Standel 25L15 Inspired Amp

[HotBluePlates]

I wanted to take a moment to post my recent build, a modified version of a Standel 25L15.

Many thanks to Tubenit for the wonderful cabinet he made! 

The original amp has 2 channels with a common set of Treble and Bass controls. I ditched the 2nd channel, and added a Contour control which shifts the center of the midrange dip, giving the amp a wide tonal range. I also added a control to the underside of the preamp chassis which controls the depth of the midrange dip, akin to a Raw control in some amps. Next to this is a Treble Cut control somewhat like those in Vox amps.

The preamp and power amp chasses are connected via XLR cables, with power and signal kept separate.

The power amp chassis incorporates locking pots for output tube bias and balance, so any two 807's can be used without matching. Also present are bias/balance test points and a ground lift switch. Circuit ground has been floated off the chassis throughout.

I did try to use a pentode/ultralinear/triode switch for the output tubes, but overlooked some details about how the output tubes are powered in the stock circuit. Unfortunately, I had to remove that, which left the power amp chassis wiring messier than what I'd prefer.

As I get the inclination, I'll clean up that wiring, and will be adding some cable clamps to the inside of the cabinet to route the cables/power cord. I also wanted to use radio grill cloth initially, but it's very lightweight fabric so I used typical guitar amp grill cloth. I will likely add the radio grill cloth over it in the future so it gets the needed support. I also have some ideas about the handle I'd like for this, but will likely have to have it custom made.

Enjoy!

[HotBluePlates]

Picture showing the light-up control panel didn't turn out as it appears in person. The LED light seems to overload the camera phone somewhat... The actual color is a little lighter than royal blue and turned out quite nicely. For reference, the metal backing the control panel is champagne gold metallic.

[tubenit]

WOW !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!     

That turned out SUPER COOL!!!   I love the lighted control panel.  Very classy. 

So how's she sound???  You getting some of the Chet Atkins tone from it?

Very nice job, my friend. 

With respect, Tubenit

[six el six]

I really like the Solen capacitors because I'm a fan of Steve Carr's amps.

Would you comment on your use of Solen caps and why you used them?

[HotBluePlates]

So how's she sound???  You getting some of the Chet Atkins tone from it?

I am really digging it!

The sound has a great clarity. I honestly don't know yet if that is the circuit or the D130, but while there isn't icepick treble, the sound is clear enough to make a guitar's tone control useful again.

I can easily do the Chet sound, but I'd need a Gretsch, or at least some Filtertrons to really nail the sound.

I really like the Solen capacitors because I'm a fan of Steve Carr's amps.

Would you comment on your use of Solen caps and why you used them?

I used them because I'll never have to replace them, and because I already had some on hand.

The stock amp uses can caps for the power amp and preamp. I did use a can cap for the preamp (mounted under the wirewound resistors on the right side of the preamp) because there simply isn't enough room for Solens.

Additionally, there are two chokes in the power supply, mounted in the power amp chassis. I thought it would be nice to keep those over by the rectifier, and "shield" the noisy end of the power amp chassis from the power tube input circuitry with the Solens. I don't know for sure that the ends I grounded are the "outer foil", but I figured it would work well enough.

[John]

Kudos to Tubenit on the cab. I seriously doubt my woodworking will ever be up to his standards!

And nice insides too! Uh, yeah.... that wiring is just really messy. 

[Willabe]

And nice insides too! Uh, yeah.... that wiring is just really messy.  

I thought at first he wasn't gonna show us a pic of the PSU chassis until he cleaned it up a little. I don't see where it needs it?   

I think the amp is beautiful inside and out. Beautiful layout and wiring.

Tubenits work on the cab is beautiful too.

Is the dark wood walnut? Is there any stain on it or it's natural color?


                            
                                Brad        

[HotBluePlates]

I thought at first he wasn't gonna show us a pic of the PSU chassis until he cleaned it up a little. I don't see where it needs it?   

I think the amp is beautiful inside and out. Beautiful layout and wiring.

