Biasing El84's in 18 watt TMB

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WilliamBrama
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Re:

Postby WilliamBrama » Tue 03/07/17 12:36 pm

zaphod_phil wrote:
Chappy wrote:... increasing the cathode resistor will run the tubes colder at idle, in effect shifting the output's bias more towards class B. This will introduce more crossover distortion when the amp is cranked. I confirmed this with my scope. As a sidenote I've managed to kill almost all of the crossover distortion by lowering the grid bias resistors from 470k each down to less than 200k each. I'm also doing this with resistors jumpered in parallel. The output stage still has the ability to distort due to saturation and clipping, with less of the harsher distortion due to crossover.
IMO this is not such a good idea, as it fundamentally alters one of the distinctive aspects of the Marshall 18W design. First of all, as we're not doing hi-fi here, there's nothing necessarily wrong with having smallish amounts of crossover distortion. I may be mistaken, but I also reckon the crossover distortion is what helps give the 18W it's very distinct Marshall grind versus the usual kind of distortion you hear from other EL84 amps. Obviously, you don't want too much, or it will get buzzy, so it has to be a limited amount. Also, as Paul Ruby demonstrated, with good quality OTs, you don't hear a buzz. And Paul's zener/diode fix can always be used if necessary.

The other thing is that reducing the EL84 grid leak resistors (or the PI load resistors) makes the amp breakup later. Now, some folks may prefer that, in which case its fine. One of Jason's mods reduces the grid leaks from 470k to 220k, to get more clean range. However, the amp is then no longer true to the 18W design, and IMO its a pity to loose the early breakup which is one of the things that gives this amp its charm.

I should also point out that the WEM Dominator used 470k grid leaks on the EL84s along with the 220 ohm bias resistor, and everyone who's ever heard one says how great they sound!
I know I'm a bit off-topic, but I just registered and entered into this forum (Hi everyone!) to thank you all for this thread and the forum.
I have been looking for a solution for my need of a bit more clean headroom, and here's where I found the hint of the grid resistors.

I am a guitar player from Spain, I have been using a self made 18 Watter, with certain modifications, for around 12 years, after trying many other amps. And well, just to try, last week, after reading this thread, I swapped the resistors that the Jason Mod proposed.

IMHO, to my ears, the only thing I got was a noticeable improvement in the definition when the amp is not hardly crancked, what helps a lot: the amp has lost the bass "farting" when playing fingerpicking style, and still breaks as **** when I over-drive it. IMO, it's a very interesting, cheap, and easy mod.

I just wanted to say I saw very minor differences, and wanted to write this here, for the record. It's an easy to do mod, and 100% worth a try.

Even it's recorded awfully, with a phone, I guess this video helps a bit to document that the amp still got guts (no pedals on in this song, just a Telecaster jumped into the amp):

https://www.facebook.com/eugenio.pachec ... 187078278/

That's by band, Los Brazos, last friday, I dig the bit softer breakup after the mod. I bet it will remain like that for long.

Thank you all for your help and sorry for the off-topic!
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Re: Biasing El84's in 18 watt TMB

Postby JMPGuitars » Tue 03/07/17 1:04 pm

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.
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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Tue 03/07/17 1:53 pm

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.

Hi there! Thank you so much :-)

Actually yes, I measured voltages before and after, but I left the notes in the amp, I will post them asap. As I remember, there wasn't a BIG difference, by the way. I guess it's a matter of balance.
I have tried many other ways to low down the amp's breakup that were way more radical at the voltage point and didn't work as fine.

Resistor values:

- Between "A+" and "B+" in the power section: 1k5 to 2k2.
- Separated 1k resistors for the EL84s' Grid 2.
- 8k2 to 1k5 for the EL84s' Grid 1.
- 470k to 220k on the grid BIAS resistors.
- 820ohm to 1k2 in the cathode of the phase inverter.
- 56k to 47k to the Phase Inverter cathode resistor.
- 470k to 1M at the Phase Inverter's PINs 2 and 7.

I have to say that long ago I put 2 sepparate cathode resistors on the Power Tubes.

I have changed all the electrolytics from time to time, the amp has been working hard this years: around 900 o 1000 gigs. I try to maintain it the best I can.

Maybe "farting" is not the best expression: I had an excessive overdrive on the lows, they didn't sound as tight as they do now. Now I have the exact ammount of clean I wanted: I didn't want more power on the amp, so this suits perfect (to me).

I will come back with the voltages :-)
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Re: Biasing El84's in 18 watt TMB

Postby JMPGuitars » Tue 03/07/17 11:15 pm

Low-end/tightness can also be affected by your electrolytics, but it sounds like you have that under control.

Where did you end up with the el84 grid leak resistors? I'm assuming around 470K for the headroom...

Thanks,
Josh
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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Wed 03/08/17 3:55 am

JMPGuitars wrote:Low-end/tightness can also be affected by your electrolytics, but it sounds like you have that under control.

Where did you end up with the el84 grid leak resistors? I'm assuming around 470K for the headroom...

Thanks,
Josh
Yes, the overall mainteinance seems to be under control ( I'm not an expert, but everything seems to work properly).

I'm afraid I have been wrongly focusing for years on the pre amp section, mostly on the Phase Inverter: I tried lots of tubes and set-ups, but I was obviously wrong: the unliked distortion came from the power section.

The current leak grids are 220K, the amp doesn't seem to lack volume at all.
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Re: Biasing El84's in 18 watt TMB

Postby geoff 1965 » Wed 03/08/17 9:28 am

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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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Wed 03/08/17 6:12 pm

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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Re: Biasing El84's in 18 watt TMB

Postby geoff 1965 » Wed 03/08/17 6:47 pm

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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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Mon 03/13/17 5:21 pm

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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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Mon 03/13/17 5:38 pm

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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Re: Biasing El84's in 18 watt TMB

Postby geoff 1965 » Mon 03/13/17 7:40 pm

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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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Tue 03/14/17 6:54 am

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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Re: Biasing El84's in 18 watt TMB

Postby geoff 1965 » Tue 03/14/17 8:14 am

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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Re: Biasing El84's in 18 watt TMB

Postby WilliamBrama » Tue 03/14/17 12:17 pm

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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