Standel 25L15-Do you like it as is or would you like to change something

[wsscott]

I've always been a great fan of Chet Atkins, and loved the sound of his Gretsch's and his Standel 25L15 studio amp.  When I was a teenager, they were a rarity, but considered as the Holy Grail where I lived.  So I've recently been thinking of building a replica.  I've found what I think to be is a pretty accurate schematic, and the circuit looks pretty simple.  The amp just has 1 channel, 25 watts, 15" JBL P130 speaker, and Volume, Bass and Treble pots.  I don't play gigs, but I have a couple friends that do, and so I would be building this for the fun of doing it, but also making it available to buddies to use.

So I'm hoping I can get some input from those who own or have used this amp, and can give me some thoughts on things they like, or would like to change or have added to the amp.  It's designed for 807 power tubes, and I've never seen those in an amp.  I've read that 6L6's could be used instead.  But would that affect the sound that I really like.

If it was going to be used by someone who has a studio for an amp that players could use when recording, would it be an attractive addition to the studio, and is there something more that the amp needs or should have that would make it even more attractive for a studio.

Thanks for your comments.

[shooter]

would that affect the sound that I really like.

just guessing, but..
pretty sure 90% of that sound you like, came from the mans fingers n soul...

[sluckey]

HotBluePlates is the man to talk to.

[kagliostro]

To build one, you need the right information; the result will be a truly, truly clean amplifier, t'hat was the purpose of that amp

As far as I'm concerned, building one and then modifying the design wouldn't be worth the effort.

Franco

[Banjan73]

Power transformers and output transformers? Can you get some similiar ones as the original?
I believe those two components has a LOT to do to a spesific sound of an amp.

[wsscott]

Yes, I'm pretty sure I can get the transformers with the proper specs.  Agreed on their impact on sound.

I've talked to a couple of buddies who gig, and they said one thing they would like is a switch to adjust the wattage of the power output because they play in different venues where a 25 watt amp might be too much.  So a switch that could change the wattage to 5W, 15W, and 25W for example.

I think this is something different than an attenuator.  I think maybe Marshalls have something like this.  I know I've seen it on some amps, but I don't know what's going on.  I'll do some research.

[kagliostro]

To maintain the original tone I don't think a VVR will be appropriate expecially with the 25L15

To me the right way will be an attenuatore, but a very good one

Franco

[shooter]

can you post the schematic, i've looked at about 1/2 Doug's n their all sand.

[wsscott]

Shooter-sorry but I had to sign a NDA and can’t share it.

[shooter]

 


it is tube though?
self biased or fixed?

[wsscott]

Yes, its all tubes, cathode/self-biased, running 807 output tubes as the original.

[HotBluePlates]

I've always been a great fan of Chet Atkins, and loved the sound of his Gretsch's and his Standel 25L15 ... It's designed for 807 power tubes, and I've never seen those in an amp.  I've read that 6L6's could be used instead.  But would that affect the sound that I really like. ...

If you use the original 807s, you have the hassle of top caps but you can buy NOS 807s all day long for half the price of some Chinese 6L6s.

That said, you can ditch the top caps and use 6L6G/GB/GC/5881 as you please without any significant effect on the amp's sound.

I've talked to a couple of buddies who gig, and they said one thing they would like is a switch to adjust the wattage of the power output because they play in different venues where a 25 watt amp might be too much.  So a switch that could change the wattage to 5W, 15W, and 25W for example.

Chet Atkins, not Eddie Van Halen, right?  Who cares if "25w" is too loud, because you simply turn down the Volume control.

You will also find the amp's Tone Controls use negative feedback, and the amp is extremely-clean sounding unless you turn Treble & Bass full-up.  That being the case, there is zero benefit to "reducing power output."

Further, the design of the entire amp is geared to "Clean, Clean, Clean."  Each stage of the preamp and power amp handles a larger signal than the stage before, so that no stage is over-driven.

So I'm hoping I can get some input from those who own or have used this amp, and can give me some thoughts on things they like, or would like to change or have added to the amp. 

HotBluePlates is the man to talk to.

Standel 25L15 Inspired Amp

It's been a minute...

The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (R14 through R22).  Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.

If you use a JBL D130 (and not a paper-dome version), you may find the upper treble a little steely.  I added a Vox-like Cut control at the phase inverter output to reduce extreme high-treble as desired.

There is a T-filter that imposes a fixed midrange-scoop.  What if you want more/less midrange?  I added a control that reduces the amount of scoop by adding a 1MΩ pot between Ground and the resistor-to-ground in the T-filter.

What if you don't like the center-frequency of the mid-scoop?  That was a little more complicated:  I added a dual-pot and a pair of build-out resistors that allowed the resistance of the pair of resistors forming the "Top of the T" to be changed; this changes the center-frequency of the scoop.

... I've found what I think to be is a pretty accurate schematic, and the circuit looks pretty simple.  The amp just has 1 channel ...