Thanks!

On the far right side of the power amp, about halfway down, you'll see a hole where a switch used to be, as well as a number of wire ends that are only taped off. They used to run to the T/U/P switch, and I haven't really cleaned those up since removing the switch.

Also, I really did want the power amp chassis to be sorta Hiwatt-style but didn't quite land on the perfect wire routing.

One other detail I didn't point out is in the preamp chassis: the tubes are mounted horizontally, and the sockets are bolted to a subchassis plate that is shock-mounted from the preamp chassis itself. There are holes in the main chassis a bit larger than the holes for the socket for the pins to pass through for wiring.

[Fresh_Start]

Gorgeous cab and even better looking build inside!  

Thanks for sharing,

Chip

[Willabe]

On the far right side of the power amp, about halfway down, you'll see a hole where a switch used to be, as well as a number of wire ends that are only taped off. They used to run to the T/U/P switch, and I haven't really cleaned those up since removing the switch.

Meh, looks good to me.    

One other detail I didn't point out is in the preamp chassis: the tubes are mounted horizontally, and the sockets are bolted to a subchassis plate that is shock-mounted from the preamp chassis itself. There are holes in the main chassis a bit larger than the holes for the socket for the pins to pass through for wiring.

I saw that and wondered about that. What did you use for the shock-mounts? Small springs, rubber gromits, or....?

I forgot to say I love the lighted face plate. I remember your thread on that.


                          Brad    

[HotBluePlates]

What did you use for the shock-mounts? Small springs, rubber gromits, or....?

Some special rubber grommets. They're designed/rated for vibration damping.

Sockets are bolted firmly to the subchassis plate. Grommets sit in holes in each corner of the plate. Two of the mounting screws which hold ends of the turret strips I made also hold corners of the plate. The other two corners have shorter mounting screws which bolt to the chassis underneath the turret strips.

I'll have to take a picture or two of that arrangement.

[TubeGeek]

I have been waiting to see the result of this project ever since you mentioned it! 

Outstanding job! Beautiful.

I love how you think outside the box and do your own thang.

Thank you for sharing.

[tubenit]

Is the dark wood walnut? Is there any stain on it or it's natural color?

The wood is sapele.  And it has a slight Red Mahoganey stain on it from Min-wax.  It is a lovely wood to work  with.

Tubenit

[Fresh_Start]

What did you use for the shock-mounts? Small springs, rubber gromits, or....?

Some special rubber grommets. They're designed/rated for vibration damping.

Sockets are bolted firmly to the subchassis plate. Grommets sit in holes in each corner of the plate. Two of the mounting screws which hold ends of the turret strips I made also hold corners of the plate. The other two corners have shorter mounting screws which bolt to the chassis underneath the turret strips.

I'll have to take a picture or two of that arrangement.

Sounds great for amps using an EF-86 or other preamp tube subject to microphonics!

Sorry I forgot to say that the lighted control panel is WAY beyond cool.  I love it!

Chip

[sluckey]

GORGEUS! Please let us hear. And share a schematic please.

[tubeswell]

Immaculate HPB!

[HotBluePlates]

GORGEUS! Please let us hear.

I may have to do that with a digital camera, as I don't have any good recording setup here. Eventually, I need to get a mic and an interface for recording.

[Geezer]

WOW! 

THAT"S a beautiful piece!

Can't wait to hear it.......

G

[Willabe]

How about a pic of the light up control panel taken in the dark?

And a link back to the thread about making it? You know we _ALL_ want at least 1 amp with it now?      


              Brad        

[HotBluePlates]

How about a pic of the light up control panel taken in the dark?

I tried that, but the camera overload is even worse then. It's mainly about the intensity of the light vs the rest of the picture.

I'll try to get a better picture, and also remove the preamp chassis so you can see how I assembled it.

[CraigB]

Absolutely stunning!  Very beautiful cabinet (Jeff's woodworking skills are amazing) and thanks for the pics of the inner works - I guess one would need a microscope to see the "messy" part of your wiring 

[rzenc]

A.M.A.Z.I.N.G!!!!!!!!!!

