Fixed Bias Valve Amp Question...

[brensabre79]

So I managed to bag myself a rack-mounted all valve amp quite cheaply on eBay for my experimentations. It's been serviced recently and seems to be in good condition too. Which is nice. But I am fully aware I'm going to have to get some new valves in this old thing (Peavey AlphaBass). It's got the stock ones in, which I believe are from 1989.

The thing is, it is a fixed bias amp. Now I'm aware some Mesa amps are fixed bias and that you can buy valve sets specifically selected for those amps, but my question is...
[list]
[*]Do I have to get a specific set of 6L6 valve to replace what is there? I've heard of matched pairs (this has Six 6L6 in the power stage) Will I be OK getting 3 separate matched pairs or do I need to get 6 matched valves?
[/list]
Also, as I'm never one to simply buy something off the shelf and use it as intended... I wondered whether it would be easy to change to KT66 or even KT88 valves without a bias control? Normally I think this is an acceptable upgrade when you can adjust the bias, but with a fixed bias I'm not sure it would even be possible...

Any (relevant) thoughts welcome

[Beer of the Bass]

I would advise contacting Peavey for the service manual, as they're usually very helpful. I'd be surprised if there was no bias adjustment in there. The term fixed bias usually means that the bias is set by applying a negative voltage to the valve, not that there is no adjustment.
If it really does have no adjustment, then it should be possible to have a trimmer added, if it's a fixed bias amp.

Edited December 3, 2013 by Beer of the Bass

[brensabre79]

Thanks, I have the owners manual, all I could get from Peavey. But no mention of changing valves or bias though.

there is a hum balance trim control on the back of the amp, thats about it. Apparently some people have added a global bias adjustment mod to theirs, but mine has not got this.

What I really need to know first off though is can I just swap the valves with mismatched ones or do I need 6 matched valves or 3 pairs of matched valves or something?

Edited December 3, 2013 by brensabre79

[johnny_frog]

the matching of the valves depends on the biasing...
Have just done a valve replacement and bias on a marshall guitar head, 4 EL-34's in the head, but they are biased in pairs so could buy either 2 matched pairs or a matched set of four.
Hot Rox give you the option of buying as matched pairs/quads/ etc.

If it's fixed bias (no pot inside for adjustment) then I would guess you would need a matched set of 6

[Skybone]

Fixed bias means that the bias is fixed, and therefore won't need to be adjusted when you change the valves. In other words, you can change the valves yourself without having to take it to a Valve Amp Technician.

Matched pairs: Most high output valve amps work in Class AB or "push/pull" (IIRC!). Where some Class A valve amps can work with a single valve, Class AB amps need pairs. However, with a fixed bias amp, you can put 3 different sets of matched pairs (1 pair at 1 bias voltage, the other two pairs at different voltages), and theoretically, because the bias is fixed, the amp will still work without having to have a trip to the Amp Tech's.

Beacuse of this, you should also be able to use pretty much any octal base (8 pin) power valves (6L6, EL34, KT66, KT77, 6550, etc).

Also, one of the other benefits of fixed bias is that your power valves will last so much longer than in a non-fixed bias amp.

It's always baffled me why more manufacturer's don't produce valve amps with fixed bias, though the Amp Tech's are glad that they do. And yes, I own a fixed bias valve amp that I can use any octal base power valve without having to get it re-biased each and every time I change valves (saying that, I'm still running the original set of valves it came with when I bought it 7 years ago, and it still sounds fantastic).

[Beer of the Bass]

[quote name='Skybone' timestamp='1386089716' post='2295590']
Fixed bias means that the bias is fixed, and therefore won't need to be adjusted when you change the valves. In other words, you can change the valves yourself without having to take it to a Valve Amp Technician.
[/quote]
Not meaning to be contrary, but you may have the terms confused a little. Fixed bias means that the bias is set by applying a negative voltage to the grid, which in many amplifiers is adjustable. The bias voltage [i]does[/i] need to be adjusted when changing between differently spec'd valves, and it is prudent to check the bias even when replacing valves with the same type.
The other common configuration is Cathode Bias, where the bias is set using resistors from the cathode to ground. Cathode bias is more forgiving of variance in the valves (within reason), not usually adjustable and I suspect the amp you are describing which can use a range of valves without adjustment is of this type.

Edited December 3, 2013 by Beer of the Bass

[brensabre79]

I'll echo that to say as I understand the Bias does need to be adjusted for different valve types on this amp.

