2xEL34 vs. 4xEL84 vs. 2x6L6 vs. 4x6V6
I am about to begin designing a custom tube amp for a musician in my town. He pretty much knows what he wants, he is a minimalist, and he wants to use the highest quality components available. Pretty much an amp builder's dream!
The thing he doesn't have an opinion on is the selection of the output tubes. He wants something in the 30-40 watt range. Any of the output tube setups mentioned in the subject would get us there. I was just wondering if I could get some different opinions on how these different setups affect the operation of the amp (headroom, clarity, tone, etc.)
BTW, the customer plays blues with a lot of slide. He is looking for early breakup.
This. Not this exactly. The F2a is scarce and expensive. But some EL34s bring silly money for no other reason than having a swank metal base and saying Philips or Telefunken. At least for the F2a you pay for quality. And it will last, 10,000 hrs! The long-life C3g or D3a would be my choice for drivers. Yes, it's MY dream amp I'm talking about here.
6L6GC , 7581A , the best , highest components doesnt meant good sound . Mix and match . 300B = king of the tube , for me it NOT worth a penny !!! BALANCED is more important ..
'blues with a lot of slide. He is looking for early breakup' - sounds like he wants a Fender-ish sound then, go for the 6L6's?
These require more drive than the EL34 family (which will give a cleaner "Marshall" sound), so the preamp stages are working harder for a given amount of output.
Depends on how you set up the gain structure of the preamp stages though.
I'd avoid multiple pairs for reliablity reasons.
(That's my theory anyway, FWIW...)
Multiple tubes mean that you use an OPT with lower turns ratio, which is supposedly better sonically. However, you have to be careful of uneven current sharing, the danger of which can be minimised by including 100 ohm resistors between OP tube plates and OP transformer primary.
My 2c contribution, when you speak of a dream amplifier, will be that I found from tests that a beam power output stage in UL is about the cleanest. Roughly pentode output capability with about triode distortion and tolerance performance. I also did not find paralleling output tubes a particular problem unless an equivalent bigger tube is available. (Thus I would not use 2xEL84 in place of one EL34, but that is not a strong feeling. Only that it will be best to parallel two equal tubes, sometimes difficult to sort out.) But the rest of the circuit also plays an important role.
But that as far as a "dream" (hi-fi?) amplifier is concerned. Your requirements differ if early break-up is required. I do not have particular experience of guitar amplifiers, but it would appear that you would need to go the pentode output route (g2s to B+), plus some other circuit features.
Anyhow, everything you do will effect the sound of a guitar amp.
Down to the layout, parts type, cap type & brand, resistors, etc... and of course the basic circuit.
Your friend needs to worry less about the tubes used and more about everything else... put it this way, select the same tubes and build the same amp with different parts and layout? Different sound.
Note how every Fender amp has essentially the same circuit? They sure don't sound the same!! Why?
Building a good sounding guitar amp is an art that is part luck, part design, part alchemy and part the unknown.
Best to promise your friend that you have no idea what it will sound like when you are done, and that you may have to change it ten times before it sounds worth a damn.
You also have to learn how to make a guitar amp sound good, when you have an already built commercial one in front of you... before you try to build one from scratch with any idea of what you are going to get. Imho.
Btw, tubes like 300B are not a good idea for a guitar amp because the filaments don't do well being thrown around and bounced in the back of a car. Other than that, they're not a bad idea for something like a studio amp.
There is a BIG difference in harmonic spectra between pentodes/tetrodes and triodes.
Anyhow, good luck.
If you want early breakup with great guitar amp warmth then the 6V6 is the go for sure. Run 4 of them into a Marshall 50W Output transformer or Hammond 1645. Use largish screen resistors on the output tubes to give that early breakup. 1K is minimum, 2k2 or even larger can be used.
Using a Hammond 1650K or 1650N and beefing up the power supply a bit would give you the option of running 6V6, 6L6, EL34 or 6550.
The only other variation which might be considered is a VOX AC30 Clone with 4 EL84s if the "customer" likes that deep Class AB sound. (No its not Class A, that claim is just more marketing lies) .
If you have a few bucks spare then order "The Ultimate Tone Vol 5" by Kevin O'Connor and build his "standard" guitar amp from chapter 9. The Preamp gives you Clean to fairly massive overdrive and the power amp has switchable fixed to cathode bias (cathode bias is a better blues tone). There is Reverb and an Effects Loop which you can include or simply leave out.
Plenty of cash?? and want some more good reading, then order "The Ultimate Tone - Volume 3" as well. This one has more than a dozen projects to build clones of what Kevin calls the "Icons of Tone". Amongst them:
Fender Champ and Bassman
Marshall Plexi and 800 **
Vox AC30 **
Ampeg SVT and Portaflex
Traynor Bassmaster and Custom Special
Laney Linebacker **
Most of the British style clones (ones identified by ** above) use the Hammond 1645 Output Transformer but with different tube complements.
I found out a long time ago, that we all have different dreams! My dream amp is likely to be vastly different from yours. My dream amps have changed a lot as I have gotten older. My next dream amp will be all about flexibility instead of raw power, but don't tell anyone. Most of my customers are really impressed by raw power, but they BUY small amps with a zillion switches to allow many tonal choices. I have sold more "Turbo Champs" that any other amp by far, but they are hard to deal with in a live situation. I have had a few customers buy two of them and switch between them with a footswitch.
I have crossed over to the dark side, by designing a microprocessor controlled vacuum tube guitar amp. It is far from complete, and I don't have the time to work on it, but here is the idea. A P-P tube amp of about 50 watts depending on mode (triode, UL, pentode), 3 different preamp sections, clean, overdriven, and solid state (opamps). The usual controls are all done by digital pots (not trivial with vacuum tube voltage levels). Why? So that when the user finds a sound that he likes, he can store the setup, and recall it by stomping a pedal. This is really useful for live situations. No scrolling menus either (many effects boxes), just a few different colored pedals that are easy to see on a stage with bright lights in your face.
If you don't have a clear handle on what the customer wants, ask him what his favorite amps are and why. I will even go with a perspective customer to a large "hands on" music store like Sam Ash (it helps that I spent several thousand dollars there when my daughter was at home) where they allow playing with the amps (go when the store is not crowded, and have him bring his guitar and effects). Try several amps asking the customer to show you what sound he wants, and ask him what is missing, or what he would want different.
Often I can copy sections of several different amps and put them into one box.
When you think that you have it all figured out, breadboard the entire amp, and have the user play through it. If he already has a speaker in mind, use it. Audition as many different OPT's as you can gather up, with different NFB levels. Try different tubes too. EL34's and 6L6GC's can often be interchanged in the same design if the bias is adjustable. I have had customers that really like a combination that I didn't like at all, but they are paying, so they get what they want.
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