Well I finally got my universal P-P driver board wired up which I needed to further develop my 6080 amp project. I've had to put this off for weeks with my speaker work and other things like getting ready for winter in the Great White North. :( Man did this thing get busy or what!?

Fully populated this is a dual mono 6AN8A line pre-amp/phase splitter, 6SN7 P-P voltage gain stage (output triodes need more grid swing) and provision to add two more tubes to drive SET when I go on to develop one of those. DC filament supply on the 6AN8A and separate regulated B+ supplies to each channel. Choke input B+ filter. Fully adjustable NFB controls. Quality circuit.

I was listening to that magic triode sound from six, 6080's P-P-P last night late...er make that this morning. The mids and highs sounded amazing, like a SET amp. Now I'm getting there!

Rob

This is to reserve space. I'll post a picture of the tube side shortly.

You will notice that I have doubled up quite a few 1% resistors (those blue things). This could be because precision Rob needs 1/2% accuracy or it just ain't good enough. It could also be that I like to make my circuits more reliable by including live back-up components so a failure won't shut the device down.... or it could also just be that I need to go to the parts store to replenish my resistor stock. I'll let you guess which. ;)

Rob

OK, so I didn't have to reserve space. :) The layout follows good logical short signal path and keeps power supply away from the low noise input circuits which are at the top. You cannot get the volume controls any closer to the BNC inputs [real lab grade amplifiers use only BNC connectors, then you get adaptors so that consumer grade interconnects can hook to them ;) ].

Rob

O - M - G. That's neat.

What are your thought on the Lundahl Armada?

SK,

Didn't that armada loose its vacuum and sink off the coast of Spain in the late 1700's? Sorry, you will have to be more specific. I am not familiar with that name. Must do a google.

Rob

Rob,

Here you go.

http://www.meta-gizmo.com/Tri/4w/Armada.html

http://www.meta-gizmo.com/Tri/4w/magnetics.html

http://www.lundahl.se/armada.html

It can use a DH driver, DH rectifier, and DH output tube. It's DH SET on steroids. In the Listener column, they outlined a plan for some pretty elaborate multiple seperate power supplys. What is your opinion of this design? The parts alone seem very expensive.

SK,

Thanx for posting those links. That looks like an entertaining read. I'll have to go study this and get back to you. :)

Rob

As always - YOU DA MAN! :ntwrthy:

Originally posted by sasaki kojiro
Rob,

Here you go.

http://www.meta-gizmo.com/Tri/4w/Armada.html

http://www.meta-gizmo.com/Tri/4w/magnetics.html

http://www.lundahl.se/armada.html

It can use a DH driver, DH rectifier, and DH output tube. It's DH SET on steroids. In the Listener column, they outlined a plan for some pretty elaborate multiple seperate power supplys. What is your opinion of this design? The parts alone seem very expensive.

SK,

I went through the 3 pages. No schematics were shown but I followed and basically agree with the authors technical assertions. I agree that it takes a power amplifier to drive a power amplifier (if your output amp is class A2 or AB2, he cleverly forgot to mention the class). Banks of push pull pentodes in class AB1 are as perfectly happy being fed with a small driver tube as they draw no grid current here in North America as they would be in Japan. While it is true that one can yield higher gain in an amplifier stage with more efficiency using an inductor as a plate load rather than a resistor there are bandwidth issues to contend with. Inductors are inherently narrower band than a resistor. The audio band is at the bottom of the frequency scale starting at just 20 Hz and you only have to go 20 Hz up before you have doubled frequency and already you are into the next octave. By the time you climb to the top of the audio band you have covered some 10 octaves. This is a large spectrum for electronic devices to cope with. At higher frequencies like in the microwave communications bands, your C-band satellite receiver for example, it has to handle a frequency range 500 MHz (500 million hertz) wide, but it is doing this from 3700 MHz (or 3.7 GHz) to 4200 MHz (or 4.2 GHz). The ~12% percentage bandwidth change in frequency is small, so the circuits can handle this huge bandwidth with relative ease. Going from 20 Hz to 20 KHz is a different kettle of fish, representing over 100% in percentage of bandwidth change. He points out quite correctly that to make inductors like audio transformers cover such wide bandwidths well is a real challenge. He alludes at the end that for himself he plans to try to split the audio spectrum and do it in two sections with his inductor scheme to better optimize amplifier performance within the portion of the band, a smaller percentage bandwidth space, that he will cover with say two 5 octave amplifiers summed together. These indeed are sound technical concepts.

As you point out, this quality iron is expensive so it isn't an easy choice for an amplifier builder to make. You want to hear something funny? If you read the diatribe you learned that resistors are evil right? You can make an inductor work more smoothly over a wider bandwidth by placing a cheap resistor in parallel with it to de-Q it. That is also a trick that might be used to some success in making cheaper inductors sound better. There, you have a secret trick. I'm sure we could get rid of a lot of unnecessary noise if we could cover the surface of the entire earth with a 100 foot thick layer of conductive carbon...but I digress. ;)

I think what these guys are doing could indeed make a very good sounding amplifier. I also agree that it is likely that just replacing key resistors with appropriate inductors in commercial amplifier designs as discussed, one could see a sonic improvement, at least in a portion of the band that the ear and brain is most sensitive to. I think this may be part of the midrange magic associated with SET amps. Triodes and iron.

