vibrolux reverb clipping????

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frits

Active member
Joined
Oct 20, 2005
Messages
38
Location
netherlands
hi all,

I'm working on a vibrolux reverb AA270, and it is clipping in it's phase inverter with the amp turned up above 3.

now I know this is not the most clean amp fender build but in this case it looks like there is something seriously wrong in the phase inverter.

what i did so far:

-isolate the phase inverter from the rest of the amp put signal on it.... no help

-different tubes

-beefed up the psu

-changed caps and resistors

-increased B+

-looked helpless around

even that last action did not help

my question is What kind of peak to peak readings could I suspect from this phase inverter without clipping.

and what possible solution is there to get this amp more clean at reasonable volumes...

thanks in advance

F.
 
What are the idle voltages around the PI?
Do they match up to the schematic, say within 10-15%?
From the values on the schematic it looks as though the grids of the 6l6s will clip before the anodes of the PI do.
The max bias on the 6L6 grids is 48V so that's a max (ideal) pp voltage of 96V before they clip.

The anodes of the 12AT7 idle at 270V and they each drop 105V (375-270= 105) through their 47K anode resistors.
If the voltage on the grids goes low enough to just stop current flowing in the valve, no current will flow and the anode voltage will rise to 375V (rise of 105V).
In reality the anode won't make it up that far in a clean fashion so let's chop off 30V for safety, which leaves us with:

Max voltage on anode with 'Clean' output 105-30= 75V

I'm not sure how close to the cathode the anode can be pulled, but as the cathode sits at 100V at idle, even if the anode could get all the way down there it would be 270-100= 170V. I really don't know what the min. voltage between anode and cathode is but let's say it's 95V so we have:

Max negative swing with 'Clean' output 170-95= 75V

Add the two together and you have a pp max of 140V.

I don't know the specifics of the 12AT7 but I think all my assumptions have been conservative, and it seems as though the PI should manage better than '3' on the dial!!!!

Check those idle voltages.

I may have got the maths wrong, but I think it proves we have a problem with the PI...

chef
 
Thanks for your input!!

those idle voltages are reasonably good 87v on cathode 272 anode, I've got my hands on some nos 12at's so i better put them in first, (this I will do within this our) I will keep you posted!!
 
Cool.

An oscillator and a 'scope would be useful so that you can 'look' at the signal at the grid/s of the PI and on it's anodes?

Are you sure that the signal getting INTO the PI is undistorted?

What is the voltage between the grids and the cathodes?

From the schematic it looks like the idle current should sit the cathodes up just over 2V above the grids...

2.2mA flowing per side.


chef
 
> I really don't know what the min. voltage between anode and cathode is but let's say ... Max negative swing with 'Clean' output 170-95= 75V

No, the long-tail action means the neg swing is equal to the pos swing.

Yes, the pos swing is 105V unloaded and 2/3rd of that with 47K plate 100K grid or say 68V.

6L6 bias is 48V, so the driver should not be clipping before the 6L6 are totally slammed.

> should manage better than '3' on the dial!!!!

Depends entirely on the drive signal! If I put my HP 200AB's 25V signal on the instrument jack, a mere crack on Volume will blow 100 Watts. Input sensitivity is about 22mV. Many hot pickups and hard picks can beat 500mV.

But what will really make "3" hot: a linear Volume pot. The plan shows audio taper. At "3" the difference is -26dB versus -11dB.
 
Ok, the voltage swing is indeed 75 at the anodes of the 12at7 after that it starts clipping.

since this is an old amp with old speaker's and it has a small output transformer i think i got fooled by the 3 on the dial.

as for the input signal, i always hook my scope in at the guitar input and dial in that the same amount on the signal generator......

I think indeed it has an linear volume pot, I can not think of anything else.

thanks for all your thoughts on this one!
 
[quote author="PRR"]> I really don't know what the min. voltage between anode and cathode is but let's say ... Max negative swing with 'Clean' output 170-95= 75V

No, the long-tail action means the neg swing is equal to the pos swing. [/quote]

Yep, forgive me!

[quote author="PRR"] 6L6 bias is 48V, so the driver should not be clipping before the 6L6 are totally slammed. [/quote]

That's what I figured, but perhaps the reasoning was eronious!:?

[quote author="PRR"]> should manage better than '3' on the dial!!!!

Depends entirely on the drive signal! If I ...beat 500mV. [/quote]

Sorry I was assuming he was using a guitar and my workshop Mustang struggles to push 300mV on transients with feverish strumming.

Never assume, I suppose. :oops:

[quote author="PRR"] But what will really make "3" hot: a linear Volume pot. .[/quote]

Good shout! :thumb: Annoyed I didn't think of that.

chef
 

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