6505+ combo high bias current
Moderators: pompeiisneaks, Colossal
6505+ combo high bias current
I've been reading about the Peavey 6505+ combo's having low bias current, so I tested mine with an srs ma socket type tester. It's running between 54 and 62 ma. Unloaded I have 516 volts at pin 8 and -53 volts bias voltage at the tube. All of the resistors in the bias circuit read close to the schematic,,220k,,5.73k all close to spec. Kinda at a loss as to what might be going on. I tried some old 6l6's and got 62ma with them, also. Any ideas would be appreciated.
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Stevem
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Re: 6505+ combo high bias current
Was this amp owned by someone else at one time as May be it was moded and already has a 1 ohm resistor to ground off of pin 8?
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Re: 6505+ combo high bias current
Why you measure it unloaded? What's the voltage when loaded?
Re: 6505+ combo high bias current
I just took the tubes out to keep from melting the tolex. It's an upside down amp with a short, built in, speaker cord, that makes it hard to work on. I'll check it loaded tonight. No mods are on the amp. Looks new, but that bias is way high. Leveled off at 52ma after playing for an hour. Seems like I remember checking it loaded, once and voltages were 485 and 51.
Re: 6505+ combo high bias current
Loaded, it looks like 486 plate voltage, 482 screen , and -51.3 bias voltage. I'm seeing 52-55 ma with the socket ma bias tester. All associated resistors are close to spec and nothing looks burnt or modded. Not sure whats going on.
Re: 6505+ combo high bias current
Have you tried fresh batteries in your meters?
What's the heater voltage? eg 6.3Vac.
Consider verifying the bias meter current reading; measure the OT primary resistances, CT to each plate, then measure the voltage across each resistance.
Keep the meter and leads well away from the preamp area of the amp.
What's the heater voltage? eg 6.3Vac.
Consider verifying the bias meter current reading; measure the OT primary resistances, CT to each plate, then measure the voltage across each resistance.
Keep the meter and leads well away from the preamp area of the amp.
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Re: 6505+ combo high bias current
Tried new meter batteries, tester is ma readings so it's a direct inline amperage reading. Heater voltage is high. It's 7.1 volts dc. I tried some new 6l6 tubes tonight and it started out at 20ma, then after a few minutes shot up to 38 ma. Better but I heard these average 14-17 ma.
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Reeltarded
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Re: 6505+ combo high bias current
Relax. I think you are misreading everything constantly. lol -38 is pretty close to what you should see. Ummm..
I am not trusting your meter and you aren't either. Because you don't trust your meter, I also don't trust your readings.
No bias check resistors? You say you are reading mA directly but I don't think it means.... (princess bride)
What is the mV reading at the junction of the output grids? I speak jive.
I am not trusting your meter and you aren't either. Because you don't trust your meter, I also don't trust your readings.
No bias check resistors? You say you are reading mA directly but I don't think it means.... (princess bride)
What is the mV reading at the junction of the output grids? I speak jive.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Re: 6505+ combo high bias current
Wish it was the meter, that would simplify things. I have cross checked with 2 other amps ( home build and a B52 at100. Both read 6.3 heater and 25-30ma . The home build uses the 1 watt resistor , built in bias check, and it coincides with the SRS socket reading on the same amp. I increase my bias pot, on the home build and the SRS ma tester tracks perfectly. Lots of articles on the 6505+ combo having low bias current, 14-17ma seems to be the complaint. Mine is to high, and really high with the Ruby tubes. not my style of amp, but I don't want to sell it without it being right. Jumping from 20 ma to 38, last night leads me to believe something is loose or breaking down. I need a day off to spend some time with this.
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schaublin65
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Re: 6505+ combo high bias current
Hi,
I have seen more than one component check out fine when tested with a meter but fail to pass current when loaded.
Fuses and pots probably first. Resistors next. Anything wave soldered always.
Lead free solder on cheap PCB's.
Passing a resistance test is just that. Working voltage and an ammeter may be very different.
Hope this helps.
Take care...
John
I have seen more than one component check out fine when tested with a meter but fail to pass current when loaded.
Fuses and pots probably first. Resistors next. Anything wave soldered always.
Lead free solder on cheap PCB's.
Passing a resistance test is just that. Working voltage and an ammeter may be very different.
Hope this helps.
Take care...
John
Re: 6505+ combo high bias current
John, I think that'll be what I find. Started out at 20ma then after a minute went up to 38ma. Theres a 5.73k bias dropping resistor, then a 220k voltage divider to ground before the bias voltage gets to the tubes. I'm betting one of them is faulty or has a bad solder joint that creates more or less resistance when It warms up. I'll get a chance to work on it again this Sat.
Re: 6505+ combo high bias current
Greeting everybody,
I'm from Poland and I registered on this forum only because I've found this topic
So, basically, this is how things are with this amp. 6505+ 112 Combo's bias is set to high values. Your reading are perfectly fine. In my opinion, that's what differs if from 6505+ head (mostly: this + phase inverter) and that's why it sounds more 'muddy', less 'straight into your face' You can check that, by putting a variable resistor in series with that 220k resistor (this is the safe way of how to do it), unless you want a separate bias regulator for both tubes - which is a more tricky option. I did that with mine but I had to cut PCB traces. But it's not needed at all if you have a good matched pair of 6L6's.
Anyway, try to set BIAS to 18-25mA (per single tube) and see how it sounds. It makes it much closer to the "original" 6505+.
I'm from Poland and I registered on this forum only because I've found this topic
So, basically, this is how things are with this amp. 6505+ 112 Combo's bias is set to high values. Your reading are perfectly fine. In my opinion, that's what differs if from 6505+ head (mostly: this + phase inverter) and that's why it sounds more 'muddy', less 'straight into your face'
You can check that, by putting a variable resistor in series with that 220k resistor (this is the safe way of how to do it), unless you want a separate bias regulator for both tubes - which is a more tricky option. I did that with mine but I had to cut PCB traces. But it's not needed at all if you have a good matched pair of 6L6's. Anyway, try to set BIAS to 18-25mA (per single tube) and see how it sounds. It makes it much closer to the "original" 6505+.