Fender Princeton AA1164 voltage dropping resistors

[OnlyFanes]

Hey! This is my first post. I am just finishing my first DIY amp – a TAD Princeton Reverb. I see that the Mojotone schematics use different values (15K) for the two dropping resistors than TAD and Fender (18K).

Could someone please explain what these resistors do, and what would be the result if I use 15K vs 18K? The mains AC voltage where I live is quite hot, so I suspect I'll get voltage values about 15-20% over nominal. Would I benefit from using 15K vs 18K?

And: What about the 1K 1W resistor? I've read that it drops the B+ to the screens. Is that right? Could (and should) I use a different value to  decrease B+?

Thanks for any help.

Cheers!

[Willabe]

Welcome! 

I am just finishing my first DIY amp – a TAD Princeton Reverb.

Maybe should have posted before you started? 

.... The mains AC voltage where I live is quite hot, so I suspect I'll get voltage values about 15-20% over nominal.

It would be helpful to us to be able to answer you, when you say something, not to be vague, give us a little more detail, like the wall voltage. 

Read this, from Merlin's web site, on the dropping resistors you asked about in the B+ power supply;

https://www.valvewizard.co.uk/smoothing.html

And this, you'll need to have a solid understanding on good grounding, how and why, very important;

https://www.valvewizard.co.uk/Grounding.html

From Rob Robinette's amp web site, section on Princeton Reverb amp, very good over all info and he has a hi-lited Fender Princeton Reverb schematic showing the amps signal flow, and also the layout drawing hi-lited with the same colors to match the hi-lited schematic so you see how the signal flows through the chassis;

https://robrobinette.com/AA1164_Princeton_Reverb.htm

And there's lots of stuff in Rob Rob's site;

https://robrobinette.com/Amp_Stuff.htm

Sluckey's web site Scrapbook section, page 9; hi-lited -bias flow, page 12, hi-lited input jack, hi/low flow;

https://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf

And more great stuff in Sluckey's web site;

https://www.sluckeyamps.com/index.htm

Much of/most of what you need to know how to build that amp will be in those web sites.

Try and look up what your are unsure of before you post, then if your still unsure or can't find it, post your question.

[Willabe]

And please post the schematic with your question, NOT the layout drawing please.

The layout drawing is for finding where the parts are in the chassis.

The schematic shows the circuit and the flow of the circuit. Most questions will need the schematic.

You'll see this when you look at the schematic and layout drawing that Rob Rob hi-lited on the Fender Princeton Reverb, showing the circuits flow. It will be very helpful to you as a new builder.

[Latole]

Welcome OnlyFanes.

[acheld]

Willabe's post with all the Princeton references is a tremendous resource!   And for you, the "Smoothing and Filtering . . ." page on Merlin's website is what you need to know.

Please do post your schematic.   It will help us help you.  If you're building a Mojotone amp, the schematic will be easily available to you.  Speaking for myself, I can't make the jump from layouts to schematics easily.

As for your question, 15K vs 18k will yield similar  results.  But, just how similar really depends on the current flowing and the voltage.  In practice, I normally measure voltages, derive current via Ohm's law, and choose a new resistor to meet my design goals -- and then measure again with the new resistor installed.

Do you really only use Fane speakers?

[OnlyFanes]

I appreciate you trying to help and also pointing out that I'm too vague on certain aspects. The reason may be that that I'm totally new to this and that English is not my native language.

I've read many articles but am not experienced to fully understand everything I read. Thats why I'm looking for a plain answer on that these dropping resistors actually do.

Here are the schematics. The three resistors I'm referring to is located around the cap can. MojoTone uses 15K+15K+1K. Fender and Tube Amp Doctor uses 18K+18K+1K.

Thanks again!

[SEL49]

I've read many articles but am not experienced to fully understand everything I read. Thats why I'm looking for a plain answer on that these dropping resistors actually do.

Those resistors drop the high B+ voltage to a smaller voltage more appropriate for the circuit that will be supplied. The resistors combined with the filter caps also smooth out the big AC ripple that's present at the output of the rectifier.

