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Old 28th April 2015, 06:38 AM   #1
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Default Bewitch 6550 Balance Issue

Dear Forum

Greetings from South Africa - my name is Johan.

I really hope someone will be able to provide me with some input...

I recently purchased a Bewitch 6550 amp from Hong Kong. The amp sounds amazing.

Unfortunately a preamp tube blew within a day or so... no problem... I sourced another.

A few days after this I noticed a large channel imbalance between left and right speaker. I measured sound pressure at about 5db louder at left speaker when listening at 70db average range.

I have absolutely NO IDEA how to fix this. I spent a whole day swapping tubes, cables and checking speaker placement. The issue can only be with the output of the amp. No matter what I change - the left speaker is considerably louder.

I have considered adding a $50 passive variable attenuator to the channel, but I dont know if this is the best solution.

I have read that people do have issues with these amps and having the inners checked isn't a bad idea at all.

This is my first tube amp and I have no idea if this is a common issue? Is someone aware of a person in SA hat might take a look at this?

Thank you so much for reading!

Johan
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Old 28th April 2015, 06:56 AM   #2
Mooly is offline Mooly  United Kingdom
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You say you measured SPL levels ? An SPL meter in front of the speakers ?

For many reasons that is not accurate enough to determine gain problems. You must measure the electrical output in relation to a known applied input signal in order to get any idea of where problems may be. The gain should be virtually identical between channels.

There seem to be some circuit details here,
Bewitch

Its impossible to guess where any problem may be, it could be anything. A scope and signal test source should show any problem in minutes.
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Old 28th April 2015, 07:09 AM   #3
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Dear Mooly - thank you so much for the feedback. Yes, I used a SPL meter in front of speakers. About 5 cm from the grill. As precisely as possible with regards to height and distance from speakers. But I understand this is far from a decent diagnostic method.

I actually purchased the amp after the Lampizator recommendation!

I must be honest... apart from measuring simple SPL and switching out tubes, and cables - I have zero technical knowledge and experience with tubes amp (and schematics). I'd love to learn though.

Can I merely measure the voltage output at the speaker outputs on the amp? By playing some mono tone to ensure it should be the same at both terminals?

Thanks for the help Mooly!!
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Old 28th April 2015, 11:43 AM   #4
Mooly is offline Mooly  United Kingdom
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Yes, you need to create a pure sine test tone. You can use Audacity to that in just a couple of minutes.

Installing and using Audacity. A get you started guide.

Use a tone of around 400Hz which will be OK with any meter. Its probably best to use a dummy load such as a 10 ohm 5 or 10 watt resistor rather than speakers.

Play the tone and turn the volume up to get somewhere around 2 or 3 volts RMS across the resistor (use the AC volt range on your meter). Note the reading and then leaving the volume set where it is check the other channel and compare. We can calculate from the two readings what the difference in db is. Ideally they should agree very closely though.
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Old 28th April 2015, 05:14 PM   #5
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There is direct connection between 6N9 and 6N8 /plate first tube to grid second/. Every change of tubes is big issue, because parameters of new and old tubes are probably different. Old and new tubes must be identical. If You put some new tube, You must change some resistors to receive old V nominals and modes. This is work not for You, IMO....
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Old 29th April 2015, 12:02 AM   #6
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I have found that some of the Chinese Amps have quality control issues whan it comes to component values. What is on the schematic is not necessarily what is stuffed into the PCB.

I have certainly found problems in Music Angel Amps with wrong value componets fitted in one of the 2 channels. In a MA PP EL34 Amp which had (the usual) unequal anode load resistors on the phase splitter (82K on input side and 100K on the otherside) I found that one channel had these swapped.

In this instance I would carefully check each channel to see that that R4 and R5 have the same value resistor loaded.
Also check that R9 and R10 have the same value resistor fitted.
AND
R2 and R3 have the same value resistor fitted.

Cheers,
Ian

Last edited by gingertube; 29th April 2015 at 12:07 AM.
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Old 29th April 2015, 12:17 AM   #7
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You need to apply sine wave and go through each stage of each channel with a scope to find where the difference arises.
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Old 29th April 2015, 02:36 AM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by joetjie2000 View Post
This is my first tube amp and I have no idea if this is a common issue?
Try measuring the two 100 Ohm screen resistors, which are connected between the output tubes and the switch. One may be open.
Turn the amp off and let it discharge for a few minutes before measuring. Or, a connection to that switch may be bad.
The switch also can be damaged if switched when the amp is on.

Last edited by rayma; 29th April 2015 at 02:38 AM.
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Old 29th April 2015, 09:50 AM   #9
ColinA is offline ColinA  United Kingdom
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I no not think that it is helpfull to just blanket post comments like
You need to apply sine wave and go through each stage of each channel with a scope
When it is quite clear from the original post that the person it not a valve expert and is VERY unlikely to have access to sophisticated test equipment or the knowledge to use it.
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Old 29th April 2015, 01:11 PM   #10
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Thank you all so much for the feedback! Much appreciated!

I think the wise decision would be to take the amp to someone knowledgable to do a thorough check for me. I was over-simplifying the causes and possible solutions.

I know its a long stretch but is there someone in South Africa you are familiar with and can recommend?

Once again - thank you so much for taking the time to help me.
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