[John L Stewart]

John’s short note accompanying his amplifier invites us to comment & offer suggestions. I did 7 or 8 years ago & found that John was already happy with his amplifier just the way it was then. And I guess he still is. And John comments freely on the work of others.

Anyway, here is my take on some minimal changes that would result in a substantial performance improvement.

Cosmetically the photos show a clean but crowded chassis top. Not sure of the underside, the photo does not show enough detail.

John’s hand drawn schematic would not pass on a high school boy’s test. I mentioned this to him. John said he would redraw the schema but again that was many years ago. Still the same as it was in 1998.

The power output, distortion & bandwidth specs are given nowhere. We are not told what the OPT’s are but they do look like substantial Hammond iron.

John’s amp uses broadcast quality 600R to PP grid input transformers. Very expensive indeed, but thereby gets past the need for a PT.

First mod I would recommend is to add another 1N4007 & an electrolytic to essentially double the plate supply volts with a half wave voltage doubler. That would result in doubling the audio watts available. The OPT’s look to be well up to that.

John has posted the schema of a WE 140A amp along with his amp. This appears to be where he got the idea for an amplifier without a PT. The WE140A can run on an AC or DC supply. The possibility of finding a 110 vdc supply to run John's amp on at this point in time seems rather unlikely. The WE140A makes use of a pair of 25Z6 rectifiers as a full wave doubler while running on AC.

The output stage is a pair of beam tetrodes. Damping factor for the speakers is essentially non existent, so they are sure to flop around like a fish out of water. This amp does not have enough gain available to allow an effective application NFB.

Unplugging the 12SN7 drivers I recommend the 6SL7 with 100K plate resistors.
The gain improves by about 10 db. The 25L6 does not need the drive performance available from a 12SN7.

Two resistors of 270K connected from the 25L6 plates to the corresponding 6SL7 plates gets you approx 5 to 7 db NFB, depending on supply voltages. This results in a DF of about one, what used to be called ‘Unity Damping’.

Are these simple mods worthwhile? I would say so. What do others think? And what about John B who never seems to complete the myriad projects he talks about.

Would I build this kind of amp? Not likely. A PT is far less money than a pair of expensive audio line transformers.

Cheers, John S, who is still able to ID the hot end of a soldering iron!