oscillating amp

[Hunts smoothing bomb]

Hi Everone,

I built a stereo amp last year and it works very well, it has ECC81 phase splitter and 6v6 push-pull output.
My problem is when you turn the amp off its as if someone is rapidly connecting and disconecting the control grids to earth! and so the audio is modulated if you like.
It has done tihis since I built it. How can I stop it?

It doesn't affect the operation of the amp, it is just annoying!

Many thanks.

Lee

[Hunts smoothing bomb]

I know in the schematic it quotes ECC83's. I changed the valves but haven't updated the drawing yet.

Thanks
Lee

[ronbryan]

Sounds like unwanted positive feedback as the HT fades away.

I suggest you try incorporating some HT decoupling between the phase invertor stage and the 6V6 screen grids. Use something like 10k ohms in series with the HT and 22uF to ground.

Ron

[Skywave]

I don't think that this is related to your problem - but I've got to enquire - why such a low input impedance (68R) for an audio amp?

(Thinks: I might be missing something obvious here; not unusual )

Al / Skywave.

[Skywave]

Seems like a remarkable lack of HT decoupling in that cct. to me.

I'd try installing an R/C decoupling network between the common junction of the two 150K resistors (triode anode loads) and common HT. Something like 22k and 8uF as a "starting guess".

Al / Skywave.

[Hunts smoothing bomb]

OK, so what I need to do is put a resistance of around 22k in the HT+ supply to the phase splitter and then add 8uf of smoothing to the "lower voltage end" of the 22k resistor?

That would seem to make very good sense actually
I'll try it tomorrow and report back.

Many thanks
Lee

[Hunts smoothing bomb]

Hi Al,

There is a 68R resistor across the inputs because when the amp was built it was too sensitive ( I don't have a pre-amp built yet so am using the headphone out of an intergrated amp at the moment) and I tried negative feedback from the output tranny to calm things down but it severely compromised the audio quality so decided against it, it is a bit of a bodge really and I don't intend on leaving it that way permanently as it is an impeadance mismatch. I just need to work the grey matter until I come up with something different!

Cheers
Lee

[Hunts smoothing bomb]

Hello Ron & Al,

Good news,
The amp does not oscillate anymore! I took your good advice and added some decoupling between the splitter and power stages and also stuck 5600puffs of decoupling from the 6v6 screens to earth and it now just fades gracefully when turned off

New drawing attatched for those interested.

Thanks again.

Lee

[Hunts smoothing bomb]

Al,

I have attatched a copy of the schematic components I have drawn that I use with paint.
Hopefully you can save the image and then open it with paint and design your own circuits by copying and pasting.
Sorry mods, I know it's OT but I can't seem to find a way of sending an attatchment in a PM.

Cheers
Lee

[Top Cap]

Hi Lee, nice to fine another person who advocates using Paint,it's all I use now I have a good solid database of components. Your circuit has a misplace.
The anode load centre points of the phase splitters do not go to the same place, I have amended your circuit (see attached), assuming that you want both channels to be fed from the screen supplies of the output bottles.
Incidentally, I draw in bitmaps first then if I want a smaller file, I save it as a TIFF, it seems to pick up less of a speckled background that you get with jpegs.
Regards
Les

[Skywave]

Originally Posted by Hunts smoothing bomb

Al,
I have attatched a copy of the schematic components I have drawn that I use with paint.
Hopefully you can save the image and then open it with paint and design your own circuits by copying and pasting.
Lee

Thanks very much, Lee.
I have just done that : it'll probably save me hours!

Al / Skywave.

[Hunts smoothing bomb]

Goodmorning Les,

Thanks for the pointer on the mistake on the schematic, below is how it is supposed to be wired, thanks for the tip on using TIFF files, I will try it.

All the best
Lee

[Station X]

Originally Posted by Hunts Smoothing Bomb

Sorry mods, I know it's OT but I can't seem to find a way of sending an attatchment in a PM.

