Bias adjustment question

[planet10]

Originally posted by gingertube
There are a few schemes you can try to reduce the mains voltage.

Why not just consider it an opportunity to add another filter section (RC or LC)?

dave

[gingertube]

Dave,
BECOZ we want to get the heater voltage down as well as the High Voltage. Tubes take a "dim" view (joke) of heater voltages over 7 volts.
Also if we make the mains dropper as a separate box into which we plug the amp there is no need to "*******ize" the amp.
Cheers,
Ian

Edit: oops - Fell fowl of the language police. What I meant to say, of course, is that there is no need to "mess with" the amp.

[planet10]

Originally posted by gingertube
BECOZ we want to get the heater voltage down as well as the High Voltage. Tubes take a "dim" view (joke) of heater voltages over 7 volts.

Yea, DC heaters might be overkill....

dave

[planet10]

Or add a separate filament trafo (a good upgrade anyway)

dave

[n0plb]

I will get back to you whether or not I am able to come up with a 12v 6 watt bulb or not.
Dave are you needing the sch for yourself or to help with the problem? I don't have a problem either way just needing to know whether to e-mail it or put it into a thread.
I will also be searching for the zeners that was mentioned.
Keep it coming guys I appreciate all the help I can get!

[planet10]

Originally posted by n0plb
Dave are you needing the sch for yourself or to help with the problem? I don't have a problem either way just needing to know whether to e-mail it or put it into a thread.

It is for interest ... i have at least a couple 6BM8 amps to build, and i'm accumulating data. Might as well post it here -- it will be helpful for evryone to see in halping you sort it out ... (as big as you can make the GIF and keep it under 100k -- mail it to me if you don't have tools to manipule the scan)

dave

[n0plb]

First of all I want to thank Dave for modifying my schematic to fit on the forum. Thanks for your help.
I changed the power supply transformer last night and got the heater voltages back to where they should be.
I still have a very slight glowing to the plates of the 6bm8's.
Measured voltage across the plate to cathode of the tubes measures is 280 v.
Plate to ground is 300 v, and cathode to ground is 20.5 v.
I am including a schematic of the amp and hope that someone can give me some help to a beginner.

[gingertube]

20.5 V from cathodes to ground is across a common 250 Ohm resistor.That means 82mA or 41 mA per tube. 280V x 41mA is 11.5 Watts - You have to get this below 7 Watts.

Before you go to much further check the voltages on the output tube grids. One common problem with these Japanese Amps of this era is that they often used a particular Toshiba Oil Impregnated Coupling Capacitor. These caps dry out and go leaky. This can cause the Output Tubes grids to be pulled positive (as the 0.047uF coupling caps get leaky) and so the tubes conduct excessive current.

I have a STAR SA30 which looks very much the same as far as the schematic goes - I found more than half of those Toshiba Caps I checked were bad. In the end I stopped testing them and replaced the lot.

If the coupling caps are OK then you could try changing the 250R cathode resistor to 270R or even 330R. Note that the cathodes of the two channels are tied together (The "To K1" and "To K2" lines). Again this was not unusual in these amps. You will have to change resistors for both channels at one time.

Cheers,
Ian

Cheers,
Ian

[n0plb]

All caps have been changed in the unit so I hope that they are not giving the trouble. Now to find 3 watt resistors in the range that I am looking for. The ones there now are quite large in physical size and marked 3L.K.250ohm suzukiohm and are wirewound.
Will let you know how it goes.

[gingertube]

Your local electronics hobbiest shop will have bog standard 5W wire wound resistors - use those. Suggest you try 330R first and then from measurement and calcualtion as above you can decide if you need to change to 270R or 390R.
Cheers,
Ian