Tube amps & dummy loads into line level devices

[tcio]

Hi all! I have been experimenting with guitar tone for years now and the best sound I have achieved to date by far is running my home brewed pedals through a Hughes and Kettner Blues Master using the built in power soak/dummy load out to an EQ, FX unit, and clean SS power amp. I am currently working on a tube amp project and just successfully installed the dummy load w/line out and got everything working as planned.

The thing that is bothering me though is the dummy load (10 Ohm resistor) is barely getting warm. The amp is approx. 8 watts with (1) 12AX7 and (1) EL84, The resistor is a 10 Ohm 10 Watt. I had it running for about an hour and ran it through a 100 Watt SS power amp. I had the tube amp master cranked to about 8 to 8 1/2 and gain on 10.The thing just screamed and sounded great. I’m just a little nervous about that resistor barley getting warm though. Anybody have any thoughts or ideas on this?

[daz]

Depends on how many watts the load is. I have a 100 watt resistor i use for a load and with a 8 watt amp i suppose it would stay cold enough to keep your beer cold. If it's a 10 watter then i'd be scratching my head.

[d95err]

Sounds a bit strange if it's not really getting warm. What kind of heatsinking are you using on the resistor? 10W seems a bit risky (you should use at least twice the amp power rating). Are you sure the amp is really putting out 8W? Have you measured the output power?

I have a 50W heatsinked resistor mounted to the chassis as dummy load on a 15W tube amp, and if I crank the amp it gets hot enough to burn your fingers after a while.

I once used a 100W L-Pad to attenuate my Peavey Classic 30W. When cranked, the L-Pad quickly got so hot the insulation on the connecting cables started to melt...

[tcio]

Now that sounds more like what should happening. OK, my apologies. This should be 5 watts not 8. I have been working on a Gibsonette 8 watt prior to this and did quite a bit of research on the specs, tubes, etc. I must of had 8 watts stuck in my brain lol! It still should be getting hot I would think though. I am going to run some tests tonight and see what this thing is actually putting out.

Regarding the dummy load....... I am scraping for parts and had to use what I had on hand so I stripped a piece of 18 gauge hook up wire and wrapped it around the resistor real tight onto a metal corner bracket and mounted that onto an "L" bracket so it would be raised up and be in the air thinking it was going to be getting real hot.


Dummy Load 3.jpg


I remember reading exactly what you said about resistor(s) being twice the wattage as the output of the amp. I'll post my test results later today. I did fry a small potentiometer when I was testing out the line out circuit as I was trying to get the best sounding values using pots. I didn't have any small value high watt pots and ended up having to use a 1M 1/2 watt to finish. At least got in the ball park for now. So after replacing the pots with higher watt resistors the load resistor seemed to get a little bit warmer but still overall barely warm.

[tcio]

Here’s what I’ve tested so far for the EL84 and some of these are certainly not right (in Bold):

(Class A amp)

Screen Voltage = 229

Plate Voltage = 233

Cathode Voltage = 4.26

Heater Voltage = 7.0

Current Consumption: 74mA

Cathode Current: = 28mA

Screen Current = 7mA

Plate Current = 21mA

Plate Dissipation = 4.8 Watts

[d95err]

Have you measured the actual output power? 233V sound a bit low to be getting 5W output power. Not that the exact power matters much.

It seems like your heatsinking is actually very effective, which is why the resistor is not getting that hot. Good work!

[J M Fahey]

I see nothing strange on the bold type values.
I'm sure your amp puts out less than what you think.
Put a steady 1kHz tone through it and measure voltage across the load.

[tcio]

Thanks everyone for your input on this.

It seems like your heatsinking is actually very effective, which is why the resistor is not getting that hot. Good work!

Thanks for that! It's not the prettiest but I'm after results right now not appearance plus that's all I had laying around lol! I did the same exact thing when I replaced the speaker field coil on a Gibsonette. It worked great. But I am still concerned as it is not even warm. You can barely feel any warmth even when you hold the resistor tight with your bare hand while the amp is cranking.

Originally Posted by J M Fahey

I see nothing strange on the bold type values.
I'm sure your amp puts out less than what you think.

That would be great as I really wanted a 1 to 2 watt amp to begin with. But I went with the most basic yet best sounding tube amp circuit I could find online that had a 12AX7 and EL84 (that's what the H & K Blues Master amp consists of). I figured down the road if I still wanted to I could figure out how to get the wattage down. Although I have been playing around with tone in regards to amps, guitars, and pedals non stop for over twenty years and also building my own pedals for about three years now, this is my first amp build. Supposedly it is a 5 watt SE Class A amp.

