Biasing a bogen srb-40

[Carbsandcaps]

Working on a bogen stereo receiver and I’m trying to set the output bias. It has a weird current monitor setup where normally the cathode goes straight to ground but then you open these connectors and that puts a 15ohm resistor into the circuit. I’m assuming you then set the bias through the resistor to ground. But then when you reconnect it to ground doesn’t that change the bias or is it not that critical? Do you make your calculations to compensate for the 15ohm resistor?

I’m also trying to figure out the correct bias for a 7189 tube. An Online bias calculator sets it to 12 watt like an el84 but then I’ve also found them up to 17 watts including the screen dissipation. Pretty big difference in bias current. Do you include the screen dissipation in calculations?

 

[TriodeLuvr]

15Ω won't make a significant difference in idle bias. Nevertheless, I would replace them with 1Ω resistors and permanently remove the jumpers. If it was mine, I'd set bias for about 35mA through each tube. Is there a bias adjustment in the power supply? The schematic you posted only shows a DC balance control.

RCA 7189:



Jack

 

[FlaCharlie]

There is no single "correct bias" level because every amp uses different designs and they all run them at different voltages and currents. The resulting dissipation is what matters and you will find that some amps run the tubes conservatively and others run them quite hard. I have vintage amps that run 7189 / 6BQ5 tubes as low as ~8.5w and others that run them close to 12w and various points in between. 12w is the maximum plate dissipation under the Design Center Method for both the 7189 and the 6BQ5.

When you measure across the tube and calculate the dissipation that always includes both the plate and the screen dissipation. The screen dissipation is generally ignored but if you want to separate it out you need to add a screen resistor, measure the voltage drop and calculate. Then subtract that from the total dissipation.

You only show a partial schematic which shows the pot used to adjust the balance between the two tubes. There should be another pot somewhere that sets the negative voltage going to the balance pot, which is shown as -14v. You will need to adjust both pots in order to set the bias voltage correctly.

When I'm restoring an amp I try to use the dissipation level that the manufacturer intended and every one is different.

Do you have a manual which tells you what the voltage on the cathode should be with the 15 ohm resistors engaged or what the current should be?

If not, I would adjust the bias supply to -14v and then use the balance pot to get the same negative voltage on each grid. The schematic says it should be -10v (that's with the cathode grounded). Once you've done that you can measure across the 15 ohm resistors and determine the current and dissipation that Bogen chose to use. Note that the values of the resistors must be accurate. If they're the originals, probably carbon comps, they may have drifted. You can still use them to calculate the current but you need to use their actual measured values not their nominal (15 ohms) values.

Bottom line, if you can adjust it so that you have -10 on the grid with the cathode grounded you should be good.

 

[Carbsandcaps]

So here’s the power supply. I replaced the selenium rectifier with a silicone diode for the -14v which took it up to about -17v. I changed the 15k resistor out for a 20k and brought it back down to -14v. I didn’t realize that was just for balance.

 

[FlaCharlie]

Found the schematic and . . .

I see you just posted it and already adjusted it to get -14v for the supply. You should be able to adjust it correctly now. If you didn't already, I'd suggest that you might want to replace the 100uf 25v cap in the bias supply. I find that low voltage electrolytics are almost always out of spec.

 

[Dave451]

Restored one of these a while back. A good estimate for screen current for a 7189 at these voltages (350VDC plate voltage) is about 2mA, rule of thumb. So, if you biased the tubes to, say, 32 mA cathode current per tube, the plate current would be about 30mA and the plate dissipation around 10.5W, or 87% of the 12W max cited. Yes, you would have to add screen resistors to determine the exact screen grid current, but this figure may be close enough.

If you use the 15R resistors to determine cathode current, be sure to measure them for an actual value, as these old carbon comp resistors often drift significantly over time.

However, looking at the schematic shows that there is only a 'balance' control and that the "-14V" source is fixed--there is no adjustment control. You can only use the 'balance' control to even out the draw between the two tubes in a pair. If you want to actually be able to adjust the bias voltage, you'll need to add a control pot/voltage divider in the -14V supply, then you'd be able to set the same bias voltage for all tubes with the balance control to even it out in each pair. Individual tube bias adjustment would require installing the IBAM or IBBA capability.

These old units depended a lot on having matched output tubes, and this one does in stock configuration.

+1 on replacing that 100uF/25V cap--my experience has been the same on them being bad.

 

[Carbsandcaps]

All of the caps have been replaced. I found that with 20k resistor before the -14v I had -12 at the grids. With a combination of 28ohms I have -10v at the grids

 

[gadget73]

Does it not say on the schematic anywhere what the expected voltage across the test points is? Might be on the very first page of the Sams folder if you have the whole thing. Thats where it is for my Pilot 260 at least. I wouldn't be remotely surprised if the original target was for 0.5 volts, or 33 ma total.


me being me I'd probably modify it so there was a bias adjust per channel and leave the balance control in place. Wouldn't be difficult and it would give you full control over whats happening.

