Scratch built AA864 Voltages

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fevzay
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Joined: Fri May 18, 2012 12:43 pm

Scratch built AA864 Voltages

Post by fevzay » Thu Nov 09, 2017 7:56 pm

After digging around, I didn't find a clear answer to my question...

I used a '71 SF Bassman as a donor to build a single channel AA864 into a Mojotone chassis. Everything "works," but the voltages are confusing me. And...this is all new to me still.

My voltages read high compared to the AA864 schematic, but I have read this is somewhat normal given I have 125 V AC coming out of my wall. However, some of the voltages are disparately high.

I'm not comfortable with V4 and V3 plate voltages. V4 (PI) is shown as 220/205 on the schematic. I'm well over that. V3 is also a little hot although closer than V4.

A critical point. I build out only the Normal channel. Is this affecting the B+ levels to V3 as it would normally connect to three places (instead of my one)? I'm also concerned with a few grounding points. I have the V3 cathodes grounded to a bolted solder tab between the board and tone stack. Additionally, the volume pot is connected to this ground too. Is this problematic?

The gut shot posted is old. I've cleaned up the wiring layout a bit since this pic. Will take a new one in a few hours. . Thanks in advance for any tips on things to check.
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Last edited by fevzay on Sat Nov 11, 2017 4:37 pm, edited 2 times in total.

fevzay
Posts: 17
Joined: Fri May 18, 2012 12:43 pm

Re: Scratch built AA864 Voltages

Post by fevzay » Thu Nov 09, 2017 7:57 pm

I guess my main question is.... do these look normal? If not, is the next step to increase the dropping resistors from 1k and 4.7k to something a little higher, OR...do I need to recheck other things like grounding, etc.?
Last edited by fevzay on Sat Nov 11, 2017 4:36 pm, edited 1 time in total.

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xtian
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Re: Scratch built AA864 Voltages

Post by xtian » Thu Nov 09, 2017 9:19 pm

Your voltages are high, but not worryingly high. You say everything works. Does the amp sound ok? Then don’t worry about the voltages. More important is the power tube dissipation. Did you check?

And yes, your voltages will be higher because of your house 125vAC, and also because of the missing other gain stages (as you guessed).
I build and repair tube amps. http://amps.monkeymatic.com

Stevem
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Re: Scratch built AA864 Voltages

Post by Stevem » Sat Nov 11, 2017 12:45 pm

I would bias your output tubes at .042 to .045 amp which will bring down the V+ string voltage, if your ok with where the output tubes start to clip at this setting ( if your playing the amp loud enough) then your Golden!
Most times with good Amps just like it is with sex, it's best to just plug straight in!

fevzay
Posts: 17
Joined: Fri May 18, 2012 12:43 pm

Re: Scratch built AA864 Voltages

Post by fevzay » Sat Nov 11, 2017 4:25 pm

Thanks for your insight, guys.

The amp does sound good. Tubes are reading 43.5 and 46 mA. Not the best matched pair, but they work.

Going to think around with a few things today. Maybe increase the first dropping resistor from 1k to 4.7k.

Will post results tomorrow.

fevzay
Posts: 17
Joined: Fri May 18, 2012 12:43 pm

Re: Scratch built AA864 Voltages

Post by fevzay » Sat Nov 11, 2017 4:33 pm

*edit* files added to original post

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