Abbey Harmonic II

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jimmy74
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Re: Abbey Harmonic II

Post by jimmy74 »

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Re: Abbey Harmonic II

Post by RectifiedAmps »

jimmy74 wrote: Wed Feb 24, 2021 2:41 pm Yes all measurements are done without the power valves in because it is noisy and pops a lot when turning standby on and off (tried installing an x2 cap on the standby switch but that didn’t do much).
Ignore the standby and just use the power on/off switch if it’s quieter. Using the standby isn’t necessary.
jimmy74 wrote: Wed Feb 24, 2021 2:41 pm So which 22k resistor would I have to change?
The one next to your ‘-50V’ annotation on your sketch. You could try parallelling a 100k across it first, to see if that gets the bias to where you want. How are you measuring the 23mA you mentioned before?

Also, I think the other 22k in the bias circuit is actually a 2k2. The voltages on either side of it aren’t right- if it’s a 22k and the resistor to ground is 22k, then the voltage divider should drop it from -58 to around -29, not -50. Something doesn’t add up there...

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Re: Abbey Harmonic II

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I think you’re right the resistor in parallel to the 22k+cap looks more like a 3.3k

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Re: Abbey Harmonic II

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Yep, that looks right and the voltages make more sense that way too. So it’s the 22k that can be lowered slightly to get hotter bias. You could tack a 100k trimpot across it and tweak to get what you need. 30-35mA should be a good ballpark for 5881s at ca.450vdc.

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Re: Abbey Harmonic II

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Also would the fact that the old power tubes were old sovtek 5881/6l6wgc and the new ones are 5881wxt be of any substantial significance?

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Re: Abbey Harmonic II

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And also maybe one thing I should have mentioned earlier is that this amp is a 100w version

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Re: Abbey Harmonic II

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Took the voltage readings with the power tubes installed and without turning the standby off... bias voltages differ very little... -49vdc on pins 5 instead of -49.7vdc other readings are a few mv lower.

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Re: Abbey Harmonic II

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So I installed a 100k resistor in parallel with the 22k resistor and am getting 30 to 36mA using a tad biasmaster

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Re: Abbey Harmonic II

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Sounds good. Sovtek 5881s aren’t rated for more than 25W so depending on your idle anode/plate voltage you should be good to go with 30-35mA. How does it sound when you play through it (aside from deafeningly loud)?

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Re: Abbey Harmonic II

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Well turned it on and it was as quiet as a whistle... too quiet... plugged in a guitar and turned and fiddled around with the volumes and nope no sound whatsoever... now I’m trying to figure out what’s going on

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Re: Abbey Harmonic II

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Tested the output transformer primaries and am getting around 145ohms from one side to the other. Resoldered the ot wires to the tube sockets, powered it up and just fiddling around with the volumes and touching the tip of the input jack I am getting noise but at very low volume

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Re: Abbey Harmonic II

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Give the speaker plug a good wiggle in the jack, then rock each valve in its socket too. If it made noise before but now doesn’t, it’s probably a dirty or bad contact. Otherwise have a good look at all the wiring.

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Re: Abbey Harmonic II

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Yes tried tugging on the cables and tapping all the tubes and as I said I turn the volume up very high and touch the tip of input jack I get low volume earth noise... I have noticed that the standby is behaving as it did before... very abruptly turns HV on and off so much so I can see and hear it.. I’ll test the bias again after resoldering the ot primaries

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Re: Abbey Harmonic II

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Ok tested the bias and its ok... there is noise coming through the speaker when I flip the standby off... constant noise accompanied by pops... so I’ve started doing some voltage testing on the preamp tubes.. started with the tube that was giving me most grief before... very microphonic and noisy. Removing only that tube and testing the voltages I’m getting almost B+ voltages on the plates... am I wrong in thinking that this isn’t normal?

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Re: Abbey Harmonic II

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Start by measuring all the preamp cathode voltages with the tubes installed. No cathode voltage= faulty tube. Then use your voltmeter to test the preamp grids- not so much for voltage but just to see if you get a loud buzz when the probe touches them. You need to figure out where in the path your audio signal is failing to get through.

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