Before you actually turn the amp on with the tube in it, make sure that the trimpots are set for their maximum resistance.

Also, look at the changes we made for pho_boi's amp. They may be helpful in getting your amp to work first time.

[Edited by Guitarsenal on 11-24-2005 at 12:16 AM.]

Your resistors look to be 2W which should be fine. Yes, they will run hot and yours are only 40°F more than I measure.
Agreed, you will need to add some sort of ventilation and you do want to stand these resistors off the PCB by at least 1/2" (look at my prototype back at page 1 of this thread) to allow them to dissipate heat somewhat easier than when laid flat against the PCB. Space them slightly further apart and you should see the temperature drop back to what I measure.

Swapping out the 2W pairs for larger wattage resistors doesn't automatically mean the substitute resistor(s) will run cooler. It depends on what type you use and other factors.
Two pairs of 2W seemed a good cost compromise and easy to get anywhere.

My prototype SOHA resistors run at 115°C (240°F) for hours on end and have done so for quite a few weeks now with no ill effects, which is just as expected.

[Edited by mains_hum on 11-24-2005 at 07:42 AM.]

I've tried all the new production 12AU7's (JJ, EI, ElectroHarmonix) and they sound fine but they produce hihger plate voltages (>50V) than NOS tubes. Mains_hum tells me this is becasue the filaments of new production tubes are more efficient. At any rate the JJ sounds fine in Stoopid so long as you get the plate voltage down below 50V.

[Edited by Batman on 11-29-2005 at 12:48 AM.]

I'm seeing 70 volts out of the high DC part of the power supply. That's more than the 55-60v on the diagram. Am I seeing that because there is no load on the supply?

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quote:

[By Guitarsenal] I finished the power supply part of my SOHA amp:

I'm seeing 70 volts out of the high DC part of the power supply. That's more than the 55-60v on the diagram. Am I seeing that because there is no load on the supply?

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Yes. It should drop when you attach the tube's heaters.

Nice work.

[Edited by runeight on 12-04-2005 at 05:24 PM.]

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quote:

[By guitarsenal] It's showing 68 volts with the heater attached.

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Wow, that's pretty high, but not deadly.

Can you measure the AC volts on the trafo secondary when everything is powered up. And then would you kindly measure the AC volts across the heater.

Also, are you using the jfets for the CCS or are you using a CC diode?

Thanks.

[Edited by runeight on 12-04-2005 at 07:32 PM.]

Across 2ndary: 34.6 v
Across tube heater: 12.5 v
High volt out: 69.8 v
Bipolar out: 12.1 v, -12.1v

I haven't built the signal path for the circuit yet. I'm planning to use the CCS diodes, which should arrive tomorrow.

Just drilled the case, and now have metal shavings in my foot!

This is going to look pretty cool when it's done. Can't wait to hear it!

[Edited by Guitarsenal on 12-04-2005 at 07:49 PM.]

All of your measured voltages seem to me right on target. The B+ will come down when it is loaded. So let's see where it goes.

The only thing to remember is to set the trimpots to their MAXIMUM resistance before you fire up the amp. And then dial them smaller until the plate voltages are around 40V.

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quote:

[By runeight] Tip is left. Ring is right. The schematic actually shows this.

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Holy cow, it does!

Now I just need to sort out where everything gets wired to the tube, but I saw diagram a few pages back that had that.

Thanks!

[Edited by guitarsenal on 12-04-2005 at 08:23 PM.]

Basic Stoopid Hybrid Amplifier

Basic Stoopid Hybrid Amplifier 2

Thanks!

I just realized that I ordered .47uf caps instead of .047uf caps for the to ground cap of the Meier crossfeed that I plan to build into this amp. I think I'll finish building the amp on Tuesday night without the crossfeed, and add that part later.