deluxe reverb distortion

[mshowson]

Hi all.
Having a problem with a deluxe reverb clone (minus the trem).
With bass turned up fairly high and say 3/4 volume or higher, I get a nasty distortion when hitting block chords with authority.
It's a farting sort of distortion that lingers momentarily and then fades.
Now, I have built several of these before with no issues.
I have checked grounding and power connections a thousand times, tried different speaker cabs, tried different tubes, checked bias, checked voltages, replaced filter caps one by one, verified component values, traced wiring......It really behaves like a bad ground or cold solder joint but I've been over that stuff many times.
Frustrated and tired.
Anyone battled anything similar to this with some thoughts?
Thank you....Mark H.

[NickC]

I had something similar with my DR built. I followed the AB763 circuit, except put a switch to disable the NFB.

The "problem" went away after the amp had been burned in for 10 hours or so. It just gradually went away, never to return. Might have been the speaker, or maybe a cap settling in. I don't know what caused it, but I'm glad it's gone.

[el_Fela]

I have exactly the same problem. Every tube on this amp distorts like crazy, but the cleans are good enough. I've tried everything!

most of the distortion comes from the PI, and it starts distorting at 3 or 4. I have to turn the treble all the way up and the bass all the way down to get a pleasant overdrive out of this.

I wonder if this is really the BF sound...

HELP!!!

[Andy Le Blanc]

sounds like either blocking distortion or you've biased too hot...

you can reduce the value of the coupling caps in the power side from .1 to .022

check the voltage difference between the plate and screens at the tube socket
if the screens are too positive it'll show up as a distortion, bias cooler or change the screen grid resistors

[crbowman]

I will say that on my DR that I have to turn the bass down to 2 or below when the volume is over 6 or so in order to get decent overdrive. It's the dreaded farty low end that everyone talks about. It is my understanding that this is pretty common for black face and silver face Fenders. So long as you remember to turn down the bass as you turn up the volume, the overdrive character sounds great.
The OP said he's made a few of these, is this issue notably worse on this particular amp?

[el_Fela]

I'm starting to believe that this is related to the use of orange drop caps for everything except power filtering. Is this just nonsense or what? I always thought that old coupling caps were not bassy sounding at all...

[Structo]

Do you have a 12AT7 for the PI?

What are your voltages at?

[Milkmansound]

make sure lead dress is good - especially in the preamp section. I find that the leads to the treble pot and reverb driver tube can be fincky and cause oscillation which sounds like distortion. Futz with those wires using chopsticks and confirm that is not the problem.

Orange drops are fine - they look ugly, but the amp should work perfectly with them

[M Fowler]

Bad reverb transformer?

[jjman]

I recently put a post PIMV in my '71. With the channel up (for distortion of the PI) the bass must be lower than 3, with 2 being better. This was the case before the MV was installed too.

[tubesinside]

Remove the reverb driver tube and check if the distortion is still there. I have had this issue in many of my AB763 builds where the driver tube is overdriven causing ugly distortion throughout the amp (even with the reverb return pot off. Solution is to fit a 68K or higher on the grid of the driver tube....or reduce the drive to the tube.

[el_Fela]

Well, voltage readings are whitin specs.

I've tried without the reverb tube. In fact, I had that distortion problem with the reverb tube in the past, and i had to move around some wires. Sure, that thing is wild!

The news is that yesterday i found some kind of solution which was to bias the power tubes much colder that before. Couldn't measure though, my meter broke (touched the wrong stuff in Amperage measuring mode ). It sounds kinda cold now but its good anyway.

¿Why is this? ¿How does the power tube bias affect PI clipping? ¿Isn't this a power supply thing?

I know (well, I'm almost certain) it was the PI clipping because I also instaled a post PI MV by connecting a large cap and a pot between both PI outs (that cancels 180 degree out of phase signal, for those who might not know). So with little signail to the power tubes I could tell that the PI was clipping.

¿How do I know which part of the circuit starts to clip first? Well, I installed "volume" controls after each stage (temporarily) to hear how clipped the signal is coming. The PI clips way before the rest of the preamp section.

Thank you guys... for solving the misteries of tube technology

[Structo]

Pull the cover on your meter, you most likely blew the internal fuse when measuring current.

[jjman]

Remove the reverb driver tube and check if the distortion is still there. I have had this issue in many of my AB763 builds where the driver tube is overdriven causing ugly distortion throughout the amp (even with the reverb return pot off. Solution is to fit a 68K or higher on the grid of the driver tube....or reduce the drive to the tube.

I noticed the reverb driver was clipping at high channel volume during my MV install project. It was "cold" clipping which is from the tube reaching cutoff sooner than saturation. I heated up the bias after reviewing schematics of later Twins and noticing they used the same Fender reverb transformer part#. I also added a resistor in line with its cathode bypass cap to bring the reverb signal output back to the stock level. My guess is that Fender essentially did the same thing for the same reason in the later SFs.

[dobbhill]

I cured mine by replacing the Ck on the preamp gain stages with a 5uf...Got that from HAD...Easy fix for me, maybe for you...

Good luck!
D