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In Reply to: RE:Pros and cons of your 211 amp? posted by DAK on December 05, 2024 at 06:48:49
You'll never experience the joy of a 211 using Chinese parts (especially output transformers). Here's more food for thought:
The dirty secret of SE amps is that most of them produce much less power in the first two octaves than what they're capable of at upper bass and midrange. This is usually hidden by publishing specifications showing only low-power frequency response and a maximum output number taken at 1kHz or so.
For example, my A2 211 monos are capable of 25-30W (depending on bias) at 1kHz. However, that falls to 15W at 40Hz and only 6.5W at 20Hz without feedback. Adding 5dB of feedback pushes the 20Hz number to 10W. This is more than sufficient for my speaker system (which I used for years with a much smaller 6V6 SEP amp), and the musical presentation is superb. Nevertheless it highlights the typical results when using a series-fed SE output transformer. I'm using Peerless/Magnequest FS-100 specified for 30W, and I can tell you they are large and heavy. If these transformers were used with a smaller tube, such as a 2A3 or 300B, the natural power limitations of the output tube itself at midrange would render the LF rolloff less prominent. Of course, no one uses 30W transformers for an 8W output tube. Transformers capable of flat response with a 211 or 845 from 20Hz to 20kHz would be the size of cinder blocks.
My point is that you should evaluate your speaker system, listening environment and budget carefully before jumping into the large SET arena. I will probably never build another SE amp unless it's very small and designed for a specific purpose (such as a headphone amp). OTOH, I believe Class A push-pull resolves these issues, so that's a direction I want to explore in the future.
Follow Ups:
As a full range amplifier, the SET is just too weak on the bass. Only been happy coupling them with SS to make full range.
This amp I finished a month ago PP Triode KT88 fits the bill for full range.
Congrats, that's very nice, and I think it's an excellent topology. Just to be clear regarding my earlier comments, a large SET can provide a superb full-range experience if it's matched to the right speakers. My speakers are only about 95dB, but they're flat to nearly 20Hz. My main amplifier for many years was a 4W/ch SEP, and that combination performed remarkably well. In comparison, the 211 amps allow higher listening levels (although rarely needed) and a certain sonic magic only large SETs can create. I'll probably build something Class A PP in the future, but for now I've got all the hi-fi I can use. :)
I've been refurbishing an Electro Voice A-20 amplifier, which uses a pair of 6V6s in class B and makes 20-25 Watts. It has a unique driver circuit which keeps the output tubes operating at the right point (so no bias or balance controls). The output transformer is conventional but its in the cathode circuit of the output tubes and the output section is wired as a Circlotron. The Circlotron operation prevents the usual spike that would otherwise occur when the tubes go into cutoff, so no distortion at low power levels. It has excellent bandwidth.
In addition there is a feedback control which is a 2-deck device, allowing you to balance between full Voltage feedback (so the amp can behave as a Voltage source) or full current feedback, allowing it to act as a current source. For speakers like we see often driven by SETs, you set the damping control to noon, as which point its a power source (similar output impedance to an SET, but with equal amounts of current and Voltage feedback).
BTW if you google the schematic, Electro Voice published it with an error. The screen resistor also serves as the plate resistor for the driver tube. The schematic shows something else.
At any rate, 6V6s are much easier to drive than 6L6s or any of the Kinkless Tetrodes. In proper UL mode or in Circlotron mode as described above this might make for an amp that could suit the bill.
Ralph, I don't suppose you could post a marked up version of what EV actually built, re. the driver/screen grid resistor, could you?
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
The plate resistor of the driver is also the screen resistor of the associated output tube. The coupling cap coming off of the plate of the driver is driving G1 of the power tube.
The schematic shows the driver plate resistor as tied to the screen and the screens then tied to the plate of the opposite power tube. Both versions will work.
In my amp and also in the schematic the driver plate resistor is a 12K 2 Watt device. The schematic as shown was glued to the bottom panel.
Here's a PDF of the user manual that includes EV's description of the circuit and a schematic. It's a PDF, so it can be rotated by most browsers. What I found most interesting about this is the statement that the output transformer is 1/4 the usual impedance.
-With the lower turns ratio the output transformer has wider bandwidth due to lower distributed capacitance. You don't even need it to be a transformer- it can work as an autoformer too (but you have to be more careful about DC Offsets).
The original Circlotron patents employed a bootstrapped driver circuit like you see in the schematic. But I think that is higher distortion and you can get into issues with blocking distortion and preventing thermal runaway of the output tubes. EV ran into a problem with this and so reduced the output tube grid resistor value to 100K and also increased the value of the coupling cap to maintain the timing constant.
Definitely an interesting design!
I went all the way up to the 304TL with the 28 pound 1642SE hammond OPT and 1,200 volts @ 200mA. Bass is better even at low volumes, but not quite like the vice-like-grip of a PP amp. (8 ohm load on the 4 ohm secondary to raise the 5,000 ohm reflected load a bit.) Horn bass is just so physically massive. I am currently using a pair of 18" vented 97db bass speakers.
I do like percussive bass you can feel in your chest, however. I think musical choice changes the preferences.
"8 ohm load on the 4 ohm secondary to raise the 5,000 ohm reflected load..."
