Lundahl 4-30 Build

[83stang]

I've decided to ditch the dynaco and eico design. Has anybody built the Lundahl 4-30 amplifier? how is it? and can I use the Antek AN-4T360 for the stereo power supply and an Hammond 1645 for the output transformer? How would I go about getting the bias voltage? Thanks again for all your help.

[andRookie]

I built the 4-30 two years ago. Very happy with it. I did use other trannies but I then had to modify the circuit to make the amp stable. It occilliated, and we had to lower the value of the coupling caps.

[Eli Duttman]

Originally posted by 83stang
I've decided to ditch the dynaco and eico design. Has anybody built the Lundahl 4-30 amplifier? how is it? and can I use the Antek AN-4T360 for the stereo power supply and an Hammond 1645 for the output transformer? How would I go about getting the bias voltage? Thanks again for all your help.

Dude,

The 4-30 uses Mullard style circuitry. With all respect to Claus Byrith, the use of high R_P/low gm small types is (IMO) ill advised. Two very important considerations in amps with loop NFB, like the Mullard style, are slew limiting and O/P trafo core saturation. High gm small signal types provide resistance to slew limiting. My previous recommendation of 12BH7 and 12AT7 small signal types stands. The overall gain structure of the 2 selections is very similar. An added benefit of the 12AT7's presence is a superior net harmonic distortion spectrum. By all means, use CB's "fixed" bias PP O/P stage, with complete confidence.

You will have little trouble in using the AN-4T360 power trafo. Connect the 2X 360 VAC windings in parallel. Bridge rectify with snubbed UF5408s on the ground side and 600 PIV Schottky diodes on the B+ side. Install a CL150 inrush current limiting thermistor in the line between the rectifier bridge and the PSU filter. CLC filter the B+. Start with 1.5 μF. in the 1st position and adjust that value, with the PSU loaded down, to bring the B+ rail in at 440 V. No less than 5 H. are required in the choke. Pile the energy storage up in 2nd position. As the AN-4T360's filament windings are lightly loaded (no vacuum rectifiers), use 1/2 of 2X 12BH7s in each channel. Wire 1 channel to use system #1 and the 2nd channel to use system # 2. When the tubes wear out, flip/flop them.

IMO, the Hammond 1645 O/P trafo is a very poor choice. Its end to end impedance is somewhat high for this application and its power handling capability is woefully inadequate. O/P core trafo saturation is dealt with by rolling low freq. info. off at the amp's I/P. To keep the NFB error correction signal from saturating the core of a 1645, the I/P high pass "corner" freq. will be high enough to foul the music up, very badly. Sorry, TANSTAAFL.

Greinacher ("full wave") voltage double a "Rat Shack" catalog # 273-1366 to derive the EL34 bias voltage.

Pay CLOSE attention to the remark about oscillation. Notice the 560 pF. cap. in parallel with the NFB resistor. That's a phase compensation part. Its value is tied to the character of the O/P "iron" used. The proper value is determined with an o'scope and a 10 KHz. square wave. I suggest a brute force method for phase compensation. Use inductive wirewound resistors as phase splitter plate loads. That peaks HF gain. Short the NFB loop out above 80 KHz. by connecting a cap. between the "hot' speaker lead and ground. This method relies on the natural HF rolloff of the O/P trafo and an o'scope is not needed.

[83stang]

Thanks for the help, if the 1645 is a poor choice, then what can you suggest that would work better? preferably Hammond. Can you post a schematic of the 12bh7 and 12at7 preamp? I can take the bias voltage by putting the filaments in series and voltage double it right? thanks for the help, I really appreciate it

[83stang]

The Edcors look really attractive, I might go with a pair of these CXPP60-8-4.2K, they seem to be a good option at a very fair price. It's shipping here and the exchange that'll kill me, but I'll try to get some numbers, they'll probably be way cheaper then hammonds.

[Eli Duttman]

Can you post a schematic of the 12bh7 and 12at7 preamp?

With a few changes in value, CB's 4-30 schematic is what you will use. Some minor alterations, such as CCS loading the LTP's tail, will be made.

I can take the bias voltage by putting the filaments in series and voltage double it right?

If you wire the 2X 6.3 VAC windings in series, a quadrupler, not a doubler, is needed. In addition have the bias supply and the heaters connected to the same source of AC is questionable. Buy the R/S trafo previously mentioned. It costs less than 7 USD. Dedicate a 6.3 VAC winding to the heaters in a single channel.