Is this red plating?

Moe_Zambeek

This is a 6l6 in my cathode biased orange, part of a pair of small bottle tad red bases. Amp went very quiet, glowed very red for a minutes then backed off again and the amp volume came back. In that time it has burnt the labelling off the glass. I’m guessing this is red plating?

replaced the pair with the original tubes and the amp seems ok. Firstly I’m wondering if this is red plating, then wondering if it’s a fault with the tube (which had filament rattle / was microphonic) or the amp?

FWIW this tube rattled like mad in a fender combo and was returned to Hotrox for testing, they tested it and said it was fine and refused to replace it for noise and sent it back to me. Don’t buy tubes from Hotrox people.

 

RiftAmps

Yes, that's redplating. Swap positions with the other 6L6...if the fault follows the valve then it's a valve fault. If it stays with the socket then it's an amp problem.

Moe_Zambeek

Thanks @rift amps. Weirdly it stopped red plating after a minute or so. I swapped in some other tubes and no issues at all, and in fact I’d only just put the TADs in. Went back to the original tubes and no red plating so I am going to assume a faulty tube. 

I’ll try the TADs again later and see if it happens again though. 

ICBM

Yes, faulty valve. They can do that intermittently.

Your experience with Hotrox mirrors mine with a customer’s amp which was damaged by an intermittently faulty 6L6, which they refused to refund as they claimed it tested good after being returned. A ‘good’ valve does not burn out the virtual centre-tap resistors though…

Moe_Zambeek

Was it a TAD 6l6 @ICBM? I actually think these sound better than the JJs the amp came with but maybe they’re not very reliable…

£80 for a matched pair at Hotrox too, and no refund for the faulty one is pretty hard to swallow.  Plus they took almost a month to test them and I had to chase to get them back. Hotrox are definitely on my blacklist now.

ICBM

No, Tung-Sol 5881.

ecc83

Another explanation is "grid current". This is the tiny, uA current that flows in the grid leak resistor due to minute impurities in the valve. Modern valves don't seem to get a long enough 'burn in' time.

Grid current negates bias and so the valve runs hotter than the circuit design intended. Almost always a temporary issue as the crap 'boils away' after a few hours use. I am sure those of you who have set bias notice that Ia often reduces over time then settles down?

The value of the grid leak R is specified in the valve data but some amp designers just chuck in 470K or even 1M without a thought. Lower grid leak values give fewer grid current problems and generally longer valve life.

Dave.

Moe_Zambeek

Good info all round, thanks all.

This particular tube clearly had something loose inside, from new it tinkled when moved and had audible noise / rattle in another amp I tried it in. Disappointing it was delivered like that as an allegedly ‘premium matched’ tube and disappointing that Hotrox washed their hands of it but I was warned about them a few times and should have taken that on board.

Cirrus

We should start a support group for victims of their valve service.

Modulus_Amps

Hotrox aside, I had an amp in for repair this week with a rattling TAD red base 6L6's, wonder if that is a thing with these now?

Moe_Zambeek

Modulus_Amps said:

Hotrox aside, I had an amp in for repair this week with a rattling TAD red base 6L6's, wonder if that is a thing with these now?

They seem well regarded generally but we certainly have some anecdata now! Was it a small bottle 6l6? What did you replace it with @Modulus_Amps ? 

RiftAmps

I’ve had zero rattling Red Base TADs, and I buy quite a few of them. I really like the EL34s.

I get mainly get rattling JJ 5U4s and Tung-Sol 5881s. Doesn’t mean anything though…any valve can fail at any time.

Modulus_Amps

Moe_Zambeek said:

Modulus_Amps said:

Hotrox aside, I had an amp in for repair this week with a rattling TAD red base 6L6's, wonder if that is a thing with these now?

They seem well regarded generally but we certainly have some anecdata now! Was it a small bottle 6l6? What did you replace it with @Modulus_Amps ? 

Customer supplied some Groove Tubes that he has stashed away. Can't remember the exact details of what was fitted now other than them being TAD 6L6's. I might get to see them again in the next couple of weeks, it was one of those jobs where the amp lost power/clarity but was working fine with me.