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Old 3rd March 2014, 02:45 PM   #1
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Default Valve amp first build - questions

Hey everyone, apologies for the length of this post.

I've just got my first valve amp 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 an oscilloscope 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?

Any help would be much appreciated!

multimeter readings

B+ 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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Old 3rd March 2014, 03:00 PM   #2
SY is offline SY  United States
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Quote:
Originally Posted by emi23 View Post

First full switch on was terrifying
That's the right attitude! Always assume that on first switch-on, there will be sparks, smoke, and explosions.

Can you post a schematic? That will help with seeing if your voltages are OK.
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Old 3rd March 2014, 03:04 PM   #3
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Oh yes, sorry her we are:

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Old 3rd March 2014, 03:27 PM   #4
DF96 is offline DF96  England
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Naked valves! How do you stop the vacuum from escaping?

If you change loudspeaker impedance then you have two choices:
- leave the feedback completely unchanged
- move the feedback point to the output, and change the feedback resistor too (and possibly the capacitor as well).

Never switch between triode and UL while the amplifier is switched on, or even shortly after being switched off. Personally I would recommend never switching at all, as an amplifer needs to be designed for either triode or UL output (especially when feedback is used).

Remember that a single point, such as a heater pin, can't have a voltage (AC or DC). It is only two points which can have a voltage between them. With an elevated heater it is possible that an AC meter might be confused by the standing DC voltage. Or are you actually measuring half the AC voltage?

I can't see any screening (or twisting) on your input wiring. Do you get hum? Where is the ground return for the input wiring and the RCA sockets? Is it the separate yellow wire?
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Old 3rd March 2014, 04:23 PM   #5
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Hi there, thanks for the reply.

I was measuring dc voltage to ground, or ac voltage to ground in the case of the heaters. Maybe that's not that useful, they seem to heat properly. The input grounds are bussed and then connected to the volume pot and then the pcb ground with those yellow wires. Hum seems to be extremely good, probably because the inputs are so far away from everything else!

Perhaps I should just commit to a mode and feedback then. The thing is, it's my first valve amp, so I'd like to audition all the different options and see what they sound like, and you can't really compare things when it takes 20 mins to resolder everything.

What's that about naked valves and vacuum escaping?!
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Old 3rd March 2014, 04:33 PM   #6
DF96 is offline DF96  England
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Your circuit diagram shows naked valves.

Your inputs appear to be quite close to a valve socket, with its AC heater wiring. The large input loop you have created may pick up RF interference, although the metal chassis will reduce this.

The valve grid stoppers must be right next to the valveholder, not several inches away on the PCB. You may get problems from the extra capacitance of the wires, or you may get away with it.
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Old 3rd March 2014, 04:42 PM   #7
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Oh I see. Yes my valves are clothed in glass, or perhaps they're space valves.

Well I have some shielded cable I could use for the inputs, but when in UL mode with no feedback, only if I crank the volume to full I can just about hear mains hum, so I'm quite happy with the noise floor at the moment.

Yes I read about that grid stopper issue but I had already soldered the pcb so decided to try it anyway. What effect might extra capacitance cause?
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Old 3rd March 2014, 06:49 PM   #8
DF96 is offline DF96  England
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You could get instability, or better stability, or a treble droop in response - it all depends on what the feedback loop assumes.
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Old 3rd March 2014, 07:38 PM   #9
MelB is offline MelB  Canada
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Humm...measure the A/C voltage across the two yellow wires attached to the rectifier tube and see what you have.
The wire colors do not match what Hammond says they should be for that transformer.
Filament #1 should be 5v 3amp and be Yellow/Black-Yellow/Black
Filament #2 should be 6.3v 6amp and be Yellow - Yellow ???
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Old 3rd March 2014, 07:42 PM   #10
MelB is offline MelB  Canada
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Never mind..they have different things in different places....Oh Boy
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