feedback howl on hi gain amp

[conisat]

Iv recently completed a valve amp of my own design, the result is awsome!!! except for one thing when the gain controls are maxed out (2XDual pots) and the master volume is about number 3 (which is very loud) the amp will start howling , i believe this to be feedback. Does any one know how to solve this iv tried investigating a few methods without success (neutralisation, negative feedback, noise gate etc).
The design revolves around ECC82 X 2 and 12AY7 X 2 in common triode gain stages. Any suggestion on appliying feedback or neutralization on these valves would be great.
Happy building guys

[daz]

have you tried reversing the OT's primary wires on the plates? If not, definatly try that. That would likely happen even below full gain, but worth a try if you haven't

[tubeswell]

If the amp starts howling of its own accord when the vols are dimed, I'd guess it was possibly at least one microphonic pre-amp tube. Is it a banshee-like howl? Hi-gain amps tends to wear out poor pre-amp tubes (even bad new ones get weeded out) faster because of all the acoustic vibration. Get each tube and flick it with your fingernail. If it makes a jangling sound like a jingle bell it has haddit.

If it ain't a bad tube, there are other solutions for providing local NFB (like a resistor between the grid of the subsequent stage and the grid of the stage in question, or un-bypassing the stage's cathode resistor - but these will lower the gain). Try seeing if you've got a bad tube (or 2) first.

And welcome to the forum BTW :-)

[conisat]

have you tried reversing the OT's primary wires on the plates? If not, definatly try that. That would likely happen even below full gain, but worth a try if you haven't

Thanks for the reply daz. Tried that without success. Just to add a little info the amp in question is a push pull design (6L6 GC) being driven by a long tail pair phase inverter with out put transformer current feedback applied to cathode. The problem i am having i am sure is down to the preamp's large amount of gain. It is my belief that applying negative feedback on valves that are heavily (purposfully) distorting will be detremental. If any one out there can provide information on neutralization of modern valves capacitor coupled I would be greatly appreciative as i believe this to be the way forward.
Cheers

[conisat]

If the amp starts howling of its own accord when the vols are dimed, I'd guess it was possibly at least one microphonic pre-amp tube. Is it a banshee-like howl? Hi-gain amps tends to wear out poor pre-amp tubes (even bad new ones get weeded out) faster because of all the acoustic vibration. Get each tube and flick it with your fingernail. If it makes a jangling sound like a jingle bell it has haddit.

If it ain't a bad tube, there are other solutions for providing local NFB (like a resistor between the grid of the subsequent stage and the grid of the stage in question, or un-bypassing the stage's cathode resistor - but these will lower the gain). Try seeing if you've got a bad tube (or 2) first.

And welcome to the forum BTW :-)

Thanks for the reply tubeswell.
The amp only starts howlling when all gain controls are maxed out and volume is turned up to 3 (which is loud enough for medium pub gigs). It makes no difference on what guitar i am playing or at what volume the guitar pots are on.
The one thing i would like to investigate is NEUTRALIZATION of my gain valves, I have found some info on this but not enough to impliment it if yourself or any one out there can point me in the right direction that would be great.
Once this amp project is fully completed i will post the schematics for all to see it has some interesting methods and features i have never seen on any other design, and is capable of fender cleans, and overdrive like nothing ive herd.

[tubeswell]

You can attenuate signal between stages with voltage dividers - look at various hi-gain amp schematics such as PV 6560/5150, or Soldano SLO100. AX84 also has some lead pre-amp schematics along these lines. You probably will need various frequency-shaping capacitors in between stages as well, if you do this - because hi-gain amps sound better when they are more middy

The Valve Wizard also has an article on designing a triode stage with local NFB here:

http://www.freewebs.com/valvewizard/localfeedback.html

Check your pre-amp tubes for microphonics nevertheless - it is a simple test and can save you lots of bother.

[daz]

Tubeswell is far more knowledgable than I to be sure. But may i suggest a alternative method to reduce gain? (assuming you are willing to reduce gain) I tried dividers in mine for months while trying to figure out how to not only get a lot of preamp gain but while getting great tone and no issues. What i found while studying various schematics led me to try this and it worked so well that it transformed the amp from a tone i felt was just acceptable to maybe the best sounding marshall style amp i ever played, and i owned many marshalls. Just give it a try before you discount it. It's simple as putting a shunt resistor at the grid of each preamp stage. I used 100K from the grid to groun at the 2nd and 3rd grids in the pre. No series resistor otherwise you have a voltage divider, and for whatever reason it works worlds different than a divider. I don't know whether 100k will work best for you, but the way i determined it was to use a pot and see where it worked best then measure it. But 100k seemed to work best in all my amps i tried this in so that may be the best value for you too.

