Safe to use 6L6/5881 with 120mA rated PT?

[bob p]

One of the nice features in PSUD is that after plugging in the no-load voltage and the secondary winding resistance, you can vary the load on the transformer in your models. (unfortunately that web calc doesn't seem to have that functionality.)

varying load conditions can produce a huge difference in the voltages obtained, and PSUD is a very handy tool for that kind of modelling. As an example: PSUD makes it very easy to model the differences between biasing an amp into class A1 and into class AB. I've run into situations where a transformer works fine if you bias it into class AB1 and you assume an average duty cycle, but ends up being way under spec for the class A application.

in addition to the other considerations that have been suggested, such as cap values and winding resistance, I'd add that bias conditions could make a difference. some power transformers and output transformers won't hold up to class a operation. of course, it shouldn't be an issue if you decide to stay in class AB.

[bob p]

I'm not sure if you have taken standby (for more than 30 seconds) into consideration or not?

PSUD has a useful feature that allows you to perform stepped load calculations. When modelling a power supply for an amp, I'll perform two models:

the first model is constructed to simulate turn-on to the standby state, and then the application of a stepped load condition that represents standy-off at idle. this model shows the voltage changes that exist in the standby state, and how the voltages transition to idle when switching standy-off.

the second model is constructed to simulate the standby-off / idle state, followed by a stepped load to full-load/maximum-signal conditions for the output stage. this model demonstrates the transition to the worst case scenario for power supply voltage sag.

between those two models I get a good leg up on how the power supply will perform before i actually have to build it.

[nickb]

I'm not sure if you have taken standby (for more than 30 seconds) into consideration or not?

HV standby...there's another good reason not to that 'feature' that destroys your good tubes by building up cathode interface resistance. If you insist on having one then you can put it before the first caps. Otherwise you'll have to use two caps in series to get the rating up high enough. Or just wire it as mute.

[DRH1958]

HV standby...there's another good reason not to that 'feature' that destroys your good tubes by building up cathode interface resistance. If you insist on having one then you can put it before the first caps. Otherwise you'll have to use two caps in series to get the rating up high enough. Or just wire it as mute.

Another way is to use a switch to disconnect/ground the screens of the 6L6s. Yep, no reason to use the conventional standby.

[pdf64]

I think that a good 'all requirements covered' arrangement for when standby is perceived to be essential / there's already a hole labelled as such, is to use a dpdt switch to apply a signal mute alongside the insertion of a high value cathode resistor.
Thereby standby mode is silent, sufficient cathode current can be arranged to avoid cathode poisoning, high voltages are kept off the switch, and potentially damaging levels of switch on surge are avoided.

[nickb]

I had another thought about the cap rating.

Whether you have a post cap HV standby or not, it could be useful to have higher rated caps during testing e.g. no power tubes fitted. Also there is the fault situation to consider where, for whatever reason, the load current is reduced potentially subjecting every cap to 550V+. 600V caps are available.

[elipsey]

Is there anything wrong with just putting a standby switch before the first filter?

[DRH1958]

Nothing wrong with that. I think nickb touched on that method in post #18?

[eschertron]

Is there anything wrong with just putting a standby switch before the first filter?

Mostly out of my own curiosity on the matter, I looked at a handful of amps (Fender, Ampeg, PV, Randall, Marshall?, Crate) and it seems to me they're either after the first filter, or they break the current return path on the HV secondary before the rectifier (HV centertap or equiv) if used before the filter, regardless of SS or tube rectification. Maybe it's easier to meet AC switch ratings? Maybe a concern for tube rectifier longevity/safety that's been copied down the line?

[bob p]

On the subject of standby and over-voltage on caps: I take a fairly brain-dead approach to avoiding this problem. I always use a totem pole configuration with high voltage caps at the first node. that way I don't have to worry about the overvoltage transient at turn on, even in the tubes-out condition.

On the subject of alternate standby methods, I have amps that use the push-pull signal short method. while that gets rid of most of the volume, it's never really silent because there's always some mismatch at the PI.

[pdf64]

Is there anything wrong with just putting a standby switch before the first filter?

First off, consider attempting to convince the customer that technically, the traditional standby has no benefit and various drawbacks; I suspect that generally it is only included to fulfill market expectation. It's much better to use the power switch alone to switch the amp on and off as required.

A switch between the rectifier and reservoir cap (whether in the HT or 0V circuits) subjects those circuits to the significant current surges when actuated, due to the reservoir cap appearing initially as a short. That's unlikely to maximise service life of any of the components that have to cope with that current, but that's most especially the case if a tube rectifier is used. The 'hot switching' of capacitive loads on a tube rectifier is bad practice, as it will likely lead to its peak plate current being exceeded.
The 5F6A is an example of this http://bmamps.com/Schematics/fender/...5f6a_schem.gif though the 0.05uF cap may provide a slight degree of mitigation.

[elipsey]

First off, consider attempting to convince the customer that technically, the traditional standby has no benefit and various drawbacks; I suspect that generally it is only included to fulfill market expectation. It's much better to use the power switch alone to switch the amp on and off as required.

A switch between the rectifier and reservoir cap (whether in the HT or 0V circuits) subjects those circuits to the significant current surges when actuated, due to the reservoir cap appearing initially as a short. That's unlikely to maximise service life of any of the components that have to cope with that current, but that's most especially the case if a tube rectifier is used. The 'hot switching' of capacitive loads on a tube rectifier is bad practice, as it will likely lead to its peak plate current being exceeded.
The 5F6A is an example of this http://bmamps.com/Schematics/fender/...5f6a_schem.gif though the 0.05uF cap may provide a slight degree of mitigation.

Thank you. What if I do the same thing, but with silicon diodes? How about with an MOV to limit inrush current?

[SoulFetish]

Thank you. What if I do the same thing, but with silicon diodes? How about with an MOV to limit inrush current?

A MOV is used to protect against a voltage overload. I'm not sure this is offers you the current limiting protection your looking for here.
Personally, I think the solution is not incorporating the traditional standby. I use 500V and 700V film caps in all my power supplies. They are rated to handle high surge currents and I spec them to easily clear the unloaded voltage of the transformer. Even though they could probably withstand the surge at switching off standy, the current would still stress the iron and other components, so I dont use standby.

[elipsey]

A MOV is used to protect against a voltage overload. I'm not sure this is offers you the current limiting protection your looking for here.
Personally, I think the solution is not incorporating the traditional standby. I use 500V and 700V film caps in all my power supplies. They are rated to handle high surge currents and I spec them to easily clear the unloaded voltage of the transformer. Even though they could probably withstand the surge at switching off standy, the current would still stress the iron and other components, so I dont use standby.

Sorry, I meant to say thermistor. Like this: WTH Thermistor

I'm curious about using this between a diode rectifier and standby switch before the first filter because I already built one amp this way. It seems fine so far but it only has a few hours on it...

[nickb]

Sorry, I meant to say thermistor. Like this: WTH Thermistor

I'm curious about using this between a diode rectifier and standby switch before the first filter because I already built one amp this way. It seems fine so far but it only has a few hours on it...

Thermistors are commonly and successfully used for just this purpose i.e surge control in millions of pieces of equipment. The down side is you have to let them cool off between cycles.