Tube amp for biamped speaker

[navin]

Hello navin, you're absolutely correct in your argument. ... This is why there's no problem in using a tweeter that is rated at 8W continuous in a 100W system.

The problem is that although the averaged energy is low, those sparse peaks are high voltage, and that's why you need an amplifier capable of swinging the voltage to accomodate them..

thanks. so what you are saying is that the HF peaks are as high as LF peaks but less frequent?

so lets see...

it seems that I will use a 10W EL84 Class A PP for the tweeter (ScanSpeak 9900 XOed at about 2.5k) unless someone has a better suggestion for a Class A PP amp.

I now need to mate that to a good woofer tube. EL34 Class A would give 20W, Class AB would give 40W and I'm told the KT88 class A would give 40W. Which of these 3 makes most sense? Is ther a 4th option I am not aware of. One option is to use the EL34 Amp with a Class A / AB switch so I can choose between high power or high quality mode. Would this be nesscary for the EL84 amp as well.

For both amps I'd lkke to keep DC voltages low, PP mode to keep the OPTs cheap and use them in Class A to reduce crossover distortion.

 

[Giaime]

I just readed the whole thread.

I think that you should start by building the little 10W EL84PP amp, and see (hear) how it performs on the whole system (woofer + tweeter). You may be surprized

It won't be an error because either option you choose, you'll still need that amp, so why don't you start by building it?

Another suggestion: a 2A3 PP amp for the tweeters and a SS amp for the woofers that's REAL biamping

 

[navin]

I think that you should start by building the little 10W EL84PP amp, and see (hear) how it performs on the whole system (woofer + tweeter). You may be surprized

Another suggestion: a 2A3 PP amp for the tweeters and a SS amp for the woofers that's REAL biamping

the reason why i am looking to finalise on a desgin for a bigger power amp is that I will be visiting singapore (from India) in december.

I would be hoping to buy my tubes, sockets and OPTs there.

I have a SS amp that I am currently using. it is derived from the B&K ST140/202 circuit and uses 6 Hitachi MOSFETS per channel.

If the tube amps outperfrom the SS amp I intend to use the SS amp to power the subs in the HT/AV system (seperate room).

 

[EC8010]

So what you are saying is that the HF peaks are as high as LF peaks but less frequent?

That's about the size of it. They're also shorter duration.

 

[navin]

That's about the size of it. They're also shorter duration.

given that some recomend the EL84 for fullrange use it shold be adequate for HF use as well. right?

Now which tube would you prefer KT88, EL34, or another tube? The idea is to use it in Class A PP for 2 Scan Speak 18cm woofers.

 

[EC8010]

EL84 can make a very nice treble amplifier. I assume that the rush on making decisions is that you hope to take advantage of your trip to Singapore to buy stuff more cheaply than if you import it via the post. If that's the case, the most important parts to get are the transformers (and make sure you have plenty of baggage allowance for them). Typically, PP EL84 requires 8-10ka-a. (We don't have to be very precise because the reflected load from a real loudspeaker will vary anyway.) If you put two pairs of EL84 in parallel, you would double the power to 20W and need 5ka-a. 5ka-a is also useable by 2A3 and 300B, so there is a lot of choice available in this value. It might make sense to initially use PPP EL84 for bass - with a view to perhaps changing to 2A3 or 300B later.

 

[navin]

.... I assume that the rush on making decisions is that you hope to take advantage of your trip to Singapore... ...Typically, PP EL84 requires 8-10ka-a. (We don't have to be very precise because the reflected load from a real loudspeaker will vary anyway.) If you put two pairs of EL84 in parallel, you would double the power to 20W and need 5ka-a.... It might make sense to initially use PPP EL84 for bass....

Yes I am hoping to get some stuff from singapore as quality reliable tubes in India are hard if not impossible to find.

I figured I would need tubes, sockets, and OPTs.

