homemade Blues Junior sound issue

[sarkipinty]

Hello All,

I have scratch built a Blues Junior based on the following schematic
http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

due to lack of tubes I modified the power section for 2x6V6, essentially similar to this schematic
http://el34world.com/Hoffman/files/Hoffman_5E3.pdf

the problem I have is that the sound of the amp is scratchy and randomly cuts off
if I play loud, the sound is too much distorted
if I play quiet, some notes are amplified too much, some too little
its fairly hard to explain, so I include a youtube clip:

I double checked for any cold solder joints

any tips on which section of the amp could be causing this kind of issue?

thanks!

[John]

I'd swap in another 5Y3 to make sure the one you have isn't faulty, and also double check and/or reflow the solder joints on your filter caps. If that checks out good, then:


If you have 'em, swap each tube out, 1 at a time,  to rule out a bad tube. In fact, maybe do that first, it's the easiest. If they're all okay, check for leaky coupling caps; any DC voltage on the "downstream" side shouldn't be there, and will make your amp sound funky.

[sarkipinty]

pardon me for not mentioning, but I am using a solid state rectifier
I also swapped the pre-amp tubes + the power tubes to see if they are faulty

the coupling caps were a good pointer, altough I think if they were faulty (missing) it would be sqeeling and motorboating
but I checked them anyway, they seem to be ok

when I think about the circumstances, the problem occurs when giving the amp a loud signal, so its either
- the PT not delivering enough juice
- faulty/incorrectly matched OT impedance
- faulty tubes
- something with the negative feedback loop

so what i checked (in respective order):
- B+ voltage does not drop whilst playing
- OT could be a potential issue
- replaced the tubes with same result
- tried disconnecting the negative feedback, same issue

does anyone know an easy was of checking the winding ratio of the OT without disconnecting it from the amp?
(im pretty sure I used the correct one, but I build this more than a year ago and cant remember the exact winding ratio)

[sarkipinty]

after a closer look, I see that I have written the winding ration on the side, so it is 1:25
given a 8 ohm speaker load gives 25*25*8 = 5k ohm - which after some research is in the ball park

[jjasilli]

Those are not schematics, they are layout diagrams.  It is difficult to help if you don't post an actual schematic which specifies the actual transformers & other components used, and actual measured voltages @ key locations.  It is not useful to say that your amp is like some other amps.  It causes confusion and tends to draw replies that turn out to be inapplicable. 

[sluckey]

Those are not schematics, they are layout diagrams.  It is difficult to help if you don't post an actual schematic which specifies the actual transformers & other components used, and actual measured voltages @ key locations.  It is not useful to say that your amp is like some other amps.  It causes confusion and tends to draw replies that turn out to be inapplicable.

Take a closer look counselor...

[John]

Your video sounds a LOT like my old Harmony (real old) that got me started in this. I ended up replacing all the caps, and about half the resistors. I'll never know what exact component was bad, but the overhaul fixed it. I really had no clue what i was doing, or I'd have tried a few things, checked it, tried a few more, etc.


Have you tried chopsticking it?

[sarkipinty]

Those are not schematics, they are layout diagrams.  It is difficult to help if you don't post an actual schematic which specifies the actual transformers & other components used, and actual measured voltages @ key locations.  It is not useful to say that your amp is like some other amps.  It causes confusion and tends to draw replies that turn out to be inapplicable.

if you scroll down, there are the schematics

[sarkipinty]

Your video sounds a LOT like my old Harmony (real old) that got me started in this. I ended up replacing all the caps, and about half the resistors. I'll never know what exact component was bad, but the overhaul fixed it. I really had no clue what i was doing, or I'd have tried a few things, checked it, tried a few more, etc.


Have you tried chopsticking it?

yes, the most "response" from poking around with a chopstick was from the volume pot, the problem is that I already replaced it with another one (this was one of the first thing I tried)
sometimes I cut the shielded signal cables too much and there's a connection to ground which messes up the signal, so I also tried it with a unshielded wire to make sure -> more hum, but same issue

yep probably I'll try to replace the caps, because I also have the feeling it's to do with the power section

[jjasilli]

Ok, there are schematics in your post.  But you say your amp is based on them.  So the crux of my point stands.  Lacking actual specifics as to components, voltages, etc., everyone is shooting in the dark.

[jecars]

To my ear, video clip it sounds like crossover distortion. 

If so, could be caused by cold bias of power tubes. Check to see if idle current is proper (45 to 50 ma would be a reasonable idle current assuming 320 volts B+ for 6V6). 

Also, check the coupling cap values and grid stopper resistor values.  Perhaps large cap values or low resistor values found their way into the circuit by mistake.

[sarkipinty]

To my ear, video clip it sounds like crossover distortion. 

If so, could be caused by cold bias of power tubes. Check to see if idle current is proper (45 to 50 ma would be a reasonable idle current assuming 320 volts B+ for 6V6). 

Also, check the coupling cap values and grid stopper resistor values.  Perhaps large cap values or low resistor values found their way into the circuit by mistake.

the coupling caps and grid stopper resistors are the correct values

but you are right, I watched a video on cold bias, and this sounds exactly like my problem
(watch?v=sWTbFNkvBzA)

however I found that my bias is way too high
the voltage drop on the cathode bias resistor is 21.67 V, given a value of 270 ohm -> 80 mA 
the plate voltage is 318 Vdc

what I would do is try doubling resistor value and try 500ohm, what do you think?

[John]

You forgot to divide by 2 (tubes)

[John]

You're dissipating roughly 24 watts total, so 12 watt Pdiss per tube. Right in there!

[sluckey]

however I found that my bias is way too high
the voltage drop on the cathode bias resistor is 21.67 V, given a value of 270 ohm -> 80 mA 
the plate voltage is 318 Vdc

That sounds about right to me. Remember, that 80mA splits between two tubes. Those numbers calculate to just under 12 watts per tube, or about 85% max dissipation. That's a good place to be with cathode biased 6V6s.

[jjasilli]

Ditto. I was wondering about a bias issue but lacked info.  Your bias seems OK.  80mA is correct. Next step is plate diss.  W = V X I.  V = 318 - 21.67 = 296.   


W = 296V X .08A =  24W / 2 = 12W plate diss per tube.  That's OK.  Plate diss is actually lower, 'cause this rough calculation ignores screen draw. 


6V6 max plate diss 14W X 90% (hot) = 12.6;  X 85% cool = 11.9. 

[sarkipinty]

Hello All,

thanks for all the feedback and tips. in the meantime I found the issue
it is a little anti-climactic, but there was a connection issue between the input jack and the first preamp the screen resistor.

interestingly poking around the shielded cable did not produce any weird sounds, but the vibration of the amp did.
i believe one of the resistor lugs were broken "inside" the resistor, so from the outside no visible problems

but on closer inspection it came apart like uri geller's spoons...

[John]

Thanks for the followup! IIRC, Tubenit had a similar issue he talked about years ago.


Glad you got 'er done! :)