Selmer Little Giant MKIII Clone Build (couple questions)

[Bigyouth]

Hello,

I've just finished my first amp build. A clone of the Selmer Little Giant MKIII. It is working pretty well, it is certainly loud, and plays very similar to the originals I have tried. I'm yet to house it, but here are a couple pics of the build.

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1822.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1824.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1825.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1826.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1836.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1839.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1872.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1873.jpg[/img]

[IMG768]http://i163.photobucket.com/albums/t294/Edd6464/IMG_1876.jpg[/img]


Here is the schematic:

[IMG650]http://i163.photobucket.com/albums/t294/Edd6464/LittleGiantMK3.jpeg[/img]


One thing I am worried about is the voltage rating of (what I assume is) the 25uf cathode bypass cap on the single EL84 power tube going from Pin 3 to ground. The rating of that cap is 450v. Are they not usually something like 25v?

I noticed that at full out volume there was some slight cutting out when doing second-fret hammer-ons. After first testing the amp, which lasted about an hour, when I came to turn it on again later, the output was much much lower, and breakup occured far too early. After replacing the tubes one-by-one it was the EL84 that was at fault. Replaced it with another of the NOS Genalex GL tubes and it was back to normal. I haven't had the chance to play the amp since then, but I guess that it could have just been a bad tube, but I'm a little skeptical. Also, whilst I know that power valves are supposed to run the hottest, it did seem to be hotter than it should be.

I'm not sure what else could cause this. The filter caps are 400v each. I could only find a 120r resistor for the 130r resistor on pin 3 of the power valve. And for the 5.6k resistor on the filter caps I have a 4.7k resistor. Might those be an issue?

thanks,
Edward

[xtian]

Nice job! I see you even bent your own chassis. Very nice.

Voltage specs on caps are a do-not-exceed voltage. So it's no trouble to have a 400v cap where the schematic specs lower voltage, for example on your power tube cathode, and as you noted, it only gets around 25 volts.

Yes indeed, you're probably running your power tube too hot. This is an old schematic, made when wall voltage was only 110vac...wait are you in U.K.?

Anyway, measure your plate voltage on the EL84, pin 7, and the cathode voltage across the cathode resistor (you said you're using 120R) and let us know. I think you'll need a higher value for cathode resistor, like 180R or even 240R.

[Bigyouth]

Nice job! I see you even bent your own chassis. Very nice.

Voltage specs on caps are a do-not-exceed voltage. So it's no trouble to have a 400v cap where the schematic specs lower voltage, for example on your power tube cathode, and as you noted, it only gets around 25 volts.

Yes indeed, you're probably running your power tube too hot. This is an old schematic, made when wall voltage was only 110vac...wait are you in U.K.?

Anyway, measure your plate voltage on the EL84, pin 7, and the cathode voltage across the cathode resistor (you said you're using 120R) and let us know. I think you'll need a higher value for cathode resistor, like 180R or even 240R.

Thanks very much. It is about as crude as a chassis could be, 1mm aluminium sheet cut with tin sips and bent with shelving wood. Fortunately I have outsourced the cabinet'; not quite up to building one myself yet.

I am in the UK yeah, where the wall voltage is typically 230v.

Thanks for the advice. Unfortunately I won't have a chance to test the amp until next weekend.

[Leo_Gnardo]

Good work indeed! Now I'm interested, gonna have to build one of these "British Champs".

EL84 cathode resistor and plate current thoughts. The typical Rk for a PAIR of EL84 was 130R. It's possible you're subjecting your output tube to an early burn out with sky high plate current. I'd start with 300R, then add or subtract a little until you have a plate dissipation @ 11-12 watts. The bypass cap would be probably enough at 25V and with plenty of safety margin 50V. Doesn't hurt to have 400V but it's way overkill for this function.

