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30W triode?

zivhackman

Member
2010-03-13 12:26 pm
hello!
I want to build a good 30w class A or AB1 triode amplifier.
this means I have to go for PP operation, sins i dont want to mess with 1Kv...
another thing is that from my experience (which is very limited..) I prefer the older tubes (30's 40s') which brings manly 300B or 2A3..
I never used one of them, but i have a filling that i will prefer the 2A3 and from the places that i know it is cheaper to by 8*2A3 than 4*300B..

now i have a few questions;
is anyone have any experience whis PPP 2A3?
what should i go for, fixed bias or self bias? (I think I prefer self bias..)
what operation point do you suggest for the 2A3?
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
With your average 87~90 dB/W*m efficient 2-way box speaker, 30 W would yield well over 100 dB SPL. Normally ear protection is required in work places once the SPL exceeds 75 dB(A). In other words, you're aiming for ear splitting volumes under normal listening conditions.

Two 6L6GT in push-pull would deliver that.

You seem pretty dead set on using 2A3 or 300B. Unless you really need that 100+ dB SPL I'd suggest looking at using those tubes in a single-ended triode (SET) configuration. You'll get a couple of watts out of that. That'll produce 90+ dB SPL. Earplugs still required...

~Tom
 
Tom...really now! He is asking for 30...not 300watts. Tho I'm very happy with my 8watt SE job, I don't go snot nosing and tell him 30watts is too much and he should go for less. If he were asking for 300 I might say something, but only if I really had little to do.

hkthienthanh...he is asking for lower voltage designs.

If u have the money, a 4*2A3 will do a great job, look at Lynn Olson's design for instance and just double the output stage. There are many other PP triode designs, and since I'll be insisting on building a 6AS7 design, I think I'll let some one with 2A3 experience grab the stick...

But please folks, lets answer the original post without starting a flood for once.
 
With your average 87~90 dB/W*m efficient 2-way box speaker, 30 W would yield well over 100 dB SPL. Normally ear protection is required in work places once the SPL exceeds 75 dB(A). In other words, you're aiming for ear splitting volumes under normal listening conditions.
*snip*
~Tom

Next time the wife yells at me I'll need to report her to OSHA! Where's my ear protection...? Next thing ya know we'll have HESHA (Home Ear Safety and Health Administration)...so we don't play our Hi-Fi's too loud!

Even so with an average 87~90 dB/W*m efficient 2-way box speaker, you'll need at least 30WRMS to cover the peaks, unless you like to listen to background chamber music...

Ear splitting? Maybe if one listens to 100db sine waves or highly compressed Metalica, with the volume all the way up and along with the horrid distortion/clipping.

Wayne :Pawprint:
 
hello!
I want to build a good 30w class A or AB1 triode amplifier.
now i have a few questions;
is anyone have any experience whis PPP 2A3?
what should i go for, fixed bias or self bias? (I think I prefer self bias..)
what operation point do you suggest for the 2A3?

Take a look at the JJ 2A3-40 tube; it is a 300B style tube with a 2.5 volt filament. 2.5 volts on the filament will give less hum when using AC to power the filament (I like AC on the filaments). A pair of these will give equal or more power than 2 2A3's in PP configuration.
I have an amp that uses 6B4G's (6 volt 2A3) in PP configuration. This amp has ALL power supplies fully regulated with transistor pass devices. I am using UTC LS-57 output transformers because I am a fan of Permalloy cored outputs; they take less current to energize the core but both sides of the primary need to be matched closely in current draw. I am using half of a 6SN7 in cathode follower to drive each output tube; this way I can squeeze a few more watts out of the output tubes before they distort badly. Fixed bias on the 6SN7's which in turn provides fixed bias for the 6B4G's. Self bias wastes a lot of energy that you get no return with... But some people like the way it sounds better... Not me.
I am using 250 volts 60ma as the operating point for the outputs, like the RCA manual suggests. I am getting 15 watts out of the pair but this is largely due to the fact that I am running the tubes in class A2 because I can with the cathode follower drivers.
I am using no feedback, so output transformer selection is a big factor in how the amp sounds. I love UTC LS output transformers, they sound great!
As for your main question... you can get 30 watts output if you run a pair of 300B's or a pair of JJ 2A3-40's in class A2 with a robust driver in fixed bias. You can also do it with standard 2A3 / 6A3 / 6B4g's in PPP, also in Class A2. If you want to run class A1 (no grid current) with RC coupling to the output tubes then expect about 1/2 the output that class A2 will give before distortion sets in.
I would stick with the JJ 2A3-40 tube as a pair of these will pretty much give you what you want while still keeping it relatively simple.
Daniel
 
