Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mozz on August 25, 2023, 06:51:49 pm
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SE? PP? Not enough current for PP 6v6? El84 types heard they do not sound good in SE. I do have some 7189A's that could take 440v, SS rectifier, 90ma might just make it for a pair? I have lots of output transformers to choose from so no problem there. Shows 105 ma at 400v max signal.
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El84 types heard they do not sound good in SE. I do have some 7189A's
How many Valve Juniors did Epiphone sell?
I do have some 7189A's
7189 is a 6BQ5/EL84 made with 400V plate stuff. Doesn't change the tone.
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SE? PP? Not enough current for PP 6v6? ...
700v CT ---> 350v x 1.414 = 495v peak
- You'll want that 5Y3 rectifier, and maybe some extra dropping resistance if the PT doesn't already have that as excess winding DCR.
- Pencil "450v DC" supply.
- 6V6 should be able to pull its plate down to 100v, so 450v - 100v = 350v peak swing.
- Reserve "10mA" of PT capability for small tubes, screens: 90mA - 10mA = 80mA.
- Convert to peak current: 80mA x 1.414 = ~113mA
- Does our peak voltage and peak current imply a probable output transformer?
- Impedance = Volts / Current = 350v peak / 0.113A peak = ~3100Ω
- For Class AB, Ra-a = Impedance x 4 = 3100Ω x 4 = 12,400Ω.
Let's round-down to a more-common 10kΩ OT primary impedance
- Can a 6V6 pass 113mA peak with 400v or more on the screen? Yes, that should be easy (based on a brown Deluxe, Deluxe Reverb).
- Power Output = [Peak Current2 x (Primary Impedance / 4)] / 2 = [0.113A2 x (2500Ω)] / 2 = ~16 watts
- Plate Voltage Swing: 0.113A x 2500Ω = 282.5v ---> 450v - 282.5v = ~168v lowest plate volts (before sag)
... I have lots of output transformers to choose from so no problem there. ...
Got a 10kΩ primary at 10 watts or more?
If not, Hammond has the 1609 (https://www.hammfg.com/files/parts/pdf/1609.pdf) which looks like a perfect fit. Yes, Hammond says "10 watts" but they also rate it down to 30Hz. Since you won't be going that low, you can shove more power throughput, making 16-20w no problem.
If the Stancor actually delivers a little higher current than the rated 90mA, then you'll get more than the estimated 16w output power from this setup. The Stancor Catalog (page 14) (https://www.earlytelevision.org/pdf/stancor_61.pdf) says to use this PT with a choke-input power supply, in which case it will deliver 350v x 0.9 = 315v DC ---> better suited for EL84 and 6AQ5. It is wise to use a choke rated over the claimed 90mA capability of the PT.
You can pick parts other than what I outlined, and get a bit less output power. These days, most folks don't want "all the power/loudness."
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I'd take a chance with that PT in an early tweed deluxe circuit
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I came across 3 of these transformers. Forgot i made this test unit 10 years ago to test pulls. 3 resistors, about 22uf/600v in there. With a good 5Y3, i get 434v @46ma, 412v@65ma, 362v@116ma. So, nothing too outrageous. Weak 5y3 was about 40v lower at highest draw. I have made a few 5c3,5d3,5e3, princeton reverbs, etc. So maybe a brown/deluxe this time. No 12.4k output transformers but a few 16 ohm speakers sitting unused so maybe slap that on a 6.6k or 8k transformer, if needed/5ar4. Now out to the garage to find that empty/heavy Acoustic cabinet with 12" speaker cutout.