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Old 28th February 2009, 03:29 PM   #1
Marko25 is offline Marko25  United Kingdom
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Default The daunting world of tubes (where to start)

Hello everyone,

I have a previous post in the solid state section re: Class A and someone directed me to tubes so I'm hoping someone may help.

Firstly I'll tell you what I am trying to achieve; I play vinyl and last year purchased some AKG701 headphones for private listening, it was whilst playing a mobile fidelity recording that I noticed the ghosting on the opening track i.e. the track starts before it actually starts. I cannot reproduce this through the speakers however despite any volume setting. This brought me to the conclusion that my integrated amp was losing a lot of detail in the system and as my headphones plug into my amp the equipment prior to this was obviously able to extract this information.

So now my search begins for a truly transparent amplifier that allows every ounce of detail to flow through.

My questions are as follows;

1. Am I on an unfulfillable quest?
2. Can anyone suggest any kits or amp designs that would help me get to where I want to go.

My speakers are the Wilson Benesch Arcs, 88db on axis 2.83V, 200W max unclipped, 6 ohms nominal 4ohms minimum, 111db max SPL.

Many thanks in anticipation,

Mark.
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Old 28th February 2009, 07:34 PM   #2
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Mark,

Let's do a little math, 1st. It's necessary to treat your speakers as being 4 Ohms, when combined with tubed power amplification. 2.83 V. into 4 Ohms is 2 W. Therefore, your speakers are 85 dB. efficient. Paul Joppa has provided us with a useful rule of thumb. Joppa's Rule states that an amp/speaker combo be capable of 102 dB. SPL peaks, in an "average" sized listening space. That means you need at least 60 WPC.

It takes "coin of the realm" in considerable quantity to build or buy tubed power amplification of the sort being discussed. Are your pockets DEEP?

Frequently, the practical solution for speakers like yours is tubed preamplification combined with "top notch" SS power amplfiication.
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Old 28th February 2009, 08:16 PM   #3
Marko25 is offline Marko25  United Kingdom
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Quote:
It's necessary to treat your speakers as being 4 Ohms, when combined with tubed power amplification. 2.83 V. into 4 Ohms is 2 W. Therefore, your speakers are 85 dB. efficient. Paul Joppa has provided us with a useful rule of thumb. Joppa's Rule states that an amp/speaker combo be capable of 102 dB. SPL peaks, in an "average" sized listening space. That means you need at least 60 WPC.
Thats interesting, how did you come to that calculation?

Quote:
It takes "coin of the realm" in considerable quantity to build or buy tubed power amplification of the sort being discussed. Are your pockets DEEP?
It depends, if its the choice of spending say 3000 - 5000 on a commercial amp or building something far superior, I would opt for the build but only if it was right so I would build a sample amp first i.e. a down scaled version.

Quote:
Frequently, the practical solution for speakers like yours is tubed preamplification combined with "top notch" SS power amplfiication.
Yes, I have a post on the SS forum re the Class A approach and have considered a tubed pre-amp separate but was steered in this direct by some fellow forum members in order to help me achieve my desired goal.

It starts to feel as if I'm going in circles, there are so many options
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Old 28th February 2009, 08:39 PM   #4
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You could do a push-pull (PP) 6550 or KT88 amp to get the kind of power that you need. It won't be class A or anything single-ended if you need the watts (at least with a single pair of tubes per channel), but may not be unreasonable for a beginner tube project, depending on your experience and comfort level working with high voltages. Power out is dependent on your B+ voltage (among other things), so that is something to think about.

Those tubes in PP can get 60W or so in pentode config and somewhat less in ultralinear mode (Eli or one of the other experts can be more specific on this)



Search this forum for 6550 or KT88 ultralinear designs or pentode designs to get an idea.

I would imagine that one could build an amp based on the above tubes for sub-$1000 fairly easily, although putting $$ into the output transformers is usually well spent.
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Old 28th February 2009, 10:08 PM   #5
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Mark,

At the risk of boring your socks off, please be aware that high voltages are involved in tube amps. If you're not experienced in dealing with this hazard, you should acquaint yourself with the facts and precautions before getting involved. This thread contains some warning of what to expect.

