It's me again, the know-nothing that bites off more than he can chew
I'm working on a power amp build and I had a few questions. I will scan/upload a copy of my schematic upon request. Generally, the gist of it is I took an existing schematic and modified it. I am essentially constructing two 4-stage pre-amps (2 x 12AX7) and tying those into 2 EL84 power tubes. I later changed those to KT88s for more power.
I intend to run the 12AX7 plate supplies at 250V and the KT88s at close to 800V so I can extract maximum power.
I'm shooting to be able to drive a 70W speaker on this build.
Now here begins the questions.
1. Changing the power valve stage changes the output impedance appreciably. How would I measure the output impedance (beware, n00b ideas coming) - is it as simple as powering the valves and putting an ohmmeter across the signal path?
2. Relative to 1, would that change in impedance pose a problem when picking an appropriate output transformer? If so, would it be possible to use an L-pad to convert to the appropriate impedance for the output transformer?
Obviously, a passive L-pad would incur a power loss, and there are no active designs I can think of that are capable of operating at power valve ranges aside from well...power valves. But is the change in impedance caused by the change in valves in the power stage appreciable enough that if I were to use an L-pad to match the two, would the power loss be significant?
3. Right now, the circuit (which I will post in a day or so - as soon as finals are over I will be home and have access to a scanner) is configured as two independent single-ended single paths summed at the output transformer.
First of all, why is it that power valves exhibit higher power ratings when ran in push-pull configuration? Secondly, how would I configure two KT88s to run in push-pull?
4. Rather unrelated question, but why is it that voltage coming out of a transformer typically increases in voltage a certain amount after rectification? Was wondering because I need to account for this when assigning resistance to drop the B+ rail to acceptable plate loads for the valves.
Thanks in advance, and I will have the schematic up as soon as I have a scanner to work on.
Eli Fedele
(solidus)
I'm working on a power amp build and I had a few questions. I will scan/upload a copy of my schematic upon request. Generally, the gist of it is I took an existing schematic and modified it. I am essentially constructing two 4-stage pre-amps (2 x 12AX7) and tying those into 2 EL84 power tubes. I later changed those to KT88s for more power.
I intend to run the 12AX7 plate supplies at 250V and the KT88s at close to 800V so I can extract maximum power.
I'm shooting to be able to drive a 70W speaker on this build.
Now here begins the questions.
1. Changing the power valve stage changes the output impedance appreciably. How would I measure the output impedance (beware, n00b ideas coming) - is it as simple as powering the valves and putting an ohmmeter across the signal path?
2. Relative to 1, would that change in impedance pose a problem when picking an appropriate output transformer? If so, would it be possible to use an L-pad to convert to the appropriate impedance for the output transformer?
Obviously, a passive L-pad would incur a power loss, and there are no active designs I can think of that are capable of operating at power valve ranges aside from well...power valves. But is the change in impedance caused by the change in valves in the power stage appreciable enough that if I were to use an L-pad to match the two, would the power loss be significant?
3. Right now, the circuit (which I will post in a day or so - as soon as finals are over I will be home and have access to a scanner) is configured as two independent single-ended single paths summed at the output transformer.
First of all, why is it that power valves exhibit higher power ratings when ran in push-pull configuration? Secondly, how would I configure two KT88s to run in push-pull?
4. Rather unrelated question, but why is it that voltage coming out of a transformer typically increases in voltage a certain amount after rectification? Was wondering because I need to account for this when assigning resistance to drop the B+ rail to acceptable plate loads for the valves.
Thanks in advance, and I will have the schematic up as soon as I have a scanner to work on.
Eli Fedele
(solidus)