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Jeff429 11th January 2021 07:23 PM

What tube amp to build.
Hi, I'm new here.
Been reading site for a few days. Lot's of data to absorb.
I am thinking of building a tube power amp.
What do you think would go well with a VTL 2.5 preamp, has 12AT7 and 12AU7 tubes?
I've never had a tube amp before. The VTL pre was paired with an Acurus A200 driving PSB goldi's. Then went surround with a NAD T187 preamp.
So the VTL has been sitting. This system will be for another room for stereo only.
I have been in aviation avionics repair for 30 years repairing weather radars and autopilots to the component level.
There are so many plans out there, not sure what to get.
thanks, Jeff

Eli Duttman 11th January 2021 07:46 PM

The speakers to be employed govern the characteristics of the amp used with them. Power requirements "dovetail" with sensitivity. The impedance curve impacts on damping factor requirements. What speakers are going to be used in this (apparently) 2nd system?

Schmitz77 12th January 2021 12:50 AM

Eli is right here, first the power consumption is governing the design of the amp.
This could be the starting point for choosing the best design.
High power needed means low efficiency of the speakers.
If thats true, good quality will come at an even higher price.
The best is, to use a high efficiency speaker, without any impedance minima far under the nominal ohm impedance stated by the manufacturer. But those are rare and even more rare, when it comes to good sound quality. Thats why they are so sought after, that their prices have risen to the sky in the cases, when a cult badge is on the speaker.
So if you want to use low efficiency speakers (maybe the ones you already own), go for a push pull design with 50-100W output.
But always bear in mind, the higher the efficiency and quality of the speakers, the better is the achievable result. No 100W push pull design will beat a combination of single ended, maybe 8W amp and a high eff. speaker.
You can make up for the efficiency losses of your speaker, but at the same time you always pay with the achievable quality of the system. Because its technically not achievable to design a good 100W tube amp that has all those phantastic sound merits inside that equal the effortless 2W single ended design, that could play with a small, high efficiency speaker. Even when dealer people tell you the opposite, try to sell their monster and mega buck amps, they are nothing compared to a good and easy to drive speaker in combination with a simple design, low wattage tube amp. But with those, not that high could be the profit of those salesman, so they will tell you the high end is always with the mega bang, mega buck big systems. Go to an audio fair and listen how dead most of those gears sound in combination. Even at the highest levels of price range, only dead horses to ride.
But with small amps, there the live and fun factor begins.
That doesn't say its impossible to design good and heavy power rated tube amps. But its not that easy and with the higher output, there are less and less amp designs that manage to play with ease on heavy and low impedance speakers.
The good designs always have a cult following, because they are all time classics.
Nobody managed, until today, to beat those designs. And in most cases, its even nearly impossible to build them to the original standards, because so much has changed, but often to the bad side in tube electronics.
Try to recreate a Klangfilm tube amp. You only can fail, because the transformers and many other parts are unobtanium today. And thats, why people pay high prices for originals.
So how to find a good design amp for a good match to your speakers?
If you know what league your speakers are playing in, how watt hungry they are, you know the class of amps you are wanting.
In that power class, go out and look for the icon amps, the all time best, because that is what you will build. Maybe a bit less good, but go for the best designs to learn from them.
Never go for second best, because often this is only mediocre sounding combinations.
And your build project often isn't able to catch up with the originals, so its one step behind them.
So if you managed to find some of that iconic designs from the famous companies, you need a circuit diagramm, or you can buy a complete set of parts right on the internet.
That could be a start. Otherwise, you have to look at all parts for yourself, manage to build a housing and put them together.
But first, look for three designs that are excellent and famous, and then choose which one is to build the easy way.

audiowize 12th January 2021 01:02 AM

I would start with something incredibly simple and very inexpensive so you can mess around without feeling a ton of pressure or scrapping expensive parts. Something like a triode strapped EL34 single ended amp should be buildable on a cake pan with $200 in parts and will give you an inexpensive place to start.

Schmitz77 12th January 2021 01:11 AM

Before even knowing the speakers, no further recommendations could be given seriously.
It all will be speculation.
Not every speaker will be satisfied with an EL34 triode wired. Why not using an EL34 the way it was designed for? There are hundreds, or thousands of tubes that are triodes, why change one of the best ever designed pentodes to be used as a pseudo- triode?
Use a triode instead, thats the way they were designed for.

It takes two parts more to use an EL34 as pentode- too complicated of a design?
Every schoolbook says how to do it.
If the speakers manage to run with low power, there are no $200 in parts necessary.
Use two PCL82 and build an amp for less than $50. Thats enough. Try to better this design with a 100W monster is like riding a dead horse, and I don't mean the PCL amp.

audiowize 12th January 2021 01:42 AM

The triode strapped EL34 takes pressure off the output transformer for stability. It also kicks out all those pentode and UL designs out there that don't have any feedback at all, so a second bullet is dodged.

Sure, a PCL/ECL82 amp would also work.

Schmitz77 12th January 2021 01:48 AM

My Klangfilm KLV502 uses EL34 push pull and sounded much different to any triode amp.
Technically, this isn't an instable design which needed to be triodised for "taking pressure out of the output transformer". It is simply designed to work as a pentode amp, and the output transformer is specially designed to work flawless with those tubes.

We can discuss about which sound is to be preferred, but with the start of a certain level of output power, pentodes and push pull designs are a first choice.

audiowize 12th January 2021 01:54 AM

A push-pull EL34 amp would indeed not need to be triode strapped for the sake of stability, but I had suggested a single ended amp to keep things really simple.

wg_ski 12th January 2021 02:06 AM

*I* wouldn’t be satisfied with a triode strapped EL 34 (or even KT88) either, even if using horn loaded speakers. I want it louder than that. But it is a good first build to learn to use tubes.
EL 34’s can always be used to make bigger amplifiers later.
SE triode strapped requires no feedback to get useable distortion and damping. Feedback either requires a lot of luck or at least some advanced math. Build triode strapped w/o NFB with care and it won’t oscillate, regardless of how much control theory you know or don’t know.
EL 34’s are cheap, good power triodes not so much.

Schmitz77 12th January 2021 02:49 AM

If I had to tell the truth, it can take dozens of years to study tube amp design and find out whats good and whats crap designs. Hundreds of crap designs on one excellent one on the internet.
If one has a job, a family, a house and car to maintain, I would take a kit of any good tube amp with enough power out and build it.
To design, buy parts and build takes a whole lot of time and effort. And in the end it mostly don't pay off for just one amp. Thats why there are professionals who have done that. And some of them in a way and fashion, that isn't copyable today.

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