Hi guys,
I thought I would offer these two Presto 40-B pre-amps here before listing on E-bay.
These were designed to hook directly to the Presto 88A cutting amps, using their B+ and 6.3 volt filament output connections.
If you wanted to run these on their own, you need an external power supply. I measured two 88 amps and the B+ is in the 260v to 280v range. There are supplies on Ebay that can provide both the B+ and 6.3 filament for pretty cheap.
These amps use one 6J7 and one 6SJ7 tubes. Both have Kenyon P400 output transformers. One unit has a UTC LS-10X input transformer, and the other one has a Kenyon P204 input transformer. They are currently wired for 50 ohms in, and 50 ohms out.
If you look at the front panels, you see cannon connectors - easy to swap out if desired. You also see an RCA cable looping into an RCA jack. That was a mod installed in 1966 to provide a patch point to access the amp output prior to the transformer. Why they didn't just use a normalized jack, I just don't know. In the center of the panels is a hole cover. I think originally there was a switch there to select several output transformer taps. That might make a nice place for adding a gain control.
I have not run these. I think due to age they should just be recapped right off the bat.
Attached is a schematic and data for the Kenyon transformers. I don't have the info for the UTC transformer.
I'm thinking $250 each is reasonable (plus shipping). The same UTC transformer recently sold for more than that alone.
Any interest?
Bryan
Two Presto 40-B Preamps
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- EpicenterBryan
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- Location: Eugene, OR USA
Two Presto 40-B Preamps
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- EpicenterBryan
- Posts: 738
- Joined: Sun Aug 10, 2014 9:01 pm
- Location: Eugene, OR USA
Re: Two Presto 40-B Preamps
Oops, a typo there....EpicenterBryan wrote: ↑Sat Dec 23, 2023 4:00 pmThey are currently wired for 50 ohms in, and 50 ohms out.
They are currently wired for 50 ohms input, and 500 ohms output.
Looking at the transformer details shows you can change those combinations if needed...
Bryan
- EpicenterBryan
- Posts: 738
- Joined: Sun Aug 10, 2014 9:01 pm
- Location: Eugene, OR USA
SOLD - Two Presto 40-B Preamps
Sold. Thanks guys.
Bryan
Bryan