Valves required for Audio Innovations S500

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Savo

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Hi, one of the 4 valves on the back row of my amplifier only has a barely detectable glow, the others all glow brightly. I have swapped them all round and it's just that one valve which is dull. All the valves in the amp are over 15 years old and the amp has been well used over that time. The markings have worn off the valves so I'm not sure what type they are. Has anyone anything suitable for sale or advice on what and where to buy?

Thanks

Andrew

 

carlhowe

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Are they el34?

Is this amp fixed bias....which would mean a new set of valves will need a rebias.

I hope the the "glow brightly" does not mean the plates are glowing, just the heaters.

 

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Hi, one of the 4 valves on the back row of my amplifier only has a barely detectable glow, the others all glow brightly. I have swapped them all round and it's just that one valve which is dull. All the valves in the amp are over 15 years old and the amp has been well used over that time. The markings have worn off the valves so I'm not sure what type they are. Has anyone anything suitable for sale or advice on what and where to buy?Thanks

Andrew
The 4 valves on the back row are EL34 output pentodes. The amplifier runs in cathode bias so you will require two matched pairs or a matched quad.

http://www.trioda.com/index.php/en/gallery-diy-en/4561-amplifier-audio-innovations-series-500

https://www.watfordvalves.com/products.asp?search=EL34

 
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Colin151

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This amp is auto bias. You'd ideally need to replace all the tubes. The push pull class A operation in these amps runs the output tubes very hard and many EL34s can't take it and short circuit. The only recommended ones that are supposed to be reliable are EH or Sovtek EL34s. A new matched quad of EH EL34s will cost around £60 delivered from Hotrox. The other alterantive is to go for Russian 5881 tubes which give a bit less power but are tough as nails and are ultra reliable in these amps.

http://www.hotroxuk.com/electro-harmonix-el34eh.html?utm_source=GoogleBase&utm_medium=organic&gclid=CKyfjvH6pMoCFYMIwwodOAgABw

http://www.hotroxuk.com/sovtek-5881-6l6wgc-slim-base.html

If your speakers don't need the power I'd go for the 5881s to be on the safe side...

I would not recommend replacing just one of your old EL34s tube with a new or used tube. For one you don't know what the brand of the current tubes are and the amp will not work optimally with non matching tubes.

 
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crashhot

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as above, you've got another is there, just remember when you've changed the valves they will need running in for A while before they Reach their optimum , probably be a good 50 hours.

Also, as they are 15 years old I would change the lot, don't just change the one that''s gone…

 

Savo

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Thanks for the quick replies! About 15 years ago the amp nearly went on fire with a flash and burning smell etc, when it was repaired it cam back with the valves in it that it still has now which I think are EL34. I think the amp isn't sounding right at the moment, my speakers are early Snell K so I presume I'd be OK going with 5881s from hotroxuk but will it change the sound? Should I change the 5 smaller front valves while I'm at it?

Thanks again

Andrew

 

Jazid

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Maximum plate voltage on the 5881 is 400V, on the EL34 is 800V, therefore aside from anything else make sure the voltages in the AI amp are below 400 brfore proceeding. They likely are but this advice needs confirmation. I know Colin 151 has/had an AI 500 so he may have personal experience (I don't) but the 5881 is not the same beast as an EL34 on paper.

 
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Colin151

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Maximum plate voltage on the 5881 is 400V, on the EL34 is 800V, therefore aside from anything else make sure the voltages in the AI amp are below 400 brfore proceeding. They likely are but this advice needs confirmation. I know Colin 151 has/had an AI 500 so he may have personal experience (I don't) but the 5881 is not the same beast as an EL34 on paper.
It's true that the spec of the 5881 is not the same as the EL34 and on paper and it has a lower plate voltage limit, but in practice it's actually a more reliable tube to use in these AI amps than most EL34s. That's very well known. David Wright recommends using 5881s in these amps over EL34s and he knows what he's doing. He test things out for extended periods to check reliability. I chatted to him on the phone at length about this issue once. He also sold me a Series 500 a few years ago that he had retubed with 5881s.

 

Colin151

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I recently purchased one of these amps that has 5881's and it works and sounds suberb.Mike
Yes. The series 500 a very nice sounding amp for the money. The only thing comparable I've had for similar price (and that actually beat it) was a Chinese 300B SET amp that was completely reworked by local guy David Coe for me, but that's a bit of a project. The stock Chinese amp sounded very clinical and pretty poor compared to the 500.

If only I didn't have speakers that need a lot more power....

One slight modification David Wright told me he usually does to the 500 circuit is to slightly raise the cathode resistor values and IIRC this slightly decreases the bias and makes the amp easier on tubes. I've had EL34s anode plates glow like carrots with a stock amp (5881s gave no trouble at all in that same amp) but in an amp David Wright sold me that had this slight mod those same EL34s worked fine. If I had a totally stock amp, I'd play safe and use 5881s.

I believe cap and diode upgrades are possible too if desired but I never tried those. Might be nice and improve the sound further.

