plate voltage effect on tone

[pula58]

my first build (see post:

https://tubeamparchive.com/viewtopic.php?t=13650 ).

Works pretty good...but..I am now getting into tonal tweaking.

The fist preamp tube (v1) has a plate voltage(s) of 285V. If I lowered it to, say, 250 V what would change in the tone/feel of the amp? I think if I change the 7.5K power supply dropping resistor to 15K that would lower the plates to 25o or so. What might I expect from doing this?l

I ask this cause I notice that in, say, Dumble circuits the plates voltages get progressively lower as you go from output to input of the circuit. In a dumble, the fist triode even has a lower plate voltage than the 2nd triode (both in the same tube).

[iknowjohnny]

lower voltages tend towards a softer squishier tone/feel while more=a brighter crisper tone but at the cost of a more harsh high end. Some say more voltage=more dynamic, tho till the voltage gets very low i don't find that to be very noticeable. I have had V1 at 100v before like the badcats do. It is kinda nice in a high gain pre because it really makes things more round and soft compared to harsh and bright. But at that point i DID notice a loss of dynamics so i took it back up. I now find that just a basic 250 or thereabouts gives me what i want which is a good balance of all the details and nothing over bearing.

Just give it a try, it's quick and easy. I just daisy chained resistors one at a time and trying the amp after every one. At one point i had like 30k dropping resistors at least on the last node at V1 if not the one b4 that too ! Thats when i had 100v at v1. After years or experimenting with the power supply i came to like what i first started with...10k nodes with 20uf at each except the power tubes where i have 100uf each for plates and screens. I'm getting around 250v at all the preamp stages.

[tubeswell]

But the difference between 285V and 250V would not be much in terms of tone., whereas you might notice more tonal difference between 285 and (say) 100.

[Firestorm]

As long as you're thinking about what happens in a Dumble circuit, dave g had some interesting insights into that here:https://tubeamparchive.com/viewtopic.ph ... highlight=

[iknowjohnny]

But the difference between 285V and 250V would not be much in terms of tone., whereas you might notice more tonal difference between 285 and (say) 100.

True. To hear or feel (and feel is actually the biggest change) the difference you need to go even further than that. I'd say 50v just to notice a slight difference. 100 is considerable. At least thats what i noticed but it may vary depending on many factors including the design and especially a cascaded higer gain p[reamp compared to clean vintage pre.

[brewdude]

Is the 285v the voltage at the 12ax7's plate, or the B+ voltage from the PS node that feeds the plate load resistor?

I seem to like 12ax7's with a B+ of about 300v and about 200v on the tube's plate.

[iknowjohnny]

At the plate. Of course depending on the cathode value it could change a lot.

[pula58]

it's 285 at the plate. 100K load resistor, 1.5K cathode resistor, 25uF cathode bypass cap

[Roe]

it's 285 at the plate. 100K load resistor, 1.5K cathode resistor, 25uF cathode bypass cap

thats high. 260v is nice for clean blacface tones. 190 gives you more of a blonde bassman grind

[jjman]

I'd expect a little less output strength from that stage. Still clean unless it's biased way hot or cold, which it probably isn't.

[pula58]

I'd expect a little less output strength from that stage. Still clean unless it's biased way hot or cold, which it probably isn't.

could elaborate on this? Thanks!

[jjman]

Reducing the PS voltage reduces the gain (amplification strength.) In your case I don't think it would be very noticeable.

The 1st stage usually will not clip since the input signal isn't strong enough to push the grid high or low enough. This assumes typical biasing, voltages, and no "boosting" of the input.

I recently increased the PS voltage and the plate resistor on a triode stage. Also rebiased to also help maximize gain (amplification), but not reach clipping/distortion. It's a recovery stage from a parallel effects loop and it needed a little more for mixing in with the dry sound. It was a noticeable increase, but not huge.

What's you current PS voltage on that node?

[pula58]

Reducing the PS voltage reduces the gain (amplification strength.) In your case I don't think it would be very noticeable.

The 1st stage usually will not clip since the input signal isn't strong enough to push the grid high or low enough. This assumes typical biasing, voltages, and no "boosting" of the input.

I recently increased the PS voltage and the plate resistor on a triode stage. Also rebiased to also help maximize gain (amplification), but not reach clipping/distortion. It's a recovery stage from a parallel effects loop and it needed a little more for mixing in with the dry sound. It was a noticeable increase, but not huge.

What's you current PS voltage on that node?

The power supply voltage for V1 is 419V, plate voltage is 285, 100K plate resistor (1% metal film). So, plate current is 1.34mA.

[topbrent]

it's 285 at the plate. 100K load resistor, 1.5K cathode resistor, 25uF cathode bypass cap

thats high. 260v is nice for clean blacface tones. 190 gives you more of a blonde bassman grind

I agree, 285 seems a bit on the high side. I tend to like the 250-265 range for the clean punchy blackface thing.

I would put a slightly larger dropping resistor in the B+ string feeding your 100k's. Try several values to taste the difference. Try a 10k to start.


- What is the voltage on the plates of your PI?

[rfgordon]

Plate voltages can also affect bandwidth, which, in turn, changes the tone. I suspect the high end coming off a 12AX7 @ 285vdc is harsh. I prefer to run my peanuts at 150 - 175 vdc for a browner tone and, as has been noted, more dynamics and chewiness. I build input gain stages for harp at 85 vdc, so they'll overdrive easily and roll off more highs.