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Old 21st April 2011, 11:46 AM   #11
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Default A rude awakening.

So I got a little bit of a rude awakening last night. I tried to get a quote from a transformer winder and the response I got was (basically) "You don't know what you're talking about; figure out what you really need and get back to me. Oh, and this design sucks." Needless to say, I probably won't be buying anything from that particular vendor again, but it did push me a little to figure out a little bit more about what's going on here so I'm not just painting by numbers. So I hope some of you that know a lot more than me will go easy on a newbie and walk me through this a little.

Now, before anyone says, go read Morgan Jones or Pete Millet's site; got it, thanks. I've spent several hours on the Millet site (great resource) but most of the material there is over my head. I spent an hour with a buddy last night that is an "electronics guy" and he was able to walk me through some of the basics of a circuit, but he's not really an "audio guy" so he wasn't quite able to explain to me many of the nuances. So, I'm still wanting to build this circuit; I think it can be done cheaply and I think it will be a good learning experience, but I'd really appreciate some explanations of the "why" pieces, just keep it at a "Dummies" level.

First up, Power supply:
My buddy says the 47uF cap and the choke are basically filters to clean up the power. Correct? I'm assuming this 47uF cap is the one that should be increased if I am going with a smaller choke. Any danger in too large of a cap? Would a motor run cap work here? Do these two devices do anything other than filter?

Next, the other two caps in the PS; my buddy says, more filtering. One is big (220uF) and appears to be polar/electrolytic and the other is small (.47uF) and appears to be non-polar. Is this just filtering? Is this the "just bypass the big electrolytic with a small film/oil cap" thing? I've seen the "bypass" term used often, but never really understood it.

Finally, after going through the above, the power is clean/filtered (hopefully) and ready for the signal portion of the circuit. Correct? Did I miss anything important?

Signal circuit:
First the signal comes in at the "in" place and hits two resistors 220Ohm and 1MOhm wired in parallel; the big one goes to ground, the smaller one goes to the grid (I believe) of the driver tube. Correct? What does these two resistors do? What are their "audio names?" Bleeder resistor is something I've seen pretty regularly.

Once the signal hits the driver tube, electrons are shot from the cathode (bottom part of the tube in this schematic), through the grid, to the anode before the signal heads out. First stop, the .22uF cap. Iím assuming this is the ďcoupling capĒ that everyone says is the most critical one and that you should spend the money on. This is a non polar cap. Now, Iím not sure on the next part. Iím assuming the signal, i.e. the music for a dummie like me, heads on to where it hits the next two resistors; the 220 Ohm one connected to the EL84 tube and the 470K one going to ground. What do these two do? What is there special audio name? How does one arrive at those values?

Now, what really screws me up is the rest. Going back to the driver tube; Iím not sure what that 250 Ohm resistor that is connected to the cathode does or what itís called. It connects to ground, thatís all Iíve got. Also, on the ďtopĒ side of the driver, Iím not sure what that 100K resistor does or what its called, except that my friendly vendor said that was a dead giveaway for my crappy circuit. AnywayÖ Iím also not sure on that 22K resistor, what it does or is called. What is ďitĒ thatís flowing through those two resistors and why?

Continuing ďupĒ, the 10uF cap is that one that was mentioned in the previous post as something that could be bypassed with a better quality, smaller cap. Does that mean just running a smaller cap in parallel to that one and then on to ground? Also, why is one side of the cap symbol shaded and one not? After that cap is the 10K resistor, what does that one do and whatís its name? I see that it connects to where the B+ comes ďinĒ and the primary of the OT.

Going back to the EL84, that 220 Ohm resistor appears to be connected to the top row of dots (I believe that is the suppressor?) and the bottom row of dots (the screen?) and the cathode; correct? The 100K resistor is connected to the anode of the EL84 with the primary of the OT in parallel; correct? The 2.2K resistor appears to connect to the grid of the EL84 and the other ďendĒ of the OT primary; what does it do? Whatís it called? Last but not least, the 270 Ohm resistor and the 47uF polar cap coming off the cathode of the EL84. I have no idea on these ones; in fact, Iím having a hard time figuring out if they, and that 220 Ohm resistor are all connected in parallel to the cathode.

