Preamp voltage and tone Question

[tubenit]

I've been contemplating building a low wattage 6BM8 amp but with clean/overdrive channel switching to have access to a higher gain low wattage amp.

And so I have a theoretical question ..................     There are enough reality factors such as speaker, power amp, amp cab wood, etc... that at some level it's not completely a fair & realistic question. .

Having said that, I need a starting place to being thinking about the amp design and so I'd be grateful for responses that work within the confines and  framework of the theory and concept of this question, please.

I have a 6BM8 amp called the Minimalist & it's about 7 watts.   I have a cathode biased 5881 amp that is about 23 watts and it's the Carolina Overdrive Special.  They both have FX loops.

So, let's say I run the COS preamp with clean and overdrive into the power amp of the Minimalist and really like the tone I am getting .....................

Well, the COS power transformer puts out 400v on the power tubes and the Minimalist power transformer puts out 300v on the 6BM8 power tubes.

If I wanted to design an amp that emulated this sound (and NOT use a VVR which I will not do again) then I'd need a way to try to match the preamp tube voltages of the COS using a lower voltage power transformer. 

Let's say the Minimalist B+ rail dropping resistors are pretty low value already and so there is not much room there to change the node E significantly on the B+ rail and increase the node E voltages.

So my next option in my thinking is to change the plate/cathode resistors on the preamp tubes. I'd lower the values on the plates/resistors with a lower node E to try to achieve the same voltages on the plates/cathodes.

So, IF I were able to reduce the plate/cathode resistors to a lower value and that gave me similar voltages on the preamp tubes ........... would it be a reasonable expectation to have the preamp sound the same tonewise?   

What I am referring to hear is IF  there was a way to "test" both preamps thru the "same" power amp would I find a similar tone to these preamps?  I am qualifying similar to mean similar but not exactly the same tonewise.  Maybe similar enough that in a blindfold test one couldn't tell the difference?


Essentially my question is :   Does the voltages on the plates/cathodes determine the tone ............... OR does the values of the plate/cathode resistors also play a factor in tone even if the voltages are the same?


With respect,  Tubenit

[CascoSieg]

I'd think comparing load lines for each scenario might be informative, since different voltages will place the signal in different areas of the "linearity curves" of the tubes (hmmm... there's a more proper way to say that). Also, related to that, the specific pre-amp tubes used (maybe that's mentioned somewhere?) would seem to be a factor, since the same changes in voltage will produce proportionally different changes in bias depending on the tube characteristics.

[shooter]

Does the voltages on the plates/cathodes determine the tone

My understanding they determine when and how a particular gain stage distorts, or not.
if it's designed to amplify in class A, tone should not be affected.  think center biased.
if it's designed to compress or clip, tone will be affected

[Greg]

I would say that changing plate and cathode resistor values while keeping same supply voltage will result in a big tone change. However, keeping cathode and plates resistor values the same while adjusting supply voltage will not affect the tone to the same extent. I guess that is why Fender kept almost the same preamp values from deluxe reverb (lower V1 plate supply)  to twin (higher plate supply). I would use a small preamp choke. Hammond makes one around 20 henries and 2600 ohms if I remember correctly, just a small 5ma choke the size of a reverb transformer. This should keep your available preamp supply almost as high as your main B+.

[jojokeo]

I prefer a higher voltage, if possible, yielding brighter cleaner punchier play and tones. I like reaching for cleans in most cases these days as breakup/overdrive never seems to be an issue for so many other aspects that goes into things. Especially when using lower powered output tubes like with the BM8's. Lower the preamp load resistor without worry to get the specific voltage range you're looking for thereby lessening gain. Bias as you would normally with cathode/cap values. Do you remember an old forum "KOOK" (keeper of odd knowledge)??? He would say similarly and remember +/-20% tolerance rule. So not overly critical at all. And, "he who builds it is doing over those talking". I miss old DaGeezer. I've done exactly what you're wondering many times as I convert donor organ and HiFi amps, etc.


But whatever the voltage range it doesn't have to be exact. Get what you can and either way, at the end of the day you're only going to get what you're going to get unless changing the PT.
To your "theoretical" postulation ie - design criteria - I think YES to your last questions, all things being as equal as possible.


As HBP once said, (paraphrasing) "As preamp tube transconductance changes, so does the bandwidth increase which yields cleaner, clearer, punchier, and brighter tone." Now with transconductance, plate resistance also plays a roll. And notice that the highest gain factor, 12ax7 is ~100 (1600uMHOS/62k5 ohms plate) where the 12ay7 is ~45 (1750uMHOS/25k ohms plate), yields the lowest transconductance and highest plate resistance. These are also your TONE parameters that you are playing with yielding options under your control and choices to make. Hopefully this helps as a little food for thought along with the component changes aspect?


