Just like with a
Klein Bottle, its inside is also its outside
УНИВЕРСАЛ ТЕХНО-КОНТРОЛЬI decided to throw together a "breadboard" version of an all-octal Champ amp, literally on an unused 13.5 inch long bamboo cutting board, which I bought a couple of these at a "Dollar Store" last year, for just a purpose. I took a leisurely pace with this over Thursday and Friday afternoone & early evenings. With my still-fuzzy vision, I just had to be a bit more careful and patient than usual, so no mad rush to git 'r' done...
I used three octal relay sockets, and for the PSU, one of the Triode Electronics 5F2 power trannnies, and a Hammond 158M 10H, 100 mA choke, which is not standard Champ-issue
OPT is one of the 10 watt SE OPTs from Musical Power Supply on e-bay. Very similar to a 125ESE but cheaper, this particular one was made in Honduras (and not China) .
Yeah, it's kinda ugly, but in a cute sort of way
I'll post a photo or two soon...
Definitely not suitable for use around toddlers, curious pets, or folks who have an insatiable need to "look" with their fingers, since there is plenty of exposed HV AC and DC voltages lurking about. Not just a nasty shock, but potentially lethal.
The circuit is classic basic "Champ" for all resistor values, but with the exception of using a 6SL7 instead of 12AX7. and the added choke which gives me a C-L-C-R-C-R-C PSU string. Caps are two of the J/J 32 + 32 uF / 525 V units, and an added 22 uF / 450V cap at the end of the string.
I have only the volume ontrol, and no tone stack, just to keep it relatively simple for now..
I doubled up the capacitance on the B+ to the OPT section for 64 UF, the first and third sections are 32 uF. Not as "saggy" B+ as the vintage 8 or 10 uF caps, as I wanted to have a little more available DC stored-charge power.
With the standard tube lineup of 5Y3GT, 6SL7 and J/J 6V6S, it preformed as expected, nothing really special.
What did truly surprise me was that it was totally hum-free, I figured with a wooden base, and the relay sockets, and lots of "flying" wire leads, that it would hummmm like a cheap electric shaver, but it did not
Since the PT has enough spare VA capability, I wondered what would happen, if I swapped out the rectifier and output tube, and put in a Sovtek 5AR4 and a Russian 6L6WGC / 5881.
With the 5AR4 I now have 395V B+ to the OPT and 31V cathode bias across the 500 ohm 5W cathode resistor, for 62 mA cathode current.
With the 5Y3GT and 6V6, I had 365 VDC B+ and 45 mA (22.5V) across the 500 ohm cathode resistor, and about 208VDC plate voltage on both 6SL7 sections. The PD on the 6V6 was about 15W, just a tad above tie normal PD rating, but not enough to
red-plate it.
This gives me about 22-23 W of plate dissipation which is safely below the 30W max rating.
The 6SL7 driver stages also have a bit more "juice" and are running around 1.4 mA plate current, and 227V on the plates.
All I can say, is it has a LOT more power, and better overall response, but since I am using just an old cheap 8 ohm bookshelf speaker, I can't really say how it would sound through a proper guitar amp speaker mounted in a proper cab.
Next is to obtain an 8 or 10 inch MI speaker and cobble together a reasonable facsimile of a Champ speaker box, and give it a more proper listening test. And then to put it all together on a proper metal chassis, once I iron out any potential "bugs" that I may discover with use.
I'll preserve the "wooden wonder" as it is very handy for prototyping and testing new ideas, and I have duplicate parts for a more permanent build..
I wouldn't recommend taking a standard Fender Champ (or a clone) and just doing the 5AR4 & 6L6WGC tube swap, as it may then become a smoke-emitting or fire-belching "Chump" in rather short order.
The MPS OPT is fairly hefty, more so than the normal "dainty" Champ OPTs, (NEVER do this to a TF103) and it can readily handle the extra DC current coursing through its primary, and the Triode power trannie has extra VA capability than the original "just good enough" PT.
Per favore, no rompere mi coglioni about the RCA jack - I couldn't find my small stash of 1/4 inch shorting phone jacks at the time. I later did find four, but none with the "shorting" contact, so the RCA remains for now...
Update: I replaced the RCA jack with a 1/4 inch jack, so it is now completely "legitimate"
/ed B