Fender Vibro Champ Amp

[Bufo Bill]

@Kyri and @Sortedradio, thanks for adding your comments. @Kyri, I have used my variac to bring down the voltage, not quite as elegant as your solution but it will do temporarily.
Valve pins and contacts have been cleaned and tightened.
I have had a good look round, and found that the two pots involved in the tremolo circuit were bad. The 3 Meg speed pot was totally O/C, and the 25k intensity pot measured 1.5k at max impedance (although it could make it to 2k a little further back along the track). All the Servisol and canned air I could throw at them made no difference, I think replacements are called for. I may open up the three Meg pot to see if the wiper is even in there.
Cheers from Bill.

[ms660]

Originally Posted by Bufo Bill

the 25k intensity pot measured 1.5k at max impedance (although it could make it to 2k a little further back along the track)

Was that measurement with the pot disconnected from the circuit ?

Lawrence.

[Bufo Bill]

Lawrence, I left it grounded to chassis, but removed all other connections. Was that okay?
Cheers from Bill.

[ms660]

Originally Posted by Bufo Bill

Lawrence, I left it grounded to chassis, but removed all other connections. Was that okay?
Cheers from Bill.

Yes, it's always best to measure out of circuit, the parallel path across the track when in circuit is approx. 1.5k.

Lawrence.

[Bufo Bill]

I've had a play with the Variac, and if I have the transformer putting out 271-275V, this runs the 6V6GTA anode at high but safe 308-313V. The circuit is supposed to measure 342V at the anode. Can I use Resistors to achieve this reduction safely, as discussed above? The transformer is barely warm after 30 minutes of playing. The output current measures 0.015A when the meter is put in series between the transformer output and rectifier valve.
Hope this makes sense.
Cheers from Bill.

[Bufo Bill]

5Y3 went "Ppffzzztt." Perhaps that explains things. If you have a spare or know of a reliable seller, please see my request in the wanted section.
Cheers from Bill.

[ms660]

Originally Posted by Bufo Bill

I've had a play with the Variac, and if I have the transformer putting out 271-275V, this runs the 6V6GTA anode at high but safe 308-313V. The circuit is supposed to measure 342V at the anode. Can I use Resistors to achieve this reduction safely, as discussed above? The transformer is barely warm after 30 minutes of playing. The output current measures 0.015A when the meter is put in series between the transformer output and rectifier valve.
Hope this makes sense.
Cheers from Bill.

Don't forget that when determining as to whether or not the anode or screen grid voltages are higher than the recommended maximum voltage given in the valve data the cathode voltage should be deducted from the measurement if you are measuring WRT chassis.

When measuring the current flowing between either of the rectifiers anodes and the transformers HT winding the measurement should be done with a true RMS reading meter, that's because the current waveform is in the form of relatively short duration high amplitude pulses.

As an approximation in the absence of a suitable meter the RMS value of the current flowing between the anode of the rectifier and the transformers HT winding will be 1.1 times that of the amplifiers DC load current, it's that RMS value which should be taken into account when determining the power being dissipated by any added series resistance that's connected between the transformers HT winding and anode (P = I squared*R)

Lawrence.

[Bufo Bill]

Thanks Lawrence, that explains it well.
Many thanks from Bill.

[PJL]

A solution for taming volts that does not create extra heat and is easily reversed is a bucking transformer. We have 250V mains here...

It also keeps the LT voltage correct and does not mess up the recovery time after being driven hard (apparently critical in guitar amps) .

[Bufo Bill]

Interesting suggestion, many thanks PJL!
Cheers from Bill.

[Aub]

What's a bucking transformer?

Aub

[Radio Wrangler]

Bucking in general means producing a voltage (or current) which opposes and therefore subtracts from a main voltage (or current)

For example, if you had a 10:1 ratio transformer with isolation between 240v primary and its 24v secondary, you can connect that secondary to whatever you like. You might just want 24v.