According to the spec sheets I found online on an EL84, the standard operation of a Class A amp for the following in bold ones that I listed should be:

Cathode Current: = 65mA (as opposed to my 28mA)

Plate Current = 48mA (ss opposed to my 21mA)

Plate Dissipation = 12 watts - Oops! That was the Max so I guess 4.8 Watts may be OK?

Originally Posted by J M Fahey

Put a steady 1kHz tone through it and measure voltage across the load.

There seems to be a lot of controversy on how to accurately measure the output of a tube amp but your 1khz method seems to be one of the soundest that I have found out there (thanks) so I am hoping I did it right as my passion for this stuff by far exceeds my understanding. I put the voltmeter (I don't own an oscilloscope) on AC, put the positive lead to the output lead of the dummy load resistor and neg lead to ground. It read 2.7V, I squared that and divided it by the resistor value (10 ohms) which came out to 0.729. That can't be right can it?

[Jazz P Bass]

Some say the "best way" to measure the amplifiers output power is to inject the signal into the PI.
As to your test setup:
(best outcomes are with a scope)
#1 : the dummy load should match the specifications of the output transformer that is installed.
#2 : the test signal that you inject should not be higher than 100mV.(arguably lower)
some manufacturers specify the input signal on there schematics to achieve a "clean" undistorted output. Without a scope it is not possible to know when the output starts to distort (square wave).
You cannot get a valid voltage reading with a distorted signal.
A clean signal will give a valid reading (with good instrumentation)
#3 : where the volume & tone controls are set affects the output voltage reading. ie: clean or distorted.
Finally, the equation used "V squared / resistance" is true" when the measured output voltage is in an RMS format.
A good (Fluke?) meter will display True RMS Vac. A cheap one will try to come up with an average reading, which may or may not be tweaked by the manufacturer to "display" RMS Vac.

[tcio]

Crap!……. Unless there are any shortcuts for a somewhat accurate way to get a reading with a DVM without the true RMS feature, it looks like I am out of luck Thanks for that info though Jazz P Bass. That was part of the controversy I was reading about. Distortion vs clean while testing and also tube guitar amps vs Hi-Fi amps

Is there anything commonly used that you know of that is around 100mA to do the 1khz tests with? I used a cheap mp3 player lol! I know I am attempting to do this the “diy poor man style” way but I’m broke! I only have low budget DVMs without the RMS feature. The best one I have is an Extech 22-816. They served me well for the past 3 years making foot pedals but it looks like I have to invest in even more tools for amp building sooner than I expected.

BTW, I am going off from the OT’s 8 ohm tap but went with a 10 Ohm 5% tolerance resistor for the dummy load just to be safe as I heard (more controversy on this one too) that it was safer to go higher than lower on output impedance with tube amps.

Well. I’m still open for suggestions. Perhaps there’s a poor mans method out there somewhere to maybe get close?

[tcio]

I ended up doing the Gerald Weber “Quick & Easy Wattage Check”. I'm sure it’s not 100% dead on accurate but I am confident it is close. I still don’t know where I stand in regards to running the amp 100% clean vs cranked up like I intend to be running it most of the time. When I ran one of my pedals through it and cranked it up the voltage tripled. The test results were then 2 watts. Otherwise straight in and turned up without distortion it was around .7 watts. I realize this is quite a slang way to get the wattage so perhaps with professional equipment the clean route is probably more dead on but 2 watts does make more sense as the DL resistor still is not even getting warm.

So far it seems to be running fine. No smoke, pops, sizzles, or funny smells. Some friends were over last night and one of them brought a Celestion Greenback for me to try out. I plugged in one of my high gain pedals (which I tried the day before with terrible results) and our jaws must have hit the floor. Minus a lack of some minor highs, lows, and a little torque, with just a few turns of some knobs I nailed some early VanHalen tones. Now if I can just nail some early VanHalen licks lol!

When I first got the amp working and plugged it straight into a cab it had a fairly nice rhythm sound but terrible lead sounds when I started hitting the lower notes and overall sounded a little rough. I didn't tweak one thing on it before I ran it through the DL and all the other stuff. I can only image how good it should sound once I actually start tweaking the amp and adding a little more circuitry.

Thanks again everyone for your replies and info that you have provided. Every little bit certainly helps and is greatly appreciated