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-   -   Kit 34 tubes change bias (http://www.world-designs.co.uk/forum/showthread.php?t=7555)

Aro69 9th November 2017 11:41 AM

Kit 34 tubes change bias
 
Hi guys. I'm very happy owner of second hand kit34. I've bought it on Audiojumble two months ago for my Altec speakers. The seller who build this amp gave my also a box of spare tubes. The original 5u4 were changed for different need help to find out what r they. Now I started hear some noise. Not laud but it's hearable... I've swapped tubes back for 5u4 and two ec80 Telefunken for Mullard but noise left. Isn't it from el34? I was also wondering had this amp autobias? It when I change el34 will have to adjust it. If so how to do it?

John Caswell 9th November 2017 01:03 PM

Re: Kit 34 tubes change bias
 
Hi,
No no need to even think about biassing as the output valves are cathode biassed. All I would suggest is that you buy matched pairs/quads of EL34 as and when.
If you can take a picture of the 5U4 replacement and post her we can have a guess at what it is.
Noise - I would have a look at the resistors in the anode/cathode circuits of the ECF80 as after some time we did find them going noisy/changing in value. R11/12 - 430K, R15/16/19/20 - 100K, R17/18 - 2K2
If they are 1/2 Watt types change them for 1-2 Watt types.
Please report back you findings here.

John

Aro69 15th November 2017 02:12 PM

Re: Kit 34 tubes change bias
 
Quote:

Originally Posted by John Caswell (Post 87456)
Hi,
No no need to even think about biassing as the output valves are cathode biassed. All I would suggest is that you buy matched pairs/quads of EL34 as and when.
If you can take a picture of the 5U4 replacement and post her we can have a guess at what it is.
Noise - I would have a look at the resistors in the anode/cathode circuits of the ECF80 as after some time we did find them going noisy/changing in value. R11/12 - 430K, R15/16/19/20 - 100K, R17/18 - 2K2
If they are 1/2 Watt types change them for 1-2 Watt types.
Please report back you findings here.

John

Hi John. Thanks for response. I'm not exactly electronic to understand those numbers u wrote. If will have to check some resistors will need more help. I've resize pic with those tubes..

Aro69 15th November 2017 02:16 PM

Re: Kit 34 tubes change bias
 
1 Attachment(s)
This is that picture.

Aro69 30th November 2017 10:59 AM

Re: Kit 34 tubes change bias
 
Hi John. I though well have more problems too measure those resistors, but done it. All resistors keep their values at the level it shoots be. No one is broken. Noise left but must add this noise isn't constant it's gone as time to time and isn't loud the same every time...




Quote:

Originally Posted by John Caswell (Post 87456)
Hi,
No no need to even think about biassing as the output valves are cathode biassed. All I would suggest is that you buy matched pairs/quads of EL34 as and when.
If you can take a picture of the 5U4 replacement and post her we can have a guess at what it is.
Noise - I would have a look at the resistors in the anode/cathode circuits of the ECF80 as after some time we did find them going noisy/changing in value. R11/12 - 430K, R15/16/19/20 - 100K, R17/18 - 2K2
If they are 1/2 Watt types change them for 1-2 Watt types.
Please report back you findings here.

John


Richard 1st December 2017 10:39 AM

Re: Kit 34 tubes change bias
 
Hi, Is it the integrated version with input switching and a volume control? If so they may be the cause of noise. The input switch in particular can go noisy after a few years or if it was overheated when soldered. Dirty valve pins and sockets can also cause random noise. Is the noise on both channels or just one?

Aro69 3rd December 2017 08:46 PM

Re: Kit 34 tubes change bias
 
Hi. This is 100% kit34 build correct with drawings. I've got instruction. It has switch on the back left side. The noise is mainly in right channel. Should I just look for new switch?

bob orbell 4th December 2017 07:40 AM

Re: Kit 34 tubes change bias
 
Read Richard's post again, you have not answered him correctly, he is not referring to the power switch on the back. BOB

Aro69 4th December 2017 01:17 PM

Re: Kit 34 tubes change bias
 
I did and see my error. The problem is English isn't my first language and sometimes I'm struggle to understand. TBH I have no idea what it is input switch and where can I find it. Will have to study something to find out.

bob orbell 4th December 2017 02:43 PM

Re: Kit 34 tubes change bias
 
The input switch, that Richard refers to is on the front panel on the right, I hope this helps you. BOB


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