18 Watt Valvestorm TMB COMPLETED w/ Voltages and questions!

[crgfrench]

The 470k is on V2. Schem & layout look OK and agree on V1 to me.

[ViperDoc]

You are correct, I boneheaded that last “finding”. Next!

[ViperDoc]

OUTPUT DISSIPATION CHECK:

I used the methods described on RobRobinette.com to calculate some numbers for my power tubes using measurements from my powered-up chassis. Some very interesting data here.

First I used the basic calculator:
PT plate-to-cathode voltage = 329 VDC
Cathode resistor value = 125R (verified with DMM)
Calculated per EL84 dissipation = ulp...14 WATTS.

Secondly, I measured the voltage drop from the OT CT to each PT plate and found each at 13 VDC. The Resistance between the OT CT and the V5 plate was 312 ohms and to the V4 plate was 329 ohms. Using Ohm's law I calculated my V4 plate current as 13 VDC/312R =39.5 mA and 13 VDC/329R = 40.6 mA for V5. Again using Ohm's law with an average of 40 mA of plate current, I have an average PT plate dissipation of 13.2 Watts or 109.7% dissipation.

SO! Assuming this is all correct, what would you recommend? Previous recommendations included:

1) lowering PT grid reference resistor values
2) EDIT:[lowering] "CORRECTED TO" RAISING PT cathode resistor values
3) installing 7189 power tubes (I don't want to do this until I correct my voltages and power for proper EL84 operation).

Where do you think you would start first?

Thanks.

[JMPGuitars]

1) lowering PT grid reference resistor values
2) lowering PT cathode resistor values
3) installing 7189 power tubes (I don't want to do this until I correct my voltages and power for proper EL84 operation).

Where do you think you would start first?

Thanks.

No, RAISE the power tube cathode resistor value. Lowering the value will increase the output. You should be experimenting between 150 to 200 ohms, with 180 ohms being the best place to start.

That's what you do first. The grid reference resistors are more about changing your distortion/headroom sound. Biasing is done with the cathode resistor...that's why it's called cathode bias.

...and even if you did install 7189 tubes, you'd be about 100% dissipation there anyway.

Thanks,
Josh

[geoff 1965]

like Josh said 180R would be a good starting point,this will bring the dissipation down and also bring your cathode voltage up closer to 12V.
when you measure your voltage drop "centre tap to plate OT" leave the multimeter on for a good 5 mins to allow the tube to heat up and stabilize,with the unequal resistance you should have a slight difference in voltage drop for each tube.use the 20 or 200 DCV range to get an accurate reading "noticed you have 13V drop on each tube"

[ViperDoc]

Copy that! Thanks!

[ViperDoc]

I had to work overtime to find some suitable values at Mouser in 6.5-7 W wirewounds. Seems these are in relatively short supply. Any other sources you’d recommend, please PM me, thanks!

[crgfrench]

Go with 8W Ohmite Brown Devils. Sometimes the 12W ones are actually cheaper at Mouser than the 8 W ones.

[ViperDoc]

Thanks for the tip!

[JMPGuitars]

I had to work overtime to find some suitable values at Mouser in 6.5-7 W wirewounds. Seems these are in relatively short supply. Any other sources you’d recommend, please PM me, thanks!

I use 5W resistors there. 5W is already plenty. The ohmites Craig referenced are also good.

[ViperDoc]

Mouser upgraded me to 2nd Day Air so my swag showed up early!

I just swapped my 125R cathode bias resistor with a Vishay 180R 7W. The V-drop across it is now 12.8 VDC. My Plate current dropped to 33.6 mA and with a plate-cathode voltage of 335 VDC, that leaves each tube at 11.3 watts at idle.

My tubes are thanking you! Now to tone test it.

[ViperDoc]

I hear a lot of improvement since making all the changes I’ve made. The amp has great tone all around, even if it still could use a bit more headroom. But it is hiss-free, has really nice snarl when you dig in, is dead silent when you aren’t. Can’t complain! Thanks all for your help.

When I ordered this kit, I fat fingered the order to the effect of almost ordering a whole second kit, so I would like to build a second version with a few changes. That TMB tremolo looks pretty badass...

[JMPGuitars]

I hear a lot of improvement since making all the changes I’ve made. The amp has great tone all around, even if it still could use a bit more headroom. But it is hiss-free, has really nice snarl when you dig in, is dead silent when you aren’t. Can’t complain! Thanks all for your help.

