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Author Topic: how to tell if valves need replacing?  (Read 7879 times)

gwEm

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how to tell if valves need replacing?
« on: December 13, 2005, 02:38:50 PM »
After all the positive discussion regarding the advantages of valve amplifiers on this forum, I invested in a little H&K valve amp on ebay.

It arrived last night, and so far i'm disapointed... However I strongly suspect the valves are shot.

Not knowing anything about valves, I'm posting here ;)

What I experience is a tone somewhat like my lame Yamaha practise amp (complete with belt clip! and overdrive button ;) ) when the batteries are close to running out.

At low gains, the amp works as I might have reasonably expected valves to sound (violin sounding distortion, subtly sustaining notes). But at higher gains it becomes progressively more fuzzy, the tone become glitchy and breaks up in an incredibly cheap sounding way.

When I tap the valves with my finger they are not microphonic, and they glow strongly.

The unit is a 'cream machine', I brought it because H&K are not known for cr@p gear... but maybe someone knows different?
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you wouldn't use the meat knife on crusty bread but, equally, the serrated knife and straight edge knife aren't going to go through raw meat as quickly

TwilightOdyssey

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how to tell if valves need replacing?
« Reply #1 on: December 13, 2005, 02:47:39 PM »
I guess the 1st question is: is the amp all valve, or a hybrid?

Also, power tubes don't have to be dim in order to be dead ... they will still glow even if the filament inside is dissintegrated.

Can you take any pics of the amp?

gwEm

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how to tell if valves need replacing?
« Reply #2 on: December 13, 2005, 02:51:34 PM »
it has a solid state buffer on the front end.

this is then followed by a 12ax7 preamp and a 12au7 configured as a 1watt power amp.

that goes into an output transformer, and if you like into a speaker.

or you can use the built in 'power break' (actually a 10ohm resistor) and 4x12 simulator.

i was using it in the 4x12 simulator configuration so far.
Quote from: AndyR
you wouldn't use the meat knife on crusty bread but, equally, the serrated knife and straight edge knife aren't going to go through raw meat as quickly

TwilightOdyssey

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how to tell if valves need replacing?
« Reply #3 on: December 13, 2005, 02:52:33 PM »
Hmm, have you tried it through an actual speaker cab yet?

PhilKing

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how to tell if valves need replacing?
« Reply #4 on: December 13, 2005, 06:35:48 PM »
Check the make of the valves and look up the characteristics here:

http://thetubestore.com/12ax70ectyp.html

or here:

http://www.watfordvalves.com/home.asp

If your sound differs from the described characteristics, then getting new tubes is an easy upgrade.  If gain is the problem, then I might be tempted to change the valves to use a 5751, which will cut the gain to 70% of the AX7.  This will give less drive and should give a less fuzzy tone.
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tewboss

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how to tell if valves need replacing?
« Reply #5 on: December 13, 2005, 07:09:57 PM »
perhaps one thing that should be pointed out it that valve amps come alive when the valves are hot - which is usually through volume rather than just gain. alternatively use something like a tubescreamer to push the amp into overdrive at a lower level - most overdrive/distortions work best with a valve amp.

Searcher

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how to tell if valves need replacing?
« Reply #6 on: December 13, 2005, 07:31:59 PM »
I'd email the eBay seller and ask when, and if, he last changed the tubes. H&K are my favourite amps (the big, expensive ones, anyway) and I don't think it should sound cr@ppy unless there's something wrong.
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gwEm

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how to tell if valves need replacing?
« Reply #7 on: December 13, 2005, 08:07:00 PM »
thanks for all the advice

regarding the tubes, they are the original H&K items which suggests they might be getting on a bit. however, the previous owner claimed he hardly used it. certainlly the tubes are neither very noisy nor microphonic.

i'll try with a speaker this evening.

i was mainly wondering if my symptoms sound like a typical tube reaching the end of its life?
Quote from: AndyR
you wouldn't use the meat knife on crusty bread but, equally, the serrated knife and straight edge knife aren't going to go through raw meat as quickly

chrisola

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how to tell if valves need replacing?
« Reply #8 on: December 14, 2005, 10:29:42 AM »
think it could be the valves:

http://www.harmony-central.com/Guitar/Data/Hughes___Kettner/Cream_Machine-1.html

loads of actual user reviews, the amp seems to be good for a few people :)
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Floyd Pepper

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how to tell if valves need replacing?
« Reply #9 on: December 14, 2005, 10:58:21 AM »
Quote
When I tap the valves with my finger


It wont make any difference but you're supposed to tap valves with a chop stick or something similar.  Don't want you frying your fingers now.  :)
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gwEm

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how to tell if valves need replacing?
« Reply #10 on: December 14, 2005, 01:22:49 PM »
well last night i plugged the thing into my 'new' 1x12 which chrisola sold me. its sounding considerably better like this... even it its still somewhat fuzzy at times. i can see what people say now when they talk about valves reacting to your playing.

floyd pepper:
Quote
It wont make any difference but you're supposed to tap valves with a chop stick or something similar. Don't want you frying your fingers now.

i used my guitar hand hehehe ;))

philking:
Quote
If your sound differs from the described characteristics, then getting new tubes is an easy upgrade. If gain is the problem, then I might be tempted to change the valves to use a 5751, which will cut the gain to 70% of the AX7. This will give less drive and should give a less fuzzy tone.

yes mate. this seems like an incredibly perceptive comment. after trying with a speaker i can clearly hear the 12ax7 gives way too much gain in the Cream Machine. i think i'll try a 12at7.

chrisola:
Quote
loads of actual user reviews, the amp seems to be good for a few people  

yes, i was encouraged by these reviews too! the amp was just 60 pounds on ebay, including postage. the 1watt of power through your old 1x12 would be easily loud enough to make my neighbours (i live in a flat)

as i said before, though the speaker its much better, but still rather fuzzy and too much gain. as the harmony central reviews say, the built-in 4x12 emulator isn't great and now listening through the speaker it seems reasonably clear that 12ax7 preamp needs taming at least. i might as well change the poweramp at the same time.

thanks for all advice!

i must say the fuzziness gives a pretty 70s edge to my tone though... i can do a great "sad wings of destiny" sound ;)

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Quote from: AndyR
you wouldn't use the meat knife on crusty bread but, equally, the serrated knife and straight edge knife aren't going to go through raw meat as quickly

HTH AMPS

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how to tell if valves need replacing?
« Reply #11 on: December 14, 2005, 08:54:45 PM »
if you want to take the edge off an ECC83, try a 5751 - less gain (popular with SRV in his Fenders)

 :twisted:

tewboss

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how to tell if valves need replacing?
« Reply #12 on: December 15, 2005, 03:13:34 PM »
why would you want less gain???

HTH AMPS

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how to tell if valves need replacing?
« Reply #13 on: December 15, 2005, 06:00:01 PM »
some people find ECC83s too gainy/buzzy - a 5751 has slightly less gain and can smooth out the tone to suit some players tastes.

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t3h haze

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how to tell if valves need replacing?
« Reply #14 on: December 16, 2005, 12:18:38 AM »
I take it you don't have a valve based power amp. This is where you reallt get the true valve amp sound, pre-amp valves mean nothing, it's all about what valves you have in the power amp stage of your amp (what makes the amp loud, rather than what makes it distorted).