6550 SE Started

Hi Everyone,

I have the bug I have started another amplifier with some tubes I had in my box of goodies, speaker terminal, power switch, feet on, volume control and sockets all mounted, just need to fit some RCA's at the back and that's about as far as I can go until I buy a power and some audio transformers.

This will be a Single Ended amplifier, they are not as powerful as a push pull but have a nice sound to them, on efficient speakers enough volume to full a room with sonic excellence :)

Will run a solid state rectifier as much easier to get big caps for smoothing and can get away with out a choke. Will keep the B+ under 450vdc as can get caps for that more easily than if you go above 450vdc.

6550 starting out

7591a SE Amp

Hi All,

I have had a few tube bits around for a while now, I had some parts I bought from www.vt4c.com LM-130E power transformer, some audio transformers, some Japanese audio transformers from Toei OPT-10S in Akiharbara Tokyo, (If anyone knows where they moved to after the radio building closed down can you let me know please) http://www1.tcn-catv.ne.jp/toei-trans7arc-net/OPT-10S.htm

Please visit www.vt4c.com for all your DIY tube amp needs, its my favourite online store! He has lots of goodies and has a good forum page with lots of help and advice.

A selection of tubes - 6N1P, EL34, 6550 and 7591a. I started off on going to build an EL34 amp but found my transformer 6.3v winding was 2amps and the EL34 was 1.5amp each, so that was the end of it until I looked at the 7591a tubes 0.8amp each great lets build a SE amp with these.

These 7591a tubes are old! There is some history behind these, in the late 1960s my father had a Sansui 1000a amplifier, it had a burn up in the output stage, he was a TV tech of the day, he wrote to Sansui to let them know what happened and then out of no where some time later a letter and a package arrived from Sansui apologising for the problem and as a token of their appreciation of buying their product supplied 2 full sets of RCA 7591a tubes, my father passed away and these tubes he had kept came to me.

The circuit is pretty easy, look at the tube, 300v at -10 and you have 60mA so that decided away I went with the calculator and worked out the resistors and wattage.

I went to the local jay car store looking for something to construct my amp in/on and found a 2U case and a blank back panel.

I spent a couple evenings marking up, drilling holes and using my nibbling tool to make everything fit together.

I constructed the mighty birds nest on the back of the top plate, it took me about 2 hours to build it all the electrical part, I am not very fast nor was I very neat, expecting to have hum issues although I did use star grounding. 

Power up time, well that was a bit of a disaster, it whistled and made some funny noises out the speakers, so a quick power down and then I remembered I had hooked up the volume pot, joined the 2 gangs together to earth for the grid of the driver but in my haste to power up I had not earthed the pot so the grid was floating, tied it to earth and WOW! it powered up and worked fine first pop, hum was very minimal, so small I can live with it!!!

I don't know how to explain it but these 7591a tubes really sound good!!! Really easy to setup! Nice sound, punchy! A couple friends who have heard are very impressed!

Thanks to my friend Dave H for all his advice on tube power supplies! Good to have a friend who learnt on tubes when he started out in electronics.

Just starting out 

The mighty birds nest enjoy 

On my work bench

 

on my work bench

Set up in my lounge and the puzzle was to hard so put it away now :)

Grass roots - how odd to see someone use a pencil and paper these days

 

You know what this is - the night photo