I hope that's not authentic NOS EL84 you're sacrificing there. The current EL84 marketed as Genelex/Gold Lion by New Sensor is supposed to be a darn good tube but New Sensor as always cashes in on the "name brand" they bought. I'm sure you can do just as well far cheaper with JJ or Sovtek EL84's.

Another mystery of the Selmer schemo is - where's the filter cap for the 3rd hi voltage node? I'd expect to see maybe 8 or 10 uF at the junction of the 100K and two 270K resistors. Without this filter, there are going to be unexpected signals passing from one plate to t'other, possibly setting up a nice ultrasonic feedback loop. Just for fun, clip a filter cap there & have a listen. Maybe enjoy some more output tube longevity too.

[Bigyouth]

Good work indeed! Now I'm interested, gonna have to build one of these "British Champs".

EL84 cathode resistor and plate current thoughts. The typical Rk for a PAIR of EL84 was 130R. It's possible you're subjecting your output tube to an early burn out with sky high plate current. I'd start with 300R, then add or subtract a little until you have a plate dissipation @ 11-12 watts. The bypass cap would be probably enough at 25V and with plenty of safety margin 50V. Doesn't hurt to have 400V but it's way overkill for this function.

I hope that's not authentic NOS EL84 you're sacrificing there. The current EL84 marketed as Genelex/Gold Lion by New Sensor is supposed to be a darn good tube but New Sensor as always cashes in on the "name brand" they bought. I'm sure you can do just as well far cheaper with JJ or Sovtek EL84's.

Another mystery of the Selmer schemo is - where's the filter cap for the 3rd hi voltage node? I'd expect to see maybe 8 or 10 uF at the junction of the 100K and two 270K resistors. Without this filter, there are going to be unexpected signals passing from one plate to t'other, possibly setting up a nice ultrasonic feedback loop. Just for fun, clip a filter cap there & have a listen. Maybe enjoy some more output tube longevity too.

thanks for the suggestions. Very much looking forward to next weekend when I get a chance to do this, I usually spend the weekdays collecting parts.

They're not the originals I don't think no, but they are all I have on me at the moment; they're not cheap I agree. Perhaps I'll try and find some JJ's or sovteks.

Would that be something like a 100v cap for the filter? And is that to go between the 270k and the 100k resistors?

[Leo_Gnardo]

They're not the originals I don't think no, but they are all I have on me at the moment; they're not cheap I agree. Perhaps I'll try and find some JJ's or sovteks.

Would that be something like a 100v cap for the filter? And is that to go between the 270k and the 100k resistors?

I wasn't even sure Genelex made EL84 back in the day. They sure made terrific KT66, 77, & 88.

I'd use a 400V cap at the junction of those 100K and 270K caps just to be safe. If you remove the preamp tube with power on, the voltage there will rise to nearly the main B+ level. A 100V cap would be toast.

Whilst you're shopping pick up a selection of values for your cathode resistor. Selecting the proper one will be a cut & try affair.

Where are you in the UK? I used to go wandering up Edgeware Road in London, mid 80's for electronic goodies. "Smith's" was one of the best places. They had EVERYTHING. Long gone. I should have loaded up on old UK tubes, oops valves, when I had the chance. Oh well. Maybe the Green Man is still there. Good ol' London pub.

[Bigyouth]

I'm in London. Most of my parts I have gotten from barry at ampmaker, (high quality and affordable), but for the odd bit, I tend to visit the techs on Denmark street.

Most of my tubes are actually from Hong Kong, I look forward to being back there come the summer, it really is electronic heaven.

I've never seen an electronics shop on Edgware Road; probably past my generation, but the Green Man is certainly still there, great pub.

[Leo_Gnardo]

Denmark Street - I could find my way there blindfolded. For those who don't know, this is a short street just west of central London, jam packed with music stores. Sort of the way that block of 48th St just east of Times Square in NY used to be. Rose-Morris in London holds top position the way Manny's used to in NY. Long time ago Rod Argent opened his own store - keyboard oriented - smart thing to do since it's tough to make a living in the music biz. Guess what Rod, it's tough to make it in the music store biz too. Hope he's done all right.

