Preamp broken

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billybob said:
This thing is weird.
Click to expand...
And you are not wrong. there either.

The whole thing screams "gimmick" to me. Having had a look at the full schematic the actual "preamp" in the thing is transistors and IC amps; and the audio outputs are driven by IC amps.
The valves may be in the middle of the circuit but do not appear to do anything essential or even useful.
 
Just for info, these are some rather older and larger valves, the originals in a 1950 recorder I've been working on recently.
The filaments, cathode tubes and general structure are rather more visible in these:

 
Well I originally thought somewhere along the way power wasn’t reaching the tubes, but I guess it’s something else. Those tubes do look a whole lot brighter.
A friend of mine has two vintage reel to reels. With visible tubes and they get pretty warm!
 
billybob said:
Those tubes do look a whole lot brighter.
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I shaded the area before taking the photos, so the filament glow was clear. I think your photos were just in brighter ambient light so the filaments did not stand out so much from the background.

Re. voltages, you need to check the anode and cathode voltage of each valve section as well - the filament supply being on is a good thing, but that alone does not mean they are working.

If both channels failed simultaneously I'd look at power supply voltages before getting in to details on the audio path parts.
 
rjenkinsgb said:
The valves may be in the middle of the circuit but do not appear to do anything essential or even useful.
Click to expand...
they are in the signal chain to impart "tube sound" into the signal, and the current mirrors are there to linearize the output of the tubes beyond what you normally see in tube preamps, so they are adding a very slight "tube sound" without adding enough even harmonics for you to pick it out as distortion.... i'm sure the resulting change is measurable on test equipment, but so subtle that in a listening test you would hear a difference, but not be able to pick out what the difference is.

the current mirrors also act as level shifters so they can direct couple the plates of the tubes to the op amps without the phase shift of capacitive or transformer coupling.
 
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unclejed613 said:
the current mirrors also act as level shifters so they can direct couple the plates of the tubes to the op amps without the phase shift of capacitive or transformer coupling.
Click to expand...
I feel an opportunity to be educated here. Why would they want to remove the phase shift? Surely it would be near enough equal for both channels to not be noticeable anyway if they just used Cs or Ts?
 
throbscottle said:
I feel an opportunity to be educated here. Why would they want to remove the phase shift? Surely it would be near enough equal for both channels to not be noticeable anyway if they just used Cs or Ts?
Click to expand...

I doubt they do - it's probably just advertising bullsh*t, rather like pointlessly adding valves.
 
Nigel Goodwin said:
it's probably just advertising bullsh*t, rather like pointlessly adding valves.
Click to expand...
i wouldn't be surprised if some ad brochure for this preamp actually said it's a "DC coupled tube preamp" or something like that.... "audiophiles" are a highly opinionated bunch. some of those opinions are valid, but i've seen a lot of downright "snake-oil" stuff out there (very expensive snake-oil). one company makes preamps with a huge 50 lb power supply because their claim is that even for a preamp, the power supply needs to have an impedance of less than a milliohm or it will degrade the sound somehow.... and don't get me started on the "cryogenically decrystallyzed" audio cables or the "no electrolytic capacitors in the signal path" nonsense.....
 

There's a lot of gullible people out there, and many dishonest companies happy to fleece them.
 
This https://www.thecableco.com/gtx-d-g-duplex-pure-copper-receptacle-w-gold-plating.html one is good too.

 
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