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

[wsscott]

I'm working on the bias circuit in this cathode biased amp, and the schematic gives the option of making the cathode bias resistor adjustable using a 100 ohm pot plus a resistor vs. just a single value resistor.

This option calls for a 100 ohm pot plus a 68 ohm 2 Watt resistor.  I failed to order the 68 ohm 2 watt resistor, I just have 1/2 watts on hand.  But I do have a 100 ohm 2 watt resistor.  Would substituting this resistor create issues in adjusting the bias--or otherwise?

Thanks.

[sluckey]

Kinda hard to answer without access to a schematic.

[shooter]

But I do have a 100 ohm 2 watt resistor. 

more R = less I (I= E/R)
so you are "safer"
start WITHOUT the adjustability, get the basics dialed in, then tinker...so install the 100, figure out what the tubes are dissipating, figure out if they go into cut-off, figure out how it sounds......


then tinker

[wsscott]

Thanks Shooter-I figured there might be a math analysis involved that only some people would know.  Certainly not me!

[shooter]

 


it's not "math", it's the language of electronics 

[wsscott]

Does anyone know how the new Standel amps back light their front panels? I think Magnatone also has some models that are backlighted. They look really cool, and I'd like to try it some time.

[kagliostro]

HBP did it on his Standel

The light is placed around the panel

Go here
 
https://el34world.com/Forum/index.php?topic=13326.msg129564#msg129564

And read the post from Doug

Franco

[wsscott]

Thanks!

[wsscott]

I've almost completed building the Power Amp section, and wondered if there is a way to test it. This section is where the AC comes in and contains the PT, OT, 2 chokes, and the rectifier and 2 6L6's.  The on/off switch is in the preamp, not this section. Since I haven't started construction of the Preamp, I don't have a way to inject a signal, or anything else, but I want to make sure this section of the amp is wired properly.

[sluckey]

How can you continue to ask for help on this project when you won't share the schematic? I understand honoring someone's request to keep the secret, but if everyone honored that secret then you wouldn't have the schematic. Think about it!

[shooter]

wondered if there is a way to test it.

sure, no problem, just provide the following;


total expected;
 filament current/voltage
 preamp current/voltage
 PA current/voltage

then you can calculate a dummyload for testing

[wsscott]

Thanks Shooter.

[wsscott]

Assume the attached Gretsch schematic of the 6165's power amp has the AC power line and fuse attached at the PT's primary windings.  Other than that, no controls and nothing from the Preamp is connected.

[kagliostro]

To test if it works you can use the preamp of one other amp or a signal generator or arrange a partition resistor from the output of a small transistor radio

Franco

[wsscott]

I have a question about filament wiring.

I've seen some builders run the 6.3VAC filament wires from the Power Transformer to a terminal strip, and then run 1 pair of wires from there to the Pilot Light, and another set to the pre-amp tubes.  Is there any benefit to doing it that way vs. just running the wires directly from the PT to the Pilot Light and then from there to the pre-amp tubes?

Also, does it matter which pre-amp tube's filaments get wired first in the string?  Assuming you have 3 common preamp tubes, should you start with V1 which is farthest away from the PT or start at V3 and finish at V1?

[shooter]

the key to good filament wiring is keeping it "uniform" and away from sensitive AC wires or any AC source where noise coupling can happen. (inductive coupling)


so it doesn't matter to your questions as long as nothing can "couple" into or out of the filament chain n induce 60hz into the amps signal path.


believe this is Steve's example of "clean filament wiring"

[wsscott]

Thanks Shooter.  Sluckey does nice work!

I have normally used 20AWG wire for wiring up the filaments.  Is that a good choice?

[acheld]

20AWG is fine, but you could easily use 22AWG as its ampacity is about 7 amps or more, depending on insulation.

[wsscott]

Thanks for the tip.  Good to know if you're out of 20.

[Willabe]

From Merlin.

Use the example on the far right, red/white wires.

[wsscott]

Thanks.

