problems with original wad 300b

[julianr]

thanks Lord all good thoughts and info.
good link to the circuit, will definitely change cathode resistors to 1.5K.

Clive don't know how to check OPT, only 1 300b failed along with it's cathode resistor, all the rest were ok. I do not know if the resistors failed or the 300b failed.

Will wait for more comments, in any case will have to order the parts.

Julian

[Clive]

Sorry, I didn't mean to worry you. The OPT should be fine as only set of resisistors went (I'd been thinking the resistors for both valves on one channel). You can check the resistance of the windings and that they tally with the other channel. Just to be sure.

No problems Julian,
The link I gave is the 'thread' with the circuit - if you click on the image it gets slightly bigger and a lot 'sharper'.
Just in case it wasn't obvious!

Also, have a look at the long '300B PP Ringing' thread which covers a lot of information about the amplifier.

I have an e-mail bundle (~1.5MB) of old Hi-Fi World articles regarding this amplifier if you'd like them? PM your e-mail address if so.

[julianr]

Once again Lord thanks for all the info (thanks for the email) - it might take a bit of time to digest it all. But to start with must at least order and change the cathode resistors to 1.5K, then take it from there.
Julian

[Ianm2]

just to slightly illumine what happens, if you

forget the rest of the amp and imagine

the psu rail at the top, down into the output tx, down into the tube, and then the cathode resistor, and at the bottom the common, zero volt, or ground rail.

from a psu point of view, its all dc, and simple ohms law.

simple golden rule of V's, I's and R's

the cathode res. is controlling the current, and as its value has dropped, the whole thing is drawing loads of current, hence the valve going red, whether that's a cause or an effect??

[julianr]

yes that makes a lot of sense, easier to understand when you simplify it for me, thanks
Julian

[Greg.]

Hi Julian,

Take heart that your build has worked well for ten years. I'm inclined to think this is simply the resistor going wrong or the valve ending its life and taking thr resistor with it. Replace the resistor with a 1K5 option. A cheap white coffin would do. Then replace valve with a good performing one and see if the suspect valve section is stable. If so, introduce a replacement valve and fit 1K5 for all the cathode R's. Realise this resistor selection is not because there was a design fault but simply that with 1K2, the valves were being driven hard and with some we've tried and liked such as Electro Harmonix gold grid 300B, some of us found a tendancy to plate glow. Moving to 1K5 generally cured this by reducing plate dissipation to approximately 30W. A number of us favour Mills reasistors here, but you'll need to order in from the likes of http://www.percyaudio.com/ or http://www.partsconnexion.com/

Best wishes,

Greg

[Clive]

Hi Julian, make sure that whatever resistor you choose has a rating of 3 times the actual dissipation.

Clive

[Richard]

Originally Posted by Clive

Hi Julian, make sure that whatever resistor you choose has a rating of 3 times the actual dissipation.

Clive

Wow, disippation is 7W so I used 11W 1K5 white ceramic ones RS 2061473 which fit in OK and work fine.

(If you do consider the aluminium types with higher ratings such as 25W or 50W bear in mind that they need heatsinks to achieve those ratings whereas the ceramic ones are fine just in free-air.)

Re this problem, generally a wirewound cathode R will go open circuit if it fails so the valve will simply not work. Given Julian's reading of 67R I'd remove one end of the affected cathode R and re-meter looking for the reason for that 67R which could be a short-circuit bypass cap. A cathode resisitance value of 67R (for whatever reason) will cause excesive current through the valve and would account for the glowing GZ37 and failed 300B.

Rich

[Clive]

Originally Posted by Richard

Wow, disippation is 7W so I used 11W 1K5 white ceramic ones RS 2061473 which fit in OK and work fine.
Rich

11W will work fine until they fail.

It's a very well established rule of thumb to triple the rating for continuous use.