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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 23rd August 2019, 12:30 AM   #61
Salas is online now Salas  Greece
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
True, its more sensitive to apply extra, but when feasible its welcome.

This 20W amp took 14dB GNFB easily for example. I had it at 9dB initially but when I added +5dB extra feedback is sounded better. Did not get harsher, only cleaner. Note that I was using the 8 Ohm tap on 4 Ohm load so the reflected impedance was higher on the primary.
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Old 23rd August 2019, 01:10 AM   #62
TonyTecson is offline TonyTecson  Philippines
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Quote:
Originally Posted by Eli Duttman View Post
Because of current hogging, forget PPP EL84/6BQ5s in a self biased amp.

Different O/P "iron" for EL34s than 6L6 family tubes. Edcor's CXPP60-4K should be OK, but make certain to roll infrasonic noise off at the amp's I/P. A single RC high pass pole whose F3 is in the 16 to 18 Hz. range gets the job done.

There's little to be gained by reinventing the wheel. Use Mullard's original 5-20 as the starting point, for a self biased PP EL34 O/P stage.

BTW, Mullard was trying to sell tubes that they manufactured, when the 5-20 design was published. The topology is fine, but the high RP/low gm 12AX7/ECC83 is a poor choice for the LTP.
i got very good results with a 6dj8 input tube and 6sl7 ltp stage in a JJ6L6GC pp amp i made a couple of years ago...
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Old 23rd August 2019, 01:16 AM   #63
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Generally the figure of merit for global NFB is considered to be around 20dB - somewhat arbitrary, but that is enough for a 10 fold reduction in both source impedance and distortion, it also means the multiplicative products are low enough that you no longer care too much about them.

Probably nothing but a rule of thumb on the old hifi highway here in Boston.

Transformers are an excellent way to get galvanic isolation (ground loops, etc) and super high CMRR for induced hum pick up. So if you need balanced this is what I would do. (Kevin Carter at K&K Audio is who I would recommend you talk to about Lundahl's input transformers if you go this route. Yeah shameless plug, he's a friend. )

I use no global NFB these days so take that as purely anecdotal. I do however use a lot of transformers...
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Old 23rd August 2019, 12:19 PM   #64
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Quote:
Originally Posted by TonyTecson View Post
i got very good results with a 6dj8 input tube and 6sl7 ltp stage in a JJ6L6GC pp amp i made a couple of years ago...
Just wondering about the alignment of the transformers? Can you get away with the power transformer having the same alignment as the OPTs? Is that the advantage of the bell ends?
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Old 23rd August 2019, 03:37 PM   #65
Kofi Annan is offline Kofi Annan  United States
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WOW! Guess I shouldn't have gone to bed early!

Rongon-- BIG thanks to you for modeling it stem to stern. I know that must have taken a while! Thank you so much!

Quote:
Also, if the polarity of the feedback signal is in phase with the input signal, the two will be additive (positive feedback) and LTspice will go nuts.
That is EXACTLY what was happening with my LTSpice model. I swapped the phase of the splitter and was able to twiddle away with NFB after that.

Not sure why, but I was able to get about 45W out before clipping on my model with 400mV in? I have to Boot Camp over to the Windows side (from the Mac / iOS side) to make the simulation work, so I'll post mine in a bit along with relevant THD.

Any concerns about the need to balance the current between each PPP pair? I believe the earlier advice was that it may not make much difference as long as I am able to get matched quads / octets or that a trimpot may be added to assist with balance.

Kofi
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Old 23rd August 2019, 04:37 PM   #66
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Quote:
Originally Posted by Kofi Annan View Post
WOW! Guess I shouldn't have gone to bed early!

Rongon-- BIG thanks to you for modeling it stem to stern. I know that must have taken a while! Thank you so much!
No worries! I was curious to see what would happen, so it was fun. I'm seriously thinking of building a triode-wired version of this, to get maybe 8 to 10 W per channel. That would be enough for my needs. I have the iron and a chassis hanging around, so I should do it. But I have other unfinished projects in the way...

Quote:
Originally Posted by Kofi Annan View Post
That is EXACTLY what was happening with my LTSpice model. I swapped the phase of the splitter and was able to twiddle away with NFB after that.

Not sure why, but I was able to get about 45W out before clipping on my model with 400mV in? I have to Boot Camp over to the Windows side (from the Mac / iOS side) to make the simulation work, so I'll post mine in a bit along with relevant THD.
Your results will depend on the models you use for tubes and transformer. My OPT model is of a hypothetical OPT from a Dynaco Stereo 70, with measured coil resistances added from the real thing. They differed a bit from the model I downloaded, so my results may or may not more closely reflect reality than others'. I do know that a real-life Stereo 70 is capable of delivering about 20W max per channel, so when the sim gives 20Wpc I figure it's doin' it close to right. YMMV of course.