[kagliostro]

First connect the Power Tubes, then PI and down till V1

https://valvewizard.co.uk/heater.html

Franco

[HotBluePlates]

I've seen some builders run the 6.3VAC filament wires from the Power Transformer to a terminal strip, and then run 1 pair of wires from there to the Pilot Light, and another set to the pre-amp tubes.  Is there any benefit to doing it that way vs. just running the wires directly from the PT to the Pilot Light and then from there to the pre-amp tubes?

It doesn't matter, except one may be easier given your physical layout.

... does it matter which pre-amp tube's filaments get wired first in the string?  Assuming you have 3 common preamp tubes, should you start with V1 which is farthest away from the PT or start at V3 and finish at V1?

It doesn't matter, except that you will find your own preference for performing mechanical assembly.

For my own 25L15-style amp, I decided routing the heater wiring over the tube socket would give the shortest path, smallest "hum loop," stayed at roughly right-angles to all important components/wiring, and was feasible with the ~22ga teflon-insulated wire I was using.  I planned ahead to orient the sockets so that the heater-pins were horizontally aligned for this.




It's been too many years, and I don't remember which end I started with.  But today I would probably start on the left-hand side of the image, work my way across each socket, then finish with the run from the last socket to the XLR jack that I used for the power connection to the preamp chassis.

[wsscott]

I finished the build of the amp a few days ago and its in testing mode.  But no shorts or major problems. It worked. The only issue I had was that the schematic I was following was a little confusing to what I'm used to and I wired the pots in "reverse", so I had to rewire them so Full CCW is "Off".  It's really clean and bright.  I haven't checked the output wattage yet.  I've got it set for a bias of 65ma.

One thing I'm trying to figure out is the way the 2nd Input/Volume for Channel 2 works.  Input 1 and Volume 1 work normally like any other amp, and turning Volume 2 pot has no effect on its output volume.

However, if I'm using Input 2 which I would describe as having a softer/smoother tone, maybe more "jazz-like", it only produces an output signal if I turn the Vol. 1 pot past its "Off" postion.  It seems like it's sort of like a Master Volume for Channel 2.  I don't know if I've wired it wrong or this is the way it's designed.  The more I turn up the Volume 1 pot, the louder the signal, but increasing the Volume 2 pot also increases the signal too, but not as much as Volume 1 does.  So there's some kind of interaction.

One other thing in using Channel 2, is that I get oscillation and high screech if both Volume 1 and Volume 2 pots are turned up really high.

This seems to be the only "issue" for me to figure out.  Its dead quiet.

[passaloutre]

We really can’t help you without a schematic. The pots are wired in reverse because the tone stack is a negative feedback loop

[wsscott]

I understand.  I don't believe the amp I'm working with has a negative feedback circuit, but I could very well be wrong.

[passaloutre]

The Standel schematics I’ve seen have a negative feedback loop tone control circuit. Of course we don’t know what circuit you’re building.

[wsscott]

HotBluePlates-I finished the amp and have been testing it and have a question about some voltages and the bias, if you have the time.

I checked voltages yesterday, as well as the bias, and it seems like the voltages are fine for the Power transformer's secondary (356/712 VAC), and the OT (374 VAC), but the bias is too cool based on Rob Robinette’s bias calculator.  I have Plate to Cathode voltage of 349 volts, and with the 6L6GC’s that I’m using I should have a bias of around 80–85ma, but the highest I can get it using the adjustable bias pot mod that you designed, I can’t get it past 75ma.

The Power Supply's cap nodes are reading: 387, 373, 296, and 378.
The Pre-amp's cap nodes are reading: 318, 211, 274 and 287.

Also, V1’s plate voltage seems low.  I’m using a 7025 tube vs. the 12AX7, and the plates are running 93.1 and 90.4 VDC.  They’re getting their power from Node 3 of the cap and it is outputting 296VDC going into those 270K resistors.

Any thoughts.  I've looked for circuit voltages but can't find any that are relevant for this circuit.

