Bad Recifier tube?

by islanddave Wed Sep 10, 2014 11:49 pm

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Join date : 2012-10-15

Post n°1

Bad Recifier tube?

by islanddave Wed Sep 10, 2014 11:49 pm

Hello,

After about three years of service I have had my first problem with my vta st-70. One of my kt-66's was red hot and the bias was about 800mv. I could not get it to go down. Pulled all the tubes out and replaced them with el 34's. Brought bias up to 400mv and after a while the bias of all four tubes goes up. I reset it to 400mv and then after awhile the bias goes down to 298mv. Does this sound like a bad rectifier tube? It is a NOS mullard gz-37/

Thanks Dave

by nerpissad Thu Sep 11, 2014 7:43 am

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Post n°2

Re: Bad Recifier tube?

by nerpissad Thu Sep 11, 2014 7:43 am

hi dave,

a while ago I was experiencing a similar issue.
maybe you can have a look through my thread. there are a lot of helpful tips in there on where and what to look for.
in my case, it turned out to be bad solder joints on the offending tube socket.

by Guest Thu Sep 11, 2014 10:17 am

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Post n°3

Re: Bad Recifier tube?

by Guest Thu Sep 11, 2014 10:17 am

yes, check all solder joints, bias resistors, go over the whole wiring of your amp...then when you're done...go over it again...and finally...go over it again!
Bad solder I would account for 90% of tube amp problems!

by nerpissad Thu Sep 11, 2014 10:30 am

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Post n°4

Re: Bad Recifier tube?

by nerpissad Thu Sep 11, 2014 10:30 am

MontanaWay wrote:Bad solder I would account for 90% of tube amp problems!

100% of mine anyway Wink

by Guest Thu Sep 11, 2014 10:30 am

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Post n°5

Re: Bad Recifier tube?

by Guest Thu Sep 11, 2014 10:30 am

nerpissad wrote:

MontanaWay wrote:Bad solder I would account for 90% of tube amp problems!

100% of mine anyway

I may have under estimated that!! Very Happy

by **Bob Latino** Thu Sep 11, 2014 11:52 am

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Post n°6

Re: Bad Recifier tube?

by Bob Latino Thu Sep 11, 2014 11:52 am

MontanaWay wrote:Bad solder I would account for 90% of tube amp problems!

Very true .. Most people think a wiring error is to blame but on most kit built amps it is hard to make a wiring error when you have a pictorial and a color photo showing you exactly where the wire should go. On a wiring error, most kit builders can see the mistake easily.

Use good rosin core solder and make sure that connection is heated up good so that the solder flows well into the joint. Even though a solder joint LOOKS good does not mean that there is a solid connection beneath the joint. A classic bad solder connection problem is that the amp plays OK until it warms up. After it is warm the metal expands and the connection may go "partial" causing strange noises, hum etc. Another thing .. I have seen a few instance where the bad (cold) solder connection does not show up until weeks, months or even years have gone by. After hundreds of ON (heat and expansion) OFF (cold and contraction) cycles, the connection may break away under the solder.

Sometimes if you can isolate the problem to one area of the amp say one tube socket or maybe the left side of the driver board etc., you can fix the issue by simply REsoldering every connection in that area. If you have no idea where the bad connection is you can try tapping some connections lightly (with the amp ON) with an insulated item like a plastic pen to see if tapping that connection creates the noise/hum that the amp exhibits. I like to use a "Sharpie" felt tip marker with the cover over the felt tip on.

Bob

by islanddave Thu Sep 11, 2014 12:34 pm

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Post n°7

Re: Bad Recifier tube?

by islanddave Thu Sep 11, 2014 12:34 pm

Hello Bob,

Thanks for chiming in here. I haven't been on here for about 3 years, until now. I am very happy to see that you are still providing excellent customer support above and beyond!

A couple of observations here. The problems started when I took a front right kt-66 out of it's socket to clean some dust around the socket. Although the problem persisted even after I put the el 34's in.
Now with a fresh head and coffee I put the kt-66's back in and re biased the tubes, starting with the left channel and then the right channel. All good so far after four Beatles albums.I have my multi meter on the offending socket while its playing to watch for changes. None so far. I have had adjust the bias but only slightly and only when no signal is passing through the amp, and with the speakers attached. Possibly the problem with appear again indicating a cold solder joint, or maybe it was that I jostled the rectifier tube or maybe some oxidation on that offending socket?

At any rate so far so good. By the way I haven't listened to the Valve art el 34's for about three years.......they are a wonderful sounding tube! Highly under rated!

Thanks Dave

by Dynalover Thu Sep 11, 2014 12:51 pm

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Post n°8

Re: Bad Recifier tube?

by Dynalover Thu Sep 11, 2014 12:51 pm

One problem with rectifier tubes is a tube tester isn't much utility. Shorts test, yes, and if they test weak, they probably are but... The real world test is to (Carefully!) measure the B+ voltage under load against published specs. If they can pass enough current the outputs should bias fine.

by **Bob Latino** Thu Sep 11, 2014 1:36 pm

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Post n°9

Re: Bad Recifier tube?

by Bob Latino Thu Sep 11, 2014 1:36 pm

With regard to biasing problems most often ..

1. If the bias on ALL THE OUTPUT TUBES goes UP or DOWN by a fairly large amount then either ..
***A. Your line voltage is going up and down (check it from time to time) OR
***B. The rectifier is erratic or weak and should be replaced

2. If just ONE OUTPUT TUBE'S bias goes up or down
***A. That tube is bad and should be replaced OR
***B. There is a tube to socket pin non contact issue on that tube socket OR
***C. The bias resistor from pin 8 to ground is bad or has a poor contact issue (bad solder connection)

Bob

by Guest Thu Sep 11, 2014 2:17 pm

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Post n°10

Re: Bad Recifier tube?

by Guest Thu Sep 11, 2014 2:17 pm

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Last edited by PeterCapo on Tue Dec 01, 2020 10:08 pm; edited 1 time in total

by islanddave Fri Sep 12, 2014 4:38 pm

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Post n°11

Re: Bad Recifier tube?

by islanddave Fri Sep 12, 2014 4:38 pm

Hello Folks,

Turns out that my el 34's bias very nicely and have remained solid for the entire day.....so I guess that it must have just have been that one kt-66.......

This experience has made me appreciate rolling other tubes in this amp! What are the possible combinations of tubes both preamp and power that can be run in this VTA ST-70?

Thanks Dave

by **Bob Latino** Fri Sep 12, 2014 6:07 pm

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Post n°12

Re: Bad Recifier tube?

by Bob Latino Fri Sep 12, 2014 6:07 pm

Hi Dave,

1. For driver tubes in the VTA ST-70 you can run any 12AU7, 12BH7, 5963, 5814, 6189, 7316, CV4003 or ECC82

2. For output tubes in the VTA ST-70 you can run any EL34, 6L6, 5881, KT66, KT77, KT88 or 6550

Depending on your associated audio gear, type of music played and your hearing, you may or may not hear much difference between different driver tubes and/or output tubes.

I don't like to recommend "which tube should I use in my amp" but 12BH7's and the Mullard CV4003 seem to work really well. Real sleepers are the military grade 12AU7's - the 5963's and the 5814's. You can still find the latter two as USA made (RCA, GE and Sylvania) at fairly reasonable prices. For output tubes, I use the Genalex Gold Lion KT66's in my own VTA ST-70. The only downside to these is that they are expensive ..

Bob

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Bad Recifier tube?