Amp Tone

[alastair_in_oz]

I have a scratch built DR type amp, clean channel only. Been playing it for a while, getting used to the sound. The circuit is stock, mostly, except for the transformers, which are in spec.
I have found that I have to turn the bass and mid pots almost all the way down, otherwise I start to get this annoying sound, when playing the A string and low E strings. Its not a buzz or hum ,its like an overtone or resonation, or like a bit of feedback for that frequency.
Its probably been doing it all along, but I have not played and cranked the amp much since being built. I don't mind the sound with the bass and mid pots down, but I would like to be able to crank the mids to at least midway sometimes.
The speaker is a 100w C12K Jensen, open backed cabinet.

Any thoughts folks

[Tubenit]

sound

If you took the amp and ran it into a different speaker and cabinet ...... does it still do this sound?

If so .......this could be possible parasitic oscillation which can be a layout or lead dress problem ...... or a bad tube.


With respect, Tubenit

[alastair_in_oz]

Thanks for that. I will have a chance in the new year to try out my amp with some different speakers, until then I will have to wait.
I understand what you mean about the oscillation, and I have had it in the past with other amp builds, but not this one. At least not the same way.
In the past, the oscillation was usually high frequency, and related to lead dress, or instability. This amp I can turn up all the way, with the input grounded, and there is no audible oscillation.
I have tried different preamp valves, I put some old mullards in the preamp, which sound OK, makes no difference to the problem sound though, so I put the new EH back. The output tubes are brand new 6V6 EH.

I may have described the sound a little wrong. If I play the D string, it sounds nice and clear, and great with the mid and bass pots at halfway, then if I play the A string, it will sound too thick and looses clarity, and the overtone sound is like it has too much sustain. This causes the sound to be kinda "overloaded" on the E and A strings. Its like the amp is allowing lower frequencies to have some kind of extra gain. Ever accidently had a guitar too close to a cranked amp and it starts to feedback and the amp vibrates and makes a low pitched feedback type sound? Well something a little like that.
It must be some kind of feedback.

[Tubenit]

For what it's worth, ...... I have had two amps I've built that the oscillation was confined to "ONE" note and it was on the E & A strings. It was a different note in both cases. I could play any other notes on those strings without the oscillation being there. I discovered that moving wires with a non conductive chopstick resolved the problem on one amp. The other amp was a bad "new" EL84 & replacing the power tubes resolved it.

Hope you get it figured out. Again, if it does it with another speaker and cabinet ..... that would inform me that it's electrical and not mechanical.

With respect, Tubenit

[alastair_in_oz]

Thanks, I will let you know,
Regards
Alastair

[black_labb]

when you say the E and A string, do you mean all along them, or just the low frequencies?

could it be that you are overdriving one of the tubes slightly? maybe a bit of compression/distortion on the lower frequencies?

have you tried with different instruments?

[alastair_in_oz]

Hi, Yeh pretty much all along the A and E string. Has something to do with the frequency and/or amplitude. I just spend the afternoon building a speaker load and connecting to my PC which I have an oscilloscope program.
Trying to replicate the problem on the bench to see if I can get to the bottom of it.

Just an after though, my PI valve is a 12AT7, would changing it to a 12AX7 do any harm, or just overload the output tubes?

[black_labb]

the 12at7 would be less distorted assuming it is working correctly. 12at7's can pass more current before distorting than a 12ax7. try the 12ax7 to see if thats the problem, but if it is it would be a bad tube, not due to the type.

try a different guitar. it could be that the pickups are closer to the E and A strings than the high strings, overdriving something.
having mentioned pickup height's, you could be getting wolftones, which are when a pickups magnets pull the string and give the note a strange harmonic content. seems it may be the issue. you may not have noticed it on other amps as they didnt pass the signal so clearly as this amp does.

ps: where in sydney are you? i'm in sydney too, near west pymble/north ryde

[alastair_in_oz]

Hi Black_labb,

I live in Kellyville, so not that far away.
I noticed yesterday, that it happens on the E string 5th fret. Where I mentioned before it happened up and down the E and A strings, I had it wrong, it really is just in on or two one spots. So I tried it just now and it did it on the 10th fret E string, but not the 5th fret like yesterday. Maybe its just the guitar or this one string.
I just tried another guitar, and this one did not do it, but the strings are very old and dull sounding, so has a low output, its not really a good comparison
Sounds like I need to try various amp/speaker/guitar combinations and see what happens.

[pdf64]

What value capacitor have you used at the input to the phase splitter / long tail pair? Stock for a BF DR would be 1nF. That cap's value sets the balance between the highs and lows. Peter.

[alastair_in_oz]

All my values are stock as per schematic AB763

[Diablo]

Is it possible that you have positive feedback instead of negative feedback?

[alastair_in_oz]

How do I check that? Like I said all my values are stock. Is there a way of measuring the NFB?

[Diablo]

How do I check that? Like I said all my values are stock. Is there a way of measuring the NFB?

If this is a Deluxe Reverb, then you have two 6V6GT power tubes. Switch the wires from pin 3 on one 6V6 socket to pin 3 on the other 6V6 socket. After you switch the wires, if the amp oscillates badly, that wasn't the problem and you'll need to reverse the two wires again to put it back the way it was. Then again, the problem may get fixed if you had positive feedback in the first place.