Kay 704 Transistor Amp

[unknowntome]

Hi all,

I’m in the early planning stages of building transistor Kay Vanguard 704 amp.

I’m based in Australia, so I’d really appreciate advice from anyone who has built, restored, or replicated one of these regarding:

* Safe adaptation from original US 120V mains to Australian 230–240V
* Suitable modern transformer options/specs
* Recommended germanium transistor substitutes or modern equivalents

Any guidance, build notes, or experience would be hugely appreciated.

Cheers

[Stevem]

That phase splitter / driver transformer is going to be a BIG stumbling block!

[unknowntome]

Thanks for your response!

Can you explain how/why it’s such a big issue? If there a work around solution?

[R.G.]

+1 for the driver transformer, stevem. That's the key to the whole power amp. That being said, those very rarely if ever fail.

I studied that output stage for years off and on in my mania -er, enthusiasm for Thomas Vox amps, which use the same driver transformer approach. It is generally possible to replace the germanium output devices (Q4 in the schematic) with silicon devices by changing the value of a couple of the biasing resistors (R13), in case you need new/different output devices. Thomas used both kinds.

On the power supply issues:
For safety's sake, when you re-wire the AC input to the PT, change it to a three-wire AC mains cord with safety ground; remove the "death cap" (C11); rewire the incoming AC mains for three-wire standards, that being the incoming AC "LINE" to the fuse first, then the AC power switch, then the transformer primary. Neutral goes just to the PT primary. Australia may have regulations on exactly how the AC safety ground has to be attached to the chassis. I know how to do this for the USA, but Oz may be different.

On the conversion to 240/250Vac: If you can, get a new similarly sized PT that has a 240/250Vac primary and the same/similar secondary output voltage. It is a quirk of physics that a transformer designed for a given power throughput and power line frequency is very nearly the same size no matter what the primary voltage is, so a new 240/250 primary transformer with the same output voltage and power will be close to the same size. It is also possible to replace the PT with a toroidal PT with similar output voltage and power. This will in general be smaller than the original E-I power transformer.

That is, if you can find one. Failing that, your options are to use a step-down autotransformer between the amp and the AC mains or have the original power transformer re-wound to keep its size and original look. The autotransformer will be PITA to remember to use every time. Re-winding can be costly.

Replace all the electrolytics with new ones. Don't mess with trying to re-form the old ones.

There's more if you need it. Ask questions.

[martin manning]

I think this might be a clone build? In that case the transformer might be hard to source, maybe even hard to find a substitute without an original to measure or some more detailed spec.

[sluckey]

Here are all the parts for that amp in one neat box. Probably cheaper than buying individual parts.

Kay 704 parts

[nuke]

I don’t know what it would take to export one from the USA to Australia, but these amps are fairly common and not expensive to buy. They’re not particularly awesome.

The interstage transformer is the long pole. You may be able to wind your own. I’m not sure where you could source one, other than salvage from another amp.

It is an interesting choice of project.

[R.G.]

I think this might be a clone build? In that case the transformer might be hard to source, maybe even hard to find a substitute without an original to measure or some more detailed spec.

Oops. I missed that. Thanks!

Yes, if this is a clone build, the driver transformer is the hard part to get. There will not be any source other than cannibalizing one from another similar amp.

I did spend bits and pieces of decades chasing the similar Vox circuits, and actually reverse engineered the one in the Beatle models. It is possible to make a driver transformer from a small AC transformer (for the iron and bobbin) and some copper wire. The down side of this process is that it's a lot of work to gently disassemble a transformer into parts that can be re-assembled. The winding is not horrible. These things all use a winding pattern of 1/2 the primary, then both secondaries wound parallel ("bi-filar") then the other half of the primary.

Getting/estimating the number of turns is an interesting problem.

[LOUDthud]

Nelson Pass designed a power amplifier that used a really expensive Jensen transformer.

Link: https://www.diyaudio.com/community/thre ... er.216616/

The transformer has four identical windings, you use two in series for the primary and one each for transistor drive.

[nuke]

Interesting patent on this related OTL, quasi-complimentary output with an interstage transformer and an improved biasing method.

Gives some interesting background on the problems with germanium transistors of the early 1960's.

US Patent 3,142,807

https://ppubs.uspto.gov/basic/

Search for: 3142807


Here's a complete set of Vox T60 transformers. Very similar design.


https://reverb.com/item/96267247-1964-1 ... interstage

Several projects on Ampgarage restorations of T60's . Do a search.

Not the same, but similar.

[R.G.]

If you get into doing it yourself, the writeup I did for the Beatle driver is here:
http://www.geofex.com/Article_Folders/T ... former.pdf

This has the whole method in it. What would be different is that the Kay output stage probably doesn't need as much base current to the output transistors, so the driver transformer could be much smaller. The Thomas Vox "Buckingham" and "Viscount" models used a pair of germanium output transistors to produce 30W, so f you found a driver from one of them, it might just work.

[Stevem]

You might turn up a cheap old SS amp to get driver transformer out of.
The early 70s Sunn amps like the Concert Bass used one as shown in my photo.

The Sunn guitar version used the same output section.

The 66 thru 67 Kustom amps used one also , but unlike the Sunn with its 4 secondary windings ( one for each output device) the Kustom has only 2 secondary windings for all 4 output devices which is more like the Kay.

[R.G.]

I did a little looking into the schemo. The output devices (2N554) limit the total power supply to something under 16V, their BVceo. Probably about 12-ish. It's specs say 5W, I think.

This makes for a much easier driver transformer. I saw some of the pics of the insides at Tone Gems. The driver trans looks to be the size of a small filament transformer, so the donor transformer for a self wind should be easy to find.

[didit]

Will "strawman" with the Hammond 145E. Specs are: 1:1 600 Ohm both sides CT, with sensible DC resistance and 200mW power rating.

https://www.hammfg.com/electronics/tran ... io/143-146

There are a few others listed here that might also work but this one was the closest to "wet finger" estimates based on R.J.'s notes, the schematic, and transistor datasheets.

..

[R.G.]

Did some more looking.

The output transistors and the input transistor would be the most special of the bunch. The outputs (2N554) can be subbed by many other types. The trick will be finding pairs that you can actually buy. I did find a good choice, I think. The 2N1545A ought to work, and I found these for under US$4 each.
The input transistor is special because it's a high current gain type, hfe of 200+. The 2n408 and 2N591 will be pot luck.

You might consider getting it running with silicon then moving to germanium as you find transistors.