Laney LC15R Mods : adding screen grid resistors?

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lolbou
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Post by lolbou »

Hi everyone...

I'm currently re-making my 1998 Laney LC15R. At first I wanted to transform it in a head + cabinet amp, and now I'm making a new PCB with no SMDs.

I expect to make a full topic here with the cloned interchangeable PCB and descriptions...

I've read about these screen grid resistor, and someone talked about 1k - 5W.

Will it modified the biasing? Is it really needed? (I've changed the EL84 sockets in the past to some ceramic ones as the original litteraly "toffeed" (same color, but taste like coal :lol: )). Will it prevent the tubes from heating to much?

If any one carried it out on its amp, any help would be nice...

It's supposed to be a class AB amp with 2 EL84. Cathodes are connected to ground, and screen grids are straightly connected to B+ (+290V).

Thank you in advance...

ps: the schematics can be found here http://www.bnv-gz.de/~ooehmann/schemati ... y/LC15.pdf although there are slight differences/errors compared to my PCB and my schematics version (which shows dated factory mods)... Anyway the power section is the same...
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JHS
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Post by JHS »

You don't need screen grid resistors. Those Rs increase the lifetime of the tubes a bit but alter the tone too. Tubes must be changed regulary due to tube wear in such amps and IMHO I would'nt bother much about a few hours less lifetime.

JHS

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lolbou
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Post by lolbou »

Thank you JHS... I read somewhere it could be a way to improve safety (but I did not really catched out how come...), is it a true story or some internet fairytale stuff?

Anyway, I'm quite happy with no power resistors heating the chassis more and more...

Cheers!
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Post by gkaiomenos »

well i had one for repair with overheating problems
installing screen grid resistors really helped a lot
i wouldn't put 1k though cause it will alter the tone a lot as JHS said
I used 220ohm 5W just to be sure, but 100 ohm will also do the trick

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Post by lolbou »

gkaiomenos wrote:well i had one for repair with overheating problems
installing screen grid resistors really helped a lot
i wouldn't put 1k though cause it will alter the tone a lot as JHS said
I used 220ohm 5W just to be sure, but 100 ohm will also do the trick
Oh, really? Then I have to connect the resistor between the screen grid and B+ (for each tube), that's it?

I guess they're going to heat quite a lot, so finding the appropriate place won't be easy... What was your choice then?

If it's just a matter of tube lifetime/overheating, since I use the amp at reasonably low levels, is the mod worthy in my situation?

Thank you for your experience gkaiomenos!
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Post by Ronsonic »

I'll also suggest adding screen resistors. Much better tube life. 100 or 220 is pretty well standard for EL84, 1K would just choke the amp. Run the resistors to the existing screen supply. 5W is probably overkill, but no reason not to - it's good to keep things cool and derated. 2 Watters should suffice.

With the quality of tubes in this world - EL84s are not getting better - yeah, do it.
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Post by lolbou »

Ok then...

But shall I modify the biasing in some way, or should I just leave the factory setting (-11.5V) between R25 and R26?
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Post by mutetourettes »

I think the 1k @ 5W advice comes from kevin o'conner - like in this thread:

http://music-electronics-forum.com/t19495/

It's to protect the output tubes AND output transformer from bad runaway-type things that happen when e.g. the output goes open or shortcircuit (both are bad) (e.g. maybe you've got an extension speaker cable plugged in, but the cable gets knocked out of the cab...) .

e.g.
The use of non-existent or low-value screen-stops is an "ideal". They are needed in real circuits to maintain stability with variations of the tubes themselves and with layout. Increasing the values above what is needed for stability affords great protection to the tubes. In an upside-down-tube amp and/or combo amps and/or any amp you want to clip frequently and/or any amp you want to not eat tubes, the 1k standard should be adhered to.

For EL-84s, 1k5 to 2k2 should be used instead.

Have fun
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I'd be interested to know how badly such resistors would affect the sound of the amp..

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Post by tictac »

Actually Kevein recommends 2k screen resistors with EL84

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Post by phatt »

mutetourettes wrote:I think the 1k @ 5W advice comes from kevin o'conner - like in this thread:

http://music-electronics-forum.com/t19495/

It's to protect the output tubes AND output transformer from bad runaway-type things that happen when e.g. the output goes open or shortcircuit (both are bad) (e.g. maybe you've got an extension speaker cable plugged in, but the cable gets knocked out of the cab...) .

e.g.
The use of non-existent or low-value screen-stops is an "ideal". They are needed in real circuits to maintain stability with variations of the tubes themselves and with layout. Increasing the values above what is needed for stability affords great protection to the tubes. In an upside-down-tube amp and/or combo amps and/or any amp you want to clip frequently and/or any amp you want to not eat tubes, the 1k standard should be adhered to.

For EL-84s, 1k5 to 2k2 should be used instead.

Have fun
Kevin O'Connor
I'd be interested to know how badly such resistors would affect the sound of the amp..
Re read post 5 by *Wilder Amplification* on that link again :thumbsup
It's that simple and any tone loss factor is just fools who think that with some secret they can extract 1 more watt and get perfection. [smilie=lol_xtreme.gif]
If you don't have screen grid protection you are going to have to replace power tubes every year or so.$$$$$$$$$$$

The screen grid is the weak link in pentodes so if you have no protection against over dissipation then you melt the screen.
Tetrodes have a better setup with a beam focus thingo but pentodes have much higher TC which translates to more perceived grunt = earlier distortion. sadly guitar amps builders don't read the old books and pelt them to death.

If fools want that extra 1 watt of mojo tone and have the money who cares. :popcorn:
Meanwhile I'll stick to the safer option and have power valves last for years. :secret:

Phil.

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Post by DrNomis »

lolbou wrote:Ok then...

But shall I modify the biasing in some way, or should I just leave the factory setting (-11.5V) between R25 and R26?


You might need to tweak the biasing a bit, I seem to remember reading somewhere that you should adjust the bias so that the valves are dissipating at least about 70% of their full dissipation capability in terms of watts, but I could be wrong, adding a 100 Ohm to 220 Ohm screen grid resistor is a good idea since that helps to stop the screen grid from over-dissipating and burning out, especially if you crank the amp up to get the EL84 valves to produce power valve distortion, the two screen grid resistors should connect to the HT supply at the same point as the Output Transformer's Primary winding centre-tap this ensures that the EL84 power valves operate as Pentodes, hope that helps.... :thumbsup


If there aren't any installed in your LC15, a good idea would be to install grid-stopper resistors on the control grid of the EL84 valves too.... :thumbsup
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