*The attachment shows creative preamp voltage ideas rather than using a long series type of filter node. Yes, not as good of filtering to an "E or F node" but you may not notice or need that much and keep a higher voltage?

[kagliostro]

Ciao Jeff

I hope you don't consider my solution to the problem as extravagant

for me as to have a response of the preamp like in the other amplifier it is important that you do not touch anything, the components around the tubes of the preamp and the power supply must not be touched at all

Okay, you need about 100 volts less for the anode of the Power Tubes  and you don't want to use a VVR, solution that can be shared as an idea of ​​principle

So what would I do? Well, don't consider me extravagant, but in my opinion the simplest solution would be to make the power supply for the 6BM8 and compensate for the missing 100V of B+ for the plates of the preamp with a tiny transformer capable of delivering about 70V AC that will result in around 100V DC

This additional DC voltage must be connected "in series" with the anode of the final in order to reach the desired voltage resembling what is done on this PS (that use multiple windings instead of multiple PT)



keep in mind that for the preamp you only need a few milliamps of current and that therefore you just need to use a very small additional transformer

---

An alternative will be to see if is possible to arrange a PS the way is arranged this old Philips PA PS as to obtain the voltages you want, very simple solution, no doubler, only B+1 will be 2 x B+2, no problem about current because the two halves of the HT secondary with CT are rated for the full current disposable for the circuit on transformers with CT




Obviously your interest will be to feed the PT with the Lower B+ Tension and the preamp section with the Higher B+ Tension

Franco

[tubenit]

Guys, I appreciate ALL the responses!  Thank you! You've given me much to think about.


Here are some of the schematics that I've been pondering.  Don't plan to start this anytime soon unfortunately.  Maybe a 2023 build?



With respect, Tubenit

[nandrewjackson]

I've wondered about the node voltage to overall tone relationship too.


I don't have a setup to test my wondering in the real world right now.


I've built some amps/preamps using 12ax7 preamp tubes that had higher or lower voltages, depending on whatever transformers I had. Usually using the standard 100K plate / 1K cathode, without too many variations. An occasional 150K or 200K  up top, and an occasional 800R or 2K2 below, based on whatever design I was plagiarizing at the time, lol.


Id like to build a modular "breadboard" system for valve amp tweaking, I think I've seen some here on the forum.


I suspect there are indeed some "sweet spots" in the voltages for some designs.


😎✝️🎸🎸🎸🎛️🎛️🎛️🎧

[dwinstonwood]

Essentially my question is :   Does the voltages on the plates/cathodes determine the tone ............... OR does the values of the plate/cathode resistors also play a factor in tone even if the voltages are the same?

I've wondered about this, too. It's always been a concern when choosing a PT for a new project, along with what dropping resistor values to start out with (and, I tend to go back and adjust dropping resistor values once the amp is built).

In my simple/beginner's way of understanding this, the two biggest factors that will affect the sound/tone are:
1.) the tube's gain - and, what that gain level does to the next tube;
2.) the bias point - hot, center, or cold - and how that distorts the AC signal waveform.

I don't know if this will help, but Merlin's Load Line Plotter calculates a tube's gain, and displays the bias point when inputting different B+ voltages and resistor values.

It's at the bottom of this page under the "Downloads" heading:
http://www.valvewizard.co.uk/links.html

[tubeswell]

Does the voltages on the plates/cathodes determine the tone ............... OR does the values of the plate/cathode resistors also play a factor in tone even if the voltages are the same?

Read this Chapter from Merlin Blencowe's 'Designing tube pre-amps' book http://www.valvewizard.co.uk/Common_Gain_Stage.pdf (or better still get the whole book)


You'll see that load resistance (including the size of the plate resistor) and HT voltage, determines the position and gradient of the load line w.r.t. the grid curves.  The bias point - determined by the cathode current and bias method (including cathode resistor, when cathode biased) - determines the plate voltage idle point along the load line.


If the idle point occurs in the centre of the effective grid voltage range, harmonic distortion can be minimised.


If the idle point is moved closer to the right-hand end of the load line (where the more-negative grid curves are - i.e. colder bias), there is a greater likelihood of harmonic distortion, and ultimately if the bias is cold enough, the tube would be in cut-off.


If the idle point is moved further to the left-hand of the load line (closer to the Vg0 grid curve), there is more likely to be signal clipping at the grid (caused by grid-current limiting), where positive swings at the grid don't get amplified.


So the answer is yes.

[shooter]

Tone can be mathed out, but you need to understand how complex signals interact with complex math.  a 400hz signal just won't cut it.
Experimentation is closer to the real world, start with something close, experiment til it becomes "your tone"
which looks n sounds Nothing like "my tone"
EARS are your best calculator for tone, assuming you started with the basic understanding of how the circuit works to begin with.