1 Watt 8 Ohm / 600 Ohm SE amp

Hi,

I think about building a little tube amp - 1Watt of power would be enough - which can also be used as a +30dBm 600 Ohm line driver.

I'd like this to have a SE triode output stage.

I'm looking for recommendations on

(1) a SE transformer with 8 Ohm and 600 Ohm output tabs

(2) what tube I should use.

Thanks in advance for any ideas,

JH.

.
Jurgen, did you see the 'one-bottle preamp' thread?
There are two different preamps there, WE417A from CJ and E180CC/12AV7 from NYD.
Plus other tube suggestions. Fun read.

BTW enjoy your contributions. Thanks
Paul

[quote author="pmroz"].
Jurgen, did you see the 'one-bottle preamp' thread?
There are two different preamps there, WE417A from CJ and E180CC/12AV7 from NYD.
Plus other tube suggestions. Fun read.

BTW enjoy your contributions. Thanks
Paul[/quote]

I've seen the thread, probably have to re-read it again.

But I hope I've made this clear: I want to connect an 8 Ohm *loudspeaker* to the output, with the option to also use it as a line driver with 600 Ohm output.

So the main difficulty would be to find a transformer which has both 600 Ohm and 8 Ohm output tabs, and then to select a triode which will work fine with that specific transformer. (If there is any such transformer at all.)

Sorry for not being clear enough about that.

JH.

[quote author="jhaible"]But I hope I've made this clear: I want to connect an 8 Ohm *loudspeaker* to the output, with the option to also use it as a line driver with 600 Ohm output.[/quote]

There may be a way to will one into existence involving black magic. Are you willing to go that far?

[quote author="tk@halmi"][quote author="jhaible"]But I hope I've made this clear: I want to connect an 8 Ohm *loudspeaker* to the output, with the option to also use it as a line driver with 600 Ohm output.[/quote]

There may be a way to will one into existence involving black magic. Are you willing to go that far?[/quote]

Black magic?
I thought the worst case would be I'd have someone wind me one.

JH.

output transformers with both 600 and 8 ohm secondary windings DO exist, or did, anyway. eBay is probably the best route to find those. hammond makes a seperate 600 to 8 ohm matching transformer specifically for hooking your 600 ohm output up to a speaker, the 119DA.

http://www.hammondmfg.com

possibly the easiest route would be to contact EDCOR, tell them you want one of their XSE series transformers with a 600 ohm secondary winding added. they seem quite willing to do one-offs, as forum members here have found.

http://www.edcorusa.com

for the tube, prr recently had discussion about the dual dissimilar octal 6EM7 tube, which has one side that is big enough to get you your one watt output.

glad to see you back here jh!

ed

[quote author="edanderson"]output transformers with both 600 and 8 ohm secondary windings DO exist, or did, anyway. eBay is probably the best route to find those. hammond makes a seperate 600 to 8 ohm matching transformer specifically for hooking your 600 ohm output up to a speaker, the 119DA.

http://www.hammondmfg.com

possibly the easiest route would be to contact EDCOR, tell them you want one of their XSE series transformers with a 600 ohm secondary winding added. they seem quite willing to do one-offs, as forum members here have found.

http://www.edcorusa.com

for the tube, prr recently had discussion about the dual dissimilar octal 6EM7 tube, which has one side that is big enough to get you your one watt output.

glad to see you back here jh!

ed[/quote]

Thanks Ed!

JH.

check UTC pages at

www.vacuumbrain.com

I remember seeing some 600/8 outers there.

sounds like a fun project.


hmm, many 600 outs have taps.
a 50 ohm strap might not be too bad a mis match
dcr might not be low as 8 ohm tap.

yeah, check the last page in the UTC cat pages displayed in blue.

output to line and voice coil.
a whole slew of them.
and, the nice big can!
won't mount on 1 u case.
need 2 u min.
LS series!
ebay

cj

I have a pair of SE output transformers with a 7K primary, two 600-ohm secondaries and an 8-ohm secondary. So they definitely exist; you might just have to hunt around.

