• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Opinion on methods to reduce B+

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#45
limono said:
Bucking transformer is your best bet unless you want to choke the amp with series resistance in the B+ circuit. BUT it's a tube amp so 20% plus minus voltages are not a problem.. according to tube guru Tim de Pavaricini
Click to expand...
I have never tried OR simulated this but a resistor in series with the
buck transformer primary might act as a rudimentary 'regulator' as
the load goes up, the buck transformer will droop faster than the main.

G²
 
#47
AudioFanMan said:
Since my other thread has zero traction... :violin:

Swapped the 300B's and no change in heater voltages... Left side, 4.95 Right side 5.2
Anyone think .7v difference on the heaters is worth the trouble of dealing with a dropping resistor ?
Click to expand...
It isn't .7 it is .25. That in itself isn't enough to worry about.

Have you measured the AC voltage directly from the transformer on the heaters? I saw your schematic and it gets converted to DC. The rectification could be the "problem", in quotes because it isn't really. I'm using AC on my amp, PP 6B4G, and there is no hum even with the 6.3v heaters.
 
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#48
If you have ac rectified to DC supplying the tube heaters, and you measure differences on the heater voltage, why on earth would you swap tubes??
That's about the very last you should look in desperation if all else fails.
Just measure the DC from your rectifier; if these are different, then that's it.
The ac windings (if you use two) may be slightly different, or the rectifiers, or the wiring.

And I think it is clear from other replies that 0.25V delta isn't a matter of live and death.

Jan
 
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#49
AudioFanMan said:
I use an isolation transformer and variac, I dial as close to 120v as I can get every time. I also monitor it just before measuring and during....
But, that said I do see variations in B+ while at 120v in... Making me think the 274B might be a bit tired... But, even at that point, B+ feeds both 300B's
rectifier diodes (each heater circuit) and each and every time the left channels 300b is lower in voltage than the right..
Amp cooling off now. Going to swap the 300B's and see what role they play...
Also going to measure Vin and B+ in intervals to see how warmup might play a role.....
Click to expand...

Variacs don’t offer any regulation and the ac line will always be varying a bit. You’ll see this result in the b+ voltages.
 
#52
jan.didden said:
If you have ac rectified to DC supplying the tube heaters, and you measure differences on the heater voltage, why on earth would you swap tubes??
That's about the very last you should look in desperation if all else fails.
Just measure the DC from your rectifier; if these are different, then that's it.
The ac windings (if you use two) may be slightly different, or the rectifiers, or the wiring.

And I think it is clear from other replies that 0.25V delta isn't a matter of live and death.

Jan
Click to expand...
Jan,
Swapping the tubes I thought was a 2 second test to see if may be there was something going on with the cathodes....
Geez wiz... lol..
I also read some time back that some people say running DC for the heaters can un-evenly wear something out (I may not be remembering perfectly perfect) and that some builders add a switch that changes polarity..
As my measurements show I can pretty much reverse polarity simply by swapping tube location.. So I thought it might be prudent... Ok ? lol

But seriously, all sarcasm is tong in cheek... Im thankfull for all help!

The 4.95-5.2 was a typo on the 4.95 at least... Was 4.5 ...
I have recorded the voltages a bunch of times and 4.8/5.6, 4.5/5.2, 5.3/5.9
Those average to be .6 off
Regardless, Im pretty sure .6 average is not anything to worry about...

for the heck of it I will check the AC voltage before the diodes again, but if memory is correct they were identical
 
#55
So I measured the 5v AC before the diodes, both sides read exactly the same 5.6 volts.
Meanwhile, the DC on the heaters read 4.8 left side and 5.7 on the right. 0.9 of a volt...
Within a few seconds, I don't believe the variations on mains would cause this...
Since the tubes were swapped, there must be something in the circuit..

I guess time to learn how to check 8 diodes.. the only other components are the 2 6800 uf smoothing cap's
 

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#56
AudioFanMan said:
for the heck of it I will check the AC voltage before the diodes again, but if memory is correct they were identical
Click to expand...
Doesn't that tell you right there where your difference is? You have 4 diodes and a cap in the heater rectification on each side IIRC from the schematic. That's 8 diodes that all would have to be exactly the same, plus the two caps. But before I'd go and start replacing all the diodes, I'd just do the calculation on finding the right small value resistor to put on one side. .6v/1.2a = .5R. I'd go with 2W or 5W, depending on what you can find. I'm assuming here that the heater draw is for a single 300B.
 
#57
There may be something subtle connected to the capacitors. Note that your DMM reads the avaerage voltage. The DC after the rectifier will have ripple and part of that is added to the DMM's DC reading. So if the actual value of the caps is different (they most probably are +/-20% tolerance), the ripple will be different and as a consequence, the DC on the DMM will be different.
If you have a scope, check the ripple amplitude, or measure the AC on the rectified voltage at the caps.

Jan
 
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