I'm afraid it's not possible. If it's something of general interest and small enough post it in the forums. If not use PMs to exchange email addresses.

[Mike Phelan]

There is another slight (mistake?) as well - the two 3M3 grid leaks on the ECC 83 need a "blob" to connect them to the junction of the cathode resistors or they look as if they are not connected.

I agree with the comments regarding M$ Paint - after trying various proprietary cct drawing freeware I found that it never did what I wanted. I follow Les and Lee's methods more or less exactly.

[Skywave]

Originally Posted by Hunts smoothing bomb

Goodmorning Les,
Thanks for the pointer on the mistake on the schematic, below is how it is supposed to be wired, thanks for the tip on using TIFF files, I will try it.
All the best
Lee

That cct. still doesn't look right to me

Surely, the full HT from the PSU should feed the 1 k-ohm screen grid resistors direct, and the same PSU HT point should also feed the 22 k-ohm resistor, the other side of which should be connected to the 47uF cap. and the four 150 k-ohm anode loads?

That way, the triode anodes HT source only is decoupled by the 22 k-ohm and the 47uF - which is not what is happening as its drawn - but is what is needed.

As it stands, I can see that at switch-off, the time-constants on the PSU rails - and including the reduced screen voltage - are likely to put the amp. into a transitionary unstable state as the HT gradually falls. (I assume that this is not what is required )

Also, as it stands, you've got reduced HT to the screen grids (22 k-ohm R) - limiting the final power O/P. It's usual to run the screen grids at full (or very nearly full) HT.

Al / Skywave.

[yestertech]

Originally Posted by Skywave

That cct. still doesn't look right to me

Surely, the full HT from the PSU should feed the 1 k-ohm screen grid resistors direct, and the same PSU HT point should also feed the 22 k-ohm resistor, the other side of which should be connected to the 47uF cap. and the four 150 k-ohm anode loads?

That way, the triode anodes HT source only is decoupled by the 22 k-ohm and the 47uF - which is not what is happening as its drawn - but is what is needed.

As it stands, I can see that at switch-off, the time-constants on the PSU rails - and including the reduced screen voltage - are likely to put the amp. into a transitionary unstable state as the HT gradually falls. (I assume that this is not what is required )

Also, as it stands, you've got reduced HT to the screen grids (22 k-ohm R) - limiting the final power O/P. It's usual to run the screen grids at full (or very nearly full) HT.

Al / Skywave.

I concur - The HT voltage at the OP side of the choke should feed the Screen grids. The 22K should then just feed the phase splitter anodes. As it stands, there is the possibility of signal coupling via the screen grids, and indeed as mentioned, the OP power will be well down with 22K in effect in lowering the screen grid voltage. Andy

[oldeurope]

Originally Posted by Hunts smoothing bomb

Hi Everone,

I built a stereo amp last year and it works very well, it has ECC81 phase splitter and 6v6 push-pull output.
My problem is when you turn the amp off its as if someone is rapidly connecting and disconecting the control grids to earth! and so the audio is modulated if you like.
It has done tihis since I built it. How can I stop it?

It doesn't affect the operation of the amp, it is just annoying!

Many thanks.

Lee

Hi Lee, you can stop it by giving the RC timing at both grids of the ECC the same value.

Kind regards,
Darius

[daviddeakin]

Aside from the matter of decoupling, which I fully agree with, I would also propose that your phase inverter is rather unbalanced.
This is liable to generate quite a lot of 2nd harmonic, which, although not unpleasant, is still more than necessary and isn't really to be expected of a hifi amp. I would suggest reducing the anode resistor of each of the input triodes by around 15% (say 130k to 140k). The 3M3 grid leaks are also unecessarily large, 470k would be quite sufficient, greatly reducing noise.

[Boom]

There is a severe lack of negative feedback as well! It is usual for amps' of this kind to take a small amount of fedback from the output transformer and send it back to the pre-amp. That would probably do much to tame it and would also increase the frequency response.