Basics of valve amplifiers

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bostod
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#1 Basics of valve amplifiers

Post by bostod »

Hi guys. Could someone point me in the right direction to learn the basics of how these valve amplifiers work. I have an old world audio designs kt88 power amplifier.
From what I have found on Google it consists of 4 kt88 tubes for the power section and 4 small valves for the input section. Is this correct terminology or even correct. That's the sort of information I want to find out.
Secondly the 4 small input valves are mostly different with 2x Siemens ECC82 on the outer left and outer right, 1x GE 6AU6WC inner right and a Bramar 6***(cant read the rest) inner left. To my inexperienced eyes this doesnt look right. Should they all be matching or matching pairs?
Are these tubes good for the input part or are there other recommendations
Having said the above I think the amp sounds very good and the mixed valves might be like that for a reason.
Any help pointing me in the right direction would really be appreciated.
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Paul Barker
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#2 Re: Basics of valve amplifiers

Post by Paul Barker »

the two inner small valves would be same valve type, but can be different manufacture.

you mentioned elsewhere one valve lit hard then settled. That is a characteristic of some small signal manufacture, Mullard are known for it for instance but also known for excellent sound.

you can’t contemplate changing the chosen valve types, They are both quite unique and need specific settings another valve wouldn’t enjoy. Adapting that circuit much harder than a beginner could consider.
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Paul Barker
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#3 Re: Basics of valve amplifiers

Post by Paul Barker »

But improve the amp with better Pp OPT’s, I have some Sowter opt’s that would lift it. Also upgrade the coupling caps, and if youre bold enough, the resistors.

That is how you in you’re position could learn and experience which changes make which difference, and is it for better or just for differernce? Then pick the version you like.
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bostod
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#4 Re: Basics of valve amplifiers

Post by bostod »

Thank you for that. From your post it looks like I can swap out the valves for the same type and I would guess that's what the previous owner did to get the GE Bramar combination of the same type. Would swapping to a matched pair of same type but different manufacturer make much difference.
Now considering I'm a total novice, I didn't realise changing the transformers would make a big difference could you PM me with details on the sowters and how easy they would be to change.
In my ignorance I thought a bit of tube rolling was all it was about ;)
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Ali Tait
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#5 Re: Basics of valve amplifiers

Post by Ali Tait »

Changing the valves can alter the sound, a lot of people “tube roll” to obtain their preferred flavour of sound.

At a guess, the previous owner would have had matched pairs, possibly had a problem with one 6au6, and swapped it for one he had lying around. Different makes of the same type will work fine, most like to buy pairs made by the same manufacturer though.

6au6 are quite cheap, or they were the last time I looked, so a pair or two to try shouldn’t break the bank. Also lots of ECC 82 around, though NOS of the more prized names can be very expensive. Again, lots around, have a hunt on eBay.

Hotrox are a reliable supplier of valves, have a look there also. Good service too.
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Paul Barker
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#6 Re: Basics of valve amplifiers

Post by Paul Barker »

Yes I’ve got some nos Mullard military ecc82 last time I looked anything from 50 to 100 each. Not worth it, but I’m not letting mine go at the minute.

Bostod regards what makes big differences. If circuit built right valves working right.

Coupling caps, transformers rectifiers resistors would all improve any kit amp or commercial amp.

With the exception of LDA who use stuff that sounds good. The designer knows what he’s doing .
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
bostod
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#7 Re: Basics of valve amplifiers

Post by bostod »

So apart from the mallards. What would be a good ecc82 to try for reasonable money.
Same question for the 6au6's
Thanks
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#8 Re: Basics of valve amplifiers

Post by IslandPink »

I just bought a bunch of these for another project -
https://www.ebay.co.uk/itm/EF94-6AU6-SI ... Swu79demSf
They should be as good as anything , based on the quality of their other valves.
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Paul Barker
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#9 Re: Basics of valve amplifiers

Post by Paul Barker »

bostod wrote: Fri Jul 03, 2020 9:06 am Thank you for that. From your post it looks like I can swap out the valves for the same type and I would guess that's what the previous owner did to get the GE Bramar combination of the same type. Would swapping to a matched pair of same type but different manufacturer make much difference.
Now considering I'm a total novice, I didn't realise changing the transformers would make a big difference could you PM me with details on the sowters and how easy they would be to change.
In my ignorance I thought a bit of tube rolling was all it was about ;)
These Sowters. Are “ wrote:

Pr. Sowter UO82S OPTx 8Ka-a U/L taps//8R upright, zinc plated shrouds, used briefly about an hour, long leads - £150.00”
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Paul Barker
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#10 Re: Basics of valve amplifiers

Post by Paul Barker »

You’re welcome to try them and if u want them pay what I paid pay my postage. If no better send them back at you’re cost?
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Nick
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#11 Re: Basics of valve amplifiers

Post by Nick »

Just to point out that installing new OPT will possibly involve drilling new mounting holes and unsoldering and resoldering them in. You will still have the new mounting holes if you don't use them. Also to make the most of them the feedback components should be changed to match them which would require test kit.

Just saying what Paul is regarding as a simple thing to do, you may not. Sorry to be negative.
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Paul Barker
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#12 Re: Basics of valve amplifiers

Post by Paul Barker »

No problem Nick. No need to say sorry, it’s all true. Most of what we do casual enquirers seldom end up doing. we pass on our thoughts, we don’t attach a directive to advice.
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bostod
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#13 Re: Basics of valve amplifiers

Post by bostod »

Yeah probably too much for me to do. I have bought some Siemens 6au6's to try as reccomended so they will match the Siemens ecc82's.
Also bought some 6au6's rca black plates to compare. About my limit I think. will look at the kt88's when they begin to run out but that should be a while yet
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#14 Re: Basics of valve amplifiers

Post by pre65 »

If you want to know how valves work, read through this. :)

https://vacuumtubes.net/How_Vacuum_Tubes_Work.htm
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Paul Barker
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#15 Re: Basics of valve amplifiers

Post by Paul Barker »

As youve bought some alternative small signal valves, read what Stephie said on tube rolling.

“Are all tubes of the same "type" the same? No. Ever wonder why one
manufacturers tubes "sound" different than anothers? Here are four
sets of 12AX7 curves. 12AX7CHI.GIF shows a Chinese 12AX7, 12AX7TES.GIF
shows Tesla's 12AX7, 12AX7WA.GIF shows Sovtek 12AX7WA and 12AX7WXT.GIF
shows Sovtek 12AX7WXT. These curves are all different, and will sound
different in your circuit.”
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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