Filament voltage

[Fred_P]

Just completed my Stereomour build (230V version), and measured 2.3V filament voltage during the voltage test with the 2A3s in place while the mains voltage measured 226 Volts. Since the Sovtek 2A3 datasheet I managed to google specified the minimum filament voltage at 2.25 volts, I fired it up, and enjoyed my favourite CDs this afternoon.

Only thing is, I built the 45 version since I would like to use EML 45s instead of the 2A3s.
The speco of the EML 45s says the following about the filament voltage:
"Maximum deviation is 5%.  ...  More than 5% off can damage the tubes after weeks or months. We can easily recognize this, and there is no guarantee on tubes that have worked on too high or too low filament voltage"

The measured 2.3V means 8% off, and there is no guarantee that the mains voltage will not fluctuate even lower occasionally.
- There should be some ways to increase the voltage... (rectifier + cap?), may be a DC stabilizer?
- Or is it a better idea to use and external voltage stabilizer for the whole amp?
  Some of them cost as low as 50 bucks and have a user selectable 220/230/240 output voltage.
- May be some completely different approach?


Thanks 

[Paul Birkeland]

- There should be some ways to increase the voltage... (rectifier + cap?), may be a DC stabilizer?

Converting the AC filament voltage to DC will drop the voltage significantly.

- Or is it a better idea to use and external voltage stabilizer for the whole amp?

This is a very good idea.  You may be able to find an isolation transformer that can do 220:240V.

Part of the design change in Stereomour II is a power transformer with adjustable primary taps, so at 226V, you would be well within 5% of specified filament voltage. 

-PB

[Fred_P]

Thanks for the reply.
When I thought about converting to DC, I completely forgot about the forward voltage drop of the rectifiers which is not negligible at such a low voltage.

Would it be possible to retrofit a Stereomour II transformer into my Stereomour I instead of adding an external step up transformer?
Would be the most hassle free way I guess.
Not sure if it actually fits, and if it is available separately though....
 

[Paul Birkeland]

Would it be possible to retrofit a Stereomour II transformer into my Stereomour I instead of adding an external step up transformer?
Would be the most hassle free way I guess.
Not sure if it actually fits, and if it is available separately though....

It's actually a pretty large hassle, the only similarity it has with the old transformer is that it drops through the same hole in the chassis and has the same high voltage winding.  Otherwise, the heater windings for the 2A3's and driver tube are different, and the primary windings are also very different. 

-PB

[Fred_P]

I was trying to explore some options here, but it seems none of them are really promising.

As for the 220V to 240V step up transformers...
...they were not in high demand thus far, but now with Stereomour II out there having the adjustable primary taps,
the market for them just downright collapsed. :^))
Sorry, I could not resist...

Now, I am still a happy BH customer, and I like the products and the support as well. Seriously, no pun intended.
The fact that this time I feel I am stuck with an issue, does not change this in the least.

I was expecting that the amp would work reliably in my location with my choice of 45 tubes.
Since this is clearly not the case, I count on your expertise to come up with an acceptable workaround.

Regards

Fred

[Grainger49]

I have no experience with 240V devices but here a variac will put out more than the supplied voltage due to the transformer aspect of the device.  240 variacs might do the same thing.

[Doc B.]

Look for a surplus buck boost transformer. Typically they are rated for about 10% buck or boost. That should get you pretty close to what you want.

[Fred_P]

OK, so here is plan # n+1
It's gotta work out, so please support it.   

I found a reasonably priced 2A3 filament transformer:
230V primary, 2x2.5V, 2x2.5A secondary.
It has an M65 core (65mm), so it would fit below the chassis plate at the front, where the volume pot would normally be.
(I do not need the volume pot, since I am using an external volume control)

Do you see any potential issues with this solution?

Yes I know, the 12AT7 filament voltage and the B+ would still be lower this way.

The Valvo 6201 I plan to use specifies 10%, so that should be OK.
(Btw., the 6201 data sheet also mentions that in case of series filament supply
a current limiter is necessary, but I do not want to hijack my own thread...)

Raising the B+ should be easy, but I do not think it is really necessary.

