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Advice for Super Cheap & Simple 829b based amp from spare parts

Updated: Advice for Super Cheap & Simple 829b based amp from spare parts

Hi All

So, I now have 2 very nice amps working really well, my Miniwatt N3 and my big beast Audioromy FU-29 / m828a 829b based amp.

I upgraded the tubes in this to Westinghouse 829b's so I have the chinese FU29's left over. So I have the following spare tubes and was thinking, hey, why don't I make something from them!!!

- 2 * FU29 829b's
- 2 * EL84 Shuangung chinese things
- 2 * GE JAN 5670's
- 1 * 12ax7 Shuangung chinese thing

I've got shedloads of various electronic components, switch mode power supplies and some microcontrollers I would like to use for power switching and delays etc.

Case components I can make on the CNC so ignoring metalwork and concentrating on circuits, what could I do using the cheapest possible transformers.

Really I just want to make use of these things at the lowest cost and run a pair of Tannoy's off them.

I was thinking as I have the FU29's, push pull, following a schematic similar to the audioromy.

Could the EL84's be used to drive the FU29's?

For the output transformers I was hoping to use cheap toroid's. I understand the FU29's like a high load impedance, something like 8k. Would this be possible using some cheap power transformer toroids. It seems some members have had great success using toroids.

Keeping the costs to a minimum is a key here. I will aim to use a toroid for the input transformer too, bridge rectified to get to about 430v DC. I will use switch mode power supplies (meanwell's) coupled with dc-dc regulators to do the heaters. I already have one suitable to do the small preamp tubes, will need a slightly larger one to do the FU29's

So, list of bits in my head:
- Potentiometer
- PSU Caps
- Coupling Caps
- Power Resistors for Anode / Cathodes (got loads of standard resistors, may need some precision ones)
- I've got LM317's so can do bias control with these
- Input transformer (toroid)
- 2 * Output transfors (toroid)
- 829b sockets
- 12ax7 / EL84 / 5670 sockets

I have LTspice so can start playing with configurations that are suggested.

Any thoughts would be greatly appreciated. I don't mind this being a work in progress that gets upgrades later on down the road.

I like the idea of using my surplus kit because I very much like the sound of the Audioromy 829b so if I can use these in a new custom build and get anywhere near the fantastic sound of the Audioromy then I'll be happy!

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Perhaps since you already have a push-pull amp, you might like to try a single ended amp. An inexpensive way to build such an amp is to use parafeed loaded by a resistor. The output transformer can be small and very inexpensive.

Here are a couple of examples that are fun and might give you some ideas.

Audio ratbag: 829B lightbulb amplifier

Audio ratbag: Lumiere

I have used both of these amps with my Tannoy 15 Reds and they work fine.

Hi All,

So an update - I've spent the last few days being obsessed with LTSPICE.

Firstly thanks for reply ratbag, saw those in the past and thought they were interesting. I've got a nice miniwatt N3 with although not a SET is more like a SEP and sound very very good. Love it to bits.

Anyhow, I am keep on a powerful as I can make with whats available.

So messing around with LTSPICE - wow, what and amazing piece of software, love it. Really remarkable and easy to use. Anyway, I have gone mad with it and played for hours, hours and hours, learning a hell of a lot. I am more of a electronic systems engineer and low level analogue electronics is newish to me, tubes are especially new so I expect a lot of problems to be lurking in my circuits.

Anyway, I have attached all of my information for review, comment and for me to hopefully learn, fix and make.

Summary (per channel basis):
- 1/2 of 12ax7 as input phase splitter
- 1 * 5670 1st stage to gain up to ~7v pk-pk
- 1 * 5670 2nd stage to gain up to ~50v swings into the 829b
- 1 * 829b output tube in push-pull config. Each half of the tube does a push or a pull.

I am still keen on using toroids as output transformers. I believe I need about an 8k output resistance to work best with the 829b?

It would be great if the experts might want to leak some of their awesome knowledge to help me tweak this amp. I've done loads of analysis and annotated and commented this and the schematics, as attached. The LTSpice model and tube data zipped is attached too.

Cheers for any help and comments!


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