At a local ham fest I picked up 100 of 150V zeners so I thought I would
build myself a stabilised 300V HT supply - not big current, just enough
for my mic pre projects. So, I have been looking at high voltage
transistors and came across the TIP50 which are rated 400/500V. I picked
this oas it the only one whose number I vaguely recognise.

Has anyone used these for HT supplies or what else do you use?

Cheers

Ian

I think I asked this question before, but never got any
good answers. I was checking a bunch of Telefunken 12AX7's.
I got different, mostly improved readings on the 539C tester by tapping
on the tubes. What do I believe ?

greg

I was given a Presto 92-B and would like to find a schematic please.
Must be one on the WWW somewhere, but I have not turned it up.

Anyone know the difference between the 92-A and 92-B?

please remove the "REMOVE_THIS" in the e-mail address if replying,

thanks,
Patrick J.

** Have an Ampeg SVT-2 Pro ( bass guitar head ) on the bench - it uses
six 6500s in parallel push pull ( class AB) with grid bias to generate about
300 watts. Drive signal to the 6550 grids comes from a pair of 12AU7s with
half of each wired as a direct coupled cathode follower.

The 12AU7's operating conditions are:

Plate voltage = + 395

Grid voltage = - 78

Cathode voltage = - 56

Cathode current = 3 mA

Plate dissipation = 1.35 watts.

Heater / cathode voltage during warm up = -195 volts.

QUESTIONS:

How long will a cheap, Chinese short plate 12AU7 survive ?

What happens to the 6550s when the heater cathode insulation fails during
warm up ?

( The above IS what happened with this one. )

Also - what 12AU7 is the most likely to be OK here ?

..... Phil

I have a severe 60 hum being produced by
by my home brew OctomberPlexi:

http://www.freewebs.com/willispage8/octoberbuildpage1.htm

I've cured some wrong components over the past weeks in the
tone/PS circuit that certainly awoken the beast.

Now, I have this intolerable 60 oscillation that I
think I have traced to the PT ( Weber 25130 ).

I back tracked the noise to the rectifier circuit:

There are diodes in series ( D1 -> D2 on one leg, and
D3- > D4 ) for the full wave rectifier, and connecting
a scope between D1 and D2 produces this weird wave sign
that appears on B+ as a sawtooth ripple:

http://www.freewebs.com/willispage8/pwrsupp02.jpg

Here is a little more detailed document on it:

http://www.freewebs.com/willispage8/octp1/18wpwrproblem.doc

I'm convinced it's the $50 PT ... simply because I replaced the diodes
and I used the alternate higher voltage taps and I get the same
result.

Thoughts ?

Why did they discontinue this excellent tool?

Completely irrelevant to tube audio but very relevant to why I like it:

January 2002

This month�s article is not going to focus on a specific driveability
problem, but rather a specific testing technique. Have you had the
opportunity to see a Toyota Wide Range Air:Fuel sensor yet? You may have
seen one and not even noticed it. Why? Because this new type of "oxygen"
sensor looks very similar to a conventional oxygen sensor. Photo 1 shows a
Wide Range Air:Fuel sensor in a 2001 Camry with a 2.2L engine.

At first glance, it looks like a standard 4-wire heated O2 sensor. Well,
it is definitely not a standard 4-wire O2 sensor. Toyota first started
using Wide Range Air:Fuel sensors with some 1997 models in California. By
1999, their use had spread to other Toyota and Lexus models.

Wide Range Air:Fuel sensors are sometimes called Linear Air:Fuel sensors.
Both Honda (in some limited Civics) and Cadillac (1999 Catera) have used
this unique type of air:fuel measuring device. But what is the benefit?

Let�s answer that question with a chart shown in Figure 1.

In groups or 2+>
http://cgi6.ebay.com/ws/eBayISAPI.dll?MfcISAPICommand=ViewListedItems&userid=myradioroom&include...

I'm working on an old power supply that uses two 0B2 tubes. Both test
good, but one glows with an orange color, and the other blue. I have
several other 0B2s - all of those glow blue.

Is it unusual for an 0B2 to glow orange? Does that indicate a problem?

-Thanks