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Old September 10th, 2009, 02:58 PM   #1 (permalink)
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5F2-A Question for a first build

I'm getting ready to start on a 5F2-A like build as a first build. I read here and elsewhere that a 5F2-A with a second 6V6 in parallel is an nice sounding, simple amp. I'm using a combination of Dave Hunter's book and the Weber layouts to develop my layout.

Hunter's Two-Stroke amp has the signal coming into the power tubes on pin 6 with a resistor going to pin 5 (I think it was 1.5K but I don't have the book in front of me) where Weber's layout goes straight in to the grid on pin 5. Which is better or why is this?

I also read that if I add a second 6V6 power tube that I should halve the value of the cathode follower resistor. Does this sound correct?

Here is the power portion of my version of the layout (changes from the 5F2-A in yellow) does this look correct? Am I even close?
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Old September 10th, 2009, 03:18 PM   #2 (permalink)
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Pin 6 has no connection on a 6V6, so it's just a clean way to wire the grid stopper on pin 5 of that tube. IMO it's a good idea, but some people say it's not necessary in a properly designed amp; this is what Randall Aiken has to say about it, and I think he's one of the most knowledgeable people around.
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Old September 10th, 2009, 05:33 PM   #3 (permalink)
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Thanks for the link. I knew pin six was used just as a solder point and now I'll definitely add in the 1.5K grid stoppers. Any info on the cathode follower resistor value?
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Old September 10th, 2009, 09:41 PM   #4 (permalink)
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Yes, halving the cathode resistor sounds right.

Here's a schematic posted on the Angela Instruments sight.

http://angela.com/angelasupersingle-...mpproject.aspx

I built a similar amp from a kit made by STF Electronics. A very simple and nice sounding amp.

Good Luck!

Ken
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Old September 10th, 2009, 09:51 PM   #5 (permalink)
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Looks good to me. What are you using for the output transformer?
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Old September 10th, 2009, 11:59 PM   #6 (permalink)
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Ken thanks for the link.

Scott, good question about the output transformer. I was planning to use the transformer that Weber uses in their 5F2-A kit which is a W022913 but then I was reading another thread on similar circuits. Bruce from Mission Amps was pretty insistent that this transformer is wrong for this amp. He said that this is a push-pull transformer and doesn't fit in this circuit. He strongly recommended the Hammond 125ESE. Weber sells another transformer called a WSE15 which is 15 watts and has a 5K primary impedance with 4, 8 and 16 Ohm output taps (like the Hammond).

I'm leaning towards the WSE15, what do you think?

Will I need to change the circuit if I change to an output transformer with a 5K primary instead of the 8K primary?
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Old September 11th, 2009, 12:27 AM   #7 (permalink)
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Since you have two output tubes in parallel, you need to halve the primary impedance of the OT. The tweed Champ (running the output tube in around the same conditions) had a primary impedance of around 5K, so I think if you used the WSE15, and hooked up an 8-ohm speaker to the 16-ohm tap (bringing down the primary impedance to 2.5K) that ought to work pretty well.

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Old September 11th, 2009, 12:50 AM   #8 (permalink)
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How do you know all of this? I feel so unprepared.

Thanks for the info though. That seems like a simple enough solution. I'll probably wire up both the 16 and 8 ohm taps for 8 and 4 ohm cabinets. As long as I select the right wires and label them on the back of the chassis I should be good.
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Old September 11th, 2009, 01:01 AM   #9 (permalink)
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I don't have a life, but I can stuff my head with a lot of info when the subject interests me.