Fender chose 1K and 18K based on their own design criteria. Mojo chose 15K/2W because it's close enough but mainly because they don't even stock 18K (except for an 18K 1/4W). So, Mojo's choice was mostly based on their inhouse logistics.

[OnlyFanes]

Those resistors drop the high B+ voltage to a smaller voltage more appropriate for the circuit that will be supplied. The resistors combined with the filter caps also smooth out the big AC ripple that's present at the output of the rectifier.

Thanks!
Do you happen to know what increasing or decreasing the different resistor values will do?

[stratomaster]

There's also a 340V vs 328V transformer to consider.  I think the 15k is also to try and bump up the preamp voltage to compensate.

But take a look at the notes for voltages on the Fender schematic.  The tolerance provides for a very wide target, 15k vs 18k is unlikely to result in voltages that violate the original Fender spec.

[stratomaster]

Those resistors drop the high B+ voltage to a smaller voltage more appropriate for the circuit that will be supplied. The resistors combined with the filter caps also smooth out the big AC ripple that's present at the output of the rectifier.

Thanks!
Do you happen to know what increasing or decreasing the different resistor values will do?

Ohm's Law is ∆V=I*R (change in voltage equals the product of current through a given resistance).

I think with this as a basis, and by approximating the current draw as constant, you can sort out your question for yourself.

[Willabe]

Do you happen to know what increasing or decreasing the different resistor values will do?

This is why I posted the link to Merlin's web site on B+ filtering.

It goes through what the dropping resistors do.

It's only 1 page, read it. Then if you still don't understand, come back and ask your question.

https://www.valvewizard.co.uk/smoothing.html

[Willabe]

I appreciate you trying to help and also pointing out that I'm too vague on certain aspects. The reason may be that that I'm totally new to this and that English is not my native language.

I could see where you where at and where you wanted to be going in your 1st post. That's why I posted all those links. If you read through them, and they are not long, you probably won't even need us to answer any questions. Or maybe just a few.

Much of what you want to know has already been written about and is on line for free.

It is much easier for us to post a link to a well written source of information then to answer your questions 1 at a time.

When I posted those links, I did answer your questions. You need to read them. Then if you still have questions, ask. 

You still haven't posted your wall ac voltage like I asked you to.

I've read many articles but am not experienced to fully understand everything I read. That's why I'm looking for a plain answer on that these dropping resistors actually do.

We have no idea what kind of articles you read. Going by that you don't know what a simple dropping resistor does, they didn't have much information in them.

I'm trying to guide you to good solid correct information. Did you read through the links I posted for you? Did you even open them up and look at what's in them?

There is a plain answer on what a dropping resistor is for and does in that link I posted, read it.

https://www.valvewizard.co.uk/smoothing.html

[Willabe]

There's also a 340V vs 328V transformer to consider.  I think the 15k is also to try and bump up the preamp voltage to compensate.

It might be but, we don't know that, because we don't know how much sag either PT's B+ will have when fully loaded.

[Willabe]

Those resistors drop the high B+ voltage to a smaller voltage more appropriate for the circuit that will be supplied. The resistors combined with the filter caps also smooth out the big AC ripple that's present at the output of the rectifier.

Thanks!
Do you happen to know what increasing or decreasing the different resistor values will do?

This is why I want him to read the link from Merlin. He's just going to have 1 new question after another as you answer him.

That link is only 1 page and will give him a solid foundation on dropping R's to go by.

He has to be willing to do some home work on his own. I put it in his hands, he didn't even have to search for it. Doesn't get much easier than that. 

[OnlyFanes]

Thank you. I didn't get the answer I was looking for reading the articles. But I think this might be it:

In the Fender Princeton Reverb, the two 18kΩ resistors and the 1kΩ resistor in the power supply section form a chain of voltage-dropping resistors that step down the high B+ voltage. This series of resistors provides the correct, gradually reduced voltages to the amp's stages (phase inverter, preamp, and reverb), which impacts tone, headroom, and response. Here’s how each resistor functions and what changes to their values would do:

Roles of the Resistors

Voltage-Dropping Purpose:
1kΩ Resistor: First in the chain, it provides an initial drop to supply the phase inverter and parts of the preamp, setting the stage for overall power and tone.