Are you suggesting that I use EL84PP for the HF and EL84 PPP for the bass? Can the same OPT be used for EL84 PPP or EL34 PP?

I was given to understand that to ensure that 2 tubes in parallel are biased equally is diffcult so PP is preferable to PPP. Given that we need more average power in the bass I was considering 40W for the bass (presently the speaker is powered by a SS Class AB 200W amp as the room is about 300sq. ft. with a 9 ft ceiling). I was told that the KT88 can delvier 40W class A PP and the EL34 can devlier 40W class AB PP and 20W class A PP right?

I had sort of settled on using the EL34 PP for the bass (class A mode for private listening and Class AB for those occasions where i need a bit more power). Is the EL84 PPP preferable over the EL34 PP?

The one advantage I see that an EL84 PPP amp has over the EL34PP is that it can share the same power supply as the EL84 PP amp. By that I mean I can build a EL84 PPP amp and a EL84 PP amp (1 channel each) onto one chassis with a common power supply and only seperate OPTs. Right?

Q: How much do these OPTs wiegh. I assume each OPT (Lundhal) would weigh about 2kg. Given that I need 4 OPTs I am looking at 8-10 kgs.

Thanks for all the help.

 

[EC8010]

Yes, you might consider PPP for bass and PP for treble.

EL34 PP needs 6.6k, which isn't far from 5k for EL84 PPP, but it's in the wrong direction. You don't lose much power by having a load a little high, but if the load is a little low, distortion tends to go up rather fast.

A lot of people don't like the idea of putting valves in parallel, saying that mismatches cause blurring of the sound. If you think about it, a valve that uses both sides of the cathode to emit to diametrically opposite sides of a cylindrical anode is two valves in parallel. You'll have to decide what you're happy with. Your power estimates are a little optimistic for Class A. KT88: 30W, EL34: 15W. Most "Class A" designs are actually Class AB.

A good output transformer is a heavy beast. I'd expect a decent EL84 (10W) PP transformer to weigh about 1.5kg. A transformer that could transfer twice the power at 20Hz would weigh twice as much. Good luck!

 

[navin]

Yes, you might consider PPP for bass and PP for treble.

EL34 PP needs 6.6k, which isn't far from 5k for EL84 PPP, but it's in the wrong direction. ...

A lot of people don't like the idea of putting valves in parallel,...Your power estimates are a little optimistic for Class A. KT88: 30W, EL34: 15W. Most "Class A" designs are actually Class AB.

A good output transformer is a heavy beast. I'd expect a decent EL84 (10W) PP transformer to weigh about 1.5kg. A transformer that could transfer twice the power at 20Hz would weigh twice as much. Good luck!

Hey you work fast. Thanks for all this help.

what does EL34 needs 6.6k and EL84 needs 5k mean? confused.

Can one build 1 channel PPP and one channel PP Class A amp on one chassis with common power transformer and only seperate OPTs?

So what you are saying is that if I was to use the EL34 in Class A it would only deliver 15W. However a EL84 Class A PPP would deliver 20W? right?

if the 10W EL84 Class A PP transformer is 1.5kg I assume the 20W EL84 Class A PPP transformer would be about 2.5kgs. A set of 4 transformers (2 for PP and 2 for PPP) will weigh about 8kgs. right? I am hoping to afford lundhal as they are said to be smaller and lighter. Hammond are awfull huge. What transformer have you based this weights (1.5kg for Class A 10W EL 84PP).

 

[EC8010]

6.6k and 5k refer to the primary impedance of the output transformer.

In practice, if you measure, you will find that a pair of EL84 in PP delivers the first 5W in Class A, and the remaining 5W is in Class B.

There's no reason at all why you should not build amplifiers on a common chassis and share the power supply. Other than total weight.

Yes, the Lundahls will be lighter because they are C-core. I was considering EI core.