I feared that i would need a matched sextet of replacement 6L6s. I suppose they will never be running at their full potential without being able to adjust the bias, but that tends to shorten the life of tubes anyway.

[Skybone]

But as I understand it, and one of the reasons that I bought this particular amp in the first place (as well as it being incredibly versatile, sounding superb, and being built in the UK), was the fact that because it is a fixed bias design, it enables the end user to change the power valves themselves, without having to take the amp to a tech. This particular amp works in both Class A and Class AB(1), and can be switched between classes and valve configurations to acheive a range of tones and outputs, from family friendly single ended operation to pummeling your eardrums at gigs.

But, one of the great features of this amp, is that you can use pretty much any octal valve (still have to be in matched pairs), and it'll still sound superb. And I have tried it a few times, with no detriment to the amp or the volume, through this experimentation though, I found that it sounded best with the stock EL34's.

I used to have an Ampeg VL503 (guitar combo) where you could flick a bias switch on the back and then change the power valves from EL34's & 6L6's, then simply adjust the micro pots at the back to get the valves into bias using rear panel mounted LED's.

Likewise, the Peavey head should sound best with 6L6's, but with a fixed bias, you should be able to swap them yourself. However, you should get the amp serviced by an Amp Tech & get him to change the valves for you at the same time.

[Roland Rock]

I don't know the technical stuff, but my amp was modded from cathode to fixed bias. Previously I could switch power valves no worries, but now if I do it ( hasn't happened yet) I'll need to take it to my tech for adjustment. At least that's what my tech told me....

[Beer of the Bass]

[quote name='Skybone' timestamp='1386171911' post='2296719']
But as I understand it, and one of the reasons that I bought this particular amp in the first place (as well as it being incredibly versatile, sounding superb, and being built in the UK), was the fact that because it is a fixed bias design, it enables the end user to change the power valves themselves, without having to take the amp to a tech. This particular amp works in both Class A and Class AB(1), and can be switched between classes and valve configurations to acheive a range of tones and outputs, from family friendly single ended operation to pummeling your eardrums at gigs.

But, one of the great features of this amp, is that you can use pretty much any octal valve (still have to be in matched pairs), and it'll still sound superb. And I have tried it a few times, with no detriment to the amp or the volume, through this experimentation though, I found that it sounded best with the stock EL34's.
[/quote]
What sort of amp are we talking about here? I still think it sounds like you're talking about a cathode-biased amp, which can be considered "self adjusting" to some extent. I wouldn't like to suggest to the OP that he could swap any octal valve into his Alphabass with impunity (without adjusting anything), as that may not be the case.

Edited December 4, 2013 by Beer of the Bass

[3below]

One key part to this is the Peavey circuit diagram. Armed with this (and an internet / books) you can work out what the circuit will do. Many (but not all) Peavey valve (tube) amps of the 1970s / 80s can 'auto' bias for different valves (I can not say about later - have not studied circuits). Circuit diagrams will also indicate B+ (HT) voltage that transformers can deliver - many Peavey designs deliver a lot - great for KT88s, can be too much for EL34s - I have considered using Peavey Deuce / VT for donor amp in HIWATT clone build - plenty of HT available.

If Peavy can not supply the diagram (unlikely, they are very helpful above and beyond diagrams, will provide detailed info e.g. power ratings of transformer low voltage supplies circa 1980) then similar diagrams will probably help. The 6 x 6L6 configuration was pretty common with Peavey - Mace etc. The circuit diags I have show 4 x 6L6 and 6x6L6 variants if my memory serves me right.

How much power do you want? 6 x 6L6 will deliver quite a lot of volume providing your bass cab can deliver [size=4] KT88s are not cheap, so the gain (pun intended) per £ may not be that significant.[/size]

Edited December 4, 2013 by 3below

[Skybone]

[quote name='Beer of the Bass' timestamp='1386191602' post='2297098']
What sort of amp are we talking about here? I still think it sounds like you're talking about a cathode-biased amp, which can be considered "self adjusting" to some extent. I wouldn't like to suggest to the OP that he could swap any octal valve into his Alphabass with impunity (without adjusting anything), as that may not be the case.
[/quote]

[quote name='Skybone' timestamp='1386171911' post='2296719'][b]However, you should get the amp serviced by an Amp Tech & get him to change the valves for you at the same time.[/b][/quote]

[Beer of the Bass]

