New 5F6A Build Issues

Fender Amp Discussion

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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

Actually, that turned out to be faulty leads on my multimeter ! I ran out and got a new set, and all was well. I'd been repairing the old ones for several years, and it was long overdue.
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

martin manning wrote: Sat Jul 22, 2017 10:54 pm Note the 2.4 V is more like the expected cathode voltage, and since they are tied together they should be identical. Crackling could be bad solder joints, or poor contact between tube pins and sockets. Poke around with a chopstick to look for sensitive areas. Note with 180 V on the cathode of V2, the heater-cathode insulation is highly stressed. That tube should have 200 V spec for Vh-k.
Just to revisit this. So, for V2, would you suggest something like a 12ax7WC ? Or perhaps 12au7 ?
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

12AX7 is original. Period data sheets typically show Vh-k max 200V, where a few new manufactured 12AX7's including Sovtek 12AX7WA, B, and C, show 100V max, which is obviously too low when you are idling at 180V. TAD 12AX7 lists 150V, JJ 12AX7S lists 180V (which might be ok), 12AX7EH lists +100/-200 (which is ok).
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Lummox
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Re: New 5F6A Build Issues

Post by Lummox »

I'll put a 12AX7EH in there, but would it be desirable to lower the voltage on the cathode instead ?
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martin manning
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Re: New 5F6A Build Issues

Post by martin manning »

I wouldn't recommend changing the operating point of that triode. You could elevate the filament string reference voltage, but you should be fine as long as you stay with tubes rated at 200V Vh-k max.
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