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Hi,
I am in the process of building a 125 watt power amp using 6550's/KT88's. Or I should say I am attempting to build one. It is using Dynaco transformers from tubes4hifi. The input circuit is SS that a friend is going to put in. I am just doing the output section myself.
A friend suggested to put on pins 3 and 4 of the power tubes a 1K resistor wrapped with transformer winding wire(30 guage). It is a PIA(pain in the a_ _) to do but I am trying to be patient and get it done.
What does this wire do? Is it an inductor? Does it get soldered at both ends of the resistor?
I know when putting on grid stopper resistors to keep the resistor as close to the pin(solder lug) as possible.
Thanks for any info!
Follow Ups:
Your friend is suggesting a technique that is commonly applied to the anodes (and less commonly the screens) of transmitting tubes. You can use almost any type of wire for this - enameled or tinned - and it should be soldered to the resistor leads at both ends. The two resistors in this photo show the general construction method:
The concept here is that the parallel R/C combination constitutes a low-Q inductor that will impede high frequency (RF) currents and quench RF oscillation. I can't tell you how necessary this might be with those particular tubes, but I don't recall seeing this technique used in audio.
I did a full resistor winding end to end and soldered both ends. If it works then fine. If not then I wasted my time and let me tell you it's a bitch to wind the resistor but I am getting better at it. 30 guage wire
Just to flesh this out, the resistor is usually 1W or 2W, and it must be carbon comp like the photo. Resistors constructed with an internal spiral track, such as carbon film and metal film, are ineffective and may themselves encourage oscillation. Years ago, someone was selling carbon comps that were actually film resistors in disguise. It caused a major problem for a number of people working on high-power RF amplifiers at the time, including myself. I spent hours on this problem after rebuilding one particular amplifier, finally cut one resistor open and saw it had a spiral track. Replacement with genuine CC resistors solved the problem.
I've never done anything like this in an audio amp, and I don't know of any commercial manufacturer who has, either. The Q of the anode circuit (tubes and output transformer) is pretty low, and that seems sufficient to quench any tendency to oscillate. Control grids and screens respond to a simple stopper resistor attached to the tube socket pin, usually about 1KΩ at the control grid and 100Ω to 220Ω at the screen. No wire winding necessary or desired.
Hi Triode,
You seem very good at this so I am going to take your advice and go to carbon comp with no winding on it. Now! The 100 ohm goes on what pin? 3 or 4 And the 1K goes on the remaining pin.
Thanks!
Paul
Well, I wouldn't use anything at the anode (pin 3). You certainly wouldn't want to add unnecessary resistance there, and I don't believe oscillation is an issue if the other stoppers are in place. The screen (pin 4) needs a 100Ω to 220Ω stopper resistor, 1/2W or 1W. I haven't heard of anyone having issues if this resistor is a standard carbon film, metal film or metal oxide. The control grid (pin 5) needs a 1/2W carbon composition resistor about 1KΩ to 1.5KΩ, attached as close as possible to the socket pin.
These measures should keep the tube(s) completely stable. Incidentally, pin 6 of this tube is unused and can serve as a tie point for the control grid stopper resistor. If pin 1 is not attached to a metal shell (depends on the tube), it can also be used as a tie point. The screen circuit is less critical in this regard than the grid, so a little lead length won't hurt if pin 1 is not available.
Thanks!
If it uses four 6550s (push-pull parallel), then anode stoppers like those shown above may be necessary. 47 or 51 Ohm carbon, 5 turns or more, in series with all four plate leads. 100 to 220 in series with each screen lead, current is low enough that adding a choke across it isn't necessary.
Ask your friend who recommended it to explain and clarify and let us know. Perhaps he is on to something.
Pin 4 is G2, the screen grid - perhaps the 1k resistor will serve as screen stopper, but I don't understand the purpose of the 30ga wire wrapped around it, and whether, or not, it is soldered both sides. Pin 3 is the plate and it seems unlikely to me that you want a 1 k resistor between the plate and output transformer secondary winding.
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