[quote author="CJ"]check UTC pages at

www.vacuumbrain.com

I remember seeing some 600/8 outers there.

sounds like a fun project.


hmm, many 600 outs have taps.
a 50 ohm strap might not be too bad a mis match
dcr might not be low as 8 ohm tap.

yeah, check the last page in the UTC cat pages displayed in blue.

output to line and voice coil.
a whole slew of them.
and, the nice big can!
won't mount on 1 u case.
need 2 u min.
LS series!
ebay

cj[/quote]

Thanks CJ. I even think your page was the first time I saw these. I knew something like that must exist!

Seems that all of them are PP, though. Maybe I ditch the idea to go SE then.

JH.

Your right, all p to p.

Let's check Triad and Peerless.

Nothing there.

Checked Jensen Sowter and Lundahl.
Not much there either.

You could use a plate to 600, then a 600 to speaker if you want.

UTC A-23 pretty cheap. 600 to 4/8

I have a WE output with 600300/200/60 and 4 ohms.
But it is kind of big.
Takes 80 ma, so 10 ma SE might not bother it.

It's 3 1/2 by 4 by 3.
4400 ohms pri.
20 bucks and it's yours.
cj

[quote author="CJ"]Your right, all p to p.

Let's check Triad and Peerless.

Nothing there.

Checked Jensen Sowter and Lundahl.
Not much there either.

You could use a plate to 600, then a 600 to speaker if you want.

UTC A-23 pretty cheap. 600 to 4/8

I have a WE output with 600300/200/60 and 4 ohms.
But it is kind of big.
Takes 80 ma, so 10 ma SE might not bother it.

It's 3 1/2 by 4 by 3.
4400 ohms pri.
20 bucks and it's yours.
cj[/quote]

Maybe PP would really be a good idea.

I forgot to mention that I need a pair - this will be a stereo device.

JH.

Found something.

Single ended it will be.

Just ordered from BTB:
http://www.btb-elektronik.de/de/uebertrager.html

60 Euros for a 5W SE 8 Ohm type, plus 5 Euros for the extra 600 Ohm winding. Will take 1 ... 2 weeks.

JH.

Cool!
Obvoiosly a top secret project, right?

Which tube are you going to use for output?

[quote author="CJ"]Cool!
Obvoiosly a top secret project, right?[/quote]

No secret. What I have in mind is this:
I need a stereo limiter, and I need a small stereo monitor amp.

I love that LA2A-inspired limiter I've built some time ago, so I want this limiter to use the same method, opto electronic gain control, and a tube recovery amp. Then it occured to me that I might just build the pwer amp into the limiter, or add the photo resistors and control circuit to a power amp, depending on the way you look at it. Then I thought if I have extra 600R winding, I can use it as a regular line level limiter as well.
As I'm mainly going to use it on (virtual) mellotron and e-piano sounds, the next idea was that sometimes I'd like it with a little overdrive, and sometimes I'd like it clean. Sometimes I might even use it to monitor a virtual grand piano sound. Consequently, I just have to switch the opto limiting thing in and out, and I can choose between clean at a controlled, moderate volume, and overdriven. All in one box. Hopefully.

Which tube are you going to use for output?

Click to expand...

That's the big question, and I'm still looking for recommendations. First I thought of some triode. (Parallelled 12BH7s ?). But for that transformer, a EL84 pentode would be the first choice. Gives even *some* Watts instead of just one watt. So maybe running it well below its max output level (like, +20dBm instead of +40dBm, I might get a pretty clean signal in "Line Level" 600 Ohm mode. (Is this fairly realistic?) And I'd have more power for driving speakers than the 1W amp I first intended to build.

I don't know. Maybe I'll go for a triode anyway. That transformer has UL taps, so not using the full primary winding, I might even get a primary impedance where various triodes might feel comfortable.

