Totally. I have a long way to go before I feel knowledgeable on the subject but I at least know what I dont know and I have the books to remedy that.

The main thing I'm interested in is being able to make plate characteristic curves at the screen voltage of my choosing since they're nearly always at some useless value on data sheets (the JJ 6V6S sheet doesnt even state it although I could be being a tit here). How to do it is in the Kuehnel book. I'm going to write a little app that does it for a few standard tubes and I'll pass it around once I've got it debugged.

Someone loosk to have done it in matlab if anyone uses it:
http://www.normankoren.com/Audio/Tube_params.html

I do so I can output whatever graphs you need...

Hmm the curves it produces dont look right.

I think I understand the graph which is exciting to me because I feel like I'm learning something! Should be ordering my first set of parts in a few days

Cool man. I got to say your hot cat thread has inspired me to think about something I wouldnt normally go for. I have a universal OT sat on my desk burning a hole in it. It does 3.25 & 4kΩ primaries but I can double the impedance for 6.5 & 8kΩ if I dont mind losing the 4Ω tap. If I run on the 8kΩ primary then a pair of 6L6s at a B+ of 440V will give an output of about 40W. Preamp wise I'm thinking of 2x12ax7 followed by an FMV stack and then perhaps a switchable pentode stage (EF86 most likely) to fit between the the 12ax7 stages. May add a loop. Not to sure about the NFB arrangement.

Actually the load line looks a little better at 6.5KΩ primary as the class B line is closer to the knee of the 0V curve. That gives about 45W output.

Are you using the matlab program you posted?

Nah I'm being far less sophisticated than that. I'm copying the plate curves from a pdf and then drawing on them in paint following the guide here:
http://www.freewebs.com/valvewizard1/pp.html

The bit right at the bottom tells you how to estimate the ideal output power.

Looks like a power transformer I was given is from a vox ac30cc. Could be useful for a this little clean/jazz amp I'm toying with making if it works (going to test tonight - if you dont hear from me assume I electrocuted myself...). Its not got quite as much juice as I wanted (295-0-295) but I can work around that I guess. Not too sure how much current it can supply but I can guess based on what the ac30cc uses and then maybe add 20% as I doubt they went crazy and over spec'd the PT.

This looks more and more like its an AC15cc PT now. Hmm.

It still weighs just under 7 lbs though so its pretty chunky for a 15W PT.

Pair of 6V6s can work with a few simple changes in a 6L6 type circuit.

Or... and actually what my current map is you could run both/either. Mine uses a Marshall 18w PT instead of an AC15 one though, i don't know how they compare, but i know that the 18w Marshall is a pretty beefy size for its original application. I can run either 6V6 or 6L6(just have to remember which it is when i plug in the cab!) in mine, but usually i just do the 6V6s because i have a couple nice ones and the 15-20w is plenty for what i normally use it for.

Lower voltage 6L6s can give you maybe 25-30 watts and still keep within the current spec of the PT, if you set it up right. Their current demands aren't too bad at lower volts. I ran mine for a few hours at full blast to test it with the 6L6s, and the marshall style PT hardly got warm.

I only run two 12AX7s, and a solid state rectifier, though...

Just tested the no load voltages:
Red-black as primary

Brown - Brown = 28V
Purple - Grey = 323V
Purple - purple = 645V
pink -pink = 7V

the schematic for the AC15CC shows 374V after rectification under load which isnt a kick in the pants of 323x1.2=388V you'd get from two-phase full wave rectification under load. I didnt measure the outlet voltage but it might account for the difference. There are also two 82Ω resistors before the diodes. Not sure why we need limiting resistors here but it may be to get closer to the voltage a valve rectifier would have given them

I might look at 5881s. Might be able to squeeze a bit more than is sensible out of the PT with those. All I want is 2 12ax7s, an EF86 and something like 5881s.

Just knocked up a load line using the old (dangerously old...) plate curves for the tung sol 5881



I get about 25W output if I use the 8k primary on my OT and I can bias at 70% max dissipation (-20V) without pushing the class a load line too close to the danger zone.

The thing of course is that I have idea what the transformer can supply without throwing a hissy fit. I wont use the 28V secondary for anything but that isnt going to buy my a huge amount of extra HT current. Given that its fairly beefy I'd assume it can take the heater draw. The 5881 is 0.9A to the EL84s 0.76A so its not too bad. The PT in my A15 is much smaller and that runs 4 12ax7s and 2 6V6s without breaking much of a sweat although the 6v6 heater demand is pretty slim.

Fuck it. I think I'll risk the build when I have this preamp done. It shouldnt be adding too much additional stress on the PT and given its size I think it can probably take the strain. If it runs too hot I'll buy a filament tranny to run the 5881s off which should take 11W of stress off the core.

If I get a little creative I can shoehorn all of this in an epi jnr chassis:
http://cgi.ebay.co.uk/Epiphone-Valve-Ju ... 178wt_1140

This thread gets more epic everyday!

seems like it would be rather tight...

Haha yeah, having thought about it I'm gonna stick to a 17" JTM chassis for this build. I can probably squeeze a 4Wer into the valve jr chassis though.

check this crazy little 4W beast out:
http://www.ampmaker.com/4w-push-pull-ou ... 506-0.html


325V on the plate will give about 3.6W output with a 12BH7.

In other news: my label machine has turned up so I can label my chassis now. My previous attempt at using laser water deal sheet ended in disaster when the supposed clear coat safe (in fact recommended that its used) sheets melted under the clear coat and fucked up the paint job Its now more of a blue colour as the autoshop had none of the sparkle purple is stock and the front panel is a little scarred. It may come out in the clear coating so I'm not too worried but ultimately I just want to build the fucking thing now! I'll tart it up at a later date. I said I'd build a 2 channel 20Wer for a friend and he's getting antsy about it and sales tax goes up in the new year so I better get on it quick.

Preamp chassis is labelled, gonna drill the socket retainer later. Soldering to commence.