 

[navin]

so really my choices are:

option 1.
EL 84 PPP Class A + EL 84 Class A PP

option 2.
EL 34 Class A PP + EL 84 Class A PP

I was talkng to my brother in law who lives in Sing and he told me to avoid the KT88 as it produces a lot of heat. This would be a big factor in India where (a) the ambient temps are high and (b) in the case my install, since the amps will be partly hidden ventillation will be limited.

he also told that he can get custom OPTs and power trannies made for me so i should not worry about taps etc. I have asked him to look at this thread as well.

so far this is my design:
a) option 1 above.
b) all EL84s in Class A mode triode (6 per amp - 4 in PPP and 2 in PP)
seperate driver tubes for each output stage (PPP and PP) ECC 81, 83?
c) small C core OPTs, common power transformer for all driver tubes
d) 12 db crossover (can this be implemented using just 1 tube) for HF and LF that can be varied by 1 octave (1.5-3kHZ for LP and 2-4kHz for HP).

any suggestions, modifications?

now i got to find circuits for this.

BTW i will also need a preamp that can drive these power amps via 15 meters of cable.

 

[EC8010]

b) all EL84s in Class A mode triode (6 per amp - 4 in PPP and 2 in PP)
seperate driver tubes for each output stage (PPP and PP) ECC 81, 83?

Did I miss the "triode mode" earlier? You can get 5W out of PP EL84 triode mode, with the first 2W or so in Class A.

Yes, KT88 will produce a lot of heat. So will any other valve that produces an equivalent amount of output power.

If you use triode output valves (especially PPP) you will require more drive than ECC83 can provide. ECC81 might be a possibility.

Having a seperate power supply for the driver to the output stage is a good idea.

12dB/octave can be done with a single triode.

I'll probably be drummed off the forum for suggesting it, but the easiest way to find out exactly what crossover you need is to use a digital crossover such as the Behringer DCX2496 as a test bed. It allows huge flexibility of slopes, frequencies, filter types, equalization, and levels.

 

[navin]

Did I miss the "triode mode" earlier? You can get 5W out of PP EL84 triode mode, with the first 2W or so in Class A.

If you use triode output valves (especially PPP) you will require more drive than ECC83 can provide. ECC81 might be a possibility.

Having a seperate power supply for the driver to the output stage is a good idea.

12dB/octave can be done with a single triode.

I'll probably be drummed off the forum for suggesting it, but the easiest way to find out exactly what crossover you need is to use a digital crossover such as the Behringer DCX2496 as a test bed. It allows huge flexibility of slopes, frequencies, filter types, equalization, and levels.

no i was under the impression that the EL84 Class A PP in triode mode would produce 10W. Sorry. now this leads me to another question (doubly sorry for being such an idiot) - which would you prefer Class A PP pentode (10W) or Class AB PP Triode (also 10W?)


Yes the ECC 83 is for pentode and the ECC81 is for triode.

I am in India I dont have any access to a DCX2496.

I however know that a relatively simple 12db XO (currently I use a similar passive XO) can work with these drivers. as long it has variable turnover points, gain and slope (variable between 9-12db/oct) I would be able to tailor the response to my liking.

 

[EC8010]

I find that triode amplifiers tend to have a sweeter treble. I would probably use UL (43% taps) for bass.

Couldn't you order a DCX2496 over the Internet? The reason I suggest this is that although it is possible to make a valve crossover to a particular slope/frequency etc, it's not easy to make it variable. In fact, it's jolly hard.

 

[navin]

I find that triode amplifiers tend to have a sweeter treble. I would probably use UL (43% taps) for bass.

Couldn't you order a DCX2496 over the Internet? The reason I suggest this is that although it is possible to make a valve crossover to a particular slope/frequency etc, it's not easy to make it variable. In fact, it's jolly hard.

i'm now really charged about this project and full of questions.

Q: Triode Class A EL84PP = 5W, then what does Class AB give us? 10W? Is this configuration preferable to Class A Pentode which also prodces 10W? I assume UL would be halfway between pentode and triode in power delivery.

so what we have finalised is that the HF amp = EL84 PPand the
bass amp = EL84 PPP.