What do you recomment for this project: Should I go EL84, or should I use something different?

JH.

> a little tube amp - 1Watt of power would be enough - which can also be used as a +30dBm 600 Ohm line driver.

My question is: why would you ever want +30dBm in a modern studio, especially on the output of a limiter? And conversely: what use is a 1 Watt loudspeaker amp on today's loudspeakers?

Also: we usually accept higher distortion in the speaker-amp stage than in the many line-amps along the way.

Before you did the deal for custom winding, I was thinking....

+20dBm is 8V, 16 ohms is a standard speaker winding, 8V in 16 ohms is 4 Watts, which is a respectable speaker amp. So a 4 Watt 16 ohm winding connected to 600 ohms will give ample line-level, while using the 8/16 ohm taps will give a nice speaker level.

Also: in triodes, raising the load impedance reduces distortion drastically. When we are hungry for power, we load a triode with 2*Rp and accept ~5% THD. But if we load it higher, THD falls much faster than power. 10*Rp will be about 1/2 the power and 1/10th the THD.

> what tube I should use.

The usual suspects. Mu=4-5, ample plate dissipation. Type '50, 300B, 2A3.... Note that several of the TV V-sweep tubes, notably 6EM7, are in effect small indirectly-heated 2A3s. At higher voltages and impedances, any of the Power Pentodes (EL84, 6L6, EL34, 6550,...) triode-strapped make effective triodes.

You bought a 5 Watt transformer. If you want to use all that you bought, then you need a triode with 5 Watts output. For low THD, an SE triode will dissipate about 6 times its output power, so you need a 30 Watt triode. This takes you to 300B, 6550, EL34, 6L6GC; or multiple lesser tubes. The 300B (when made by WE) is very-very linear, which is why it costs more. The 6L6GC is super-available and cheap, but not so linear at normal load impedances. If you don't mind multiple tubes, a pair of 2A3 is a classic, but the directly heated cathode is a real nuisance. A 6EM7 works a lot like a 2/3rd-size 2A3 but indirect heat (plus a free volt-amp triode).

If you assume Rl=2Rp, then to get 5W in 5K load you need 315V across the tube. If we use 6L6-triode, Rp is ~2k so this is nearly right. If we use two 2A3, Rp will be close to 400 ohms, much less than Rl/2, so we can use a lower B+, maybe 250V.

Such tubes should usually be used cathode-bias. Bias voltage will be about 0.6*(Vpk/Mu). High-Mu tubes like 6L6 or 6550, assuming Vpk is 315V and Mu is 10, will need about 20V bias, 334V total supply. Low-Mu tubes like 2A3, assuming Vpk is 250V and Mu is 4, will need 37V bias, or 287V total supply. The bias voltage is also the peak drive voltage; low-Mu tubes need drivers working on large supply voltages.

We estimated 30 Watts dissipation, so the idle condition should be 315V 95mA or a little less with low-Rp triodes. Current swing will be from 53mA to 137mA.

THD at 5W will approach 5%. At 0.1 Watt (+20dBm) it may be under 1%.

Dual 2A3 or triple 6EM7 is a big amplifier (and push-pull does start to look better). A less-insane plan might be to take all the power one tube can make, and not cry about getting less than 5 Watts. Single 2A3 gives about 2.5W; single 6EM7 gives around 1.6 Watts. Reduce voltage and current in proportion to square-root of power: about 200V 75mA for one 2A3 working in 5K, or 170V 55mA for one 6EM7.

PRR - thanks for your long reply. Much food for thought!

[quote author="PRR"] My question is: why would you ever want +30dBm in a modern studio, especially on the output of a limiter? [/quote]

No, I don't really need +30dBm. That little WCF on my LA2-style compressor is good enough. I just noticed that the 1W output amp I used
for my Variable Mu compressor (UA175-based) isn't _that_ much bigger either. And here we're in a power range that could already move some air with the right speaker.

