One or two bias test points?

[drz400]

Me too.... Anybody know a decent set of plans / schematic??

Ron

After dealing with surplus testers I finally splurged on this
I'm pretty happy with it
http://www.maximatcher.com/operate.htm

Otherwise make yourself a 50W power amp with bias resistors, use a scope and a signal generator to check the match with signal applied, bias resistors in cathodes to check idle current. wack on them with signal applied (tap lightly) to check for shorts and put resetable fuses in the cathodes.

[jtmjtm45]

some one told me Weber may be thinking of putting a tester kit out.Try him!

[ToneMerc]

I've often wondered about the wisdom of two test points for fixed bias in a push pull amp. Even with the best of "matched" tubes there is always a few ma difference. So you set it so one's a little hot and the others a little cool compared to your goal. What's the point of that? After all is said and done (redplateing aside) don't we just set bias so it "sounds right"?

Why not put a single one ohm resistor between a common tie point for both cathodes and ground, then measure the combined current as a starting point; then set it where you like it without having the pair over limits?

Comments appreciated

Dan

Digging up an old thread here....

I'm building a 6G6B Bassman(actaully 2) and I would like to keep it externally as stock looking as possible and any additional holes to a very miminum. Since there isn't a ton of real estate I have been debating for the last few days on the location for the bias test points.

Earlier to today I was thinking the exact same thing that Dan mentioned above.

Here's what I think I'm going to do, mount the 1 ohm resistor on each power tube, but only install one red test point in the chassis. This still would provide full functionally for resistor biasing from the inside of the chassis, but obviously you can't measure both tubes at the same time externally. However, you could move the the non TP tube to the TP socket and check it there.

Since this is my personal build I don't really mind this scheme, but I wouldn't sell an amp this way. The another build for a client will have the 2 tests points(wherever he chooses) and a tube rectifier option.

TM

[martin manning]

How about this: Put a 1-ohm from each cathode to an isolated test point, and a one-half ohm (or two 1-ohms in parallel) from the test point to ground. The single test point will give you averaged current per tube (as mV) from the outside, and measuring from either cathode pin to the test point will let you measure each tube individually from the inside. And, you can always lift each tube slightly and probe the cathode pins to get individual current draw from the outside, but don't tell anyone I said to do that.

[gingertube]

Here is a pic during construction of one of the amps I built .

1 Ohm in each cathode to ground. The 2 cathodes wire to the red 2mm sockets (multimeter probe size)b the black socket is the common ground, the trim pots are the individual bias adjustments for each tube.
Put meter between 1 red and the balck to set each tube to about the right idle current then finally put multimeter between the 2 red sockets and adjust for 0V ( that is absolutely matched idle currents).

If building a fixed bias amp from scratch there is absolutely no reason not to put in adjustment for each output tube.

The blue things are the Bourns 10 turn bias trim pots.

Cheers,
Ian

[Reeltarded]

You also use smaller g wire for heaters and twist them into a quad that splits to two? That is very interesting. Am I seeing that right?

[gingertube]

No its just one heater chain - I twist the 2 heater wires right up to the socket and join the twisted pair running to the next tube socket on the socket pins.
This pic may show it better.
The wire is 16 AWG by the way good for 7 to 8 Amps which is at least double what this amp drew (4 x 6V6 + 4 x 12AX7 + 1 x 12AT7)
The PCB to the left is a London Power Power Scaling Board.

On little hint - see the switches, they are the type which are provided with a plate with ON/OFF labels which key into a slot on the bush, don't through those plates away, stick them on the inside and just before final asembly wipe some 5 minute araldyte on the plate wings. They then become anti rotation devices for the switches.

Cheers,
Ian

[gingertube]

Here is the top pic but after wiring.
The 1 Ohm resistors are on teh actual bias measuring sockets. Note that contrary to a lot of folks recomendation I deliberately use 0.5W for those resistors so that they act as cathode fuses in the event of a tube short.
Cheers,
Ian

[tubeswell]

Another vote for 2 bias TPs. I don't care which one I actually use on the day tho'. Its all ballpark science anyhow

[Reeltarded]

Here is the top pic but after wiring.
The 1 Ohm resistors are on teh actual bias measuring sockets. Note that contrary to a lot of folks recomendation I deliberately use 0.5W for those resistors so that they act as cathode fuses in the event of a tube short.
Cheers,
Ian

Obviously a great deal of thought beyond common fare in the production amp world..

Thanks! Excellent! I have a long way to go.

[Structo]

Is araldyte a two part epoxy glue?

[ampgeek]

Gingertube: Hope you don't mind my asking..but...what is the P/N and source for your bias adjustment pots?

I found a handful of similar examples in a box of loose parts that came my way a few years back and am about to use the last two.

Can't for the life of me find them (or similar) at our usual parts haunts.

Many thanks,
Dave O.

[gingertube]

Bias Pots are the 32mm (1 1/4") types although there is no reason that the smaller 19mm (3/4") types could not be used.

32mm are Vishay 70Y Series
19mm are Vishay 43P Series or Bourns 3006 Series.

Vishay make both 19mm and 32mm panel mount adaptors.

The Vishay 43P Series is available in a panel mount version and this is the cheapest option.

I bought them from Farnell (now Element14) her in Oz but Mouser etc should have them.

One more hint, screw the trim pots max anti-clockwise before fitting, you can them use a multimeter during wiring to make sure that you get max bias volts (min idle current). That way, clockwise adjust always increases idle current.

Yes araldyte is 2 part epoxy

Cheers,
Ian

[ampgeek]

Thank you sir!
Cheers,
Dave O.

[Firestorm]

Here's something I've started doing to avoid dealing with the spindly legs on the cermet pots: the board at the bottom holds the pots (in their little panel mount housings), the grid load resistors, bias load resistor(s) and bias safety resistors (in case a pot opens). In this case, it worked out that the bias adjustment board sat nearly right across from the coupling caps so the colored leads could have been even shorter (didn't seem to need to be, tho).