Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on October 04, 2011, 01:22:12 pm
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Hi, I am building a SE amp with an El34 , 275V at the plate and an OT of 40 turn ratio.
The math doesn't give me the result I'd like to ( my girlfriend neither but this is another ball game hey hey ).
I explain: 25 w max dissipation for an EL34 and 275V at the plate. 275 squared divided by 25= 3,025 ohm reflected impedance
OT ratio of 40= 1,600 OHM , with a 4 ohm speaker = 6,4k OHM primary impedance, which is the double of the reflected impedance of the EL34........to match perfectly the impedance, I should use a 2 OHM speaker :w2: Hooking up a 4 ohm speaker would run the tube colder or hotter ?
Any suggestion, idea ?
Colas
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Your math looks correct to me. The OT primary impedance, together with the supply voltage, determines the maximum power output. So, in your case, you'll get more like 12W output from the stage with a 4 ohm load. For a given load impedance, the only way to increase the power output with a given OT is to raise the supply voltage.
You can, as you pointed out, use a 2 ohm load (by paralleling two 4 ohm speakers, I presume), which will effectively lower the primary impedance.
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You can also double the output tubes to halve the impedance
Kagliostro
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Kagliostro, by doin' so, ( adding a seconde output tube ), the power will drop too much, according to the power tranny too weak for 2 tubes. I guess I'll use two 10in 4 ohm speakers in parallel
Colas
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OT ratio of 40= 1,600 OHM , with a 4 ohm speaker = 6,4k OHM primary impedance, which is the double of the reflected impedance of the EL34........to match perfectly the impedance, I should use a 2 OHM speaker :w2: Hooking up a 4 ohm speaker would run the tube colder or hotter ?
Any suggestion, idea ?
Colas
It is not as critical to perfectly match impedance of OT and power tubes on a guitar amp. many Fenders are not matched perfectly.
The information on data sheets are assuming you are building a HiFi amp and the transformer is large to accommodate a much lower bass response.
When you plug an external speaker in a tweed style fender, there is an even bigger mismatch.
You should be very safe running 4 ohms of speaker.
In smaller amps I like the sound of 2 10" speakers. They seem to have less "fart", more punch, and a greater perceived volume compared to a single 12. Also hum will be less noticeable.
Ray
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Kagliostro, by doin' so, ( adding a seconde output tube ), the power will drop too much, according to the power tranny too weak for 2 tubes.
do you mean there isn't so much filament current in your PT ?
about B+ don't you can "play" with bias ?
Kagliostro
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The heater supply is at 6.7V, ( el34 and 2X 12ax7 ) so it would handle another tube. The B+ though dropped from 480V to 275V when I fitted in an EL34, perhaps another EL34 will drop the B+ too low, what do you think ?
A solution would be to replace the GZ34 by a diode bridge, yielding more high voltage ??????
Regards
Colas
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>A solution would be to replace the GZ34 by a diode bridge, yielding more high voltage
That's what I did on the Little Brown One, full wave not a bridge. Problem solved. I don't see any reason to desire a tube recto in a SE amp anyway. SE don't sag like PP.
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the reason why I wanted to keep the GZ34 is to use the 5V tap form the PT....................
anyway this small tranny and the even smaller OT were ordered 2 years ago to build a champ and I will stay within their limits
Colas
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Not using the GZ34 takes 250mA load off the 5V which translates to roughly 10mA off the primary (at 120V). If your PT is already marginal, a few more mA ain't gonna hurt you none. More B+ is gonna be the only way to get more out of that tube + OT combo. I built my full wave into an octal plug so that I can go back to a tube recto, although I really need to change the cathode resistor if I revert. Diodes are certainly the cheapest solution, maybe not the best solution. Marginal iron, OT or PT is still marginal.
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This is an interesting thread...especially for someone who knows little of the technical maths behind these things! Speaking of which...
...What's the formula/maths behind bobmegantz post? So I know for future reference.
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I can't tell, sorry !