As a side benefit, I could use a commercially available 230V delay timer
in front of the original transformer to delay the B+ (at least for the 45s).

Does this sound like a plan, or is it time I started working on plan # n+2 ?   

[Paul Birkeland]

Do you see any potential issues with this solution?

Yes, you'll be running a bunch of AC wire runs all over the place where they weren't before, and you're adding power transformers where there were none before.  This will likely lead to noise issues that will be very difficult for us to help you track down.

What happened to getting a 220V to 240V transformer?

[Fred_P]

Not much...

In theory they are a nice idea, but do you have any concrete model on mind?
Even if I find one it would come without a case, and even if I find a case to mount it into, I have another box lying around in the end.
It would obviously not fit inside the amp itself, or you would advise me not to do so.

But the actual problem is to find one.
The only ones I saw so far were complete tube amp transformers with many secondaries, including a 240V one.
If such a one could provide enough current on its 240V winding for the whole amp is also a question.


But if I take a few steps back again to get the birds eye view:

Which countries did you actually have on mind when you decided for a 240V transformer?
Except for some exotic countries like Nauru or the Cook Islands (no offence), there aren't many.
At least all of Europe and most of Asia as well as South America is 230V, or even worse 220V.
I believe it is safe to say that the vast majority of your customers is not located in a 240V country.

With due respect, I can conclude that the whole issue stems from a less than optimal choice of transformer.

And even though I may be the only one raising the issue, I am definitely not the only one to suffer from it.
Most non-US customers will also face the same problem to some extent, even if they are not aware of it.
And the fact that Stereomour is your most popular kit, does not make things better.

To be fair, I must also say that your kits are very affordable, high quality, and you do a lot to support the DIY audio community.
This is all true, and I do not intend to blow the issue out of proportion.
Nevertheless I need a solution, and that solution must come from you.

My contribution is that I am willing to take a reasonable amount of additional costs.
But I still see _you_ in charge of the issue.
Therefore I kindly ask you again to take due responsibility.

Thanks for your understanding,

Fred 

[Paul Joppa]

I'm on vacation at the moment, but just saw this thread. A few comments:

1) Until our latest products (Seductor and Stereomour II), we have only offered 240v designs, not 220 or 230. We made this choice a long time ago, because over-voltage is more common than under-voltage, and is more likely to reduce tube lifetime. Also, at least initially, most of our customers outside the USA were legacy-240v countries, mostly the UK (see #2).

2) There are in actuality 220v generators and 240v generators; both of them now specify "230v" even though their power generation machinery - and voltages - have not changed. The power line voltage specification has asymmetric deviations, instead - for example, 230v +10%-6% in the UK and 230v +6%/-10% in many other countries. This is a political solution, not an engineering solution, in my opinion! In any case, many homes are not actually within the specified limits.

3) Most tubes, including those we supply, will operate happily at +/-10% voltage. A few specify 5% or even 4%, but their operation is not really different - the manufacturer is asking for a tighter control so that the other specifications have less variation. This reduction in variation is not critical to the operation of our amps.

4) Tube amps have traditionally had a +/-10% expectation - this has been standard practice since the 1930s (when AC powered gear became ore common than batteries). Note that this means 216-264v for our 240v transformers.

5) The 45 will have slightly higher voltage on the filament because it draws less current.

6) I am confident the amp will operate correctly on your power line, and that any effect on tube lifetime will be much smaller that the normal variation between one tube and another.

All that said, I do (as an engineer) sympathize with the desire to optimize the operation. We are addressing this issue with the new power transformer - but it does cost more because of the greater number of taps and terminals, so I can't say whether we will expand this practice or not.

[Grainger49]

So no comment on the use of a Variac?  It could solve your problem.

[Fred_P]

@Grainger

So no comment on the use of a Variac?  It could solve your problem.

Thanks for the hint.
On one hand, I would prefer a solution that does not require any additional external components lying around.
If there will be no other way, I may still consider this solution though.
On the other hand, I did not find any such device with 230V primary that is available in the EU and does not break the bank.
If you have any specific model in mind, please share with me.


