Faulty tube?

matejS · 25th August 2007, 04:42 PM

Hi!

I've a SRPP output stage (preamp) and I've noticed that one channel plays a little but louder. I've checked the circuit components and (soldered) contacts and all where OK.

I found out that in the point where B+ should be halved on one channel it is not. I've changed the tubes (left tube to the right socet and vice versa) and the problem is tube related.

It seems that one half of the tube is not the same as the other. Do I have a faulty tube? Is this a common problem?

Thanks,
Matej

SY · 25th August 2007, 04:47 PM

In order:

Probably, and it depends on the tube/brand/vintage. What are you using?

matejS · 25th August 2007, 04:49 PM

Quote:

Originally posted by SY
In order:

Probably, and it depends on the tube/brand/vintage. What are you using?

Electro Harmonix 6922

anatech · 25th August 2007, 05:35 PM

Hi matejS,
Sounds like a defective tube for sure. Normally they are good, but this one appears to be bad.

Unless ....

If you have no feedback, expect gain differentials. You have to understand that the two tube sections are individuals. They are in no way matched to their mate. There are some circuits that require trimming if no feedback is used. If you expect close gain matching and distortion performance, then you must provide variable circuit elements that allow you to tune your circuit for each tube. This is the same no matter what brand and age of tube you are using.

-Chris

KaDe · 25th August 2007, 10:13 PM

In the classic SRPP stage the tubes need to be matched very careful.
If you have some spare tubes the first step is to select them for half B+ at the test-point, and than check for same gain level.
Some dealers offer SRPP-matched pairs, but they are expensive.
The DIY solution is what Chris recommends.

- can you post your circuit Matej - ?

matejS · 25th August 2007, 10:16 PM

Quote:

Originally posted by KaDe
- can you post your circuit Matej - ?

Yes, no problem. Here it is:

- Matej

KaDe · 25th August 2007, 10:31 PM

very classic indeed

what you can DIY then is to add adjusters to either R1 or R4 to provide bias correction

Klaus

anatech · 26th August 2007, 02:26 AM

Hi matejS,
Yes. Clearly you need a way to adjust for gain differences in the first triode.

A commercial example of a circuit with no adjustment is the Counterpoint SA-1000. Takes a while to find tubes matched close enough. The SA-12 / SA-100 amp is another offender with no adjustments for gain matching (VR3 needs to be removed and shorted).

-Chris

richwalters · 30th August 2007, 09:16 PM

bit late, the ECC88 has a restricted heater # cathode potential with cascade operation.
Perhaps cathode pin 3 is touch faster on warm up than other section.

richj

25th August 2007, 04:42 PM	#1
matejS diyAudio Member Join Date: May 2007	Faulty tube? Hi! I've a SRPP output stage (preamp) and I've noticed that one channel plays a little but louder. I've checked the circuit components and (soldered) contacts and all where OK. I found out that in the point where B+ should be halved on one channel it is not. I've changed the tubes (left tube to the right socet and vice versa) and the problem is tube related. It seems that one half of the tube is not the same as the other. Do I have a faulty tube? Is this a common problem? Thanks, Matej

25th August 2007, 04:47 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	In order: Probably, and it depends on the tube/brand/vintage. What are you using? __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

25th August 2007, 05:35 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi matejS, Sounds like a defective tube for sure. Normally they are good, but this one appears to be bad. Unless .... If you have no feedback, expect gain differentials. You have to understand that the two tube sections are individuals. They are in no way matched to their mate. There are some circuits that require trimming if no feedback is used. If you expect close gain matching and distortion performance, then you must provide variable circuit elements that allow you to tune your circuit for each tube. This is the same no matter what brand and age of tube you are using. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife

26th August 2007, 02:26 AM	#8
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi matejS, Yes. Clearly you need a way to adjust for gain differences in the first triode. A commercial example of a circuit with no adjustment is the Counterpoint SA-1000. Takes a while to find tubes matched close enough. The SA-12 / SA-100 amp is another offender with no adjustments for gain matching (VR3 needs to be removed and shorted). -Chris __________________ "Just because you can, doesn't mean you should" © my Wife

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25th August 2007, 10:13 PM	#5
KaDe diyAudio Member Join Date: May 2007	In the classic SRPP stage the tubes need to be matched very careful. If you have some spare tubes the first step is to select them for half B+ at the test-point, and than check for same gain level. Some dealers offer SRPP-matched pairs, but they are expensive. The DIY solution is what Chris recommends. - can you post your circuit Matej - ?

25th August 2007, 10:31 PM	#7
KaDe diyAudio Member Join Date: May 2007	very classic indeed what you can DIY then is to add adjusters to either R1 or R4 to provide bias correction Klaus

30th August 2007, 09:16 PM	#9
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	bit late, the ECC88 has a restricted heater # cathode potential with cascade operation. Perhaps cathode pin 3 is touch faster on warm up than other section. richj

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