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auralflea

Valve Amps and switching tubes

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This kind of follows on from PAGs posting last night - slow, that's me :nuts:

Firstly, a big thanks to Si and family for putting up with me in the middle of a bank holiday weekend - bringing a few tunes was the least I could offer in return, mate :D!

Thanks for your time and sharing the knowledge :).

As you the reader may be aware, I recently acquired a Tube Technology Unisis valve amp via the dreaded flea-bay. Whilst sounding very tasty, overall sound was really very dark, with bass seeming quite excessive as I got it settled in and got over the initial 'glow' - definite signs of bass bloom, to the extent that I needed to swap speakers from my Castle Conway IIs to my less bass-heavy Rega Jura's. Next step included 'socking' the ports to bring listening to acceptable bass levels in my little room.

Then early last week, one valve had a little hissy fit, went purple and started to get stinky :shock:. Needless to say, off switch operated very quickly - fortunately I was only a foot away at the time.

I had already decided a new set of valves was called for. So, last week the full complement arrived - 8 x EL84s, 4 x 12AX7's and 2 x 5751's. Replacement across the board was with Electro-Harmonix valves (big up to Tube Town Germany - super prompt and very good pricing at c.£60 for EL84s + 12AX7s - 5751gold E-Hs coming off e-bay very promply as well ).

As this amp requires biasing of the EL84s (in pairs) from underneath :shock:, I felt a bit intimidated so opted for asking PAG as a previous owner of this amp to lend a hand (literally). Must admit it is a bit scary seeing the amp powered up whilst standing on its side and having to have hands in where you know 500V is running around :zip:. I am very glad I opted for help on this - bravura is all well and good, however I am pretty sure I would have bollixed things doing this on my own with only the manual to guide me :dunno:.

So, what we dicovered - the valves in place were well past their best and biasing was somewhat low!!! [standard setting for this amp is -17V - biasing was found to be -13.6V :? iirc].

In went the new valves without hesitation and biasing reset to -15V, to allow for burn-in and settling - apparently should see the bias settle at/near -17V after the 20 hour run-in.

What a difference!

Bass bloom gone! Sound tightened up overall and much crisper than previously. Appreciating that this was all through PAGs (unfamiliar to me) very tasty Audio Physics.

As a base comparison, also ran Si's own TT Unisis Mk2 - sound patently similar although his tubes were obviously settled in with bass in place.

Right, well made up with the improvement, but really felt the full test would be in situ in my own system (couldn't wait to test at home).

After a very pleasant visit with a very hospitable family, back on the road to check at home.

First up the Rega Jura's.

YES! Improvement remains improvement :oj:

Sound is crisper and cleaner than before. Bass definitely reduced tremendously, in fact I would say it is in reality a bass-light sound at the moment. So first thing, out came the smelly old socks :lmao:. The difference in sound quality after switching valves is truly apparent. Given that there is a burn-in period of some 20 hours here during which time the bass will improve, I am guite happy to let it bed in with time.

Next up, putting the Conway's in circuit :^ and seeing what's what. I will post an update for those interested (if any.......:whistle:)

I suppose another possibility in due course is one I would never previously have considered...small tweaks in bais setting to change sound (although that is not something I will be rushing into :gag:)

Thanks once again to Si and family for a pleasant day - it even made braving those rainy motorways worthwhile :lmao:

Kevin

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Glad to hear it matey!

I think the biggest difference was substituting new tubes for the old ones, they where a tad tired, also, the bias was out which didn`t help. The Unisis is sensitive to correct bias, and the volt will settle to around -17v after the tubes have settled down. You`re ok with -18 to -15 anyway for it to be spot on, so i don`t think you will need to fiddle too much, just give it a check now and again between settlement.

Looks brand new now after the polish too, the previous owner certainly looked after it!

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PAG wrote:

Glad to hear it matey!

I think the biggest difference was substituting new tubes for the old ones, they where a tad tired, also, the bias was out which didn`t help. The Unisis is sensitive to correct bias, and the volt will settle to around -17v after the tubes have settled down. You`re ok with -18 to -15 anyway for it to be spot on, so i don`t think you will need to fiddle too much, just give it a check now and again between settlement.

Looks brand new now after the polish too, the previous owner certainly looked after it!

:oops: Oops, forgot about the bonus amp polishing facility offered in my visit, knobs and all :lmao:. Yeah, you're right about this being a looked after amp. I certainly scored it lucky. After that polish and my previous rather perfunctory wipe down, it still looks brand new! Think I will post a piccy later today to show off your hard work :D

That and the excellent bacon butties ;-)

Ta for the help and advice.

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audioboy wrote:

Very interesting read, love all the emotions:shock::P:shock::D:):(:oj::nuts::gag:

Without them, you'd be forced to read what I actually wrote......(Oi loikes a bit of colour, me)

Edited to add:

Given My off-beat (and often misguided) sense of humour as well as the potential for misunderstanding here of even the most paltry comments - as evidenced on innumerable occasions - I would rather telegraph my punches tbh.

Kevin

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Aural Flea wrote:

audioboy wrote:
Very interesting read, love all the emotions:shock::P:shock::D:):(:oj::nuts::gag:

Without them, you'd be forced to read what I actually wrote......(Oi loikes a bit of colour, me)

Kevin

I know, thats why i said i loved them:P:D

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very intersting chaps :) i think i may have to come to visit you in the very nearfuture Si..i'd like to change my valves just for confidence of knowing they are right. so what were these valves then Kevin? 50 quid sounds very appealing to my pocket :)

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Aural Flea wrote:

I had already decided a new set of valves was called for. So, last week the full complement arrived - 8 x EL84s, 4 x 12AX7's and 2 x 5751's. Replacement across the board was with Electro-Harmonix valves (big up to Tube Town Germany - super prompt and very good pricing at c.£60 for EL84s + 12AX7s - 5751gold E-Hs coming off e-bay very promply as well ).

Just looking at the bits we took out - the 5751s actually replaced 12AT7WA's. I wonder how much difference that makes to the overall sound??? Have looked at the table of specifications for each on TDSL - there is a difference but I cannot interpret what it is - any takers for an explanation? :geek:

Bourney - I bought electro-harmonix throughout m8.

8 x EL84 and 4 x 12AX7 from www.tube-town.de cost Euro 114.50 incl p&p 14.50 (about £75).

2 x 5751 (electro-harmonix gold) bought via e-Bay for just under £20 incl p&p.

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