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Should I change tubes in 410-HD


Jeff Kaplan
Posted on Tuesday, March 15, 2005 - 05:27 pm:   

Hi there. Love this site. Came across it because I was looking for info on repairing a '74 Music Man 410-HD I've had in storage for close to 20 years. It died with a screech and after a lousy failed repair experience I just parked it in the basement.

i took it to a local guy recently and apparently some caps or resistors had come loose and just need re-soldering. it's as loud as I recall it.

The 20 year old Ruby EL34s tested fine. But, I've heard that 6CA7s will give me a ballsier sound so I bought a set. They immediately seemed to get to hot and I could hear them goin "tink tink tink". I shut the amp off after about 15 minutes and put the EL34s back in.

I know the 6CA7s need to be bias but here's muy concerns.

1. I'm nervous about having such an old amp tinkered with.
2. If I don't like the 6CA7s I will need to go and have it re-biased back the the EL34s.
3. Have I overheated and damaged the 6CA7s?

The tech, Dan Torres of Torres Engineering in San Mateo is very experienced and I trust him based on that. I'd love some opinions on the risk of stuff going wrong and whether there is sufficient difference in the sound that I should risk the change.

Anyhow, I've learned alot by reading these posts and truly appreciate the effort you all put in so thanks for any and all responses.
Jeff Kaplan
Posted on Tuesday, March 15, 2005 - 05:30 pm:   

Back for one more point. The 6CA7s I have are Electro Harmonix.

Thanks again.
Jeff Kaplan
Posted on Tuesday, March 15, 2005 - 05:43 pm:   

Can't find an edit option so I've got one more question.

What should bias be set for the 6CA7s? After reading through the threads my head is spinning with the settings for different amps. I believe Dan Torres told me he sets them at about 12 because of the high voltage.

michael kaus
Posted on Wednesday, March 16, 2005 - 05:09 am:   

THe original spec was around 25mA idle current PER TUBE. 12mA is cold and will probably distort. I will say however that I don't endorse the method of measuring bias current at the 1o ohm resistor. THis just gives you a total of the two tubes os .5 volts or 500mV which is divided by the resistor factor of 10 to give 50mV of bias idle FOR TWO TUBES. It's too easy to have a hot and cold tube together. 21-23mA is just fine and they will honk. Mike.
Jeff Kaplan
Posted on Wednesday, March 16, 2005 - 11:01 am:   

Thanks michael. The 410 is a 4 tube amp (I'm sure you know that). Am I correct to assume it is still 21-23mA per tube for all four.

I would value your opinion on the "tink tink tink" I heard and whether that indicated damaging overheating and whether there will be an obvious difference in the sound quality with 6CA7s vs. EL34s.

Thanks again.