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I wonder why the extra gain stage for this amp? It seems like this circuit would probably work with harder to drive or low mu tubes.
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I wonder why the extra gain stage for this amp? It seems like this circuit would probably work with harder to drive or low mu tubes.
Are those transformers potted or just covered to look like they are potted.This is one i'm leaning to and will build it with some pioneer iron i acquired from Tinkerbelle to see ( hear) how it sounds with 6v6's .
Also will make some adapter so 6BQ5's can be used in the same amp and maye 12AU7/6FQ7 adapters also
Got the SM-30 transfomers in last evening will get some photos and measurements on it later tonight . The outputs are 32 , 16 and 8 ohms , the primaries are split and measure , 203 and 190 each .
Whatever i do with them they will make a very nice looking amp and sounding amp .
High-gain triode input stages place a relatively large capacitive load on the preamp due to Miller effect. I would use a cascoded triode or pentode input stage unless I was very sure that only low-output-impedance preamps would be used. The 6SN7 input stage is a reasonable compromise because of its low gain.For a power amp only for EL84 outputs, my vote would be a 12AX7 gain stage and a 12AX7 split load inverter. For power amp only with 6V6 outputs, I'd go a 12AX7 gain stage and 6NS7 or 6FQ7 split load inverter. I know...nothing new under the sun in those suggestions, but it works so well.
The HF-V60 is one i'm leaning to and will build it with some pioneer iron i acquired from Tinkerbelle to see ( hear) how it sounds with 6v6's .
Also will make some adapter so 6BQ5's can be used in the same amp and maye 12AU7/6FQ7 adapters also
Got the SM-30 transfomers in last evening will get some photos and measurements on it later tonight . The outputs are 32 , 16 and 8 ohms , the primaries are split and measure , 203 and 190 each .
Whatever i do with them they will make a very nice looking amp and sounding amp .
You will find that radio tuning capacitors don't have enough range in many cases, depending on feedback divider resistance values, so you might want to add a capacitance decade box or substitution box to your shopping list. Also, be aware that other elements of the amplifier may need tuning for acceptable squarewave fidelity --- particularly voltage amplifier bandwidth, rolloff shape, and output transformer damping. And expect interaction. This whole area tends to be about as sticky as tube amplifier engineering gets.I was always hesistant to make changes in a feedback loop until I read a handy trick: get some tuning capacitor, feed the amp with a square wave and observe the output. Then adjust with the tuning capacitor till you get the best square wave at the output and subsequently measure the capacitance of the tuning capacitor and replace with fixed values.
You will find that radio tuning capacitors don't have enough range in many cases, depending on feedback divider resistance values, so you might want to add a capacitance decade box or substitution box to your shopping list. Also, be aware that other elements of the amplifier may need tuning for acceptable squarewave fidelity --- particularly voltage amplifier bandwidth, rolloff shape, and output transformer damping. And expect interaction. This whole area tends to be about as sticky as tube amplifier engineering gets.
Compression mica trimmer varicaps with the required range will do.I would qualify that statement with some variable capacitors don't have the needed range . I have silver plated variable capacitors over 1600 PF that i use to make crystal radios , they are four section and 430 pf per section so the can be wired in series or parallel to get the capacitance you need . Most caps in AA-5 radios have a 365 PF tuning cap and a 100 to 180 pf section for the oscillator which would give about 465 pf to tune mosts amps .
http://peeblesoriginals.com/catalog/15.php
The HF-V60 is one i'm leaning to and will build it with some pioneer iron i acquired from Tinkerbelle to see ( hear) how it sounds with 6v6's .
Also will make some adapter so 6BQ5's can be used in the same amp and maye 12AU7/6FQ7 adapters also
Got the SM-30 transfomers in last evening will get some photos and measurements on it later tonight . The outputs are 32 , 16 and 8 ohms , the primaries are split and measure , 203 and 190 each .
Whatever i do with them they will make a very nice looking amp and sounding amp .
Thanks for the reply, I prefer to stick close to orginal designs unless there is a compelling reason to make changes. The early Sansui designs have quite a good reputation, quality on later Sansui gear leaves lots to be desired.You'd use a 6SL7 or 12SL7, or a 6SC7. A 6SN7 is like a 12AU7.
I copied an Andio Note PP EL84 amp circuit. It had a 6BL8 for input/splitter duty. I am not sure if that is the one you will be using but the circuit that i used had too much gain. It needed tons of -FB to tame it , and frankly i just did not like listening to negative FB. I scrapped that circuit and switched to the Scott 299 circuit which was more in tune with my needs.View attachment 1026479 View attachment 1026476 View attachment 1026478
I'm building a clone of the Audio Note kit 4 - picked up the PCB on ePray and did surgery on that. Mains transformers and output transformers are oversized and I will be able to change the end stage from the 6V6 to the 6L6 but output impedance will change and I may have to adjust the feedback loop. I was always hesistant to make changes in a feedback loop until I read a handy trick: get some tuning capacitor, feed the amp with a square wave and observe the output. Then adjust with the tuning capacitor till you get the best square wave at the output and subsequently measure the capacitance of the tuning capacitor and replace with fixed values. The other thing to check is the overall phase shift and that can be done with a dual trace oscilloscope - even a cheap USB one will do that.
Am attaching the circuit diagram of the Audio Note kit 4 and of the Audio Note P2-PP (third version - in it the 6SN7 was replaced with the ECC83/12AX7) which has the 6L6 and is 20 Watts. I also upload another schematic which I think is for the Audio Note P1-PP but am not sure although the schematic makes it quite believable. (give away are the floating paraphase resistor values and the general configuration of the finals. It is interesting to see that they have brought back the 47K trimpot in the floating paraphase inverter.) The 6V6 has about 10 dB feedback, the gain is not excessive. Sensitivity is approx 250mV for full output which may be usefull when driving from an iPod etc.