AC30 query

knuckleberryfinn

I have just brought a non-working AC30 from 1974 back to life, and everything is now working fine except the vibrato and tremolo.  I'm going to do some more work on it over the weekend, so hopefully I'll be able to work it out.  Strangely you can hear the tremolo effect after the power switch is turned to off, as the sound fades.  So the oscillator is definitely working, it's just not being applied properly.  

But enough of that for the moment.  My immediate query relates to the cathode bias resistor.  Most of the schematics indicate a 47R or 50R resistor here, but none state the wattage.  Because the HT voltage is higher in this circuit than in the valve rectified ones, I have fitted a 56R 7-watt resistor and am reading the requisite 10V across the resistor.  It sounds incredibly good - very hot, but amazingly rich.  So I think it's okay as far as the biasing goes, but am wondering if 7 watts is enough.  Can anyone tell me?

Phil_aka_Pip

V=IR, so I=10/56 ~0.2A
P=IV, so ~2W

I think you're OK. I'm sure if I got it wrong, @ICBM will correct me.

EDIT Alternatively you could write P = V.V/R = 100/56 still giving you ~2W

jpfamps

The bigger the better for cathode resistor. 7W should be fine.

To greater power handling I often wire 2 resistors in parallel.

The problems with the trem / vibrato in JMI era AC30s is usually the 10k plate load resistors on the 12AU7 in the trem channel going open circuit.

knuckleberryfinn

Oh yeah, Ohm's law.  I don't think I've ever actually used it, even though it's the simplest and most useful thing in the world.  Thanks for showing me how.

knuckleberryfinn

jpfamps said:

The bigger the better for cathode resistor. 7W should be fine.

To greater power handling I often wire 2 resistors in parallel.

The problems with the trem / vibrato in JMI era AC30s is usually the 10k plate load resistors on the 12AU7 in the trem channel going open circuit.

Thanks JPF.  The 10K plate load resistors check out fine.  I'll investigate further...

ICBM

Make sure it does have a 12AU7/ECC82 in the trem mixer position (V3) as well, you often find a 12AX7 there and it doesn't work.

I would go for a 10W resistor for the cathode. There's no disadvantage to using the biggest one that will physically fit. You can fine-tune the value by using the original 50-ohm and adding another smaller one in series, too - there should be a spare tag right next to it on the strip. That's how I did my one, ending up with 65 ohms (15-ohm 3W added).

Are you sure the HT is higher though? Mine isn't. With the 22-ohm resistor in the power supply it's the same as a valve-rectifier one (320V) at idle - it just doesn't drop as much at full output.

knuckleberryfinn

There was a 12AX7/ECC83 in the trem position, but it made no difference fitting a 12AU7/ECC82.  I'm finding it difficult to trouble shoot this amp as I have no accurate schematic or layout, and I've never worked on anything other than Fenders before.

As for voltages, I was looking at the unloaded readings!  Oops.  In the power amp I've put two 390R resistors in series because the closest schematic I could find had an extra 390R instead of a choke.  I did it like this because I thought the choke might not work, but it does, so actually the HT voltage on the screens of the EL84s is only 304V, so less than normal. I think I'll just leave it as it is (instead of bypassing one of the resistors) as it sounds incredible.  Now if I can just get the tremolo to work...

Cirrus

I built a little voltage reducing transformer for mine as it blew fuses every now and then, brought the HT down to 315v and used a 56 ohm 10 watt resistor.

Is that 304 in relation to ground or the ~10v on the cathodes? Your AC30 is almost running the EL84s IN SPEC! I bet they last a billion years.

Or months. Who knows.

knuckleberryfinn

It's 304V in relation to ground.  It certainly smells and feels like it's running hot - unbelievable heat coming off the new valves - but I know these AC30s are renowned for it.

Phil_aka_Pip

http://music-electronics-forum.com/t24561/

Cirrus

Out of interest, what are the valves actually biasing at? I can't remember the numbers off the top of my head but I think mine runs at about 14w/ tube. No "accidents" since I cooled it down from the 19w it was at before.

Voxman

knuckleberryfinn said:

I have just brought a non-working AC30 from 1974 back to life, and everything is now working fine except the vibrato and tremolo. 

  Are these actually two separate effects or one and the same thing?  I know the control says vib/trem but I thought one knob was for 'rate' and the other tremolo depth - or did the latter actually move it between two different effects?  I ought to know as I had an early 70's AC30 myself, but (senior moment mode strikes again!   >:D<   ) I'm damned if I can remember! 
  

Phil_aka_Pip

I think it's tremelo. (Vibrato is fluctuation in pitch, which Leo Fender never understood )

ICBM

It's both, that's why it has a Vib/Trem switch!

And why the circuit is some complicated, needing three preamp valves… which is then why none of the modern AC30s (apart from the 90s-00s AC30TBX 'Korg' reissues) or clones have it which kills a big part of the character of the amp, in my opinion.

The other 'knob' is a three-position speed control on the old ones, and a fully variable rate on more modern ones.

knuckleberryfinn

Yes, definitely both (though I've never heard either in this amp!) because the vibrato (pitch) circuit looks completely unfamiliar to me.  I honestly have no idea where to start.

ChrisMusic

ICBM said:

It's both, that's why it has a Vib/Trem switch!

And why the circuit is some complicated, needing three preamp valves…

So how on earth do you do vibrato (i.e. shifting pitch) with 3 valves ?  I thought that was the domain of bucket brigade devices and asynchronous memory algorithms ?

ICBM

It works by phase-shifting the signal and then alternately mixing it with the original, which gives the result of slightly 'stretching' the frequency as it pans back and forth between the two. It only gives a gentle vibrato, not a deep pitch-shift type.

thermionic

Similar to a univibe then I suppose. It intrigues me how a phase shift tricks the ear into hearing a pitch shift.

The Vib-Trem in my old AC30 never worked (I knew next to nothing about amps then) and I don't think I've ever heard a decent example of the sound.

Phil_aka_Pip

@ICBM I learnt something. And now I want one. In a pedal, made with proper valves, or course.

ICBM

thermionic said:

Similar to a univibe then I suppose. It intrigues me how a phase shift tricks the ear into hearing a pitch shift.

Exactly. It's like a sort of pseudo-Doppler effect. As the mix pans between the original signal and one which has been phase-shifted backwards, it's like the second one has taken slightly longer to arrive - so the result is to stretch the time interval between the same points on the waveform and thus apparently lower the frequency. (And vice versa as the pan goes back from the shifted signal to the real-time one.)

ChrisMusic

ICBM said:

It works by phase-shifting the signal and then alternately mixing it with the original, which gives the result of slightly 'stretching' the frequency as it pans back and forth between the two. It only gives a gentle vibrato, not a deep pitch-shift type.

That is just brilliant.  Such elegant simplicity.