Have I got this right?

[Rob11966]

Hi Folks,

I just wanted to check that I have understood this correctly before I put my soldering iron to the Lundahl transformers.

I am specifically looking at the correct configuration of the secondary windings of the output transformers.

The amp is an SE 845 with plate voltage 920v. I have 8 ohm speakers.

Here is a link to the tube datasheet -

http://www.pentalaboratories.com/pdfs/845.pdf

Here is the link to the transformer datasheet -

http://www.lundahl.se/pdfs/datash/1620_3_7_9202.pdf

I believe that I need to connect the transformer secondary connections as per the diagram "C" on the Lundahl datasheet.

Can someone confirm that this is correct?

Apologies if this seems rather mundane or obvious but I thought that I should check.

Cheers,

Rob

 

[Robert McLean]

If you are using the LL9202 then yes, connection C is correct, giving 11K impedance, a good match for the tube spec sheet 9K.

(LL1620 will also give about the right impeadance connected B, but only is rated at half the power.)

 

[Rob11966]

Sorry Robert,

I did not specify the transformer.

I am using the LL1620. Connection 'C' will give me an impedance of 6K and connection 'B' will give me an impedance of 11.5K. The tube data specifies 9K at 1000v, so at 920v or thereabouts it will be a little less, so I thought that connection 'C' at 6K would seem to be a better match than connection 'B' at 11.5K? However, I can see that the tube specifications fall somewhere between my two options. Is there any good reason to move towards one or the other (higher or lower impedance), or is it just best to choose the closest match (i.e connection 'C' at 6K).

As an aside, the designer has designated this transformer at 38:1 which obviously chorresponds to one of these options but I am not sure which.

Here is the original design schematic if this helps.

http://www.audiodesignguide.com/New845/New845v2.html

Thanks for the help so far.

Cheers,

Rob

 

[gingertube]

Rob,
Andrea specified 38:1 winding ratio.
That means an impedance ratio (turns squared) of 1444:1

That would indeed give
11,552 (nominally 11K5):8 Ohms
BUT it also gives
5,776 (nominally 6K):4 Ohms.

My (educated) guess is that Andrea was using 4 Ohm speakers thus the primary load on the 845 would be nominally 6K.

Since you are using 8 Ohm Speakers then the connection option C to give 6K:8 Ohms is indeed correct.

In any case 6K is the correct load for an 845 with 900V between anode and cathode.

Note: I was careful not to say a 900V B+ rail. With the fixed bias arrangement shown they are the same thing but if cathode biasing is used for the 845 then the voltage across the tube (anode to cathode) will be 120 to 150 volts less than the rail volts and a lower primary impedance again (about 3K5 to 4K) would be required.

Hope this helps,
Cheers,
Ian

 

[Rob11966]

Hi Ian,

Thanks for the comprehensive reply.

It sure does help.

I have learnt something as well!

Cheers,

Rob