SSS to negativ bias voltage???

[soeves]

Hello from Denmark

I am reparing a SSS 002 that I bought some years ago. Found a dead filter cap and thought that was it. But when checking up on bias voltage on powertubes I measured -181 volts on pin 5. In my amp - 361 volts arrives from rectifier and is divided by two 220 kohm resistors on the one side and two 220 on the other side. The pin 1/5 on the powertubes is connected to the end of these resistors. But voltage is at each point -181 volt. The incomming -361 volt is - after feeding the two resistorpairs - led further/on the underside of board to 820 kohm resistor. This resistor is connected til a potmeter and further downstream to ground. The voltage for the tubes is NOT influenced by the potmeters position.

I cant figure out what is wrong. The amp used to be fine?

[martin manning]

But when checking up on bias voltage on powertubes I measured -181 volts on pin 5. In my amp - 361 volts arrives from rectifier and is divided by two 220 kohm resistors on the one side and two 220 on the other side. The pin 1/5 on the powertubes is connected to the end of these resistors. But voltage is at each point -181 volt.

My guess is your driver tube is not conducting and needs to be replaced.

[soeves]

Thanks for answering! I am running it without any tubes, when meassuring minus 181 volts on pin 5 on power tubes sockets.

[martin manning]

You won’t get the right bias or bias adjustment unless the driver tube is installed. I would put all of the power amp tubes in and see what you get.

[soeves]

Okay! Thanks. Will do that. It just bothers me that I dont understand how the biascircuit works. Any suggestions for some reading?

[martin manning]

The drivers are cathode followers, which you can read about here: https://www.valvewizard.co.uk/accf.html They are AC coupled and fixed bias (see at the bottom of the page). The DC voltage on the cathodes will be a couple of volts above the voltage on the grids, and it is that DC voltage that sets the power tube bias. You want to use a well-matched tube there.

[soeves]

Thanks again! I have now inserted a ecc81 in v7 CF, and see a voltage change on pin 5 from minus 181 to minus 82-87 depending on the adjustment on the black pot. Still to low. I realized that the amp earlier had 12ax in the v7 and also that the builder has fitted two times two 220 kohm in parallel at the point where the minus 361 volt comes from the rectifyer. I am a bit confused now?

[martin manning]

I believe you will have to reduce the 100k resistor feeding the bias pot. Try paralleling a 220k and see if that gets you closer.

Is there some reason you want to use ECC81 instead of ECC83 for the CF tube?

[soeves]

I fitted all the tubes in, with ecc81 in v7. On Standby the volt on pin 5 on power tubes reads ca 80 volt. When I shift standby switch to “on” the voltage drops to 45 volts aprox, depending on the pot meter. So everything seems right when all tubes are active. I just dont understand the circuit!!

[martin manning]

There is an issue with that circuit in standby. See if you get a negative voltage at the screen filter positive terminal in standby. You might want to fit a 1N4007 diode across the screen filter capacitor (with the cathode to positive) to bleed that off.

[Richard1001]

I fitted all the tubes in, with ecc81 in v7. On Standby the volt on pin 5 on power tubes reads ca 80 volt. When I shift standby switch to “on” the voltage drops to 45 volts aprox, depending on the pot meter. So everything seems right when all tubes are active. I just dont understand the circuit!!

It is normal operation to see -80 volt in standby. The driver tube is wired as a cathode follower. This means that the voltage on the cathode follows the voltage applied to the grid. The voltage on the grid is set by the voltage divider (resistors 820k and 130k, or in your case 820k, 100k and a 50k pot)

With the amp in standby, there is no positive supply voltage on the driver. Without the positive supply the anode voltage is +/- 0 volt instead of the B+ of 420 volt when the amp is on.
Without the supply voltage, the tube will try to follow the grid voltage, but can not supply enough current to achieve this. So it will only come up to -80 volts.

With the B+ applied, the tube can deliver enough current to achieve te correct voltage drop across the cathode resistor to follow the grid voltage. In your case -45 volt.

Since there is still a current flow in the tube when the amp is in stand by, the tube will pull the voltage down on the power supply capacitors. (They get loaded bij the negative bias supply trough the driver tube.) A reverse wired diode over the supply voltage to ground will prevent a negative voltage on the supply capacitors in standby.

[Richard1001]

I read you are using a ECC81 / 12at7 tube for the driver. This should be an ECC83 tube.

This circuit is not designed for a ECC81. The isolation from filament to cathode is less than that of an ECC83. (90 vs 200 volt)

I would recommend changing the cathode resistors back to 220k and making the resistance from the cathode's to ground the same value. (The original schematic uses 470k resistors on all of the power tube sockets from pin 1 (the grid resistor) to ground. And put in an ECC83 tube.

[rootz]

What part pf the bias circuit don't you understand?
The principle is largely the same as with a normal fixed bias amp, you just supply a different stage with the bias voltage. But still a fixed bias amp. The driver tube 'forwards' the bias voltage to the power tubes, because it is DC coupled. The drivers cathodes are not grounded, but connected to the full negative voltage. Otherwise the circuit would not work. If you don't understand, please point out the parts you don't understand in the schematic. Upload here. There's quite some folks here that will help you!
I've tried ecc81 and ecc83 as drivers in the SSS amps (mostly 005). IMHO there is no contest: the 83 sounds more fluid, a bit more compressed and just more right than a 81. Dumble definitely got that right. YMMV.
I see Martin already mentioned to fit a protecting diode. Screen node goes negative in standby? check that. In standby of the 005 the plate node for the driver went negative. Killed two screen caps that way. Those are absolutely not designed to accept any more reverse polarity than mere Volts. And they probably see more in standby. So check that.

[martin manning]

IMO the better solution, if you have a 60V bias tap on the PT, is to convert to the bipolar driver supply originally proposed here: https://ampgarage.com/forum/viewtopic.p ... 11#p368411, and for reference the current version which is described in the .pdf linked here: https://ampgarage.com/forum/viewtopic.p ... 08#p365408
It eliminates the negative voltage problem, and establishes the bias voltage in standby mode. Quite a few people have been using it successfully over the past 10 years. Note the only differences between the original and the latest version are a 47R current limiting resistor added to the 60V feed, and a 150k bleeder resistor to ground on the positive side.

[soeves]

Thanks for all the replies. I understand now that the cathode on the drivertube delivers bias for the outputtubes. And the cathode on driver tube follows grid voltage witch is -80 volt delivered from rectifier via 820 kohm resistor and pot meter. When drivetupe is connected til B+, the voltage changes til aprox -45 volts that are available for powertube grid.

I asked AI about the function of this setup and was informed that the drivertube secures adequate flow of electrons to powertube grid, which in traditional setups can be a problem at high volume because of the 220 kohm resistor between rectifier negative volt and the grid of the powertubes?

Can anyone explain HOW the drive tube secures stable bias voltage til output tubes? The drivertube cathode gets it negative voltage from the same supply and through the same 220 kohm resistors as the outputtube grid. So what does it do that increases the flow of electrons?