diytube.com

Just wondering if I can use the poseidon circuit design to upgrade the Mr. Liang T2 model I bought from eBay. The amp has been having a problem lately it keeps on burning a cathode resistor. I want to upgrade it to poseidon, so that I can take advantage of the bias adjustment features. The T2 model is pretty much the same as the modified mullard circuitry, except for the 5K output transformer with 4, and 8 ohms output.

I really appreciate your help on parts recommendations. Thanks

Is this a Mark III clone, or a whole other amp? Do you have a link to where I can check it out visually?

Hi Shannon, thank you very much for responding to my questions. It is very much close to the improved mullard 30 watt amplifier using the Dynaco A-420 OPT. can be found here http://www.triodeel.com/dynamull.gif

The T2 however, is using a KT88 for the power tubes instead of the EL 34 found on the original design.

Here is the schematic of the Mr. Liang

There is another hobbyist by the name of Lorenzo of Fullerton successfully modified this T2 with your boards, but he did not keep much records of the parts substitutions. However, he did sent me the schematic of the constant voltage B+ and the Bias supply modification using one of the 350 tap from the power transformer to deliver a constant -39 to -56 volts DC for the bias supply to accommodate your board requirements.

Thanks again.. Yellow_Light_Colorz_PDT_12

Can you tell me more about the power transformers? We could basically rebuild your amp like a KT88 Mark III, I think.

The power transformer's secondary winding are (2) 3.15-0-3.15, (2) 5V/4A, 350V-300V-260V-0V-260V-300V-350V at approximately 350mA.

It would be very Nice, if we can turn this amp into something like Mark III. I think if we can accomplished the conversion, it will help a lot of people who are until now still facing the difficulties in finding 6N11 and 6N6 replacement tubes, as I have read on audiokarma, and other diy audio forum.

Which cathode resistor are you burning? One on the output tube, or one on a driver tube somewhere?

The one on the output tubes. I believe it was the R16. I have taken out the board already in preparation for the poseidon boards. If there is any hope in upgrading this to the mark III design , I think the power transformer, and the output transformers are still in good shape. The resistor will just burn into ashes slowly. I did noticed the quality of the parts used on the boards were borderline, commonly found on this type of amps. Most of the power resistors on the driver board are beginning to burn (turning black from maroon). I was thinking on upgrading the parts, but it will be just useless, because the 6N11 tubes are not easy to find. It will be very nice if the poseidon boards can be matched with the power and the output transformers.

Thanks...

I would sort out the smoky resistors first. You shouldn't be burning out resistors in that position. Was the amp built for KT-88's or EL-34's? Also, are the voltages inside the amp close to spec? You might try and squeeze a 10 watt resistor in there and see if things stabilize. If you want to use the Poseidon, you will likely get rid of that resistor and capacitor anyway, although you can opt to keep the cathode bias instead of converting to fixed bias.
-Paul

Thanks Paul, before I took out the board from the chassis, I made sure that all of the voltages were within the specs. The B+ was 400V, for the KT88's anodes the readings were around 357 to 368 volts and about 370 for the screens (pin 4). According to manufacturer this amp was designed to run on KT88 . I sent out several emails to the manufacturer in Hong Kong, but their only response was the schematic as shown on this thread (probably so, because I bought this amp on eBay).

Several attempts were made on the resistors: I replaced the resistors with 10 and 20 watts, then experiment with ceramic types (U.S. made ) the result was the same. Although the ceramic can't be burnt to ashes, it would still get extremely hot. There is a guy by the name of Lorenzo residing in Fullerton, CA (won't respond to any email right now, too busy writing an output transformer for tubes calculator program), did a successful conversion of this amp to Poseidon boards, but he did not keep much records of his parts adjustments. He warned me not to use the Poseidon parts straight out of the BOM, because of the excessive bias voltage appearing at about -2.56V even with the bias pot at its lowest.

Thanks,
Mike

Ok, I don't see any issues with using the Poseidon, except that you are using cathode bias currently, and the Poseidon is setup for fixed bias. If you wanted my 2 cents, I would convert the amplifier to fixed bias first. You would remove the 470uf cap and 250 ohm resistor, and then connect a 10 ohm 1 watt resistor between pin 8 and ground on EACH tube. Next, you will need to add a small 40-50VAC transformer and a negative bias supply, then a pot to bias each tube. Basically, you can copy the bias scheme for the Dynaco MK IV, but double up on the bias setting pots. This will get you "around" those scorching resistors, and you can easily measure the current each tube draws. You will want to have have .55-.60 volts between ground and the pin 8 end of each 10 ohm resistor.

When this is done, then you might consider the Poseidon, otherwise, there will be no change in terms of burning out resistors. Also, just for the sake of being meticulous, you might want to measure the DC voltage at pin 5 on each KT88 while the amp is running and report back. If you have a coupling cap that's leaking voltage onto the grid(s), you can easily draw more current than you should.