Anyways, after i did this the entire amp's sound and feel changed dramatically and everything just sounded and felt so much more right. An i had used dividers always before this, and tried them everywhere i could try them and with different values. Nothing worked till this. As simple as it is please give it a try because it literally was the turning point in my amps.

[conisat]

Tubeswell is far more knowledgable than I to be sure. But may i suggest a alternative method to reduce gain? (assuming you are willing to reduce gain) I tried dividers in mine for months while trying to figure out how to not only get a lot of preamp gain but while getting great tone and no issues. What i found while studying various schematics led me to try this and it worked so well that it transformed the amp from a tone i felt was just acceptable to maybe the best sounding marshall style amp i ever played, and i owned many marshalls. Just give it a try before you discount it. It's simple as putting a shunt resistor at the grid of each preamp stage. I used 100K from the grid to groun at the 2nd and 3rd grids in the pre. No series resistor otherwise you have a voltage divider, and for whatever reason it works worlds different than a divider. I don't know whether 100k will work best for you, but the way i determined it was to use a pot and see where it worked best then measure it. But 100k seemed to work best in all my amps i tried this in so that may be the best value for you too.

Anyways, after i did this the entire amp's sound and feel changed dramatically and everything just sounded and felt so much more right. An i had used dividers always before this, and tried them everywhere i could try them and with different values. Nothing worked till this. As simple as it is please give it a try because it literally was the turning point in my amps.

I am hoping NOT to reduce gain in the pre amp, and i find with my design a very low grid leak resistors work best also. As it stands my voltage dividing pots are grounder on pin three via a variable fixed resistor through a 100 nf cap, pin two then goes to grid with an 82k to ground.My thinking on this is so when the gain pots are all the way off the variable resistor is set to give the minimum voltage level to grid for clean sounds( ie 3v peak to peak for 12ay7, 10v peak to peak for ECC82) the large cap grounds all signal voltages but isolates any grid leakage which is given the low 82k resistance to earth. I have never seen this done before but can assure you the tones on my amp is very nice.

[tubeswell]

A schematic would be handy

[conisat]

A schematic would be handy

currently i am offshore and have no access to my own computer, if i draw the section of my schematic and scan it to this computer can i attach it to this blog??

[tubeswell]

yep you can post .pdf files (or a whole range of different file types) using the 'manage attachments' button when you are writing a new post.

[conisat]

yep you can post .pdf files (or a whole range of different file types) using the 'manage attachments' button when you are writing a new post.

Here it is. (hopefully)
this is the type of interconnection i described previously, there are four of these in this amp connected via DPCO relays giving me a foot switchable 3 channel amp head.

[tubeswell]

Good that you figured out how to post stuff here, but in order to try and understand what might be going on in your amp, a more complete schematic is required. I can't tell anything much from that except to wonder why there is a relay to switch out the pot, and why you have an 82k grid load resistor in front of the next gain stage (when I can't see the next gain stage), and why you don't have a gain control. The thing you have labelled 'gain' is not a gain control the way you have it shown - it is just another pot in series with your other pot which is in series with a cap to ground. So that way you appear to have no absolute control over gain per se, (and very crude control over tone). Your 'gain' pot needs its own ground to function as a gain control.

[conisat]

in order to try and understand what might be going on in your amp, a more complete schematic is required.

Unfortionaitly I have not time to post the full schematic at the moment that is just a snippet.

wonder why there is a relay to switch out the pot

This amp has three channels one clean one moderate gain and one full on gain. That is just a demonstration to give you an idea at the moment, a complete schematic will be posted probably next week.

and why you have an 82k grid load resistor in front of the next gain stage (when I can't see the next gain stage)

This is not a load resistor but a grid leak resistor the grid will collect stray electrons which will build up on the grid if not given a ground path. The capacitor below the variable resistor will inhibit this as it will not pass DC.

control you have labelled 'gain' is not a gain control the way you have it shown - it is just another pot in series with your other pot which is in series with a cap to ground. So that way you appear to have no absolute control over gain per se

The gain pot is a 1 meg linear pot when fully in off position OR the relay is changed over the variable resistor underneath acts as a divider and is set to give the desired voltage output (just enough to give the following stage the required amount to reach desired clean level out put).
obviously fully turned to the on position the signal output of the preceding valve is available the drive the following valve to maximum and any where in between.

(and very crude control over tone).

The amp has a tone stack after the first valve (basically identical to the one found in most fender amps). In addition to this I have added a 3pole 4way switch who's function is changing the value of coupling capacitor between stages (sorry i did not show this on diagram) 3 stages have a choice of four caps ranging from 1nf to 33nf the resulting tonal variation of this design takes it from a bass response of approx 2 or 3 hz high pass to 100hz even without adjusting the tone controls. (Ps it is a make before break switch to eliminate switching sound)

Hope this clears a few issues up for ya. I appreciate the feedback.