Red Plating and Bias

[C Moore]

Below my post is a quote from tubeswell, from another thread...
It made me curious. I do not think I have ever seen/observed tubes that were conducting, but did not have a negative bias at their grids. Do tubes "red plate" pretty fast if their heaters are working, their anodes have a normal voltage supply, and there is no bias voltage at the grids?
Thank You

"If your bias supply is from a ~40V tap, then 47uF and more is fine because you are not concerned about the time it takes for the cap to charge up. As to the voltage rating, it depends how you configure the bias supply voltage divider. 50V would be under the target if you were rectifying the bare 40V (40 x 1.4142 = 56.6), but maybe not if you drop the voltage supply with a divider first.

However, if your bias is from the HT supply (like a vintage BFPR) then you want a cap that charges up much faster, (because its dependent on the same supply as your B+, so it has to be ready to go when your B+ is) so go with 8-10uF. You can get a steadier supply if you use 2 of them in a Pii filter. The voltage rating should be a minimum of 10 or so above whatever voltage is going to hit them."

[ToneMerc]

Below my post is a quote from tubeswell, from another thread...
It made me curious. I do not think I have ever seen/observed tubes that were conducting, but did not have a negative bias at their grids. Do tubes "red plate" pretty fast if their heaters are working and their anodes have a normal voltage supply?
Thank You


However, if your bias is from the HT supply (like a vintage BFPR) then you want a cap that charges up much faster, (because its dependent on the same supply as your B+, so it has to be ready to go when your B+ is) so go with 8-10uF. You can get a steadier supply if you use 2 of them in a Pii filter. The voltage rating should be a minimum of 10 or so above whatever voltage is going to hit them."

I can tell you from first hand experince that they will glow red.

I asked one of Marshall experts months back why don't you see 50uF or 100uF capacitors in the bias supply of early Marshalls and that was basically the answer he gave me.

TM

[martin manning]

It's a problem for some seconds if the main filters charge faster than the bias caps, and if you increase the filtering by distributing it over more than one bias cap, you're still going to be increasing the time required for the bias supply to stabilize.

Why not just take the bias supply at the PT secondary, before the standby switch? This solves the problem nicely, unless the user hits the standby to soon.*

Some Marshall circuits did take the bias after the standby, but the JTM45 and 2204, e.g., did not, and still used only 8 and 10uF bias caps (respectively) in their Pi filters. Methinks the reason they used the smaller values was to lower the cost.

*Note that even if a separate bias tap is used, hitting the standby too soon after a cold start can result in the power tubes running under-biased for some time if the bias filters are too large.

[C Moore]

It's a problem for some seconds if the main filters charge faster than the bias caps, and if you increase the filtering by distributing it over more than one bias cap, you're still going to be increasing the time required for the bias supply to stabilize.

Why not just take the bias supply at the PT secondary, before the standby switch? This solves the problem nicely, unless the user hits the standby to soon.*

Many Marshall circuits did take the bias after the standby, but the JTM45 and 2204, e.g., did not, and still used only 8 and 10uF bias caps (respectively) in their Pi filters. Methinks the reason they used the smaller values was to lower the cost.

*Note that even if a separate bias tap is used, hitting the standby too soon can result in the power tubes running under-biased for some time if the bias filters are too large.

Thanks -
I cannot be the only guy that leaves/wires the SB switch in the off position. So after reading the post by "tubeswell" in the thread about BF Fender bias caps, it made me wonder how fast lack of bias becomes a problem. Like I say, I have never observed red plating do to lack of grid bias. Well, I have, but only when there has been an amp that was not working properly.
Thank You

[PCollen]

Below my post is a quote from tubeswell, from another thread...
It made me curious. I do not think I have ever seen/observed tubes that were conducting, but did not have a negative bias at their grids. Do tubes "red plate" pretty fast if their heaters are working, their anodes have a normal voltage supply, and there is no bias voltage at the grids?
Thank You

If you do not have a negative bias voltage on the control grid of the power tube in a grid-biased circuit design, then the control grid is doing nothing to limit the flow of current between anode and cathode and the tube will red-plate.

[C Moore]

Yeah, I understand the need for bias voltage at the control grid. I was just curious how fast red plating occurs in relation to bias cap and filter cap charging time. I never considered it until I read the other post I referred to. Like I said, I either leave the SB switch in the off position, or at least hit it at the same time as the power switch.
It has never created a problem that I know of. But maybe I have been getting away with murder in this regard.
Thanks

[tubeswell]

It is more precautionary than worrisome. The heaters need to be warmed up for tube current to flow, so there is some grace time for the bias to kick in if you are just going from cold switch on.

Also, different rectifiers could affect how quickly the B+ gets to maximum (if the heaters are already warm) e.g.: SS and 5U4G will power up faster than 5AR4.

[PCollen]

Yeah, I understand the need for bias voltage at the control grid. I was just curious how fast red plating occurs in relation to bias cap and filter cap charging time. I never considered it until I read the other post I referred to. Like I said, I either leave the SB switch in the off position, or at least hit it at the same time as the power switch.
It has never created a problem that I know of. But maybe I have been getting away with murder in this regard.
Thanks

You should turn ON the power switch, but leave the amp in STBY for a minimum of 30-60 seconds, and preferrably a couple minutes if the amp has been subject to a cold environment, to let tubes warm up and bias voltage stabilize before applying DC voltages to the tubes. Nobody needs to be in THAT big a hurry to have to flip both the power and standby switches simultaneously. If you don't have a standby switch, turn the amp on but don't play through it for a minute or two.