happy land for octals and pentodes

[Andy Le Blanc]

Spent the time this week prototyping an amp with a 6sj7 pre and 6sn7 inverter.

The first hurdle was heater hum... the 6sn7 was problematic, hummy pain in the arse.
The solution was DC heaters... so I opted for a interesting way to get it.

The simplest solution using the least parts was a cascade voltage doubler.
Just two diodes and two electrolytic caps, uber simple to rig in the amp.

And... to run the two 6v heaters in series on 12v dc.

The drawback is that the 6sn7 are actually designed for this, with a "controlled heater warm up period"... it like waiting for the old black and white TV.

So... I was able to shorten the time at turn on by switching to 12sj7 and 12sn7 in parallel. Great fun being able to use brand new NOS, great tubes.

It also opens the door to good variety of tube types, very flexible for many NOS types.

The result was a 12sj7 pre and a 12sn7 inverter, I opted for a longtail inverter... and the pentode pre,
once you nail the voltage relationship between the plate and screen, the screen grid voltage window with sharp cut-off pentodes is surprisingly small,
they are lovely with 5% harmonic distortion figures.

[matt h]

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[davent]

I've a couple amps that will take 6sl7's, my hifi (with dc heaters) uses them for the inverter, then i built a universal 5f2a that will take 12a_7's, the 6eu7 and 6sl7, 6sn7 and the usual suspects for octal output tubes. With the 5f2a, 6sl7's are much noisier then 12a_7's or the 6eu7, this runs elevated ac heaters. With those same 6sl7's run in the hifi amp, no such noise.

[matt h]

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[davent]

Hi Matt, I've no voltages notated on my old computer schematic but the voltage was derived by a 330k:68k//10uF voltage divider off the screen node, guessing somewhere between 60-75 volts. (270ex transformer)

Well on a second search i found a paper printout that i did notate, 70vdc.
dave

[matt h]

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[Phil_S]

once you nail the voltage relationship between the plate and screen, the screen grid voltage window with sharp cut-off pentodes is surprisingly small, they are lovely with 5% harmonic distortion figures.

So, Andy, are you going to share, or just keep us in suspense?

[matt h]

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[Jack Hester]

Please post your schematic, as I would like to see your heater voltage doubler. I've just buttoned up my Guild Thunderstar, and was intrigued by the doubler in the power supply. 190vAC rectified/doubled to 490vDC. I would like to compare the two, plus I'm always interested in amps built around nice old octals. Thanks for sharing your prototyping. Have a good one.

Jack

[davent]

Also interested in seeing what you came up with for the preamp. Have an amp based on the Octal One (6sj7) but did tweak that tubes component values to brighten things up, was pretty dark sounding as per the schematic.

[Andy Le Blanc]

The PT that I'm using does not have a center tap for the 6.3 fil's. so I was stuck.
Even the old data sheets for the tube type show a preference for a center tapped AC source and then elevating the heaters with DC as a last resort
And... I really wanted to be able to use all the tubes I've got hanging around the bench.
A separate DC supply at 12v was the more flexible resolution, for more tube types.

With sharp cut off pentodes, the same as with power pentodes, there is a very old argument, to use a voltage divider instead of a series resistor,
to set the screen grid voltage relative to the plate.

measure the difference between the plate and screen, keeping the screen negative relative to the plate.

with a simple variable supply (just like adjusting bias) you can find, maintain and adapt ( the sweet spot to your ear)
for the tubes your working with and other pentode types

[Jack Hester]

Really good info. Have you had an opportunity to make a schematic? Please post one when you are able. Thanks.

Jack