Good evening all, Is it possible to replace an ech35 with a 6k8 valve. bowler

You may not get quite the same result but they appear pin interchangeable. http://www.tubedata.info/

Pin one ECH35 is metallisation. Glass 6K8 has no shield; Metal type its pin one. Make sure pin one is grounded, on both.

Metallisation can be replaced. Do not extract metallised tubes by the envelope, as that and failed bond between envelope and base, can break the connection between pin one & the metallisation.

Hello. For what it is worth, I looked up the equivalent for the ECH-35 in my equivalents book, (by Babani 1960.)

It lists as drop in equivalents the following types.

6E8
6J8
6K8
6P8
6TH8
ARTH2
CCH35
ECH3G
ECH33
OM10
X61M
X62
X65
X66
X147

They recommend that when used in a high frequency stage, that re-alignment is desirable to ensure peak performance.

I would like to add that this information comes from an English book, so that some types listed might be unknown here.

Wayne.

What can happen and Astor JJ and a few others will do it: If you swap a 6A8 with a 6J8 that will compress the BC band. That's why my previous comment.

The different construction of the valves leads to different capacitances etc and that will tend to alter things. That why "don't expect the same result".

Off topic but a classic is the often cantankerous 6SA7 Pentagrid Frequency Changer. A large part of what you know about the other octal Frequency changers does not apply to it: It's very different.

Hi All , Thanks for your replies re the 6k8 valve. Would the ECH33 be a better replacement for the ECH35. bowler

I believe ECH35 superseded ECH 33. The screen like a JJ is normally fed from a voltage divider: ECH35 is not. Control grid bias is slightly different and it has the bigger top cap of the older series like 6D6.

Data: Frank's Electron Tube Pages http://www.tubedata.info/

Many years ago when I did radios, and finding other converters to be very expensive, I bought some Canadian metal 6K8s via Tube Depot.

I subsequently used these to replace a 6A8, 6J8 (various radios) and an ECH35 in an HMV. Never had a problem with them and I didn't need to make any changes other than to lengthen a top cap lead - the metal 6K8 is shorter.. In fact, I found a slight performance improvement.

This from the NZVRS:

https://nzvrs.com/wp-content/uploads/2018/12/RCNZ-notes-on-substitution-of-6A7-and-6A8-by-6J8-or-6K8.pdf

The main thing seems to be that the oscillator plate voltage of 6K8 should not exceed 100V.

No mention of changes in station positions on the dial, but since the change from 10 to 9kHz spacing very few are original anyway.

Very useful document!
If the original was an ECH35, the dropper for the oscillator should already be there, but still a good idea to check the voltage on the cold side of the osc. plate winding.

Interesting with the metal tubes. That may not be just an RF tube thing the owner bough for new I think 6F6 glass tubes for a Midwest. to replace the metal ones in the Push Pull (Yes four 2 & 2 as the power output into two electrodynamic Jensen's, Woofer & Tweeter) liked the warm glow. However, as far as I am concerned those metals were better.

One thing I have found is that the audio is not necessarily improved, by RF in the output stage and in more than one instance, I added an RFC, sort of Plate detector style to stop the OP tubes amplifying it.