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Author Topic: Revalving my amp  (Read 4024 times)

Toe-Knee

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Re: Revalving my amp
« Reply #15 on: September 09, 2012, 12:49:45 PM »
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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Toe-Knee

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Re: Revalving my amp
« Reply #16 on: September 09, 2012, 12:54:27 PM »
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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HTH AMPS

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Re: Revalving my amp
« Reply #17 on: September 09, 2012, 12:58:07 PM »
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).


_tom_

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Re: Revalving my amp
« Reply #18 on: September 09, 2012, 06:29:07 PM »
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!

Toe-Knee

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Re: Revalving my amp
« Reply #19 on: September 09, 2012, 06:37:59 PM »
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.

« Last Edit: September 09, 2012, 06:42:46 PM by Toe-Knee »
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jpfamps

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Re: Revalving my amp
« Reply #20 on: September 09, 2012, 08:56:43 PM »
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).

_tom_

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Re: Revalving my amp
« Reply #21 on: September 09, 2012, 09:31:37 PM »
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..
« Last Edit: September 09, 2012, 09:33:39 PM by _tom_ »

jpfamps

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Re: Revalving my amp
« Reply #22 on: September 09, 2012, 10:11:16 PM »
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.