The sound of the 500 is not far off the famous AI 2nd audios IMO and is quite similar in character. The 2nd audios have advantages in some special areas like purity, natural presentation and total lack of any grain/fatigue but the 500 swings more and sounds a bit more ballsy.

 

Fourlegs

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David Wright has been mentioned and I would certainly recommend him for checking out and doing work on AI 500 Amps. I was speaking to David yesterday and he reminded me that much of this kit is getting on for 30 years old and has components in it that simply start to need replacing after that period. Of course the great thing is that this can easily be done and my 2nd Audios are indeed going back to him to sort out some age related issues.

 
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Savo

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This amp is auto bias. You'd ideally need to replace all the tubes. The push pull class A operation in these amps runs the output tubes very hard and many EL34s can't take it and short circuit. The only recommended ones that are supposed to be reliable are EH or Sovtek EL34s. A new matched quad of EH EL34s will cost around £60 delivered from Hotrox. The other alterantive is to go for Russian 5881 tubes which give a bit less power but are tough as nails and are ultra reliable in these amps. http://www.hotroxuk.com/electro-harmonix-el34eh.html?utm_source=GoogleBase&utm_medium=organic&gclid=CKyfjvH6pMoCFYMIwwodOAgABw

http://www.hotroxuk.com/sovtek-5881-6l6wgc-slim-base.html

If your speakers don't need the power I'd go for the 5881s to be on the safe side...

I would not recommend replacing just one of your old EL34s tube with a new or used tube. For one you don't know what the brand of the current tubes are and the amp will not work optimally with non matching tubes.
I ordered the 5881s from your link. :) & plan to change the ECC88s & 83s next month, when my wife allows me more pocket money.

Cheers

Andrew

 

Savo

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Sorry for posting again but I just noticed the link above for the 5881s is for the slim base 5881WGC, is this right or should it be the standard base 5881WXT? I've already placed the order but the shop says I can change before 3pm

Thanks

 

Juancho

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Yes. The series 500 a very nice sounding amp for the money. The only thing comparable I've had for similar price (and that actually beat it) was a Chinese 300B SET amp that was completely reworked by local guy David Coe for me, but that's a bit of a project. The stock Chinese amp sounded very clinical and pretty poor compared to the 500.If only I didn't have speakers that need a lot more power....

One slight modification David Wright told me he usually does to the 500 circuit is to slightly raise the cathode resistor values and IIRC this slightly decreases the bias and makes the amp easier on tubes. I've had EL34s anode plates glow like carrots with a stock amp (5881s gave no trouble at all in that same amp) but in an amp David Wright sold me that had this slight mod those same EL34s worked fine. If I had a totally stock amp, I'd play safe and use 5881s.
Interestingly David Wright says on his Website "It may be 'good practice' to change the cathode resistors to a higher value when fitting Sovtek 5881/6L6GC WXT but it is not essential and it will cost money if done by a repair shop. Plugging and playing is perfectly finePersonally I’ve rebuilt/upgraded four AI500s now and all used 5881s with the original cathode resistors and all had given long tube life and no ‘red spots’ were showing on the anodes so the valves were happy bunnies.

On point of interest with regard to output valve failure was that early versions had an extra cathode resistor known as R1 on the circuit diagrams. R2 the main cathode resistor is a hefty 10watt 220R device. R1 was deliberately chosen as a 0.25watt 15R device. In normal operation 130ma is drawn through the resistor, giving a voltage drop of 1.95v and dissipation of precisely 0.25watts. If an output valve fails, current increases, and R1 theoretically overheats and prevents further damage.

This may have been dropped from later versions if in practice it didn’t fail quickly enough to prevent damage to e.g. the mains transformer or possibly because the resistor itself gave trouble when operated at its max rating.But it struck me as an elegant attempt to offer protection without resorting to a fuse.

David

 

Colin151

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Sorry for posting again but I just noticed the link above for the 5881s is for the slim base 5881WGC, is this right or should it be the standard base 5881WXT? I've already placed the order but the shop says I can change before 3pmThanks
I don't thnk it matters really. I've seen series 500s fitted with both types. The standard base one might be taller. I'm not totally sure. Same spec so no worries which one you choose.

 

Colin151

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Interestingly David Wright says on his Website "It may be 'good practice' to change the cathode resistors to a higher value when fitting Sovtek 5881/6L6GC WXT but it is not essential and it will cost money if done by a repair shop. Plugging and playing is perfectly finePersonally I’ve rebuilt/upgraded four AI500s now and all used 5881s with the original cathode resistors and all had given long tube life and no ‘red spots’ were showing on the anodes so the valves were happy bunnies.

On point of interest with regard to output valve failure was that early versions had an extra cathode resistor known as R1 on the circuit diagrams. R2 the main cathode resistor is a hefty 10watt 220R device. R1 was deliberately chosen as a 0.25watt 15R device. In normal operation 130ma is drawn through the resistor, giving a voltage drop of 1.95v and dissipation of precisely 0.25watts. If an output valve fails, current increases, and R1 theoretically overheats and prevents further damage.