There are really 2 channels, 2 Volume controls.  But one is hidden on the underside of the preamp chassis, and they share the T-filter and Tone control section.

I didn't need 2 channels, so I ditched the input 12AX7 and used a 7-pin 6AV6 (or 6AT6 if desired).  Once again, you can buy NOS all day long for cheap, cheap cheap.

[kagliostro]

If you want to go on the cheap there are G-807 and if you want to go cheaper than that, 1625

Franco

[Willabe]

Main design points we know;

1. Real class A 25w power amp with 807 tubes (6L6GB with different pinout/bottle).

2. There's 2 chokes in the power supply.

3. Each stage can take more input power then it's given, so it doesn't distort.

4. 15" JBL D130

5. T filter mid range scoop.

6. NFB around the tone controls.

Questions about design still not clear;

1. Do we know how many and what type/# of preamp and PI tubes?

2. And which type of PI?

3. NF around the power amp?

4. Grid bias for power tubes?

[wsscott]

HotBluePlates-If you use 6L6GC's instead of the 807's, what changes to its wiring would be needed?  Obviously you would no longer have the anode cap to deal with, and it would free up some space, so might make the build more efficient.

[HotBluePlates]

1. Do we know how many and what type/# of preamp and PI tubes?

12AX7 (1 stage per channel) ---> 12AT7 ---> 12AU7

2. And which type of PI?

Split Load / Concertina / Cathodyne. (pick your favorite name)

3. NF around the power amp?

No.

4. Grid bias for power tubes?

It's a cathode-biased amp.

HotBluePlates-If you use 6L6GC's instead of the 807's, what changes to its wiring would be needed?

Octal socket instead of 5-pin socket.

No other changes necessary.  You're not trying to get higher power output from the 6L6GCs, even if the tubes themselves can dissipate more heat.

[wsscott]

HotBluePlates-would you still add the Bias Adjust and Bias Balance pots if you use the 6L6GC's instead of the 807's?

[shooter]

would you still add the Bias Adjust and Bias Balance pots

It's a cathode-biased amp.

[wsscott]

Maybe it’s just a single pot to balance the tubes so that the bias point for both tubes is the same.

[Willabe]

would you still add the Bias Adjust and Bias Balance pots

It's a cathode-biased amp.

The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (R14 through R22).  Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.

So, I think HBP made his adjustable grid bias and balance to match up nos 807 tubes he bought?

[wsscott]

BHP-If you wanted to use an OT with 3 taps and selectable for 4, 8 and 16 ohms, what would you recommend on how to wire the circuit?  Would this affect the NFB circuit?

Also is the T-filter that you refer to a Prescence control, or something else?

[HotBluePlates]

HotBluePlates-would you still add the Bias Adjust and Bias Balance pots if you use the 6L6GC's instead of the 807's?

Objection:  Asked and Answered, because

The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (R14 through R22).  Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.

An amp does not need to be fixed-biased to have bias-adjustment, and an amp does not need to be fixed-biased to have bias-balance.  Click the links guys, and look at the schematic.

... Would this affect the NFB circuit?

3. NF around the power amp?

No.

No Negative Feedback around the power section.


Guys... you're killing me with the repeated questions about stuff that was already answered.  Maybe that "pretty accurate schematic" ain't so accurate?

When I built my example of the 25L15, I received information on the circuit from someone who was provided access to an original by a collector, and documented every detail of that particular amp (there is a possibility that other individual examples were tweaked for their original buyer).  Now the 25L15 is not "magic" and you can cop the sound close-enough using a blackface Fender amp set for a clean sound.  But the only negative feedback in the amp is inside the preamp to make the Treble/Bass controls function.  Apparently Bob Crooks didn't feel power section negative feedback was necessary to damp the speaker.

[wsscott]

I've noticed on a lot of older amps, ie. from the late 50's to mid-60's, like the Standel, and the Valco/Gretsch/Supro, and probably some others, that the chassis' were made with their ends being made with blocks of wood instead of the aluminum/steel of the chassis being bent up to form the end pieces like the rest of the chassis.

Any thoughts on why they did it that way.  Was it to provide for a solid mounting surface that wasn't of a conductive material, and maybe helped avoid ground loops?

[passaloutre]

I think it’s just easier to manufacture. I’ve made chassis this way, with old safe deposit boxes, cut to length and plugged at each end with blocks of wood. Inspired by my Supro Thunderbolt.

Also, why is everyone always so cagey about this schematic? These amps are 60 years old.

[HotBluePlates]

... Also, why is everyone always so cagey about this schematic? ...

In my case because:

... I received information on the circuit from someone who was provided access to an original by a collector, and documented every detail of that particular amp ...

Right or wrong, the gentleman who provided me the information on this amp asked that I not make it public.  He was concerned he might get sued by the modern Standel company (as unlikely as that is/was).

I gave him my word, so that's that.