Suspect Output Transformer

[StevenPituch]

Hi All,
I thought I was almost finished with my AB763-2 Deluxe Reverb build, but I ran into a problem.  I was adjusting the bias on the matched JJ 6V6S tubes and I thought things were going well.  Both power tubes were within a half milli-Amp in Plate current.  Then I smelled smoke and shut it down.

I could not see any problems visually so I turned it on again but the Plate currents were 25 and 16 milli-amps now, so I turned it off.  After a thorough inspection I realized that I had a cold solder joint at one of the grid pins.  The tube had suddenly lost it's bias.  The tube appears OK, but I disconnected and tested the leads of the primary of the output transformer and one side was open.  I assume the overcurrent burned out the primary of the output transformer.  It was a Mojotone 768SP recommended in the Hoffman literature and had an 8000 Ohm primary split.  I ordered a Mojotone 773 to replace it as it was cheaper and it is 7000 Ohms for the split primary, which is listed in places as a replacement for the Deluxe Reverb.

I am about to install the new output transformer but the resistance of the splits in the primary are 67 and 78 Ohms.  I am worried that this will significantly unbalance the plate currents of the output tubes as it is a 16 Ohm difference.  Before I cut the wires on the new transformer, I wonder if I should call Mojotone on Monday and ask for a new transformer.  So my question is: Is this output transformer defective?

Regards,
Steve Pituch

[shooter]

are 67 and 78 Ohms

that shouldn't be a deal breaker, most trannies I get have "imbalance"

The tube had suddenly lost it's bias

that should NEVER happen, I'm continually surprised.

I never put in PA tubes until I have verified the bias at the socket, and beat on everything in the circuit while monitoring the grid pins.   Usually I do it a couple times cuz it's fun 

[sluckey]

Before I cut the wires on the new transformer, I wonder if I should call Mojotone on Monday and ask for a new transformer.

Don't cut the wires! Tack solder it in place and check the amp. If it doesn't work ***AND*** you know there are no other issues, then contact Mojo.

[kagliostro]

So my question is: Is this output transformer defective?

Maybe or maybe not

Consider that if you measured the resistance of the winding, instead of the impedance, that difference may happen, what is important is the number of wire turns

If you are winding a transformer and you wind a first coil

then wind a second coil on the first coil, the diameter of the nucleus is being increased

so same number of turns is equal to a more long winding that will have more resistance

as to verify the real situation you must perform a voltage test

connect a low AC voltage to the secondary of the OT and measure if the two branch of the primary are at the same voltage
(pay attention you will have hundreds of volt on primary)

Franco

[trobbins]

You may want to add a poor man's fuse to each 6L6GC cathode for some interim insurance - at least it will provide a simple cathode current measurement point.  Something like a 10 ohm, 0.25W metal film would likely pop itself if a valve was continuously biased full on.

[PRR]

> the resistance of the splits in the primary are 67 and 78 Ohms.

Absolutely normal.

> am worried that this will significantly unbalance the plate currents of the output tubes

How? Normal plate current is, what? 50mA? 0.050A? So one side drops 3.3 volts and the other 3.9 volts. Say it is a 400 volt B+. One side is 396.7V and the other 396.1V. Less than 0.1% different. On tubes which never match better than a few % (and don't need to!). On the element (plate) which has the LEAST effect on plate current.

Worry more about how your signal grid got off its leash.

[Backwoods Joe]

No worries! That xformer should be fine.

[StevenPituch]

Thanks everyone for the education.  I like to learn something new every day, and this amplifier project has not disappointed me.

1) First pf all, yes, I won't make that same mistake again with the grid wire.  Once is enough.  But it could have been worse.

2) I understand now that turns ratio is more important than DC resistance, and why.  Here are some examples of what I've observed with the bias set to -35 Volts:
     a) V7 (JJ 6V6S): Icath=17.6 mA, Iscrn=.5 mA, Vplate=460 V, P=7.9 Watts, V7eff=56%.
     b) V8 (JJ 6V6S): Icath=19.8 mA, Iscrn=.6 mA, Vplate=458 V, P=8.8 Watts, V8eff=63%.

3) When I swapped tubes the grid currents only changed by .2 mA, so the tubes were not causing a lot of the variance.

4) I swapped the wires from the transformer at the power tubes and the speaker and the grid currents didn't change.

So yes, the transformer is fine. The only problem was a "loose nut in operator's chair".  But I am smarter for the effort so it was worth it.

The screen current seems to be a bit low compared to other sources, but I'm thinking that it must be OK.  The bias may be set a bit low, but in my small room testing it, the volume is blaring at "5/8ths of 1", and with a WGS G15C speaker it sounds so good compared to my Fender Mustang I ver 2.  I slowed down the vibrato with a capacitor change, and I love how it sounds.  The only thing I don't like right now is the reverb.  I might have plugged the tank in backwards.  I get a springy sustain after I mute a string, but mot much large room clear echo sound that I was expecting.

So any advice on the reverb is welcome.  I've included a picture.  I scratch built the cabs.

Regards,
Steve Pituch
Hallettsville, Texas

[PRR]

> The bias may be set a bit low, but in my small room testing it, the volume is blaring at "5/8ths of 1"

Bias has almost nothing to do with maximum power. Like a car engine idle RPM doesn't affect the top speed.

If bias is cold, the sound for small signal can be "hoarse". But you may not be able to verify this in a small room where you can't get to large signal.

The VOLume knob setting for "BLARE" has almost nothing to do with bias or power. It is about how hot the guitar is and how much gain you put in the amp. If it is "BLARE" at 6% on the knob, even in a small room, you may have more stages than you need.

[StevenPituch]

Hi all,
Ok, the vibrato and reverb seem to working well now.  I had lots of pulsing, but rerouting the wires to the reverb and vibrato pots reduced it quite a bit.   I made a video showing the current degree of pulsing. I show the position of the four pots (volume, reverb, speed and intensity) in the video.  A guitar cable is not plugged in.   
This is way more volume that I need in the room.   When the volume is turned down to “1”, the pulsing is much less.   I tried repositioning a lot of wires. But this amount of pulsing did not decrease. 
Is there anything else I can try or should I accept this is it now is?

Steve

[sluckey]

Try putting a diode across the INT pot.

[StevenPituch]

HI All,
I've been spending my time trying different things.  Slucky, the diode knocked maybe 1 db off of the hum.  I'm thinking it took half the ac wave out.

Ok, so this is were i'm at.  After a lot of reflowing solder, the amp now is quite stable.  I think that my concern is called excessive "hiss".  When the vibrato channel Volume=8, Speed=4, Intensity=6, and Reverb=0, and the Reverb and Vibrato are both off, the hiss is very low.  When I turn on the vibrato, the hiss is still low but if i really listen what I hear is the vibrato modulating the hiss.  The hiss is the same volume but it is just slightly modulated.  I studied the schematic and I'm thinking this is normal behavior.  The vibrato should work on hiss or actual music.

The problem is when: 1) I turn up the Reverb pot to 7 to 10, _AND_ , 2) I turn the reverb on.  The hiss gets much louder, and when the vibrato is on, the loud hiss is modulated.
Other than the hiss with the reverb, the amp sounds great, both with and without the reverb and the vibrato.

I have been studying articles on grounding of audio amps in a effort to educate myself.  Here are two articles I found interesting.  I've tried a few of the suggestions, but they haven't helped.

http://www.valvewizard.co.uk/Grounding.pdf

http://www.electricalfun.com/WorkbenchFun/reverb_tank.pdf

Any suggestions appreciated.

Steve