NAD - Peavey ValveKing MH20
- Thread starter RVA
- Start date
jtcnj
Ambassador of the Crossroad
Happy new amp day, glad I could help in the process with funding via relieving you of the Orange MT.
Sorry to hear of a bit rocky start to your relationship.
Kind of like the first date or two with a new hottie and things didnt go well, but you still want to hit it real bad so you hang in there.
Damn that seems like 100 years ago.
Assuming you bought used, maybe or maybe not the failed tube is a symptom, not the problem's cause.
Keep the back off, work up to pushing the amp hard and watch those power tubes for red plating.
A little variation between valves on bias is not uncommon.
When you say, 27/30 is that in mv at a 1Ω test point?
How does that work out in plate dissipation, or do they give you a chart for where to read, interpret, and set it?
High plate voltages (at around 100% plate dissipation) are fairly common for push pull EL84 cathode biased circuits, so shorter tube life is expected, but I dont know what is typical in fixed bias.
EDIT: I know 70% is the usual base line, I meant I dont know if it is common to run fixed bias EL84s on the hot side.
Sorry to hear of a bit rocky start to your relationship.
Kind of like the first date or two with a new hottie and things didnt go well, but you still want to hit it real bad so you hang in there.
Damn that seems like 100 years ago.
Assuming you bought used, maybe or maybe not the failed tube is a symptom, not the problem's cause.
Keep the back off, work up to pushing the amp hard and watch those power tubes for red plating.
A little variation between valves on bias is not uncommon.
When you say, 27/30 is that in mv at a 1Ω test point?
How does that work out in plate dissipation, or do they give you a chart for where to read, interpret, and set it?
High plate voltages (at around 100% plate dissipation) are fairly common for push pull EL84 cathode biased circuits, so shorter tube life is expected, but I dont know what is typical in fixed bias.
EDIT: I know 70% is the usual base line, I meant I dont know if it is common to run fixed bias EL84s on the hot side.
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OP
RVA
Ambassador
Thanks and goo analogy!
Yes it is. That is the readings at either socket. It only has one bias pot so you must split the difference.
I do not understand this. Please explain
jtcnj
Ambassador of the Crossroad
cathode vs fixed bias, is just the characteristic inherent to those designs.
Generally speaking, you check or set the bias in fixed bias amps, not in cathode biased (but it can be done to tweak the circuit - just not part of typical tube change service procedure).
What do the numbers 27/30 come from?
You measure something, then compare it to an intended target value, and adjust toward the target.
Where do the target values come from?
Plate dissipation in watts is the current through the tube (cathode to plate, not counting screen current) x the plate voltage, so each circuit will be different- based primarily on the plate voltage for that circuit design, and the variation in tolerance of the components.
It is based on the wattage the tube is rated for, 12w for EL84.
so, if the target is 70% or 8.4w, dividing that wattage by the plate voltage for that amp, will yield the current that should be flowing, which you measure by one of several methods.
The builder has done the above math for you and given you a test point and a target range to adjust to.
That number is a guideline, use your ears and watch the plates at higher settings.
I dont hear any difference +- 5% or so.
I may be off on explaining the math on the fly but the interaction between target wattage, circuit plate voltage and the current to obtain that wattage is the point.
Generally speaking, you check or set the bias in fixed bias amps, not in cathode biased (but it can be done to tweak the circuit - just not part of typical tube change service procedure).
What do the numbers 27/30 come from?
You measure something, then compare it to an intended target value, and adjust toward the target.
Where do the target values come from?
Plate dissipation in watts is the current through the tube (cathode to plate, not counting screen current) x the plate voltage, so each circuit will be different- based primarily on the plate voltage for that circuit design, and the variation in tolerance of the components.
It is based on the wattage the tube is rated for, 12w for EL84.
so, if the target is 70% or 8.4w, dividing that wattage by the plate voltage for that amp, will yield the current that should be flowing, which you measure by one of several methods.
The builder has done the above math for you and given you a test point and a target range to adjust to.
That number is a guideline, use your ears and watch the plates at higher settings.
I dont hear any difference +- 5% or so.
I may be off on explaining the math on the fly but the interaction between target wattage, circuit plate voltage and the current to obtain that wattage is the point.
OP
RVA
Ambassador
Loose power socket was a poor description. I re-tentioned the spring contacts by closing them up a bit. The sockets themselves are not loose. They are pcb mounted. I also replaced the tubes. It was working well yesterday.
I do notice that the 5w setting on the Peavey is not as loud as the 3w setting on the Blackheart, but it may be comparable to the 15w Panama. I guess amp output varies
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OP
RVA
Ambassador
OP
RVA
Ambassador
The manual has nothing about the inside of this thing. I had to watch someone bias a classic 30 to get the idea of how to do the bias on this. While it has a bias pot, it does not have a defined probe point
