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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Sun 07, 2019 2:59 am 
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Brett_Buck wrote:
Flipperhome wrote:
"Solve a problem with extraneous oscillatory elements by adding *more* oscillatory elements? " Yes, it's standard practice in analog systems from audio to video to RF. What do you suggest? Just leave the problem there?
You aren't "fixing" anything in this case. But far be it for me to impede you, do whatever you want.

Brett
I don't know what your problem is but you're the one who said "Solve a problem with...?" so apparently you think there's a 'problem" to be solved. Tell you what, why don't you explain the problem and then tell us your better solution?


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Sun 07, 2019 3:33 pm 
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Back to the original question. The power transformer is good for an EL34 amplifier, but voltage is too high for 6V6 or EL84, which the output transformer would be suited for. I suggest you trade it for something in the range of 260-300V (520-600V CT). The output transformer is designed for a 25Wguitar amp, but should work over a wider frequency range at 12-15W, which is what you can expect from 6V6 or EL84.

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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Sun 07, 2019 8:52 pm 
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Tom Bavis wrote:
Back to the original question. The power transformer is good for an EL34 amplifier, but voltage is too high for 6V6 or EL84, which the output transformer would be suited for. I suggest you trade it for something in the range of 260-300V (520-600V CT). The output transformer is designed for a 25Wguitar amp, but should work over a wider frequency range at 12-15W, which is what you can expect from 6V6 or EL84.


You are right. When the transformer is plugged to a 116.8VAC outlet I get 390-0-390VAC. So, I got two options.
One adding a hi wattage resistor to drop the voltage across the plates of the EL84/6BQ5.
Second option would be to move to the 6L6GC since it can handle up to 450V in Class AB operation. But I think I might need another output transformer since the plate-plate resistance is 5.6K in this case.

I need to find which one is a cheaper option, getting a new power transformer or the output. I guess the former would be.

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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Mon 08, 2019 6:58 pm 
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Flipperhome wrote:
Brett_Buck wrote:
Flipperhome wrote:
"Solve a problem with extraneous oscillatory elements by adding *more* oscillatory elements? " Yes, it's standard practice in analog systems from audio to video to RF. What do you suggest? Just leave the problem there?
You aren't "fixing" anything in this case. But far be it for me to impede you, do whatever you want.

Brett
I don't know what your problem is but you're the one who said "Solve a problem with...?" so apparently you think there's a 'problem" to be solved. Tell you what, why don't you explain the problem and then tell us your better solution?


He doesn't have one because he hasn't a single clue what he's talking about. Using corrective networks to flatten impedance curves for loudspeaker is a standard practice, and has been one since at least the late 1930s. Harry Olson dedicated a substantial portion of his 1940 book "Elements of Acoustical Engineering" to such corrective networks, and the physics that governs the loudspeaker is the same today as it was in 1940.

Barry H Bennett wrote:
I think this more or less applies to most of the topics around here by the time we all get done with them ;-)

..........beware of bored engineers, techinicians, and hobbiests with too much time on their hands...... 8)


Well in this case Barry, it isn't a "bored engineer, technician, or hobbyist"; I'm a bored physicist, and I first learned of the technique from Olson, arguably the finest engineer ever hired by RCA. I'd still like either of you to demonstrate a better fix to the impedance curve problem, particularly the rising impedance at higher frequencies which can cause many amplifiers to become unstable. That is if you're capable of coming up with a better solution, which is doubtful.


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Mon 08, 2019 9:37 pm 
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There's no impedance curve problem. The guy has a transformer, some tubes and a speaker. You can argue all you want about speaker and acoustic "history". It is totally irrelevant. If you want to correct all the "todays misinformation" out there, write your own book, i need a good laugh.

I'm gonna go with the above mentioned EL34's with a different output trans, or 6v6/el84 and a different power trans.


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Mon 08, 2019 11:02 pm 
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It appears that physicists must not have a sense of humor. The day I argue about speaker acoustics will never come. <wink >

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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Tue 09, 2019 1:57 am 
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You might be able to use your power transformer and output transformer if you use a choke input filter instead of a capacitor input filter. They put out lower voltage and have better regulation as well.


