Problem with biasing Twin reverb replica

[tedsorvino1]

Hello kind people of the forum. It's the first time I'm writing.

I've got a mid 70s Acousta amplifier (silverface Twin reverb replica). I'm trying to blackface it. Most things go well. The rectifier is a bridge rectifier because of the Power Transformer. It hasn't got a normal channel. There was an active parametric eq (much like super twin) that made things bad, so I took it out.

Now I've changed the bias to a blackface bias adjust by using the hofmann layout. My readings were ok with the silverface one (around 30- 50mA MAX) with the silverface one , but I decided to follow the advice of removing some extra capacitors and resistors and keep it basic (like a blackface one).  Everything works fine on pin 5, without power tubes plugged. I've got readings from -33v fully counterclockwise, to -66v fully clockwise. So around -45v is the average.
But when I plug my tubes there are problems .

 Nothing really good has happened. I raise the value of the resistor (I went even up to 15k ohm...) on the bias circuit and the bias reading doesn't come that higher. Maximum 30mA, while there was a huge red-plating developing (with a 15k ohm resistor). And a rather small number 10-15 mA with a 1k/ 5w resistor. My tubes are ok...rather old ELECTRO HARMONIX but still ok.

Then I changed my readings to mV (even if it's not correct to do so- I did just out of curriosity) and when the current is 15mA the voltage is around 110mV. Pretty scary in a way.

Here're two pictures of the bias circuit

https://imgur.com/a/geaiB
https://imgur.com/a/BdqaD

The bias pot is a 20k linear pot.
Lug 1 has a 22k resistor attached , to ground.
Lug 2 (center) goes to the crossroads of 2 220k Bias splitter resistors.
Lug 3 goes to a 1A/1200v diode (+ side) and a 47uf/100v , the diode (- side) goes to a 1k ohm/5w resistor (the resistor I keep changing values- 1k is where i start from) and the resistor goes to the power tranformer (55v).

My plate voltage on pin 3, is around 430v with the tubes plugged in and around 470v with the tubes unplugged.
They are simple Electro Harmonix 6l6gc (I think they are 30w max). Usually two of them. In an amp made for 4. Placed in sockets 1 and 3. I'm aiming for 42-47mA (I think I can achieve this range with a 25k/ 5w resistor but I will probably burn the tubes and maybe more, so I'm not trying it.

The bias probe I use is similar to this one: https://www.amazon.com/Eurotubes-Bias-Probe-Octal-Tubes/dp/B019PAVXPG
and just measures current.

Is there any problem with any NFB capacitor or PI capacitor.  I changed them some days ago and things worked fine. I guess.....How can I check them for leakage in an easy way.

Any ideas where things may have gone wrong would be much appreciated.
Cheers

[92Volts]

Red plating suggests high current-- even while you're measuring 15ma--  which sounds like  a problem with how the bias is measured, not applied.

If you have a resistor in series with the cathode, you can measure the voltage (often in the millivolt range) across that resistor and calculate the current using that information. This is the most common method. It gives you combined plate plus screen current, and those bias probes advertise they don't do this, but it's also reliable... while these bias probes don't seem right.

The claims those bias problems make sound like they install your multimeter *in series* with the plates-- that's how multimeters measure current, it must go through the meter.

Therefore, they "shouldn't" contain a resistor (it's just not relevant to how they operate) and a DC voltage reading is meaningless without a resistor. Actually, it shouldn't be possible since a meter won't conduct current in voltage-reading mode, and will interrupt everything.

At least use a traditional cathode resistor (and DC voltage reading) to verify the results you're getting from the probes. If it's a small resistor, you can leave it in place even if you decide to use the probes in the future.

[sluckey]

Everything works fine on pin 5, without power tubes plugged. I've got readings from -33v fully counterclockwise, to -66v fully clockwise.

That bias voltage range is fine for 6L6s. There is no need to be modifying anything in the bias circuit.

Then I changed my readings to mV (even if it's not correct to do so- I did just out of curriosity) and when the current is 15mA the voltage is around 110mV. Pretty scary in a way.

Don't do that! If your bias probe instructions say to set your meter to measure mA then do that. Or if your bias probe instructions say set your meter to read mV then do that. But don't be switching back and forth between mA and mV!

I think you should use a fresh new set of 6L6s.

[tedsorvino1]

Thanks 92volts for the reply.
The probe was working some days ago reading current mA...but yes, let's assume it's problematic.

