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Old 5th March 2013, 09:17 PM   #1
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Default Help choosing which Amplifier to build

Hi there guys!

First i'd like to introduce myself - I'm Luka, coming here from Serbia, a tube amp enthusiast, but so far i've only had experience with guitar amps, hoping to go HiFi as well
Fell in love with the vacuum inside the glass some 6 years ago with my first tube guitar amp (Epiphone Valve junior) and it didn't take me too long to start modding it. Then followed some Marshall builds - 18w EL84 PP cathode bias, 100w EL34 PP fixed bias, Fenders with 6V6 and 6L6s in PP configuration, before deciding i'd definitely love to hear what my turntable sounds like when going through a tube amp.


I've done some homework for this, but would need your help guys in order to go the right way here from the start.

So:
Coming from guitar stuff, as you'd expect i'm mostly into blues and classic rock sounds - BB King, Buddy Guy, John Mayall and the Bluesbreakers, ZZ Top, AC/DC, Thin Lizzy, Gary Moore, Pink Floyd, and so on, but also like some, let's say quieter sort of genres. Also, i am a big fan of classical music, but the more powerful side of it - think Wagner and his heavy brass sections rather than Verdi... you get my drift.

This leads to the question what sort of amp is best suited for delivering these tones.
The speakers i'll be using are Mordaunt-Short Avant 906i floorstanders that are rated between 4 and 8 ohms and be able to handle 120w while having 90db sensitivity.
Click the image to open in full size.

I've also managed to find their ratio of impedance and frequency that i've read is quite useful for deciding whether to go SE or PP:
http://liga-zvuka.ru/data/multimedia...nt_Short/5.PNG


From what i've read and my experience with guitar amps - SE should be more punchy and immediate, with more detail and sparkle but suffering on the noise field, lower bass response, lower power and more distortion - on the other hand PP amps should be more rigid, quieter, much more powerful, but also slightly duller than the SE types. (i do realize that at this point there are people that'll say that a well designed SE amp doesn't need to be low in bass or noisy or what have you, or that the PP can have the clarity and sparkle of an SE amp, but i'm talking about simple, available, DIY options, using pretty much limited resources in terms of high quality, not a huge budget, and somewhat limited knowledge).


This is my view of the situation - i have a Marantz 2226b receiver that isn't too powerful, but sounds great with my speakers, so i decided i don't really need a very powerful amp. So i'd be willing to sacrifice power with return in "niceness" - so natural way of thinking is SE.
Also, due to budget constraint i've decided that the best way would be to go with these great Russian 6P3S-E military tubes, Russian equivalent of 6L6, that sound amazing given their pretty low price and ruggedness, but also due to the fact that 6L6 should be in theory a tube that's suitable for any kind of music. (please correct me here if i'm wrong). I have also considered KT88, but to be honest i don't know how well would it behave while playing tight and punchy riffs of AC/DC or Whitesnake - no concerns with classical though...

To add to the issue - i tend to think that a single 6L6 in output stage would somewhat be insufficient for driving the speakers while not being too distorted and compressed for the heavier genres, so:
I've thought of using 2 6L6 tubes in parallel to provide me with more power while retaining that SE nice tone.

Am i getting this part right, or am i just overly doubtful about a single tube in SE power stage?


I realize that i've written a lot, while expressing my opinion which is almost entirely made on assumptions (and experience in guitar sound, which i am uncertain how much i can use to get the HiFi sound i desire), so please comment on anything i've written here, i'd love to hear your experiences and recommendations.

Also, take a look at the attached russian schematic that i've found that uses 2 beam power tetrodes (6P3S) in parallel SE amp and let me know if this is any good...


Hope i didn't bore you to death with my theoretical/philosophical approach to this topic, but i have absolutely no experience with anything else going through tube amp other than a guitar or a bass (which is still a guitar )

Thanks a lot!

Best Regards,
Luka
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File Type: gif 6p3C.gif (28.0 KB, 243 views)
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Old 5th March 2013, 09:27 PM   #2
multi is offline multi  Australia
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HI
With 90db speakers don't even think of SE amps.
I would think Push pull 6l6 would suit the type of music you enjoy better anyway.
Phil
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Old 5th March 2013, 09:33 PM   #3
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Phil,

Thanks for the reply!

You reckon that SE even with double the tubes in output would struggle with 90db speakers?
What would "suffer" the most in that case? Would it be needed to run the amp really hard, introducing too much distortion/ lack of power/ weak bass/ or something else?
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Old 5th March 2013, 09:46 PM   #4
roline is offline roline  United States
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Just a pic of a 6L6 based PP amp with 6P3S tubes. Do you want to make one from scratch? This is a toner transfer etched in the sink version.
You can upgrade with Svetlana 6550=C= tubes, they can take higher screen and B+ voltages. The 6P3S tubes preferred 6.6kRaa output iron, the 6550 can handle 4.3KRaa output iron with minimal feedback.
Attached Images
File Type: jpg keg3_6L6.jpg (186.2 KB, 234 views)
File Type: jpg P6220376.jpg (929.0 KB, 227 views)
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SO many tubes, SO little time!!!
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Old 5th March 2013, 09:53 PM   #5
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Wow, that amp looks great!

yeah, i was going to build it from scratch...
I have a guy here in Belgrade that winds transformers, so i'll "design" transformers that i'll need - or order to my specifications rather than design.