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: Apr Wed 17, 2019 3:12 am 
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Audioman wrote:
You might be able to use your power transformer and output transformer if you use a choke input filter instead of a capacitor input filter. They put out lower voltage and have better regulation as well.
I like this idea. Also look into lowering the voltage on the primary of your power transformer with a capacitor circuit or a bucking transformer. Get the secondary voltage down to something the 6V6s are comfortable with, less than the 315 volts max rating. Then carry on.

Start by applying 120 volts to your power transformer and measuring the output. You might be fighting an uphill battle with the above strategy. But you don't know unless you try, and remember the voltage will drop when a load is connected.

Bigger holes in the chassis can be drilled with a hole saw if you can't find a proper chassis punch. Mount a piece of wood in the chassis to keep the drill centered in the existing hole, and then grind away and make the two holes. Start slow and get the groove even. Sparks will fly. Finish with a round file.

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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: May Sat 11, 2019 3:37 am 
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George Anderson of Tubelab has a few circuit boards that sound very good.One is a nice little push pull 6bq5 cathode biased amp.he also has a couple of Single ended circuit boards if interested.He uses a CCS 10m45 and 12At7 driver, can be wired triode Ul or pentode.Boards are about 45 dollars bare. Nothing to engineer and a decent option.


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: May Sat 11, 2019 4:12 am 
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westcoastjohn wrote:
Also look into lowering the voltage on the primary of your power transformer with a capacitor circuit or a bucking transformer.
A capacitor dropper will only work in series heater devices without a power transformer.

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Last edited by FStephenMasek on May Fri 17, 2019 9:29 pm, edited 2 times in total.

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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: May Fri 17, 2019 8:56 pm 
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FStephenMasek wrote:
westcoastjohn wrote:
Also look into lowering the voltage on the primary of your power transformer with a capacitor circuit or a bucking transformer.
A capacitor dropper will only work on series heater devices without a power transformer.
There are circuits that will work. I think. :D


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 Post subject: Re: Best choice for building a tube amp, 6L6GC/6V6/EL84?
PostPosted: May Sat 18, 2019 12:23 am 
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IMHO,

From what I gather so far, the power transformer "may" be a bit too high voltage for 6V6 but the OPT is wrong impedance for the 6V6.

How about this combination: use push/pull parallel 6V6 that should drop the load resistance to half so the OPT becomes usable. Use choke input on the power filter and a thermionic rectifier like a cheap and robust 5U4 FWIR there is a 5 volt winding. Remember the transformer readings may not be fully loaded so there will be some drop...There is more than enough current from one 6 volt winding to power the (4) 6V6's and a suitable phase splitter/preamp by selecting tubes with lesser filament current requirements. Take the remaining 6 volt winding and use it to buck the primary of the power transformer if needed but watchful that the "working 6 volt filament windings" are not dropped too far below reasonable specs for all the tubes, check the 5U4 too.... Note the filament rectifier will have at least a 30 volt drop depending on load but should be robust enough to provide current surges.

I suggest using two chokes in the supply the first a "swinging choke" then feed the OPT then second a conventional choke then feed the preamp/inverter stages.

Avoid using a power dropping resistor at all costs especially in the output to the 6V6 this will cause sags in the B+ to the 6V6.

Use a series voltage divider as a minimum load across the B+ at the second choke, allow about 10 to 20ma, tap down to provide B+ to preamp/inverter, add a 33 to 66 mf cap that source.

If desired to use fixed bias for the 6V6 it can be derived from the center-tap of the power transformer, reposition that 2nd choke to the CT, place suitable resistors usually a combination arriving at 10K or so to select a bias source. A 10mf will be O.K. if a little residual hum is present. The "return" bias will also knock down the effective B+ so again, more happy, happy.

Conclusion I think you have what you need, on the cheap. Don't go nuts buying into matched 6V6 get a handful of used and match within 10% of the trans-conductance, you will be good to go.

There is a lot of latitude in wiring up the associated tube circuits, leave lots of room on the chassis and use 14 ga filament wire twisted close to the chassis.

As always,

YMMV

Chas

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