Starting a new power amp project (100 watt 6883B pentode amp)

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Freo-1

Roger has inspired the creative impulse to attempt a DIY project.  After doing a lot of research for an affordable power tube that is affordable and of high quality, settled on the 6883B tube for this project.  The 6883B is the 12 volt version of the 6146B tube.  Its still readily available, reasonable cost, and very well built. 

The 6146 is a RF transmitting tube.  Early attempts to use the tube for audio and guitar were less than stellar.  RCA made a theater amp in the 50's that employed the 6146, with less than good results.  So, RCA converted these units to the 6550 tube. In 1964, RCA introduced the 6146B, which was a complete re-design of the 6146.  RCA claimed 33% increase in output power, along with increased reliability.  Although it was not always a drop in replacement for some of the older transmitters, it proved to be more reliable than than older 6146 tubes for units that were designed from the ground up to use it.

The key to using this type of tube is to make sure one pays attention to the screen voltages.  For the 6146/6883B tube, this means a separate screen supply not to exceed 250 volts.  As long as that is followed, the tube should work flawlessly a s a push/pull output tube.  Designs can get up to 100 watts from a pair. There is a gent in the Philippines who has made quad mono blocks that output 200 watts a channel.  I also like the "dark heater" feature, which is supposed to increase longevity and run the tubes cooler.

The input tubes will be a pair of 6BL7 TV tubes.  They have similar characteristics to 6SN7, expect they are a much stouter tube (they were vertical deflection tubes back in the day).  Expect to have this done late 2014/early 2015. 

Thanks to Roger for providing the spark to try and build another DIY product. 

/mp

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Enjoy, good luck & share your results.

Freo-1

Thanks, and will do.  Should be ready by the end of the year. 

Looking to see if Roger has any insight/pointers to offer up. :thumb:


Roger A. Modjeski

That is a good choice for an output tube. Ampeg used 6 of them in the first SVT-300 amps getting 100 watts per pair. It does require a low screen voltage as you noted. Lets us know how it works out.

Freo-1

That is a good choice for an output tube. Ampeg used 6 of them in the first SVT-300 amps getting 100 watts per pair. It does require a low screen voltage as you noted. Lets us know how it works out.




Thanks, Roger. I have grown fond of tubes with plate caps.   I'm going to use a plate voltgae of 575 volts, and screen voltage of 225 volts.  I reckon it should get at least 80 watts out of a pair.  The output transformers will be spec'ed at 6.6 K CT, and good to 120 watts.  Will likely start at 50 ma bias, and look at values between 40 and 60 ma.  I'm keen to see how the 6BL7 works out as input/driver tubes.

Roger A. Modjeski




Thanks, Roger. I have grown fond of tubes with plate caps.   I'm going to use a plate voltgae of 575 volts, and screen voltage of 225 volts.  I reckon it should get at least 80 watts out of a pair.  The output transformers will be spec'ed at 6.6 K CT, and good to 120 watts.  Will likely start at 50 ma bias, and look at values between 40 and 60 ma.  I'm keen to see how the 6BL7 works out as input/driver tubes.

Be sure to respect the plate dissipation of the tube.  At 575 volts and 50 Ma that's almost 30 watts. Where did you get the 6.6 K plate load? I also think a 6BL7 is a bit much for a driver. Isn't the mu rather low?

Freo-1

Be sure to respect the plate dissipation of the tube.  At 575 volts and 50 Ma that's almost 30 watts. Where did you get the 6.6 K plate load? I also think a 6BL7 is a bit much for a driver. Isn't the mu rather low?

I found a schematic of a 100 watt 6146 amp from the old Acrosound manual that used the TO-350 transformer.  That had a CT of 6.6K.  The plate dissipation of the 6883B is 35 watts, according to the spec sheet.  I figure at 30 watts, it should be OK.  The 6BL7 mu is 15, which is pretty close to the 6SN7 (which is 20).

Freo-1

Here is the link to the Acrosound 6146 with the TO-350 transformer:

http://www.the-planet.org/dynaco/Misc/acrosound.pdf

Roger A. Modjeski

Here is the link to the Acrosound 6146 with the TO-350 transformer:

http://www.the-planet.org/dynaco/Misc/acrosound.pdf

Which one are you building from the Acrosound lit?  I wouldn't run a 35 watt tube at 30 watts, more like 20 if you want life.

Freo-1

Which one are you building from the Acrosound lit?  I wouldn't run a 35 watt tube at 30 watts, more like 20 if you want life.

It's loosely based on the one in page 11.  The transformer is 6.6K.  I also checked the Hammond site, and 6.6 K was the recommended value. 

Good point on the tube wattage.  Guess I could bump it down to 40 ma or so.  I'm running the 1625 tubes at 40 ma. wit no issues at all.

Roger A. Modjeski

You will need that separate screen winding if you plan to do ultralinear.

Freo-1

You will need that separate screen winding if you plan to do ultralinear.

Not a big fan of ultra-linear.  The plan is to employ pentode, and use seperate power supplies for each stage of the amps. 

Roger A. Modjeski

ok on the no UL and lower screen.  how many driver tubes per channel given  your choice of a low mu tube. Are you replacing the SL7s with BL7s?

Freo-1

ok on the no UL and lower screen.  how many driver tubes per channel given  your choice of a low mu tube. Are you replacing the SL7s with BL7s?

The front end will have one driver tube and one phase splitter.  It's a proprietary circuit based on using 6SN7/12SN7, which also has a MOS-FET in the circuit for the driver tube.  This is a tried and true front end, as this is basically the same circuit used in the 1625 mono blocks.  My buddy has used this base front end for over two dozen amp projects he has completed for his customers.  It measures very well.   He has also used this front end with 5687 tubes with great success.  I'm fairly confident the front end will not run out of voltage before the output tubes.

Roger A. Modjeski

its the gain that i wonder about. Is there going to be feedback?

Freo-1

its the gain that i wonder about. Is there going to be feedback?

Yes. We will play around a little to work out the optimum amount of feedback.