18 Watt Lite Test Readings?

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Re: 18 Watt Lite Test Readings?

Post by JMPGuitars »

tatter75 wrote:
Sun 05/26/24 11:15 am
JMPGuitars wrote:
Sun 05/26/24 10:52 am
I would suggest what I already suggested...higher wattage, and between 470Ω and 1KΩ. You're way above what's needed.
Do I need to go higher than 3 watts?
No, that should be fine. I meant higher than 1W.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Excellent, thanks. Hopefully this should fix my lamp issues.

Now onto biasing. I've added zener diodes and my voltage has come down to around where it should be depending on mains power fluctuations.

Here are my new voltage readings and I entered the numbers into the RR bias calculator (hopefully correctly) and this is what I'm getting.

Are these running a bit hot? And if they are should I just increase the cathode resistor?
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Re: 18 Watt Lite Test Readings?

Post by Bieworm »

Ok I misread. I thought you were sayibg 70V voltage drop. Swap the 1M for 100k and see where that brings the voltage

The plate voltage is still 374V??? This xoukd need more zeners to lower it to 345V.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Bieworm wrote:
Sun 05/26/24 12:23 pm
Ok I misread. I thought you were sayibg 70V voltage drop. Swap the 1M for 100k and see where that brings the voltage

114% plate dissipation is too high. Try a 180ohm resistor. You should land around 85%, so even higher resistance might be necessary
My stash of 3 watt resistors is pretty thin on the ground otherwise I would do. I’m going to try 1k and 480ohm.

The parallel experiment was a good thing to try though.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Bieworm wrote:
Sun 05/26/24 12:23 pm
The plate voltage is still 374V??? This xoukd need more zeners to lower it to 345V.
I’ve been measuring the B+ to bring that down not plate voltage. As shown in the attached image.

That is now 340-355v depending on what my mains is giving.
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Re: 18 Watt Lite Test Readings?

Post by Bieworm »

The B+ is the highest DC voltage in the amp. The EL84 plate voltage can not be higher. If it is, then you have wired the zeners wrong. I know that diagram is confusing, but watch it closely.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

So now I know that B+ and plate voltage are the same thing and perhaps my zener diodes aren’t in the right place.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Bieworm wrote:
Sun 05/26/24 1:16 pm
The B+ is the highest DC voltage in the amp. The EL84 plate voltage can not be higher. If it is, then you have wired the zeners wrong. I know that diagram is confusing, but watch it closely.
I’ve added them in series with the resistor in between the first and second filter caps. I thought that was correct?
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Re: 18 Watt Lite Test Readings?

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Like this:
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

This is where I've added the zeners. I checked and double-checked that it was in the right place but obviously they aren't having any effect on the plate voltage. When I check the voltage in-between the two filter caps they are dropping the voltage by 25volts.

According to my layout where should they go?
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Re: 18 Watt Lite Test Readings?

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Look closely:
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

I think the point at where the voltage is sent to the output transformer means that the diodes aren't doing anything.

It's difficult to tell with only a partial schematic.

I think I need to move the connection to the output transformer?
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Re: 18 Watt Lite Test Readings?

Post by Bieworm »

tatter75 wrote:
Sun 05/26/24 1:42 pm
I think the point at where the voltage is sent to the output transformer means that the diodes aren't doing anything.

It's difficult to tell with only a partial schematic.

I think I need to move the connection to the output transformer?
Look at the schematic and layout from the 18 watt Tremolo TMB reverb in the downloads
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Bieworm wrote:
Sun 05/26/24 1:57 pm
tatter75 wrote:
Sun 05/26/24 1:42 pm
I think the point at where the voltage is sent to the output transformer means that the diodes aren't doing anything.

It's difficult to tell with only a partial schematic.

I think I need to move the connection to the output transformer?
Look at the schematic and layout from the 18 watt Tremolo TMB reverb in the downloads
Thanks, it makes sense now.

I have some spare terminals where I’ve wired in my SS rectifier so I may be able to move the output transformer wire to there and connect the diodes to it etc.
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Moved the output transformer connection,the zener diodes and resistor to a central point off the main board and now my plate voltage has dropped to 345v.

I've attached my test readings and entered them into the RR bias calculator, still seems to be a tiny bit too hot but I think if I change my cathode resistor from 150ohm to 180ohm I should be where I need to be?
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Re: 18 Watt Lite Test Readings?

Post by JMPGuitars »

Probably. Most of us start at 180Ω with modern voltages. Some need to go a little higher, so it wouldn't hurt to also get a 200 and 220Ω resistor just in case.
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Re: 18 Watt Lite Test Readings?

Post by Bieworm »

And to get thing totally on par with our methods: ditch the small cathode cap and swap it fir a 1000 to 2200uf cap😎
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

Bieworm wrote:
Mon 05/27/24 3:40 pm
And to get thing totally on par with our methods: ditch the small cathode cap and swap it fir a 1000 to 2200uf cap😎
Will this change the sound or is it more for reliability purposes?

And what voltage should I go for?
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Re: 18 Watt Lite Test Readings?

Post by Bieworm »

tatter75 wrote:
Mon 05/27/24 3:49 pm
Bieworm wrote:
Mon 05/27/24 3:40 pm
And to get thing totally on par with our methods: ditch the small cathode cap and swap it fir a 1000 to 2200uf cap😎
Will this change the sound or is it more for reliability purposes?

And what voltage should I go for?
Better low end. 63V
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Re: 18 Watt Lite Test Readings?

Post by tatter75 »

I've swapped in a 1k 3 watt resistor on the lamp and that is a lot brighter now, still not really bright but I don't want to go any lower and damage another one, so as far as I'm concerned that's sorted now.

I've also swapped my cathode resistor from 150ohm to 180ohm and that has brought my plate dissipation down to 96.7%, those readings were taken when my mains was kicking out 245v though so that is the max they will ever be. I will try and take a reading when it is giving 230v, I expect the plate dissipation to be around 85%.

EDIT: took readings while the mains voltage was at 235v, plate dissipation was as expected at 86%.

Still waiting for my 1000uf 63v electrolytic capacitor though. Once that is in I think I'm pretty much done (famous last words :lol: )

The amp is sounding great now, much less stiff and harsh than before.

Massive thanks to you guys for helping me out and putting up with my stupid questions!
Last edited by tatter75 on Thu 05/30/24 10:00 am, edited 1 time in total.
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