That came out really nice, did you build and paint the chassis too?
Finished my second "PriestyAMP" project, a single ended amp based on a Blackface Champ aa764. It has the following design features I incorporated:
- Adjustable Cathode Bias
- Fan Cooling
- Foot-switchable "Turbo Knob" feedback lift boost
- Sounds best for Rock and Blueswith American sounding 6V6 (JJ 6V6s operating on its limits) or British sounding EL34 (Groove Tubes)
- Preamp tube Groove tubes ECC83S (Great for Blues to Metal depending on volume setting) or 12AU7 for Jazz/Country (with 6V6).
- Brutally distorted with ECC83S/KT88 combination
- "Turbo Knob" gives noticable bite and mid-boost when engaged
I've probably made tons of errors, which will no doubt be pointed out forthwith by the experts, but I think the Amp plays good, so I'm putting this out there to help any other people looking to try a single ended amp with more than 5 watts.
Last edited by priesty; 06-06-2010 at 11:07 PM.
That came out really nice, did you build and paint the chassis too?
It looks like you started with a hammond chassis and painted it and lettered it yourself. The paint and the lettering looks very good.
I have a few comments and questions for you.
1. I like your schematic. Its well drawn, clear to read, and the objects are well proportioned in size. Can you tell me what software you used to draw it?
2. I like the footswitchable NFB loop. Do you get any pops when you use the footswitch?
3. Since you're using such a wide variety of tubes, I was wondering if you've given any thought to using a selectable switch for the primary OT impedance.
4. The lettering looks nice. Am I correct in assuming taht its a peel and stick sheet that you put through a laser printer? I'm interested in hearing about it.
Looks like a nice build. Congrats!
1. Schematic drawn at great length on MS Word. If you make a few standard components and copy and paste, you can make it look good, but it does take time!!
2. The foot-switchable NFB boost has no pops at all, I put this down to the fact the stock NFB resistor (2700ohm) is always in series with the switch. There is no "opening" of the NFB loop, just switching in the high value pot. The foot switch is actually closed for the non-boost option, shorting out the "Turbo knob". With no footswitch, the "Turbo knob" is active for use for say, practice or recording. Important to use a stereo insulated jack for this!
3. Impedance - the Hammond 125FSE has multiple tappings on the seondary, I use a 3 position marshall style impendnace selector to alter impedance as required.
4. As you noticed, used a Hammond blank chassis and cage, cut holes to suit. It was powder coated by a friend who does it professionally. Lettering was waterslide decals printed on laser printer, like old Airfix kits!
Thanks again for your comments!!!
Last edited by priesty; 06-08-2010 at 11:18 PM.
Great looking amp! Thanks for all the good ideas.
thanks for sharing that with us. I did not know that there are more elastic types of powder coating out there.
Having just given up using MS Word for texts and after twenty-five years of using CAD and I can tell you that I am totally amazed that you drew that schematic on MS Word. Hats off to you Sir!
"Airfix" decals = Lasertrans, am I right?
Inkjet Papers and Fabrics, Window Stickers and Other Arts and Craft Materials
Glad to share my "research", especially on this forum seeing I got lots of help from here with my first amp!
priesty, very kind of you sharing all this with us. not to mention your build, superb.
the decal paper tip just gave one of those eureka moments...i've been using silk screening and it's such a cumbersome process.
do you or anyone else know if one can safely apply automotive polyurethane varnish on top of the decal to protect it? or will it destroy the decal?
Very cool looking amp there priesty 8-)
Building a better world (one tube amp at a time)
I have used "CLOU" nitro-cellulose, which is intended for joiners and cabinet makers in Germany, over "Lasertrans" decals on a guitar headstock with acceptable results ...well it was perfect for the low-tech, "warts and all" feel that I was going for!
It was before I had an air compressor, so I used a brush with some care. It did smudge a little in places where I became careless. It has been fine sitting on there a year now and has seen some temperature change abuse too.
The edges of these kind of decals will show up in certain light conditions, so the brush was really useful to build up a few layers around the decal first helping to feather-the-edge-in a little. You will still see the edge if you are looking for it.
Take a look at the images to see if the smudging is acceptable for you and bear in mind that an air gun should give a much better result.
That's a fantastic looking amp Priesty !! And does it sound like you hoped for as well ?
I'm getting very curious to hear what it actually sounds like. Could you do a clip ?
I'm planning to do a SE amp myself, I have already all the hardware including a Heyboer OT but haven't got the time right now.
Hurricane - I havn't been able to borrow a scope to check output, but it was so loud when trying it through a Marshall 4x10" cab that you couldn't be heard when shouting to the guitarist! Amp runs with B+ of 440V at anode load of 5K and withEL34, about 30mA idle current.
Looking forward to it .I will get a clip next time I visit him.
I like the adjustable cathode bias , i bet its sweet , you've put your own stamp on the looks too , well done mate .
BTW , I put the hammond 125fse in a 600 champ at the end of changing to a weber alnico speaker and various cap make tries and changes . That OT really opened the whole amp up , its nice isn't it ?
My father said " don't let the world change who you are" i was surprised as i felt it was probably a very good idea
priesty, one question please, if I may, about the decal work.
The decal paper is opaque on the back. How do you align the decal over the chassis so it's perfectly aligned horizontally and with the pots, etc? If you don't mind sharing that little secret, of course.
Thanks for sharing, overtone. I'll try some experiments, hopefully soon if my day job allows me, and will share the results as well.
