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Old 11th May 2021, 9:19 am   #41
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: 807 tube amp, any ideas?

Just be careful looking at noise specs. Some low-noise RF devices are real stinkers down at audio, some are terribly microphonic. Often devices only get specified in the frequency range their manufacturer was targeting them at.

Also the noise behaviour can vary quite a lot depending on the impedance of the circuit/transducer driving them.

It's not as simple as low noise versus noisy devices, the frequency range and the impedances they are embedded in get in on the act.

THE low noise transistor beloved of designers of mixing desk inputs for studio microphones, was originally designed and marketed as a motor driver device.

Knowing which devices work well in applications outside their advertised markets and data sheet conditions goes under the heading of 'trade secrets' Of course when your competitor buys one and opens it up, they wonder why you used that one... and after a bit of testing, they find out. If type numbers are sanded off, then this really motivates them. Plastic cases are dissolved (nasty!) and die are photographed to try to identify what they're using.

All this goes on with transistors, it went on with valves.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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Old 11th May 2021, 9:48 am   #42
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Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 1,338
Default Re: 807 tube amp, any ideas?

Thanks David,
Yes I agree with your comments. As I have access to literally thousands of NOS valves I have the luxury of being able to test 10 or 20 to find if they will work
1.) at all,
2.) possibly,
3.) or total write off,

Circuitry will not change significantly, because cascode and srpp have proven themselves over many years to work OK. Buying lots of really cheap valves is one way around this quandry, as one can select and check each valve for suitability, with enough left over for replacements.
There may be a few resistor or capacitor changes made to suit impedance or frequency issues. R's and C's are very cheap today!!.

This present discussion is excellent for "blokes like me", because I am a pretty poor designer in every respect, but if I or nzoomed don't ask somebody with really wide experience, or bounce it off the "boffins", where does one look today?? Buy a very expensive CD player with 14 bit capability? that is junk 3 weeks after the warranty expires? Pardon to those digital converts .

This is NOT an attempt to take over this thread, because I think everybody who reads it and has an interest, will learn from it.

Remember too, that thinking outside the box, and using RF stuff that the Hi-Fi crowd have pooh-poohed is why I go there.

with respect

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Old 12th May 2021, 3:25 am   #43
Join Date: Feb 2016
Location: Tauranga, New Zealand
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Default Re: 807 tube amp, any ideas?

Originally Posted by joebog1 View Post
I would think you wouldnt drive a power amplifier directly in any case.
Some sort of "pre-amp" would be required, especially if a magnetic cartridge is employed. You will need RIAA filtering to make it work correctly. FAT high gain pentodes are sort of out of favour, although I use them in my phono stage. 6BR7,s. Modern ( ish) especially designed and constructed for low noise and low microphony. They have been changed out for CV4006. Same valve just different manufacture ( still Brimar UK ) but much lower noise again, at least in my application.

The Hi-Fi phoolies ( yes I am one ) havent discovered 6C4's yet. Very common, very cheap and are half a 12AU7A. VERY low noise too. They work superbly in SRPP or CASCODE stages in Hi-Fi setup's. If you want to play hi-gain, try 6AT6 or even 6AV6 single triodes. LOTS of gain, but with large gain you get large noise.

I am still building my power amp up, but that will be rapidly followed by my pre-amp, which will be cascode, and using RF twin triodes that were designed specifically for low noise black and white TV tuners way back, AND designed with cascode amplifiers in mind. Yes a tad higher frequency than Hi-Fi, but they have a screen between triodes in the same bottle. wound heater coils, and specifications showing which triode works for the top and bottom halves of the circuit. Check out 6BQ7A's to start with. If you want big fat
( octals ) valves only, I think you will pay big fat prices for anything decent and GENUINELY new old stock. My above hints on valves is because they are common, cheap, and available at hobbyist prices ( I am an old age pensioner, so read peasant).

Yes your right, I am also going to be working on an integrated preamp and RIAA stage.

I will be using E88CC/6922 for the RIAA stage and a simple preamp using a EF86 as a triode for the first gain stage and then an ECC82 and ECC83 in a baxandall feedback tone loop, I was going to start another thread on this actually about bypassing the tone stack, should I want to.
See here is you scroll down.

As far as Ef37A /6J7 pentodes go, they are a pretty robust tube and in some ways are better then EF86 as I believe they are not anywhere near as microphonic.
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Old 13th May 2021, 4:46 pm   #44
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Location: Wiltshire, UK.
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Default Re: 807 tube amp, any ideas?

If you want really low-noise input stages at audio, wire multiple identical valves in parallel [or - simply - strap the two halves of a classic twin-triode together and get the job done in one convenient envelope].

I've seen this done on a reel-to-reel tape recorder's playback amplifier. The same approach also works with transistors [the AC107 was used this way - in common-base - as a low-input-impedance low-noise 'head amplifier' for a ribbon mic].
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