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Help choosing which Amplifier to build

Bulatovic

Member
2013-03-05 6:59 pm
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 :D
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.
[IMGDEAD]http://ocenil.ru/photos/mordauntshort-napolnaya-akustika-mordaunt-short-avant-906i-black.jpg[/IMGDEAD]

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/Mordaunt_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 :D)

Thanks a lot!

Best Regards,
Luka
 

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Bulatovic

Member
2013-03-05 6:59 pm
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?
 
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.
 

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Bulatovic

Member
2013-03-05 6:59 pm
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 :confused:

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?
 

Bulatovic

Member
2013-03-05 6:59 pm
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...

Here is a set of monoblocks I built a few years ago with the help of the guys on this forum (particularly Tubelab) http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-50.html#post2259756

that has to be one of the best looking amps i've seen! :eek:

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

To go class A or AB?
 

chrish

Member
2003-10-20 2:43 pm
Sydney
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 http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-45.html#post2179774 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
 
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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.
 

chrish

Member
2003-10-20 2:43 pm
Sydney
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.

While it may be true that you might only be using a watt or less, the OP has said he likes to listen to classical music. Much classical music has a high dynamic range. The problem you will have is that listening to the quiet passages, all is ok, but when the whole orchestra or choir comes in - it sounds like crap with high distortion. Also, the OP has 90dB speakers. My Tubelab Simple SE amp (7-8 watts?) sounds good on my 93dB speakers, but limited to low volumes, especially with orchestral music. Personally, I would consider this power (7-8 watts) about my minimum for these 93dB speakers. Effectively, with 90dB speakers, you would require about 15 watts for similar volume levels.

Building a flee power SE can be fun and teach some valuable skills, but the OP has said he has built some guitar amps, so probably does not need this building experience. Moreover, a flee power amp on 90dB speakers is not going to be very satisfactory in the long run, in my opinion. I think the choice of 6L6, EL34 or KT88 push pull is a realistic match for 90db speakers with modest volume requirements. From memory, doubling output power increases dB by 3, so a flee power 1 watt amp is going to give 90dB max. 2 watts gives 93dB, 4w-96dB, 8w-99dB, 16w-103dB, 32w-105dB, 64w-108dB etc...
 
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So, how do i use the test in order to see how many watts do i really want? Play the test and read the AC voltage from the speaker terminals - then do the RMS calculation based on the results or some other, less basic way?

Yes, use a full spectrum pink noise and assume 4 ohms. When you're adjusting the volume for music before the test, make sure to use loud music :cool:. Once you have a power number you can decide for yourself how much extra power headroom you want to give yourself.

chrish said:
While it may be true....etc.

I mentioned a possibility but advocated a measurement.
 
Here is a set of monoblocks I built a few years ago with the help of the guys on this forum (particularly Tubelab) http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp-50.html#post2259756

My next project.........using trioded KT88's........I've got all of the parts, just need to find the time.....This project has everything I'm looking for in a PP design...diff front end and LTP with CCS, mosfet followers....I love it!

Back to the OP's question......When I think about cranking ZZ Top, Floyd or classical music through 90db sens speakers with a flea powered SE amp, I think he'll be happier with a PP amp IMHO.
 
Is there any schematic you'd recommend going from for the beginning?

A Mullard 5-20 based design is not a bad place to start. Here are a couple of designs that were developed about the same time as Chrish's amp. Both are PP KT-88 designs, although it would be easy enough to substitute 6L6 or equivalent output tubes.

Tubemack's project is a modernized 5-20 design with a CCS for the LTP and the Opus project adds mosfet followers.

http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html

http://www.diyaudio.com/forums/tubes-valves/161702-opus-5-0-modern-mullard.html
 
For output transformers, you will have more class A output with a 6.6k than a 4.3K, but you will have more output power with a 4.3K. Most of my amps are 6.6K, the exception is the 6L6 amp with 6550's and 4.3K outputs. I played around for about a year with different transformers, tubes, coupling caps, eedback ideas. the 4.3K outputs required 6550 or KT88 's to drive them (with little feedback).

Look for a Dutch side by Triode Dick and this amps "BOB", a KT88 amp driven by Russian ECC99 LTP, a good combination. His site is in Dutch and English.
The Russian PIO caps are a good component for the price.
 
Thanks for all the input guys! I really appreciate it!
Sorry it takes me long to answer, as i'm doing extensive research on the topic, trying to figure out what would fit me best...

So far, the main contender is Mullard 5/20, but on the other hand - what about ST-70 made to fit 6L6?
I'd stick with more "old" approach to my first build - tube rectification, fixed bias circuit with simple potentiometer to alter the voltage/current, as i am not too familiar with voltage regulating mosfet/jfet and other stuff... plus i am a bit of an old fart if you know what i mean - when it comes to audio equipment... for some reason i don't believe in semiconductors no matter how much better they can be :)

A completely different way - more wallet friendly *but would it also be ear friendly?* would be UL version of El Cheapo, or Baby Huey, or some variation of Mullard 4/10 or even ST-35... but how much cheaper would it really be? And how much sound would i "lose"?


Also, take in consideration that i'd be using it to play CDs, Vinyl and radio, maybe even computer connection or ipod - so what about the preamp section? Would it be best if it were an integrated amplifier, or the preamp for my needs would be too complicated and a bit stupid to put in the same package with (let's say i choose) 5/20 type of amp?


- In case i go with 6L6 5/20 or ST-70 version, would i be ok with something like 350-0-350V 320mA HT, 6.3V 6A, 5V 3A? Would a single GZ34 do the trick of rectification or would i need to split it in two separate B+ supplies with 2 rectifiers, coming from the same power transformer, followed by 2 150mA chokes? or would that be an overkill?

Though i'd like to go big from the beginning, i have to admit that my budget can't go too far, in terms of - i don't really have a limit, but wouldn't splash out too much for something i don't really need - so if it turns out that i'd be using a third of a potential of an El Cheapo - why would i go way bigger with 6L6... But i guess there's no easy way of telling, is there?

But one thing is certain - i want to build me a fool-proof, simple amplifier that will be suitable for the music i'm into.

I'm often not listening to the music with the amp too loud, so i suppose that i'm more for sound rather than power, but it's nice to have some power reserve when i want to blow my mind off :)
 
Well - an update is in order.
I've played the range of music i usually listen to and found that AC/DC (the loudest of all the things i've played) during the chorus was pretty much constantly in the 14 - 14.5V AC range when measured on the speaker terminal. Taking that to be peak voltage it gave me around 10W RMS, but i'd be on the lower side of power i suppose if i was to go for 10w amp, right?
That's the very point where i don't want my amp to let me down in terms of power and bass - so.. i guess that push pull EL84 won't do me much good, would they?
This is the loudest i'll play, and it's not happening that often, but i need an amp that can take me there to meet my mood.