Help for power tubes bias circuit

[Bonzai_Musik]

Hello,
I just build a clone of a portaflex 15B from ampeg, and I've got a pretty big issue on my bias circuit, I only get around -6V on my power tubes control grids, which I am guessing is way too low (it is the first time I'm doing a fixed bias amp). A good indication of that is that my tube are red plating ^^'.
So I'll link, the schem, layout and pics of my build and if anyone can help me out I'd appreciate it !
And Happy New Year to all !

 

[Bonzai_Musik]

With the trimmer is sweeps between -5 and -6 V

 

[Matador]

Accidentally used 5K for TR1?
C101 or C102 installed backwards? The + side of the caps needs to be at 0V potential.

 

[Tubetec]

Dont apply HT with such a low grid voltage , you can damage the tubes or burn the transformer primary easily .

best figure out whats going on with the tubes out for now , establish that the required bias voltage is there before trying it with tubes in

At a quick look the first filter cap is the wrong way round , but there seems to be other mistakes too.
Which are the wires to the tube grids ?

The positive end of both caps goes 0V ,

 

[Tubetec]

So , reverse the first filter cap ,
add an extra link between the positive side or lower end of that capacitor and Ground ,
Theres a yellow wire joined to that spot , if thats the grid wire it needs to connect with the junction at the top ( or negative terminal)of the second filter cap
Make sure you have the pot wired as a rheostat .
You may have damaged the cap with the wrong polarity ,
although ususally the blow up or bulge.
That should get you going .
I can probably add some rough sketches on the tag board pic if your stuck.

 

[Bonzai_Musik]

Right so simple, don't know how I missed the caps orientation !
The yellow wire connects the cap to the trimmer lead that goes to ground, the blue wire goes to the 270K resistor which goes to the 1K then the tube grid.
And yes my pot is wired just like a variable resistor.
Thank you very much for the help !

 

[Tubetec]

Ah yes I see how its done now , I think you need to flip the second filter cap around also as its sees - V on its positive terminal as it is .
Do a quick resistance test with the meter across the caps while out of circuit .they may have been damaged.
If you can verify the capacitance value with the multimeter or LCR ,even better .

 

[Admiral-dk]

PLEASE use NEW Capacitors ..!!!!.. as those shown most likely are DEAD now - or at least Damaged ..!!!!
They are so cheap and the posible dammage done by reusing them can be new Transformers etc.

+ as mentioned - Do NOT have the Output Tubes installed before you have measured at least -40 to -60Vdc. on Pin 5 on both sockets.

Set the BIAS to the maximum negative Voltage (probably around -50V to -60V) before you put the two 6V6's back in.
With a Scope on the Output Jack + a Dummy Load connected the same place (I prefer to use the 16 Ohm, with a 16 Ohm Dummy) - adjust the BIAS so you have almost no Cross-Over-Distortion - just before Clipping.

Another option, would be to 'Lift the wire from Start-Ground to V4 Pin 8 and put a 10 Ohm in series. This allows you to easily adjust the BIAS with a Meter.

Per

 

[CJ]

I bet the caps are fine, there is close to zero ripple current in the bias circuit. As long as they are not budging they are fine. Heck if you leave them reversed long enough you can run them backwards.
Tubes should be fine, they operate close to red plating anyway if you crank the amp. In fact that will speed up your burn in time so you will not have to readjust you bias.
I am running cathode bias on my portaflex along with 6384 power tubes with a western electric 5214 opt from 1945 and it sounds pretty good.

 

[Bonzai_Musik]

Hey,
So it works just fine now, set the bias around 10W for each tubes.
But I have some pretty good amount of noise which I think comes from the power tubes, I think I may have damaged them.
This clip is with all pots turned down.
Could you confirm that ?

 

[CJ]

That's a 12ax7,/6sl7, start pulling preamp tubes with the noise going, starting at stage 1 and working your way to the phase inverter, don't be afraid of loud pops as you pull the tubes, try spraying offending tube socket with some type of hazardous chemical. Lightly sand with 320 wet or dry the tube pins , if no joy, try new tube or borrow tube from reverb socket if no new tubes because of Stoopid Peeutin.
If solid state preamp, replace u1 tl072 unless Mouser did not anticipate supply chain failures.

I bet the spray and pray will work.
.unless you used some crusty carbon comp plate resistors

 

[Admiral-dk]

I will agree with CJ that electrolytes are the least sensible polarised capacitors - but I still wouldn't trust them after they have been powered up more than a few minutes in reverse ...!

Power Tubes that has 'Red Plated' have a high risk of having Gas inside .... I have tried to reuse them a number of times after this kind of treatment and in most cases they end up Failing very badly ...!!!
I started repairing music equipment (mostly guitar Tube Amps) in 1990 - so I have repaired thousands of amps since.
I would only try Tubes that have 'Red Plated' again - if they actually can pass a test in a Quality Tube Tester after the fact .... and even then about 50% fail badly, when driven hard for long enough .... though admittedly very dependent on how hard the Amp actually runs the tubes.

Just my two cents .... for what ever they are worth ....

Per

 

[CJ]

I did have one amp make that noise due to a bias cap. Fender used to use a cap rated right at 50/50 on a -52 volt line, like I say that is such a low power circuit that you can get away with that. First thing everybody says is " you gotta upgrade the v rating on that cap" like those dead guys did not know what they were doing for years and years on thousands of amps,


I had a friend with a sunn solarus or Solaris, that had Millard el34's, he always had it running wide open, he was like Geezer Butler on steroids. Use to drop the E sting to D, only guy I have seen do that, you can play cool chords with that tuning, darn whisky took the guy away, one night at a house party his power switch gave up the ghost. This guy could get violent when things did not go his way, especially on the JD , i pulled the chassis and got a spoon out of the kitchen and bridged the contacts on the switch while lying on the floor. Got him through the last couple of songs.
That amp in a dark room you could see a red glow on the plates along with the blue gas light effect. Thing ran like that for probably 10 years, yeah, the Russian stuff ain,t gonna cut it if you were to try doing that.


Spell checker likes Millard instead of Mullard, wtf?

 

[Admiral-dk]

OK - when I mentioned 'Red Plated' - I mean strongly glowing Red in full daylight ...! => glass melting after about 5 minutes ...!

Not a faint glow in the Dark ....

The Blue Light normally means nothing / has no influence -> I can easily believe that they will emit Blue Light and play without any problems for their whole life (even decades).

Per

 

[CJ]

The plates are large chunks of metal, they will be ok, it's the screen grids that get cooked, fine wires running in triode,

 

[Matador]

I did have one amp make that noise due to a bias cap. Fender used to use a cap rated right at 50/50 on a -52 volt line, like I say that is such a low power circuit that you can get away with that. First thing everybody says is " you gotta upgrade the v rating on that cap" like those dead guys did not know what they were doing for years and years on thousands of amps,

I always wondered about that: I've had a handful of these caps pop when the diode shorts (this is rare). The 60VAC (ish) winding can go up to 70V or so if the wall voltage is hot: I never understood why a 100V part wasn't used here, but at the time it may have been a large cost/size difference. 100uF/100V is a standard part I swap into that position.

 

[Bonzai_Musik]

Actually after playing on a proper cab for a while, I don’t think the problem lies in the output tubes, the tone control affects the noise, any ideas on what to check first ?

 

[CJ]

tighten the pots, spray if they are old, clean chassis where pots bolt in, check gnd wires in tone circuit, check for leaky coupling caps by looking for dc on the pre amp tube grids, (unless they are supposed to have DC on them)