There's a bunch of great info in the threads here, and on other tube audio forums. A lot of classic tube audio books are downloadable online...
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Old September 11th, 2009, 10:08 AM   #10 (permalink)
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So here is what I have now. Would I be crazy to think about adding a presence control? I like the effect that it has on an amp like a Bassman but I don't know how much impact it would have on an amp like this. It seems like all I would need is a pot and a cap of the right value and a little bit of wire. What do you think? Do you know what kind of values would be appropriate? I was thinking about a 5K pot and a 0.1uF (250v) cap.
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Old September 11th, 2009, 02:14 PM   #11 (permalink)
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That 5K and 0.1uF combo is a proven one for Presence. If you want to be really cool, you could change the feedback resistor to a pot and add a cap to get a Resonance (low end) control as well.

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Old September 11th, 2009, 02:24 PM   #12 (permalink)
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That sounds like a cool idea, what kinds of values would be required? I may have to rename this circuit the "Scope Creep". :o)

Do you know where I could find some recommendations for tube spacing and control pot and input spacing? Is it better to have the tubes closer together so that the wire runs are as short as possible or is it better to have a little space between certain components?

Here is the whole thing so far:
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Old September 14th, 2009, 12:16 AM   #13 (permalink)
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The "Scope Creep" -- I like it!

For the "resonance" control, the capacitor value depends on the value of your feedback resistor (22K here). How about switching to a 25K pot, and try out different caps between 0.1uF and 100pF? The idea is that the whole 25K is always in the circuit, but you wire one end of the cap to the wiper, so it bypasses more or less of the resistance as you turn the knob.

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Old September 14th, 2009, 04:14 PM   #14 (permalink)
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I'm still having a problem figuring out the OT input and output impedances with parallel power tubes from your earlier post.

One 6V6 tube has a load resistance of 8500 Ohms (according to ampage.org) so wouldn't two in parallel produce a resistance of half that value or 4250 Ohms? The impedance on the primary of the OT is rated for 5000 so do I still halve the impedance on the output tap? Am I looking at the wrong values?
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Old September 14th, 2009, 04:45 PM   #15 (permalink)
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Would a resonance control look something like this? This would be used in place of the 22K resistor on the board?
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Old September 14th, 2009, 04:46 PM   #16 (permalink)
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Well, the *ideal* load resistance depends on the screen voltage, and the examples in the tube manual aren't applicable to all designs.

When designing for max power, you want the load line to intersect the "knee" of the characterist curves, and that knee goes up as you raise the screen voltage, and goes down as you lower the screen voltage. When Fender went to fixed bias and higher screen voltages in the tweed era, the primary impedances went down ("steeper" load line) to compensate. The lowest screen voltage I see for the 6V6 examples is 285V, and my 5F2A runs about 300-310V.

All that boring junk said , you could try hooking them up to the 5K primary and not bother halving the speaker load. Might even sound good!

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Old September 14th, 2009, 04:47 PM   #17 (permalink)
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Quote:
Originally Posted by PeteG45 View Post
Would a resonance control look something like this? This would be used in place of the 22K resistor on the board?
Break the connection between the left and middle lugs, and yeah, should do it.
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Old September 15th, 2009, 12:39 AM   #18 (permalink)
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OK, getting ever closer. Here is my updated layout. Can you look at the NFB loop/resonance control and tell me if it looks right? I also changed the config of the grid stopper resistors on the power tubes, do these look correct? I think that I had them wrong in the previous layout that I reworked.
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Old September 15th, 2009, 01:05 AM   #19 (permalink)
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Looks good! I totally missed the grid stopper thing in the previous diagram, but you've got it right, now.

I think it'll be fun to dial in those cap values.

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Old September 15th, 2009, 09:01 AM   #20 (permalink)
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Yeah but I'm trying to come up with a way to make it easier to switch out the caps. I might start out with two small leads terminated with alligator clips so I can quickly change back and forth between the caps. Then, once I've settled on a value, I can go back and solder the cap into place.

The wire returning from the volume pot to pin 7 on V1 looks unusually long. Would it be better to route it under the board between the input grid stopper resistors and the cathode bias resistor? I'm not sure how this is all going to line up because I don't have the board yet and because this will be a head type chassis (with the tubes and transformers on top and the jacks on the back).
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