Anyone built a low cost parafeed SE amp?

[Mick Bailey]

Removing DC from the OT frees up the possibility of using a wider range of ungapped transformers for SE amps. Champ-sized transformers are relaitvely inexpensive, but running a KT88 (or larger) means increasing OT cost. A choke loaded plate with capacitor coupling to the OT means that a much smaller transformer can be used, as the winding needs only to carry AC and not the constant DC of a conventional SE amp. The core can also be smaller.

But, the expense gets transferred to the choke.

Has anyone got practical experience of using low-cost alternatives to audio chokes, and what's the verdict on parafeed SE amps for guitar use?

[Gingertube]

Hi Mick,

In short - NO. I did one for HiFi with a Nickel Core Tranny and "flash" cap but it hardly seemed worthwhile for guitar. If I'm building SE for guitar then I want low power and a 6V6 is usually what I go for. If I want to do "arty-farty" I use a couple of 6J7 for the front end and an 80 for the rectifier..
For SE guitar I also often use an old electrodynamic speaker for glorious tone/compression, they tend to come with the approriate output tranny bolted to them.

Cheers,
Ian

[Mick Bailey]

Some of the jazz players like the big bottle SE amps and I get it when I hear them play, especially with vintage archtops. I think it suits that style pretty well and the higher plate voltage and additional power of the larger tubes gives the extra headroom, tonality and volume. I've got along with the hammond 125FSE but that's quite a lump for the output produced. I experimented with a lighting choke and motor capacitor - the cap was fine, but the choke didn't like the DC at all.

What's the repair and building scene like in Adelaide nowadays? I thought of setting up when I lived there, but 23 years ago there wasn't enough work at that time to cover overheads (that was when the big recession hit and there was no money around).

[Steve Conner]

I think a standard power supply choke would be worth a go. Just calculate how many henries you need to get your desired bandwidth against the reflected load impedance of the speaker.

A 100V line transformer might be worth a try as the OPT, or even a toroidal mains transformer (or two with primaries in series and secondaries in parallel)

[Mick Bailey]

Nice idea with the line transformer - I'll give it a go. Shame about the lighting chokes - they're so cheap compared to 'proper' chokes. Cheap for a reason. I'll crack on and come up with something with a standard choke for the time being. I've used mains transformers as a temporary substitute when testing amps with suspect OPTs. Sometimes they sound better than the original transformer.

[Gingertube]

Mick,
Repair and building scene has not changed in 25 years, same couple of guys building, one of whom I've been avoiding doing buisiness with for that same 25 years. Same guys doing the repairs too.

I do a few jobs as hobby work, tend to get the high end audio guys and the guitar guys who want something special restored (Just finished a (1965?) copper front panel AC30 and a 1968 Ampeg Echotwin). Tend to be few and far between since I'm a design EE in the day job and sometimes I get home having had electonics "up to the back teeth".

For SE at higher power you may wish to look up the "Mudlark" which was published in Silicon Chip (the magazine which said "We will never publish and Valve Amplifier design"). It was a parallel SE 6L6 HiFI design using series connected flouro balast chokes (approx 4H each from memory). Some clever tricks in the power supply too. The output trannies it used should be readily avalable. See here Silicon Chip Online - MUDLARK A205 Valve Stereo Amplifier. If you have trouble tracking it down PM me. I have some less than stellar scans of at least part of the article and the schematics.

Cheers,
Ian