Fine, but in your first reply you also said these things:
[quote name='Skybone' timestamp='1386089716' post='2295590']
Fixed bias means that the bias is fixed, and therefore won't need to be adjusted when you change the valves. In other words, you can change the valves yourself without having to take it to a Valve Amp Technician.
[/quote]
[quote name='Skybone' timestamp='1386089716' post='2295590']
Beacuse of this, you should also be able to use pretty much any octal base (8 pin) power valves (6L6, EL34, KT66, KT77, 6550, etc).
[/quote]

This gives exactly the opposite impression to the last quote about taking it to a tech.
I'm a bit concerned that you have a basic misconception about the meaning of the term "fixed bias" and that you could be passing on this misconception. There is a good explanation of the different types of bias here;
[url="http://www.aikenamps.com/WhatIsBiasing.htm"]http://www.aikenamps.com/WhatIsBiasing.htm[/url]

[Skybone]

Thanks for setting me straight on that, I am forever in your debt and bow to your superior knowledge and wisdom.

[brensabre79]

OK, thanks for all the info

I now have the schematic for the power amp. I'm guessing 3 matched pairs will suffice. I was told that this is a non-adjustable fixed bias amp and as such, without modifying to add a bias adjustment swapping the 6L6s for another will not be possible.

[Beer of the Bass]

I have had reasonable results using non-matched valves (mine were two matched pairs) on a four-valve power stage. I just used one of each pair on each side to keep things roughly symmetrical, then checked the bias using a bias probe to check that none of them were exceeding 70% dissipation at idle. If you have a bias probe or a friend with one there would be no harm in going for this approach with some new 6L6s, and if the bias checks out OK you wouldn't need to do anything else. You might well get away with just sticking in new 6l6s and checking visually that non of them are red-plating, but this would be a little more risk-taking that I would go for personally.
If any of them turned out to be running too hot you may need to tweak the bias supply. I would think the R9/R14 network would be the area to look at if the bias turns out to need adjusting.

[brensabre79]

Thanks Beer, so [i]IS[/i] this a Cathode Bias then?

Edited December 16, 2013 by brensabre79

[Beer of the Bass]

The Alphabass schematic you posted is very definitely fixed bias, as the negative voltage bias supply is shown. Cathode bias amps don't have this.

[3below]

Start looking for adjustable bias circuits - e.g. [url="http://hiwatt.org/DIY/bias.gif"]http://hiwatt.org/DIY/bias.gif[/url] Thank you MR Huss, or other amp circuits. It will soon be obvious how to design such a circuit. Next implement it (and the dropper resistors and test points) and make diy biasing easy. It is not that difficult. The pcb on the Peavey of that era should have nice big wide copper tracks and large components. These amps are great for repair / mods for those reasons alone.

[brensabre79]

[quote name='Beer of the Bass' timestamp='1387226575' post='2309665']
The Alphabass schematic you posted is very definitely fixed bias, as the negative voltage bias supply is shown. Cathode bias amps don't have this.
[/quote]
Thanks! I was getting a bit lost with the explanation above as I have no bias adjustment... So it's fixed & non- adjustable.

[quote name='3below' timestamp='1387227183' post='2309678']
Start looking for adjustable bias circuits - e.g. [url="http://hiwatt.org/DIY/bias.gif"]http://hiwatt.org/DIY/bias.gif[/url] Thank you MR Huss, or other amp circuits.
[/quote]

Thanks! So I'll have to mod the amp to get the most out of the valves in it basically. One for an amp tech as it's way beyond me to start messing about inside a valve amp. I imagine that it's pre-set pretty cold to be very easy on the valves then?

I posted in the amps section about this amp, I'm not sure how much more clean volume I will get from this by changing the valves and/or modding the bias circuit. Currently it's a little bit underpowered for my needs - I can just about get the volume i want, but its a bit too distorted at the loud end of the set. Which is a shame because I love the clean sound of this amp, I just need a little bit more headroom.

[6v6]

[quote name='3below' timestamp='1387227183' post='2309678']
Start looking for adjustable bias circuits - e.g. [url="http://hiwatt.org/DIY/bias.gif"]http://hiwatt.org/DIY/bias.gif[/url] Thank you MR Huss, or other amp circuits. It will soon be obvious how to design such a circuit. Next implement it (and the dropper resistors and test points) and make diy biasing easy. It is not that difficult. The pcb on the Peavey of that era should have nice big wide copper tracks and large components. These amps are great for repair / mods for those reasons alone.
[/quote]

It's even simpler than that IMO, looking at the bias supply in the schematic, there's already a potential divider where you could add in a potentiometer very easily (between R14 and R9, possibly tweaking their values a bit to provide a sensible range of adjustment on the pot), or just tweak the values without the pot if you expect the new tubes to last another 25 years

This mod should be very easy for any competent tech IMO, you don't even need them to add cathode resistors and test-points unless you want to enable DIY biasing in the future.

