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Fred Nachbaur SET amp: input capacitor
Fred Nachbaur SET amp: input capacitor
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Old 20th December 2017, 05:42 AM   #101
PRR is offline PRR  United States
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Quote:
Originally Posted by Carlp View Post
...I wonder, then, why certain tubes ...virtually always have the blue glow while others ...virtually never show it ...
I do not know. (Have not noticed.) The 6550 is often run at 600V, the EM7 at 140V; but 6AQ5 is more often 250-300. Maybe plate holiness has a lot to do with it.

But what I really want to show is something I found: how much glass is in a tube (circa 1930!). About 59% of the '27, '24, '45 types in 1930 is glass. So, very important, mainly that it be serviceable and cheap.

The list of ingredients for glass is long; but glass makers had all kinds of (not-so) secret recipes to adjust liquidity, strength, hardness, to cut key contaminants, etc.

ELECTRONICS: Electronics Engineering magazine beginning in 1930
http://www.americanradiohistory.com/...cs-1930-11.pdf
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Old 20th December 2017, 04:39 PM   #102
Carlp is offline Carlp  United States
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Fred Nachbaur SET amp: input capacitor
Quote:
Originally Posted by PRR View Post
The 6550 is often run at 600V, the EM7 at 140V; but 6AQ5 is more often 250-300. Maybe plate holiness has a lot to do with it.
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That was one of my thoughts, and from what I've read plate holes ARE involved. Perhaps, too, it's because of higher energy in the electron flow and/or higher current itself allowing more electrons to escape (through plate holes) and making the fluorescence stronger. Hmmmm. Guess I'll do more reading.
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Old 31st December 2017, 06:45 AM   #103
logotrikes is offline logotrikes  Australia
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Quote:
Originally Posted by Zen Mod View Post
my friend Fred was clever an practical guy

that cap was deliberately small

you can even go without it .
In a rush to get one of my mono Minibloks working I completely forgot to remember to include C6, and when I did notice I'd omitted it I figured that there might be some DC blocking on the output of my Marantz CD player. (maybe)

It's good to see some expertise in the mix (Cheers, Zen Mod) recommending that it can be left out. I'd always thought this but wasn't keen on deliberate omission. I'm a believer of "as simple as possible but no simpler" and really what could be simpler than Fred's Miniblok...?

I know just enough to build it, nothing more, and this was the tremendous appeal about Fred's design, and not only that, but his easy-to-follow method of testing. Just the ticket for a wood duck like me...

The one I have running now sounds pretty good, (it's a little over-driven at the moment, according to my tests and feedback from the forum, but seems fine) and I can't wait to finish the other one to check for imaging, depth and soundstage etc...
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Old 31st December 2017, 02:25 PM   #104
Carlp is offline Carlp  United States
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Fred Nachbaur SET amp: input capacitor
Unless you know all of your sources have DC blocking at the output, best to keep that cap in place. And as ZM hinted, its size is dependent on the ability of the output transformers to handle low frequencies. If you're using the cheap Hammond OPTs, stick with 0.1uF. If you have larger/more capable OPTs you could increase it.

BTW, seems like it's possible that if you have DC on the input and no C6, that might well drive you into distortion by changing the bias. When you say "overdriven" I assume you mean you are getting distortion early on in the volume pot position?
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Old 31st December 2017, 04:25 PM   #105
logotrikes is offline logotrikes  Australia
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With regard to 'overdriven', I was referring to the difference between Fred's values and the ones I was measuring, and also to conversations with yourself and Allensoncanon. That was it really. I'm not hearing any distortion at my suitable volume level, but I'm assuming that my tube is being driven at the higher end of its rated value. I've yet to complete the second monoblock.

With regard to C6, I'm using custom wound OPT's, which I hope are better than the generic Hammonds, so probably no need to increase the value of C6...
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Old 3rd January 2018, 03:04 AM   #106
logotrikes is offline logotrikes  Australia
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My bad, I should have written that since I'm using a better OPT than the generic Hammonds, I should consider a higher value for C6 as suggested by Carlp. I've got some PIO Russian caps in my stash, maybe I've got a couple suitable...

