4 output tubes for 30W amp ???

[badoumba]

Hi

This is my first post in this forum.

I try to design my first guitar amp - lokking for building something similar to Kingsley 30W amps.
Looking at the specs these 30W Kingsley boutique amp, I noticed that they use 4 output tubes for 30W amplifiers. How can we manage to overdrive the output valves on such configuration? Nice power stage overdrive happens when we reach the amp power limit, don't we? 4 valves for 30W is far from the capacity limits.

Thanks for reading!

[Cantplay]

If the tubes are running in coscode, or plates strapped triode then thats certainly possible.

John

[Leo_Gnardo]

Nice power stage overdrive happens when we reach the amp power limit, don't we? 4 valves for 30W is far from the capacity limits.

I don't know what kind of output tubes Kingsley uses, but 4 x EL84 or 6V6 can definitely put you in the 30W zone. Also if the B+ is low, that will be a limiting factor in how much wattage the amp produces, no matter what output tubes. If it's overdrive distortion you're looking for why not plan on a 15W amp?

[badoumba]

@Leo

I would like actually to build a 15-20W amp, so I can push the output stage up to it's limit for a bluesy sound. I was just wondering on a design point of view why do they use 4 valves for 30W when 2 could be enough, especially for an overdriven sound (unless the overdrive is set on the pream stage as I asked on a second post).

[Structo]

Probably a more complex tone with four output tubes vs two.

That is a guess but you can certainly build a great sounding amp with two 6V6GT's,

The 4x EL84 amps I have played seem real punchy and not really vintage sounding.

[Phil_S]

For an overdriven bluesy sound at around 20W, I am thinking you want a Fender 5E3 Deluxe, or a Marshall 18W. These are two classic designs that are well documented -- schematics and layout are easy to get on the internet. You can find many examples on youtube.

You can easily get 30W from 2 EL34 or 6L6, but not from EL84 or 6V6. You will need 4 EL84 or 6V6 for 30W.

[badoumba]

In his nice pdf file "Designing Vacuum Tube Amplifiers and Related Topics", Charles R. Couch suggests that the best option is to go down to 25W with two EL84, 6V6, or even better (much more dissipation) 6JN6. This comforts your comment Phil_s.

The point behind this question is : with 4 6V6 for instance, you can get 50W easily. If we want to saturate these valves at 30W, do we have to lower their B+ or is it best practice to add a power attenuator as last stage?

[Phil_S]

Ordinarily, we don't see 50W designs for a pair of 6V6 or EL84. There must be a reason for that.

[Firestorm]

Available output power is mostly controlled by two things: tube type and B+. OT impedance can have a small impact, but it's best not to try to manipulate power in this way. To get a nice bluesy feel at 30 watts, I'd say to use 4x6V6 at about 300V, cathode biased, with a load of about 4K. The cathode bias will add some compression, but for more, use a tube rectifier (good quality 5AR4 or 5U4) or add a small sag resistor in series with the supply and rectify with SS diodes. If you want the flexibility to run more powerful output tubes like 4x6L6, you can't get away with a single tube rectifier and you'll need a multitap OT. Though if you build for 4x6V6, you can swap to 2x6L6 without changes provided your cathode resistor is stout enough.

[badoumba]

Thx Firestrom

So according to you guys, what leads the choice for 2 powerful tubes or 4 less powerfull tubes for the push-pull stage? i.e. does the number of tube changes something at equivalent power?

[Phil_S]

badoumba: you continue to ask about the number of tubes. This has be answered a number of ways. Not all tubes are equal in capacity to generate power out. The answer is that it depends on what tubes you select.

EL84 and 6V6 are low power tubes, about 8-10 watts per tube.
6L6, EL34, and most of the KTxx tubes will give you 15-25 watts per tube.
The exact amount of power depends on lots of other factors including the particular circuit and the transformers selected.

You have asked about a particular kind of tone and you were very good about giving us an example of what you want. That question has also been answered. Forget about whether you use 2 or 4 tubes. Read the answers that have been given! That is where you will find your answer.

[Leo_Gnardo]

How many holes ya wanna drill? How many sockets ya wanna wire up? It's OK to decomplicate your life and answer 2 for the power section. Why some build amps that have quads of EL84 or 6V6, there's some tone factor to those tubes. And that's the only answer I can think of. Unless ... they had a big overstock of those tubes & associated OTs or access to same for a knockdown price.

[badoumba]

Phil_s

I clearly understood all the answers which have been given to me and I all thank you for this - my questions have actually been answered very well. I just try to understand a little more about design choices (here, why using 4 valves with power limitations when 2 could do the job). All of this seems clear to you - I opened my first book about amps 2 weeks ago.

@Leo_Gnardo, just saw your answer, thanx

[JoeCon]

I think the answer to the question about number of tubes vs. power is really about tone.

I want more power than 2 EL84's can provide to play with the drummer in my band. Now I could go to 2 EL34's for the increased power BUT I want the EL84 chime so an amp with 4 EL84's would give a bit more power but keep the same tone I'm looking for.

You can get similar output from 4 vs 2 tubes of different types but each will have their own sound in the circuit you build.

[Zippy]

There is also a matter of coloration and breakup characteristics. Generally speaking, EL84 and EL34 are similar, 6V6 and 6L6 share similarities.

If you prefer the character of the EL series, then you decide whether you need power and/or complexity. Same for 6_6 tubes. It's Euro vs American sounds.

Start somewhere and get to know how the various tubes sound in the common configurations.