[shooter]

plates are running 93.1 and 90.4 VDC.

the signal coming in is 150mV, it'll swing maybe 2VacPP, those #'s are fine.

[wsscott]

Shooter--How did you calculate the 150mv incoming signal value?  Just so I can learn something.

[shooter]

no calc, just measure the average guitars output, you'll probably find it's ~~~ 150mV

[HotBluePlates]

One thing I'm trying to figure out is the way the 2nd Input/Volume for Channel 2 works.  Input 1 and Volume 1 work normally like any other amp, and turning Volume 2 pot has no effect on its output volume.

However, if I'm using Input 2 ... it only produces an output signal if I turn the Vol. 1 pot past its "Off" postion.  It seems like it's sort of like a Master Volume for Channel 2.  ...

When I built my iteration of the 25L15 I omitted the 2nd channel.  Somewhere I have a paper copy of the schematic, but it is buried somewhere in a box.  Below is the version of the preamp schematic that has been floating around for a long time (and which is probably right for the 2nd channel).

Notice that:
  1.  The 2nd channel Volume is wired "backwards" like a tweed Deluxe: its wiper is the Input and the non-grounded outer-lug is the Output.

  2.  The 2nd channel Volume Output is connected to the Input of channel 1's Volume control.  There is an isolation-resistor in between, mainly to keep Channel 2's Volume from affecting the response of the T-filter tone circuit (whose response would change if you turn Channel 2's Volume control high, due to bleed-back through Channel 2's coupling cap to the plate load resistor for Channel 2's input triode).

So what you're describing sounds "normal" for this circuit.  You gotta turn the Channel 1 Volume high to keep it from reducing the output of Channel 2.

Recognize that this "2nd channel" was not accessible via a control-panel jack: it was hidden on the underside of the preamp chassis (ditto for the Volume control for the 2nd channel).

   - The legend was that the 25L15 was only intended for use by "a Star."

   - Supposedly "no-2nd-channel" was a desired feature: now no other guitarist could "sit in" and "plug into the Star's amp."

   - But there was a "hide-a-jack" 2nd channel, if "the Star" wanted to let someone else play along through their amp.

   - But the sound from the 2nd channel was supposed to either be quieter, or be able to be completely shut-off if "the Star" turned off the main Volume control.

[HotBluePlates]

... the bias is too cool based on Rob Robinette’s bias calculator.  I have Plate to Cathode voltage of 349 volts, and with the 6L6GC’s that I’m using I should have a bias of around 80–85ma, but the highest I can get it using the adjustable bias pot mod that you designed, I can’t get it past 75ma. ...

349v x 0.075A = about 26 watts ---> plenty hot enough for 807s or 6L6GBs

I wouldn't bother trying to run it hotter.  It won't really get you any more "goodness" in this amp.

Or you can shrink the value of a fixed-resistor in the output tubes' cathode circuit to increase the plate current.

The Pre-amp's cap nodes are reading: 318, 211, 274 and 287.

Also, V1’s plate voltage seems low.  I’m using a 7025 tube vs. the 12AX7, and the plates are running 93.1 and 90.4 VDC.  They’re getting their power from Node 3 of the cap and it is outputting 296VDC going into those 270K resistors.

Close enough for Rock n' Roll.
(or even Chet Atkins tunes...)

(296v - 90v) / 270kΩ = 0.76mA ---> plenty enough current per triode

As Shooter says, a guitar's pickup probably delivers only ~100-150mV and if the stage gain is ~60 then the output signal is only 6-9v.  There's plenty of headroom for an Input Stage.

[wsscott]

HBP--Thanks for the input.  Looks like I've got the wiring done correctly and it does sound really clean--even through the cheap Fender Champion 20's speaker!  I'm trying to decide what speaker to get for this amp now that it looks like it deserves something nice.  I'm probably going to go with a 12" vs. 15", but which make, I don't know.