I don't know the method you mention. Some people, like Gerald Weber talk about a rather dangerous method involving the center tap, but my PT hasn't got a center tap.

The cathode is pin 8 on the socket (connected to ground), I think.
What kind of resistor do I use?   
Any range of small values?
And how do I calculate the current (mA) by measuring the resistor for mV? How does this method works?

[tedsorvino1]

Thank you Sluckey for the reply.
Maybe I 've harmed the bias probe by changing the way it opperates, even if I don't believe it can be that sensitive.
But maybe something doesn't work well anymore with that.
On the other hand even if I'm using new tubes, my readings are low, so in order to go up to the range I want, I need to change the resistor. And I'm scared of red plating new tubes.
I mean in order to achieve this range I need to use a 25k resistor, which is rather huge for the bias circuit.

[92Volts]

You would insert the resistor between the cathode and ground. Ohm's law states V=IR or Voltage=Current*Resistance. Use a 1 ohm resistor and 1 milliamp equals 1 millivolt. Use a 10 ohm resistor and 1 milliamp equals 10 millivolts.

If each tube's cathode gets its own resistor, each reading tells you the current through that tube. If both share a resistor, the result will be the combination of both tubes' current.

The center tap method likely uses the center tap of the output transformer, which you do have. I assume it is more dangerous though, and you don't need the benefits it might offer (telling plate current apart from screen current just isn't that important in guitar amp use)

[sluckey]

Everything works fine on pin 5, without power tubes plugged. I've got readings from -33v fully counterclockwise, to -66v fully clockwise.

That bias voltage range is fine for 6L6s. There is no need to be modifying anything in the bias circuit.

I repeat... YOU DO NOT NEED TO CHANGE ANY RESISTOR IN THE BIAS CIRCUIT.

If your tubes are redplating then you either need a fresh new set of tubes, or there's something wrong with your bias probe or meter, or you are not using the probe correctly.

Here's an easier safer method to measure bias current. Install a 1Ω 1W 1% resistor between pin 8 and ground for each 6L6. Measure the mV on pin 8. Since the resistor is 1Ω the measured mV will be the same as the mA. You should be able to easily set the bias to your desired 45mA.

BTW, I've read your thread on the amp garage forum so I'm familiar with what you've done.

[tedsorvino1]

Thank you both Sluckey and 92Volts. I used a new set of tubes. The readings were the same. I will try the 1Ω method you mention and as soon as I ve got results I will let you know. Can I leave the resistor on pin 8 and ground or should I remove it everytime I finish with bias?

[92Volts]

The resistor can be left there. A larger resistor would impact the bias and/or available power from the amp, but a 1 or 10 ohm resistor shouldn't have a meaningful effect.

That bias voltage range is fine for 6L6s. There is no need to be modifying anything in the bias circuit.

I repeat... YOU DO NOT NEED TO CHANGE ANY RESISTOR IN THE BIAS CIRCUIT.

If your tubes are redplating then you either need a fresh new set of tubes, or there's something wrong with your bias probe or meter, or you are not using the probe correctly.

It sounds like the issue isn't that tedsorvino1 can't get enough negative bias to prevent redplating (which suggests bad tubes or need for changes-- even if the changes are just to correct a mistake).

They're seeing a low number, and turning up the bias to compensate, which gives red plating. So I'm leaning towards a measurement problem.

[sluckey]

Set the bias pot so you have -50V on pin 5 of each 6L6. Are the tubes redplating? How does the amp sound?

[tedsorvino1]

Well guys. I did a test measurement with a 100Ω 1/2watt resistor and it seems it's working well. I take 4.5V  which means 45mA x 100.
So I guess it was a problem with the probe. Sometime by Monday I will buy some 1Ω 1watt 1% ones just to make sure everything is fine.
I checked pin 5 and give me -37.5V for the same bias setting.

Thank you both for your great help.
While we are at it I ve got another question regarding capacitors. Because people told me to check my PI - NFB capacitors. And I know they are new. So I decided to measure voltage around their pins.
The 0.1uF NFB Coupling cap takes about 67V and gives 0.4V and the 0.001uF P.I. Coupling cap takes about 61V and gives below 0.20mV (while the measurement doesn't stay steady). Do these measurements seem correct to you?

[92Volts]

That sounds much better!

