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Old 6th August 2008, 03:49 PM   #1
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Default Questions about tweaking the Miniblok SET...

I recently completed the Fred Nachbaur Miniblok SET. Actually a pair of monoblock Minibloks, yuk yuk! Maybe not the greatest amp to build, but it fit my criteria of being reasonably uncomplicated and cheap to build. And although I've owned tube gear for many years, I'm a relative beginner at building with tubes. So I also wanted the experience.

I used new, not used or NOS, bargain resistors and caps. All values used are correct, but I suppose quality could be suspect. I used Radio Shack power transformers of the specified values. Edcor OPTs. All my voltages are correct per the checklist. The amp played with no problems from the start.

I bought some NOS Raytheon 13EM7s, and later some NOS GE, but with a coin type base. Can't really tell a great difference in sound when changing tubes. Maybe the quality of parts used elsewhere prevents me from hearing any difference here???? Or maybe there is no difference to be heard.

My question is that although I'm hearing a nice soundstage with great imaging when listening through my Signets, I have to say I'm very disappointed with the SQ. Knowing that the speakers are capable of some extremely good SQ, I'm wondering if I can make parts changes in the circuitry that will improve this at all. My amps are constructed to easily allow such changes.

Any suggestions and/or help is much appreciated.

Here's the circuit;

Cheers, Greg
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Old 6th August 2008, 04:16 PM   #2
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Hi Greg,

if you just want to replace components without changing the overall design, my suggestions would be:

- use a low ESR type capacitor for C8 in parallel with a film cap of ~1uF
- I would also use a film cap parallel to C3 (WIMA or SOLEN are relatively cheap and sound good to my ears).
- The coupling caps C6 and C7 are also good candidates to be replaced by good film capacitors (polypropylen Solen or WIMA or "boutique caps" if you like to spend money).
I wonder if the coupling caps are not a bit small? Depending on your source 0.1uF for C6 sounds a bit undersized. I am not used to fixed bias topology but 0.015uF for C7 looks also smallish to me (waiting for the more experienced people here to comment)

Trying different coupling caps and paralleling electrolytic caps with film caps made significant changes to the sound of my version of gingertubes "Baby Huey".

I also wonder if decoupling the power supply for the small signal triode from the power section would help. Maybe by a simple Pi filter (again, waiting for the more experienced people to comment).

This is just a starting point from a not-so-much experienced member. I hope I could help.

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Old 6th August 2008, 04:45 PM   #3
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Thanks so much for the suggestions. Just what I'm wanting to hear. I'm trying to pick every brain I can.

My first "goal", of course, was to complete a working amp. After that happened without any problems to speak of, I can't resist trying to find what will help in terms of SQ. The sound right now is just dull and lifeless. Of course another help will be more efficient speakers. That's another project to come.

Regards, Greg
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Old 6th August 2008, 06:51 PM   #4
Pyre is offline Pyre  Canada
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The amp should not sound dull and lifeless at all. I built one of these a while back and it sounds nothing of the sort. I think its one of the greatest amps to build for your first project.

You are very correct the amp needs sensitive speakers. I was running mine into 94db/1M kef 104a/bís and it sounded great.

I made a few changes to the circuit. I think I posted a link to my homage to Fredís circuit a while back when someone else was building one. I can send you a copy when I get home from work if you like.

I can remember that I took out C6 and the 100k pot (everything I run had volume already), ran straight to the grid with a 1M to ground. I also added 220uf to the power supply. It was a matter of potential longevity not sound but I increased the values of C4 and C5 to 60V. I used a separate transformer for the heaters and ran them DC.

Have you checked the voltages you are running? Make sure that you have 12-13 volts on your heaters (between pin 7 and 8), 215 on the plate (pin 2) and -35 bias (pin 1). Any major variation from these could result in your lifeless sound.

I have not tried the coin base tubes or GEís but I found a big difference in sound between a set of NOS Realistic gold pin lifetime tubes and a NOS set of black plate Silvertone tubes. The Silvertones blow the Realistics out of the water. Maybe I should write them and take that up on that warranty.
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Old 6th August 2008, 10:24 PM   #5
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Thanks Pyre!

Yes, my voltages are all within a whisker of recommended. I'm taking all advice under consideration. My common sense says this is the thing to those with more experience and knowledge than I.

Feel free to post any and all pertinent info. You can be certain it will be appreciated.

Thanks, Greg
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Old 7th August 2008, 03:55 AM   #6
rman is offline rman  Canada
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I have built two EM7 amps (not the miniblock) both use ac on
the heaters with no problem. Also PS decoupling between
stages should not be required as there are only two stages
and they are out of phase. My first thought was Fred's
choice of operating points is not optimal, but another poster
wrote that his sounds fine.

I have used a variety of used 6Em7's such as RCA, Zeinith,
Sylvainia. They vary in voltage readings in the same socket
but all sound fine. I have not tried realistic.

Sorry I couldn't give any positive ideas, but I don't think these
two ideas are the place to start.

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Old 7th August 2008, 03:32 PM   #7
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Thanks Rolf!

Maybe I'm mistaken for thinking this isn't a huge thing, but my heater voltage(AC) is about a full volt above recommended. Will this cause any problems?

Thanks, Greg
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Old 8th August 2008, 07:29 PM   #8
Tweeker is offline Tweeker  United States
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It will shorten life. You can put a pair of 1 ohm resistors in line, one on each side, to reduce it.
Be sure your foil hat has a good low impedance ground.
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Old 8th August 2008, 09:35 PM   #9
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Originally posted by Tweeker
It will shorten life. You can put a pair of 1 ohm resistors in line, one on each side, to reduce it.

Thank you Tweeker! I will do that very thing.
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Old 9th August 2008, 08:31 PM   #10
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Followed through on some advice given here. Will do more experimenting as I can.

Used some caps I had on hand. Replaced a no name electrolytic C8 with an Elna Cerafine and bypassed it with a Wima film.

Replaced C6 with a Wima, as well.

Bypassed C3 with a film.

Added resistors to drop the heater voltage to 12.5V from about 13.8V.

In high frequency clarity, the change is very noticeable. It's as though I removed a blanket from the speakers. That's how the sound was previously, as though the speakers were blocked by something.

My next thing is to use more suitable speakers. Possibly a full range, single driver design of some kind in the 100dB sensitivity range. And add an appropriate subwoofer system. My Signets have a sensitivity of 89dB, so I'm sure that change will let the amp work more as it should and allow a fair assessment.

Thanks to those who helped!!

Best regards, Greg
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