Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: trioid on March 21, 2011, 06:26:08 AM

Title: Bad 12AU7 and the Extended FPIII Circuit
Post by: trioid on March 21, 2011, 06:26:08 AM

Hello folks who understand the circuit much better than I. The situation is that one triode section of a Telef. 12AU7 managed to start putting on a lightening show about 10 sec. after powering up. That show lasts for about a second. Does not seem to affect the sound, but if that is a grid to plate short or something like that, will that cause damage (I am guessing not).

It is only one of the triodes in that tube, and the problem follows the tube (swapped R --> L and the arc show is definitely the tube....)

Thanks for the education.....
Cheers,
Jim

Ps. I am buring in some EML 300Bs - the standard version, not XLS, not mesh plate. They sound very nice.

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: Doc B. on March 21, 2011, 07:33:31 AM

Yes, an arcing tube can cause other things to break, particularly the solid state devices on the PC boards. Best bet is to replace it.

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: trioid on March 21, 2011, 09:04:41 AM

Hi Doc, I will do that!

Jim

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: trioid on March 21, 2011, 04:44:05 PM

Hi Doc and others,

Too late.

The bad tube must have fried one channel -- it sounds like a sick transistor radio, using an known good RCA cleartop. LEDS all appear to be running, nonetheless. The other channel is running great.

I assume that is an issue with the extended portion of the circuit? If so, any recommendations on replacement parts (I have spare 2N2222s only)? I will run voltage checks if needed....

rats,
Jim

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: Doc B. on March 22, 2011, 05:25:31 AM

Yes, do measure voltages and report back with any that are way off.

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: trioid on March 23, 2011, 04:41:08 AM

Hi Doc and all interested parties --

Here are my observations and measurements.

First (and perhaps irrelevant) I noticed a

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: Doc B. on March 23, 2011, 07:21:17 AM

A scorch between pad I and O indicates that the transistor next to them may have been damaged. That could produce the white noise you are hearing. Might be worth replacing it and the associated metal can transistor. B end is the cathode follower IIRC, which would be the MJE340 and 2N2222A. Replacing the transistors can be a bit difficult. A solder sucker is indispensible for cleaning up the pad holes before you install the replacement transistors. If you don't have any luck with the repair it may be easiest to just build a new board and replace the existing one.

Title: Re: Bad 12AU7 and the Extended FPIII Circuit
Post by: trioid on March 23, 2011, 08:40:05 AM

Hi Doc,

Yup. I will get a MJE340 and I have the other transistor already. Worth a shot at the old board first.

Thanks
Jim