First amp build - a Tubelab simple push-pull - diyAudio
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Old 28th February 2014, 08:34 PM   #1
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Join Date: May 2013
Default First amp build - a Tubelab simple push-pull

Hey everyone, apologies for the length of this post.

I've just got my Simple P-P up and running. I'm 25 and it's the first electronics project I've taken on, besides a few guitar pedals, so I've got quite a few questions I was hoping one of you more experienced builders could help me out with.

I built the amp, wired it all up, tested out all the grounding and continuity and then switched it on in various stages. First full switch on was terrifying but all went OK, in fact there was so little noise I thought the amp hadn't worked at all. In triode mode it's not really powerful enough to get enough volume out of my speakers (celestion ditton 33s) without pushing into distortion, but in ultralinear mode it sounds fantastic! And I haven't even set up negative feedback yet.

I went for a symmetrical layout and took the risk with wires from pcb to valve sockets. I chose a large chassis so I could space things out a bit, and put the inputs on the front side like the great Ongakus (http://www.sibatech.co.jp/kondo/img/Ongaku.jpg), to minimise noise. I also took the power supply off-board so I could position it near the rectifier and power transformer. So far the risk seems to have paid off, this amplifier is totally silent. Except when playing music that is…

Transformers - Hammond 370HX and 1650FA
Choke - Hammond 159S
Valves - JJ GZ34, JJ EEC81s and JJ EL84s
Power switch - DPST rotary on front
100k alps volume pot
3-way input selector switch
all resistors, capacitors and pcb from George at tubelab.com
Transformers, chassis and valves from Philip at bluebellaudio.com (UK based) who has also given me lots of valuable advice.

Questions:

- firstly, do all my voltage readings look correct?
- should all the heaters be reading 6vac or 12vac? or could it be my cheap multimeter?
- is a 3v variation between the different output valves acceptable?
- I don't have the means to test out what value capacitor to use for negative feedback, I have some 470pf silver mica caps, will they do the job?
- I've grounded the PCB to the chassis with a wire to the left negative feedback terminal, is a screw-down terminal safe for this job?

- I cut holes for 4 and 8 ohm speaker posts before I realised I could only hook one up to feedback at a time. So my workaround is a 3 position ON-OFF-ON 250VAC@15A DPDT feedback switch, with the top ON position for 8 ohms and the bottom ON position for 4 ohms. Is this a good idea or is there a better solution? Will I be able to use this switch with the amplifier on?

- I've bought 2 heavy duty 4PDT switches which I plan to make a 3-way triode-UL-pentode switch system for. They claim to tolerate 250VAC@15A and 380VAC@10A. Is that sufficient? Will I be able to use the switches with the amplifier on?

multimeter readings

High tension - 319 volts
Left ECC81/12at7
1- 202v
2- 104.7v
3- 106.6v
4- 2.8vac
5- 2.8vac
6- 104.7v
7- 0v
8- 0.87v
9- 0v

Right ECC81/12at7
1- 193.4v
2- 110.3v
3- 112v
4- 2.8vac
5- 2.8vac
6- 110.4v
7- 0v
8- 0.85v
9- 0v

Left lower EL84
1- 0
2- 0
3- 11v
4- 2.8vac
5- 0
6- 0
7- 317v
8- 0
9- 319v

Left upper EL84
1- 0
2- 0
3- 10.8v
4- 0
5- 2.8vac
6- 0
7- 318v
8- 0
9- 320v

Right lower EL84
1- 0
2- 0
3- 11.4v
4- 2.8vac
5- 0
6- 0
7- 315v
8- 0
9- 317v

Right upper EL84
1- 0
2- 0
3- 11.4v
4- 0
5- 2.8vac
6- 0
7- 315v
8- 0
9- 317v

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