OT: Show us your DIY tube amps

[alancohen]

An interesting writeup on the cathode follower circuit:

Cathode-follower power amplifiers

Some experpts:

Cathode-follower power amplifiers
Cathode-follower-based output stages are rare—surprisingly rare. A conservative guess would be that 99.9% of tube power amplifiers use the grounded-cathode amplifier configuration, with the load attaching at the plate, not the cathode; the output tubes working as amplifiers, not buffers. Yet the cathode follower produces far less distortion and a much lower output impedance than the comparable grounded-cathode amplifier does when working into the same load. Furthermore, the cathode follower is not straddled with the Miller-effect capacitance that the grounded-cathode amplifier is burdened by. So, the big question is, Why isn’t the cathode-follower configuration used more often?...

...As for output impedance, there isn't much debate. The cathode follower's output impedance equals:
Zo = rp / (mu + 1)

whereas the grounded-cathode amplifier's output impedance equals:
Zo = rp

Thus, the higher the mu and the lower the rp, the lower the output impedance. This means that a 300B-based SE amplifier would sport an output impedance of only 0.4 ohm, not the usual 2 ohms, with a KT88, with its higher mu, even greater reductions in output impedance can be expected.

It seems to me that if you can design a workable CF circuit, it would be the cat's meow. It's interesting why more amp designers have not taken on the challenge. I have been playing mine for 6 months and it's rock solid and very sweet. Bass response absolutely kicks for a 3W steady state amp. That's why I'm building another one.

Transcendent Sound Cathode Follower Tube Amp

No affiliation, just spreading the good word on an ideal DIY Altec power amp.

[Alien_Shore]

It is interesting that we don't see more of these designs. Is the only real drawback to CF the need for high-current tubes?

[gortnipper]

Strong language and strong opinions.

Yours? You seem awfully sure of yourself.

How long have you been building tube amps, and how many electronics businesses have you founded, and then sold for a profit?

I'm sure Dennis could care less what you or anyone thinks, but you're putting me in the position of defending him, so I feel obligated to respond.

Jeff is well known for his strong opinions, and for his strong defense of his own friend - another Dennis.

But, IMHO this is a big can of worms best left unopened on an Altec forum - there is plenty more of where this thread is likely to go on the Asylum...

[alancohen]

But, IMHO this is a big can of worms best left unopened on an Altec forum - there is plenty more of where this thread is likely to go on the Asylum...

Agreed. This was started as a "Show us your DIY tube amp" thread and should probably stay that way.

[analog addict]

Jeff is well known for his strong opinions, and for his strong defense of his own friend - another Dennis.

But, IMHO this is a big can of worms best left unopened on an Altec forum - there is plenty more of where this thread is likely to go on the Asylum...

Agreed. However, I didn't start with slagging off on other opinions. There are a lot of ways of doing things, and all methods have their own degree of validity.

And now I remember, this is the fellow who wanted to argue about crossovers as well.....

[mah]

Authority comes from the validity of information.

Assertions alone are not worth much.

[alancohen]

It is interesting that we don't see more of these designs. Is the only real drawback to CF the need for high-current tubes?

In a CF the output tubes provide current amplification, but not voltage. It needs a fairly high gain driver stage. Here's Broskie's thoughts on design:

Cathode-Follower Power

As far as I can gather from Mr. Rosenblit's design, he's connected two drive tubes in series to provide the high gain necessary to supply the output tubes the voltage required.



It's only a 3W amplifier, but due to the ultra low output impedance there is power to spare to feed bass surges. It kicks like a 10W amp. It's still not a match for an 87dB speaker, but for a 102dB Altec it is ideal. The low noise and low distortion is critical with 117dB 288s where you here everything.

[RonSSS]

I can wire better than that. What a crock.
And when I go drag racing, I use 100% throttle.....every time.

Ron

[mah]

Cathode-Follower article. Look under the 1955-59 section for Multivalve Cathode-Followers. Separate links for parts 1 & 2.

The Wireless World Archive

Lots of good info on the Douglas Self site.

[cradeldorf]

I'll try to remember my camera tomorrow and get you a picture of the tube we have here at the college... It's about 18" tall and about 8" in dia.