I bought a replacement 6U7G because the top cap was missing (wire still poking out however), and I was sent was an RCA instead of the Sylvania I had originally wanted (they couldn't find it apparently)

After installing the new tube and making sure it was seated securely, I noticed that tapping any of the parts on the chassis, eg TX, tubes, etc causes the circuit to start oscillating or otherwise ruining the output, but tapping the new 6U7G set it right (usually) - I put the original Sylvania back in, a dodgy'd up a way to secure the top lead to the wire where the end-cap used to be, and I don't get these issues.

Is it a bad unit? (It was NOS sealed in the box) or are the specs just different?

High gain pentodes can be prone to microphonics where mechanical vibrations excite the elements of the valve leading to oscillation.

Maybe you have been supplied with a badly microphonic valve. The 6U7G is a very old valve type and anything allegedly "NOS" could have had a hard storage life.

Very true GTC.

I had some 6L6's left to me from a deceased friend. Philips NOS in original boxes, very nostalgic, but that's all.

With delusional thoughts of pure Fender sound from the 60's I decided to replace a pair of Harmonix 6L6's with the 'real thing."

Both microphonic once the volume is cranked up beyond 3/10. Very disappointed, because I always considered modern valves
inferior. It did not occur to me that these components are fragile. Ultra thin grid wires etc. suspended in space being bounced and jerked
around all over the place during their lifetime must eventually succumb to mechanical stress.

Storage care is paramount here, being in a cardboard NOS box is no guarantee that they have had a soft ride over the years.

Thanks guys.

Looks like the RCA is going on the shelf as museum decoration.

I had assumed that being NOS, there would have been no problems... Lesson learned

Mawdryn! Joe Oost sells good valves at good prices, he would probably check to see if they are microphonic . He always asks you not to pay before you check the valve either. I have two x NOS 6U7s but dont particularly want to part with them. If all else fails my email is open.

6U7 aka 6D6 aka #58 Different heaters & bases but same animal; Would have to be one of the worst "S" type valves for causing absolute chaos. If it has no effective shield, you can expect massive instability, as oscillation, especially when is in the IF train.

Now just because the top cap fell off, for a variety of common reasons does not mean its stuffed. if there is ire sticking up & the envelope is not damaged it can be fixed. and its much easier if you have the top cap that fell off. That sort of repair is common here.

I did write articles for the radio Club on it and replacing the Metalisation on Philips tubes, the later was in "Radio Waves (HRSA)".

All you need is High strength Araldite; a 1mm drill (sometimes); Scalpel or similar; Solder and Soldering Iron and an offcut from a cap. If the base is loose: insulation tape.

Files will have been kept.

Marc

Hi Guys,

Thanks for the input. There was enough wire poking out of the top of the old tube for me to make electrical contact with the connecting lead. Not ideal, I know, but the apart from the top cap, the old tube still works well.

I don't have the old cap unfortunately, otherwise, yeah I would have glued it back on.

Maybe I could pry the cap off the new 'bad' tube and solder/glue that to the working tube?

You can get a top cap off a dud tube. You may need to unsolder it.

One cleans the grid wire with the scalpel whilst supported. Then you tin it. It is best to fork any new wire around the old: Tin it as well beforehand.

You kink the wire to take off any heating tension.

One cleans of all of the old balsam from the cap. & cleans the glass around the pip & makes sure there is a clear hole for the grid wire in the cap.

Where needed you crimp on & solder the grid wire first. Then you wipe Araldite onto the inside of the cap & offer it to the envelope. Then you quickly solder it (minimum time & heat). That will accelerate the reaction of the Araldite.

If the envelope is loose in the socket (not Philips metalised) wipe Araldite around the top of the base where it meets glass. Then put insulation tape around the socket base extending onto the glass, to stop the Araldite running. Then sit it in a socket

Not at all difficult.

If base & envelope separate on a Philips metalised tube, it can break the wire grounding it: That has to be fixed.

Thanks Marcc.

I've now transplanted the cap from the ebay tube to the original.. looks good so far.

Loose valve bases & envelopes are common.

Extracting them by pulling on the envelope is the common cause and a no no!

That is also how a lot of Metalised Philips tubes & others similar get their grounding wires broken. That and loss of metalisation is often fixable. I have a Breville running with a repaired & recoated ECH35.

Hi Marcc,

None of the tubes have a loose envelope as far as I can tell, so no troubles there.

Marc re the coating on Philips valves. I tried gal paint once and that had no measurable circuit when it dried!
Looked good but that was all.

do you have a favourite mix or a industry product that is satisfactory for a couple of valves?

Fred.

Fred this high heat metallic tape might work from Bunnings.

https://media.bunnings.com.au/Product-384x384/311879f4-aa20-4c4a-abcd-c75ee2e6beff.jpg

I have to run off to a meeting but I did write up my method for that ECH35 & it went into radio waves. Of course I will still have it archived.

I will endeavour to dig it out. but it involves conductive glue.

The 6U7G / 6D6 / 58 was one of the first "super-control" RF pentodes and was designed for better AGC operation. This was achieved by varying the pitch of the control grid wires from top to bottom, thus making different levels of the space between the cathode and the anode exhibit different gain.

This construction was difficult to make, which is why there tends to be large variations in performance between examples of the same valve. It's also why they tend to be microphonic, as well as notoriously unstable when not shielded.