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zivhackman

Member
2010-03-13 12:26 pm
With your average 87~90 dB/W*m efficient 2-way box speaker, 30 W would yield well over 100 dB SPL. Normally ear protection is required in work places once the SPL exceeds 75 dB(A). In other words, you're aiming for ear splitting volumes under normal listening conditions.

Two 6L6GT in push-pull would deliver that.

You seem pretty dead set on using 2A3 or 300B. Unless you really need that 100+ dB SPL I'd suggest looking at using those tubes in a single-ended triode (SET) configuration. You'll get a couple of watts out of that. That'll produce 90+ dB SPL. Earplugs still required...

~Tom


I alredy have a SE KT88 wich I use it as a triod..
I have a 96 db speaker and it steel lacks power in big classical muisic..
now I want an amp thet culd drive 90 db speaker and will have that extra headroom that I need..

I'm realy not dead set on using 2A3 or 300B..
if I would know other strong triod from that period of production i will defenetly go for it!!
 
I am using 250 volts 60ma as the operating point for the outputs, like the RCA manual suggests.
Daniel

One more thing that I forgot to mention last night....
With the 2A3-40 250 volts is too low a voltage to get most power output; 250v is good for the original 2A3. With the 2A3-40, I would use 400 volts B+ at aound 80ma. You can push it to 100ma but that would be right at the limit for the 2A3-40's 40 watt plate dissipation.

The UTC LS series of transformers use a permalloy core also. In fact, the main difference between UTC's LS and HA series of transformers is size and weight. The HA series was for "portable" equipment and made to be as light as possible without sacrificing fidelity. The cases on HA transformers are thin compared to the heavy cases of the LS series. If you look at the literature, most HA transformers have an electrically equivilant LS transformer.
Daniel
 
@danfrank: i thought lower voltages would lead to higher currents and thus even more hum if using AC. (read Morgan Jones). So why do you say that 2.5VAC is better for the 2A3-40

It's the potential across the filament that determines how much hum you get from this source in a DHT or DHP.. 2.5V tubes work fine from a hum perspective, (5V tubes usually don't) however you should ask tubelab about the intermodulation distortion spectra he measured with ac heating on 2.5V dhts.

The 2A3-40 is very similar to the JJ 300B and a single pair will easily give you 30Wrms in pushpull.

As a frame of reference my system runs out of steam somewhere north of 106dBSpl at my listening position, but I normally run in the low/mid 80s. (Low power with very efficient speakers) IMHO the power level proposed is perfectly reasonable for a speaker of that efficiency level.
 
Kevinkr:
I recall sometime in the past that you said that the JJ 2A3-40 tube is a good tube for durability, but to buy them after they have been tested and burned in so it weeds out the few where the filaments burn out.
Can you recommend a place that does this and sells the 2A3-40 for a good price?
Thanks, Daniel
 
Try 4П1Л instead of 2A3, you will be surprised...
 

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Kevinkr:
I recall sometime in the past that you said that the JJ 2A3-40 tube is a good tube for durability, but to buy them after they have been tested and burned in so it weeds out the few where the filaments burn out.
Can you recommend a place that does this and sells the 2A3-40 for a good price?
Thanks, Daniel

Not so much for filament failures as for generic infant mortality and in order to get ones that are well matched and stabilized. You can get these from New Sensor amongst others.