Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Marshmallow on May 29, 2025, 11:11:57 pm
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Hi all,
Long time lurker. Have built a few amps from here. I've been digging around online but haven’t found much on KT88 Parallel Single-Ended (PSE) builds. Has anyone here built one or seen a solid schematic/design for a KT88 PSE amp? Thinking of making a Vibro champ reverb with two kt88 pse.
I have two single-ended output transformers on hand — both are 30W, 1.5k ohms — and I'm thinking about building a KT88 PSE around them. I’ve never built one before, so I’d really appreciate any advice, experience, or references anyone could share.
Thanks in advance!
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Same as for SE KT88 but load impedance reflected through the OT* is halved, and if cathode-biased, the cathode resistor range is halved. Your PT and OT need sufficient power rating to cope with the current (basically 2 x the power rating of a single KT88).
*Or you could have a separate OT and speaker for each '88.
You probably want to double the capacitance for C1, C2 and C3 in the attached schematic to help minimise hum, or find a really big HT choke** that you can sub instead of R2 on the attached schematic.
**which you may not be able to find.
For parallel KT88s, the MV pot on the attached schematic probably wants to be no bigger than 100kA (especially if you're going for 100% Pdiss), otherwise you may experience too much grid current
But IMO, a parallel KT88 SE amp would weigh a lot and see a lot of power wasted to get a teeny bit more dB gain. YMMV
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I would recommend individual cathode bias components for each KT88, rather than a single shared one. The KT88 isn't exactly a cheap tube, so if one fails you don't really want it to take out the other one too!