Fender Champ & triode power ?

I'm in the process of giving a Fender SF Vibro Champ
too many switches & things.

So while it's out of its box, why not also a triode/pentode switch ?
(actually a triode/beam-power switch)

Anyone ever tried this yet ?

Circuit is like AA764:
http://www.ampwares.com/ffg/schem/champ_vibro_aa764_schem.gif



FWIW, main reason I opened it is to add a pot that can dial the NFB out.
Triggered by a recent EC-article where he did that with a Super.


Bye,

Peter

Haven't tried it, but it should work fine. You'll want a screen grid resistor, of course, somewhere between 100 and 1k ohm should be OK.

For additional flavor, you might try a tone stack bypass switch between the bottom of the mid resistor and ground, maybe with 1 meg across the switch. Or maybe just change the mid resistor to a 1 meg pot, then you could progressively remove the tone stack from the circuit.

Just some thoughts...

some mods I can suggest if you haven't already done them is:

have a switch with a middle position off. and have the option of changing the first cathode cap to .68uf-no cap-25uf...

put in a 25k pot in place of the 15K resistor for mid control.

have a switch to change out the 250pf to a 500pf. unless you have an additional blocking cap this will make a pop though.

I don't see why the pentode triode thing would be bad. although I've never played an amp with this option on triode mode. it never sounded as good. those where bigger amps though.

use a good switch as the screen gets tied to about 250 vdc

Thanks all for the reactions & additional mod-suggestions !

*** tone-stack ***

The mid-lift I tried already and volumewise it's a good jump.
At the tonelizard-site I read the treble-pot would more or
less keep it's functionality but not really so here. The tone
gets quite a bit darker.

I'll keep this switch though and yes, while at it why not add the mid-control.
I'll add it in the form of a footswitch & 25k pot.
The 1 M pot is a nice combination though.
And I forgot about that 1M across the switch, good to have that, thanks.


*** feedback ***

Yesterday I changed the feedback-circuit (2700 & 47 Ohms):
keeping the 2700 and replacing the 47 Ohms by 100 Ohms
and a 100 Ohms lin pot across it. The wiper of the pot goes
to the 2700 Ohms resistor.

The effect is zero... turning the pot gives no audible difference.
After some q&d calculations (see also RDH-4, pages 334/335)
I think I know why.
The open loop gain of the second 12AX7 stage & the 6V6 is
simply too low.
So no or less feedback won't raise the outputlevel much,
say only by 3 dB.

So the NFB-pot has no real use.
I'll be trying to lower the 2700 feedback-resistor, so the
pot does something. Range is then from no feedback
(wiper to ground) to more feedback (fully CW),
about 7 dB change in output level.


*** triode-switch ***

Nothing yet, will add. Actually strange one doesn't see this
more often - probably it's not really a worthwhile addition ?
I'll try though and add that screen-grid resistor.


*** other ***

have a switch with a middle position off. and have the option of changing the first cathode cap to .68uf-no cap-25uf...

Click to expand...

That sounds like a nice one. That 0.68 is a Marshall-thing, right ?



BTW, if you're wondering if I'm totally messing up & drilling
this Vibro Champ into pices, no.
No additional holes in the chassis; I've made a metal backplate
which is clamped between the two wooden side panels. The existing screws keep it in place.
On this panel I've added things like the NFB-pot and I
will add the mid-pot here as well. There's also a 3R9 power resistor
on the inside of this panel and some speaker-jacks and a jack for the
vib-footswitch etc.
Everything can be restored again - FWIW for a SF...

Thanks again,

Peter

Don't feel bad about hacking it up. I do it all the time!
You might try a prescence control, although I never liked them much.
I just got an email from Kendrick.
They want $999 for a single ended 6550 pratice amp!
Man it better sound nice. Thats about 500 bucks a watt!
cj

use a good switch as the screen gets tied to about 250 vdc

Click to expand...

Sure ! This SF-amp seems to be quite a Vibro Champ on steroids:
371 Vdc B+ and 362 Vdc at the screen grid.
I understood not all 6V6GT's like that. FWIW, I have a Radiotron in it.

Peter

I'm running 440 vdc on some v6's.
Love the smell of burning paint when I'm playing!

Don't feel bad about hacking it up. I do it all the time!
You might try a prescence control, although I never liked them much.

Click to expand...

I'm trying to find a balance between a nice tone machine with possibilities and a thing with too many bells & whistles.

With so little difference between the stock closed & open loop gain it'll be a subtle presence-boost if done in the NFB-area, right ?

I just got an email from Kendrick.
They want $999 for a single ended 6550 pratice amp!
Man it better sound nice. Thats about 500 bucks a watt!

Click to expand...

Oemph. That'll be the most expensive Watt on the planet... They better include some spare tubes for that price.

Bye,

Peter

I'm running 440 vdc on some v6's.
Love the smell of burning paint when I'm playing!

Click to expand...

Aaa ! :thumb:
Makes me feel better, that 371 is not that insane after your 440. How about your caps, 450 V types ?

Peter

yes, 40 yr old deluxe reverb

yes, 40 yr old deluxe reverb

Click to expand...

Cool. Original caps ? And given the Mackie-mess in another thread, be sure to post pics when they go ! :wink:


But serious, talking about voltages - the heater voltage is quite low in that Vibro Champ: 5.5 V, that's more than 10% off. I'm too lazy to correct that but I was surprised, especially with the 230V mains we here here i.s.o. the 220 of a while ago. (well, the backplate says 200-250 V).

Make a dc heater supply and it will bump up a little.
Having space for the large caps might be a problem.
I think the whole eletrolytic change out deal is overrated.
At least in guitar amps.
I measured the caps for leakage and capacity and they are fine.

Make a dc heater supply and it will bump up a little.
Having space for the large caps might be a problem.

Click to expand...

Thanks, will try that, at least for the preamp tubes and well. that single 6V6 won't be that hard to take along here as well. For some reason I always had thought this AC->DC wouldn't work - but that was wrong.

I think the whole eletrolytic change out deal is overrated.
At least in guitar amps.
I measured the caps for leakage and capacity and they are fine.

Click to expand...

Could imagine that - must say I've never had a big HT cap go. It's just that there's that feeling they'll likely go when you least need it. But then there's no end...

Bye,

Peter