Weired amp noise

[pbman1953]

I recently picked up a a YBA 3 Custom.  I've been updating older parts to newer ones. I've completed most of the changes besides the filter caps. There is one weird issue that has stayed from before and after the updates.

When tapping anywhere and I mean any anywhere, even ever so slight a tap, a noise comes out of the speaker like you're clearing your throat. You know that Eh Chem sound that you make? That's the sound that comes from the speakers if you tap any where.This issue exists even with all controls at zero. Now I'm sure you're thinking microphonic tube or bad tube in general. I swapped them all one at a time. And remember controls are zero. Even if I lightly tap a transformer it happens, it's so strange. I reinforced grounds with no change

Has anyone ever experienced such a thing?



Also, there are 2 screen resistors, one to each pair of tubes off the bias adjustment. It was suggested to me to remove them and have a 1k 5 w resistor off the bias line so each tube would have it's own resistor. Physially there are 2 lines off the bias line and I'm suppoed to add 2 resistors , to each line then connect each resistor to the pin 4 of each tube.


Here's the explanation but when i did this , the bias volatge was way too low to adjust so I put it back

Right now your screen voltage supply feeds two tubes directly and the other two tubes it goes through those 47 ohm resistors. Pull those out and put one 1K 5W ceramic resistor on each output tube's pin 4 connecting to the screen supply. So instead of two connected directly and two going through 47 ohms resistors, all four will be the same, each having its own 1K resistor


Here's the schematic-

http://www.freeinfosociety.com/electronics/schematics/audio/traynor_customspecial_yba3c.pdf


Thanks

[Dave]

Sounds like a cold solder joint to me.

Dave

[DummyLoad]

bad solder joint.

[Willabe]

Hi pbman1953,

I think you have g2 (screen) and g3 (suppressor) mixed up with each other.

Traynor put the -bias supply on the --suppressor-- grid, g3, on some of his amps, not on the --screen-- grid, g2, giving g3 a more negative voltage than if it were hooked up to the cathode, which is at ground. Look a little closer at the schematic, you'll see it, the -bias goes to g3 pin # 1 and is separate from the + screen supply, g2, pin # 4.  The EL34/6CA7 tubes have g2 (screen) connection at pin # 4, and g3 (suppressor, EL34, beam forming plates, 6CA7) connection at pin # 1. American types like 6L6, 6V6, 5881, 6550, and some others, all have the same pin out, with the beam forming plates being internally connected to the cathode, at pin # 8 only, with no separate g3 pin out access.

Traynor along with Garnet used this type of wiring, sometimes. Garnet is said to have used it more often. You can't do this on most of the more common guitar amp octal tube types from the USA (because they don't have a separate g3 pin out, like EL34, 6CA7 types).

From what I've read, some like this type of g3 wiring schem, some don't, what else is new?     What is the pay off for this hook up, I don't know. Is it better to tie g3 to the cathode (ground), I don't know that either, but I'm sure someone here does.

IMO, yes get rid of the 2 - 47R screen resistors and put in a separate screen grid (g2) resisitor for each power tube, in this case as you said, 1K, 5W, (which is a very common value for EL34/6CA7) should work well, although IIRC, HBP said 2W is all that's needed. Flame prof type should be used.


    Hope this helps you,           Brad            

[pbman1953]

Dave & Isotone,

I hit every possible connection and no change


Willabe

You're 100% right, I looked at the schematic and the board again. The thing is that I installed the 4 resistors. After,  I went to readjust the bias and noticed that the resistors took away cathide current.  There's an adjustment pot on the board and it kept going clockwise looking for more current. At this time I took out the 4 - 1k's 47's and have the just 2 1k's in their place , like the 47's were positioned. Still the bias control is at 4 o'clock. I'm sure by upgrading the bias caps has sometinhg to do with this. I have a Bias Rite meter and I was always told to bias for 60-70% of the plate disapation. At the settting now for the 4 tubes-  12,14, 15 ,18-watts . The tubes are out a bit from each other and the amp still has alot of power.

This amp can handle 6550's. but I think all you do is change a resisitor or 2. The transformer still have planty of power. I'm reading easily 550 plus. Do you know about this too?

Thanks

Rich

[pbman1953]

  Seems that 2 power tubes are bad.

I need suggestions because I know 6550's and KT 88's but not any others. This amp has 6CA7's/EL34's

I've used Doug from Doug's Tubes, good guy, and he suggested, for bass, the Ruby EL34BHT.

Any other suggestions? This amp can go to 6550's too but I'm not sure what needs to be replaced.  Possibly the bias resistors? Not sure
Measuring with my Bias Rite the transformers have plenty of juice. I'm reading from 526-575 volts
Thanks

[Willabe]

No I'm not the guy you want for this. Just wait for one of the other guys to chime in, they'll help you.

           Brad