Now, my buddy tells me pretty much all the caps and resistors that connect to the ground are pretty much there for filtering; is he right? Also, another thing that got me in trouble with my vendor, how do you figure out that mA rating for your transformers?

Lots of questions I know; Iím sorry there are so many, but I am trying to learn. Iím currently in the middle of an all expenses paid trip to an exotic desert local, courtesy of Uncle Sam, so my resources are somewhat limited. Iím hoping to tackle this project once I get home and Iíd really like to understand it a little better. Thanks in advance for your patience.
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Last edited by bigjppop; 21st April 2011 at 11:48 AM.
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Old 21st April 2011, 12:09 PM   #12
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The transformer I am using is

Primary :
0-115V-230V


Secondary :
1) 300V-0V-300V @ 125mA
2) 0-5V @ 4A
3) 0-12V @ 3A

The first cap after the rectifier is limited by the maximum that can be used for the rectifier tube. If you choose to use solid state rectification, then you can basically use an "unlimited" amount capacitance :-)

The limiting factor for the choke would be the maximum current supported. Once you have exceeded the current rating, it will go into saturation and will not work as well. For the RH84, I run my output tubes really hot at 50mA ( I am using mil-spec Russian tubes) and the 12AT7 at around 15mA, giving a total of 115mA.

You are right in saying all the caps connected to ground are for "smoothing", all except one, the 47uF 63V one at the cathode of EL84. It is used to provide a low impedance path for AC and to increase gain of the output tube. The larger and better the cap is the better the bass response will be. Bypassing the caps means placing a smaller value cap but of higher quality in parallel with the larger caps. I use a combination of motor caps and electrolytics for power smoothing. I also include a small value cap around 400pf to filter out RF hash.

The signal cap should be a high quality one. Any polypropylene would fit in nicely, choose your poison. Make sure its voltage rating exceed the B+ by a factor of at least 35-50%.
The other critical cap would be the one at the cathode of the EL84. A good one to use there would be a ELNA Cerafine. I use 100uF 63V there.

Hope this helps.

Last edited by quikie22; 21st April 2011 at 12:28 PM.
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Old 21st April 2011, 12:22 PM   #13
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Now for the signal Circuit:

22K ohm resistors = Plate load resistors

100k ohm resistor = local feedback resistor @ Plate-plate feedback @ Schade Feedback loop

250 ohm = cathode resistors to provide biasing for the tube.

220 ohm to EL84 = grid stopper to prevent oscillation and control grid current

470k ohm = grid leak resistors

Your theory re tube function needs a little correction.

The cathode is heated by the filaments till electrons are boiled off it and form a cloud of electrons around the cathode. Due to the + voltage exerted at the Anode/plate, these electrons are drawn to the plate continuously. (basically how a rectifier works).

In a triode, there is a control grid G1 placed in between the anode and cathode. If the grid is not charged, the electrons flow past the grid easily and directly to the plate. However, bias the grid appropriately, then apply a varying waveform (ie music) to the grid, the charge on the grid changes and the electrons are repelled or allowed to pass through accordingly to the plate. Technically this is how the signal is amplified.

Pentodes and tetrodes have extra grids for other functions eg screen grid.
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Old 21st April 2011, 11:40 PM   #14
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Thanks bigjpopp for asking something I (and probably a lot of other noobs on this forum) always wanted to know, and quikie for answering and sharing knowledge. I made the mistake of ordering a polypropylene for the 47uF at the EL84 cathode and when it arrived it was as big as the output transformer. Sounds great, but need to replace that with an electrolytic if it's going to go in the chassis!

Returning to the question of bypassing the 10uf electrolytic to ground, how do you work out the value of the smaller high quality bypass cap? I'm now trying out the other circuit on the website, the RH807 and he's got a 33uF there (anyone tried that circuit, by the way?). You didn't mention the 1M resistor - I've seen this sometimes hardwired onto the RCA in to 'clean up' the signal, would this be the same thing?

As for the power supply filtering, after the choke is it just the more the merrier? I've seen giant electrolytics in old amps as big as beer cans.

