Line Out For Kingston

[lowell]

I am putting a line out on a Kingston amp and am unsure of what resistor values to use. I know I need to take the signal off of the speaker/output transformer, and then use a ten to one ratio voltage divider. I'm just wondering what values to use for the resistors.

[Chuck H]

I know I need to...

use a ten to one ratio voltage divider.

How do you know this? I don't know your electronic chops so I'm not trying to insult, I'm trying to help. The ratio will depend on the output voltage of the power amp. If you want to idealize you wouldn't use the same ratio for a Champ (5 watts into 4 ohms) as you would for a Plexi into a half stack (100 watts into 16 ohms).

The Champ has an output of 4 or 5 volts. So a 10:1 divider would only give an output of half a volt. You would be much better off with a 5:1 ratio for a Champ. But the Plexi puts out 40 or 45 volts. So a 10:1 divider would give you 4 to 4.5 volts. So you would be better off with a ratio of 40:1.

So 5 watts into 4 ohms = 5:1
100 watts into 16 omhs = 40:1

I bring it up because the 10:1 ratio is common lore, but doesn't tell the whole story. If you actually got your ratio using the correct info then I hope I haven't offended you. I attached a watt/load/volts conversion table (courtesy of Enzo) that can help you determine the correct ratio if you need it.

For values I like to try to keep the load resistor under 600 ohms. Why? Because many line level outputs are designed to also accept headphones. Most of which are 600 ohms. As a side effect, many inputs are designed to accept a signal with an impedance of 600 ohms or less. I have had some trouble above the 600 ohm range but you can go lower without problems. Except that you do want the series R to be large enough to separate the line out circuit from the speaker load, which is typically very low impedance like 4 to 16 ohms. So lets say you do need a 10:1 ratio. If you used a 100ohm/10ohm divider, that would be fine for the line out. But your divider is a low enough ohm value to suck about 1/14 of your power (with an 8 ohm speaker) from the amp and even smoke the resistors in the line out drawing current if they are underrated. So I like to start with a 500 ohm load and figure the series R by the ratio. So a 10:1 ratio with a 500 ohm load would use a 5k series resistance. 5500 ohms in parallel with an 8 ohm speaker will draw no appreciable power but I like to use 1 or even 2 watt resistors in this circuit anyway because of spikes in the OT and just in case there is ever no load plugged into the amp at least the line out circuit will survive.

My last Line out was a 5:1 ratio. I used a 1 watt 2200 ohm series R and a 500 ohm linear taper pot (1/2 watt rated) for added versitility. The 2200/500 combo is close enough. Getting close with standard value parts is fine. No need to be exact here. A .047 cap across the load helps keep spikey business from the OT from the input of whatever your feeding the signal to. I saw that on the Chieftan and thought it was a good idea.

Chuck

[MWJB]

Further to what Chuck says, speaker impedance is also a factor...using a 680ohm load (near enough to 600) a 5W amp with a 16ohm output would use the same divider (6.8K dropper, 680ohm load) as a 15W amp with a 4ohm load, as voltage at the speaker would be in the region of 8 volts or so in both cases.

Also line level is 0.775VAC @ 600ohms, but as Chuck says, no need to be too exact here, as long as your voltage out is around 0.5VAC to a little over 1VAC that'll be fine for mixer/PA.

So your formula is (Load R/(Load R + Drop R)) * output voltage = 0.775VAC...ish.

So for instance you have a 50W amp with a 8 ohm speaker & 20v at the speaker...

680/(680+18000) = 0.04.

0.04 * 20v = 0.8VAC...close enough and bear in mind that it's the ratio between the 2 resistors that's most important, so 6.8K & 180K would work too, if you had those values lying around.

Another route is to use a dropping resistor 1/40th to 1/50th of the following volume control, so 2.2K dropper for a 100K PA/mixer fader. For the same 50W 8ohm amp, your load resistor would be 91ohms to get 0.794VAC out - 91/(91+2200)= 0.04 again.

10:1 ratio will work fine for a 5W/16ohm amp, 10W/8ohm amp, 15W/4ohm amp, 25W 2 or 4 ohm amp, 50W 20hm amp.

[Chuck H]

Further to what Chuck says, speaker impedance is also a factor...using a 680ohm load (near enough to 600) a 5W amp with a 16ohm output would use the same divider (6.8K dropper, 680ohm load) as a 15W amp with a 4ohm load, as voltage at the speaker would be in the region of 8 volts or so in both cases.

I tried to imply the method. And that it's volts, not watts, that we're dealing with for a line out signal. But also that watts will be dissapated in the circuits resistors. Thats why I provided the chart.

Also line level is 0.775VAC @ 600ohms, but as Chuck says, no need to be too exact here, as long as your voltage out is around 0.5VAC to a little over 1VAC that'll be fine for mixer/PA.

Yes. Thank you. I didn't give a firm target and I should have. FWIW I usually aim for about 1 volt output at full tilt boogie. Most inputs will handle it fine and you still have something to work with when the amp isn't cranked. When I go with a "level" pot I use a linear pot and design 0db at 5 on the control with the amp blasting. That way I can turn it up when the amp is quiet or down if an input is finiky.

Chuck

[lowell]

cool that's great info... and nice chart!

this 5 watt amp into 8ohms gives 6.3vac.
So i'm going with a 4k dropping and 600ohm load resistor.

I guess if we always want to stay around 600ohm load we could just multiply 600 times the VAC to get our dropping resistor right? In my case multiplying 600X6.4vac gives 3.8k for dropping resistor value.

[lowell]

Ok I was having trouble w/ this amp until I realized the secondary on the output transformer was not grounded. This amp came in w/ a 2 prong power cord and I upgraded to a 3-prong...

My question is can an OT in an amp w/ a 2-prong cord not be grounded and work right? I'm thinking not, maybe someone was inside this amp and forgot to reconnect that or something. Anywho the amp sounds twice as powerful now.

[Chuck H]

Well what is "not grounded"? Is there NO lead on the output jack from the bottom of the OT secondary wind? Or is the jack not using a second ground lead and instead possibly relying on the chassis connection? Some amps do not ground the OT secondary. They actually have the jack isolated from the chassis and the bottom of the OT secondary is connected directly to the speaker - lug. In fact some amps do not even have an output jack and just have the speaker wired directly to the OT secondary. Three prong or two, most schems do not differentiate between circuit ground and chassis ground. They can be two different things but not often in tube gear. The bottom of the OT secondary does not need to be connected to any proper ground. It only needs to be connected to the speakers - lug. Though most amps DO have the output jacks - lug grounded. And thats fine too. I always do it that way. If both ends of the OT secondary have always been connected to the speaker, I can't figure how grounding the bottom of the OT secondary would change anything besides hum perhaps. Was one of the OT secondary wind ends simply not soldered to anything?

[lowell]

Hi Chuck,
This amp had the OT directly wired to the speaker... no speaker jack at all. When I grounded one side of the OT to the chassis the amp came alive... it was working before this but sounded anemic and had way more hum than it does now...???

[Chuck H]

Hmmm. Perhaps grounding that end of the OT eliminated an ultrasonic oscillation or something. That would explain the increase in power and dynamics.

Chuck