ID these valves

ICBM

ecc83 said:

No IC becuase  6.3V "rms" means the same heating power as 6.3V DC.

Yes, but if you rectify and filter 6.3VAC - which is how it's done in most of the amps I can think of with DC supplies derived from an AC one used for some of the other valves - you get nearly 9VDC. Having woken up properly now I suspect that may be the cause of the trouble.

ecc83 said:

Most of B's heater supplies are regulated and I think that protects the filaments from mains surges. As you say, no hard data but if you are going to put valves on a PCB DC is SO much easier to use.

I've said this before I know, but I genuinely wonder if the subtle hum from AC filament heating is part of the 'warm' tone of classic valve amps, and the lack of it from certain modern amps which are specifically designed to have low background noise is one of the reasons I don't like them...

ecc83

ICBM said:

ecc83 said:

No IC becuase  6.3V "rms" means the same heating power as 6.3V DC.

Yes, but if you rectify and filter 6.3VAC - which is how it's done in most of the amps I can think of with DC supplies derived from an AC one used for some of the other valves - you get nearly 9VDC. Having woken up properly now I suspect that may be the cause of the trouble.

ecc83 said:

Most of B's heater supplies are regulated and I think that protects the filaments from mains surges. As you say, no hard data but if you are going to put valves on a PCB DC is SO much easier to use.

I've said this before I know, but I genuinely wonder if the subtle hum from AC filament heating is part of the 'warm' tone of classic valve amps, and the lack of it from certain modern amps which are specifically designed to have low background noise is one of the reasons I don't like them...

Well the power valves are AC heated. Now, 6.3V rms gives you ONLY 8.9V PEAK OFF load and the ripple would be horrible and 100Hz. After some serious RC filtering you will struggle to get back to 6V. B's do it the right way with enough AC  volts to give a regulated output. I seem to recall the 12BH7 runs on "raw" rectified 15V but it is an output stage and one of the quietest amps on the planet.  You might like a bit of muck in your signal IC but many of the Home Recording guys don't!

Dave.

ICBM

ecc83 said:

Now, 6.3V rms gives you ONLY 8.9V PEAK OFF load and the ripple would be horrible and 100Hz. After some serious RC filtering you will struggle to get back to 6V.

Most like Marshall don't bother, they just use a bridge rectifier (with no heatsink, so it overheating and desoldering itself is the more common problem than valve failure, in fact - pure bad design) and a plain cap filter...

DiscoStu

Just a thought- can they be checked with a multimeter and if so, how?

ICBM

DiscoStu said:

Just a thought- can they be checked with a multimeter and if so, how?

The only thing you can check is the filaments, which will tell you if they're definitely dead, but nothing else.

ecc83

ICBM said:

ecc83 said:

Now, 6.3V rms gives you ONLY 8.9V PEAK OFF load and the ripple would be horrible and 100Hz. After some serious RC filtering you will struggle to get back to 6V.

Most like Marshall don't bother, they just use a bridge rectifier (with no heatsink, so it overheating and desoldering itself is the more common problem than valve failure, in fact - pure bad design) and a plain cap filter...

Hah! One solution is to use a doubler circuit to get about 12V loaded and then a regulator but you only get enough juice for the input valve but that is the critical one. Yes, I know PIs can be hummy! Had one punch through the cathode once in an A100. Fortunately the speaker was 4x12xV30s they did not suffer for the few seconds burst. My ears took quite a while to recover though!

Dave.