Best Regards
R.

[HotBluePlates]

Absolutely stunning!  Very beautiful cabinet (Jeff's woodworking skills are amazing) and thanks for the pics of the inner works - I guess one would need a microscope to see the "messy" part of your wiring 

Thanks Craig!

My inspiration for the preamp wiring style was the tube Tektronix scopes. While not nearly as dense (or using the cool ceramic strips with silver-lined notches), the mounting strips on either side of the row of sockets with wire running down to the socket tabs allows a pretty compact package.

Forgot to (re-)mention one thing: the original amp uses a 12AX7 input tube, but has 2 channels. When I chopped to 1 channel only, I used a 6AV6 instead of a 12AX7. You can buy NOS blackplate 6AV6's all day long for $3 (and occasionally less). Since the other tubes are 12AT7 and 12AU7, it worked out for a pretty inexpensive tube lineup. Even NOS 807's can be found for quite a lot less than comparable 6L6GB's.

Another tip: both chassis are aluminum and painted (maybe not obvious with the preamp chassis). The gentleman advising me has a sandblasting booth, but since I don't, he recommended using 100-150 grit sandpaper to scuff the surface. He also recommended not using primer, but simply painting the hammertone or metallic paint directly on the surface. I tried it and gave several days for them to dry before any real handling. Paint has held up well, no chipping or flaking, even where bolts were screwed down to the surface (the bolts do slightly flatten the hammertone right where they press on the surface).

[DummyLoad]

beautiful & well executed.

--DL

[quayhog]

Very neat work and classy layout.  I bet it sounds real good.  Do you have any schematic?  Especially the power supply. 

Jonathan Horne

[HotBluePlates]

Okay guys, some new pics...

First up, some detail on the preamp chassis and the shock mounting. You may note the socket mounting holes are not all the same; the input 6AV6 needed a different rotation than the 9-pin sockets to keep the grid support rods vertical when the chassis is in the amp. All preamp tubes are mounted horizontally, and the tops point towards the baffleboard. Ventilation is not an issue with these tubes.

The first pic shows the preamp tube suchassis which holds the tube sockets and has enlarged openings for the vibration-damping grommets.

Second pic shows an overhead view (actually the back of the preamp chassis with the subchassis in place.

Third photo is a view of the subchassis suspended off the main preamp chassis. There are holes in the main preamp chassis which are larger than the socket holes, and roughly concentric with the socket holes on the subchassis. The tube socket holes extend through these partway into the preamp chassis.

I used some high-damping PVC grommets from McMaster-Carr to mount the subchassis to the main chassis, stacked 2-high at each corner. The first of the grommets is pressed into each corner of the subchassis, the second sits underneath as a spacer. Tightening down those mounting bolts is a little tricky, as the bolt pulls against the grommet only, so you have to be careful how much torque you apply.

[TubeGeek]

Very nice way to shockmount the preamp tubes!

Do you notice any difference?

Impressive attention to details with this amp.

[HotBluePlates]

Next set of pics shows some detail of the mounting of the LEDs for the light-up faceplate.

If you noticed the overhead shot of the preamp chassis above, the faceplate is clear acrylic, 1/8" thick. It is engraved on the side against the metal chassis, in reverse.

I downloaded Inkscape (a free drawing package) to lay out the control holes locations and the font/position of letters/numbers. I'd already figured out the control spacing as part of the mechanical layout planning for the build. I was able to flip the image after getting everything positioned the way I wanted, and saved in a format acceptable to the engraving shop that did the panel for me. They used their laser to cut the control holes.

The LEDs are positioned along the right edge of the control panel.

It might look slapped together disassembled, but I put a layer of electrical tap on the chassis just beyond the edge of the panel, then have the LEDs laying against the edge of the panel, and another layer of tape over the whole package. If I were going to make this into a production amp, I'd have to figure out a prettier packaging/mounting method, or find a way to encase the whole package. But the amp is for me, and no one sees the tape unless the amp is disassembled.

