Values of the screen grid resistors influence sound

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musicheals
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Yes, I will give it a try without negative feedback....
My B+ is around 20 volts less than a typical 18 watt Marshall. Does this make a big difference? And if so, what will be the difference asides from a bit more power?

Thanks a lot...
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Re: Values of the screen grid resistors influence sound

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musicheals wrote:
Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
My B+ is around 20 volts less than a typical 18 watt Marshall. Does this make a big difference? And if so, what will be the difference asides from a bit more power?

Thanks a lot...
I don't think these values/voltages are arbitrary. It's more important the proportions are matching. My first Tremolo TMB has a B+ 15V over spec. I think it sounds awesome. Think about it... there is nobody on this planet that can tell the difference between 15 watt or 18 watt by ear. They're both equally loud watts.
About the NFB loop... with the loop the transition from clean to mean is very abrupt. Without the loop it's more gradual. You reach the max clean volume at 3 or 4 on the dial.. then it starts to break up more and more.. without getting much louder. It's the key ingrediënt of an 18 watt.

And for loudness.. don't underestimate the roll of the speaker.. and don't forget to give a well broken in V30 a chance!!! I'm glad I did eventually.
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Thanks a lot. I use a 2/12 with V30s and a 1/12 with a G12H and I like both very much. I absolutely agree in things power. I never needed more than 18 watts, even on open air stages...
You wrote about the right proportions in voltages. Could you please give me more detail or maybe and example for my B+ 324 volts?

Thanks a lot
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Re: Values of the screen grid resistors influence sound

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I would copy the LTP PI and power amp of a standard 18W and bias the power tubes correctly. See how that sounds...
I have built an amp with the 2nd gain stage that's pretty much like yours, with the cold clipper and all... but did the tone stack after the 2nd stage. I really like that amp. I put a 1M gain pot between the 1st and 2nd stage + a single tone knob as tone stack-y thing.
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

18 Watt amps have a typical B+ target of 345VDC (+/- 5V). Within that range gives you the ideal tone and dirt. If you go above 350VDC, you get more headroom and less dirt. If you go below 340VDC, you get darker/muddier sound/dirt, and less headroom.

You could make comparisons to this (power amp section only): files/JMPGuitars_18_Watt_Tremolo_TMB_Re ... _Chart.pdf (those are the target voltages I like)

Your 324VDC B+ is about a 6% drop, so consider the relationships based on that when comparing to the chart.

Thanks,
Josh
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Great! I will definitely try that and calculate the voltages to my amps voltages. Also I will also try it without the NFB.

THANKS A LOT
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Here's a short clip I recorded yesterday in the living room. Only a Strat and a cable. Speaker is the G12H. I used a sm57 right into the laptop. The volume was reduced to about 1/2 watt.
I admit the treble is a bit much. Especially today when I practiced with the band. I had to roll back the treble control to 9 o'clock :?
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Re: Values of the screen grid resistors influence sound

Post by zaphod_phil »

musicheals wrote:
Sat 05/15/21 11:57 am

I admit the treble is a bit much. Especially today when I practiced with the band. I had to roll back the treble control to 9 o'clock :?
Have you tried pointing the mic off-center from the speaker's dust cap? That usually helps tame the treble; although I thought the clip sounded pretty darn fine on my laptop speakers :)
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Re: Values of the screen grid resistors influence sound

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I don't think it's too bright at all. Sounds pretty nice to me
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Re: Values of the screen grid resistors influence sound

Post by zaphod_phil »

musicheals wrote:
Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
hurray Then you will get the real deal Marshall 18W sound. NFB tends to make the tone dull, so you lose detail :(
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Have you tried pointing the mic off-center from the speaker's dust cap? That usually helps tame the treble; although I thought the clip sounded pretty darn fine on my laptop speakers :)


Yes, of course I tried that. In this case the mic is off center and with an angle of about 50 degrees...
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

zaphod_phil wrote:
Sat 05/15/21 12:32 pm
musicheals wrote:
Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
hurray Then you will get the real deal Marshall 18W sound. NFB tends to make the tone dull, so you lose detail :(
Maybe this will lead to even more highs 8O
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Re: Values of the screen grid resistors influence sound

Post by Bieworm »

musicheals wrote:
Sat 05/15/21 3:19 pm
zaphod_phil wrote:
Sat 05/15/21 12:32 pm
musicheals wrote:
Sat 05/15/21 9:40 am
Yes, I will give it a try without negative feedback....
hurray Then you will get the real deal Marshall 18W sound. NFB tends to make the tone dull, so you lose detail :(
Maybe this will lead to even more highs 8O
Get rid of the NFB loop. It's not only eliminating treble..
If you still think the amp is too trebly you can tweak that in many other ways.
- You can put a bigger bright cap, but there's already a 500p in there. Try a .001uf and see what that brings. ( try parallelling another 500p)
- try a snubber cap across the plates of the PI. 100p is a good value to start with. ( don't try to alligator clip on your tube pins!!! Do this where the wires between pin 1 and 6 are soldered to a turret)

Right now alligator clips are your friend.

Maybe try these things first...
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

I cut the NFB and have to say, that the tone gets indeed more complex and the breakup is more evenly. Great!
To get less treble, I tried a 220K in front of the second tube's grid ( right after the volume control). This worked quite well, but I got mor hiss then. So instead of the resistor I used a 220pF on that grid. The added hiss went away. But I'm not happy 100% with this. Maybe there might be a better place in the preamp? A 47p snubber between the PI anodes I had already...

Thanks a lot
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Re: Values of the screen grid resistors influence sound

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schem adjust.png
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