Quote:
Originally Posted by Kofi Annan View Post
Any concerns about the need to balance the current between each PPP pair? I believe the earlier advice was that it may not make much difference as long as I am able to get matched quads / octets or that a trimpot may be added to assist with balance.
Paralleling spreads out the differences of individual tubes' current draw, so it should be possible to mix 'n match positions of the 8 EL84s to get reasonably good balance. With an EI or C cored OPT you can have a few milliamps of imbalance without much penalty. With a toroid core OPT it's more likely that you will have to be more careful about keeping current balanced between the primary windings. Beyond that things get complicated, and I'm not knowledgeable enough to go beyond that level of understanding. Maybe someone else would be willing to dive in here and clarify?
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Old 27th August 2019, 08:49 PM   #67
Kofi Annan is offline Kofi Annan  United States
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Hey! Sorry for the couple-day hiatus, but I got distracted with non-amplifier events.

I am still working through the BOM for the power supply and such, but wanted to share my working copy now (attached as Excel file and picture). Please feel free to review and mock my choices as you see fit.

Kofi
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Old 28th August 2019, 02:11 AM   #68
AllIsParadox is offline AllIsParadox  United States
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I noticed your (Kofi) concern about product liability, so I will make a few un-requested comments, and hope that I don't offend anyone with the free advice. I practiced as a trial lawyer for fourteen years, and retired from the practice in 1996, burnt out.

Basic: YOU are responsible for what YOU do, incorporated or not. A couple of attorneys I knew had to be bailed out because they confused "liability for personal actions" with "liability for company actions", and really screwed up their clients' lives. Screw-ups of the first order. The whole point of hands-off stock ownership (and that literally means HANDS OFF, so no involvement in management) is that hands-off shareholders are only liable for corporate actions up to the value of their shares. If things go south, they can throw their hands up, say So Sad, Too Bad and walk away, free of further obligations.

If you are in the middle of it, like being the CEO, or the builder or designer, then you do not get that protection. Insurance is kind-of nice when your damages are covered. What makes insurance indispensable is the blood-sucking pit-bull defense attorneys the insurance companies have on staff who will fight to the last drop of the plaintiff's blood to keep from paying a penny, and better yet, the insurance company pays those defense lawyers, not you. Coverage for a one-off policy may be relatively inexpensive. The insurer may require you to get a Licensed Professional Engineer to bless your work. If you cannot afford to do this, or no insurer will cover you, that is a huge hint from the Universe that maybe you shouldn't do this (so take the hint). Build it in your own home for your own use, keep adequate homeowner's insurance up to date, and you are probably covered. Build it for use in a commercial environment, and liability exposure can run into the millions.

My first boss made almost a million off a bolixed-up farm harrow with a home-made hitch that crippled and maimed his client. His client got the other two-thirds of the settlement. Please don't be naive enough to think that actual innocence from negligence will get you out of it - it won't, and your homeowner's insurance won't touch it, not if you intended to build it for a commercial environment. This is not legal advice. This is just my own opinion, formed after hard, sad, experience. Listen to your attorney, but don't skip the product liability insurance. If you design and build it for commercial use, no form of incorporation can protect you, but insurance will.
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Old 28th August 2019, 09:02 PM   #69
Kofi Annan is offline Kofi Annan  United States
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AllIsParadox,

No offense taken whatsoever. Thank you so much for your reply.

I have taken your comments very seriously-- so much so that I was able to sign up for product liability insurance today, which I believe will cover me for the issues you represented here. I also have a couple of calls out to lawyers to review my situation and offer further advice and / or recommend possible changes to this policy as needed.

As you mentioned, it was actually quite inexpensive and will not only provide coverage but peace of mind at a very reasonable price.

Here's a follow-up question for you:

Quote:
Please don't be naive enough to think that actual innocence from negligence will get you out of it - it won't, and your homeowner's insurance won't touch it, not if you intended to build it for a commercial environment.
I'm assuming you mean that if any damages are incurred by the customer that related to my negligence, my product liability insurance will pay out up to the maximum policy amount. Is that correct?

Again, thank you so much for your response. You told me what I needed to hear and I appreciate it.

Kofi
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Old 28th August 2019, 09:27 PM   #70
TonyTecson is offline TonyTecson  Philippines
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Quote:
Originally Posted by OldHector View Post
Just wondering about the alignment of the transformers? Can you get away with the power transformer having the same alignment as the OPTs? Is that the advantage of the bell ends?
the OPT's were mounted vertically on its short side, the power traffo is mounted horizontally on its longer side, so the coil axis are 90 degrees...
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