Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on January 03, 2011, 08:49:58 am
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Hello everyone,
I am new to the forum so I apologize if I'm bringing up a previous topic but I searched the forum and found a few close idea's but I still have questions.
Im looking into a build of Da Geezer's Little Wing amp non-reverb and have a few questions about the iron used to build it. The original schematic say's it used the weber stand alone reverb power transformer and a 5y3. Now I could be wrong here but I thought a 5y3 needed 5V not 6.3V like the current spec sheet says the Weber transformer has? Another schematic I found has a 270-0-270 used with a 5R4 and another has the same with a 5y3. Both the center-tapped examples appeared to provide the same voltages and the original was a little lower.
My question about these different examples is about what iron can be used. I currently have a Hammond 272DX rated at 300-0-300 @144ma on hand and would like to know if with the right rectifier choice could get the same voltages or if I'd need to zener things down a bit.
I also have a 1750E on hand which should make the grade in the 6BM8 version of this amp im just wondering what choke should be used in this build.
Anyway those are my questions for now and as a new member I hope you guys can help out the new guy on this one. From what I've heard the Little Wing is a hell of a little amp and I look forward to giving it a try.
Thanks,
Neill
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The 5Y3 does, in fact, need 5 volts for the heaters. I don't know whether the weber xformer has 5 volt taps or not.
The voltages are not set in stone. 30 more volts is not going to catch anything on fire. The tone may change slightly (at 30 volts probably not enough to notice), but it will be fine.
Besides, I always like to make minor changes in other people's designs. It feels more like my own if I change something.
PRR always says that everything from the old days was built at 30 percent tolerance. In the 300 volt range, 30 volts comes out to about 10 percent, so you're doing good.
You are not going to find a 5 volt rectifier (readily available) that will drop your voltages any more than a 5Y3. It is the least efficient in its category.
Dave
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Sorry for the differing info, but we tend to throw out updates "on the fly", leaving the old schems in place & trying to note changes where we can. All of the schems worked OK, they are just "different" as we tweaked the circuit & shared our ideas.
I have attached what I believe to be the best of my versions (the one that was used for the sound clips, which had a GZ34 installed at the time, so the voltages would be a bit higher)
I used a separate 5v/3A tranny for the recto heaters (the one that Doug sells in his store)
The PT you have will work fine, although you may have to tweak the B+ rail resistors a bit to get things in line(?), or it may be just fine with a 5Y3 or 5U4.....the higher or lower voltages aren't "bad", just different resulting tones. Just don't get "too" high on the little 6BM8 plates. Staying in the ballpark of what EL84's like would be safe.
Choke is a standard small Fender, again, the one that Doug sells.
Feel free to ask questions....I (or others) will be glad to answer if I can.
Geezer
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So looks like all my answers got answered in that reply from Geezer. It seems to me that the Hammond I have might be a little much voltage wise. The other thing to note is it's rated for 115VAC from the wall and I get wall voltages higher than that so it would sit at like 315-0-315 and getting close to your voltages would probably require me to zener down the secondary. Thats no big deal though. Another question that pops up for me is about Musical Power Supplies 275-0-275 transformer. Those are rated 100mA and have both the 6.3V and 5V windings. I've used some of their transformers before with good results do you think that might be a decent match for a build like this?
Another thing i'd like to know about is the tubes selected for V1 and V2. I know the 6BM8's take care of the PI but are you using 12ax7's in the first two positions or something lower gain in V1? I specifically would like to know if a 12ay7 might be a decent match in V1 for one of these amps.
Thanks again for the help it's cool to see a forum with such great support.
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The 275v MPS PT would work great.
I commonly use 12AY7's in V1.....any variety would work in any position, just sub them in to your taste! :smiley:
G
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That sounds good to me. I have a few spare parts around and im trying to do things on the cheap as im still a starving student. In your power section could I use a 32x32 can cap using one before and the other after the choke? I also have a few 22 and 16uf caps around that I'd use for the rest of the filtering. Im just wondering how much of a difference those values might make if any. I know most components are rated at around 20%, 10% or 5% tolerances so I doubt my choice of values could make too much of a difference on things.
Just curious if the Idea of being frugal here might be faulty on my part.
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In your power section could I use a 32x32 can cap using one before and the other after the choke? I also have a few 22 and 16uf caps around that I'd use for the rest of the filtering. Im just wondering how much of a difference those values might make if any.
Those will be fine
Just curious if the Idea of being frugal here might be faulty on my part.
Nope, I do it all the time....go with what you've got!
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Cool that sounds good to me. One last question I think. If I use the 272DX PT I have would you say a few 15V Zeners @ 5W in series with the center tap would bring me closer to your noted voltages? Say like 3 or 4 of them. I know the 272dx gives me like 315-0-315 unloaded with my wall voltages.
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Hi Geezer,
Check your cap values for a typo for the bass and mid in the schem. Shouldn't you have the .02 (larger cap) going to the bass and the .0047 going to the mid locations?
ncusac, depending on your voltages keep you screens at or below 300v and use a suitable screen resistor as needed. If your plate voltages are on the high side, you likely will need a higher value of cathode resistor staying w/in the power dissipation of the 6bm8s. I ended up w/ a value of 680r keeping the tubes happy.
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Sounds like the zener idea would work then. I know they never reduce the B+ by exactly thier rating but i can fiddle before putting the tubes in until I like the B+. Also I was thinking another option might be to scrap the 6BM8's and use a pair of 6v6's. I know 6v6's like working with the 272DX and I could probably still get away with the 1750E with them if I keep the output similar to a tweed deluxe or princeton. Just means adding another tube for the PI but I have a number of tubes on hand and would have had to buy the 6bm8's anyway.
Let me know if that sounds doable or if it would be completely off the mark trying to make a Little Wing like this.
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I essentially built a 6V6 Little Wingish type amp for my son and called it a Texas Raptor. Cathode biased Marshall topology with paralleled V1.
The 6BM8 tube has signficantly more breakup than the 6V6 at lower volumes. So if you switch to 6V6, it will sound different (not necessarily worse but different). I have built and played both versions.
You could also do a 6BM8 version with VVR on the power amp and LTPI and keep the voltages down easily to a comfortable level. Dana Hall's VVR's are inexpensive and easy to install, IMO. I personally would do that before using zeners.
http://www.el34world.com/Forum/index.php?topic=6899.0
With respect, Tubenit
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That looks like a pretty cool appraoach. I see you seem to have included a sew tone/gain switches and is the last one a set prescence switch removing the NFB? Those little tweaks might be more of what im looking for too since I really dont know what im looking for and options let me find it. Do you think the 1750E transformet I have for the output will make the cut? that 1620 is a 20W versus the 15W of the 1750E. And the primary is lower too isn't it 6.6K?
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So I think a 6v6 version of the Little wing might be my most affordable option I just have one more inquiry.
How would you go about incorporating an effects loop into the little wing?
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Passive or active effects loop?
Did you try the SEARCH function and plug in "effects loop"?
with respect, Tubenit
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How would you go about incorporating an effects loop into the little wing?
Oops....Tubenit beat me to it, but I'll post my detail anyway......passive version
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Thanks for the info guys looks like what I have searched. What kind of differences might I get using an active effects loop?