Perplexing issue with a AB763 Deluxe Reverb clone, low frequency buzz

Thread Starter

Planobilly

Joined Jan 23, 2016
8
Hi,

Perhaps someone here could give me some ideas of how to solve this issue. The amp is a new build I just finished. After working through a few issues the amp ran normally for about two hours.

It now buzzes when I play the low E string on the guitar. The issue is in the amp. I have changed speaker cabinets, guitars, all the tubes, and changed out most of the tone stack. I have been over the amp pretty thoroughly. I have sort of run out of ideas. I can not see any issues on my oscilloscope at any point in the circuit.

The issue is gain sensitive, both on the guitar volume control and the amp volume. Increasing the bass pot control also makes it happen quicker.

Here is a schematic...I do not have the tremolo circuit built the same exact way but I have V1, V3, and V5 tubes removed.



Thanks,

Billy
BTW, I can do most any troubleshooting and can post photos/video ect.
 

#12

Joined Nov 30, 2010
18,222
Check for mechanical vibration in the laminations of the output transformer...or any other mechanical vibration.
 

Thread Starter

Planobilly

Joined Jan 23, 2016
8
I can not seem to find any mechanical vibration. After I first started up the amp there was an issue with the with the reverb recovery circuit which had a failing cap. After fixing that I played the amp for two hours testing a couple of new speakers. The amp sounded great. I assume it is possible that I just did not hear the buzz but I find that hard to imagine.

Today I made the video below with the amp hooked up to a Weber 12" 30 watt DT12 speaker. You can clearly hear the issue. Later I tried a new Weber NeoMag (Neodymium) 12" 65 watt which sounded better I assume due to different bass response.

If in fact the amp is actually working correctly based on the above schematic, what changes should I make to reduce the bass response of the circuit?

I assume cathode capacitor values and/or some sort tone stack change.

Need ideas.


Thanks,

Billy
 

jvcmarine

Joined Mar 11, 2012
15
Sounds like one of two things, I can't hear the audio very well on my crappy laptop, but it sounds like parasitic oscillation or blocking distortion.
Post a picture of the chassis, lead dress is most likely the culprit.
Think of the grids and the wires connecting them as little antennas. Wiggling them or moving them or re-routing them will most likely solve the problem. Providing you have installed the correct component values. Make sure the grid stoppers are wired on the tube socket with short leads.
Check your grounding scheme. I just reread your post, you say you made a change in the recovery stage and then you noticed the issue. What changes were made in component positioning and or lead dress. Pull the reverb tube.
This is a common issue with home builds.
Hope this helps.
 

JoeJester

Joined Apr 26, 2005
4,390
Do you get the "error" when you connect the instrument to the Normal Jack and remove V2, V3, V4, and V5?

You can eliminate alot of the circuitry if that works properly.
 

Kermit2

Joined Feb 5, 2010
4,162
Yes. We need to perform a process of elimination.
Try different inputs. Try a different guitar.

I personally think it is in the instrument. A pick up is loose and rattles when a resonant note is struck.
 

jvcmarine

Joined Mar 11, 2012
15
Yes. We need to perform a process of elimination.
Try different inputs. Try a different guitar.

I personally think it is in the instrument. A pick up is loose and rattles when a resonant note is struck.
He stated he has tried a different guitar, but not the other input channel.
 

jvcmarine

Joined Mar 11, 2012
15
Hi,

Perhaps someone here could give me some ideas of how to solve this issue. The amp is a new build I just finished. After working through a few issues the amp ran normally for about two hours.

It now buzzes when I play the low E string on the guitar. The issue is in the amp. I have changed speaker cabinets, guitars, all the tubes, and changed out most of the tone stack. I have been over the amp pretty thoroughly. I have sort of run out of ideas. I can not see any issues on my oscilloscope at any point in the circuit.

The issue is gain sensitive, both on the guitar volume control and the amp volume. Increasing the bass pot control also makes it happen quicker.

Here is a schematic...I do not have the tremolo circuit built the same exact way but I have V1, V3, and V5 tubes removed.



Thanks,

Billy
BTW, I can do most any troubleshooting and can post photos/video ect.
 

jvcmarine

Joined Mar 11, 2012
15
One more thought here, I didn't realize the Deluxe used a tube rectifier, put a silicon rectifier in there for testing. (two 1N4007 into an octal base). Power supply is also in the signal chain.
 

