A couple of questions

[jimboyogi]

Hi All,

I'm just finalizing a schematic for a new single-ended build of mine, and I wanted to ask a couple of questions.

1 - this will be an amp head, so has a speaker jack and an impedance selector switch. But because it's a head, i'm worried that it is more likely to be accidently run with no speaker connected/plugged in to the jack.

I'm using an insulated mono Cliff style speaker jack, with switching on both terminals. When a plug is inserted, both contacts are open, when there is no plug, both contacts are closed. I was wondering if this gave me a chance to put a resistor across these terminals to act as an emergency load in case of no speaker being plugged in, to protect the OT etc.

Do people do this? Is there more to it than this?

The two impedances available to the speaker jack will be 4 and 8 ohms, and the amp puts out a max of 10W. So I was planning on using a 5.6 ohm/10W resistor for the job.

2 - Bleeder resistors. They sound like a good idea to me. I am using 3 filter caps (all 47 uF), 2 of which are isolated from the HT when in standby mode. So I figure that I have to put a bleeder resistor in parallel with at least 1 cap in each of the 2 standby zones. My voltages are ~ 350v down to 280V across the power supply. If I use 1Meg resistors, 0.5W, how long will it take to discharge? Should I use a lower resistance/higher wattage?

Thanks for any assistance

[Steve Conner]

1: Yes, people do that, but it only protects against the case where a plug isn't inserted in the amp's speaker jack. It won't help you if you forget to plug the other end of the cable into the speaker cabinet, for instance. A 220 ohm (say) resistor, permanently connected across the OT secondary winding without any kind of switching arrangement, works fine.

2: I use 100k or 220k 2 watt bleeder resistors, but I've mostly built bigger amps in the 30 to 60 watt range.

[jimboyogi]

Thanks Steve, greatly appreciated.

Re. 1 - I see your point about my suggestion not protecting against all of the other things that can prevent a speaker being connected. A 220 ohm across the OT secondary it is. Will a 5W do?

Re. 2 - How do I calculate the required wattage of the resistor?
Is it: W = V^2/R = 350 x 350/220,000 = 0.56 W?
Times ~2 for safety?

[Wilder Amplification]

Thanks Steve, greatly appreciated.

Re. 1 - I see your point about my suggestion not protecting against all of the other things that can prevent a speaker being connected. A 220 ohm across the OT secondary it is. Will a 5W do?

Re. 2 - How do I calculate the required wattage of the resistor?
Is it: W = V^2/R = 350 x 350/220,000 = 0.56 W?
Times ~2 for safety?

350 is your B+ on the primary side. Voltage on the secondary side is MUCH lower than that. Just to give you an idea we'll use the voltage output of a 100 watter on a 16 ohm load, which is about 40 volts -

W = V^2/R = (40 x 40) / 220,000 = 0.007W

With the high value of the resistor drastically limiting the current it can pull at output voltage you can see that using a resistor value that high ain't gonna dissipate that much power at all. With your build a 1 watt should be more than enough.

[Bruce / Mission Amps]

350 is your B+ on the primary side. Voltage on the secondary side is MUCH lower than that. Just to give you an idea we'll use the voltage output of a 100 watter on a 16 ohm load, which is about 40 volts -

W = V^2/R = (40 x 40) / 220,000 = 0.007W

With the high value of the resistor drastically limiting the current it can pull at output voltage you can see that using a resistor value that high ain't gonna dissipate that much power at all. With your build a 1 watt should be more than enough.

oops... Yeah but Steve said 220 ohms across the speaker load side, not 220,000 ohms. That was the high voltage PT secondary cap bleeders he was talking about.

Secondary on the OT:

40^2 / 220 = 7.27 watts
I use a 470 ohm @ 5 watts and that always seems to be just fine.

[mooreamps]

Hi All,

I'm just finalizing a schematic for a new single-ended build of mine, and I wanted to ask a couple of questions.

1 - this will be an amp head, so has a speaker jack and an impedance selector switch. But because it's a head, i'm worried that it is more likely to be accidently run with no speaker connected/plugged in to the jack.

I'm using an insulated mono Cliff style speaker jack, with switching on both terminals. When a plug is inserted, both contacts are open, when there is no plug, both contacts are closed. I was wondering if this gave me a chance to put a resistor across these terminals to act as an emergency load in case of no speaker being plugged in, to protect the OT etc.

Do people do this? Is there more to it than this?

The two impedances available to the speaker jack will be 4 and 8 ohms, and the amp puts out a max of 10W. So I was planning on using a 5.6 ohm/10W resistor for the job.

2 - Bleeder resistors. They sound like a good idea to me. I am using 3 filter caps (all 47 uF), 2 of which are isolated from the HT when in standby mode. So I figure that I have to put a bleeder resistor in parallel with at least 1 cap in each of the 2 standby zones. My voltages are ~ 350v down to 280V across the power supply. If I use 1Meg resistors, 0.5W, how long will it take to discharge? Should I use a lower resistance/higher wattage?

Thanks for any assistance

let me tell what I do.

1. I run a 100 ohm pot off the 16 ohm tap of the output transformer. This keeps a constant low impedance load on the O/T. While not a "perfect" match, it's better than using a shorting jack for the speaker plug, which will guarantee you will back kick the power tubes when plugging in and out with the power on.

2. Bleeder resistors. Two schools of thought.

2.1 Keeping an cap fully charged increases it's life. Perhaps the main reason why Fender did it this way. The downside, well the cap(s) will retain the high voltage charge and they will bite.

2.2 Bleeder resistor makes the amp safer to work on when you are building and testing it. You can either calculate the rate of discharge, or measure the rate of discharge with a meter. I tend to use a meter because being bite by a 47uF cap fully charged to 300 volts is just not as much fun as it used to be.

However, to calculate the rate of discharge of a 1 meg ohm bleeder across a 47uF cap ; for example ; is the value of resistance times the value of the cap ; or T=RC ; "times 5". The "times 5" is the five time constants for either charge or discharge. Very simple... So ; do some quick math ; and the calculated discharge time for this circuit comes out to
235 seconds. I'm pretty this is right given what I measure on some of my HV power supplies. Personally, I run with a value of 390K ohms at 1 watt for a 300 volts bleeder. This value does not get too hot, and I don't have to wait all day before chassis is safe to work on.

When selecting a value for a bleeder, it's important to understand how to calculate the rate of discharge, and how to calculate the wattage rating of the resistor.


Here is a link that is almost pretty good on RC time constants.
http://www.tpub.com/neets/book2/3d.htm




-g

[jimboyogi]

Thanks to all of you for your replies, every time I come to this forum I learn at least 1 new thing!

Re. the permanent resistor (220 or 100 ohm) across the OT secondary I have another 2 questions.

1 - As the impedance of either of these 2 resistor values is so much higher than the ideal load (speaker), how do they not create problems for the output valve?

2 - Does having this resistor permanently connected have any effect on the tone of the amp when a speaker is connected?


Thanks again

[Steve Conner]

1- They do create problems for the output valve. If you lose the speaker connection at full volume, you still run the risk of melting the screen grid. The resistor only protects the OT from breakdown. I don't know of any way to protect the screen grid without complicated circuits that generally ruin the Zen-like simplicity of tube amps.

2- Not as far as I know. In an amp with no NFB, it might damp the bass resonance and presence peak slightly, but that's often a good thing.