And conversely: what use is a 1 Watt loudspeaker amp on today's loudspeakers?

Click to expand...

Well, I don't like it too loud. When the sound is loud enough to mask the mechanical noise from playing the keyboard, that's just the comfortable level I like to work.

Also: we usually accept higher distortion in the speaker-amp stage than in the many line-amps along the way.

Click to expand...

Yes, that was part of the idea. I don't really need +30dBm line level. I was hoping for low distortion in Line Amp mode, and gradually increasing pleasant distortion in amplifier mode. What I don't know is how noisy it will be when I run a power amp with low level. (Well, I guess I'll have to work with feedback than, instead of just reducing the input level.)

Before you did the deal for custom winding, I was thinking....

+20dBm is 8V, 16 ohms is a standard speaker winding, 8V in 16 ohms is 4 Watts, which is a respectable speaker amp. So a 4 Watt 16 ohm winding connected to 600 ohms will give ample line-level, while using the 8/16 ohm taps will give a nice speaker level.

Also: in triodes, raising the load impedance reduces distortion drastically. When we are hungry for power, we load a triode with 2*Rp and accept ~5% THD. But if we load it higher, THD falls much faster than power. 10*Rp will be about 1/2 the power and 1/10th the THD.

Click to expand...

Very interesting idea!
I wasn't aware that just using a higher impedance is a good method to get the desired results. Isn't there a problem with running tube power amps without load (or with a higher impedance than intended) ? I admit I still haven't understood this entirely. I mean, *when* it can produce overvoltage that can damage something, and when it does not. Does this only apply to PP?

At higher voltages and impedances, any of the Power Pentodes (EL84, 6L6, EL34, 6550,...) triode-strapped make effective triodes.

Click to expand...

That would have been my first choice, but without knowing if it's a good one or not. Just to use the EL84 for which the transformer was intended, and maybe try to connect it as a triode. Is there any substantial disadvantage, compared with "real" triodes?

JH.

It is sorta common to build a low power tube amp, and just add a headphone jack to make it also a headphone amp. You could just add a TRS or XLR connection and it will also be a line amp.

600 ohms isn't necessarily the key. All you need is an output impedance low enough to drive 600 ohms. So you could just make a conventional low power amp, with enough gain, and have a power amp, line amp, and headphone amp depending only on the output connection.

Here is an example:
http://www.bottlehead.com/et/adobespc/S.E.X./SEX.htm

This is a 6EM7 amp with a headphone out and 15dB of gain. It is parallel feed output, and I think it uses 70V line matching transformers for the output.

The cool thing about having a 600 ohm output is that you could get lots more gain into 600 ohms than into 8.

The Radio Shack $6 70V line matching transformer has a primary with 8k, 4k, 2k, and 1k taps, and a secondary with 16, 8 and 4 ohm taps. You could do something like use the 8k tap on the primary and the 16 ohm secondary to an 8 ohm speaker to get a 4k load impedance (and a 22:1 stepdown ratio), and then for 600 ohm use, keep the 16 ohm secondary tap, but use the 1k primary tap for a really high load impedance, but an 8:1 stepdown ratio - and therefore ~8dB more gain.

Or maybe you could have a conventional power amp with an extra gain stage that would allow you to get 30dB or whatever. Like an integrated amp with a phono section.

Or you could use a parallel feed output arrangement and simply switch the output between 2 different output transformers.

Cool project. Let us know how what you end up doing.

jsn

Your tube choice will dictate OPT choice, since everything revolves around that DC current.

DC dictates the lam used, which is the starting point of transformer design.

[quote author="CJ"]Your tube choice will dictate OPT choice, since everything revolves around that DC current.

DC dictates the lam used, which is the starting point of transformer design.[/quote]

So that means when BTB says they designed that transformer for EL84, I better use this, or something similar.

And what about loading with a too high impedance? Or leaving the output open? Will it do harm in that case? (Excuse my ignorance, but I really don't know.)

JH.