[6v6]

[quote name='brensabre79' timestamp='1387272360' post='2309987']
Thanks! I was getting a bit lost with the explanation above as I have no bias adjustment... So it's fixed & non- adjustable.
[/quote]

Yeah, "fixed bias" just means you apply a fixed negative voltage to set the idle current/operating point of the tube, contrary to what was mentioned by a previous poster "fixed bias" means you often (almost always IME) need to make an adjustment when changing tubes, unless you always use the exact same tubes as the manufacturer-designed bias circuit values were chosen for, even then it's not certain to be exactly right.

Cathode bias is, to an extent, self adjusting, but is only common in low power guitar amps, and even then you sometimes need to change the cathode resistor to avoid destroying tubes prematurely.

[quote name='brensabre79' timestamp='1387272360' post='2309987']
Thanks! So I'll have to mod the amp to get the most out of the valves in it basically. One for an amp tech as it's way beyond me to start messing about inside a valve amp. I imagine that it's pre-set pretty cold to be very easy on the valves then?
[/quote]

Well not necessarily - at a minimum, you need an amp tech to measure the idle plate dissipation, and figure out if the bias needs adjusting. Without actually measuring it, you have no way of knowing if it's set cold, hot or just right. If you're lucky, all will be fine, otherwise the tech will either adjust the fixed resistors slightly, or install a potentiometer to enable easier biasing in future.

To make it safe for DIY biasing, you'd need cathode resistors and test points installed, but this won't be necessary if you always get a tech to bias it, as there is another (less DIY friendly and much more dangerous) way of measuring the idle current via the output transformer.

[quote name='brensabre79' timestamp='1387272360' post='2309987']
I posted in the amps section about this amp, I'm not sure how much more clean volume I will get from this by changing the valves and/or modding the bias circuit. Currently it's a little bit underpowered for my needs - I can just about get the volume i want, but its a bit too distorted at the loud end of the set. Which is a shame because I love the clean sound of this amp, I just need a little bit more headroom.
[/quote]

Unfortunately, there's only one (expensive) way for you to find out how much more headroom you will get

[brensabre79]

Thanks 6v6. I think getting a tech to put in a bias adjust pot and some test points will be a good move IF the amp can go loud enough for me. I suppose first step would be to get a tech to take a look at how it is performing with the current valves in.

So, anyone know a good valve amp tech in Sussex? Having checked the sticky, they all seem to be relocating, or no longer taking repair work! The guy I normally use had my old practice amp for a year before returning it to me unfixed so I'm not inclined to go there again!

[brensabre79]

I found this Bias mod tutorial for the 5150 amp. the circuit looks alarmingly familiar!

http://www.gearo.com/Files/5150biasmod.pdf

Now I understand what you're all talking about i can probably have a pop at this

I would also need to get some bias testing equipment, aren't these simply adaptors that you put under one of the tubes to measure voltage/current?

Edited December 18, 2013 by brensabre79

[6v6]

[quote name='brensabre79' timestamp='1387377475' post='2311267']
I found this Bias mod tutorial for the 5150 amp. the circuit looks alarmingly familiar!

[url="http://www.gearo.com/Files/5150biasmod.pdf"]http://www.gearo.com...5150biasmod.pdf[/url]

Now I understand what you're all talking about i can probably have a pop at this

I would also need to get some bias testing equipment, aren't these simply adaptors that you put under one of the tubes to measure voltage/current?
[/quote]

Yep, that bias circuit looks basically the same.

Note if you are going to do this yourself that the components and pot need to be properly secured to the board (not like the photo in the pdf with some sticky pads, it will break off and leave your tubes with no bias, which is like running your car at max revs in neutral for want of a better analogy, you will trash your tubes quickly)

And yes, you can get some adaptors to measure the bias current, or a cheaper way is commonly to install some low tolerance (1%) 1ohm or 10ohm resistors inside the amp, from the power tube cathodes to ground. This is a little more involved than the bias mod so maybe adaptors with the resistors built in are the way to go.

EDIT: Also read up on how to safely drain the power supply filter caps, they stay charged for hours in some amps and will give you a potentially lethal shock if you touch them.

Edited December 18, 2013 by 6v6