12AU6 versus 6AU6

You got it Reeek! The larger amp probably uses a 6.3 volt parallel heater circuit, the smaller amp uses a series circuit. Add the numbers of all the tubes together (50 + 35 + 12) and the sum is the amount of voltage drawn by all the tubes, the rest is scrubbed off by a large resistor wired in series with the filaments.

Putting the 12AU6 into the 6AU6 slot it won't generate enough heat to boil the electrons off the cathode and it won't operate properly. But you allready figured that out!

BTW, those little pentodes are great sounding tubes! And plentiful and inexpensive. They are great tubes to use for fabricating great sounding circuits.

 

I'm no expert but the tube complement you mention sounds like a series heater aka widowmaker/acdc/transformerless amp where the tubes are directly powered by the line voltage without powertranformers.Since the chassis can be "hot" depending on how the plug is inserted using these amps can apparently present a electrocution hazard depending on what else you may touch.Some of these amps have built in isolation transformers and folks generally recommend them for these little amps if they do not have them.If this info is new to you do some research on google.alt.guitaramps using the above terms especially series heater/acdc/widowmaker.

I've got a few of these Harmony and Kay etc. amps that use 50C5 and such tubes where the numbers add to approaching 115.They can be great fun for lower volume distortion tones.My Harmony 303B has been quite nice,complete w/UL sticker?!? and the big flange for a power tube socket which the amp don't got that I found floating around inside it!
I've managed to use these amps wo/electrocuting myself but I am pretty careful about not grabbing other guitar's strings,metal parts,mikes,and sundry while also touching my guitar strings,metal parts,etc.Oh also don't play barefoot on basement concrete floors,outside,grab metal pipes either.

Edit:I just reread and saw the reference to the isolation transformer in the first post.Still the "series heater" hazards were new info to me at one time and some folks might benefit from this info.

 

Hey there- I happen to have like 20-30 6au6's I pulled from an old oscilliscope that I pillaged. However, I have no use for many of the tubes. The thing was barely used and well maintained.

Most tubes test good-new

Various brands: Tung -sol , many GE, RCA, Sylvania

I could send them to ya if you want to pay S+S.

Later.

 

email sent!!

 

I wonder if this is the "little sister" amp mentioned in the opening post:

http://lectrolab.wordpress.com/lectrolab-models/r203/