[wsscott]

I wanted to know what the Output wattage of the amp is at the speaker, so I connected an 8 ohm dummy load and injected a 150 mVrms at 1Khz signal.  Using my scope I adjusted the sine wave until it started to flatten and then backed off for a clean wave form.  At that point it is reading 11.3 Vrms.  So if you square that number and divide by 8 ohms, you get right at 16 watts.

That's pretty far off from the 25 watts output its supposed to produce.

Yet your math shows 26 watts.

So what am I doing wrong?

[shooter]

349v x 0.075A = about 26 watts ---> plenty hot enough for 807s or 6L6GBs

Tube's dissipation,  in watts

So if you square that number and divide by 8 ohms, you get right at 16 watts.

Audio signal,  in watts

[wsscott]

Ahhh, so then what is a "25 watt amp" being measured for, ie. tube dissipation or audio signal?

[shooter]

so then what is a "25 watt amp" being measured for,



 


the buyer, salesmen just do what salesmen do


tube dissipation is for the tech/builder, so we know that $100 tube set won't let all the magic smoke out before the guitarist can get all giddy.


I build a 23W (measured at speaker), 5 guitarist said "NO WAY that's a 23w amp, it's gotta be closer to 40W"  so I converted 23wrms to PP and they were happy 

[wsscott]

OK, now that's the answer, ie. multiply by 1.414 to get PP, and its what I wanted to see!

[passaloutre]

Turn your 150mv signal up until the output *just* starts to visibly distort on a scope. Then measure your output voltage.

[wsscott]

I"ll give that a try.

[HotBluePlates]

... 16 watts.

That's pretty far off from the 25 watts output it's supposed to produce. ...

Differences of Power can be expressed in Decibels:  10 x log (16w / 25w) = -1.94 dB

Differences of Loudness can be expressed in Decibels:  +10dB is commonly perceived as "doubly-loud."

Speaker Sensitivity can be expressed in Decibels:  dB SPL at 1 meter, for 1 watt of applied power


You will find it is pretty easy to find a speaker whose Sensitivity Rating is 2dB higher than some other speaker you might use.  Which then means you can easily make up the loudness-difference of "16w instead of 25w" with a more-sensitive speaker.

[wsscott]

I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be. 

Or maybe my method of measuring the output is wrong.

[shooter]

maybe my method of measuring the output is wrong.

more like an "understanding" of the many ways in which Watt's gets used to equalize apples n oranges

[passaloutre]

I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be. 

Or maybe my method of measuring the output is wrong.

Again, you measured the output power with an input signal of 150mv. That's a completely valid thing to do; the answer wasn't wrong, it just wasn't what you actually wanted to measure. The amp was outputting 16W under those conditions.

But generally, when someone says "this is a 25W amp" they are either: (A) referring to the maximum clean output power, or (B) more interested in marketing than physics, as shooter alluded to.

I'm thinking your amp can put out more than 16W of clean output, but to measure that you need to turn it up until *just* before visible clipping. Even this is an inexact science, as harmonic distortion occurs well before the clipping becomes visible on a scope

[wsscott]

Sorry, I probably wasn't clear.  The 150mRms input signal was with the Volume pot all the way down, and it raised up to 16VRms by my turning up the Volume pot, until at that point it showed beginning of distortion on the scope.

[BrianS]

oops.

[sluckey]

Calculate your power output using the peak voltage (15.9V, right?), then convert that to RMS...

Why bother? He already measured 11.3V_RMS across an 8Ω load. That equates to 15.96W_RMS. No conversion needed.

[wsscott]

Stuckey's correct.

[HotBluePlates]

I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be. 

Maybe they don't really make "25 watts."  Maybe that was just a guess.  Or maybe the output section makes 25w with a higher supply voltage (which then allows same-tube-current to create a larger voltage drop across the OT primary, and same-current x more-voltage-drop = more-power).

I never bothered measuring mine.  It sounded good when I finished assembling it, and that was proof-enough for me.


If the amp seems to be working right otherwise then I recommend you stop "looking for problems," and just enjoy the amp.