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Solid State Rectifier vs Tube


Zoki

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Hey Gang,

 

I got this old tube amp from a friend who got it in a lot auction on ebay buying some other gear. The only identifying feature, because it's not in a cab of any sort is the little LA - 100R on the chassis which to the best of my research seems to be a Lafeyette possibly made by Univox in maybe the late 60's. The guts are real pretty and look hand wired (I'd post a pic but never figured out how to do that here). It's unclear if the thing works or not though because it needs tubes, but in doing the research (6973 power tube, 12ax7's, a6av6 and 7247) I came across these weber copper cap subs for the 6CA4 rectifier tube. I'm wondering if anyone could tell me what the difference might be to the tone and the amp assuming this thing fires up when I get the right stuff in there...

 

Thanks in advance for any help!

 

Z

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Wasn't that "Unicord," not "Univox?" If it's like the one that I've seen, it's got the notorious "death cap," which, if it fails, makes the chassis "live" with AC. Make sure you rewired that AC section and install a 3 prong cord as well as replace all electros.

 

There is no difference in "tone" from SS to tube rectifier. The difference is in power supply sag, with a tube being markedly more "saggy" than SS. However, I believe the Weber copper caps contain resistors to simulate tube rectifiers, so the difference SHOULD be negligible.

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Hey M,

 

Wow - thanks for the heads up about that cap - had an old Kustom that loved to shock the cr@p out of me at shows if I forgot to check which side the power switch needed to be on for the room - not fun....

 

So Unicord? Yeah - could be - had done just a quick search on it and didn't come up with too much - in fact trying to find the 6973, 7247 and the 6av6 at the same place online is proving to be a difficult order to fill....

 

So this whole sag thing - I've been reading up on it and trying to clarify it in my mind for reference - is this say the difference between maybe a Marshall tone and say a Fender Silver face kinda thing - Marshall being more saggy or am I off the mark there?

 

Thanks again for the warning about the death cap!

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had an old Kustom that loved to shock the cr@p out of me at shows if I forgot to check which side the power switch needed to be on for the room

Yeah, it's a blast.

 

So Unicord? Yeah - could be - had done just a quick search on it and didn't come up with too much - in fact trying to find the 6973, 7247 and the 6av6 at the same place online is proving to be a difficult order to fill....

I think. Yeah, some of the old oddball tubes are getting very difficult to locate. With some, you can sub, given appropriate compliment component value changes.

 

So this whole sag thing - I've been reading up on it and trying to clarify it in my mind for reference - is this say the difference between maybe a Marshall tone and say a Fender Silver face kinda thing - Marshall being more saggy or am I off the mark there?

I guess most folks wouldn't be thinking of sag when they think of traditional Marshall, because the plexis were SS rectifier. The JTM45 and the 1974 18 watt had a tube rect, however, though those really aren't what most people are thinking of when they say "Marshall tone."

 

Fender's saggiest amps were the tweeds, then the browns, then the bf. SF models tended to be a bit stiff, although the lower output models definitely do the sag thing, very noticeably. Push a champ from any year hard and you'll definitely feel and hear sag. Not so much so with a SF Twin, particularly the ultralinear models.

 

The best way that I can think of to describe sag is that when you're pushing the amp hard, you feel the output drop a bit and then catch up. If you were playing the same amp quietly, you wouldn't notice this much, if at all. Dig in or hit a hard power chord, even at low volumes, and you'll feel and hear it again, as the power supply hesitates under load and then catches up.

 

SS rectified amps will still sag, but it's really limited to the power transformer and output tubes, so it's much less noticeable, and you won't have the dramatic kinds of power drop that you can have with tube rectified amps pushed hard.

 

I generally describe my preference (SS rec) as having the amp stand up and push back when it's pushed hard.

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A friend of mine is a collector of old radios (from the first half of the 20th century, pre-transistor). Quite often he will have to replace tubes when fixing up an ancient beauty. Those old tubes are readily available as used tubes which are tested and guaranteed to work and as NOS (New Old Stock). A search for "antique radio tubes" or supplies should bring up a rather large list of suppliers for tubes that are no longer produced.

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