2nd & 3rd Harmonic Distortion curves for 5B/255M (807)

[James D.]

I have been playing with a design for a friend using 5B/255M valves. These are basically a loctal version of 807 beam power tetrodes. My initial design and breadboard was a PP amplifier and it performed very well - but I thought I should knock up a SE version to compare with, in case he prefered that. The SE was not bad but was clearly worse than the PP. To try and see a little of what was going on here I measured the distortion in both modes and found the PP to be much, much lower for 2nd harmonic and about half as much third harmonic too, so I'm not surprised that the PP sounds better (in this case).

So then I thought I would try the out of fashion Tetrode Class AB2 PP just for fun. So I tweaked it to Class AB2 added a smidgeon of feedback (6dB) and sat down to listen.... I was very surprised! It sounds really good. Actually really really good! Close to the A1 pp and better than the SE! So to measuring again. Surprise it has a nice distortion spectrum! Second higher than A1 PP but much less than SE, third nicely less than second... hmm...

My speakers are fullrange about 7ohms impedance in a reflex cabinet and about 95dB sensitivity. Source battery powered CD player. All amplifiers fixed bias and IT interstage. Driver tubes 6N6P. PP amps are PP throughout.

Here is the graph of the measurements.

[Brett]

Hi James,

Interesting post, and one which basically tallies with both what I've read, and what I've measured myself in the past. The 807 is one of my fave tubes (I have boxes of them in all flavours). The STC 807 datasheet (5.5M) might be of interest if you haven't already seen it. Excellent data with lots of graphs of different output stage configurations and distortions for 2H and 3H

If you have the facilities, it's very interesting to note the higher harmonic content in the different configurations, and how they change with level.

I'd be interested in seeing your schem for the A1 PP if that's possible.

BTW: do you have a source for 6N6P's? I've been trying to get my hands on some to try for a while now.

Cheers

[EC8010]

Interesting. Were you able to measure distortion at lower levels with the AB2 configuration? And, what Ra-a did you use? (I have some 5B/255Ms, you see.)

[James D.]

Hi Brett,

I hadn't seen that data set for the 807. Nice document! Thanks for the reference.

I had shown the schematic for the A1 pp in this thread
Simple IT PP . The input pot into the input transformer is not an optimum design...but it works after a fashion. I'm having some multiple tapped autotransformers built to tryn as a TVA (I guess AVA, or ATVA) for the real version.

On test I ended up changing the OPT tap to make it Ra-a=7K instead of the 11K shown in the diagram. Otherwise it's as shown. The pp balance pot was adjusted for best sound which was also minimum distortion ( as near as makes no difference) and it was with about 8% dc imbalance...

I have a great source for all sorts of valves. It's a chap in Germany, here is the URL
Ask Jan First . Jan is a great chap and very helpful. Please mention my name if you use him.

Hi EC8010

The Ra-a was 7.5K also in AB2. I didn't measure any other harmonics as my kit at home doesn't do it. I guess I could take it into work and put the Spectrum analyser on the output... Work is a bit busy at present so it might be a couple of weeks before we get to that. I'm curious as to the extent of crossover distortion and to the upper harmonic signature too. Through my speakers I doubt I pushed it beyond 10 watts at most. I was surprised at how relaxed it sounded and how good it's imaging was. I'm very impressed with the 5B/255Ms. BTW I didn't run them very hot - only 450volts on the anode and 250 on the screen grid. Also after 40watts the 3rd harmonic climbed away but the second had peaked and dropped down by about 30% after that...

ciao

James

[EC8010]

Hello James,

very few spectrum analysers have the resolution to look a long way down on their own, but if you hang the spectrum analyser off the oscilloscope output of the distortion analyser, it adds the dynamic ranges together. This subterfuge allows me to look reliably down to -90dB, and occasionally further, limited primarily by oscillator purity.

Of course, at this point, you start thinking, "Wouldn't it be nice to have an AP System 2, or even a 1, surely they are due to turn up second-hand soon. Please?"

Hello Brett,

my (extremely lethargic) printer is starting to think about the possibility of printing the first of the 43 pages of 807 data sheet. It looks like a real gem - thanks for the link.

[fdegrove]

Hi,

EC...mind the Rain Forrest.

Cheers,

[EC8010]

Originally posted by fdegrove
Mind the Rain Forest.

Hello Frank,

that's terribly 80s of you! In the 90s we worried about skin cancer due to the hole on the ozone layer. This year we simply worry about WWIII. Whoops, wrong thread.

[Circlotron]

Originally posted by Brett
The 807 is one of my fave tubes (I have boxes of them in all flavours).

One of my favourites too. I have just one, brand new in box made by RCA in 1972. Anyway, I once saw a cct that used a pair of 807's running at about 700 volts or so and zero bias and had *all three* grids of each tube strapped together and driven with a transformer (grid current BTW) and it was supposed to get 120 watts at 2% distortion. It was a modulator for an AM transmitter.That's all I can remember now. Never seen that approach used by anyone.

[7N7]

Originally posted by EC8010


This year we simply worry about WWIII. Whoops, wrong thread.

My three principal worries are

a) Trying to find 8116s/YL1071s at sensible prices
b) The outrageous level of tobacco tax in England
c) Why can't I "get more" smilies??

7N7

[pedroskova]

Originally posted by EC8010


WWIII. Whoops, wrong thread.

...as in Dubya Dubya III

Whoops, wrong thread.