Bare Knuckle Pickups Forum
Forum Ringside => Tech => Topic started by: _tom_ on September 06, 2012, 09:52:09 AM
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My amp has been sounding quite lacklustre for a while (seems "harder" to play, with less gain and power, just sounds a bit flat and lifeless without a boost) so I think it may be time for a revalve - plus I'm pretty sure the power ones are about 5 years old! It's a Laney GH50L. Currently got Svetlana Winged C EL34s (think they're these (http://watfordvalves.com/product_detail.asp?id=1481) ones) and JJ 12AX7s, and I'm pretty happy with the sound when they're all working fine. Wondering if there's anything else that's new which I should look at as I've been quite out of touch with guitar stuff for a while now?
Oh and if I replace my old matched pair (also bought from watford valves) with a new matched pair could I get away without re-biasing?
Cheers :)
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If you can get the numbers off the valves (or the boxes if you still have them) and get exact replacements from Watford Valves you won't need a re-bias. Your pre-amp valves should be fine.
I'd get a fresh set of Svetlana =C= EL34s :)
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Ah nice I do still have the boxes so I'll give Watford Valves a call soon :) Would the preamp ones not be the ones causing the lack of gain/life without a boost in front then?
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'flat and lifeless' sounds to me like power amp valves to me but replacing the V1 pre-amp valve will probably be all you need if you want to reinvigorate the pre-amp tone.
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Yep, what Elliot said :)
The Tung-Sol ECC-83s are nice if you want some more sparkle.
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I'd also recommend getting them from Hot Rox instead of Watford Valves.
I've dealt with Hot Rox a fair few times and those I know that have used Watford Valves have told me nothing but bad reports.
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I've used both and were pretty similar, though Watford were a bit slower I think. Can't remember, its been so long since I bought anything guitar related other than strings! Have to say that not having to get the amp re-biased if I give Watford the numbers on my valves is very attractive as it's an extra expense to get it biased (can't/have no real interest in doing it myself).
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But the numbering system may well be different, if Tom wants to avoid a re-bias.
I get my valves from Hot-Rox or Karltone:
www.karltone.co.uk
www.hotroxuk.com
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I prefer Hotrox too, have done for a number of years.
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I've used both and were pretty similar, though Watford were a bit slower I think. Can't remember, its been so long since I bought anything guitar related other than strings! Have to say that not having to get the amp re-biased if I give Watford the numbers on my valves is very attractive as it's an extra expense to get it biased (can't/have no real interest in doing it myself).
Biasing is ridiculously easy. More so than changing strings.
I taught Brow how to do it over the phone the other day its that simple.
Regarding the various places to get valves. In my experience even if you get the valves from Watford that means nothing. I've had matched "pairs" from them on a number of occasions that they state are matched within 1-2mv of each other than have a 13mv difference. Just a little something to consider.
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My experience when buying new output valves a years or so ago:
Hotrox - friendly, interested, helpful, nothing too much trouble.
Watford - arrogant, rude, stupid, the worst phone conversation I've ever had with a company I was prepared to buy from.
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I get mine from Karltone - hes is such a nice guy and a gentleman, has all the time in the world to talk you through options if needbe and give you advice. He even offered to help me rebias my amp by talking me through the steps on the phone-now how many sellers would do that!
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Just remembered someone's got some Tungsols for sale in Seconds Out, knockdown price for practically new tubes.
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I've used both and were pretty similar, though Watford were a bit slower I think. Can't remember, its been so long since I bought anything guitar related other than strings! Have to say that not having to get the amp re-biased if I give Watford the numbers on my valves is very attractive as it's an extra expense to get it biased (can't/have no real interest in doing it myself).
Biasing is ridiculously easy. More so than changing strings.
I taught Brow how to do it over the phone the other day its that simple.
Regarding the various places to get valves. In my experience even if you get the valves from Watford that means nothing. I've had matched "pairs" from them on a number of occasions that they state are matched within 1-2mv of each other that have a 13mv difference. Just a little something to consider.
^ +1
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Ok so a bias at the place I took it to before costs about £35.. would that be better spent on a Bias Rite (not sure I want to poke around a live amp with a multimeter)? tbh I am still wary of messing around inside an amp but I guess if it's easy to do..
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It definitely would.
You can get some that also read the plate voltage as well as the bias current which makes it even better as you don't have to open the amp up at all unless the trim pot is internally mounted.
If its the latter you may be better off with just a regular old multimeter. If you have any issues or concerns at all feel free to PM me and i'll gladly work through it with you.
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I did a bit of googling and it seems the bias pot in the gh50l is behind the treble pot so you will have to open the amp up.
However these are the bias probes that I was talking about
Eurotubes Pro One $89
https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=108&sub_category_id=109
You may be able to find something in the UK but those are the ones that I've heard a lot of good things about and you may as well grab some valves from there too seen as you are already paying the shipping.
Doug is super friendly and gives great advice.
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Even though a bias job is not an especially difficult or time consuming job, you would do well to remember that you're working on a live amp with voltages that could kill you. Show respect to the situation, be prepared and do some reading up - ask as many questions as you want, even the ones you think are dumb and pointless. Better safe than sorry.
Speaking from my own perspective, when someone brings an amp to me for a bias job, they invariably get a wee service too - spray out the pots and bases with aerosol spray (Servisol) and I'll check the voltages through the amp against a schematic to make sure everything is ok, plus of course a visual check (moreso with old amps) as the amp can be working, but have a charred component ready to fail (moreso in newer amps with questionably low rated resistors in certain places).
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Ugh this is going to be expensive! Didn't realise the bias tools were that kind of money, i thought the bias rite was about £50!
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You could just do it with a good old multimeter but then youll have to take more measurements and probably add some resistors in.
Also this is going to be a one off cost so it isn't too bad when you look at it that way as it will last you for life.
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You can make a bias probe that aids measuring HT and cathode current quite cheaply, BUT this would involve wiring something to the amps HT so has mucho potential for hazard.
Tube Town have an inexpensive kit for measuring cathode current in octal vales:
http://www.tube-town.net/ttstore/index.php/cat/c174_Bias-Adjustment.html
Obviously this won't help you measure the HT, but if the bias pot is internal then you would have to open the amp up anyway (all caveats regarding high voltage apply of course).
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Ah i guess one of those + a multimeter would make it a lot more affordable! What is HT, why do you need to measure it and how would you measure it?
I do actually need to open up my amp at some point soon to spray some servisol into some scratchy pots so i guess i could do it all in one..
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HT stands for "high tension" and is the high voltage supply to the amp. In the US it's referred to as B+, so you will see this terminology used in texts about amps, and on internet fora.
In a typical 50W valve amp the HT will be around 450-500 VDC, and needless to say is quite hazardous.
Ideally you need to know both the current draw through a valve and the HT voltage on the anode so you can calculate how much power the valves are dissipating at idle (W=VI).
Knowing the power dissipation is important to avoid imperiling the valves by running them too hot.
The usual recommendation is to bias the valves between 50 and 70% of their maximum dissipation. I tend to bias towards the lower end of this in amps with higher HT and solid state rectifiers, whereas you can bias a bit hotter in amps with valve rectifiers.
Ideally the HT should be measured with the valve drawing current as the HT can sag quite as the power valves are biased to draw more current. Unless you have a bias probe that will help you do this, then you will need to run the amp open and measure the HT of the valve pins or first filter cap, which exposes you to some potential danger.
Alternatively you can "guestimate" the HT from the schematic diagram and use this to calculate the power dissipation. This shouldn't be too far out and is usually good enough for rock and roll.