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Asking for guidance on 6L6 PP build/design selection
Asking for guidance on 6L6 PP build/design selection
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Old 24th November 2020, 07:52 PM   #1
Captain archer is offline Captain archer  Netherlands
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Default Asking for guidance on 6L6 PP build/design selection

Hello to all reading this,

I would like some guidance on the design decisions that follow in my 6l6 amplifier build:

Two years ago I decided that I would like to build two Monoblock 6l6 based amplifiers. At the time (with a certain budget restrain) I ordered components that would allow me to build "Williamson" Ultra Linear style amplifiers with bias points slightly adjusted for a somewhat lower supply voltage. I completed two chassis, each with a power supply transformer, OPT and 5 tube sockets installed.

Then came the quite naïve realization (after carefully reading many articles on the Williamson amplifier) that my choice of output transformer might become a severe design limitation. At the time (again due to budget limitations) I chose the Hammond 1620 output transformer. My reasoning was that the 20 Watt power limitation was not going to be too big of a deal as my B+ supply voltage was going to be somewhat lower anyway (and thus the resulting output power).

After reading up on the specifications required for a Williamson output transformer I concluded that the frequency bandwidth of the Hammond transformer (30Hz --> 30Khz -1db) and leakage inductance might become a severe issue.

Also the inductance of the primary winding was unknown, so I decided to contact Hammond in order to get some numbers on the primary inductance in Henry after the realization that the output transformer needs to be one of a certain high quality in order to stabilize the Williamson design. Williamson stated in his article that the primary inductance must be no less than 100H in order to function well and Hammond confirmed that the 1620 could achieve an 106.5H inductance (under certain conditions of course).

I started looking all over the Internet to find people that have pulled off a Williamson build with somewhat inferior OPT's. I found one instance where somebody used the Hammond 1620 in a successful build by reducing feedback and some other (somewhat peculiar) design choices [img1]. It strikes me the the first half of the input 6SN7 seems to be starved and the concertina anode/cathode resistors seem to be quite low in resistance and thus allows for a higher current. Another "unique" aspect seems to be the grid stopper resistors on the 6SN7 drivers and 10k grid stoppers on the KT66. The rather high bypass 1nf capacitor across the feedback network also strikes me as odd (any comments?).

The second design I found was not a Williamson type per-se but a design by Acrosound using (correct me if I am wrong) a floating Paraphase type design [img2]

My questions then would be:

* 1: who can say (from experience or knowledge) which of these two circuit designs (Williamson or Acrosound) has the highest chance of being stable with the given OPT?

* 2: Can somebody with more experience comment on the design [img1] and what the key aspect is that makes it work and keeps the phase shift in check with the Hammond 1620. Also can we improve upon the design (to simplify it somewhat again)?

Thanks in advance,
Pim
Attached Images
File Type: png IMG1.PNG (267.6 KB, 253 views)
File Type: png IMG2.png (448.2 KB, 255 views)
Attached Files
File Type: pdf 1620.pdf (309.4 KB, 24 views)
File Type: pdf 1620A.pdf (984.2 KB, 14 views)
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Old 25th November 2020, 07:53 PM   #2
Parafeed813 is offline Parafeed813  Netherlands
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No experience with Hammond transformers, nor with this specific implementation of the Williamson amp.
Also, I'm more of a tweaker than an theoretician. Sorry.
However, I did build a few amps similar to these two over the years.
I have read about instability in the Williamson, but haven't had that problem myself.
Maybe because I keep the GNFB low. For high levels of GNFB, the OPT needs to be of very high quality.
The feedback bypass cap depends on circuit design and characteristics of the OPT. So, either stick to a known design, using that specific OPT, or determine it empirically: hook up load resistors and a o-scope, feed it a square wave and try different cap values to find the one that gives the best wave (reduces/eliminates overshoot and ringing without rounding off the square).

One question you should ask yourself: how much power (and damping) do you really need?
The 1620 OPT is not the biggest transformer, EI-78 if I'm not mistaken.
The trusty old PP EL84 amp next to me here uses EI-84 sized OPTs.
UL 6L6 might be a bit much for what those 1620's can handle. Triode mode would be my choice here, or 6V6's. Triode mode already reduces distortion and output impedance, so you could get away with a little less GNFB.
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Old 25th November 2020, 07:58 PM   #3
TG is offline TG  Ukraine
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Here's the original Hafler&Keroes article on the Acrosound amp. 1620 is fine for the job.
Williamson amp is definitely less stable.
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File Type: pdf Hafler_Keroes_UL_Amplifier.pdf (1.45 MB, 48 views)
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Old 25th November 2020, 08:21 PM   #4
Captain archer is offline Captain archer  Netherlands
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Thanks for the replies.

