diytube.com

Just thought I give this link about an interesting way to set up UL on a non UL output. I suspect that it can (easily?) be done with some of the high voltage mosfets (e.g. IRF820)

http://www.single-ended.com/6550amp.htm

Here's a SS version of an artificial UL tap:
http://geek.scorpiorising.ca/GeeK_ZonE/ ... pic=4706.0

Tube version is simpler.

Geek wrote:Here's a SS version of an artificial UL tap:
http://geek.scorpiorising.ca/GeeK_ZonE/ ... pic=4706.0

Tube version is simpler.

Thanks, interesting. He's doing a lot more though - reducing the B+ on the G2 while maintaining the UL plus using feedback. Do not know if I would care for that complexity. Actually if it comes to this one does not need tubes, a tube can be mimicked with solid state too. Thanks to a guy who has not posted for some time on DIYaudio.

AmadeusMozart wrote:He's doing a lot more though - reducing the B+ on the G2 while maintaining the UL plus using feedback.

Ummm, that's what the one I linked does too

Geek wrote:

AmadeusMozart wrote:He's doing a lot more though - reducing the B+ on the G2 while maintaining the UL plus using feedback.

Ummm, that's what the one I linked does too

Apologies - I was not clear. Your post shows a design with considerable voltage drop on g2, more so than the one I linked to. My link can be relatively easy replaced with a mosfet. However reverse voltages may make that protection is required.

I want to keep things robust and easily fixed - the design I linked to has a flaw: the filament of the UL tube needs to be at high enough voltage to prevent a flash over between cathode and filament.

I'm having trouble coming up with an answer to "Why?" on this one.

If someone wanted to experiment with a "retrofit" where a new UL transformer was out of the question, then would the cost/benefit seem reasonabe on paper before it was built?

This seems like one of those "because I can" ideas that would not be built from scratch if a good UL SE tranny was available from the start. It's kind of a Rube Goldberg machine on paper.

20

I do not recall the website but it reported a study that many pentodes / beam power tetrodes like/need to have the g2 voltage lower than the plate voltage. This is one way achieving it, another is just using the UL tap on the transformer with either some zener configuration or some gas voltage stabilising tube (OA2 etc). Both have sonic impacts and have a critical selection of capacitors.

The first schematic is for experimenting with different levels of the tap, standard is 43% and that will give the least distortion. Another standard perception is that at 20% you'll get the most output. However this varies from tube to tube, beam power tubes leaning more towards the 20% for nicest sound and pentodes more towards 40%. There are even remarks that pentodes are not as good in UL and only beam power tubes should be using UL.

It is just that someone who wants to experiment has now the opportunity to do without having to buy a multitude of transformers.