Fender Supersonic 60 combo Question

[DRH1958]

32mV is the correct bias according to the schematic.
If the plate voltage is that low (should be about 480V), this is probably why it won't bias any hotter.
You will need to figure out if the plate voltage is low because of the power transformer.
Does the heater voltage measure 6.3VAC at the pilot light? Does TP10 measure 27VAC? 180VAC at TP1 ?

Here's the data on the replacement tranny: http://www.hammondmfg.com/pdf/EDB290VEX.pdf It gives the hot-hot voltage on the high voltage winding secondary as 342 V. The schematic gives 180 volts to ground at test point 1 as g1 pointed out, so that translates to 360 volts hot-hot. The secondary voltage out is 18 volts lower in the replacement tranny. This doesn't account for the total difference but if the line voltage is low, that could do it. See if you can get a measurement of your wall voltage in addition to g1's tests to see if that is contributing to this.

EDIT: I should have also said Hammond gives the primary voltage as 240v on that data sheet, so anything lower than that will lower your voltage. So if you measure only 220 volts, then that translates to only 313 VAC out of the secondary. That will account for your low voltage, 47 volts AC lower than spec. This gives ~60 volts DC lower when rectified.

[g1]

That would explain it as the line voltage in Thailand is supposedly 220V, not 240V.
However, in the original post it was stated that the amp was 'working great' with the Hammond installed.
So I would suggest you try the amp with the bias set for 32mA and see if it sounds ok. It will probably sound fine.

[Gingertube]

See g1's schematic post.
The supersonic has a bad habit of whistling at you when gain 1 or gain 2 is set high on the burn channel. Why? Coz the 3 cascaded gain stages run from a common power supply node labelled 'X' on that schematic. Fail of your design review mister Fender.
As soon as the bypass cap on that node starts to go slightly high in impedance (with age) that channel starts to whistle.

On the last supersonic I saw this in, I simply added a 470nF/630V film cap from the top of R40 to 0V. That is, made sure that B+ node was bypasses as good as I could make it, done right at the critical point.

I post this coz the 1st time I encountered this it took me hours to nut out what was happening and how to fix it.
Cheers,
Ian

[gtto]

I don't know if it makes a difference but I set all the knobs and switches on the amp to what was recommended on the schematic. I got a plate voltage of 435 and was able to bias the tubes to 35 and 38mA. I have no idea if this changed because of that or because of something else. Wall voltage is 220. When I test tp10 though I get 71VAC. I don't know if I am doing something wrong because that is obviously way off the 27 it should be. When I test R156, the bias potentiometer, I only get -45.7VDC when it should read -54.5 . As I previously stated I am new to all of this. Learning fast but not fast enough. Question. When I measure the pilot light do I use the chassis as ground or do I have to measure across both of the wires on the lamp? Thanks.

[gtto]

I think it's time to give up......I have been playing for a few hours and rechecked the bias. Now it is 33mA and 17mA! Obviously something is wrong and it is beyond my abilities. Problem is finding someone here that can figure it out. What a drag.

[nickb]

I think it's time to give up......I have been playing for a few hours and rechecked the bias. Now it is 33mA and 17mA! Obviously something is wrong and it is beyond my abilities. Problem is finding someone here that can figure it out. What a drag.

If the bias is varying then check to see if the voltage on pin 5 of the 6L6's is varying too as it controls the bias current.

There may be a bad contact to any of the 6L6 pins - inspect and resolder anything suspicious.

If they are steady then it might be a poor contact in the 6L6 tube sockets - inspect them for damage and gently bend the sockets a little tighter with a suitable pointed tool in necessary. Also try cleaning them with contact cleaner.

If either is varying then remove the coupling capacitor to that grid i.e C71 or C72 in case it has gone bad and see if that makes it stable.

Maybe you already tried new 6L6's - you wouldn't be the first to have one with an intermittent internal contact.

[g1]

AC voltage at the pilot lamp is across the 2 wires, not to chassis.
If heater voltage is quite low, it may explain your fluctuating conditions.
Also, just to make sure, bias should be checked with no signal, and after tubes stabilize a minute or so if you have been playing.