The two squares to the right, above and below the chassis mounting hole are rubber squares which keep the LEDs from backing away from the panel edge. I made those by building up layers of a rubber tape until they were about 1/8" tall (same as the control panel).

I took 6 ultra bright blue LEDs and filed 2 opposing sides, on the same plane as the leads, until the LED was 1/8" thick in that direction. The I filed the rounded bulb of the LED to take off much of the bulk in that direction and give a generally flat surface to fit against the control panel edge.

The end LED package is 6 LEDs, which the leads connected in parallel. A little bit of insulation stripped off other wires I used during the build insulates the + and - leads as they daisy-chain between each LED.

There is a hole that looks black and part of the upper rubber backing square where the wires for the LEDs exits the chassis. I attached the wires to the last of the LEDs closest to that hole and added a bit of heatshrink to protect those wires where they pass through the hole in the chassis. Inside the chassis, this twisted pair runs to 6.3v + and - terminals on the power XLR jack, with a dropping resistor in series with one of those wires. I found the resistor value not critical for LED brightness.

The original Standel had pilot light bulbs at either end of the control panel to make it light up, but I was concerned about putting 6.3vac wiring near the input jack so I lit my panel from only one side. If you're looking for it and in a dark room you can tell the side by the power switch is a little brighter than the input jack side, but it's probably not a big enough difference to warrant adding a pack of LEDs to the other side of the panel.

[DummyLoad]

are those dale rn65 resistors? should be pretty quiet if they are.

--DL

[HotBluePlates]

I took another picture of the control panel lit, but am still having my camera give me fits.

There is a bright halo-effect that comes through on the pic which doesn't exist when you're in the room looking at the panel. I've figured out the camera is "too smart": it thinks the composition of the photo is too dark and actually keeps the shutter open longer to compensate.

As a result, the lettering comes out very bright, when it is a more subdued blue in person. Where the lettering looks near-white in the letters, with a halo of an almost turquoise color, in person the letters are blue. The panel itself has a faint bluish glow, because the acrylic material is not a perfectly uniform thickness.

[HotBluePlates]

Very nice way to shockmount the preamp tubes!

Do you notice any difference?

Well, I did this because the original 25L15 also shockmounts the preamp tubes in a similar manner because they are fairly close to a big D130.

I can't say if I notice any difference, because this is the only 25L15 I've built. So I honestly don't know if it's worth going to this trouble.

are those dale rn65 resistors? should be pretty quiet if they are.

Yes, I pretty much used RN65 resistors everywhere except where higher power ratings were required. There I used Dale wirewounds. I also used some smaller Dale metal films where 1/2w ratings weren't required, like the 1Ω resistors for output tube bias measuring.

The amp is near-silent at full volume, enough that it would make you think the signal cable from preamp to power amp is unplugged. You have to have your ear right at the speaker to hear any noise, and what noise it has is quieter as you turn the volume down. Part of the benefit of a low-gain topology.

[DummyLoad]

it is a beautiful work of art - tubenit is my woodworking hero.

is the attached similar to what you built?

--DL 

[HotBluePlates]

Yes, although there are some errors/changes in that schematic.

First, I omitted the 2nd input (the lower one in that drawing) and all components associated with it up through the 2nd volume control.

I used a 4 section cap can, with all sections at 20uF instead of whatever filter caps the schematic shows.

The T-filter between the first and second stages shouldn't have the resistor that's crossed out. In my amp, the Contour control also changes the value of the 220k resistors in the filter to shift the frequency of the mid-dip (with some added build-out resistors to limit how small each of those becomes), and adds a pot in series with the 30k resistor to allow you to reduce the amount of dip.

The markings are confusing, but I'd interpret the 1st 12AU7 section as having a feedback output which runs to the Treble and Bass controls, which then feed the 2nd 12AT7 section which is cathode-coupled to the 1st 12AT7 section. So the tone controls are part of a feedback loop.

