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Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on April 25, 2026, 12:07:18 pm »

Thanks.

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Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by HotBluePlates on April 25, 2026, 10:47:18 am »

What type of non-electrolytic, 600V caps would you use in a build of this Standel, ie. polypropylene, polyester, 715P or 716P, Orange Drop, Mallory, etc.?

You mean for coupling caps?

I used no-name polyester coupling caps.  They do the job well enough, are not overly-large, and the amp sounds good!

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AmpTools/Tech Tips / Re: Welwyn W20 Series Wirewound Resistors - Voltage Limits

« Last post by HotBluePlates on April 25, 2026, 10:39:07 am »

I have some W21 3W and W22 7W resistors and was planning to use them as HT node droppers and power tube screen resistors ...

But now looking at the datasheet they have quite a low "Limiting element voltage" ...

Or does this mean the voltage ACROSS the resistor? As those W22's pop up everywhere but are only rated for 200V

The voltage ratings make sense if you look at the resistance available, and the wattage rating:

   - W21 is rated for 100v across the resistor and you might use a 470Ω or 1kΩ for a screen resistor, right?
   - 100v / 470Ω = 213mA and 213mA x 100v = 21.3 watts ---> way over the 2.5 watt rating

   - W22 is rated for 200v across the resistor, and 6 watts at 70ºC.  Say you use it as a 1kΩ screen resistor:
   - 200v / 1kΩ = 200mA ---> your tube's screen melted long ago.
   - 200v x 200mA = 40 watts ---> far exceeds the power rating of the resistor, may last through temporary overload but will burn if sustained long enough.


The screen resistor is dropping volts when there is high screen current draw to keep the screen from melting.
Look at a tube data sheet for a graph of plate current with different G2 voltages, and you see very large reductions of plate current when screen voltage drops 50v or 100v (and screen current drops proportionally).

While there are likely brief blips higher, the Mullard EL34 data sheet suggests sustained screen current doesn't exceed ~50mA at full power output.  That would be 50v dropped across a 1kΩ W22, and 2.5 watts in this 6w resistor.


The voltage rating limits start making sense when you consider that wirewound resistors are rarely available above ~10-20kΩ, and tend to be wound in low resistances, passing relatively high current, and mostly low voltage-drops across the resistor itself.

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Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on April 25, 2026, 09:13:39 am »

What type of non-electrolytic, 600V caps would you use in a build of this Standel, ie. polypropylene, polyester, 715P or 716P, Orange Drop, Mallory, etc.?

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Tube Amp Building - Tweaks - Repairs / Re: A Question about system design

« Last post by pbman1953 on April 25, 2026, 06:21:25 am »

... the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  ...

The Volume control is hooked to the output (wiper) of the Treble control.

... on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why ...

You may be focused on these two models, but that tone stack (and the Volume control's position right after the Treble output) is the same on nearly all Fender amps from the mid-60s onward.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.  ...

Look up a "Bridged-T filter."  Its function is to impose a mid-scoop on a sound, while leaving the higher & lower frequencies less-reduced.

The standard Fender tone stack is sort of a modified Bridged-T filter, with some parts made variable to adjust the mid-scoop and overall sound of the amp.

The Bass control is a variable resistor, and sits between 2 caps whose other end are connected.  When you "turn the Bass up" you really make the 0.1µF cap appear in the circuit as though it were a 0.147µF cap.  That's because the resistance between the caps falls to zero, the caps are placed in-parallel, and the total capacitance goes up.



One form of a Bridged-T filter would have a cap-to-ground.  But sometimes the user doesn't want to "scoop all the midrange out" so they add a resistor between the cap & ground.   That's the "Middle" control in this circuit.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound. ...

What would be the benefit of having this tone stack?

If you were playing guitar rather than bass, it would be more obvious.
The standard Fender tone stack is not much use for bass, though even with guitar turning down the Middle and Bass controls (and having the Treble control below half) just about zaps all the signal.  But that's not really intentional, but just a byproduct of this circuit being arranged for convenience, low cost and low parts count.

After I asked the question, I researched more and yes, this tone stack was very common to Fender.  Thanks for the detailed write up!

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Tube Amp Building - Tweaks - Repairs / Re: A Question about system design

« Last post by HotBluePlates on April 24, 2026, 03:46:35 pm »

... the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  ...

The Volume control is hooked to the output (wiper) of the Treble control.

... on the Fender Super Twin and Studio Bass amps, is after the one half of the v1 tube from the input. After the bass control the signal goes to the Volume control and on to the second half of the  v1 tube.  My question is why ...

You may be focused on these two models, but that tone stack (and the Volume control's position right after the Treble output) is the same on nearly all Fender amps from the mid-60s onward.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound.  ...

Look up a "Bridged-T filter."  Its function is to impose a mid-scoop on a sound, while leaving the higher & lower frequencies less-reduced.

The standard Fender tone stack is sort of a modified Bridged-T filter, with some parts made variable to adjust the mid-scoop and overall sound of the amp.

The Bass control is a variable resistor, and sits between 2 caps whose other end are connected.  When you "turn the Bass up" you really make the 0.1µF cap appear in the circuit as though it were a 0.147µF cap.  That's because the resistance between the caps falls to zero, the caps are placed in-parallel, and the total capacitance goes up.



One form of a Bridged-T filter would have a cap-to-ground.  But sometimes the user doesn't want to "scoop all the midrange out" so they add a resistor between the cap & ground.   That's the "Middle" control in this circuit.

... My question is why, because that bass control determines the volume gain of the whole amp. If the bass control is all the way down there's almost no sound. ...

What would be the benefit of having this tone stack?

If you were playing guitar rather than bass, it would be more obvious.
The standard Fender tone stack is not much use for bass, though even with guitar turning down the Middle and Bass controls (and having the Treble control below half) just about zaps all the signal.  But that's not really intentional, but just a byproduct of this circuit being arranged for convenience, low cost and low parts count.

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Other Topics / Re: Meme of the day

« Last post by shooter on April 24, 2026, 03:15:00 pm »

...

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Tube Amp Building - Tweaks - Repairs / Re: Groove Tubes Soul-o 45 Rev B 1997 8 18 blowing main fuse

« Last post by wsscott on April 24, 2026, 02:10:56 pm »

My buddy cleaned the tube sockets and a few connections, and that seems to have fixed it.  He let it run for over 2 hours with no issues, and then used it in rehearsal for about 2 more hours and no issues.

So the owner is picking it up this weekend.  Glad it just needed some cleaning of the sockets.  It apparently had been sitting for several years, so that probably was a factor.

Thanks again for everyone's help.

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Tube Amp Building - Tweaks - Repairs / Re: Negative Feedback resistor value for bass

« Last post by pbman1953 on April 24, 2026, 10:38:32 am »

Based on your experience and help from the past, I'll leave it


Thanks!

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Tube Amp Building - Tweaks - Repairs / Re: Negative Feedback resistor value for bass

« Last post by sluckey on April 24, 2026, 10:32:27 am »

Try it and let your ears decide. What is your goal? The stock values work very well, especially for a clean bass power amp. Nice balance between fidelity, stability, and reliability. Most of the people you see tinkering with NFB are guitar players that are trying to get a bit of dirt in the power amp. That's OK for guitar, but not so for bass. IMO, a bass power amp should be powerful and clean. The original circuit already gives you that.