Biasing El84's in 18 watt TMB

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geoff 1965
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Re: Biasing El84's in 18 watt TMB

Post by geoff 1965 »

hello,
interesting! we were recently talking about this,see "lite2 & lite3 difference" regarding the 470K to 220K resistors.
i ended up using a 180 ohm bias resistor in my lite2 to bring dissipation down to 11.25watts with a plate voltage of 355 volts.
i'm with zaphod_phil on this one,you have to stay within certain parameters or you start altering the character of the "18 watt" and i would'nt want it to sound like a "jangly vox" etc.
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WilliamBrama
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Re: Biasing El84's in 18 watt TMB

Post by WilliamBrama »

geoff 1965 wrote:hello,
interesting! we were recently talking about this,see "lite2 & lite3 difference" regarding the 470K to 220K resistors.
i ended up using a 180 ohm bias resistor in my lite2 to bring dissipation down to 11.25watts with a plate voltage of 355 volts.
i'm with zaphod_phil on this one,you have to stay within certain parameters or you start altering the character of the "18 watt" and i would'nt want it to sound like a "jangly vox" etc.
It's interesting. Now that I have tested this mod twice (I have 2 amps, for live and rehearsall), I can hear a tighter bass response and a way more balanced tone.

I don't find then jangly at all: it may remind me a bit more of a bigger Marshall, I'd say. But, once again, it's just one opinion against many others. Maybe we just have different tastes for sound.

I like low gain pickups (singles or filtertrons), and I use a pair of 25 watt G12M's. Tubes are JJs.

I also tried a bunch of cathode resistors, now I have 280ohm resistors for each tube and 47uF bypass caps. I will post voltages asap, I don't have my notes yet.

However, I could not have been luckier to find this forum: this thing is now exactly what I wanted. Sounds loud enough, drives beatifully and has enough clean headroom to meet my best expectations.

I would recommend a try to anyone who has an spare hour (and a need for more clean headroom).
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geoff 1965
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Re: Biasing El84's in 18 watt TMB

Post by geoff 1965 »

fair comment!
everyone is different and if you are happy with it that's what counts! i'd like to know what plate dissipation you have with 280 ohm resistors?
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WilliamBrama
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Re: Biasing El84's in 18 watt TMB

Post by WilliamBrama »

geoff 1965 wrote:fair comment!
everyone is different and if you are happy with it that's what counts! i'd like to know what plate dissipation you have with 280 ohm resistors?
Here I am again :-)

It's as if it had a single 140ohm resistor: plate dissipation right now is a bit more than 10 Watt.
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WilliamBrama
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Re: Biasing El84's in 18 watt TMB

Post by WilliamBrama »

JMPGuitars wrote:Welcome to the site! I'm curious, what values did you change to? Did you test your voltages before and after?

BTW- if you're still experiencing "fartiness" at all, that can be a sign that your electrolytics may need replacing. They typically have a life of 2000 hours, which is easy to surpass in a decade of playing.
Here I got my notes.

There's not a big difference in the power section, as I said: 0.6 volts less at the power tubes cathodes (from 10 to 9,40). Apparently same plate dissipation: around 10watts.

BUT: V1 has gone from 120 to 130V, and V2 anode 1 from 178 to 205 volts and anode 2 from 178 to 210volts. I didn't remember such a big difference, but here it is :-)
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geoff 1965
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Re: Biasing El84's in 18 watt TMB

Post by geoff 1965 »

hello,
10 watts PD and your resistor changes give you better clean headroom and i notice your V1 &V2 plate voltages are quite a bit lower than the forums voltage chart.do you know what your B+ and el84 plate voltages are?
i like to make notes for future reference "builds"
thank's
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WilliamBrama
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Re: Biasing El84's in 18 watt TMB

Post by WilliamBrama »

geoff 1965 wrote:hello,
10 watts PD and your resistor changes give you better clean headroom and i notice your V1 &V2 plate voltages are quite a bit lower than the forums voltage chart.do you know what your B+ and el84 plate voltages are?
i like to make notes for future reference "builds"
thank's
Hi Geoff,

Yes, I got all the main voltages, just for the record:

A is 314V
B is 295V
C is 273V
Plate Voltage (measured between plate and cathode) is 304V.
OT throws 311V
PIN 9 on Power tubes says 292V
Cathodes are at +/-9,5 V

Both of my amps were built partly out of junk parts, the PTs came from Tweed Deluxe projects: so I had to drop some volts down with a sort of a "solid state tube" rectifier (as Webber Copper Caps do).

So, actually, I could rise voltage quite a bit (but I think I won't do so, lol).

I was happy before, now I just can't wait to play it again hurray
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geoff 1965
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Re: Biasing El84's in 18 watt TMB

Post by geoff 1965 »

yeah you have plenty of room to play with the voltages if you wanted,so basically running the tubes "cooler" and the resistor mods give you more headroom,good to know.
i wonder if you brought the el84 plate voltage up to the "sweet spot" around 345V would you lose headroom?
thank's for the info!
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WilliamBrama
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Re: Biasing El84's in 18 watt TMB

Post by WilliamBrama »

geoff 1965 wrote:yeah you have plenty of room to play with the voltages if you wanted,so basically running the tubes "cooler" and the resistor mods give you more headroom,good to know.
i wonder if you brought the el84 plate voltage up to the "sweet spot" around 345V would you lose headroom?
thank's for the info!
Back in 2001 I messed a lot around with voltages, and I didn't get such a sweet spot.

I guess it's just a matter of balance, but, well, maybe next week I can jump the sag resistor and check it out again. I bet It won't make a big difference.

I also tried 12ay7 and 12at7 tubes, both at V1 and V2: you can notice a change, but if you put the volume up there's a moment in which you get to the same overdrive as if it was a 12ax7 (at least that's what it seems in my case, I might be wrong or a little deaf). So, I didn't win clean headroom itself, it was something as the "going up to eleven" joke: you just had to turn the pot a bit more to reach to the same problem.

This mod has definitely raised the clean headroom: I have the same volume pressure with a bit of a cleaner sound. Or that's what I can perceive.
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