When you change that 100ohm to a 1 ohm, be sure to rebias. That resistor and the 4.5v across it are large enough to impact the bias, with an opposite effect when it's removed. Not sure about your NFB cap question

[tedsorvino1]

I will certainly do since I'm willing to leave them on the pins. Thanks for your help 92Volts.

[sluckey]

What brand and model meter do you have?

[tedsorvino1]

It's this one

https://www.atel-electronics.eu/produkt.php?hash=02933

[tedsorvino1]

My readings are good with the 1ohm 1% cathode resistor. Biased the tubes around 40mA and the amp sounds good.  I still wonder if there is a problem with the caps. But anyways...thanks once again guys for the help.
Ps. I follow the forum and some of the advice I ve read has help me a lot durin this amp renovation,and  other issues. Especially Sluckey, who is super active, has given to many readers, really big help on many issues. Cheers.

[sluckey]

Permanently installing 1Ω resistors on the cathode is a good move. I have them installed on most of my amps.

I think it's all good. I would not suspect the caps. I think the caps were mentioned early on before anyone knew what was really going on. Besides, you already replaced them, right?

It's possible that your meter function for measuring current has been damaged. You can test it to see. Just put a 220Ω resistor in series with your meter and a 9 volt battery. Set meter to measure mA and be sure to use the proper jacks. Your meter should read approx. 40mA. (You can use any battery/resistor combination and just calculate the current. Compare your meter reading to the calculated current.)

[tedsorvino1]

I did the test Sluckey. Instead of a 220Ω resistor I used a 100Ω one and the reading was around 84mA. I think it's good enough.
Since you 've mentioned the caps do the readings (on board) I mention before for these two particular caps seem normal to you? Cheers

[sluckey]

The 0.1uF NFB Coupling cap takes about 67V and gives 0.4V and the 0.001uF P.I. Coupling cap takes about 61V and gives below 0.20mV (while the measurement doesn't stay steady). Do these measurements seem correct to you?

These readings are fine.

But the coupling caps that were in question are the caps between the PI plates and the output tube grids. The in-circuit voltages for those caps would be about 240v on one side and about -50v on the other side. I don't suspect those caps either.

[tedsorvino1]

Thanks Sluckey once again. 235v on one side, -45v on the other. So I guess things are good. Cheers

[tedsorvino1]

Is there anyplace on the net where somebody can find nominal voltage values for every part of the AB763 circuit (between caps and resistors etc.)?

[sluckey]

Yes. Right here. Click on the "Tube Amp Schematics library" link at the bottom of this page (or most any other page on this forum). Select the "Fender" button. Then scroll way down the page. Near the bottom you will find a link for the "Fender_Twin-Reverb-AB763-schematic.pdf ". Voltages are listed on the schematic. That's all you need.

I could have just posted the direct link to the schematic, but I chose to show you how to find it instead.

[tedsorvino1]

Thank you Sluckey for the info.
The thing is that all the schematics reggarding the Twin Reverb amp do not include the readings around the locations we were talking about (plus many other in different locations on the circuit).
I guess they don't really matter that much- (is there any other reason for not including them?)...but still...it would be great if we could find a complete nominal voltage list.
Thanks anyways.

[sluckey]

The thing is that all the schematics reggarding the Twin Reverb amp do not include the readings around the locations we were talking about

Sure they do. You just gotta know how to interpret the info. The numbers I gave you a couple messages back came directly from that schematic.

[tedsorvino1]

Any help on how could I start learning this interpretation skill?

[sluckey]

Learn how to read a schematic.

[tedsorvino1]

Thanks man. I ve already started. It seems like it's a long way. Cheers.

[sluckey]

You'll learn by doing it. I've seen several people on this forum that have no formal electronics training and were intimidated with schematics. But they have come to be quite comfortable working with schematics since hanging out on this forum. I'm sure they have also read a lot of info either from printed material or just on the net. If you have a desire to learn more, you will. Having a project in hand (such as this amp) will provide a lot of motive to learn more.

[bnwitt]

I recently put together this layout of a twin reverb which has the voltages from both the official layout and schematic.  I did this because not all of the official versions are completely legible. I also made this voltage chart for checking a twin reverb for actual voltages.

[tedsorvino1]

Thanks for the great help and advice Sluckey. Hopefully I will work my way around learning more about the beautiful world of tube amps. Anyways this site and forum are an excellent source of knowledge
Really good work Bnwitt. It would be interesting if somebody had made a layout or chart of all the voiltages before and after any component of the circuit. But I guess with the appropriate knowledge people can work their way around it.
Cheers