Fender Vibro Champ Amp

[Valvepower]

Got that.

A recent 6V6 adventure…. I have recently built a JTM45 or should that be JTM20 using 6V6 valves in output stage running in fixed bias giving 21 watts at the onset of clip. This was in pursuance of getting the 5F6A/JTM45 sound less some of the power, but still using proper octal bottles. It did work, however, it’s still darn loud!

This uses the Radiospares 300-0-300V mains transformer and the 18 Watt ‘EL84’ output transformer. Under no load conditions the 6V6’s have 370V on the anode, which I felt was enough and wouldn’t like to go further!

Well, to the reason for the post… Doing this amplifier this had me looking at the Fender AB763 Deluxe circuit which according to the Fender circuit has 415V on the anode. Crikey, these 6V6’s are being pushed.

Terry

[Bufo Bill]

In the schematic linked by Terry it has that 330nf Cap marked on it, and it also has the "Death Cap" marked down, also missing on my version. It is also missing on the amp itself, so thats one job I've been saved.
My anode Voltage was at 417V!
Cheers from Bill.

[Bufo Bill]

Just been comparing the voltages I noted down before that resistor blew to Terry's schematic. Instead of 315V at the transformer output I was reading 366V.
At the 6V6GTA anode I'm getting 417V instead of 342V.

[ITAM805]

Hi Bill, just a thought regarding high voltage readings... Fender amps are shipped with the mains tx set to 230V, our mains down here at least, often reaches 253V, so you need to reset the transformer wiring to 240V if possible. You say it's a 1979 model in which case it may not be possible

Here's a short video of what you need to do (and if your model will allow)

https://www.youtube.com/watch?v=7fw58fj-z8c&t=141s

[Bufo Bill]

Thanks for the link. Sadly my transformer is much older (not even PCB era).
Is there anything I can do to lower the voltages safely, or am I just going to have to be a regular customer of the Valve sellers on ebay?
Cheers from Bill.

[Aub]

Is it possible to use series resistor (s) in the H.T. rail, or is the transformer getting too warm due to high mains voltage?

Cheers
Aub

[ITAM805]

So is the burnt out 1k resistor the one from the HT to the screen grid? If so you may have an iffy 6V6 or smoothing cap, or even the 12AX7 in the tremolo circuit?

[Bufo Bill]

Aub, ITAM807, all good suggestions, I'll check things out as soon as I can. It is the HT to Screen GRID Resistor that has blown, I will check the smoothing capacitor tonight. Still no 1k's on the door mat today.
If anybody can explain how the tremolo circuit works, I would be very grateful.
Cheers from Bill.

[ITAM805]

Here you go Bill - https://www.youtube.com/watch?v=1kV2nz4-fXw

[Bufo Bill]

Originally Posted by ITAM805

Here you go Bill - https://www.youtube.com/watch?v=1kV2nz4-fXw

Thanks buddy, I like a bit of Uncle Doug, but hadn't come across this one before.
Cheers from Bill.

[Bufo Bill]

Next door neighbour arrived with 1k's posted to them by mistake this morning. We have it running now. There are still a couple of very poorly resistors that await replacement. The signal seems very intermittent, and comes through weakly at times and proper strength at other times. I'll make sure I've tested all the components and then start looking for bad connections.
Cheers from Bill .

[Valvepower]

Bill,

I pretty much look as if it’s all been covered.

I agree dropping the HT if nothing else comes to light as regards mains voltage setting etc. Personally I’d investigate fitting a resistor in between 100 and 220 Ohm in series with the 5Y3 anodes (pin4-6) off the top of my head it would need to be at least a 5 to 7 watt part as it will dissipating quite a lot of power. These need to be securely mounted on a tag panel or strip.

I had a similar issue with an Push Pull EL84 amplifier build where the HT was too high and EL84 anodes were too far the wrong side of 300V.

Sounds if you have intermittent joint somewhere possibly a solder joint or valve base connection. In a recent restoration of a Clavioline I came upon a dodgy joint that had been there since it was first made in 1963!

I had to explain the basics of a Tremolo circuit for a future BVWS article on an 'odd-ball' guitar amplifier and it was interesting to see the You Toob video.

Regards
Terry

[G6Tanuki]

Anything more than 315-320V on the anodes of a 6V6 is asking for trouble in the long term; as noted upthread adding series resistors either in the links from the power-transformer to the rectifier anodes, or in the connection from the mains-transformer to earth, will help tame things.

[In practice it doesn't matter whether you use resistors in each rectifier anode-feed, or one resistor of the same value but twice the wattage between the power-transformer centre-tap and earth - electrically they give the same result, so which you use is entirely a matter of physical convenience and thermal dissipation]

I'd also check the valveholders; if there is a poor or intermittent contact between the anode-pin of the valve and the anode windings of the output transformer this can rapidly kill a valove: if the anode connection is lost, all the electrons being given off by the cathode then get collected-up by the screen-grid - which is not designed to handle the current and can end up being melted by the energy of this unscheduled electrode-bombardment.

[Bufo Bill]

Okay, today I replaced the feedback loop resistor and the resistors in the 12AX7 oscillation circuit. We have no tremolo and has never worked for me. So could the problem be a faulty 12AX7 as suggested above? Any way to check other than buying a substitute?
Cheers from Bill.

[Uncle Bulgaria]

I second cleaning the valveholders and pins. Polishing those and making sure they're tightly gripping the valve pins (especially likely to be necessary if it's been stored so that the chrome's suffered as you mention above) could make a lot of difference to intermittent problems, as well as be the cause of more significant failures as in post no. 33.

[Bufo Bill]

I will definitely do that. What's the best way to clean those contacts in the holders? I mean how do you get at them?
Cheers from Bill.

[Uncle Bulgaria]

Originally Posted by Bufo Bill

I will definitely do that. What's the best way to clean those contacts in the holders? I mean how do you get at them?
Cheers from Bill.

I don't know the innards of your amplifier, but there are some good general valveholder tips on the vintage-radio.com part of the Stenning Empire: https://www.vintage-radio.com/repair...omponents.html

Tightening is as above - I use a small screwdriver. For cleaning, I've got a can of Servisol 10 and one of De-Oxit, either of which would be my first port of call. A paper towel dampened with a contact cleaner can be abrasive enough to polish the pins. They can be very tarnished, when the wet'n'dry is good. Plugging and unplugging the valves as in the page linked above, transfers the cleaner to the holder and cleans the contact surfaces.

[Bufo Bill]

Thanks, that's a great help.
Cheers from Bill.

[Kyri]

I use a variable transformer with a cheap digital volt/ammeter added to it, calibrated to make sure it is reading right. That allows me to put the correct voltage into my hifi amp which is designed for 117V and since my household voltage is nearer 250V then this transformer with measurement has been very useful. I can even adjust it over time as I notice variation in the input voltage.
I happen to have a vibro champ (late 70's, must dig it out and check exactly!) and some other amps and never considered this issue (and so far didn't measure valve voltages). From now on I will use the variable transformer on them all to ensure I am not cooking all my old gear, and may take the time to measure some voltages too. A very informative thread thanks everyone!

[sortedradio]

For cleaning the valve base pins I spray with De oxit or Servisol, leave for a few minutes, and then insert & reinsert an old valve a few times. In cases of stubborn dirt or oxide I use cocktail sticks. After cleaning I check the tension of the socket with a piece of wire the same diameter as a valve pin, and re tension as required.