Just starting to look at set amplifiers. The biggest expense seems to be the output transformer. Would an oversize conventional output transformer be workable? I have a rather large one designed for a 200-250VA output, via 8 - 6ca7s in push-pull. Transformer dc current saturation shouldn't be an issue(?). Would that be workable using a 6c33c output circuit? Have matching giant PS transformer with plenty of capacity and voltage selections. Way overkill but have them for no cost start.
By conventional, I think you meant PushPull output transformers. Even if it's oversized, you can't normally use a PushPull Output in SET amps since the primary winding is usually balanced for DC cancellation in PushPull Output. The standing DC from the SET circuit will magnetize the core in the PP output so it will wreak the frequency response from such iron.
You can use parafeed method but a CCS on such output but all requires extra expense and complexity. That's why the SET optimized outputs are normally more expensive than the more common PP output. 6C33C draws a lot of DC bias current in SET design with very low Rp. Thus requires special type SET transformer in the 300-600ohm primary winding impedance. Last time I checked, Electra Print still making 6C33C SET outputs.
You can use parafeed method but a CCS on such output but all requires extra expense and complexity. That's why the SET optimized outputs are normally more expensive than the more common PP output. 6C33C draws a lot of DC bias current in SET design with very low Rp. Thus requires special type SET transformer in the 300-600ohm primary winding impedance. Last time I checked, Electra Print still making 6C33C SET outputs.
If you have a P-P OT, then you could put an un-driven pentode on the other P-P primary side to balance the DC currents. (acting like a CCS)
For more power, but still some SET like character, the pentode could be driven by a phase splitter as well, assuming the 1st P-P side is triode driven. The triode's lower Rp will control the AC voltage while the pentode will balance the average DC and provide additional power output.
If you then put a CCS tail under the two tubes, then the pentode will have to mirror the triode voltage control even closer by providing exact complementary P-P current. 2nd harmonic distortion in the controlling triode will upset the DC balance some at high signal output, but should be tolerable for an ovesize OT.
For more power, but still some SET like character, the pentode could be driven by a phase splitter as well, assuming the 1st P-P side is triode driven. The triode's lower Rp will control the AC voltage while the pentode will balance the average DC and provide additional power output.
If you then put a CCS tail under the two tubes, then the pentode will have to mirror the triode voltage control even closer by providing exact complementary P-P current. 2nd harmonic distortion in the controlling triode will upset the DC balance some at high signal output, but should be tolerable for an ovesize OT.
Last edited:
Ok, you either "bias" out the flowing dc current, or put an air gap in the transformer, correct?
what would you expect the primary winding impedance of my existing transformer to be, a guess...... existng operation point was 645vdc x 8 tubes at 20 to 40ma per tube.
what would you expect the primary winding impedance of my existing transformer to be, a guess...... existng operation point was 645vdc x 8 tubes at 20 to 40ma per tube.
Last edited:
Balance out the DC to Cancel the DC bias (yes) in the P-P output or gapped core (not necessarily air) in the transformer. There are something called Parafeed Single Ended Output (less core gap and more inductance that must be used along with Parafeed Choke).
I have 6C33C SET and it sounds good (goes down to 20Hz @ -0.5dB) with Tango Iron at full 12W RMS.
Using P-P iron in SET requires you to experiment a lot or have already experienced a lot in that regards.
- Status
- Not open for further replies.