EL-37 tube conversion (Ampeg 835ish)

[glebert]

I bought this amp as a barn find project, no tubes (no original cabinet or speaker either). Not even sure how close this schematic is, mine might actually be a 830, but there are no schematics for those, and it seems the early Ampegs were somewhat idiosyncratic.

Power transformer tested OK. I don't have any EL-37, and they seem pricey, I was planning on converting to 6550s (because I have a stash of used ones). Heater current is slightly higher for 6550 (1.6 vs 1.4 amps). The original design looks like it uses an OC3 regulator for V6 to create the screen voltage (dropping 105V from the plate, it seems). Any reason to keep that if I am going to 6550s? They aren't expensive or rare or anything, I just don't have one on hand.

Thoughts?

Also, rectifier is listed as 5u4, I'm assuming that is what is commonly called a 5U4G, and that a 5U4GB wouldn't be appropriate?

835 Bassamp.pdf

You can read the unreadable tube designations better on this schematic for a similar amp.
835N Schematics 1.pdf

[mozz]

All 5U4gb have a higher current rating, otherwise will work fine for the older 2 variations.

[Helmholtz]

The EL37 was superseded by the newer EL34. Specs are essentially the same.

[Leo_Gnardo]

EL37 always intrigued me - some I salvaged from very old Ampegs had the classic coke bottle shape, interior parts assembled with tiny nuts and bolts. Last I saw any for sale, early 2000's someone in Cyprus had a stash and was flogging them on ebay at $125. I reckoned they were leftovers from British presence on the island, probably radar or communications gear spares. Around the same time also found a couple in a vacuum tube stall at Tokyo's Akahibara, even more expensive. (I hear the old electronics parts market there has vanished.)

Thanks Helmholtz, I wasn't sure whether they were more closely related to 6L6 or EL34. In those old Ampegs I settled on EL34 for replacements after trying 6L6 and finding better power output with the EL34's, a better match to the OT primary I'm sure.

[Helmholtz]

Both the EL37 and the EL34 are real pentodes (the "L" in the name stands for power pentode) having about twice the Gm of a 6L6, so require less PI drive.