Also, ignore the series heater connection implied on the schematic; I supposed it would work for you if the output tubes idled at 75mA each (and so provided 150mA for the series heater string), but my best information is that while the first iteration of the design may have done that, even early examples didn't. I was given access to information from an early 25L15 that was reversed-engineered, and my understanding is it used a.c. heaters in the preamp. I worried I might get hum form having the a.c. heater wires along with the clean B+ inside the same power cable between chassis, but that didn't happen.

From what I can see, the plate and cathode resistors for the 12AT7 are wrong, but they're not orders-of-magnitude-wrong. But I'm having trouble reading what they say on the schematic there.

[DummyLoad]

From what I can see, the plate and cathode resistors for the 12AT7 are wrong, but they're not orders-of-magnitude-wrong. But I'm having trouble reading what they say on the schematic there.

i read 47k plate and 390R for the shared cathode R.

--DL

[kagliostro]

I don't know if this can be of some help

However ...........

K

[jazbo8]

Awesome looking amp, love the build and cabinet. Here is some good Sunday reading >> Standel 25L15

[HotBluePlates]

From what I can see, the plate and cathode resistors for the 12AT7 are wrong, but they're not orders-of-magnitude-wrong. But I'm having trouble reading what they say on the schematic there.

i read 47k plate and 390R for the shared cathode R.

Yeah, those are wrong values.

[HotBluePlates]

I don't know if this can be of some help

However ...........

The first schematic is a preamp based on the Gretsch Chet Atkins amp, with a guess for how the power supply and output stage works in the Standel, and is entirely different from the 25L15. Doesn't mean it won't be a good-sounding amp, it just has absolutely nothing in common with the amp it's trying to clone.

The second schematic from Weber is ... different. I can tell they started with some partial information about the original 25L15, but probably re-thought and did things very differently than in the original. Everything is almost similar, but if I tell you any one part is similar, I'll have to also tell you the three ways it's different. So I'll say Weber probably decided to rearrange things and give a modern take on the amp.

If anyone truly wants to build one of these, I'll pass your contact info on to the person who provided me the info from an original, documented 25L15. He's tight-lipped on the stuff because apparently the modern Standel company hassles people who make clones or something similar, even though there's no patent on the circuit. I think he wants to avoid a silly lawsuit and protect the owner of the amp who made it available to him to document.

[HotBluePlates]

Here is some good Sunday reading >> Standel 25L15

Funny that one of the schematics listed there is from a thread on this forum where we tried to guess what's in the amp.

There are errors in the power supply, preamp as noted above and some differences in how I did the cathode bias section (mine allows for bias adjust and balance, but as diagrammed you only get balance).

Also unfortunate is that they're too small to read.

[teisco88]

You have built a gorgeous tribute amp.  Bob Crooks is smiling.  The rest of us are envious (at least, I am).  I am determined to attempt a 25L15 this year--New Year's resolution.

[jazbo8]

Here is some good Sunday reading >> Standel 25L15

Funny that one of the schematics listed there is from a thread on this forum where we tried to guess what's in the amp.

There are errors in the power supply, preamp as noted above and some differences in how I did the cathode bias section (mine allows for bias adjust and balance, but as diagrammed you only get balance).

Also unfortunate is that they're too small to read.

A shame about the schematics... but nevertheless a good profile on this lesser known Californian company - a pioneer in the fledgling musical instrument, PA industry.

[kagliostro]

Hi HotBluePlates

Thanks for your clarifications

I've also found this other schematic from Weber, the Pennebaker 25 that is the precursor of the JP25

K

[twbranch]

That is beautiful. I have been trying to find a good wood to use for a "woodie" amp. I love the lines on this one. I want an amp I can put in the living room that doesn't stand out as an amp.