Hi priesty, you can slide them around after you've removed the backing paper? I've never used decals and would like to try it, I'm sorry for the newbie questions.
Thanks for all the useful info and great pics - really interesting.
Could I ask where you sourced the 1K 3W bias pot? I have been looking around and all I see are very expensive ($30 and up) choices.
I like how you made the bias meter switchable, too.
I have a couple of comments.. I like the fact you put your fuses on the A/C primary.. If you could find a power switch that was double-pole for both A/C hot and A/C return, that would be spot-on for handling primary power.. Otherwise, looks and sounds very good....
As opposed to putting them on top of the cabinet or in the volume pot wiper circuit?? Or what?
OF COURSE at least one fuse goes in the AC power primary. It's a matter of safe AC power wiring to always use a primary fuse, unless you have tested and type-certified the power transformer to have limited fault currents and heating. Not putting at least one fuse in the primary is a huge safety/fire hazard.
Saying "I like the fact that you put your fuses on the A/C primary." is like saying " I like the way that you keep your rifle muzzle pointed away from other people and yourself." This is one of those little IQ tests that Mother Nature sets out for us. The punishment for failing the test is often removal from the gene pool.
Two fuses is more problematical. One of these primary fuses will always blow before the other, as there is no good source I can think of for matched fuses. So if and when one blows, you now have to examine both to find out which one blew, and to find out of the second one was damaged by the overcurrent. If you have a line/neutral power line setup, the fuses always go in the hot/line side of the AC power.
If one is going to put in more than one fuse, it would be smart to put one fuse in each secondary winding. This protects the power transformer, the single costliest item in most amps, from secondary overloads, as no primary fuse can do. Don't you agree?
Yes, breaking both line and neutral is a good idea for any power setup, but it's practically mandatory where the AC power line has only floating lines on both side, neither being a neutral. If you have only two wire power, a blown fuse can still leave the amp insides hot to the remaining power wire. It's important to know WHY you do things too.If you could find a power switch that was double-pole for both A/C hot and A/C return, that would be spot-on for handling primary power.
Looks very nice, and although the microphone clipped a lot, you can hear that it's a good-sounding amp.
I couldn't help but notice the 1M grid resistor on the output tube. While cathode bias is more forgiving than fixed bias, that's a lot higher than the 220k recommended for KT88, or the 500k recommended for 6L6/KT66. It might be a good idea to reduce that value in case you encounter an unstable or gassy new-production tube.
Thanks for your comments, nice to get some feedback.
If I can clear up my rationale for the Fuses and Mains Switching:
Fuses: To save space on the build, I used an IEC mains socket with integral fuses on live and Neutral. These are located in a fuser "Drawer" included in the socket body. SCHURTER 4301.0501 from Farnell.
Switch: I don't know about other countries, but in the UK supply feed Neutral is tied to Earth and is always identifiable from Live. Morgan Jones book "Building Valve Amplifiers" he does mention that two pole mains switch could be fitted across Live and Neutral, but draws atttention that in such an arrangement, the shorting of the live contact due to mechanical failure would appear to be OK, as item would be still switched on and off by the Neutral half of the switch, but in both "Off" and "On" states the Live supply is still present. Morgan Jones reccomended switching the live circuit only as this was the safest and most reliable if the live/neutral polarity is known.
I think that says more about you and possibly your colleagues (look it up: colleague not colleges) than it does about the electronics.
OK. Shall we delve into this technically again? I did the math and pulled up the tech references for you. But I can do it again.
Did you not get that primary fuses are for protecting (a) the box and electrical system against fires and electrical safety issues and (b) only secondarily protecting the power transformer?. And did you not get that fuses **also** on the secondaries are to protect the power transformer? These are two different purposes, as I explained to you at some length.
The size of the short circuit current on one winding versus the other is not an issue. The objectives in the use of the fuses is an issue. You really do need to be able to embrace more than one idea at a time, Gary.
Did your colleges [sic] think about both uses of fuses? I'm guessing that if your colleges [sic] are as good as you think they are, they'd be able to follow the two different uses. And how to size them. If you presented the issue that way, and not in a way that begged the question.
Do you need me to copy the write up from the earlier set of posts that you just decide not to talk about any more. I can do that if that would help you understand.
Or would you rather not talk about this issue at all?
Yes, it's that way in the USA as well. However, we also have to worry about the electricians that wired the buildings getting live and neutral mixed up in the sockets, and so I always treat live and neutral as though they may be flipflopped in power switching. I recommend breaking both of them with the power switch. I think your decision to do so is good practice.Switch: I don't know about other countries, but in the UK supply feed Neutral is tied to Earth and is always identifiable from Live.
Yeah, I'd agree with that - with the caveat that "if the live/neutral polarity is known" is a big issue. I have come to not trust electrical power wiring. In my experience, about 5-10% of new construction outlets are wired with line and neutral switched. I carry one of the plug-in outlet testers with me for just this issue when I'm setting up. I highly recommend it for everyone.Morgan Jones book "Building Valve Amplifiers" he does mention that two pole mains switch could be fitted across Live and Neutral, but draws atttention that in such an arrangement, the shorting of the live contact due to mechanical failure would appear to be OK, as item would be still switched on and off by the Neutral half of the switch, but in both "Off" and "On" states the Live supply is still present. Morgan Jones reccomended switching the live circuit only as this was the safest and most reliable if the live/neutral polarity is known.
Also, the two-pole switch failing with one side shorted is possible, although I think that one pole fail open is more likely. That is just my opinion and experience and is not by any means definitive.
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