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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 27 Aug 2017, 09:43 
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The strange title sort of indicates that this project that will take some time, is made up in Sweden using Russian tubes (6V6 = 6P3S and 6SL7 = 6N9S).
The reason this amp was planned was a cheap Chinese PCB holding the simple 6V6 PP-amp found at DIY Audio Projects Forum.

Got the tubes, at least the OP transformers and may have the power transformer. Missing a few tube sockets, but just ordered them from Finland. A crazy thing is that different PCBs use different diameters for the socket pins but even if there sockets really don't fit a bit plier-work fix everything.
Have to inspect my component stores, I may or may not have all components. When check the component list on eBay, all resistors except the 6V6 cathode resistors are 1W.

But before cutting the heavy aluminium sheet I have and drilling holes (got a "nice" wooden box), there are several issues I must handle before starting another project. Well, I actually scrapped a 6V6-SE project and a 832 SE-project and an EL84-PP project so I think it is OK to start this one.

The issues and questions:
1. The cathodes of the 6SL7 must be wrongly connected or just wrongly drawn. I marked, in blue, how it must (?) be. Me is correckt?!
2. Got physical small 270R/5,5W resistors. Should work but with slightly lower power OP. Correct?!
3. No big matter, but 620p FB cap ...??
4. The blue figures are from sources that sell the PCB and complete kits. Looks OK?

I have managed to build so many amps that I can say none of the issues are any issues but want to check if anyone else have had any.

I finally noted that someone asked about a potentiometer, using the amp as a integrated amp. I read that and will take proper action, like make sure I have a preamp ready. Else I will sort out the information in other threads before I go.

Attachment:
6V6PP.jpg


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PostPosted: 27 Aug 2017, 11:20 
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SIGH! I have started and almost finished the 6V6-SE ....

Here's the PCB with sockets. IF I manage to drill holes with some precision I may attach the board to the chassis screwing the top plate to the sockets.

Attachment:
6V6PP-02.jpg


Here's the underside and (the not soldered power resistors. They will OF CORSE be monted 1/2" from the PCB.

Attachment:
6V6PP-01.jpg


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PostPosted: 27 Aug 2017, 12:20 
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Soundbrigade wrote:
1. The cathodes of the 6SL7 must be wrongly connected or just wrongly drawn. I marked, in blue, how it must (?) be. Me is correckt?!
Believe it or not, the schematic is drawn correctly. In this arrangement, the driver is biased through the output transformer secondary. The DC resistance drop is small, so the 1kΩ acts as a bias resistor, but the AC generated in the transformer secondary is impressed upon the cathode of the driver as feedback. Personally I'm not a fan of this arrangement as the DC bias current of the driver is magnetizing the core of the output transformer thus reducing dynamic range. I usually like to keep the driver bias operation independent of the feedback.
Soundbrigade wrote:
2. Got physical small 270R/5,5W resistors. Should work but with slightly lower power OP. Correct?!
Should be fine.
Soundbrigade wrote:
3. No big matter, but 620p FB cap ...??
This is just to reduce the feedback factor (Aß) at high frequency to eliminate the possibility of oscillation. The value is not critical as it operates entirely outside of the audio frequency range. 620pf should be more than adequate.
Soundbrigade wrote:
4. The blue figures are from sources that sell the PCB and complete kits. Looks OK?
Probably, yes.

Let us know how it goes.

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PostPosted: 27 Aug 2017, 14:16 
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Quote:
Believe it or not, the schematic is drawn correctly. In this arrangement, the driver is biased through the output transformer secondary.

Thanks a lot of informing me about that. Else I would work the circuit board over with a scalpel ....

I get it that most (all) Dynacos are more or less designed to be simple and give a lot of Watt for your pennies, so I don't expect sound quality Extra vaganca. But I'll keep you posted.

Btw, OP trannies are from a company in Sweden and propably surplus from a "better" commercial Swedish tube amp, whereas the power transformer are an old Lundahl transformer. Have to check that I get the correct voltages however.

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PostPosted: 27 Aug 2017, 14:19 
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Suncalc wrote:
Soundbrigade wrote:
1. The cathodes of the 6SL7 must be wrongly connected or just wrongly drawn. I marked, in blue, how it must (?) be. Me is correckt?!
Believe it or not, the schematic is drawn correctly...... as the DC bias current of the driver is magnetizing the core of the output transformer thus reducing dynamic range.


I've seen (and use) this for SE, where the DC going in the opposite direction de-magnetizes the core slightly, but never seen it in a PP arrangement :confused:

Prehaps someone was going for a low parts count?
(I've seen some otherwise GREAT commercial designs crippled for the sake of saving 4 resistors >_< )

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PostPosted: 27 Aug 2017, 14:50 
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It's not seldom you come across schematics (SE) where the transformer secondary is attached to the cathode, resulting in some extra feedback.

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PostPosted: 28 Aug 2017, 10:32 
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I have already made an note about it, but appearantly, the amp requires somewhat higher voltage to sing. Hooking it to a standard CD-player doesn't result, maybe, in the earthquake effect wanted.
So looking around, I have come to a few ideas:
a) simple one stage low-voltage tube amp (ECC85 or even ECC88)
b) a simple one-stage amp based on a pen-size tube like 6247
c) silicon!!!!!
or
d) use a well-known and tested design - 4S, the universal preamp using ECC81/82

The best choice is propably the 4S univ-amp. No fiddle with special PSUs ...


Btw - ordered most of the stuff today.

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PostPosted: 28 Aug 2017, 22:16 
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hah hah, this site never cease to stimulate, the humor in me.

"if you don't know where you are going, any road will get you there"
it is a good thing you signed the name --Lewis Carroll,
have you really thought of that statement carefully, lets start with the word"there"


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PostPosted: 29 Aug 2017, 00:49 
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That's from Alice in Wonderland. Alice walks along a road that divides and sees the Cheschire cat.
- Which way should I go, asks she.
- Where are you going?
- I don't know.
- Any road takes you there, answers the cat.

I have a modified version of my signature:
If you don't know where you're going - you may end up somewhere else.

That is very true for us who're into electronics and tubes. We start off nice and fine and 5 - 10 years later, the projected that was so wellplanned ended up as something completely different. What was supposed to be a 6BQ5-PP ends up as a 6L6-SE ...

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PostPosted: 29 Aug 2017, 01:41 
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"Po schwedsko-russki" will be correct :)

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