This may have been dropped from later versions if in practice it didn’t fail quickly enough to prevent damage to e.g. the mains transformer or possibly because the resistor itself gave trouble when operated at its max rating.But it struck me as an elegant attempt to offer protection without resorting to a fuse.

David
Interesting about the resistor issue you mention. It's good to know that the 5881 definitely works fine in a stock amp. I guess it would pay to change the cathode resistor values to slightly decrease the bias if you actually intended to use EL34s. I guess you'd still get a bit more power with EL34s than 5881s, about 25 watts? . I believe 5881s only delivers about 14-15 watts in this amp so for some speakers using EL34s might help a little, but I appreciate that's only a slight potential max volume increase.

In that stock amp I tried EL34 in one of the tubes anode plates started glowing like a carrot after a few minutes and the sound in that channel started to decrease... Turned it off as soon as I noticed it!

One good mod worth doing (if you have efficient enough speakers) is to change the amp from Ultralinear to triode coupled mode. That roughly halves the power but the sound is more refined. I believe some stock series 500s came factory wired in triode mode. I should have got you to fit a switch to choose between the modes on the 500 you serviced for me recently but I sold it on as my modded 300b amp sounds better anyway (thanks again for that).

Using 5881s, the power drops to around 7 watts in triode mode and that's going to be a bit too low for many speakers... Probably only a good idea to run it in triode mode with EL34 (giving around 12 watts)

 

Colin151

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David Wright has been mentioned and I would certainly recommend him for checking out and doing work on AI 500 Amps. I was speaking to David yesterday and he reminded me that much of this kit is getting on for 30 years old and has components in it that simply start to need replacing after that period. Of course the great thing is that this can easily be done and my 2nd Audios are indeed going back to him to sort out some age related issues.
Yes any AI amps are getting on now. I'd actually be a little surprised if any series 500 had lasted that long without having had some repair work done on it at some point. The amp does run very hot indeed and the chassis doesn't dissipate heat well. Resistors etc can burn out and it's a good idea to replace them with more sturdy higher wattage ones. Some of the caps would also likely need replacing after that time too of course.

David Wright told me the bridge rectifiers can go bad in the 2nd Audios and that can lead to hum. The amps have hum buckers which can be tweaked but if hum persists and is excessive then a bridge rectifier service and new output tubes can help. Which output tubes did your 2nd Audios use btw? 2a3s or 6B4Gs? I prefer the sound of the 6B4G versions of these amps, but opinions vary. I'd like to get another pair of the 6B4G ones. The 2nds are really lovely amps. I'd also like to get a pair modded with some more neutral coupling caps as I found the stock amps with the Jensen PIO caps a bit too warm and relaxed sounding at times.

 

Juancho

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Interesting about the resistor issue you mention. It's good to know that the 5881 definitely works fine in a stock amp. I guess it would pay to change the cathode resistor values to slightly decrease the bias if you actually intended to use EL34s. I guess you'd still get a bit more power with EL34s than 5881s, about 25 watts? . I believe 5881s only delivers about 14-15 watts in this amp so for some speakers using EL34s might help a little, but I appreciate that's only a slight potential max volume increase. In that stock amp I tried EL34 in one of the tubes anode plates started glowing like a carrot after a few minutes and the sound in that channel started to decrease... Turned it off as soon as I noticed it!

One good mod worth doing (if you have efficient enough speakers) is to change the amp from Ultralinear to triode coupled mode. That roughly halves the power but the sound is more refined. I believe some stock series 500s came factory wired in triode mode. I should have got you to fit a switch to choose between the modes on the 500 you serviced for me recently but I sold it on anyway as my modded 300b amp sounds better anyway (thanks again for that).

Using 5881s, the power drops to around 7 watts in triode mode and that's going to be a bit too low for many speakers... Probably only a good idea to run it in triode mode with EL34 (giving around 12 watts)
The problem is there's so many different types of 5881, with different power outputs. I'm currently upgrading an AI 500 and swapping the valves from 5881s to EL 34s gives little or no subjective difference in volume. The EL34s are JJ, but the 5881s are illegible so I cannot check their specs. But there's lots of mods you can do on these amps to make them much better than standard. You could experiment with switchable triode /pentode mode, or a variable feedback switch. One simple mod I've just discovered which really opens up the sound is to remove completely the output tubes cathode bypass capacitors.

 

Colin151

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The problem is there's so many different types of 5881, with different power outputs. I'm currently upgrading an AI 500 and swapping the valves from 5881s to EL 34s gives little or no subjective difference in volume. The EL34s are JJ, but the 5881s are illegible so I cannot check their specs. But there's lots of mods you can do on these amps to make them much better than standard. You could experiment with switchable triode /pentode mode, or a variable feedback switch. One simple mod I've just discovered which really opens up the sound is to remove completely the output tubes cathode bypass capacitors.
I like the sound of your mods. I'm a real sucker for adjustable feedback switches and triode/pentode switches. Beginning to regret selling my last 500! I'm already missing the sound. I'll have to look for another...