18 Watt Marshall Help

[Structo]

Hey,
I have a home built 18 Watt Marshall amp on my bench that belongs to a friend. He had me finish it up because he had some wiring errors that he couldn't sort out.

I got it up and running and it sounds great!

I'm somewhat new to cathode biased amps, I just built one recently and this one makes only two that I have worked on.

Anyway, I was checking the bias on the 18 watt and I measure:

10.68v on the cathode resistor which is 125 ohms.

So that comes to .085ma or 42.5ma per tube.

I measured the B+ voltage to the EL84's and got 359v.
So I x E = P comes out to 15.25 watts per tube.

According to specs this is high right? Should be 12 watts max?

I'm thinking that I need to change the cathode resistor to 150 ohms.
That should get it closer to spec.

Does that sound right?
Thanks, Tom

[Alexo]

Remember to subtract the cathode voltage from the plate voltage - you want the plate/cathode voltage, not the plate/ground voltage when you calculate plate dissipation.

Also, remember to subtract screen current from the current on the cathode. I'll hazard a guess at 3ma per tube.

So you should really be multiplying 39.5ma x 349 for about 13 3/4 watts per plate, which is of course, still too high.

150 ohms is a common value for folks to use when biasing these amps in the modern era. Screen resistors are also a good idea, if the amp doesn't have them already.

[Sonny ReVerb]

Your calculations look good to me (except it should be .085 amps or 85ma). The screens dissipate maybe a watt or two of your total. The 18 Watt is known for working the EL84's hard. That's part of the reason it sounds the way it does. But, some of the 18W gurus (P. Rowley for one) recommend the 150 ohm cathode resistor to ease the strain on the power tubes. They also recommend upping the screen resistors a bit. When I built my 18W TMB I used the 150 ohm cathode resistor and also put a 1k resistor prior to the split for the two 100 ohm screen resistors. Sounds good to me.

Doh - what Alexo said... +1

[Structo]

Thanks.
Yes the .085ma was a typo.

I had a brain burp and realized after I posted that, that I should have subtracted the cathode voltage.


Also I noticed this guy put a 1000 uf bias cap on this amp.
Surprisingly, it sounds pretty good.

I have tried to research that as well but I'm not finding much on how or why a certain value is used. I see 25uf used a lot on cathode biased amps.

I have written an email asking where he got that value from.

In looking at a lot of schematics, I see 50uf frequently used and also a 500uf occasionally.
In the layout my friend used, it is the one made by Gabriel and Mark Huss, it calls for a 500uf.

So at this point I don't know if it was a mistake or if somebody advised him to use that value.

[Deric]

The huge cap is a common trick to tighten up the bass a bit.

Search "huge cap" or "huge cathode cap" over at 18watt.com and you should find lots of info.

[Phil_S]

Cathode biased amps should be biased at 100% of design spec rating. The pair of EL84's should be running at 17-18W at idle. 42mA per tube is not out of line, but you might consider using a 150 ohm resistor.

Cathode bias is self limiting. As you drive a signal into the tube, the cathode bias resistor is going to suck down the plate voltage and keep things in line.

The 70% rule is for fixed bias amps. In a fixed bias amp, the grid voltage is constant. No so for cathode bias.

[Structo]

OK, but I thought the max P rating for a EL84 was 12 watts or is that only in SE amps?

I was looking in my Duncan TDSL for the ratings..

But I did see where it mentioned in AB1 P/P with a higher plate voltage and it was 17 watts max P.

I read it mentioned quite a bit at 18 Watt Forum the 150 ohm resistor was recommended to prolong tube life.

I believe this amp has the Heyboers from Hoffman.

Also there was talk about upping the screen resistors from the stock 100R to 1K on the board then to 100R at each socket.

[Alexo]

Cathode biased amps should be biased at 100% of design spec rating.

Just to be clear, you can bias to whatever you like, I do like 100% sometimes, but others may wish to go easier on their tubes, which is fine, if they're getting a tone that they are happy with at 80 or 90%.

The pair of EL84's should be running at 17-18W at idle.

100% dissipation would be 24 watts, combined, as EL84's have a maximum plate dissipation of 12 watts.

Cathode bias is self limiting. As you drive a signal into the tube, the cathode bias resistor is going to suck down the plate voltage and keep things in line.

The 70% rule is for fixed bias amps. In a fixed bias amp, the grid voltage is constant. No so for cathode bias.

Cathode bias is self-limiting in that higher signals will allow more current to flow across the cathode, raising the cathode voltage and biasing the tube colder - hence the need for a hotter bias setting in a cathode biased amp. It's this cooling bias that limits things, more so than the drop in relative plate voltage.

A 1000uf, or greater, cathode cap will smooth out those voltage fluctuations, letting the power section behave more like a fixed bias amp.

[Structo]

Alexo, thanks for your help.

But, I think you will find as I mentioned that a EL84 can output over 15 watts in AB1 P/P with over 300v plate voltage.
At least that is what was said in a tube manual I read.

As in the 18 Watt Marshall's case, it has a plate voltage of 348v.

I'm sure some EL84's are better at handling the higher voltage than others but, I will keep it below 15 watts each to prolong tube life.
These are some Russian EL84's from Sarge I believe so I don't want to send this amp back and have the tubes only last a few hours.

[Alexo]

But, I think you will find as I mentioned that a EL84 can output over 15 watts in AB1 P/P with over 300v plate voltage.
At least that is what was said in a tube manual I read.

...

I will keep it below 15 watts each to prolong tube life.

Hold on a second - two EL84's can put out 15 watts of power at 300 volts or so, yes. But that's output power, which, while it's related to plate dissipation, is a different thing entirely.

Just because an amp is putting out 15 watts does not mean that its tubes are biased to 15 watts. For example, look at a single-ended EL84 amp, biased to 90% plate dissipation - 10.8 watts - but only putting out 5 watts of power to the speaker.

All EL84's max out at 12 watts plate dissipation. What they put out in terms of watts that end up at the speaker is a different story.

I really recommend trying to keep plate dissipation under 12 watts per tube in this amp.

[Structo]

Yeah I just looked at the specs again from a different source.

The first data I looked at was not clear about if it was one tube or two.

The 15-17 watt figure was for two tubes in AB P/P at 300v.

Sorry for the confusion.

Thanks again.

[Phil_S]

Hold on a second - two EL84's can put out 15 watts of power at 300 volts or so, yes. But that's output power, which, while it's related to plate dissipation, is a different thing entirely.

Thank you for putting a fine point on this. You said it a whole lot better than I did. I was talking about plate dissipation and thinking about output power. My bad, sorry.

[Structo]

Yikes I was assuming they were one in the same.

That surprised me because I thought, why in the world would they call it a 18 Watt if it was closer to 30 watts output.
So I guess that the power drop is due to power loss because of inductive reactance and heat?

Got any great website or something that I can read about this?

I guess I thought those specs were what the output at the speaker was because historically, amp makers have always claimed that power in there specs.

For example, a two 6L6GC amp, is frequently called a 50 watt amp.