#12

Joined Nov 30, 2010
18,222
I didn't realize the Deluxe used a tube rectifier, put a silicon rectifier in there for testing.
No! I just rebuilt a Deluxe Reverb last week and the voltage on the 500 volt capacitors went to 515 volts during one test and I was using a 5Y3 rectifier because I didn't have a 5U4 or a GZ34 in stock. If you use silicon diodes the voltage on the capacitors will be even higher.
If you want to play with different rectifiers, use a Variac to turn down the power line voltage.

The vacuum tube rectifier not only keeps the B+ down where it was designed to be, it provides, "sag" under high volume loading, and that sag is part of what musicians like.
 

Thread Starter

Planobilly

Joined Jan 23, 2016
8
Hi guys,

Thanks ever so much for all the feedback. After a considerable amount of testing I finally track this down to a bad tube socket. I could not find the tube socked issue by normal methods of moving the tubes around with the amp on.

I disconnected things stage by stage and finally discovered the cause.

I was a bit surprised that a bad connection in a tube socket would cause the amp to oscillate in this manner. Having said that, tube amps can display some pretty strange noises.

Thanks again,

Billy
 

Thread Starter

Planobilly

Joined Jan 23, 2016
8
Here are some photos of the amp.

Photo before I redid the heater wiring.


Photos of new heater wiring.


Photos of the finished amp


I built everything myself, from the chassis to the wood case to putting on the Tolex. The CNC milling on the chassis and the CNC holes in the turret board were a bit of a challenge. I only have a small CNC mill to work with. Writing all the G Code twisted my head around a bit...lol

Billy
 

jvcmarine

Joined Mar 11, 2012
15
Hi guys,

Thanks ever so much for all the feedback. After a considerable amount of testing I finally track this down to a bad tube socket. I could not find the tube socked issue by normal methods of moving the tubes around with the amp on.

I disconnected things stage by stage and finally discovered the cause.

I was a bit surprised that a bad connection in a tube socket would cause the amp to oscillate in this manner. Having said that, tube amps can display some pretty strange noises.





Thanks again,

Billy
 

jvcmarine

Joined Mar 11, 2012
15
Nice work there Billy, nice clean tag board and lead dress,,, are those NOS sockets, and which one was bad??? Caig Gold is great for good conductivity on these.
 

jvcmarine

Joined Mar 11, 2012
15
No! I just rebuilt a Deluxe Reverb last week and the voltage on the 500 volt capacitors went to 515 volts during one test and I was using a 5Y3 rectifier because I didn't have a 5U4 or a GZ34 in stock. If you use silicon diodes the voltage on the capacitors will be even higher.
If you want to play with different rectifiers, use a Variac to turn down the power line voltage.

The vacuum tube rectifier not only keeps the B+ down where it was designed to be, it provides, "sag" under high volume loading, and that sag is part of what musicians like.

What manufacturer and model were you using for 500 volt caps????
 

JoeJester

Joined Apr 26, 2005
4,390
I was a bit surprised that a bad connection in a tube socket would cause the amp to oscillate in this manner.
When you inspected the "defective" socket, did you notice the cause? For example, did the tube seat properly into the socket? Was an individual female connection spread open too far ... causing an intermittent connection?

In a different system, I had a relay socket, female connection, that didn't make connection and prevented the emergency generator from starting.

In your case, a good inspection, or wiggling (safely) individual socket pins, might have found the problem. Then you could have found the exact pin that was "defective." Good job anyway.
 

#12

Joined Nov 30, 2010
18,222
One of the jobs I did last week was a Magnatone. All measurements done, all parts installed to bring it back to original condition, all pots squirted and all jacks replaced. So I plugged it in for the "Human" test. A dirty noise. Spontaneous and intermittent. Doesn't respond to changes in input. Doesn't respond to the hammer test. I had to wiggle parts until I got to the phase splitter tube. Spread socket, pin 2.

Poked it tighter with a tiny screwdriver, spray, jiggle, rinse, repeat. It cleaned up after 3 passes but I had selected an exact duplicate 9 pin socket in my stock drawers, just in case.

Tomorrow it will get on the Brown Truck to go back to California.

And now the local denizens know another of the jobs I do on the side.
 

JoeJester

Joined Apr 26, 2005
4,390
And now the local denizens know another of the jobs I do on the side.
One does what one must do. You are spreading your experiences here is a good thing.

Back in the day, at the training centers they would do different things to simulate a failed component. I remember one problem, back in 1977, where a triode wasn't conducting, the indication was the B+ was higher than it should have been. The instructor was shocked when I told him the tube wasn't conducting and needed replacement. He asked if I wanted to make other tests ... I said, no. I looked at all the other wave forms from the front of the equipment and this single tube is the source of the failure.

Your warning about changing the VT Diode to diodes reminded me of that problem. One never knows what triggers a memory.
 
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