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.