Quicksilver conversion

[andyjevans]

8417 are a very high gain high voltage modern valve (ie. low impedance) quite unlike anything else and a similar family to the modern 7591, which again neither resembles a 6V6 nor 6L6.


.....you still have 2 other issues:
1. The gm of 8417 is at least double of any other tube you use. So you will have a drive issue. You either lose a lot of power or you redesign the driver. Reducing the GNFB could help with gain but you have to see if the concertina can swing enough for your tubes.

The reason I suggest the EL38 is because the gm is the same, thus avoiding any need to change the input section for more gain. I use it in triode where it sounds excellent. That's an option.

 

[dgta]

The reason I suggest the EL38 is because the gm is the same,

No it is not. 8417 has gm of 23, whereas EL38 is at 14, only slightly more than EL34 at 11.

https://frank.pocnet.net/sheets/129/e/EL38.pdf

 

[andyjevans]

No it is not. 8417 has gm of 23, whereas EL38 is at 14, only slightly more than EL34 at 11.

https://frank.pocnet.net/sheets/129/e/EL38.pdf

Yes - you're right. The 8417 is completely unfamiliar to me and I thought I looked up the right data on Franks but I saw a gm of 12. Not sure what the hell that was. I use the EL38 in triode where the mu is around 14. The EL34 is more like 10.5 in triode if I'm reading that correctly. So the EL38, which sounds better than EL34 in triode, also enables a driver stage with a bit less gain. It's a good tube and not expensive, though more common in Europe.

 

[dgta]

Andy, this thread is about a 35W pentode with very high gm and its possible replacements in the QS. Like 6550/KT88. EL38 (in triode or otherwise) is in no way suitable.
Why don't you open your own thread and tell us about the EL38's wonderful qualities?

 

[andyjevans]

Andy, this thread is about a 35W pentode with very high gm and its possible replacements in the QS. Like 6550/KT88. EL38 (in triode or otherwise) is in no way suitable. Why don't you open your own thread and tell us about the EL38's wonderful qualities?

Yes - I just dropped in on this thread out of curiosity. I build 300b SE amps mostly. I went through a stage of EL38 outputs with DHT drivers like 10Y. I did start a long thread on all this, which has now lapsed. There's a new thread on driving a EL84 in triode with DHTs like the 30. Same basic idea - higher mu output stage with a DHT driver. All that is different from this Bogen amp, which isn't something I'm into. That's your territory, and I leave all that to you. I don't use either PP or pentode outputs.