@Paul Joppa

I'm on vacation at the moment, but just saw this thread. A few comments:

Thanks for your reply, really appreciate it.
I am sorry I disturbed your well deserved vacation.
We have plenty time for any further comments/discussions, so for now please enjoy your vacation.

I also inserted my comments below, but no urgent reply is needed.

1) Until our latest products (Seductor and Stereomour II), we have only offered 240v designs, not 220 or 230. We made this choice a long time ago, because over-voltage is more common than under-voltage, and is more likely to reduce tube lifetime. Also, at least initially, most of our customers outside the USA were legacy-240v countries, mostly the UK (see #2).

While this certainly holds true, the key here I believe is "we made this choice a long time ago". Please see next point.

2) There are in actuality 220v generators and 240v generators; both of them now specify "230v" even though their power generation machinery - and voltages - have not changed. The power line voltage specification has asymmetric deviations, instead - for example, 230v +10%-6% in the UK and 230v +6%/-10% in many other countries. This is a political solution, not an engineering solution, in my opinion! In any case, many homes are not actually within the specified limits.

The asymmetric deviations you mentioned were in place during a transition period between 1995 and 2008.
Since 2008 the voltage is nominally 230 V ±10% in the EU. ( 207V - 253V )

One could of course also argue that the required 5% cannot be met regardless of the transformer used, if the mains voltage deviation exceeds the required secondary deviation.   
In my home however, the voltage is a very steady 226V which means less than -2%.
In my specific case, the reason why the 5% is not met on the secondary side is that the primary winding is made for 240V instead of the nominal 230V.
With a 230V primary winding my secondary voltages would be well within 5%.

3) Most tubes, including those we supply, will operate happily at +/-10% voltage. A few specify 5% or even 4%, but their operation is not really different - the manufacturer is asking for a tighter control so that the other specifications have less variation. This reduction in variation is not critical to the operation of our amps.

In this case the tube manufacturer states that more than 5% deviations will damage the tube quickly and this will void the warranty.
I am not so much worried about the amp than the tubes themselves. Pls. also see point 6)

4) Tube amps have traditionally had a +/-10% expectation - this has been standard practice since the 1930s (when AC powered gear became ore common than batteries). Note that this means 216-264v for our 240v transformers.

The nominal voltage since 2008 has been 230V +/-10%, 207-253V in all of EU.
For a transformer with a 240V primary winding, the actual secondary voltages will have a deviation between -14% and +5%.
So over-voltages will definitely be avoided, but more than -10% under-voltage is possible.     

5) The 45 will have slightly higher voltage on the filament because it draws less current.

I have been thinking about this myself. I will simply install the 45s and measure the voltage.

6) I am confident the amp will operate correctly on your power line, and that any effect on tube lifetime will be much smaller that the normal variation between one tube and another.

While this makes sense, somehow it still contradicts the requirements by the tube manufacturer:

"Filament voltage deviation is maximum 5% (0.13V for 2.5V tubes).
More than 5% off can damage the tubes after weeks or months.
We can easily recognize this, and there is no guarantee on tubes that have worked on too high or too low filament voltage."


In the end, point 3) and 6) seem to suggest to me to simply ignore the 5% requirement.
While I believe I understand your reasoning, I am still not fully convinced to ignore such a hard statement from the manufacturer.
Especially considering the price of the tubes.
I would like to enjoy many hours of music from this amp/tubes without any major issues or annoyances.

[Doc B.]

Let's keep this as straightforward as possible. Please try the EML 45 tube in the amp and measure the filament voltage as PJ has suggested. With actual data we may be able to offer a more well informed suggestion about how to approach the issue, if we know for sure that there is an issue. At this point the thread is conjectural in nature and we really can't offer anything more than theoretical answers to theoretical questions.

[Fred_P]

The filament voltage measures 2.37V with the 45s.
The mains is currently at 228V, 2Vs higher than at the first measurement with the 2A3s.
The 2.37V is 0.13V (5%) lower than the nominal value.

While this is definitely an improvement, I would still not leave it like this in the long run.
The reason is simply that if the mains fluctuates, the secondary fluctuation will start from -5%.
Say, if the mains fluctuates +/-5%, the secondary will be at 0% to -10%.

Any thoughts, suggestions?