But you can also connect it in series with an onwards feed of the mains to something else. Depending on how you connect it it may be in adding phase and will boost the ongoing voltage from 240v to264v (240+24) but if it is in subtractive phase, it will buck the ongoing voltage to 216v (240-24v) This is what is meant here.

You can also put the secondary in series with the mains ongoing AND the feed to the transformer. This makes it into an autotransformer.

Note that the secondary carries the full load current, but not the full load power.

This is efficient use of a small transformer and a few secondary tappings can give a good number of voltage variations.

David

[Aub]

Thanks David. All clear now.

Aub

[stevehertz]

Originally Posted by PJL

A solution for taming volts that does not create extra heat and is easily reversed is a bucking transformer. We have 250V mains here...

It also keeps the LT voltage correct and does not mess up the recovery time after being driven hard (apparently critical in guitar amps) .

In opposition to what is said there, guitar amp manufacturers went back to using valve rectification around 30 years ago to re-introduce 'sag' in the HT when the amp is driven hard. This provides a degree of 'natural compression ' and other tonal artifacts much loved by guitarists.

As an aside, the subject of guitar amplifier sounds covering distortion characteristics and tone is vast and one that is discussed and debated in many places by tonal obsessed guitarists. A guitar amp isn't just an amp that distorts like a buzzy fuzz box, it's one that morphs from crystalline clarity into 'nice sounding', harmonically rich distortion, with different designs providing different types of distortion depending on the needs and desires of the musician. In short, distorted guitar amplification is a huge, healthy industry of dedicated manufacturers and designers.

[Bufo Bill]

Success! I was using the earlier schematic which was the wrong design. They upped the Voltages even more on the later design, so unbelievably 417V at the anode of the 6V6GTA was as designed! Ouch!
I now have an anode Voltage of 300V when subtracting cathode Voltage. I used resistors in series from the transformer secondary wires as suggested.
Bill.

[Bufo Bill]

Well we seem to have more valve troubles. The replacement 1k resistor blew, so following the tip off from ITAM807, I started checking valve voltages, having checked smoothing caps recently. The ECC83 responsible for the tremolo oscillator seems poorly to say the least. This seems to fit in with the poor Tremolo function. Both anodes measure 320 VDC, while the grid on pin 7 seems to have 150V flowing on it. Checked the 470K resistor on the 1, 6, & 7 valve pins which seems within spec.
If you want to follow along and check out a schematic, it is No. AB764 not AA764 as I alluded to earlier. Remember I have lowered the HT to 315V from the transformer.
Cheers from Bill.

[Valvepower]

Hello,

To give heads-up I’m still very slow with the typing post the trigger finger pulley release operation on my left hand a week ago so I'm still carefully typing mainly one handed!

It maybe it’s Terry and the obvious but when you say 1K resistor you mean the HT resistor between the 40 and 20uF reservoir and bypass/filter capacitors?

There is a high voltage on the anode of V2b which may cause the V2b valve to break down but seeing there is 68K in the cathode I feel there couldn’t be enough current flowing via the anode to the cathode of V2b to cause the 1K to pop unless there is another path.

Maybe the screen (g2) of V3 6V6 is going short? If this valve has had a rough ride due to the excessive HT it could be failing?

The [original manufacturer] valves we used at Roost in the 1970’s were far more robust than the modern valves, however I’d say that as an old timer.

I remember Robin Trower killing a set of Russian thin tube/bottle GDR EL34’s in Marshall JCM100 in more or less one gig

Sorry waffling on now!

Terry

[Bufo Bill]

Terry, thanks for persevering, you've been a real help. I will check g2, I will also have another look at the capacitors.
Cheers from Bill.

[ITAM805]

Hi Bill, I concur with Terry, your 6V6 would be the main suspect. Replace the 1k and take the 6V6 out of circuit and measure the voltage across the resistor, this will reveal the current flow through the remaining circuit and prove if the valve is duff or not

[Bufo Bill]

Hi, with the 6V6GTA out of circuit there is 6mA across that 1k resistor. I'm afraid I need help from here to draw conclusions on the state of the 6V6GTA.
Cheers from Bill.