Silverface Reverb Bias Issue

[erock0138]

Hello all,

I aquired a Silver Face Deluxe Reverb in a trade, and am having some issues with it. It's an early model, but I've researched the year of make, and came up with mixed results on transformer numbers.

Anyways, It's a AB763 Circuit. With a hum balance, not a true bias. I have a 72 Fender Bassman Ten that I just modded to be true bias with steps from http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm

I was going to do this on the the Deluxe reverb, but am a little confused as to why the pot configuration is different.

On the site mentioned before, and in my Silverface Bassman, the resistor is on Lug 2 of the pot. In reference to the attached picture, the resistor is on lug 3 to ground.




My whole reason in doing this, is because the amp is now redplating pretty bad. It has 2 6v6 power tubes, and I measured 40 mA of current on each tube, Calculated to have 20 mA +- for safe disapation. I obviously haven't played through it, but I put in a new set of tubes just to make sure the older ones weren't bad, and I got high current readings before I turned the amp off to prevent damage to the new set. The "Balance" pot is turned all the way down, and the lowest mA is 40 per tube.

I also unsoldered one leg of the coupling caps to check for DC leakage, and there wasn't any.


Any suggestions on why the pot configuration is different? Or why my bias is so high?


Thanks in advance,

Eric

[52 Bill]

Anyways, It's a AB763 Circuit. With a hum balance, not a true bias. ...but am a little confused as to why the pot configuration is different.

The "Balance" pot is turned all the way down, and the lowest mA is 40 per tube.

Any suggestions on why the pot configuration is different? Or why my bias is so high?

The photo shows a true bias adjustment pot. If you have turned it all the way down, that is why the tubes are red plating. With the standby off you should still have the negative bias voltage working. Read the voltage coming from the bias supply board, then read the voltage at the wiper of the bias pot. Turn the pot through its full rotation. What happens to the voltage level at the wiper?

I don't think that I have ever seen a Deluxe Reverb with a bias balance control.

[Enzo]

Why do you think your bias is wired as a hum balance? AB763 schematic shows a plain old adjustable bias, not a balance control. Note the common feed to both tube grid resistors. Whatever goes to one goes to the other. Your photo seems to agree. In other words it already is what you want to turn it into.

Which way are you calling all the way down?

Pull the power tubes if it makes you uncomfortable to run them this way. Now what voltage is on pin 5 of each power tube socket? What range of voltage does the control give you? The schematic gives about -35v as the target.


How did you measure tube bias current?

With tubes installed, what voltage is on the plates?

[erock0138]

Which way are you calling all the way down?

The lowest turn of rotation that gives me the lowest current draw, being clockwise.

How did you measure tube bias current?

Fluke 189 Meter, and bias sockets from Hoffman Amplifiers

With tubes installed, what voltage is on the plates?

395

[erock0138]

I was under the impression that all silver faces had the Hum Balance circuit, until I saw this configuration and was confused as to why it wasn't.

The lowest the bias will get is 40mA.

Turning the pot the opposite direction gives me upwards of 50+ mA. (Didn't go any further than that)

[Enzo]

And the key question was the bias voltage at the sockets. WHich is?


OK< socket adaptors is fine. 395 sounds to me like it is being dragged wown by the excess draw. At 20ma, it will probably rebound.

[erock0138]

And the key question was the bias voltage at the sockets. WHich is?


OK< socket adaptors is fine. 395 sounds to me like it is being dragged wown by the excess draw. At 20ma, it will probably rebound.

The highest negative voltage the pot will allow is -26v, turning opposite direction gives me -15 and below.

As it sits with -26v I'm getting 42.8mA of bias current.

[erock0138]

Also, I'm thinking I'll have to change the resistor soldered on the back of the pot. But am curious as to how it's drifted so much, or there's another problem area I should look into.

To my knowledge, the amp hasn't been played in a very long time, and never maintained very well. Though all seems to be original.

[Enzo]

Well there is your problem, the bias voltage is way too low.

Did you measure that resistor? usually they go high, not low. I'd be much more concerned over the 470 ohm resistor on the othe side of the rectifier. See if that hasn't gone way high. And also make sure the DC on the bias filter cap is smooth, with almost no ripple.

[erock0138]

Well there is your problem, the bias voltage is way too low.

Did you measure that resistor? usually they go high, not low. I'd be much more concerned over the 470 ohm resistor on the othe side of the rectifier. See if that hasn't gone way high. And also make sure the DC on the bias filter cap is smooth, with almost no ripple.

I measured the 470 ohm resistor on the bias supply board, got 474 ohms. Looks good.