The original article is very interesting (going to read it in detail). Do you have any experience or ideas about using a 6SN7 in the first stage of this design to reduce the total gain and thus increase stability?
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Old 25th November 2020, 10:09 PM   #5
Parafeed813 is offline Parafeed813  Netherlands
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The long design thread 6L6GC AB2 Amp ultimately lead to a 6SN7 LTP into another 6SN7 LTP.
It gives the proper amount of gain for PP 6L6.
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Old 26th November 2020, 08:53 PM   #6
rongon is offline rongon  United States
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Asking for guidance on 6L6 PP build/design selection
Parafeed813 is right about starting the design process by deciding what parameters you actually need.

1) What speakers will you be using with this amp? The more sensitive the speaker, the less output power will be required from the amplifier. The smoother the speaker's impedance curve, the less damping will be required of the amplifier.

2) How big is your listening room? The larger the room, the more power output you will need from the amplifier. The smaller the room, the less power needed.

3) Related - How far away from the speakers do you sit when you listen to your system? The farther away, the more power you will need to achieve a certain playback sound pressure level (SPL) at the listening position. The closer you are to the speakers, the louder they will sound to you, so the less output power you would need from the amplifier.

If you don't need gobs of power, you may be happy with triode-wiring your 6L6s. You will probably get about 10 watts per channel that way.

Triode wired 6L6s will run cleaner than in pentode, so you may be able to get away with less negative feedback around the amplifier.

Triode mode = more damping over the loudspeaker than pentode. That's another reason why you should be able to get away with less NFB if you go with triode mode.

Less NFB = less problems with stability, therefore less need for an OPT with super-low leakage inductance, etc.

Have you ever seen the "Albert Einstein" amplifier? That's an amplifier someone supposedly made for Albert Einstein, based on the Williamson design but using 6B4G directly-heated triodes as the output tubes. That would give you an idea of what might be possible with what you have.

Click the image to open in full size.

As you can see, it's basically a Williamson, with a slightly different cathodyne/concertina phase splitter. It also employs no global NFB loop.

Whether an amp something like that would work for you completely depends on what kind of speakers you use and the size of your listening space. But maybe?
--

Last edited by rongon; 26th November 2020 at 08:57 PM.
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Old 27th November 2020, 02:16 AM   #7
Tom Bavis is offline Tom Bavis
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I measured a Hammond 1620 a few years ago, primary was 230 Henries at 10V, 20 Hz. Leakage inductance was 8.6 mH at 4ohms (secondaries in parallel), 20 mH at 8. The new ones with 4-8-16 taps may not be as good, but will let you use the 16 Ohm tap for feedback, which was not a choice with the early one unless you wired for 16 Ohm speaker. Below is impedance (Z & theta) vs. frequency, with 4 and 8 Ohm terminations. I suspect the new ones would do well at 16 Ohms, worse at 8, even worse at 4
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File Type: gif 1620.gif (31.8 KB, 187 views)

Last edited by Tom Bavis; 27th November 2020 at 02:32 AM.
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Old 27th November 2020, 02:30 AM   #8
Tom Bavis is offline Tom Bavis
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Einstein's amplifier would make an excellent phase-shift oscillator if you applied any feedback... even unintentional feedback if the power supply impedance is high. There are three identical R-C coupling networks cascaded. At some low frequency, each will have a 60 degree phase shift - voila - 180 degrees! There's a reason the Mullard circuit has been used so widely - one less R-C network than the Williamson.
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Old 27th November 2020, 06:56 AM   #9
zintolo is offline zintolo  Italy
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You have a 6.6 kOhm Raa 43% UL output transformer and 6L6GC output tubes.

With 350V across the tube and 80 mA bias you will be at 28W dissipation (close to the 30W) at idle and you will hit the g1=0 at around 160mA with plates around 85V, still in class A1, reaching exactly 20 Wrms.

Has anyone ever used 6L6GC in those conditions?
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Old 27th November 2020, 06:14 PM   #10
Captain archer is offline Captain archer  Netherlands
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@Tom Bavis: Those measurements on the Hammond 1620 are very useful, I indeed have the older version of the transformer (not the A one). I have been thinking about the Mullard circuit as I have 2 red can 6SJ7 tubes laying around. The only problem is that I do not know the state of the tubes and I do not have a tube tester at hand.

@zintolo: I will be using the 6L6 (old version) in this circuit with a max dissipation of 19Watt (some 380 volts @ 50ma).

I will be honest in that I find it hard to pick a topology for this amplifier. The Mullard will be the most stable, The H&F circuit is nice but may give too much gain, The Williamson might become unstable with the given transformer.
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