When I ordered this kit, I fat fingered the order to the effect of almost ordering a whole second kit, so I would like to build a second version with a few changes. That TMB tremolo looks pretty badass...

Thanks. There's going to be a few tweaks to the Tremolo TMB docs in the next couple days as I find time to finalize my build. Nothing major, mostly a few resistor value changes.

Thanks,
Josh

[Bieworm]

I hear a lot of improvement since making all the changes I’ve made. The amp has great tone all around, even if it still could use a bit more headroom. But it is hiss-free, has really nice snarl when you dig in, is dead silent when you aren’t. Can’t complain! Thanks all for your help.

When I ordered this kit, I fat fingered the order to the effect of almost ordering a whole second kit, so I would like to build a second version with a few changes. That TMB tremolo looks pretty badass...

Eventually once you have that tremolo TMB you're not going to care for your other amps anymore. It is THAT good!!! Me too I am doing research on my next amp.. I was considering doing another tremolo TMB, maybe 36 watts and built in reverb. But I let the idea go for 3 reasons.
1: I have a Vibro King for higher volume situations. I'm afraid I won't be using that one anymore when I have a 36W Tremolo TMB.
2: I honor my 18W tremolo TMB that much... I might end up not using it because of the new build.
3: I fear the next tremolo TMB build will not have the mojo my current one has. After all it sports an early 70s celestion G12h30 which I could blow up in a 36W situation.

Sooooo...after many nights of sleep I decided to build the Tone King Imperial MK2. The reverb won't be copied for I will juice it with a 100% blackface reverb circuit. The trem tube is absent and replaced by an IRF840. Sorry for that guys, but I want to know if that will compete with the tube variant (no thumping?)
The project is really in advanced stage. My layout is nearly ready and I am about to order the components. Still deciding if I want to house it in a fender or an 18Watt cab... I have a fender type 3x10 blackface combo cab available.. but the faceplate is going to be the problem...

[ViperDoc]

I’ve been laying awake at night about my ~200 VDC PI plate voltage. Just kidding. But I’m going to swap out those 100K Carbon film PI plate resistors with some carbon comps, maybe 110-115K. Maybe.

[JMPGuitars]

I’ve been laying awake at night about my ~200 VDC PI plate voltage. Just kidding. But I’m going to swap out those 100K Carbon film PI plate resistors with some carbon comps, maybe 110-115K. Maybe.

That's a good plan if your goal is to lower the voltage.

Maybe try some 82K instead.

[ViperDoc]

So I clearly have a misunderstanding about how to manipulate voltages. Does the larger plate load resistor cause a greater drop across it, or does it block it from going across it? If the former, it seems counter intuitive.

EDIT: After taking a walk, I realized I don’t fully understand the relationship between the heater voltage and the power rail B+ voltage. It is the B+ that drops across the plate resistor to determine the PI plate voltage, yes? If so, I’ve just been looking at it all backwards.

[JMPGuitars]

So I clearly have a misunderstanding about how to manipulate voltages. Does the larger plate load resistor cause a greater drop across it, or does it block it from going across it? If the former, it seems counter intuitive.

EDIT: After taking a walk, I realized I don’t fully understand the relationship between the heater voltage and the power rail B+ voltage. It is the B+ that drops across the plate resistor to determine the PI plate voltage, yes? If so, I’ve just been looking at it all backwards.

The heater voltage is a constant, and not what we're discussing.

The B+ nodes feed the plates. The plate resistors resist that delicious juice. Less resistance = more juice. If you give more resistance (higher resistor value), you get less juice.

Since you're concerned that your PI plates are left a little thirsty, you want to give them more juice.

[ViperDoc]

Thanks. I was simply seeing it all backwards. That helps me better understand the power supply-tube relationship somewhat better. I thought the tubes were creating the voltage, but now I can see how the B+ commanding the plates makes a whole lot more sense. I notice Friedman uses a lot of 220K plate load resistors on his preamp tubes, and he’s known for higher gain amps. Obviously it’s only a piece of the puzzle. How does that work?

[JMPGuitars]

Thanks. I was simply seeing it all backwards. That helps me better understand the power supply-tube relationship somewhat better. I thought the tubes were creating the voltage, but now I can see how the B+ commanding the plates makes a whole lot more sense. I notice Friedman uses a lot of 220K plate load resistors on his preamp tubes, and he’s known for higher gain amps. Obviously it’s only a piece of the puzzle. How does that work?

That's meaningless without context. What's his B+ voltage? What are his plate voltages? A resistor out of context doesn't tell us anything.