Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on August 08, 2018, 07:01:55 am
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So last year I bought a revere t 100 for a great deal.when I got it there were multiple mech issues so I decided to part it out. Tube comp was 2x 6j7 a 6sj7 ,6k6,6v6,6x5. It was turned into a se guitar amp which I tweaked for a long time but could never get a great tone. It's been sitting until recently I acquired a nice 5k/8ohm se ot. When I put it in it sounded so much better I decided to test the impedance of the it which came in the revere that was designed apparently as a se 6v6 internal amp or. Well the thing tests 17k@8ohm. I was floored. What would be the reason for being so seemingly mismatched.
The way I measured the impedance was to input a varied to the primary and adjusted to 1volt on the secondary which gave me the ratio. I then multiples by 8ohmas to get 17k and change.when I tested the 5k I get 4.85k and others I tested gave me expected results so it seems I am calculating correctly.
Any thoughts
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The way I measured the impedance was to input a varied to the primary and adjusted to 1volt on the secondary which gave me the ratio. I then multiples by 8ohmas to get 17k and change.when I tested the 5k I get 4.85k and others I tested gave me expected results so it seems I am calculating correctly.
There seems to be a step missing in your calculations. Once you get the voltage ratio you must square that number to get the impedance ratio. Then multiply the impedance ratio by 8 to get the primary impedance for an 8Ω output.
For example, say you had to put 30V into the primary to get 1V on the secondary. This gives you a 30:1 voltage ratio. Now 302 = 900, so the impedance ratio is 900:1. And if you call the secondary 8Ω, then 900 x 8 = 7200Ω primary impedance, ie, 7200:8 ratio.
So, what voltage did you have on the primary that gave you 1V on the secondary?
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That's what I did. I forgot when I posted this morning. The ratio was 46.5:1 volts squared gives you 2162.25 x 8ohm gives you 17298 or 17.3k to 8ohm
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The same method gave me expected results on other ot. This one had me scratching my head because it was basically a se 6v6 and at 8ohms I would expect 4k to 9k from what I have read. I guess this is why the amp sounded like a four year old girl whining rather than a 23 year old sex kitten purring. I can only suppose it was meant to keep distortion down? Just seemed weird
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Maybe the OT was used with a 4Ω speaker. That would reflect a 8.6K load back to the tube.
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Yeah I thought about that but the speaker clearly says 8ohm. I don't know. I built the amp before I had a variac so I made the assumption since revere put this ot with a 6v6 that they would work well together. I spent many hours trying to get this thing to sound right and never could completely dial it in . 17k makes it clear to never take for granted things are compatable. :BangHead:
Now I get to rip out all the connections and start again. I gotta feeling it's gonna sound fine now. Perhaps I will start with the original supro spectator schematic albeit I am using a 6L6.
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https://www.tubesandmore.com/schematics/revere/t-100
https://www.tubesandmore.com/sites/default/files/schematics/revere_t_100_pg22-20.png
https://www.tubesandmore.com/sites/default/files/schematics/revere_t_100_pg22-1.png
It can drive an external 3.2r speaker. It has an internal 10r dummy-load in most positions. This comes to near 5K. Try a 2.4r speaker. (Three 8r parallel?)
In 1952 you could sell ANY crap as a home tape deck, even at $180 (1/10th the price of a Ford), and find buyers.
https://www.radiomuseum.org/r/revere_t100.html -- pictures of ads
Revere was a copper-knocker who went into budget movie cameras. The tape recorders were a small part of a small business.
https://www.madeinchicagomuseum.com/single-post/2015/08/02/Revere-88-Movie-Camera-by-Revere-Camera-Company-1940s-1
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Thank you for the response. I have seen those sources. But I didn't really study the schematic I just glanced at it overall . And took for granted the 8ohm speaker and the 6v6 meant I could pop these in an amp together for harmonious impedance match. Next time I will consider circuit effects and test the ot impedance before just putting it in some thing and expecting it to sound good.