Late posting of NAD

[ivan H]

Ivan nice clean work. Let me throw you a bone.

Steve.

View attachment 14950

Thanks plexi. I followed this scheme with a couple of minor differences. The cathode resistor for the 2nd half of the 12AX7 is moved to beside the 1st half's cathode resistor & the feedback series resistor is moved to run from the output jack straight to the cathode lead's turret. The lead from the pot to tube socket follows exactly the same path as in the diagram, just above the board instead of below. For that lead I used a solid core, 18 gauge wire that stays where positioned. I know that you know that the capacitive coupling between that lead & the 3 components in close proximity to it is one area to "tune" for good tone. Cheers
Edit; the feedback series resistor value I'll experiment with, the 6V6's 220k grid reference resistor I'll change out for 270k. Thanks muchly. Cheers

 

[syscokid]

The two 68k resistors are "grid stopper" resistors (when the Hi input is used) & so are most effective when placed right at the socket. I know most amp manufactures have mounted them either on the jacks or the board, but, for stability of the tube they"re best mounted as close to the socket as possible, same as with power tubes. Cheers

Thanks. On my two Marshally amps that I've wired up, those resistors are right at tube socket. And, with shielded wiring, too. A couple of the many things that I've learned over at the Metroamp forum. I'm sure the gurus over there explained why, but I can't remember it.

 

[ivan H]

Thanks. On my two Marshally amps that I've wired up, those resistors are right at tube socket. And, with shielded wiring, too. A couple of the many things that I've learned over at the Metroamp forum. I'm sure the gurus over there explained why, but I can't remember it.

I intended using twin core shielded on this for those leads, but the local electronics outlet only had very thing, flimsy poor quality stuff that I didn't want to use. Its very quiet so I probably wont bother changing it. Cheers

 

[syscokid]

I just found some very good info at Aiken Amplification regarding the grid stopper resistors. And it's pretty much what you said.

FWIW, here's the link to that info:
Grid Resistors - Why Are They Used?

 

[ivan H]

I just found some very good info at Aiken Amplification regarding the grid stopper resistors. And it's pretty much what you said.

FWIW, here's the link to that info:
Grid Resistors - Why Are They Used?

The Aiken site is a wealth of good information. Another good site on the theory of tube amp design is the Valve Wizard site. For books on tube theory, The Radiotron Designers Handbook, by F Langford Smith (4th edition is best but any edition is good) is hard to beat & considered by many to be the tube "bible". The author, Fritz Langford Smith was an Aussie who did a lot of design work for military's way back in the day. Covers tube design of all manner, not just audio & is extremely in depth.
Oh, @jtcnj, the cabinet is also from Weber & the tolex is the old Fender "blonde" type. Mojotone offer a similar cab slightly cheaper, but it made sense to get both the speaker & cab together & save on shipping costs to Oz. The chassis I got locally. I'm not tooled up for cabinet making. Cheers
Edit: Should any member "be" tooled up for & have the necessary skill sets for cabinet making & tolexing, & the time & desire to take on such projects, for any future projects I would rather give my money to a fellow forumite than a business. I mean, shipping cost from the US is shipping cost regardless. Cheers

 

[syscokid]

I intended using twin core shielded on this for those leads, but the local electronics outlet only had very thing, flimsy poor quality stuff that I didn't want to use. Its very quiet so I probably wont bother changing it. Cheers

I was going to bring my Allen Amp Sweetspot to a jamfest 3 days ago. The same one from post #18 in this thread. Earlier in that day, I tested the amp to make sure everything is operating fine, and the darn thing had a massive hum. No time to figure it out , so I grabbed a different amp. Today I decided to take a look at it, and all it was, was a loose input jack. I pulled the chassis to properly tightened the jack. But.... while I had the chassis out , I decided to take the opportunity and your advise, and rewire the grid resistors in V1 like you shown on yours. I also had some extra 2 conductor with grounding shield cabling, and took advantage of that, too.

What you think?

 

[ivan H]

Well done. Very nice build too. Nice electronic component selection in there. Bet it sounds great. Cheers

 

[syscokid]

Well done. Very nice build too. Nice electronic component selection in there. Bet it sounds great. Cheers

Thank you... and it does sound great. Just picture a Fender Princeton Reverb ala the Blackface era, with a Mid control that can gradually bypass the tonestack for more Tweety mids and more gain. A throbbing tremolo circuit. 18 watts with the 6V6's, or 25 watts with 6L6's (and a plugin solid state rectifier to replace the tube rectifier).

 

[ivan H]

Oh yes, that would sound good. The mid control would be a nice addition, the BF amps having less prominent mids. Very nice indeed. Cheers

 

[chilipeppermaniac]

Well, new amp build actually.
So I had built a little Fender Champ-ish type amp in a head format about a year & half ago, but used larger transformers that allow other tubes (EL84, 6L6, EL34 etc) to be used. While a decent amp, the bigger output transformer didn't sound quite the same as a champs & more importantly, didn't react like the smaller champ OT. So, I decided it would be better to rebuild that amp into something like a Marshall (Slash model) SL5 (only using the EL34's full potential) & build up a "proper" Champ. This is the result.View attachment 14920 View attachment 14921 View attachment 14922
For this one I used a reproduction of the tiny, high load impedance output transformer as used in the very early 50's model tweed champs, seen hereView attachment 14923 (the chassis is chromed & there is a reflection showing on it). I have a couple of pics taken during the build;View attachment 14925 View attachment 14926 View attachment 14927
Board drilled & turreted. I used my own layout rather than follow Leo's.View attachment 14928
& a shot showing the guts. I also used a different grounding scheme than the originals. The only other deviations from the original tweed circuit is the addition of a 470 ohm screen grid resistor & a 1k5 metal film (beyschlag) grid stopper resistor on the output tube. Tubes used are NOS, NIB items. An early 60's GE 5Y3WGTB rectifier, a Philips ECG JAN 6V6GT & a Sino 6N4 12AX7. All voltages are within the correct range though the 6V6 is biased a little hot (about 120% max dissipation). Being a very rugged tube it can handle this no problem & shows no sign of red plating at idle or anywhere through to full clip, & does sound great, though I might bias it back down go 100% max dissipation in the interest of tube life. The speaker is an alnico Weber 8A125.View attachment 14931
Volume goes to 12. Get a bit of that into you Mr Tufnel. One more picView attachment 14932
Cheers

Never mind the amp, I wanna hear about that sweet looking Strat, Ivan!!!!

PS. Now you really have me wanting to build my very own Bassman amp instead of spending huge $$$ on either old build and possibly unreliable vintage units, or the newer foreign made PCB less than ideal current production stuff being sold in stores today.