• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

John Swenson's (In my eyes revolutionary) preamp!

Status
Not open for further replies.
Jump to Latest
#22
Can anybody suggest a suitable buffer (maybe with ECC88...)?
Click to expand...

IMO, you will do better building a cathode follower around either the current production ECC99 or the NOS 5687.

There are a number of ways to enhance CF performance. Bipolar power supplies are 1 way. I lean towards CCS loading; a 6AU6 makes a suitable current sink, as does a FET.
 
#23
audio attenuators

If all one wants to do is build an attenuator/preamp, then I would certainly agree with the link Tim provided. The beam deflection tubes are cantankerous beasts, sensitive to magnetic fields, difficult to align for linearity (offset and common mode deflector bias) and they pushed the edge of manufacturability at the time, due to precision element alignments needed. Their big advantage was the extreme linearity achievable from the deflectors. (Although a simple LTP and current source tail come close.)

For attenuator duty, a nonlinear control can well be advantageous, like an audio taper pot. instead of a linear one. And there are many, cheaply available, all but ignored, dual control TV tubes available. Some tubes:

Dual Plate, Dual Suppressor control: 6BU8, 6KF8, 6MK8, 6GS8, 6HS8, 6LE8
(these tubes are almost like the Beam Deflection Tubes in operation but don't use a sheet beam, biasing up the suppressors common mode voltages to about +15V can sometimes get more linear transfer curves for the attenuator input if desired) (I've gotten some of these for $0.50 when on sale)

Dual Grid Control (dual control pentodes):
6888, 7AK7, 6BV11, 6GY6, 6HZ6, 6AS6, 6DB6, 6DT6 ($0.50 on sale often too)

Pentagrid converters: 6BY6, 6CS6, 6BA7, 12BA7, 6BE6,12BE6, 5750, 6SA7, 5915

Don
 
#25
Hello All,
I see my little experiment has wound up over here as well!

My ultimate goal for this is to make an integrated amp with the 6JH8 as the "power" tube. The preamp was just a quick and dirty way to get something built to see if I could get any sound out of it at all without having to buy the output or interstage transformers.

As to some questions on the output impedance, I just measured it at several different volume settings and they all came out to be right around 2K, the value of the plate resistor.

I just got some 6JH8s in and tried them out in the same circuit, they give quite a bit more gain, but the distortion is higher, I can hear it but haven't had time to actually measure it yet.

Well thats it for now.

John S.
 
#27
I ran a spectrum of the 6JH8 last night, its similar to the 6AR8 except it has 10db more second harmonic. That increased second harmonic is quite audible.

In the next couple days I plan to try some trim pots on the deflectors and see what happens at several different bias points.

For those of you that seemed to be shocked that a preamp could have a 2k output impedance, it works fine for me. Of course I'm driving 45 SETs that have an input impedance of 250K, and only a few feet of IC, so there is no need for a big buffer after the BDT.

John S.
 
#28
Curious that the 6JH8 has more distortion... the curves on the datasheet look somewhat cleaner. But then again, it is just a datasheet. Have you tried running either tube at lower plate voltages? My BDTs will arrive sometime next week, so as soon as I can slap together a cheap power supply, I'll be able to contribute more results.
 
#29
I have tried using B+ voltages in the 169 to 240V range. My supply was supposed to be 250V but it really came out to 240V. I have not noticed ANY difference in performance with changes in B+. At one point it was accidently running at around 60V and was still sounding great. I have not had the time to run spectrums with both types of tubes over different supply voltages, I've been mostly listening to it!

I have noticed that the design is quite sensitive to wire dress. At one point I had been messing around with different resistors inside and had managed to move a bunch of the wires around inside and it sounded much worse. I did a spectrum and it was terrible. I moved the wires back to the way I had originally had them and it was back to being much better. I haven't figured out WHICH wires were the ones causing problems, I need to spend some time playing with that.

Next time I'm going to make it on a plate that gives me a little more room to play with component location and wire dress.

John S.
 
#31
Hi,

this is interesting, but is it just an alternative or better than a passive or tvc? I guess we'll have to see what Bas thinks when he builds and tests it. Bas what are you using at the moment, that you could use as a benchmark against this?

Thanks
Raja
 
#39
Hmmm, DTS processing with valves. I really want to see that!
Click to expand...

I read that someone made a tube based decoder that transforms stereo into 5 channels (front right, front left, center, back right and back left). The basic idea came from the articles by Michael Gerzon http://www.audiosignal.co.uk/Gerzon archive.html and an other guy, don't remember the name. He claims he also put in some theory of the algorithms used by Ray Dolby, and now has a product that does make stereo sound much better, not completely accomplisching dolby digital, but getting close. He did not want to share the schematic, and when I sad I was interested in buying one (i would like to at least try it out) but made a mention of "reverse engineering", he said he would put the 'processing circuit' in a box and drop resin into it... Than I asked if it would be at least possible to listen to it, but did receive no reply.

But, back to the thread. Bas, where did you buy the 6AR8's... I know they are available on ebay, but probably there are (even) cheaper sources. I could buy some, and have another project on the shelf, there to show me it is not time to die, yet.

Erik
 
Status
Not open for further replies.
Top Bottom