And finally, my B+ is running a little high - do I just put a big wirewound in series on the DC or is there a more elegant way of dropping this a little (by about 30V)?
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Old 22nd April 2011, 05:42 AM   #15
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Quickie22,

I REALLY appreciate the walk through; things are starting to "gel" in my understanding of this project.

I do have a question regarding the heater wiring for this project; I've read that you want to twist your heater wires and that you want to run them away from something, but I can't remember what that something was. Also, for wiring the 6.3V heaters on the EL84 and the 12AT7, do you just wire all three tubes in series? Heater wire comes from the transformer then connects to ______ then to _______ then to________? Any tips of the trade on this?

I also want to second carbondated's question; how do you decide on the size of your "bypass" cap?
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Old 22nd April 2011, 09:43 AM   #16
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Quote:
Originally Posted by carbondated View Post

1) need to replace that with an electrolytic if it's going to go in the chassis!

2) how do you work out the value of the smaller high quality bypass cap?

3) You didn't mention the 1M resistor

4) As for the power supply filtering, after the choke is it just the more the merrier? I've seen giant electrolytics in old amps as big as beer cans.

5) And finally, my B+ is running a little high
1) Yes, a good cap is important. The polyprop 47uF is probably superb, but you need to choose an appropriate sized one for your chassis. There are a ton of decent caps out there. As I said, choose your poison and correlate to your needs/requirements.

2) an often quoted ballpark value would be anything from 100 to ten times smaller than the main capacitor. I usually use whatever small value I have eg 0.1uf-0.47uF

3) the 1M = grid leak resistor. Also helps reduce "pops" when plugging in the source cables if the amp happens to be on (but I usually turn off my amp before meddling with the wires)

4) get PSUD2 from duncanamps and simulate your power suppply with whatever values you want and look at the warnings. then decide what you need. PSUD2

5) are you using a tube rectifier? If yes, choose one with a higher forward drop voltage. Otherwise, if you are using the cap input filtering (CLC) in the power supply, then you may consider using choke input (LC) filtering instead.
CLC filtering gives RMS VACx1.3 (theoretical including losses). LC filtering will give around VACx0.9
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Old 22nd April 2011, 09:51 AM   #17
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Quote:
Originally Posted by bigjppop View Post
1) I do have a question regarding the heater wiring for this project; I've read that you want to twist your heater wires and that you want to run them away from something, but I can't remember what that something was.

2) Also, for wiring the 6.3V heaters on the EL84 and the 12AT7, do you just wire all three tubes in series? Heater wire comes from the transformer then connects to ______ then to _______ then to________? Any tips of the trade on this?

3) I also want to second carbondated's question; how do you decide on the size of your "bypass" cap?
1) Tightly twisting the outgoing and returning wire will cause whatever magnetic field induced by the current flow to cancel each other out as much as possible. You also want to run them as far away as possible form the "signal" part of the circuit eg signal wires, input terminals etc...

2) I am using 12.6VAC. I run a pair of wires direct to the 12AT7. I run another pair to the EL84 but I run these in series through both tubes (dividing the voltage to 6.3V per tube assuming equal current through both filaments). Seems to work fine for my system.

3)Refer my post above.
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Old 22nd April 2011, 10:58 AM   #18
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Back to the original topic... I've often heard it said that a pentode makes for a better driver when using this kind of plate to grid feedback. My understanding is this type of feedback strategy makes for a very low input impedance at the output stage, which is difficult to drive with a triode.

I'd be very interested to see some proposed modifications to the RH84 schematic. I believe a 6AU6 has been suggested, but I'd love to see some actual component values around it.
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Old 22nd April 2011, 11:55 AM   #19
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Thanks, quikie22! Ty_Bower, here's the link to Kitic's own modification -- 807, KT66 or 6L6 running at 350 or higher (up to 500!) volts B+.
RH 807 - Tube Audio ...... RH DESIGN
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Old 22nd April 2011, 11:58 AM   #20
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Quote:
Originally Posted by carbondated View Post
Ty_Bower, here's the link to Kitic's own modification -- 807, KT66 or 6L6
Thank you for the link, but Alex's schematic still shows a 12AT7 for the driver. I am looking for a recommended solution using a pentode driver.
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