I do not understand how he dislikes quiet metal film resistors to noisy, old technology carbon resistors in the grid of the first amplification stage though. What also don't understand is how the tubes have to be directly heated cathode jobs. I see no reason why indirectly heated cathode tubes which were introduced somewhat later along the historical thermionic learning curve should not be the tubes of choice rather than the ancient, crude designs from the earliest days of wireless. Nevertheless, my ears have witnessed the mighty 300B in action as an output device and were suitably impressed.

My recommendation? Not sure. :dunno: Technology and compromises create miriad options. There are as many choices for amplifier design these days as there are cars to buy. You have to look at the kind of driving you do, if you just use it for pleasure or to haul stuff. You have to look at your budget, etc. There are low cost ways to get new tube sound at very low power. There is a P-P kit amp using a pair of ECC82s's I think (the combo small signal triode and Beam power pentode in one 9 pin envelope) that sells for about $150 for a monoblock. Great value can be had buying vintage classic tube power amps on ebay. Dynaco's are terrific BTW IMO as starter amps. Get an ST-70 or a pair of MK-II, or MK-III monoblocks, but none of these are SETs. A large part of the reason these old Dynaco's sound so good is that they have high quality output xfmers. As a casual amplifier designer among my many technical interets, I'm facing the same miriad options and choices of circuit topologies right now trying to get a handle on what I feel will best make a top notch contribution to what is available, with price consideration making my decisions more difficult.

How much $$$ is this guys new SET kit anyhow?

Hmmm. Maybe I should design a low cost SET product....but the iron is so expensive. :(

Rob

Wards,

You humble me. That is a cute smiley I hadn't seen before. Your patience with me may pay off yet my friend. I'm glad you're not in a big hurry.

Rob

I don't understand most of the stuff you say, but it's still entertaining reading :D

DOT6: Nobu Shishido (Tokyo) suggested that to get maximum beauty from a DHT it takes (literally) an amplifier to drive the grid of the output tube.

I thought that was an interesting statement.

Anyway, I'm not that technically competent so I might be able to understand things more clearly as time goes by.

What also don't understand is how the tubes have to be directly heated cathode jobs. I see no reason why indirectly heated cathode tubes which were introduced somewhat later along the historical thermionic learning curve should not be the tubes of choice rather than the ancient, crude designs from the earliest days of wireless. Nevertheless, my ears have witnessed the mighty 300B in action as an output device and were suitably impressed.

Are you refering to DH SET tubes and amps? Supposedly much of the sound signiture of DH SETs is in the simplicity of the curcuitry. I think one difference is that the music signal is not split in two and it does not use negative feedback. Also, there is a minimal (or direct) connection between the anode of the driver tube and the grid of the output tube.

Sorry, I didn't mean to hijack your thread, but I'm definitely becoming more interested in these technical details.

Originally posted by sasaki kojiro


I thought that was an interesting statement.


-----------------------
SK, Hmmmm, too bad the quotes do not appear. I covered that. What was meant was that in Class A2 where the grid of the SET output tube is driven positive and starts to draw grid current it takes a really stiff driver stage to power it. That means you need a power amplifier as a driver, not just a voltage amplifier. This can be accomplished with a cathode follower driver (no interstage transformer) or plate coupling a driver tube to the output tube with a step-down interstage transformer. In both cases the driver tube must be beefy. Rob
----------------------

Anyway, I'm not that technically competent so I might be able to understand things more clearly as time goes by.



Are you refering to DH SET tubes and amps? Supposedly much of the sound signiture of DH SETs is in the simplicity of the curcuitry. I think one difference is that the music signal is not split in two and it does not use negative feedback. Also, there is a minimal (or direct) connection between the anode of the driver tube and the grid of the output tube.

-------------------------
SK, Yes I was mostly discussing SETs, except in my recommendations about buying classic tube amps like Dynaco.

I think a lot of people get confused and think it is the very same electrons from the source that go through all this circuitry and pop out the back of the amplifier with added balls to drive the speaker. Not true. If it were the point of a minimum component amp would make absolute sense. One thing is for sure, in the case of a power supply for a vac tube amp the one that allows more components will potentially result in a better amp...if...the extra components are used for intelligent voltage regulation. The output of an amplifier is only as pure as it's power supply.

It is true that negative feedback changes the sonic charateristic of an amplifier. NFB is a neat trick that is often employed to make a bad amplifier design work. Designing an amp that works well without NFB is harder but results in a better amp when just a minimal amount of NFB is employed. Sometimes the well designed amp has a very pleasant sonic signature without any NFB.

I commented that I didn't see why appropriately chosen indirectly heated more modern tubes shouldn't be as good as the directly heated (DH) ancient designs. Rob
--------------------------------

Sorry, I didn't mean to hijack your thread, but I'm definitely becoming more interested in these technical details.