Since PP amps need more components it might be a better idea for me to go with PCB version rather than point to point, which i adore and have plenty fun designing and wiring...

Hmmm - i always thought that 6L6 type of valve likes to see around 4k on primary of the OT when working in AB1 class

Is there any schematic you'd recommend going from for the beginning?

The first question that arises is - what sort of bias to use? Cathode or fixed?
Also what sort of phase inverter and first stage?
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Old 5th March 2013, 10:00 PM   #6
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I have tube amps for guitar. I have SS for hi-fi. (I have some tube hi-fi amps too )

A tube amp will cost much more than a SS amp.

Why not learn something new and build a solid state amp?
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Old 5th March 2013, 10:05 PM   #7
chrish is offline chrish  Australia
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Here is a set of monoblocks I built a few years ago with the help of the guys on this forum (particularly Tubelab) 6L6GC AB2 Amp

Here is final schematic.
Attached Files
File Type: pdf 6L6 6SN7 MOSFET amp.pdf (70.8 KB, 97 views)

Last edited by chrish; 5th March 2013 at 10:25 PM. Reason: Add .pdf schematic
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Old 5th March 2013, 10:19 PM   #8
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Quote:
Originally Posted by counter culture View Post
I have tube amps for guitar. I have SS for hi-fi. (I have some tube hi-fi amps too )

A tube amp will cost much more than a SS amp.

Why not learn something new and build a solid state amp?
I hear you! It's just that there is something rather magical in that silly vacuum that makes me go all bonkers! Can't help it... and my bank account feels that too unfortunately...

Quote:
Originally Posted by chrish View Post
Here is a set of monoblocks I built a few years ago with the help of the guys on this forum (particularly Tubelab) 6L6GC AB2 Amp
that has to be one of the best looking amps i've seen!

So i should be on the right path using 6L6 in a push-pull configuration.

To go class A or AB?
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Old 5th March 2013, 10:43 PM   #9
chrish is offline chrish  Australia
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I edited my post above to include the schematic, as it may be difficult to find in that thread. The schematic looks a little complex, but is not really. It is divided in to 4 sections. Three of them are simply regulated power supplies. The main supply is 400 volts. You could simply purchase a printed circuit board for this power supply, I think there are several available if you look. Even if you build another design, I think a regulated supply on a printed circuit board is the best/easiest implementation. The two other power supplies are simply a regulated -ive and +ive supply. I used voltage regulator tubes simply because I had not used them before and was a sucker for the pretty glow. If I were to do it again, I would use solid state and printed circuit boards (I have learned to fabricate my own since building). I would guess with a little investigation you could find some simple regulated supply printed circuit boards that would work just fine.

The interesting 4th part of the schematic is the amp. Tubelab designed a DC coupled dual differential amp driver stage using (in my case) two 6SN7 tubes. You could just as easy substitute with a smaller 9 pin tube, but I was wanting to use octal tubes for the hell of it and aesthetics of the final build. The two differential amps have a constant current tail using 10M45 constant current chips. Any constant current circuit of your choice could be substituted here, I believe there are probably pc boards or even fully built units available. This is linked to the push pull pair via a MOSFET source follower. There is much discussion on the thread linked to about this feature of the design. In my amp, they are simply loaded with resistors, but some have also loaded them with constant current devices which would lead to even lower distortion. I have not used any feedback, though the design has sufficient gain to implement that if desired.

Tubelab suggested a small balance circuit to make sure the voltages on the second DC coupled triode stage are equal, that is this small part of the circuit 6L6GC AB2 Amp and could be removed for simplicity (as it is in previos versions of the schematic on that thread).

You will also notice on the very bottom left of the schematic a small circuit that makes a virtual ground for the heaters through two 100R resistors and lifts that voltage via a voltage divider to protect the heater/cathode voltage limit and to reduce noise.

Even if you do not choose to build a similar design, I hope it at least gives you something to compare against.

Cheers,

Chris

Last edited by chrish; 5th March 2013 at 10:53 PM. Reason: Extra info
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Old 5th March 2013, 11:06 PM   #10
12B4A is offline 12B4A  United States
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I think any amplifier recommendation is not meaningful until you measure how much power you are actually driving the speakers with at the loudest you think you will ever want them to be. There are plenty of 0dBFS test tone audio files out there that will inform you of the level after you have determined the volume setting with your current amp. You might discover you've been listening at only a watt or less and a flea powered SE might fit your desire.
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