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StPhoenix's picture

Bias reading of 17 and 24 on 70's 65 212

Hi Guys,

I'm getting bias readings 7ma apart for the tubes in my 70's 65 212 (with PI). This happens no matter if I use a pair of 6CA7's or EL34's. When just warming up the reading is 1 ma apart, but after 20 mins or so it reads as described. I've chosen 17 and 24 as a 'balance' for now, as the correct setting is around 22ma according to the forum. The tubes and amp sounds great, no problem there.

I'm using a meter I bought on ebay that sits between the tube and the socket and shows ma per tube. Handy.

Is this normal? It came this way when I bought it.



mm210's picture

If you SWAP the tube

If you SWAP the tube positions, does the high tube and low tube FOLLOW the tube or stay with the socket?

StPhoenix's picture

Stays with the socket

Stays with the socket regardless of which tube (or what type, ie EL34, 6CA7)

mm210's picture

I'd check the 330k resistors

I'd check the 330k resistors and the 1.5k resistors in the bias path going to pin 5. They are what control bias voltage to the tube. Make SURE you remove them to measure dc resistance. Mike.

StPhoenix's picture

Seems to be fixed...but not sure how

Thanks Mike. The problem seems to be resolved, but I'm not sure what caused it in the first place. I did a complete cap job in the interim, then hit the tubes and sockets with a good bit of contact cleaner.

On retesting the values were only 4 mA apart.I started swapping tubes around (I'd purchased two matched pairs of 6CA7's from Eurotubes - great support btw) and eventually wound up with two that stayed within a mA of eachother after 30 mins with volume all the way up. Also no voltage creep, which was a first.

It's possible I mixed up the tubes somehow in the initial tests, but I'm not sure. That could explain the difference in mA.

When I put all four new 6CA7's from in my HD 130, each pair reads consistently with eachother, but different to the other pair. Hopefully that makes sense. In short, one pair reads 12mA apart from the other pair.

I'm not sure if that's the amp or the tubes, but on the HD 130 there's voltage creep all over the place. One tube went up 27 ma in 20 mins! That's not a typo. I haven't turned the amp back on since, as I wanted to get the 65 running right first.

I'm going to order a matched quad from Eurotubes and see if that solves the problem in the HD like it did for the 65.



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