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Old 6th December 2010, 09:36 PM   #1
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Default 30W triode?

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?
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Old 6th December 2010, 11:00 PM   #2
tomchr is offline tomchr  United States
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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
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Old 7th December 2010, 03:13 AM   #3
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I think you can use 813 triode or 833
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Old 7th December 2010, 07:02 AM   #4
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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.
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Old 7th December 2010, 07:56 AM   #5
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Quote:
Originally Posted by tomchr View Post
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
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Old 7th December 2010, 08:10 AM   #6
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Quote:
Originally Posted by zivhackman View Post
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

Last edited by danFrank; 7th December 2010 at 08:13 AM.
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Old 7th December 2010, 09:16 AM   #7
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@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
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Old 7th December 2010, 10:16 AM   #8
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No, it's the voltage swing that modulates hum into the circuit.
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Old 7th December 2010, 01:03 PM   #9
jlsem is offline jlsem  United States
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Are you sure that the LS series uses Permalloy for the core? I believe the HA line was the only UTC product that did so.

John
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Old 7th December 2010, 01:36 PM   #10
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Quote:
Originally Posted by tomchr View Post
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!!
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