My OPT's have a rated value down to 20Hz, although I think that might be optimistic.


For the first round, I'll make the second monoblock the same as the first so I'm on a level playing field, and then I can make changes one by one. I doubt that with my cloth ears and tinnitus I'll hear much difference, but you never know...
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Old 27th January 2018, 03:20 AM   #107
logotrikes is offline logotrikes  Australia
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Hi Folks,

Finished both Miniblok monoblocks. The first one worked right out of the box as described elsewhere, but I committed the cardinal sin of not testing the second one. The first is fine so I just copied the wiring for the second one, checking and double checking as I went. The second one gets power, the tube lights up but there's nobody home, not a cracker. I removed the cover and replaced the signal wire connections cos I thought the fault might be there. At this stage I still haven't run a meter over it, other household tasks demand my attention, so it sits there all forlorn.

I'll get to it this coming week but I've got no idea where to look if Fred's testing procedure doesn't reveal anything amiss. All the transformers are potted and if it happens that any of them aren't working, especially the OPT then it means a new OPT, cover and potting mix.

A couple of pix.. err, no. I don't know how to add them. Something about adding the URL for the location of the pix. They're on my desktop , that's all I know...

Can anyone advise?

Cheers, Martin
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Old 27th January 2018, 03:13 PM   #108
Carlp is offline Carlp  United States
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Fred Nachbaur SET amp: input capacitor
Sorry to hear that, Martin.

First thing I'd do is swap tubes to ensure one isn't dead. But see my caveat in the next paragraph before you even do that. If it's still the same, then I'd go through Fred's testing procedure. You are VERY likely to find the problem that way. Likely problem is a bad connection (especially ground). If you don't find it that way, or find the problem but can't figure out how to fix it, post back with the info.

BTW, I'd highly recommend against trying to run a new (or new to you) tube amp without any testing. You could destroy your speakers that way (e.g. if you have DC on an output) or the output transformers, etc. Better to wire an 8+/- ohm power resistor of at least 10watts across the outputs and then run the tests. If it passes all tests and you've checked for DC on the outputs and found no more than tens of mV levels, then I'd wire it up and try again. If there's still a problem, swap input cables and see if the problem switches sides and is therefore BEFORE the amp. No problem? Switch speaker cables and see if there is a speaker problem. BTW, if the dummy load resistors heat up, particularly without a signal, there's probably DC on the output.

Note that just copying wiring is prone to problems, particularly phase related problems, but also potentially serious problems.

Good luck. I look forward to your next post.
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Old 27th January 2018, 03:18 PM   #109
Carlp is offline Carlp  United States
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Fred Nachbaur SET amp: input capacitor
In fact, even if you swap the tubes and that indicates a bad tube, you should still go through Fred's testing. It is important but also will help you understand more about tube amps. It was one of the reasons for trying this amp as a first project. The circuit is quite complex as single-ended tube amps go (he uses a creative but unusual grid bias on the output stage, a voltage doubler PS and an interesting PS transformer setup to mention a few), so I wouldn't normally recommend it as a first project. It is entirely Fred's descriptions that make it worth trying. He was VERY thorough and easy to follow.

Note the OPTs should protect against any DC on the output UNLESS there's a problem with the OPTs...
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Old 27th January 2018, 03:55 PM   #110
logotrikes is offline logotrikes  Australia
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Thanks for those kind words Carlp. The very first thing I did was change the tube. No difference, so I swapped the RCA cables over. No change, in that the other amp kept playing. Then I re-soldered the input leads just in case. I used a watchmakers illuminated eyeglass to check for dodgy connections. Nothing that I could see. Checked the wiring against the other amp. All looks the same. All done before I looked for my meter and couldn't find it. Ah, bugger it, I'll look tomorrow. Tomorrow came and I had other things to do so I'll fish out my meter from its hiding place and get stuck in during the week. I so wanted, and expected, to be listening to Dire Straits as soon as both amps warmed up. But it was not to be...

PS, the one thing I didn't do but should have was swap the speaker wires to the other speaker, to rule in or out a dodgy speaker. I did check the speaker leads to the speaker and everything looked ok...

Last edited by logotrikes; 27th January 2018 at 03:59 PM.
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