Eli's response may not seem vary encouraging but he's only telling it like it is. With tube gear it's normal to see speakers of higher efficiency. 85dB is low and, as Eli rightly says, will require an amp of high power (for tubes) to be able to drive it with ample headroom for transients. 60w for an SS amp is no big deal, but for a tube amp it is, in terms of both cost and technical challenge.

A Class A specification makes things more difficult still, because it means low efficiency and high quiescent current, that is, the OP stage is running flat out all the time, even with no music paying, and you get out relatively little power. A 60w Class A amp is a monster IMHO!

You also need to be aware that such a tube amp will be big, will give off lots of heat and will need protection to prevent it from being touched by people, especially children, who are unaware of the dangers.

In conclusion, I would echo Eli's advice that you either invest in high efficiency speakers that are more tube-compatible, or you try to find a better SS solution for the speakers you have.
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Old 28th February 2009, 10:13 PM   #6
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Mark,

Boywonder's remarks about Class "AB" KT88s are spot on. As the owner of a Harman/Kardon Citation II, I can state, without reservation, that PP KT88 amps are FINE, when done properly.

To give you some idea of the costs you will encounter, a UK winder of magnetics, Sowter, offers the suitable model U070 O/P trafo. That "iron" costs 180.42 UKP plus VAT each.

Practical considerations, especially safety, dictate that you build some sort of preamp prior to attempting a near 500 V. B+ power amp. Some commercial receivers and integrated amps have a preamp-out/amp-in feature. Should the equipment you currently possess have said feature, DIYing a "hollow state" line stage to work with it would (IMO) be a very good idea.

As for how I arrived at 60 WPC, that's easy. Each 3 dB. increase requires a doubling of power. If 1 W. yields 85 dB., 64 W. yields 103 dB.
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Old 1st March 2009, 08:27 AM   #7
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There's something not right here:
My earlier reference books (and there are alot of them about) quotes o/p stages with power 10-15W is suitable for the average living room; large rooms and small halls 15W upwards. I think Joppa's rule is based on MI and not hi fi. Please correct ?
To me the size of the average living room in an European house has actually shrunk half the size of a 1950's grandee.

My little 25W amp has considerable prowess when compared to the 150W bemoth, the latter requiring a serious approach to dedicated bench work and I question whether the time and expense is worth it.

richy
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Old 1st March 2009, 01:01 PM   #8
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Quote:
I think Joppa's rule is based on MI and not hi fi. Please correct ?

It's HIFI. Pairing 2A3 SET amps with high 90s horn speakers is a good example.

Remember, we're discussing a Rule, not a Law. PJ has posted remarks about listening space size and musical tastes altering the calculus. You can discuss the matter further, with Mr. Joppa, here.
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Old 1st March 2009, 01:06 PM   #9
Marko25 is offline Marko25  United Kingdom
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Thanks everyone for your responses, albeit not necessarily what I wanted to hear.

Quote:
Search this forum for 6550 or KT88 ultralinear designs or pentode designs to get an idea.
Yes I think I will thanks.

Quote:
Practical considerations, especially safety, dictate that you build some sort of preamp prior to attempting a near 500 V. B+ power amp. ....... DIYing a "hollow state" line stage to work with it would (IMO) be a very good idea
I think the pre-amp idea may be the way to go for me, at least to start with so I can get a feel for tubes.

Quote:
please be aware that high voltages are involved in tube amps
Thanks for the warning, no worries about that, I am very aware and have worked with mains and live unfused mains before although not power supplies like transformers. After being electrocuted a couple of times, I am quite cautious now
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Old 1st March 2009, 01:57 PM   #10
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Quote:
Originally posted by Eli Duttman


As for how I arrived at 60 WPC, that's easy. Each 3 dB. increase requires a doubling of power. If 1 W. yields 85 dB., 64 W. yields 103 dB.
...subtract 6dB for listening in stereo, and since a majority of power is consumed in the bass region, subtract a little more for room gain, given the bass is being reproduced by a direct radiator.
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