Biasing El84's in 18 watt TMB

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ajkimba
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Biasing El84's in 18 watt TMB

Postby ajkimba » Sun 01/08/06 2:06 pm

Is it possible that one would have to go as high as "over" a 200 ohm biasing resistor to get dissipation to 12 watts at idle or a little less?

I had a 125 ohm, was red plating one tube, dissipation was very high 15.5 watt per tube.

Went to a 150 ohm, that gave me 14.2 watts per tube at idle, too high.

Looks like I'll have to go to at least a 200 ohm to get it hopefully under control.

My plate current stays around 320v but as I increase the cathode resistor's value, the voltage increases across that resistor instead of going lower...is this normal?

I'm dividing voltage across biasing R by resistor value to get bias, then substracting the that current from the plate voltage and multipling by bias.

Could something be wrong with the tubes? They are NOS, brand new. I only have this problem with this set of new tubes, all other tubes are fine with a 125 ohm.
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phsyconoodler
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Postby phsyconoodler » Sun 01/08/06 3:45 pm

I had the same problem with some NOS tubes.I had to up the cathode rsistor a lot and finally just went with some new EI tubes.The NOS tubes could not handle the voltage above 300v.That is not the case with all NOS EL-84's,just the ones I had.If I use a zener diode to drop the B+ they work fine,but the sound is not the same.Good,but not the same.I wanted to make it work with all EL-84's so i just ended up with new production tubes.They sound phenomenal!
The ones I had trouble with were Amperex.
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Postby markh » Mon 01/09/06 11:27 am

I have not had any problems so far using Fender/Sovtek (the beater test tubes), NOS Sylvania, or sarge's Polams at 130-something.

I agree that some NOS EL84s take offense at higer voltages, but not all by any means.

You can certainly use as high of a cathode resistance value as you need to without hurting anything, but it may sound funky if you put a diffferent set of tubes in. :wink:

--mark
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Postby zaphod_phil » Mon 01/09/06 11:42 am

As black chicken recently pointed out in another thread, the WEM/Watkins Dominator uses a 220 ohm cathode resistor. The Marshall 18W was copied from the Dominator, so that's the original value. IIRC the Dominator also ran at slightly lower voltages.

If you take a look at the Lite IIb schematics, you will see a 150 ohm cathode resistor is used, as well as an alternative way of powering the screen grids, from Jason's Mods. That will help a lot. I never run the cathode with anything less than a 150 ohm resistor, and I've even used 180 ohms. The tone in all cases sounds great to me.

There's no way I'll risk harming my Mullards! :)
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Postby ajkimba » Mon 01/09/06 11:50 am

Might be one of the NOS tubes, they are brand new.

When I pull one tube and check voltage across cathode R, one tube at a time, I get 9.1 volts on one and 12.68 on the other...?

Then together I get like 14 something volts, maybe the one tube pulling 12.68 on it's own is bad...?

That's the one that red plates.
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Postby KT77 » Mon 01/09/06 12:44 pm

badly matched tubes, sounds like......
I usually use separate 1k screen grid resistors and separate cathode resistors and caps (usually around 270-300ohms each) so I can use what ever Iwant...matched or not
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Postby Mr_dB » Mon 01/09/06 5:28 pm

phsyconoodler wrote: The ones I had trouble with were Amperex.
I have a selection of NOS and old pulls to choose from, and the only ones that redplated on me were Amperex too (Zenith-labelled Dutch-manufacture). Mullard, M-OV, Czech, Yugo, Russian all work fine in my unmodified 18W trem.
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Postby Plexi » Tue 01/10/06 12:56 am

These amps run the tubes pretty much on the edge. So its very important to measure the cathode resistor..don't just go by whats on the resistor. many 125ohm resistors can measure 117 to 115ohm.. and when your already on the teeter totter balance of running the tubes..this can put them over the edge. I use 130..and measure them too.. but it will also matter on the tube..some can take it some just can't.

Richie
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Postby Chappy » Wed 01/11/06 9:28 pm

Ajkimba

I just played around with this resistor to. One gotcha is that 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. I currently have two resistors in series 120ohm and 30ohm. I'm using a jumper to switch between 130 or 150 ohms on the fly. Although there is more crossover distortion, As Zaphod has pointed out I am hard pressed to hear any difference. Maybe there would be an audible difference between 125 and 220ohms?

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. I lowered the values of these resistors vice using the Paul Ruby zener diode fix. It has lowered the total output power of the amp a little though.


I'll post some scope pics with comparisons later if anyone is interested.

Chappy
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Postby Plexi » Wed 01/11/06 10:34 pm

I have been doing something similar.. I use 330k in place of the 470k..but also use 82k and 68k for the plate resistors in the PI.. But the PI i use or style of 18 amp,is more along the line or style as the 36 watter.

The amp has more clean headroom..and still breaks up when cranked,but does not have any of the buzziness or crossover type distortion.

Richie
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Postby zaphod_phil » Thu 01/12/06 5:22 am

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!
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Postby Chappy » Thu 01/12/06 7:59 am

Phil

Your right, not such a good idea if your trying to duplicate the original 18W design.

I'm just tinkering around with a testbed amp to learn what changes affect what tone etc all in the quest of understanding. I'm happier for now, with the lower valued grid leak resistors. I can get a bit more clean out of the TMB at lower levels, but I still get a lot of distortion with the amp cranked.

Chappy
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Postby Ethan » Thu 01/12/06 8:27 am

AJ
The fact that only one tube is redplating leads me to think your problem lies somewhere other than your bias (cathode) resistor.
Swap the power tubes and see if the problem follows that tube or stays in the same socket. If it stays there, check to see if your coupling cap is leaking DC onto the grid of that power tube. Even brand new caps can be defective and cause this problem (although it's rare), and if you're using NOS caps... well, let's just say it's not the most prudent location in an amp to use NOS caps.
Anyhow, I'm assuming you're using new parts, but it's time to start looking elsewhere in the circuit because a too-low cathode R would more likely affect both tubes and not just the one.
Double check ALL voltages in your output stage.
Good luck-
Ethan
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Postby johnny5 » Tue 01/31/06 2:01 pm

Plexi wrote:I have been doing something similar.. I use 330k in place of the 470k..but also use 82k and 68k for the plate resistors in the PI.. But the PI i use or style of 18 amp,is more along the line or style as the 36 watter.

The amp has more clean headroom..and still breaks up when cranked,but does not have any of the buzziness or crossover type distortion.

Richie
what effect does the mismatched PI plate loads have on the amp? i noticed that on the 36 watter as the cap across the PI plates. interesting...
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Postby rockable » Wed 02/08/06 5:40 pm

I had a new set of matched 6550 SEDs that I bought from a very reputable dealer to put in a Sunn Sorado Bass Head that I rebuilt for my son. (It isa fixed bias amp.) When I started to set the bias, I couldn't get them to go much below 60ma and I was shooting for 40-45.

I talked to the dealer who suggested that I increase the resistance in the bias circuit and I did, multiple increases, and could never get the current to come down. Finally, he asked me to read some hand written numbers on the flaps of each tube box. Both said 60 or 62 ma, as I recall.

He said, "Send them back and I'll send you a new pair. Those won't ever bias below about 60ma." Sure enough, he sent me a new pair that had numbers of about 35ma on the flap and I dialed them in to the value I wanted with no additional resistance.

The lesson I learned is this. Even though I had two pairs of supposedly identical tubes, they were different. I guess the grid was not capable of shutting the tube down below some inherent value that was built in during construction.

I'd suggest trying another pair. Sounds like you have a set that is just not going to work.
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