Fender Bronco Amp - tremolo ticking sound

[Bushbaby]

Since I restored this little amp some time ago and after watching a Youtube 'Uncle Doug' video about restoring an old Fender Blackface Vibro Champ (the same amp as this), it made me wonder if the 6V6GT output valve was overpowered (as Uncle Doug commented about the practice that the Fender Manufacturer would do with their smaller amps). Its meant to be a 5 watt amp which should harmonically distort very easily yet mine has never seemed to been like this. 

Following Uncle Dougs guidance, I initially checked the 6V6GT plate resistor; the voltage drop across the resistor and plate voltage - 537R (should be 470R), 23.2V and 343V, respectively. Following this through, this gives an output of 43.2mA to which if my calculations are correct is 14.8 Watts output from the 6V6GT, which seems a wee bit too high. Therefore, I replaced the 470R plate resistor with a 620R resistor (2 watt), and values as determined before where found to be 24.6V drop across the resistor and a plate voltage of 350V. Again, if my calculations are correct, this is an output of 36.2mA which gives an output of 12.6 Watts which seems much better.

It sounded really good BUT I am sure I heard some sort of noise which has made the amp sound a little microphonic (Its like the jack plug is 'loose' in the socket type sound) and also with the tremolo speed and intensity turned up to 10 on each control (with no input) I can definitely hear a low level ticking sound coming from the speaker. 

Does anyone have any idea what I should initially evaluate? Has the tremolo ECC83 got a problem now that wasn't there before?? NB - I had recapped all of the electrolytics when I originally restored this.

Many thanks in advance

Rob

[Murphyv310]

Hi.
Id not worry about over running a 6V6 they are remarkably tough and long lasting. I repaired a PA amplifier at Andrew Barclays in Kilmarnock around 12 years ago, this amp used an EF37 6V6 and 5Z4, it had never been turned off in over 50 years and no one could say it was ever fixed. It's condition made me think it was untouched. It was thick with dust and other debris. The fault was a broken lead from the output transformer. After fixing the lead it was perfect. Those valves I would say must have held a world record for life and use. I cleaned up the amp while there and did voltage readings and was quite surprised that none of the valves were low emission. I've also run 6V6s at 400 anode volts with no I'll effects

One question does this amp have a mechanical reverb unit?

[Murphyv310]

Incidentally.
The power dissipated is not the output power of the Amplifier. The power dissipated is much higher than what is available at the speaker.

[Murphyv310]

Hi.
I don't see a plate resistor in the diagram. There is a cathode resistor of 470 ohms. Increasing the resistor will alter the operating conditions of the output valve. You are altering the bias on the valve.
Many people will make changes to a circuit to improve reliability but think this way. These amps were common and never known to eat 6V6s. Do you use it for hours every day? I suspect no so I'd replace the cathode resistor with the correct 470 ohms. If you really are worried the 6V6 then reduce the HT. You could add a 100 ohm 10 watt in series with the centre tap of the HT winding of the mains transformer that goes to chassis, this will reduce the HT a little but in all honesty I'd leave it as it should be.

[westcliff]

Hi Rob,

I have a Deluxe Reverb with a ticking vibrato.  I used to have a copy of the Fender service bulletin which described a fix, but I can't find it just now.  You may find the linked Youtube video helpful.

https://www.youtube.com/watch?v=9GAkTDwp1BY

Gus.

[Murphyv310]

Hi.
I remember the ticking now watching the video. Don't remember it on the small practise amps TBH. The last Fender I did a couple of years ago was a 70s amp and I have to say the wiring wasn't as good as earlier ones, also I didn't like the black paper based group board, poor quality item that was severely leaking electrically in that particular amplifier.

[Bushbaby]

Thanks both
It doesnt have a reverb unit but does have a black paper board (not brilliant!).

Best Regards

[westcliff]

It's the vibrato (tremolo) in my Deluxe Reverb amp that's ticking.

[Bushbaby]

I believe that the deluxe has an optical photo unit in the tremolo section but there isn't one in the Vibro Champ / Bronco. I am wondering if its an close lead with electrical leakage or that the ECC83 I put in has gone microphonic. I was told (I cant remember who?) that it didn't matter what make of valve was put in this section as it doesn't form part of the direct sound output. Is this correct?

Best regards

[Murphyv310]

Two things to remember.
An ECC83 is an ECC83 whoever makes it, it's a thing that audiofools believe. OK there are a few poor qualty ones around but often Philips, Mullard, Minwatt, and Mazda are all from the same factory. Telefunken are excellent as are Tesla. Some of the ones by Bentley Acoustic, Colomor and Pinnacle are just rebadged Mullards.
Microphonic valves don't tick, they make a noise when tapped, why did you sub the ECC83? Was it faulty? Did the issue start after subbing the valve or changing the output valves cathode resistor or was it there beforehand? If any of those proceedings started the issue then reverse them and see what happens.
Ill reiterate that some on the Internet do mods that are unnecessary and many owners do them because they see it on the net, if the manufacturer thought it wise well they would have brought out a service bulletin.

The dressing and run of the leads is possible as is decoupler being low value.
In fact it's quite possible it is the ECC83 due to pick up, just swap the two of them over.