Two 18kΩ Resistors: Positioned further down, these resistors provide additional voltage drops to the preamp and reverb stages for precise voltage management.

Series Voltage Drop: The entire series gradually reduces B+ voltage to levels appropriate for each circuit section, ensuring stable operation and affecting each stage's tone and response.

Effects of Changing Resistor Values
1. Increasing the 1kΩ Resistor (e.g., to 1.5kΩ or 2kΩ)
Lower Voltage to Phase Inverter and Preamp: A higher resistance in the 1kΩ resistor drops more voltage across the whole chain, reducing B+ to the phase inverter and downstream stages.
Warmer, Compressed Tone: The amp will break up sooner, creating a softer, vintage-style tone with reduced headroom.
Potential for Noise: If the voltage drops too much, it can lead to instability and noise.

2. Decreasing the 1kΩ Resistor (e.g., to 500Ω)
Higher Voltage to Phase Inverter and Preamp: Less resistance means less voltage drop, resulting in higher B+ to the phase inverter and preamp stages.
Brighter, Cleaner Tone: The amp will have more headroom, faster attack, and a punchier, defined response, especially at higher volumes.
Increased Component Stress: Higher B+ voltages can stress tubes and capacitors if they exceed their ratings.

3. Increasing the 18kΩ Resistor Values (e.g., to 22kΩ)
Lower Voltage to Preamp and Reverb: Higher resistance in these resistors lowers the B+ to the preamp and reverb stages, specifically affecting preamp dynamics.
Warmer, Compressed Preamp Tone: The amp’s preamp will have more saturation at lower volumes, producing a slightly darker, more vintage tone with early breakup.
Possible Instability: Too much voltage drop in the preamp could lead to excess noise and instability.

4. Decreasing the 18kΩ Resistor Values (e.g., to 15kΩ)
Higher Voltage to Preamp and Reverb: Reducing resistance here lowers the voltage drop, delivering higher B+ to the preamp and reverb driver stages.
Cleaner, Brighter Preamp Tone: Higher voltages provide more headroom, yielding a tighter, clearer tone with faster attack.
Stress on Components: Raising B+ too high might wear out components faster if they are rated for lower voltages.
How the Resistors Interact
Increasing the 1kΩ resistor: Lowers voltage across the entire amp chain, affecting every stage after it and making the amp warmer with less headroom.
Increasing 18kΩ resistors: Primarily lowers voltage to the preamp and reverb stages, resulting in a compressed, vintage tone specific to the preamp.
Decreasing any resistor: Delivers higher voltages to the stages they supply, increasing clean headroom and brightness, though stressing components if voltage goes too high.

Summary of Effects
Resistor Change   Voltage Effect   Tone Result
1kΩ to 1.5kΩ+   Lower entire chain   Warmer tone, early breakup
1kΩ to 500Ω   Raise entire chain   Brighter, punchier, more headroom
18kΩ to 22kΩ+   Lower preamp voltage   Darker, compressed preamp tone
18kΩ to 15kΩ   Raise preamp voltage   Cleaner, brighter preamp tone

These adjustments provide control over the Princeton’s tonal and dynamic response, enabling a range from softer, vintage sounds to tighter, punchier tones. Be cautious of component limits to prevent overstressing the amp.

[Willabe]

I gave you the links for everything I posted, you post a big article with no reference who wrote it and where to find it on line. 

The three resistors I'm referring to is located around the cap can. MojoTone uses 15K+15K+1K. Fender and Tube Amp Doctor uses 18K+18K+1K.

Ok, now you understand what the dropping R's do in the B+ power supply filter cap chain.