Looking around some more, I noticed these 220k resistors (right next to coupling caps) looking a little cracked, one measured 260k, the other cracked as soon as i started to desolder. I replaced them with new, correct value ones and the problem still persists. Only -27 v bias voltage. The bias capacitor has almost no ripple as well.

Any suggestions where else to start looking?

Thanks so much again.

[Enzo]

OK then,maybe a couple tests. Pull the power tubes, no need to stress them. Disconnect the wire from the diode/cap to the end lug of the bias control. Unsolder it from the pot. NOW what voltage is across the cap? If it is still only 20-some volts, we have to figure why. If it now rebounds to 40v or more, then something was loading it down.

Power off and discharge things.

While that wire is still off, measure resistance from the now empty end lug of the pot to chassis. We ought to get roughly 20k. Do we? And from the wiper to ground, do we get a range of 10k to 20k?

And I suppose, disconnect the wire from the wiper and measuer resistance to ground. SHould read open.

Oh, and one I should never forget. Maybe do this before disconnecting anything. You can stil leave the power tubes out for this. Power up and ready to play (minus 6V6s), ground your meter. Now with the red probe, and set for DC volts, let's take a couple readings. Look at the eyelet board, the layout has right in the corner near the bias pot two high voltage wires, the red in the corner (420v) and a yellow in the next eyelet (415v). You can measure those two voltages and see if they are present. I don't care if they are off, as long as it is 400v-ish. Now poke your red probe onto the eyelet board itself, NEAR the yellow wire eyelet. NOT touching the eyelet. DO you get any voltage reading? Yes, as an insulator, the board should not have a voltage on it, but they get conductive sometimes. We can deal with that, but let us find out. Also, move along to the eyelets where the two coupling caps meet the two 220k resistors. Probe near to them as well to see if any stray voltages are happening. It is a long shot, but it happens more often than you'd think. EVery amp tech has faced it if he has been around for any length of time. And it takes only seconds to check.

[erock0138]

OK then,maybe a couple tests. Pull the power tubes, no need to stress them. Disconnect the wire from the diode/cap to the end lug of the bias control. Unsolder it from the pot. NOW what voltage is across the cap? If it is still only 20-some volts, we have to figure why. If it now rebounds to 40v or more, then something was loading it down.

Yes, with the bias supply wire disconnected, it put me back up to 47 volts.


Power off and discharge things.

While that wire is still off, measure resistance from the now empty end lug of the pot to chassis. We ought to get roughly 20k. Do we? And from the wiper to ground, do we get a range of 10k to 20k?

Yes, I get 22k empty lug to chassis, and range of 11k-22k.

And I suppose, disconnect the wire from the wiper and measuer resistance to ground. SHould read open.

Yes it does.

Oh, and one I should never forget. Maybe do this before disconnecting anything. You can stil leave the power tubes out for this. Power up and ready to play (minus 6V6s), ground your meter. Now with the red probe, and set for DC volts, let's take a couple readings. Look at the eyelet board, the layout has right in the corner near the bias pot two high voltage wires, the red in the corner (420v) and a yellow in the next eyelet (415v). You can measure those two voltages and see if they are present. I don't care if they are off, as long as it is 400v-ish. Now poke your red probe onto the eyelet board itself, NEAR the yellow wire eyelet. NOT touching the eyelet. DO you get any voltage reading? Yes, as an insulator, the board should not have a voltage on it, but they get conductive sometimes. We can deal with that, but let us find out. Also, move along to the eyelets where the two coupling caps meet the two 220k resistors. Probe near to them as well to see if any stray voltages are happening. It is a long shot, but it happens more often than you'd think. EVery amp tech has faced it if he has been around for any length of time. And it takes only seconds to check.

The red and yellow wire read 475 vdc with no tubes.
Right next to the eyelet for the red wire, I got .5 vdc on the insulator. That was the only significant voltage reading, the other places I checked were very minimal.

[g1]

Erock: If you are going to respond to questions within someone's quote, you need to put your responses in bold or something, otherwise it is hard to notice you have added something within the quote. In post #12 I didn't notice at first that you had added some answers into the quote. Ideally it would be best not to insert anything into a quote, it can be hard to spot. The format you used in post #4 is much easier to follow.

[erock0138]

Ah, I apologize! I didn't even notice until now, posting on my phone from my shop can complicate things. I will now take notice, thanks

[erock0138]

I just compared voltages to schematic in the preamp tubes, (Was mainly focusing on power supply area) And my voltages on pin 6 and pin 1 on the 12at7 closest to output tubes are upwards of 400+ volts. According to layout on link below, should be way beneath that. I'm assuming I would see some drop with tubes installed, and biased correctly, but I didn't think it'd be that significant of a drop. Am I wrong? The amp has no tubes installed right now when I took those voltages.


http://music-electronics-forum.com/a...763_layout.gif