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Building a Valve Amplifier Part 1: Design, Components and Layout

Dave Ives
I currently look after production at AB Open. I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.


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September 11, 2019 09:38

How do you get the 325v dc with this 2x 115v transformer? The circuit asks for a 2x 300v trtansformer.

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September 11, 2019 16:14

@Jdev99 230 volts (2 x 115v) is the RMS voltage on the transformer. But the peak voltage of an AC sign wave is the RMS voltage times the square root of two. 230v x 1.4142 = 325.266v

June 25, 2019 12:27

I'd like to see this project redone as a stereo amplifier.

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June 27, 2019 07:12

@Bonzadog the stereo version is on the bench waiting to be tested. Watch this space!

April 17, 2019 13:35

Hi Dave, I'm a bit confused about how you would build the power transformer with the two transformers you list. If you loom at the schematics for the classic CLC power supply on the RH84rev2 page the it would need a 300V-0-300V/120mA, 5V/3A and 6.3V/2.5A. The 117-6060 seems to be a isolating transformer and can't provided you with the 300V-0-300V.

Something like this seems ideal for this purpose:


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March 18, 2019 13:17

Thanks Sue - that's really useful information.

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March 1, 2019 08:54

Hi Dave,
Awesome project. Even more awesome is that these parts are available from RS.
A couple of things to note about the toroid HT transformer:
1. Mains type toroids can have a primary to secondary coupling up to 20 to 30 KHz, so are effectively full audio bandwidth for any noise coming in on the mains supply.
This is compared to a standard EI laminate transformer which has the primary and secondaries in separate sections on the bobbin - coupling there should be 1 KHz at best (worst?).
2. Toroids can also produce an audible hum if the mains sine wave is distorted, e;g; by computer switch mode power supplies. Check the mains supply with a DVM on it’s DC setting if it has a peak function.
By comparison the EI lamination layup gives a nominal gap so the transformer is less susceptible to small amounts of DC in the supply.

The HT should be applied ONLY once the valve heaters are warmed up. I would suggest a delayed relay and 555 (or other) timer for a few seconds turn on. A zero switching SCR could be used, although that might be adding too much “sand” to the project. A mechanical relay (with snubbing across the contacts) gives a nice satisfying clunk :)

Single ended amplifiers need well smoothed HT supplies, otherwise the ripple will modulate the signal.

Hope this helps.


April 2, 2019 08:51

At the risk of self-promotion, I have a 555-based circuit here passed on for interest only. Note the 100µF cap after the rectifier - do not reduce this to 68 or 47 µF if that's all you have - circuit doesn't work properly.

March 1, 2019 08:54

@Sue Interesting information on toroids, I was not aware of these characteristics - thanks for sharing.

March 4, 2019 09:04

All the part number links above result in a server error. The part numbers are RS component part numbers. I had to look these up on

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March 4, 2019 09:04

I'm following this project. Looks fun. I would have to modify the HT side to work from 110V American A.C. though.

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February 26, 2019 10:29

Hi Dave, This looks like a great project, do you have any idea of what the finished project might cost? ( a general idea of what you mean by "an economical route to take" ) Thx in Advance. Greg

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March 18, 2019 13:17

@gtnorth Hi - I don't at the moment but I'll let you know when we've progressed the project further.

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