Fender princeton 5C2 low volume

[pontiacpete]

This amp is putting out a max of 3.5vac on the output into an 8 ohm load. I've tried almost everything here and have just about given up.
So I thought I'd ask for help.
Here are the voltages on the tubes, do these look good?
6SC7
Pin 1 0v
Pin 2 119vdc
pin 3 -.5vdc
pin 4 0
pin 5 139vdc
pin 6 1.6vdc
pin 7 6.5vac
pin 8 0

6V6
pin 1 0
pin 2 0
pin 3 355vdc
pin 4 231vdc
pin 5 0
pin 6 0
pin 7 6.5vac
pin 8 14vdc

I have a good signal up until pin 5 of the 6SC7. Pin 1 the wave is not so good.
on the output this is what it looks like, below
Thanks for any input!

[Leo_Gnardo]

Given the early Princeton is nothing more than a Champ in a bigger box, you're getting what's expected - the exception being you're driving 8 instead of 4 or 3.2 ohm load. Is your OT seeing its expected speaker impedance? Also keep in mind, the bandwidth of cheap small Champ type OT's is execrable, but that's part of their charm. With a test frequency of 500 Hz, you'll see full power. Below 400 Hz, power drops off considerably, lucky to see a clean 1 watt at 100 Hz. Remember Champs/Princetons were designed to be affordable=cheap low power low tech "student" amps that could take a thrashing & keep working for years.

Output tubes' screen grid is on the low side, you could squeeze a pinch more power out by raising that. OTOH altering the power supply to get more power, then it's not the original item any more is it? Your choice.

[pontiacpete]

Thanks Leo, Good catch on the load. Should have known that after seeing so many of these.
So with 3.5vac across a 4 ohm 3.5^2 /4 ohms = 3.6 watts no? Shouldn't a champ or this kind of princeton make about 7 watts?
And why is my sinewave look so wonky?

[Leo_Gnardo]

Power's a function of hi voltage, and with "modern" Champs 64 & newer, B+ 425V I scarcely see 5W at clip, 400 Hz on up. With 355 on your tube's plate, and that big 25K resistor knocking screen grid down to 231, I'd say you're doing OK at 3.5W. A hard clipped single ended output looks just like the output waveform you're seeing. Ever't'ings parfekly narmal.

Two quick n dirty ways to cheat some more power out, 1) swap out the 25K in your power supply for something a lot smaller, say 2K2. Or B) side-branch a supply just for your screen grids, say a 2K2 resistor & 22 uF cap, leave the 25K in place to drop voltage for your pre tube. Of course, everything you change in a simple amp will be audible, but that's the fun of experimenting.

I dunno where "they" get a figure of 6 or 7 watts from a Champ/Tweed Princeton. On a good day, with the wind at their back, clipping hard, just about barely. Kind of like the speedometer on my old '62 VW bug. Speedo topped out at 85 mph. As if! The only way that car would make 85 is if it fell off a cliff.

[pontiacpete]

Thanks again for the explanation and suggestions.
I did try moving the screen supply between the 25K and the 500 ohm resistor and I have the OT red wire attached to the pin 8 of the 5Y3. This seems to help a bit. I'll mess with this some more.
So the thing to do is get the right speaker, maybe have this one re-coned at 4 ohms. Can't help but think that the transformer and speaker are not original. This speaker is an old jenson alnico, looks original, but it's measuring 10 ohms
Are you or is anyone here, familiar with the tweed OT numbers? this one has 80-144-1C and 928130 stamped into it.

[Enzo]

But VWs can do 85 cliff diving with almost no fuel usage.

[pontiacpete]

It seems that the output transformer and the speaker are not original,,, Speaker has one of those dual cones, the little horn in the middle above the dust cover.

[Leo_Gnardo]

It seems that the output transformer and the speaker are not original,,, Speaker has one of those dual cones, the little horn in the middle above the dust cover.

The less we know, the less we know. But we can figure out. That's a "whizzer cone" in your speaker, gets you a little more high frequency zippity doo out of the speaker. They were popular 50's & 60's, for hi fi folks too cheap to add a tweeter.

My way of figuring out "what's the impedance of an unknown OT" is try the usual 4, 8, 16 load resistors, measure the output voltage at clipping, apply P=Vsquared/R, and the load R selection that has highest power is the winner. Then you know what impedance speaker fits the OT. If it turns out your wizzer 8 isn't a good match, and/or you're tired of hearing distortion products coming thru too loud & clear via the wizzer cone, there's some choices for cheapie but goodie 8's out there, and most will cost you less than reconing the one you have. Jensen MOD 8's cheap in 4 & 8 ohm, another fave is Weber's 8" ceramic at $33. AFAIK neither has a 16 ohm version if you need that, but Celestion does offer a 16 ohm 8".

- - - - - - -

VW's over the cliff sorta reminds me of those Chinese high diving piggies . . . or ski jumping sumo wrestlers.

[TimmyP1955]

Trim the whizzer off with an Xacto knife. If it still sucks, dope the cone. You've nothing to lose but a little time.