Last time I looked on Edgeware, there was a Radio Shack, and a couple of stores that catered to the disco-DJ with glitter lights and subwoofers and little else. Just a little south of the old electronics area, and around the corner, right by the Marylebone Flyover was a funky little shop that had old tube hifi & PA gear of allsorts.

Haven't been there (yet) but Hong Kong is the spot. Lots of dealers and besides the shops, they're flogging everything on Ebay. I have been to some of the shops in Tokyo, jammed with all our favorite goodies. Plus the Akahibara - "If we don't have it, you don't need it" - every component imaginable. North Korea sends their agents there to buy parts for military experiments. Haven't they heard of Mouser? Just order online - it's fast and easy!

[Bigyouth]

Finally at that time of the week. I can now start testing voltages and I'll post my findings shortly.

I'll start with sorting out the cathode resistance, but for the 10uf filter cap, is it supposed to go in between the 270k resistors and the 100k resistor, what about the polarity? Or does the positive end go between the resistors and the negative run to circuit ground?


thanks,
Edward

[Bigyouth]

Okay, I've done some measurements and here are my calculations.

Plate voltage = 285v (pin 7)
Cathode voltage = 8.09v (pin 3)

Actual plate voltage = 285 - 8.09 = 276.91v

8.09/120 = 0.06741

Plate dissipation = 276.91 x 0.06741 = 18.666v

That is certainly too high.

If I put a 100r 5w cement resistor in series with the 120r then I get:

8.09/220 = 0.0367

276.91 x 0.0367 = 10.16w

With 200r I would get 11.2w which seems more suitable since it is cathode biased.

[Leo_Gnardo]

for the 10uf filter cap, is it supposed to go in between the 270k resistors and the 100k resistor, what about the polarity? Or does the positive end go between the resistors and the negative run to circuit ground?

Yes. + to resistor junction and - to ground or it will turn into a little smoke machine. I can't understand how that amp could have run without it. Who knows, Selmer may have left it off the schematic on purpose just to flummox us? Or another misteak?

Good going on the Rk selection. 200R sounds like it's just the ticket.

One of my customers just stopped in, he's selling off his big old amps & starting to buy 15W and under, in an effort to save his ears. Might be a copy of one of these Selmers in his future.

[martin manning]

Remember you are calculating cathode current, ~10% of which is from the screen. Adjust the 12W limit up 10%, and then say you want to be ~90-100% of that so 12-13W using cathode current is ok.

[Skev]

Hello BigYouth and others members. Sorry to disturb You.I saw the subject a month ago, i ve just built a clone too, mine works quite well but many details to correct. I saw the cathode resistor should be rised to 200r, did you try that? About mine, i will change the volume log pot for a linear one and probably try this new cathode resistor mod because the el84 is too powerful and hot to be ...Honnest !! Lol.
Strangely my amp only works when the whole head is upside down. I will check my connections . In normal position the tube don t even glow, maybe problems with sockets).
Excepts those details, this toy sounds pretty good i plugged it in a 12" 8ohm speaker ( since a change the outpout transformer in order to have a 2,5 ohm output and a 8ohm output).
Thanks for reading


[/list]

[Stevem]

I would get some of whats called Fish paper and glue it down under those Y shaped soldered resistors for saftys sake!

[Skev]

Hi everybody. I m building a little giant clone too it seems that i ve solve mist of the problems by myself but this time i need help. I ve a ground loop causing 50hz ( eu frequency) presence in my el84 circuit.
I tried higher values for first reservoir cap just after the ez80 , it just atenuate a little but the hum is still very present. I try to isomate the more that i could this "reservoir" cap ground but no result. I even tried a resistor between the ground of the two 32uF but nothing happen.
Could you help me please?