300B Tube Problem.

[Chris65]

My BeePre is working well, & when an opportunity to pick a pair of Electro Harmonix 300B at a good price, I couldn't pass.
I have been using Genalex 300B but it's always good to have back-ups.

Anyway, put them in & one was noisy (lot of hiss) but passing signal, briefly. Then nothing, just the noise, no signal getting through.
The other is ok. Filament works & measures 1.5ohm. Both were sold as unused, tested from a private seller. Put the other 300B's back in the amp & still working as it should, so not the amp.

Anything I can try, such as reheating the pins? Or is it just bad?

[Doc B.]

Did the filament stay lit when it stopped working? The reason I ask is that there is enough variation in tube pin diameter between brands that sometimes pins 1 and 4 of the socket get stretched from a tube with fat pins and after that the thinner pins of a different tube can lose contact, causing the filament yo go cold. You can just tweak the contacts a little if that is the issue.

[Paul Birkeland]

Try measuring the voltages with the offending tube.

The voltages to look at are:

POS Out
OB
OA
The non-grounded side of the two 8 Ohm resistors on that channel (measure both sides, one will be 0V, one will be something else)

-PB

[Chris65]

Thanks for the replies.
Doc, yes filament worked at all times, not especially bright compared to other tubes, but that could be just characteristic of these.
It only worked very briefly, say 5-10sec, then the noise. Checked other tubes in the amp, swapped side to side, removed the silicone ring to ensure good pin contact, still the same.

PB - I'll measure those but I've contacted the seller & he's offered a refund, so no real big deal. Said he had them tested 2 weeks ago, but of course testing doesn't necessarily mean the tube will work ok.

[Chris65]

Measured voltages:
                       LH (bad)               RH (good)
Pos Out            9.83                     9.90
OB                   144.8                  110.5
OA                   147.1                   145.9
N/G side 8Ω      5.0                       5.0

Dropped in a good pair of 300B for comparison and OB measured 96.6/98.6VDc.

[Paul Birkeland]

Yeah, you have one that is drawing filament current but not conducting.

FWIW, I have "fixed" a few 300B's like this by resoldering the connection of pin 2 on the socket itself.  I'd contact the seller to ask if they would mind if you try that before returning them.

-PB

[Chris65]

Thanks Paul, I may try that with his agreement.

[Chris65]

Quick update: tried heating the pin & adding a little solder. Unfortunately, no difference, so back it goes.
Thanks for the help anyway.

[Chris65]

A follow-up - I returned the EH 300B to seller some months back & forgot about them. He then this last week put them back up for sale along with another 2 pairs as a package. I managed to buy all 3 pairs.
Got them home, I planned to sell the known good EH 300B, but just for fun I put them in my newly finished power amp (get*set*go) which can take 300B. And they work perfectly!

Never had this situation where a tube won't work in one amp, but will in another. Any ideas why the one tube will not work in the BeePre?

[Henrik]

My apologies for reviving a long-dead thread.

However, I am experiencing pretty much this exact problem with a pair of TJ Full Music 300Bs, and I would be extremely curious to hear any further (after)thoughts on the matter?

Brgds,
Henrik

[Paul Birkeland]

If you have the same problem where a high OB voltage follows a 300B, then you should consider reheating the solder joints on each tube pin, as well as the possibility that you just have a dead 300B.

[Henrik]

Thanks, will try that. Is there a maximum heat that the tube will endure, or is it a best guess effort?

[Paul Birkeland]

Don't get it so hot that the glass melts.  (1500C/2500F)