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Old 5th December 2004, 12:55 PM   #11
GeirW is offline GeirW  Norway
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Gday.

Quote:
Save the schematic to file and zoom in with Acrobat Reader
The only shematic i found was a .gif
What do you mean. am not quite awake yet.

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Old 6th December 2004, 10:36 PM   #12
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Okay, I think I'm up for it or at least willing to give it a try. Right now I have a single 12BH7A which I think is similar to a 12AU7. Therefore, this is what I will start with. So, with a 12BH7A being a dual triode I'll use one section for each channel going to an EL84 in triode mode. Nothing fancy here but just a simple SE design. Also, I've been using SS rectifiers till now but maybe I switch to tube, maybe a 5U4, 5Y3 or something similar.

Does this sound like a good start? I 'll try to reread the Tubecad article and will ask a few basic questions as time goes on. Wish me luck. I'll make a new post when I ready...
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Old 7th December 2004, 07:01 AM   #13
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Quote:
Originally posted by RockysDad
Okay, I think I'm up for it or at least willing to give it a try. Right now I have a single 12BH7A which I think is similar to a 12AU7. Therefore, this is what I will start with. So, with a 12BH7A being a dual triode I'll use one section for each channel going to an EL84 in triode mode. Nothing fancy here but just a simple SE design. Also, I've been using SS rectifiers till now but maybe I switch to tube, maybe a 5U4, 5Y3 or something similar.

Does this sound like a good start? I 'll try to reread the Tubecad article and will ask a few basic questions as time goes on. Wish me luck. I'll make a new post when I ready...
Sounds good.
Luck.
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Old 7th December 2004, 07:55 AM   #14
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Yes, read John Broskie's article again. It's quite good.

I have quickly drawn something which you may or might not find useful Basically I am trying to show how the grounded cathode amplifiers join together to form an amp.

Everything to the left of C2 is the grounded cathode amplifier input/driver stage (with the 12BH7), and everything to the right of it is the grounded cathode amplifier output stage (with the EL84).

In the schematic, you will see R3 as the plate load resistor for the 12BH7, R2 as the cathode resistor (for self-bias) and C1 as the cathode bypass resistor (optional). R4 is the grid leak resistor, and R1 is a gridstopper (aimed at kiling parastic oscillations).

Basically the same things are present in the output stage. You will recognise R5, R6, R7 and C3 in the same setup (and performing the same function) as R1, R4, R2 and C1 in the first stage respectively.

C2 is the coupling capacitor, it's purpose is basically to stop DC from reaching the EL84's control grid, while passing the AC signal.

R8 is a resistor (I don't know what it's called) which is aimed at preventing oscillation, and limiting the screen current. I believe the connection between the suppressor grid and the cathode is made internally on the EL84, but it probably wouldn't hurt to make it externally.

The output transformer, T1, takes the place of the plate load resistor for the output stage.

The grids of the pentode are numbered from the cathode to the plate. so g1 is known as the control grid, g2 is the screen grid, and g3 is the suppressor grid.

Valve pins are numbered starting from the key/gap in the base, clockwise from the bottom view.



A few words on sizing of components:

After reading and understanding the article, you should be able to calculate the values of the plate load resistor and cathode (self-bias) resistor for a given B+ and operating point.

Grid leak resistors (R4 and R6) are generally sized as large as possible (check the datasheets for the 12BH7 and EL84), to prevent reduction in gain of the preceeding stage. Note that in AC terms, R6 is in parallel with R3.

Gridstoppers (R1 and R5) are sometimes not required. Usually, if required the value is determined experimentally. Common values are between about 100 ohm and a few kOhm.

C2 must be sized such that the low frequency cutoff the RC combination of C2 and R6 does not affect the audio band. As such, the value of C2 (in Farads) is 1/(2*pi*F*R6) where F is the -3dB point required.

C1 and C3 are cathode bypass capacitors, and will also introduce a low-frequency cutoff if used. The calculation of this value is a little involved, so bear with me. The cathode bypass capacitor (C1 or C3) sees the self-bias resistor (R2 or R7) in parallel with the resistance looking into the cathode of the valve (kind of like the plate of the 12BH7 seeing R3 and R6 in parallel). Thus, we can apply the same formula we used to size C2, where the required capacitance was 1/(2*pi*F*R), however the value of R now must be calculated.

Now, the resistance looking into the cathode of a valve is (RL+Ra)/(mu+1), where RL is the plate load resistor, in this case R3, Ra is the plate resistance of the valve (you will need to get this from the datasheet), and mu is the amplification factor of the valve (also from the datasheet). This is in parallel with the cathode resistor R2. So the value or R stated in the paragraph above is composed of these two in parallel (add the values as resistors in parallel and substitute into the formula)

On the -3dB point to choose, some designers choose 1Hz, some choose something higher like 5Hz. As long as it's low, it shouldn't be too critical. (The bandwidth of the OPT will almost certainly limit the frequency response of the amplifier).


