Steptoes amp.

[Alex Kitic]

PS. Alex, your 813s are in the mail. I have sent them with the GPO as the price charged by the specialist couriers would feed a family of four for a week, so please let me know when they arrive.

Thank you for letting me know
If sent by post, they are probably going to be here in 2 weeks or so. If properly packed they should survive, and if you declared a low value, there should be no complications with the customs office...

I'm very much looking forward to receiving them, and of course I'll let you know as soon as they arrive.

[Steptoe]

Dear DTB, thanks for that info, which has filled me with joy. I was starting to get despondent when the kitchen table was sagging under the weight but from what you say that should be a good thing.
Is it correct to say that wiring up the primaries in series will increase the inductance but decrease the capacitance and vice versa for parallel connection? If so, will having four secondaries in parallel adversely affect the loudspeaker? Also, is it possible to measure the inductance using a multimeter and if so, how? Sorry to pose so many questions but the more I get into this the more complicated it becomes. With best wishes, Steptoe.

[pre65]

Sorry to pose so many questions but the more I get into this the more complicated it becomes.

Oh how true that is.

[pre65]

I'm trying again with the 6Y6g output valves.

At the moment I've got a plate voltage of 114v and 54ma per PSE valve.

The LH channel has 57ma & 48ma.

[pre65]

I changed the dropping resistor from 1K to 470R to increase the HT, but because the current has also increased the HT has not gone up as far as anticipated.

So plate is now 130.5v (144.9v - 14.4v) and current per valve 65.5ma.

Not as nice as anticipated but sits nicely on the curves.

[pre65]

On the original build, I had calculated the power valve cathode resistor bypass cap to be 1000uf, but I could only find three, and substituted 470uf.

This morning I have found the missing 1000uf cap, so all four have now been changed.

Playing something with a bit of bass at the moment and the amp now sounds a bit more "solid" in the lower areas.

I think this is it now for triode mode connection.

[Alex Kitic]

I think this is it now for triode mode connection.

Are you going to try the pentode connection with an Rfb resistor as suggested before?

And LM317 instead of the cathode resistor on the power tubes?

[pre65]

I think this is it now for triode mode connection.

Are you going to try the pentode connection with an Rfb resistor as suggested before?

And LM317 instead of the cathode resistor on the power tubes?

Probably not, as time constraints, plus other projects and things such as gardening and "having a life" ( ) keep getting in the way.

[Alex Kitic]

PS. Alex, your 813s are in the mail. I have sent them with the GPO as the price charged by the specialist couriers would feed a family of four for a week, so please let me know when they arrive.

Dear Steptoe, the 813s have arrived safely! No physical damage on the tubes, and the filaments show continuity on the DMM.

I wasn't expecting to receive the parcel so early: it seems that the post is an excellent courier, at lower cost. No problems with the customs.

The April 1944 date on the JAN boxes means they are 70 years old! Looking at the date I was somehow transported in the time just before D-day... I'll enjoy these pieces of history.

BTW, any advice on cleaning the tube pins? For a moment I was horrified by the lack of continuity on one tube heater - but scraping the probe on the pin has shown continuity with a beep.

Thank you so much!

[Greg]

BTW, any advice on cleaning the tube pins? For a moment I was horrified by the lack of continuity on one tube heater - but scraping the probe on the pin has shown continuity with a beep.

There are a couple of methods. Rubbing the pins with a fibre glass pencil works well, but might not be an Item easy for you to find where you live. Another method is to stand the valves in a shallow container filled with Coca Cola for a few hours, or even a solution designed to remove tarnish from silver cutlery.

[Andrew]

BTW, any advice on cleaning the tube pins? For a moment I was horrified by the lack of continuity on one tube heater - but scraping the probe on the pin has shown continuity with a beep.

There are a couple of methods. Rubbing the pins with a fibre glass pencil works well, but might not be an Item easy for you to find where you live. Another method is to stand the valves in a shallow container filled with Coca Cola for a few hours, or even a solution designed to remove tarnish from silver cutlery.

The cola is a good idea clean anything that stuff!

[jack]

Coca Cola works because it contains traces of phosphoric acid, which is the active ingredient in this case.

You can buy phosphoric acid from eBay - its easier & cleaner than using Coke but you mustn't drink it

A 10% solution works very well for minor cleanups. I use 30% from cleaning rusty radio chassis and bits of my beehives that are heavily corroded.

[Neal]

Wasn't Wonder Wheels also recommended as some point on the old WAD board?

[Alex Kitic]

Well, it seems that I am not yet out of the bush...

I have decided to clean the contacts only with a baby wipe, since they are great at removing dust and greasy residues, after which procedure contacts are rather shiny and responding to the probes well.

Thus I have tried the tubes in the amp, first without rectifier to check the filaments (ON, great glow, just like they should) - and after that with the rectifier tube in place.

One of the tubes works perfectly well, starts drawing current without problems... and is very similar in current draw and bias voltage to the Haltron and RCA tubes I was using.

The other National Union tube is arcing violet arcs from the bottom part, near the socket! The arcing is rather violent and audible...

WHAT NOW? Can this be solved somehow?

I am wondering whether the problem is in the top cap (maybe poor contact there)? The top caps are new, of course, but the tubes are old, and maybe some contact is not good? Still, I do not think that is the cause, since the plate is not in contact with anything on the bottom part of the tube... the only problem possible could be "not drawing current" which could put the g2 under a lot of stress since the CCS is that a current flow is established, and the g2 could therefore arc towards other electrodes?!

Any ideas?

PS
The National Union tube Paul has sent me is rather weak, it's bias voltage only gets to 38-40V (other tubes are about 60V) and it seems to be unable to draw 100mA, since both the voltage across the transformer primary, and the voltage across the voltage dropping resistor above the CCS in the cathode circuit show low values...
Could it be revived somehow (i.e. heat for a longer time without B+ and then apply B+)?

[Alex Kitic]

"Degassing Older Electron Tubes
Electron Tubes that may have gassed up over time may be partially degassed by putting them in the equipment and running them for several hours with only filament voltage applied. After the initial filament-only degassing, the electron tube should be operated for an hour or so at reduced plate and screen voltages. This allows the Getter to soak up and hold any residual gasses. In directly heated filamentary tubes, the Getters are generally zirconium-bearing gasses."

I have found this procedure - but I am not sure whether it can be done with 813s since they are directly heated and with TT filaments? Also, Paul has mentioned that the 813s should not be operated without B+ for longer than a couple of minutes.

Anyway, if this procedure can do any good, I could just put the two troublesome tubes in the amp (the probably gassy violet arcing NU, and the low emission NU) and operate them without the rectifier and the rest of the tubes for a period of time (i.e. a couple of hours)...

After that, I could put back the rectifier, since the voltages I am running those tubes are well below maximums both for anode and g2...

Any opinions?