6U8 Revisited

wrenchone · 24th July 2010, 10:54 PM

I have a little Magnavox amp (ripped out of a console) that uses a pair of SE 6V6s. I was thinking of taking this little amp to Schade-land by adding an extra socket and using a pair of 6U8s, with the pentodes on input duty, and the triodes sitting in as middle-weight source followers. I've noted that the classic p-p amps using the 6U8 as input amp and phase splitter tend to starve the pentode and use a highish load resistor. I want to back off and bias the pentode at ~10ma, with a lower value for plate load. I figure that the gain will be about the same, but the linearity should improve. Comments?. To complete the travesty, I'm thinking of using the 6AU5 in place of the 6V6 for output tubes. The power transformer appears to be sized to run the rest of the console in addition to the power amp, so I'm not too concerned about the extra current draw. There's also plenty of room underneath for some extra stuff.

wrenchone · 24th July 2010, 11:07 PM

Here's a picture of the beast in question - old, scabby and shabby, but it should clean up ok.

tubelab.com · 25th July 2010, 03:13 AM

Some of my first tube amps after a long period in sand land used 6U8's to drive a P-P pair of KT88's or 6L6GC's. The circuit was the typical pentode input stage and triode phase splitter. I made several of these amps for HiFi and guitar amp use. The circuit and tube choice evolved as I experimented.

I bought a bulk pack of 100 NOS 6U8's real cheap. All were run through a mutual conductance tube tester and the few duds tossed. I found that the 6U8 and the 6EA8 can be quite variable, and some have higher distortion than others. This is probably not an issue if you are building one amp and have a few tubes. The repeatability thing bothered me so I would up switching to the 6BL8.

I'm sure you know that there are several dollar tubes that scream "use me" for this sort of thing.

wrenchone · 25th July 2010, 03:38 AM

Well yeah, I can think of a couple of numbers off the top of my head, but I wanted something humble and well-trodden upon. The 6BL8 would work, but I wanted a triode with a little more mu. I also wanted to see how well the 6U8 pentode would work with the bias values originally set in the data sheet rather than as a pitiful starveling.

DualTriode · 26th July 2010, 01:42 AM

Quote:

Originally Posted by wrenchone

Here's a picture of the beast in question - old, scabby and shabby, but it should clean up ok.

I like what you are doing, saving the dinosaur from the tar pit. Please post more photos of your progress.
DT
All just for fun!

wrenchone · 26th July 2010, 02:03 AM

BTW., I'm looking at the 6AU5GTs I have in house, and I don't really believe the max plate rating of 10W, as I have 6AQ5/6005 tubes with much smaller plates and the same max power rating. We'll see about that...

tubelab.com · 26th July 2010, 02:42 AM

Quote:

We'll see about that...

I have seen about that. I have several hundred 6AQ5's. I had even more but some were so crusty with green corroded pins that they wound up in the trash. The corroded pins break off in the socket. Several got "tested" before they got tossed. Many 6AQ5's are slightly red in a dark room at 10 watts. There is only a few more watts between the spec, and sucking air! The plate hot spots and the glass in only a few mm away so it melts at the location of the red spot, and then a crater appears and woosh no more vacuum.

The 6AU5 is similar to the 6AV5GT and has the same pinout. The plate structure in each looks the same. The 6AV5GA has a much bigger plate. I had one single 6AU5 for tesitng. It is a well used HP (RCA) branded tube. I could crank about 15 watts of dissipation through it in triode mode before a faint glow showed. I got several more 6AU5's at a hamfest, but never tested them. I am sure that they are more than capable of saturating those OPT's.

wrenchone · 26th July 2010, 05:13 AM

My thoughts exactly. With those small transformers, it doesn't pay to get too frisky. Replacing the transformers negates the point of the whole exercise. If I replace everything, why buy the silly thing in the first place? Tying a pink ribbon around the neck of this particular jacka** is the whole point...

wrenchone · 26th July 2010, 05:14 AM

Oh, and doing it cheaply is the other point....

Chris56000 · 27th July 2010, 08:17 PM

Hi!

I've got a circuit for a quality Phase Splitter using a single ECF82 valve which you might to try - originally from a 1950's Wireless World. I'll post it in due course!!!

Chris Williams

24th July 2010, 10:54 PM	#1
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	6U8 Revisited I have a little Magnavox amp (ripped out of a console) that uses a pair of SE 6V6s. I was thinking of taking this little amp to Schade-land by adding an extra socket and using a pair of 6U8s, with the pentodes on input duty, and the triodes sitting in as middle-weight source followers. I've noted that the classic p-p amps using the 6U8 as input amp and phase splitter tend to starve the pentode and use a highish load resistor. I want to back off and bias the pentode at ~10ma, with a lower value for plate load. I figure that the gain will be about the same, but the linearity should improve. Comments?. To complete the travesty, I'm thinking of using the 6AU5 in place of the 6V6 for output tubes. The power transformer appears to be sized to run the rest of the console in addition to the power amp, so I'm not too concerned about the extra current draw. There's also plenty of room underneath for some extra stuff.

25th July 2010, 03:13 AM	#3
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	Some of my first tube amps after a long period in sand land used 6U8's to drive a P-P pair of KT88's or 6L6GC's. The circuit was the typical pentode input stage and triode phase splitter. I made several of these amps for HiFi and guitar amp use. The circuit and tube choice evolved as I experimented. I bought a bulk pack of 100 NOS 6U8's real cheap. All were run through a mutual conductance tube tester and the few duds tossed. I found that the 6U8 and the 6EA8 can be quite variable, and some have higher distortion than others. This is probably not an issue if you are building one amp and have a few tubes. The repeatability thing bothered me so I would up switching to the 6BL8. I'm sure you know that there are several dollar tubes that scream "use me" for this sort of thing. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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25th July 2010, 03:38 AM	#4
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Well yeah, I can think of a couple of numbers off the top of my head, but I wanted something humble and well-trodden upon. The 6BL8 would work, but I wanted a triode with a little more mu. I also wanted to see how well the 6U8 pentode would work with the bias values originally set in the data sheet rather than as a pitiful starveling.

26th July 2010, 02:03 AM	#6
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	BTW., I'm looking at the 6AU5GTs I have in house, and I don't really believe the max plate rating of 10W, as I have 6AQ5/6005 tubes with much smaller plates and the same max power rating. We'll see about that...

26th July 2010, 05:13 AM	#8
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	My thoughts exactly. With those small transformers, it doesn't pay to get too frisky. Replacing the transformers negates the point of the whole exercise. If I replace everything, why buy the silly thing in the first place? Tying a pink ribbon around the neck of this particular jacka** is the whole point...

26th July 2010, 05:14 AM	#9
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Oh, and doing it cheaply is the other point....

27th July 2010, 08:17 PM	#10
Chris56000 diyAudio Member Join Date: Jan 2009 Location: Sutton Coldfield, UK	Hi! I've got a circuit for a quality Phase Splitter using a single ECF82 valve which you might to try - originally from a 1950's Wireless World. I'll post it in due course!!! Chris Williams

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