That effectively cuts the inductance in half raising the F3 an octave.
I did the same thing with the same OPT for a 211 build but I wasn't using it for bass.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The inductance remains constant and the load doubles which is not critical when the tube being loaded is a triode. If you double the load and keep inductance constant with a triode, not much bad is going to happen. The belief that if you double the load, you must double the inductance stems from the use of a pentode as the tube which has an inherently high Rp making the reflected load work against the inductance to determine LF behavior. When the tube being loaded is a triode, the rP is typically 1/3 or less that of the reflected load so it dominates the impedance the inductance must work against.
I am not saying more inductance is not a good thing. Just that how you determine the required inductance is based on tube Rp for a triode and reflected load for a pentode.
dave
if you double the reflective impedance by up-tapping the OPT you need to double the inductance to keep the same relationship between the reflected impedance and the total load impedance as seen by the output tube.
and I should have left the F3 part out.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
While what you say is technically correct, it glosses over the basic idea that the inductance has more to do with the characteristics of the drive impedance and not the value of the reflected load.
Taken to the extreme by your argument, a plate choke would need infinite inductance to match the reflected load. So would a line out transformer driving the grid of a tube. This opens up the whole large signal vs small signal can of worms which simply confuses the underlying concept. I think we can all accept that for a given tube, the more current that needs to be delivered to the load, the more inductance is needed. The idea that doubling the load requires 2X the inductance puts far too much emphasis in a FOM that isn't universally applicable.
dave
.
Have Fun and Enjoy the Music
"Still Working the Problem"
figure of merit
.
Have Fun and Enjoy the Music
"Still Working the Problem"
"If you double the load and keep inductance constant with a triode, not much bad is going to happen."
It's not just about the tube and the load line. When the reflected load impedance increases, AC voltage across the transformer also goes up. That causes earlier saturation, particularly at low frequencies where core flux is greatest. And on the subject of triode VS pentode, it seems to me that NFB (required in the case of the pentode, but not the triode) levels the field when analyzing the source impedance seen by the transformer.
The load line becomes the shape of a beach ball in the presents of low frequencies and all of the other frequencies are caused to follow that beach ball shaped load line leading to reduced power and increased distortion. That's why I don't use them for bass.At 200Hz the load line becomes that of a resistor, nice and straight keeping the power up and the distortion down.
I drive my JBL 2231A woofers with a SS amplifier.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 12/05/24
Insufficient inductance should present as a loss of amplitude at low frequencies, even at very low power levels. That's not the case for the FS-100 transformers I'm using. They're flat to below 20Hz at levels like 1W. It's only at higher levels where the low frequencies suffer. I believe this indicates transformer saturation.
I agree, that sounds like transformer saturation.
Check this out from VoltSecond.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
This may be the image you are looking for.
My site is back up and I hope to have more new stuff in it fairly soon. I also have a few inductor design articles from friends of mine that I posted on the new site.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
This is what grabbed me the first time I read it years ago.
"It does not matter if your speaker does not go down to 31.25 Hz or not. If you are sending low frequency information to the amplifier, the amplifier will make the plate voltage move. If the plate voltage moves, the tube must spend bias current to charge and discharge the primary inductance as well as the actual primary load impedance. If the plate voltage or current "clips" because of low frequency plate excursions, the sound will suffer."
This changed my outlook and I took steps to keep the low frequencies out of my single ended midrange/tweeter amplifier. The sonic results are very positive.
Thank you.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Excellent method to make the issues go away. Active Bi and Tri amp-ing solves a lot of issues!
What ever is picked for the lowest frequency, to support transients the iron will need to deliver full power at 1/2 that frequency.
Good job with the cross over!
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
The FS-100 transformers are 10K. I've always known a higher reflected load would result in a more favorable load line, but I think it must be very difficult to make a conventional transformer like that without sacrificing upper frequencies. I suppose this is the downside of the 211 and similar tubes, as opposed to those with a lower Rp.
Are the amorphous type output trans any better and worth the money?
Thanks for sharing TK. Very good points to consider. I am using some fairly efficient speakers, Klipsch Forte 2, and Altec 19. While I am impressed by good bass i don't listen to much popular music but a flat response to 30 hz would be nice. Anyway, I have not bought a Chi-fi amp or owned one, but it has been almost 20 years since they have been in the hifi market and I wondered if by now they have gotten their act together? My plan is to get something of a donor amp and put better transformers and upgrade other parts. It would save time and effort on doing a complete from scratch build. Speaking of class A PP, I just acquired an AES AE 25 amp which is PP class A 6550 tubes. It sounds great but I noticed I had some ear fatigue after about an hour. My SE amps don't seem to do that at the same listening levels.
"Speaking of class A PP, I just acquired an AES AE 25 amp which is PP class A 6550 tubes. It sounds great but I noticed I had some ear fatigue after about an hour."
That shouldn't be, and it's not the result of the output stage topology. Could be failing PS caps, incorrect bias, less than stellar transformers, or a number of other things. The first thing to do when this symptom shows up is to measure IMD. High levels of IM sometimes masquerade as "detail" or "nuance," but long-term listening creates fatigue.
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