 

[Carbsandcaps]

Well I decided to put a potentiometer in place of the resistor for the -14v just to play around with it and I did get about .525v on the cathodes and was able to balance all the tubes almost exactly but unfortunately they aren’t the tubes I’ll be ultimately using becaus they’re el84’s. But it ended up being almost -11 volts on the grid.

 

[Carbsandcaps]

I would I’ve to do individual adjustments but I don’t know if I want to go that far with this one.

 

[gadget73]

If both channels are reasonably close and they balance out OK, don't worry about it. I've got an amp with one single pot that does all 4 tubes. They were bought as a matched set and they track pretty well together still. There just wasn't any reason to go further. Some of my other stuff has poorly matched tubes and needs more adjustment in order to tune things in correctly.

its still not matched even with the idle bias set the same but its the best I can do without buying tubes.

 

[Carbsandcaps]

If both channels are reasonably close and they balance out OK, don't worry about it. I've got an amp with one single pot that does all 4 tubes. They were bought as a matched set and they track pretty well together still. There just wasn't any reason to go further. Some of my other stuff has poorly matched tubes and needs more adjustment in order to tune things in correctly.

its still not matched even with the idle bias set the same but its the best I can do without buying tubes.

I put in the only 7189’s I had. 2 different brands. The closest I could get it is about .45v (30ma) balanced on one set and .6v (40ma) on the other set. I’m not sure what “acceptable” is on this but that’s as close as I could get them.

 

[FlaCharlie]

but unfortunately they aren’t the tubes I’ll be ultimately using becaus they’re el84’s. But it ended up being almost -11 volts on the grid.

No reason not to use your EL84s. Plate and screen dissipation specs for the EL84 / 6BQ5 and 7189 are exactly the same. And if you're running closer to -11v vs -10v that means you're running the tubes a bit more conservatively.

 

[Carbsandcaps]

No reason not to use your EL84s. Plate and screen dissipation specs for the EL84 / 6BQ5 and 7189 are exactly the same. And if you're running closer to -11v vs -10v that means you're running the tubes a bit more conservatively.

That’s kind of what I was thinking. It’s running at about 380v which isn’t out of the realm of the el84 and not quite at 7189

 

[FlaCharlie]

That’s kind of what I was thinking. It’s running at about 380v which isn’t out of the realm of the el84 and not quite at 7189

As I mentioned in your other thread about the EL84 vs 7189, voltage is just one element of the equation. Voltage x current = Dissipation. Voltage alone or current alone is not critical, unless it's really extreme.

Dissipation is the key factor and the two have exactly the same dissipation ratings. So if the dissipation is in spec either tube will work fine.

 

[gadget73]

40ma at 350 volts is 14 watts plate+screen. A little hotter than I'd prefer to see, though technically within the design max rating of 13.2+2.2 or 15.4 watts.

but yeah a lot of the rating difference in a 7189 vs a 6bq5 is just the rating system involved. Design center v design max. I have seen a 7189 sheet in design center which is what most of the 6bq5 sheets are and the numbers are the same except for the plate voltage. 400 vs 300. If you look at the design max 7189 numbers they allow for 440 volts. Basically design max is 10% over the design center numbers.

 

[FlaCharlie]

Yeah, I wouldn't run them that hot either. There's nothing to be gained from doing so and the tubes won't last as long. But he hasn't said exactly what his measurements are.

Apparently, the EL84s biased at ~11v but it's unclear how much current that is because we don't know if the 15 ohm resistors are in spec. From the pics all the resistors seem to be the original carbon comps and they typically drift in value. I'd replace them with some new 10 ohm 1%. He also hasn't said what his measured plate voltage is. With modern wall voltages it's likely higher.

Personally, I wouldn't go over 12w of total dissipation per tube and no more than ~10.2w total if they're new production.

 

[TriodeLuvr]

Dissipation is the key factor and the two have exactly the same dissipation ratings.

Mullard lists the "limiting values" of the EL84 as 12W/2W, anode and screen. Sylvania shows the maximum for the 7189 as 13.2W/2.2W. That's 10% greater dissipation, for whatever it's worth.

Sylvania 7189





Jack

 

[Carbsandcaps]

The plate voltage is 370v on the button. The resistors all measure out to about 16.7 ohms. So about 10% off. I ended up putting the matched el84’s back in (actually Soviet 6p14p tubes). There’s about -11v going to the grids. The voltage though the resistors is about .55v which come out to about .33ma@ 370v which is about 12.2 watts dissipation through the tube.

My math may be off please correct me if it is. It’s on and playing now. No red plating as of now.

 

[TriodeLuvr]

I always use 1% resistors for this purpose, usually 1Ω. The DMM measurement is then 1:1, i.e. 1mV = 1mA.



Those are 10KΩ adjustment pots with screwdriver slots, not 1/4" phone jacks.

Jack