Anyone know anything about valves?

[Retropin]

Ive never got the sound to work on the peep show machine and over the last few weeks ive been mucking about with it. It has a valve amp connected to an opto receiver valve.

All I get from the amp is noise, ive realigned the opto receiver and adjusted the slit that allows the light to fall on the sound strip on the film (16mm) and I do get some harmonics when film is playing, but it is very very noisy.
I know nothing about valves and cant get my head around them.. my old man is an electronics engineer and started out at Mullards in the 50's.. he rebuilds valve amps etc but he lives in Nth Wales and emailing back and forth is a slow process..LOL.
OK, so what I have is a very noisy amp, but if we turn the amp over to see the circuit, I have some issues..



Top RHS are 3 loose wires.. the black wire I didn't really have an idea where that went to but is was the perfect length to go to ground(-ve side of cap).. also solder connection here looked like it had something come away.
The 2 RED wires have got me really puzzled.. they come from the transformer which has written on it..6V ( heater).. 300V ( straight to valve)..CT ( centre tap?).
I put my meter across them and got nothing... looking at the transformer, they seem to come from where CT is written so I ganged them and ran to chassis ground.. not a good idea as quickly I could smell a burning smell and wires were hot to touch.. so... turn on again and quickly short these 2 wires out and I get a spark which means they have potential,
But still I get nothing on my meter..
Went back to these 2 wires today and noticed that my meter was tripping out.. it only goes to 700Vac and intial reading before it trips out is some 1400Vac across the 2 wires.. that's why I couldn't get anything across these.. its too high a voltage.
So now im REALLY confused! What is a voltage this high for? all valves seem to be connected correctly

[Marty Machine]

1400v seems beyond excessive for that tiny amp.....even 500watt guitar amps i work on only use 600-800vdc at best.

What is the number on the valve? as the first digits will confirm what the heater voltage should be.
e.g. a 12AX7 (typical dual-preamp) requires 12v heater voltage, while a 6L6 (output valve) requires only a 6volt heater voltage.

Is there only 1 valve or more?

I'd also replace all those hi-voltage electrolytic caps, they dry out and start conducting(shorting) DC voltages to ground.
You can remove the Negative/GND end off the chassis and measure using DC meter from the negative pin to chassis, if you read ANY DC volts, your cap is passing DC and needs replacing before it smokes/blows.

Hope it helps a little....
MM

[Retropin]

Cool.. I knew someone would know something about valves..

All valves have a 6V heater and after looking up the specs none need a whopping 700V.. all are 300V which is present and correct.
I have:
6v6Gt
6X5GT
6AU6WA
 and another I cant read.. need to borrow my wifes glasess.

I did a bit of reading and only requirement I could find for a voltage near 700V is a swinging choke but that would be on the power supply and not the amplifier yes?

[Marty Machine]

It's almost like something (a choke?) has been removed altogether, leaving behind those 2 red wires, dunno exactly.

[Retropin]

I appreciate your input Marty... im on a steep learning curve here with valves and having to dig out of my memory all my AC theory I did when I did my electronics course... its very true, if you don't use it you lose it as im looking at my old notes and struggling to make sense of them... oddly enough, at the time, I much preferred AC theory to DC!

It seems that my opto multiplier might require the high voltage as most are a high voltage high impedance device..... wish I knew more about these things

[Mr Pinbologist]

I do a lot of work with valve amps, but its hard to know whats going on with those cut wires without seeing the amp in the flesh.

As Marty said, the electrolytic caps can dry out and cause all kinds of issues. The coupling caps in the signal path can also cause a lot of issues if they become leaky, putting several hundred volts into the grid pins of the next stage, which is never a good thing!! I had that issue on an old 1962 Moody guitar amp i fixed recently.

I'm guessing the octal socket i can see at the top of the pic is the socket for the 6V6??

[Zedman]

Ditto the advice already supplied. Remote servicing a unit without schems sounds like a challenge.

MmmmMoody- nice!

Z

[Retropin]

Ditto the advice already supplied. Remote servicing a unit without schems sounds like a challenge.

MmmmMoody- nice!

Z

Lol- its always a challenge and working without schematics is all part of the game... if you wait to find schematics for these kind of machines, youll never start work on them.. Peep show machine was a pile of bits when I got it and ive had to guess/ learn every part of the path... only the sound to go now

[ddstoys]

Valves are way way way before my time but recently replaced the valves in my old jukebox and wow what a difference.

  Best of luck Gav ill hold off on sending my new addition wouldn't want to drag you away from this little beauty

[Retropin]

Valves are way way way before my time but recently replaced the valves in my old jukebox and wow what a difference.

  Best of luck Gav ill hold off on sending my new addition wouldn't want to drag you away from this little beauty

Nah.. send it up mate... is all good. I know what im doing with yours..LOL

[big dog]

The voltage is double what it should be - suggest he's running a 115 V transformer on 230 V.