Dropping B+ to simulate the equivalent of a 5Y3, sans sag?

[Firestorm]

17W dissipation?! That's gotta be redplating. What's your cathode resistor and your cathode voltage?

As to the bleeder resistor, I'd just parallel the main filter. Maybe 220K at 2 or 3 watts, or 470K at 1 or 2 watts.

[Structo]

Quite a few amps use balancing or equalizing resistors when the first two caps in the filter are wired in series.

The helps balance the voltage to them since no two caps are exactly the same.
Incidentally, the act as bleeder resistors when the amp is powered off.

Attached is the filters on a D'Lite amp.

The first two caps are in series and the two 220K resistors are the balancing resistors.
These are great since they bleed down the residual voltage in just a few seconds making it safe to work on the amp.

[dehughes]

Bleeder Resistors:

I'll put in bleeder resistor(s) across the first filter cap....thanks.


Tube Dissipation:

413V (plate) - 26V (across cathode resistor) = 387V plate to cathode

26 (volts across cathode resistor) / 300 (ohm cathode resistor) / 2 (number of tubes sharing the resistor) = .0433A or 43.4mA plate current each tube

387v  x .0433A = 16.77 watts plate dissipation per tube

No red-plating, oddly enough (Tung Sol reissue 6V6...), though there was much more noticeable hum coming from the amp, compared to when they're run at about 363v on the plate and 22v across the 300 ohm cathode resistor (12.5w dissipation).

[tubeswell]

There are a couple of ways to knock it down apart from different rectifiers or PTs besides zenering the CT (in respect of which, you don't have to use a 10W or 20W zener - you can actually use several 5W zeners in series and play around with the values 9V, 12V 15V to get the voltage drop more-or-less exactly where you want it). You can put in a voltage regulator, or chunky anode limiting resistors between the HT winding and the rectifier anodes.

[dehughes]

There are a couple of ways to knock it down apart from different rectifiers or PTs besides zenering the CT (in respect of which, you don't have to use a 10W or 20W zener - you can actually use several 5W zeners in series and play around with the values 9V, 12V 15V to get the voltage drop more-or-less exactly where you want it). You can put in a voltage regulator, or chunky anode limiting resistors between the HT winding and the rectifier anodes.

Clever. Thanks! Any recommendations as to the Zener types (make, model?) or to the resistors on the HT winding (type, wattage, suggested rating?).

[Firestorm]

Tube Dissipation:

413V (plate) - 26V (across cathode resistor) = 387V plate to cathode

26 (volts across cathode resistor) / 300 (ohm cathode resistor) / 2 (number of tubes sharing the resistor) = .0433A or 43.4mA plate current each tube

387v  x .0433A = 16.77 watts plate dissipation per tube.

When you read the voltage across the cathode resistor, you are seeing the effect of screen current in addition to plate current. Screen current will be a small fraction of plate current, but you should take it into account. Do the same calculation with the voltage drop across the screen resistor and subtract that current from 43mA to find the actual plate current. I think you'll find that you are still running the tubes in Class A territory and you really can't run a 6V6 in Class A at that voltage. Either Zener the voltage down, or try a larger cathode resistor until you get the operating point where you want it.

[Firestorm]

You know, the other wrinkle is that we don't really know the plate dissipation of these Russian tubes. For years, all the tube data they publish has been a straight copy of the RCA manual. Maybe the tubes can dissipate 18W. Maybe they can take sustained plate voltages of 500V. We just don't really know. Anybody willing to burn up a few sets of tubes to find out?

[jjman]

My SFDR runs the 6v6s at ~427v or so. The scheme calls for 415v. So your (net) plate voltage is not high IMHO.

The plate voltage should only be scrutinized after the idle has been properly calibrated to an appropriate wattage via the cathode resistor. This bias adjustment will change the voltages on all the tubes so it should be done before fine tuning the final dropping resistors. The preamp stages draw very low relative current so changing the droppers, and thus the current in the preamp, won’t throw off the voltage measurably at the beginning of the rail (the 6v6 plates.)

I find that 6v6 screens draw less than 2ma at idle with a 1k resistor on them.

I have a (diode rectified) 2 x 6v6 cathode biased head and it runs 326 net plate volts and ~12watts ea. A Tweed Deluxe is purported to run about 13watts ea at idle at ~328 net plate volts.

[M Fowler]

I just got back from the lakes so what I was talking about is calculating the voltage.

Another way to do this is take the plate voltage you want for your power tubes below the max tube voltage divided by 1.3 and that will tell you the secondary voltage needed for from your PT when using a GZ34. Then you will know what voltage reverse zener diode you need to order.

Mark

[M Fowler]

320-0-320

Conversion factor for 5AR4 is 1.3 times one leg of the AC.

320 x 1.3= 416v rectified.

6V6 class A has max plate voltage of 350 and class AB has 250.

So if you want your plate voltage at 325 to be on safe side with the 6V6 plates then divide 325v by 1.3 = 250 primary PT. Because 250 x 1.3 = 325 rectified with 5AR4.

Mark

[Structo]

In my 6V6 amp I have a 125 v 25 watt stud mount zener on the CT.

That thing gets pretty hot, so I'm not so sure you can get away with using a 5 watt or low power zener.
Also if you use a zener don't forget to use a .01uf snubber on it for noise.

[dehughes]

Wow. Cool info guys. Thanks. I'll go give the amp a nice workout with the 5Y3 and see if I dig that. If so, then I'll just adjust the B+ to the preamp tubes to taste and call it good. If I don't dig the 5Y3, then I'll have to install a 5AR4 again and go the Zener route...

Here's to simplicity!

Or, Here's to Peavy Backstage amps!

[tubeswell]

Hi dehughes

You can use JJ6V6s - they will go to 475V or even 500V easy and are 14Wers.

As to the voltage regulator ricks there's info on the Aiken site, and as far as limiting resistors go, Merlin B has a helpful article here (his new book is out now BTW)

http://www.freewebs.com/valvewizard/fullwave.html

[M Fowler]

Well then based on what you and others have said and the fact that I read the new JJ 6V6 can handle 440 volts on the plates bias at 80% then you don't need to drop anything since you have approximately 416vDC rectified with 5AR4.

Need to get the bias correct if red plating.

Mark

[tweedeluxe]

Another option (one I have used several times now) is to use Dana Hall's VVR circuit. The cathode-bias version is posted somewhere on this site.

Add another resistor or trim pot on "top" of the 1 Meg potentiometer to set the maximum operating voltage. Works like a charm.