Why the Mojo amp kit uses 15K instead of 18K is anybodies guess. But like I said before if the Mojo power transformer under full load, all tubes in, has a lower dcv than the Fender, they might have wanted to get the preamp dc plate voltage back up to where the Fender would be. Or, they just like the souund of the preamp with a little higher dcv than with a lower dcv.

Really, 15K/18K at the 1st and 2nd stage preamp won't make much difference, it will be a slight difference. Those preamp stages pull such low current, 1mA to 2mA, that the R there doesn't drop much voltage. The more dcv it drops, the bigger the difference in sound. It will be a subtle difference in tone.

The screen dropping R, 1st R, will make the most difference as the article said, because 1. the screen pulls more current than the 1st/2nd stage preamp tube and all the small bottle tube will draw current through that screen dropping R too. 

So put in what you like, up to you to decide which you like better.

[Willabe]

I didn't get the answer I was looking for reading the articles.

Yes, you pretty much did get the answer for what you asked.

This is what you asked;

Could someone please explain what these resistors do, and what would be the result if I use 15K vs 18K? The mains AC voltage where I live is quite hot, so I suspect I'll get voltage values about 15-20% over nominal. Would I benefit from using 15K vs 18K?

And: What about the 1K 1W resistor? I've read that it drops the B+ to the screens. Is that right? Could (and should) I use a different value to  decrease B+?

You were posting in reference to voltage with the dropping R's. Nothing on amp tone, response, power, ect. So I posted the link for Merlin on smoothing and filtering, that covers dropping R's and voltages.

That's why I'm looking for a plain answer on what these dropping resistors actually do.

After reading the Merlin link and the article you posted, you should now be able to understand, there is no "plain answer on what these dropping resistors actually do." Because, the dropping R's are not isolated from the rest of the amp.

Simplest 'plain' answer; they drop dcv. The Merlin link on 'smoothing and filtering' explains this.

So, true answer, or more complete answer is; Those dropping R's do several things and cause several things to happen. And because they do much more/affect much more, and your posting in reference to voltage, that is why I wanted you to read the Merlin link.

The Merlin link tells you they drop dcv, they decrease ripple working along with the filter caps, they along with the B+ nodes filter cap provide isolation from other amp stages, ect. And there's a drawing at the very bottom that shows the dcv's dropping after each B+ filter cap/dropping R stage.

Until you understand how the B+ power supply chain works, you aren't ready for talking about higher/lower acv on the tubes plates affecting the tone, response, power, etc. You learn how the power supply works 1st, then you learn how it also will effect the amps tone, feel/response, clean/distortion/compression, sustain, frequency response, power, etc. 

Changing a B+ power supply dropping R effects several/many things in the amp. So, you may have to find a balance between the ripple reduction/B= filtering and amp tone/response and that may take compromise.

And: What about the 1K 1W resistor? I've read that it drops the B+ to the screens. Is that right? Could (and should) I use a different value to decrease B+?

The Merlin link explains this.

What about the 1K 1W resistor? I've read that it drops the B+ to the screens. Is that right? Could (and should) I use a different value to decrease B+?

Decrease it, why? From what? Can't answer, you never gave us ANY of the amps dcv's.

Do you happen to know what increasing or decreasing the different resistor values will do?

Merlin link explains this. Short answer; It will raise or lower the dcv in the B+ filter cap power supply chain.   

Could someone please explain what these resistors do, and what would be the result if I use 15K vs 18K?

Merlin link explains this. Short 'plain' answer; They drop dcv in the B+ power supply chain. Using a 15K dropping R will drop less dcv.

True/more complete answer is; Those dropping R's do several things and cause several things to happen. In the Merlin link, he covers the smoothing and filtering part.

The mains AC voltage where I live is quite hot, so I suspect I'll get voltage values about 15-20% over nominal. Would I benefit from using 15K vs 18K?

Can't answer because you never posted your wall acv.

[Willabe]

1kΩ Resistor: First in the chain, it provides an initial drop to supply the phase inverter and parts of the preamp, setting the stage for overall power and tone.

No, this 1st R drops the dcv to the power tube screens and reverb.