[sluckey]

<RANT> This bias circuit is dangerous! I've seen several Weber schematics that will allow the fixed bias to be adjusted to zero. I hate seeing Weber release this stuff into the wild. They have a lot of customers who will not see the danger nor know how to make it safe. </RANT>

[plexi50]

Man you dont play around HBP! That is one work of perfection. Gorgeous 

[DummyLoad]

<RANT> This bias circuit is dangerous! I've seen several Weber schematics that will allow the fixed bias to be adjusted to zero. I hate seeing Weber release this stuff into the wild. They have a lot of customers who will not see the danger nor know how to make it safe. </RANT>

not to mention that the bias taps are directly off a wiper - dirty or flaky pot - say hello red & boom.

--DL

[kagliostro]

This is an improved bias circuit I've draw following Merlin's councils

It is applied to an Ibanez Amp that is being converted to a Fender 6g2

The 5k6 resistor (with one other value) was present in the original Ibanez bias circuit and his actual value is specific for this situation

As bridge was used a very small bridge unit, not 4 x 1N4007 diodes

R1 function is to limit the voltage excursion as to avoid to have 0v for bias

R2 function is to prevent problems if the pot wiper contact fails

D1 function is to obtain a quick charge of C2 at startup via P1-D1 than, at switch-off, D1 blocks the quick discharge of C2 and C2 discharges slowly via R3 trying to prevent the fuse of the HV to blow if switched off and on rapidly

R3 allows C2 to discharge slowly at switch off

With a 32v AC winding and this resistors and pot values the bias range is -24v / -42v

I Know, it is more complicated than an usual bias schematics, but it uses only 1 bridge (or 4 diode) and 2 resistors more than an usual bias schematic, so, why not ?

I hope this isn't considered Off-Topic as I inserted this info only as a refund for the publication of the Weber schematic with that bad bias circuit

K

[sluckey]

Well we definitely took a sidebar from the Standel thread. 

I like your full wave bias. And you certainly don't need to atone yourself for Weber's masterpiece. I can't believe someone at Weber hasn't changed all those drawings with that particular bias circuit. I know they can't control it after it's in the wild, but they still have that stuff on their website too.

[HotBluePlates]

I've also found this other schematic from Weber, the Pennebaker 25 that is the precursor of the JP25

Again, it's different. How?

Most of that mess before the input stage 1MΩ isn't in a 25L15. Dunno why they chose to do that, but it's some form a tone network.

Original 25L15 had 2 channels, although the 2nd hidden channel was somewhat neutered compared to the one whose input jack is on the top. If you're willing to buy a 7-pin socket and a handful of 6AV6's, you can use them instead of the half-12AX7. The 12AX7 has a wrong cathode resistor value and a much smaller coupling cap. Don't know why. The treble, bass, contour circuit appears copied from the solid-state Standels, has values not used in the tube version, and from my experimentation the contour will do almost nothing; I know, cause I copied the contour from the solid-state amps then had to use an entirely different approach when it did nothing. The amp is missing the T-filter tone network in the 25L15. The cap from 12AT7 grid to ground, and across the plate load are not present in a 25L15. The cathode resistor of the 1st 12AT7 is wrong, and the 25L15 doesn't use the 2nd half of the 12AT7 as a split-load. The 25L15 doesn't use a long-tail inverter or differential amp, and so it uses the 12AU7 differently. The 25L15 is not ultralinear, although you could do that with some circuit redesign. The 25L15 is cathode biased, and has a balance circuit akin to the Williamson amp (I copied the balance and adjust I posted in the thread on adjustable cathode bias almost entirely). The 25L15 power supply is laid out entirely differently, and doesn't use any cap as high as 100uF and has 2 chokes.

So this is what I mean when I said about the earlier Weber that I think they had some information available, but designed from basically a blank sheet of paper using the same tubes, and made a lot of guesses or updates about how they incorporated the circuit. Neither of the 2 Weber Standel-type amps were ever put into production in kit form. Don't know why.

[kagliostro]

Many thanks for this large explanation

And, yes, they didn't go with production of this two kit

the kit that they produce actually with 807 (only one) is the Lauren

but this is all one other amp

K