and clean , tight and solid(no fart) bass all the way down to A1.


Overall, it has quite enough clean head room, can cut thru the mix with backing tracks.


Also include schematic with voltage measurements

[HotBluePlates]

Congrats on a successful build!

[kingplank]

Since there is an unused tube socket here, I am tempted to add an effects loop. Can anyone suggest a simple schematic? Very tight space there.

[tubenit]

There is a rather extensive collection of on board FX loops in ARCHIVES:  http://el34world.com/Forum/index.php?topic=10208.0

You also might be able to consider a "one tube reverb" IF you have interest in that?
http://el34world.com/Forum/index.php?topic=7957.0

Is the V1 tube, the "unused" tube?

With respect, Tubenit

[kingplank]

Thanks tubenit
Wow, so many choices. I think the series/parallel is a good option , dual gang pot is good too due to space limitations.
Yeah, v1 is unused, so is v2b
Gerald

[tubenit]

OK, some thoughts for you to consider.  I am NOT saying you should use any of these ideas just giving you something to ponder.   

With a clean tone amp, I like reverb as well as delay.  With an overdriven amp, I prefer delay & will use an FX loop usually active (but also used passive with success).

IF I am understanding you, it appears you have V1 and V2a available?   IF so, you could consider adding reverb using V1 & V2a.  (You could also just use V1 for a "one tube reverb").

When I first starting adding "one tube reverb" to amps,  I took a known layout design and stuck another tube in a V1 position and then added to the layout board on that V1 side of the board.  It worked fine.  You have to use shielded wiring to go to the insertion points especially IF they are long runs.

So, you could consider using terminal strips OR make a small layout board to add into your already packed amp chassis.  See photo for idea. I have mounted small boards on the side of the chassis and have had that work OK.

So, look at the schematics for the Fender style reverb (on the right) using the 3.3M insertion point resistor  VS. the reverb on the left which has a smaller sized insertion point resistor.  I like the one on the left because it doesn't attenuate the signal as much.  In fact, if you follow the signal chain in your schematic, you could use the 270k resistor as an insertion point (A) OR add a 220k to 470k resistor right before the phase invertor (B).

Just ideas to consider. Not pushing an agenda for you.

With respect, Tubenit

[tubenit]

Now if you wanted an FX loop, I think you could probably take a similar approach and use V1 tube.  Make a small board or use terminal strips and use shielded wiring where needed and especially on long runs.  You'd use the chassis back panel near the V1 tube to mount FX send and return jacks in a vertical position.

The other option is to use the Metro Zero Loss FX loop.   https://store.metropoulos.net/products/zero-loss-fx-loop-kit

I might be inclined to go that route because of how compact it is and it has excellent reviews.  You'd remove the V1 socket and install it in that position using that area of the back chassis panel to mount it.  You'd use shielded wires to the insertion point (which would be insertion point B) right before the phase invertor.

Again, another option to consider.  Personally, I think of a Standel amp as a clean tone & would probably do the reverb instead of FX.  However, with EL84's in your amp ……….. maybe an FX and delay or reverb in the FX loop would be "better"?

With respect, Tubenit

[Ed_Chambley]

Congrats on a successful build!

Glad to se a post from 2020.  Hope you are well.