Now we have to determine what Class of operation we need and what mode (Tri, U/L, Pent). Right?

ok so lets include this in our design.

can we tap the HF (PP) EL84 OPT at Triode AND Pentode and similarly we could tap the bass PPP amp in U/L AND Pentode mode via a 2 pole switch will this add too much complexity and/or noise?

When the HF PP amp is in Triode mode (5W) the bass PPP amp can be run in UL mode (15W?).

...and when the HF PP amp is in Pentode mode (10W) the bass PPP amp can be run in Pentode mode as well (20W).

as far as the 2496 is concerned I could import one it would cost $500+ after duties and taxes but what would i do with once i was done? it would an expesive investment for one amp/speaker set.

 

[navin]

EC8010,

I need more advice.

see my post #5 in the link below. am I right in assumig those power ratings?

http://www.diyaudio.com/forums/showthread.php?s=&postid=773951#post773951

thanks.

 

[EC8010]

Navin, I used to be a lecturer and had to remind students to read my handouts carefully and/or to go and read in the library... You can get about 2W out of a PP pair of EL84 triode-strapped in Class A. If you push them into Class AB, you can get 5W.

Please note ray-moth's comments on the other thread.

I'm afraid this project sounds as though it may be a bit beyond your present design capabilities and I don't have the time to hold your hand all the way through. I suggest you find a tried and tested design that looks similar and build it.

 

[navin]

You can get about 2W out of a PP pair of EL84 triode-strapped in Class A. If you push them into Class AB, you can get 5W.

Please note ray-moth's comments on the other thread.

I'm afraid this project sounds as though it may be a bit beyond your present design capabilities..

EC8010 I never expected so much help leave alone handholding. I am gratefull for all the time and knowledge you have so generously volunteered. I hope I did not offend you. If I did I never meant any offense. Sorry.

I never intended to design the amp. I just intended to learn enough to decide which tubes, OPTs and sockets I should get.

Now I have to really decide if should I get 4 EL84 and 4 EL34 or 12 EL84? So this will cover tube and socket selection

Also I have to decide what modes I intend to run the tube. Class A Triode would be nice but if I need more power I should have an alternate (Class AB Triode? or Class A Pentode?). So this will cover OPT selection.

You are right 2 EL84 in PP Class A triode will give me 2W and if pushed will give 5W (Class AB). I assume 2 EL34 or 4 EL84 will deliver about twice that.

 

[EC8010]

No offence taken. I would suggest that you treat this as a learning project and go for all EL84. This will allow you to use a common power supply at a slightly lower voltage (300V vs 430V), making it slightly less dangerous and a little cheaper. If you use grid bias on the output valves, you will be able to experiment with adjusting the bias between Class A and Class AB.

 

[navin]

No offence taken. I would suggest that you treat this as a learning project and go for all EL84. This will allow you to use a common power supply at a slightly lower voltage (300V vs 430V), making it slightly less dangerous and a little cheaper. If you use grid bias on the output valves, you will be able to experiment with adjusting the bias between Class A and Class AB.

fantastic. so what we can do is bias the tubes to Class A and if I need more power bias them to Class AB. Do you know where i could find a suitable schematic. Mye design skills are not adequate to design such an amp on my own.

I am so used to 200W SS power amps that I am nervous about these 2W and 10W tube amps. My brother in law who has more expereince says I should not worry.

In any case it would be nice to know that one can try 2W and 5W and if that dont work increase that to 5W and 10W respectively.

My room is about 300 sq. ft. with a 9 ft. ceiling and the speakers are 88-89db. 1 wall has wooden panels, 1 is a window and 2 are brick. The room is quite live (except for the curtains and our bed).

What SPLs can I expect with Class A triode (2W/5W)?

Again. I do thank you for your time.