Bell Labs HiFi amp rectifier question

[g1]

And they do not list node voltages. Their numbers do not seem to add up very well, so those plate voltages may not agree with their cathode voltages when everything is calculated out.
Are the cathode voltages all in the ball park, or are any out of range?

[Randall]

OK, so I rewired the filter caps the way they were originally, and now hum is gone. Thanks guys! Amp sounds pretty good with mp3s, the guy is a vinyl guy, so hopefully he will be happy. I have spent a great deal of time on this, and I have a second one to make match this one, including modifying for changing preamp circuit and tubes. This one has a noticeable noise floor when turned up to half or beyond, but I suspect this is not a problem with the circuit, but a fact of life. But then, it is pretty loud at this point. Not that different from my 6V6 equipped guitar amps, right?

Pin voltages on a variac to correct for B +

V1 pin 8 - 0.74v

V2 pin 2 - 119v
V2 pin 5 - 119v
V2 pin 6 - 1.5v

All cathode Voltages seem within spec.

[g1]

So going by all the other voltages you listed, and the schematic resistor values, that would put pin 6 of V1 at about 164V. Is that what you are measuring?

[Randall]

I'm seeing 143v on pin 6 of V1.

[g1]

Your numbers seem a lot more realistic to me than what is shown on the schematic for plate and cathode voltages. High value cathode resistors like that are not going to result in the high currents that would create such low plate voltages. Less current through the voltage droppers results in higher voltages at the preamp supply nodes.
Once you get the other running, you'll have a better idea of what is normal.

[Randall]

OK, supposing the other one runs at approx the same voltages, and we consider that to be the norm, should these be considered for a step down transformer to put them at older line supplies of approx 117v? The voltages I have given here are with Variac compensated to adjust to the schematic's B+, so they will be higher plugged directly into the wall. I will add that both amps have all American RCA and Sylvania tubes, so I wonder if they are OK running higher than depicted, unlike some modern tubes.

[g1]

When considering stepping down the line voltage, the first thing I look at is heater voltage. Is it way high? Then at filter caps voltage ratings, are they being stressed? Then idle dissipation of power tubes, are they running way hotter than with vintage line voltage? Tube voltage ratings I don't worry about, none of the manufacturers ever seemed to.
Bias and filter cap voltage are easy to accommodate, especially if a filter recap is being done anyway.
If everything is way over spec., then a bucker xfrmr is in order.

[Randall]

Plugged into wall, heater V is 6.74 vac. Filter caps are rated at 450vdc, so they should be fine. Idle dissipation on a NOS pair of RCAs is 42.8 mA and 40.5 mA. Plate voltage is 337V, cathode voltage is 18.3 v. I'd be interested in opinions on these and whether I should run a variac or bucker transformer. I see a 8 amp variac with a volt meter on eBay new for $115 shipped.

edit:

I swapped a new set of JJ 6V6s and with variac adjusted for 6.3 vac filaments I get the following readings:

Plates volts = 311v
diss. = 41.3mA and 41.4mA
Cathode volts = 18.1v

So by my math (311v - 18v) * 41.3 mA = 12.8 watts dissipation even after the line voltage is dialed down to 116 vac. Should I maybe be thinking of increasing that 220 ohm cathode resistor?

[The Dude]

It's only one man's opinion, but I would say yes on upping the cathode resistor. 41mA per tube is a bit much, and that will go up with more typical line voltage.

I'm sure you're more than aware of this, but when in doubt, I consult the old trusty Weber Bias Calculator Page.

Weber Bias Calculator

[g1]

The heater voltage and plate voltage doesn't seem to bad to me. Lots of modern stuff runs the heaters at that level.
Just dropping the idle dissipation should do it, but depends on the customer I guess.

[Leo_Gnardo]

Should I maybe be thinking of increasing that 220 ohm cathode resistor?

Well you could, but those JJ 6V6 are pretty tough, they'll take a plate power well in excess of what other 6V6's do. Right where it is, is just fine in my book. If you're tending towards caution 240 to 270 ohms in the cathode ought to do. What I find is any competently working tube in self-bias mode, sets its own bias just fine, it's practically a miracle.

It's the filament reading that catches my attention. Not unusual these days to see 6.7V, but if you use your variac (and perhaps later substitute a line bucking transformer) to dial that down to 6.3V you'll probably find everything else falls into line, plus you'll get a very long lifetime out of the tubes.

[Randall]

Those readings are with the voltage dropped down to set the filaments at 6.3v (post 23) which ended up being 116 vac line. I'm not going to be using the more robust JJs, he has some real nice 60's NOS RCA 6V6GTA's. Remember, this is going to be a vinyl junky's home stereo. I want to run these amps cool enough so he can turn it up and not have it distort, does that make sense, or am I kidding myself?

I put in a 300 ohm cathode resistor, and ended up with these readings, and no hum at a 116 vac line.

V plate - 334
Vc Cath - 19.7
Diss - 27.4 mA & 32.0 mA

So once again close to 65 and 70%, which is a nice place to be for this amp and those old tubes I think.

[Tom Phillips]

I don't think you need to lower the line voltage but if you decide to lower the line voltage then I recommend that you install a small bucker inside each chassis or make an outboard adapter bucker. I.e. I would not give a customer a variable transformer. It is true that a variable transformer can be set just right but it can also be set very wrong. Best not to count on anyone setting or re-setting it. This is just a general comment.

[The Dude]

I agree with Tom. I don't think you need to lower the line voltage. 6.3V is normally considered a +-10% figure for filaments. I wouldn't be horribly concerned about 6.74V filament voltage (when plugged into the wall- post #23). There are plenty of old Fenders out there built with lower primary PT's and it doesn't seem to be an issue for them.

[Randall]

Hmm.. I made an outboard bucking transformer box for a guitar player last summer precisely because I didn't want him or his crew to be able to adjust it and burn down his vintage Marshall amp. And this amp WAS a problem because the voltages were too high and it would eat tubes, get too hot, or sound too harsh. But I painted myself in a corner with this HiFi customer when I explained the vintage/modern line power thing. So now it's a big deal to him, and he thinks it's cool to have his own variac, and I'm sure to him it will make it sound a little better! And I trust his electrical chops to 'set and forget' in one location more than I did the guy in a big rock band, moving gear around all the time.

I'm not concerned about 6.7v filament voltage either, I am using it as a reference point to adjust the line power, which then agrees with the 117 vac on the schematic. If this were an old Fender, which we want to distort, I'd probably not worry about it much, but on a 65 year old Hi-Fi amp, I feel better about running it at what it was designed for. And so does this customer, there would be no talking him out of it now.

I sure do appreciate all of the advice and wisdom I find here!