Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cups on March 08, 2011, 02:43:25 pm
-
Hi all.
Put a three prong cord in a YBA-1 Bassmaster, changed a few pots, you know, upkeep type stuff. Pluged 'er in and she sounds great but the voltages are 10 to 20 volts higher than the schem indicates. I measured the AC from the wall and it read 123V. Higher than the 115V posted on the schem. B+ is 455V when the schem indicates it should be 440V.
Can we just assume the voltages are due to higher mains voltages?
-
that's well within 20% tolerances and can be because of cold biased power tubes as well.Absolutely not an issue at all.
You need to know what the bias is set at and if it's within limits then stop worrying and enjoy the amp.
Non- issue.
-
Checked the bias. It read -38 which was pretty much spot on. Thank you for your response. Back it goes into it's chassis.
-
That's not how you check bias.That's only a bias voltage reading.What CURRENT is running through those tubes?You need to replace the cathode ground wire with a 1% 1 ohm 3 watt resistor and measure the mv reading across that resistor to know what the bias 'milliamps' really are. -38v just tells you what the bias supply is putting out.It does NOT tell you what the tubes draw for current.
-
You just made me have a lightbulb moment. The only real experience with bias is with Cathode Bias. I've read and read about fixed bias and frankly never really understood it. I guess I'm a hands on learner. I'll reread some stuff and see if it sinks in.
Thanks again.
Patrick
-
20% is plenty close.
> the AC from the wall and it read 123V. Higher than the 115V posted on the schem. B+ is 455V when the schem indicates it should be 440V.
1969: 115V in, 440V out, 3.826 ratio
2011: 123V in, 455V out, 3.699 ratio
Your voltages are actually "low", after allowing for "115":123.
There's a 3% discrepancy, and I'm not gonna worry about it. Needle-meters were often read to 5% and rounded to the nearest 10. Tests may have been done without actually checking the line voltage, which had been 115 around 1960 but often 117 in 1970, and wide variations in old buildings such as Traynor worked in.
The tubes Pete was buying gave a happy current with whatever Screen voltage and -38V on G1. Tube production has changed a lot in 40 years. As phsyco says, read the *current*. Best to stick a small resistor in the cathode lead and read the voltage across it.
Bias a fixed-bias guitar amp for plate dissipation medium-large compared to tube rating. Unlike self-bias, there is no advantage in pushing 99% rated dissipation, but too cold isn't best either.
You have a 25W-30W dissipation tube, and it feels most of that 455V. 0.1 Amps 100 mA each tube would be 45.5 Watts and way too hot. 0.01A 10mA per tube would be 4W, cold; the amp will work but rough on soft sounds. 0.05A 50mA is 22.75W, a nice upper limit for a EL34 stage amp, with 30mA-40mA a good lower goal.
-
Well; read pertinant info in TUT2 and 3 again and put some 1R resistors on the cathodes. I have 36ma on one tube and 42 on the other.
Can't believe I'm finally starting to understand fixed bias. One step at a time.........