[gtto]

AC voltage at the pilot lamp is only 4.5VAC. What would cause this?

[Enzo]

Are you actually monitoring the mains voltage at 220vAC? Or are you assuming that is the voltage because it is supposed to be that?

[gtto]

Yes, I checked. It is 220. I put in an old tube I had which biases higher than one of the new ones so maybe something is wrong with the tube but that doesn't explain the low voltage heater voltage. I realize from some of the replies above that the transformer may be a cause of the low plate voltage and some of the other issues but i don't know if anything can be done about that short of changing it if that is necessary.

[DRH1958]

Are you totally sure of that 71 VAC on TP 10? All other voltages are low except that one which is wildly high. It should be 27 VAC. Are you sure your battery in your meter is good? The meter was set to the proper setting? Low battery will give false readings. It's strange it's that high because you said you only had -45.7 volts at R156 bias supply trimmer which is low. This winding that is so high feeds the raw AC to the bias supply, so it should be too high neg. bias voltage, not too low. Something is not adding up here. You could also try TP 11 which is the other half of that secondary winding, also should be 27 VAC to chassis. This winding also supplies the +/- 16 volts for your op amps and other solid state circuitry. Try test points 12,13 to see if you get +/- 16 VDC.

To my way of thinking it seems as though you should do whatever you need to bring up your voltages, especially since your heater voltage is so low. At 4.5 VAC for your heaters, your tubes won't conduct as well as they should and this adds to your problem of low plate voltage.

[DRH1958]

I also figured out why your heater voltage is low. The data sheet says it can supply 3.75 amps of 6.3 volt power. You have eight 12A_7 tubes at .3 amps each for 2.4 A. Then two 6L6s at .9 amps each is 1.8 amps, so a total of 4.2 amps. So this combined with low line voltage gives you low heater voltage. I figured with only 220 volts in you have a reduction of, 220/240=.92 factor lower. So 6.3V X .92=5.8 volts out just because of low line voltage. This itself is the lower end of where you want to be and being overdrawn on amperage, the result is your 4.5 volts on the heaters.

[g1]

Are you totally sure of that 71 VAC on TP 10? All other voltages are low except that one which is wildly high. It should be 27 VAC. Are you sure your battery in your meter is good?

I think the glitch in that reading may have to do with the diodes there and the type of meter in use (cheaper non-rms type). I bet if he disconnected that winding and measured, the AC voltage would be in the ballpark (but low) for that winding. Measuring there through a cap might also give the correct reading.

With 4.5VAC at the heaters you are going to have trouble. Especially if you go somewhere that the line voltage is a bit low.
The problem is that the Hammond transformer needs 240V on the primary and you are only giving it 220V, and made much worse by the over-current that DRH1958 mentioned above.
If you don't want to change the power transformer to a 220V model, you should at least buy a separate filament transformer to give you 6.3VAC at the heaters.

[gtto]

Tis getting so frustrating! g1....yes I am using a cheap meter! The reason I changed the transformer in the first place instead of using a voltage converter was because in Thailand they use both the flat pronged AC plugs they use in the US and the round European plugs. I was afraid that if I left the 110 transformer in the amp someone could mistakenly plug it into the 220 outlet and fry everything. It seems the best option would be for me to get the proper Fender export transformer. To the best of my knowledge this is 230V. Would that be OK or would I run into the same problems? Also, given the current situation is it ok to use the amp as it is or can something be destroyed? I notice that if I am playing for a few hours the high gain channel loses volume and gets distorted which is what happened originally to cause me to change tubes. I don't know if this is because of the transformer or is something completely else.

[DRH1958]

I wouldn't operate the amp for now mostly because of such low heater voltage but also because you don't really know what voltages you have for sure with the cheap meter. All other voltages will be low as well.

I would think just 10 volts low on the primary will be OK since it's the Fender tranny for this amp. Make sure the high voltage secondary is 360 VAC between hots and you have at least 4.2 amps for the heater current. Many export trannies do have a 220 volt tap, so look for that. Also, make sure it's rated for 50 Hz. power. I'm not exactly sure if this is essential but you could inquire at Fender or wherever you get the tranny from.