Geez, this is a long post... I started writing and couldn't stop! Don't worry it it doesn't make any sense. It will after some time. Read the Broskie article, and download the datasheets for the 12BH7 here (that's the sheet for the 12BH7-A, but don't worry, the difference is in the heater warm-up time) and EL84 here.Those were links to pages on Frank Phillipse's websites (which ought to get some sort of award )

Hmm... I don't know what to say anymore (A very good thing )

Good luck!
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File Type: pdf 12bh7-el84.pdf (15.4 KB, 1106 views)
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Old 8th December 2004, 05:02 AM   #15
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Audiousername, thanks for the writeup and schematic! I was thinking along the same line as far as a design goes. Just your typical RC coupled SE design is probably the best place to start if you are a beginner, I suppose. Then once everything works and a baseline can be established then tweeking can take place.

I've been through this with the Foreplay preamp I built awhile back. The same philosophy of enjoying the journey vs. just getting to the destination is the same. But this time some bookwork has to take place which requires a little more disipline and time. The part I am trying to figure out now is dealing with the load lines. I'm still having trouble trying to understand how the voltage swings vs. current available and how distortion is being calculated. I need to try a few more examples and maybe it will sink in then. But I will try to fill in most of the blanks from your writeup and will ask more detailed questions later. So thanks again and now back the my articles...
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Old 10th December 2004, 05:25 PM   #16
GeirW is offline GeirW  Norway
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Gday.

@RockysDad.
Do you have plotted in the values on the 12BH7-84 schem,
RD. Can you post them here.
First i was going to replicate a Tanberg 84 design,
but for several reasons, have now totally forgot that project.
I still have some spare parts, that should go along fine
with this amp.
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Old 11th December 2004, 03:53 AM   #17
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The Decware Zen is really quite good... we have gotten to calling clones "Mulligans"

dave

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Old 12th December 2004, 12:07 AM   #18
icebear is offline icebear  Norway
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Default Have buildt the Decaware

Hi

I have built the Deacaware SE84SC. I checked the spec, and tried to clone it. I bougth the parts at a local dealer here in norway (Demostenes) and rest of the parts at the huge electronic dealer Elfa. Total for me was around 600 usd. So mybe I did not save as much by cloning it??? The OPT is originaly 9.8k (I believe), but it then favoures low impedanse speakers. My speakers are quite stable at 8 ohm, so I'm using 6,8 k opt, and it should be approx. the same?? I'm running it on the Bastanis Prometheus speaker. It's a 100dB/1w dipol, and I'm not lacking any power. The sound is detailed and quite forward, but not agressive. It has a good tone. I recomend it if you have easy running speakers.
Can you believe it: One cap and one resistor in the signalpath in the amp. The speaker have a resistor in parallell and one cap in series with the tweeter. Not much :-)

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Old 12th December 2004, 09:34 AM   #19
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Default Re: Have buildt the Decaware

Nice "Mulligan", Bjorn.

Quote:
Originally posted by icebear
Total for me was around 600 usd. So mybe I did not save as much by cloning it???
Your OPTs are much larger than those in the original! I'd imagine they'd have cost a fair bit. (In the original, they are small enough to fit in under the chassis at the front)

Quote:
Originally posted by icebear
Can you believe it: One cap and one resistor in the signalpath in the amp.
That's a bit of a marketing ploy by Decware... Don't forget the OPT: it's an awful lot of curled up wire between your speakers and output valves. Anyway, if you had a direct coupled design you could get rid of that "one cap and one resistor"!
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Old 12th December 2004, 10:49 AM   #20
icebear is offline icebear  Norway
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Default Re: Re: Have buildt the Decaware

Quote:
Originally posted by audiousername
Nice "Mulligan", Bjorn.



Your OPTs are much larger than those in the original! I'd imagine they'd have cost a fair bit. (In the original, they are small enough to fit in under the chassis at the front)
]
I know. It's made by Novarro (italian). Most likely not the best around, but ok. Cost 60 euros a pice.

Quote:

That's a bit of a marketing ploy by Decware... Don't forget the OPT: it's an awful lot of curled up wire between your speakers and output valves. Anyway, if you had a direct coupled design you could get rid of that "one cap and one resistor"!


Yes. I'm aware. But still, if you look into most of the amps around, it's fully packed by a lot of components. If you can say that an upgrade to black gate caps makes a difference, how aboute eliminating it?

I have been using an OTL before the Zen, (Trancendent T16) and I think the sound is somewhat similar. The clear grainfree and dynamic presentation. It is defenatly better than my Mullard 5-20, which got a nice tone but not as good 3D and not as clear presentation.

BjÝrn
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