For an adequate power transformer in a tube guitar amp, all the info and teachers say: you have to total the amp's quiesant current.
My calculations total 190mA suggesting a 200mA transformer.
Can anybody explain to me how the amplifier companies (Fender, Marshall, etc.), get by with a 90mA transformer in the same or similar amplifier circuit I'm calculating?
Am I missing something or doing something wrong?😕
My calculations total 190mA suggesting a 200mA transformer.
Can anybody explain to me how the amplifier companies (Fender, Marshall, etc.), get by with a 90mA transformer in the same or similar amplifier circuit I'm calculating?
Am I missing something or doing something wrong?😕
You should be working out the power requirement from each winding.
Then totaling the power to find the maximum total power.
Now the difficult bit:
what is the average power requirement for the type of duty the amplifier will be required to meet?
Then totaling the power to find the maximum total power.
Now the difficult bit:
what is the average power requirement for the type of duty the amplifier will be required to meet?
Yes, duty cycle often gets ignored. If 100% (e.g. Class A or preamp), then you need a transformer with an AC rating about 2-3 times the DC current. For Class B domestic audio you might get away with AC=DC current rating. For a guitar amp used for gigging I would assume 100%!
A 90mA AC current transformer could supply perhaps 80mA DC intermittently, and maybe 50mA continuously. This assumes capacitor input PSU, as usually used with valves. Choke input is better (AC=DC) but gives you lower voltage output.
I note that there is a healthy market for replacement transformers for guitar amps. Certain conclusions may be drawn from this?
A 90mA AC current transformer could supply perhaps 80mA DC intermittently, and maybe 50mA continuously. This assumes capacitor input PSU, as usually used with valves. Choke input is better (AC=DC) but gives you lower voltage output.
I note that there is a healthy market for replacement transformers for guitar amps. Certain conclusions may be drawn from this?
I've been trying to configure a DIY guitar amp. About 30W. I've got 3 power transformers:
An old Stancor 560VCT@200mA, A fairly new Hammond 273DX 700VCT@90mA ADC(though the factory specs say 104mA) and an old one I haven't removed from its Wurlitzer chassis.
I've gone through the calculations for a 3 x 12AX7's and 2 6L6GC's. That should be approx. 9mA for the AX's and, if I'm reading GE's data sheet correctly, 63mA for each pentode.
Totalling 135mA.
An old Stancor 560VCT@200mA, A fairly new Hammond 273DX 700VCT@90mA ADC(though the factory specs say 104mA) and an old one I haven't removed from its Wurlitzer chassis.
I've gone through the calculations for a 3 x 12AX7's and 2 6L6GC's. That should be approx. 9mA for the AX's and, if I'm reading GE's data sheet correctly, 63mA for each pentode.
Totalling 135mA.
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I'm OK on the voltage with either of the better transformers. I only need +427VDC (tube rectified using a (5U4B or an 5T4) its just the mA's that keeps confusing me. i guess the Stancor would be the better one to use.
Do you really need 30W for a practice amp? Remember that a 30W guitar amp goes a lot louder than a 30W hi-fi.
That's Fender's specs on their tweed and princton reverb amps. Like I say calculations keep confusing me. I keep figuring 135mA or 155mA Iif I add the reverb unit).
Their schematic specifies a 325-0-325 V, 375 VDC @ 70 mA(DC) power transformer with a 5Y3 rectifier using 6V6's. Their Va is slightly higher than the 6L6's.
Their schematic specifies a 325-0-325 V, 375 VDC @ 70 mA(DC) power transformer with a 5Y3 rectifier using 6V6's. Their Va is slightly higher than the 6L6's.
Sorry, I meant that the 6V6's Ia is slightly higher than the 6L6's.
By the way the Princeton's output power is rated at 22W.
By the way the Princeton's output power is rated at 22W.
Your figures sound a bit conservative. Just eyeballing this schematic:
http://www.davidsonamp.com/sf/images/princetonreverbb1270.gif
Three 12AX7s = 3mA
Reverb driver = 10mA
Two 6V6s each biased close to 14W = 68mA (probably an overestimate)
Total = 81mA.
I am probably overestimating the power valves by 20mA or something, so Fender's 70mA recommendation doesn't look so unreasonable, assuming non-continuous full output power.
Merlinb I want to thank you tremendously!
You opened my eyes to the figures I was overlooking and pointed out the ones I need.
The only calculation that still puzzles me is where the "screen to anode current ratio" can be found or calculated. One of my research sources says this is also needed to find any screen current necessary to add.
Again thanks.
Boy! Audio electronics is quite a bit different than basic computer electronics.
You opened my eyes to the figures I was overlooking and pointed out the ones I need.
The only calculation that still puzzles me is where the "screen to anode current ratio" can be found or calculated. One of my research sources says this is also needed to find any screen current necessary to add.
Again thanks.
Boy! Audio electronics is quite a bit different than basic computer electronics.
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