Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

RE: Answer from the peanut gallery

Posted by tube wrangler (M) on August 12, 2012 at 16:18:43

In Reply to: Answer from the peanut gallery posted by Ray Moth on August 11, 2012 at 17:09:23:

Lew, your question is fully relevant.

Ray-- your post is outstanding-- it is 100% on the money.

As Ray relates, G2 has enormous current demand swings. The Mosfet approach has been used by several of the better P/P amp builders over the last several decades-- they used it to compensate-- partially-- for power supplies that are inadequate..

I have an old McIntosh MI-200 Industrial amp--a pair of them.

At one time, I had quite a few of them, but sold-off all the prettier ones, and kept a beat-up pair. This sported the usual "MAC" front-end-- a 12AX7 and a 12AU7-- too much gain, and too little fidelity.

The rest of the amp was really cool! The output stage was a pair of 8005 DHT's. These were operated in Class AB-Class B, depending on the output level. Standing idle current per 8005 was set at 60 ma., each, and plate supply voltage was 1000 VDC. This was from 4 5U4 tubes, operated in a Full-Wave-Bridge. Max. output was rated at a continuous 200 watts with a very wide bandwidth-- up to 200 KHZ. Peak output was over 480 watts.

WHY? It was designed as a Shaker-Table amp for Gold Mining operations. You used a signal-generator input-- you could thus select any frequency fron 2 HZ on up-- and run the transducer that shook the separation table at whatever vibration turned out the most Gold..

Usually, these had 2 ohm, 4 ohm, 8 ohm, 16 ohm, 32 ohm, and 500 ohm outputs.

Movie Theatre guys-- especially in Canada-- as in Calgary, Alberta-- had glommed onto these things to run Drive-in Theatres, and also to run indoor theatres which seated 1500 or more people.

I liked the thing, so I fixed-up the power supplies with some nice PhotoFlash caps instead of the usual soggy-sounding electrolytics. That alone really perked the thing-- it fixed a lot of output stage problems right off.

First thing I did was eliminate the input 12AX7 and 12AU7 stages-- GONE! I got rid of all NFB.

The input stage I made out of a 6BZ7 twin triode. I used a Mosfet, like Ray does-- in this case as a CCS on the common-cathode. I had connected the two cathodes together, and ran one grid grounded, the other "hot"-- that was the amp input.

The Mosfet CCS made the thing come alive more-- it was a really good amp by then.

Then, I took the Mosfet out, and VERY CAREFULLY adjusted the common-cathode resistor until I got exactly equal plate currents in both sections-- at idle.

Well, that took care of the remaining nasties-- small-signal humps and bumps due to the CCS-- which was GOOD, but now things were even better.

I still have it-- buried somewhere with enough tubes to run it for decades.

The output transformer in this thing is one of the best ever built, and it sounds like it! It sounds almost like a SET-- it's transparent, and fast-- it has dynamics and it has a very wide bandwidth.

I judge it by far THE BEST amp out there-- until-- the little 2A3 SET-- which bears zero resemblance to any other amp-- especially "usual" SETs--which are usually quite dead dynamically, and sport severely deficient bandwidth and speed. Most decent SETs do have a sort of transparency, and a certain "rightness" to the rhythm, Pace, and Timing of music, but I find them boring dynamically.

The SET I use has overcome all of these problems, and is dead-reliable and its performance is always repeatable. It TRASHES the Big MAC in every listenable musical parameter. It is FAR more dynamic than any other amplifier, period. Another unusual thing about it is that this terrific sense of unlimited power is present right down into-- and through-- the lowest Octaves, yet they are blazing fast and super-lively. Also, the entire range, including the bottom octaves has all the colors and dynamics that are present in the recording being played.

That's why I have the MAC buried somewhere---.

The MAC Finals Supply was choke-input. There were 4 separate supplies--the other 3 were the usual Center-Tap arrangements... I recall one might have been choke-input, the others CLC. 5Y3's were in abundance.

The finals were driven by a T.V. Horizontal-amp tube-- for all practical purposes, consider it a KT-88.

A great feature of that MAC amp was its output transformer-- which had windings that loaded all the driving stages. That is why it sounds close to a good SET-- all the time-delays caused by having too many stages had been compensated-for--- timing-wise-- by designing the output transformer windings with the needed reverse-delays. It all came out just like a single stage!

MAC made this thing in the 50's-- it is BY FAR their best-ever amp, but is also a 4-chassis monster and a great Room-Heater. Two channels of it occupy 4 chassis-- with umbilicals-- total weight is just under 200 lbs.

As busy as this thing is-- it is quite reliable. It can run for 2 or 3 years more or less continuously, and can be switched on-and-off at will with no tube problems and without tube servicing-- something few enjoy today.

I think it is about 300% better than any SET today-- except for the one that was actually designed right. That SET resembles no other amplifier.

A lot of discussion has ranged over Ripple, Hum and noise.

I realize that one can obtain very low Ripple, and a very quiet background by using lots of inductance, High-DCR, and large capacitors in a Power Supply.

This can be a matter of preference. If you're after just TONE-- you can get it right with any kind of power supply, and you may wish a "quiet" background. If you want a Female voice to sound like the Lady-- you can get that also.

What you don't have is DYNAMICS-- all kinds, Micro and Macro.

But, you CAN compensate, and GET BACK the MACRO-- by using an amplifier that has a lot of watt-power. To get it that way, you must utilize an inefficient speaker that will USE all that power. A lot of it will go to waste heat-- but there will still be plenty enough to Hard-Drive the speaker.

While this can be impressive, it is not real dynamic life-- as in REAL life! It's just a Power-Driven approximation-- a false POWER PRODUCED dynamic-- it's not what the musician actually did.

The other way is to decide what is an ideal power supply. That would have NO inductance, NO capacitors, and ZERO DCR.

It would also have NO FORCED way of providing Low-Impedance-- that is-- by regulator. Instead, it really needs NATURAL occurrence of the same parameters. Those should be designed-in, not forced into compliance after the fact.

The difference in sound quality is simply staggering if you haven't heard it happen.

Low Ripple, Low hum, and Low Noise are all things that we all want.

The problem arises when we seek to obtain those admirable characteristics by wrecking the amp's performance. We're fixing one thing by CAUSING other problems.

An interesting case is HUM. Long ago, while building movie theatres for special theatre owners who wanted cost-no-object performance (yes, that is rare), it was discovered that a small amount of A.C. Hum acted as a signal carrier-- that is, the HUM keeps everything in a system-- all the parts, all the wiring, all the solder joints, all the transducers IN A READY STATE. It is a signal carrier of the best possible kind.

Since the entire audio system is ALREADY IN MOTION-- when a small cue that is either alone, or is buried in signal, occurs-- it is simply and effortlessly carried by the entire system. Nothing has to "stop and start"-- as it has to do with a system that has a totally "black" background.

People get fooled by this. In a "quiet" system, NOTHING happens UNTIL a threshold of signal has built up-- and THEN-- it lets go. Many mistake this phenomenon as "fast", and say that the amp has "speed". Actually, it can be VERY SLOW and STILL APPEAR to be fast because once the threshold is hit, you do get sudden energy release.

Trouble is, you just missed MOST of the music! Ir simply isn't there!

So, what to do about HUM? Simple: tolerate as much as possible, but don't overdo it. The sound quality difference will amaze you!

---Dennis---

Edits: 08/12/12

This post is made possible by the generous support of people like you and our sponsors:

The Cable Cooker

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---