Deville Transformers Reuse

[king_me]

Hello !!!

 New to the forum and ive been searching high and low but maybe someone here can help me. Looking to build a Hoffman AB763 One Channel or Bassman 6g6b circuit  out of a fender deville chassis that has a damaged tube board. Not looking to save the circuit board but would like to save the transformers, chassis and 410 cab. Jus need to know if the transformers are good for either of the builds. Thanks for reading this and hope some can answer this or point me in the right direction.

[sluckey]

Transformers will be fine for a 6L6 AB763. But the PT is designed for a FWB rectifier so you should just use the Deville HT power supply. Forget about a tube rectifier.

[king_me]

Thank you sluckey!! Any schematics on that portion would be appreciated if you have on hand. If not thanks for the reply.

[sluckey]

https://el34world.com/charts/Schematics/files/Fender/Fender_hotrod_deville.pdf

[king_me]

my research journey continues before my great build. Now that i found the correct wiring of the donor PT. Wanted opinions of going with a fix bias or a adjustable one. The build that i am going for is the Hoffman AB763 One Channel(no rectifier tube per the PT i have). Thoughts and suggestion are very much appreciated.

[kagliostro]

Go for an Adjustable Fixed Bias 

Franco

[king_me]

THank you guys for the response that i have received. Need help again in my planning. Since i am using my existing Transformers trying to plan my work accordingly.I am more of a visual person so building the power section so i can populate a board.  Where i am stuck is on the diode section. Trying to figure out the connection points. I see it then i dont see it lol. Attached rough board layout.. Also not sure if i need the 4.7 ohm resistor to ground??   

Thanks for reading this!

[sluckey]

Your diodes and yellow cap are wrong. This is right...

[king_me]

Thanks Sluckey! Man that cap also threw me off. I understood the schematic but for the life of me could not build.

[king_me]

Quick questions on the correction made by slucky on the power section. Should i see a variety voltages like seen in the pic? i know it will vary from what is seen at least i would think. What would cause it to be the same voltages at each point (490vdc)? building this https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf. tubes not red plating but 470s or not surviving. Going to double check every connection tonight.



Thanks!!!

[sluckey]

If the amp was working correctly, you would see a variety of voltages. Since your voltages are all the same this means that no current is flowing. This usually means no tubes plugged in or a wiring error. However, your tubes are plugged in. You say the 470 are not surviving. Does that mean they are burned up? Maybe even totally open now? This would indicate excessive current flow until the 470s burned up (Your output tubes were likely burning up too!). Then current flow would cease and your voltages would all be about the same.

My concern is why the 470s burned up. Most likely due to very low or no bias voltage. Measure the negative voltage on pin 5 of the output tubes. What have you? It should be about -36v for 6V6s or about -50v for 6L6s. Pull your output tubes and don't put them back in until the bias voltage is correct.

[king_me]

thanks Sluckey for the info. yes the 470 started to smoke killed power as soon as seen. The resistors did not go completely open but they did lose resistance 250ish.. Replaced with new ones and pulled the tubes i do see variety of voltages now 448 460 477 VDC. At pin 5 i was around -31.1 and now im max out at -38.4 and does go the other direction if i turn the pot other direction. (6l6GC tubes). Still chasing before i put tubes back on.

[sluckey]

6L6s want more bias voltage than that. There is a 22K connected to one side of the bias pot. Change that to 27K, 33K, even higher if needed. You want a bias range that covers about -40v to -55v. Pull the 6L6s and don't put them back in until you have the negative bias voltage in the correct range.

[king_me]

replaced with 220k and maxed out pot -47.3. Should it be good ? And will i lose the -47 once i turn on the standby?

[sluckey]

replaced with 220k and maxed out pot -47.3. Should it be good ? And will i lose the -47 once i turn on the standby?

No that's not good. I just realized the HRD uses a voltage doubler in the bias circuit. You'll have to adapt a voltage doubler to work with the Hoffman circuit.

Do this. Measure the AC voltage between the two brown wires on that PT. Report back and give me some time to come up with a bias circuit that will work. It's past my bedtime but I'll take a close look tomorrow morning.

[tubeswell]

The actual Deville 4 x 10 combo service manual and full schematic

[king_me]

Thanks Tubeswell i appreciate the resource. I have a snippet of what i have. Between the two brown wires i have 57Vac and between the Brown and Brown/green 28Vac

Currently i have the Brown and Brown/Green configuration. I did not build out the rest since i thought it was used for the IC chips and such. Also i do see on the schematic i should be around -49Vdc which i do. Attached what i built maybe i didn't transpose the schematic correctly?

Thanks again guys for this experience!!!

[tubeswell]

Looks like the right bias source voltage to me.

[sluckey]

Bias winding is perfect for Hoffman's board. Just connect the bias winding as shown. Don't use the BRN/GRN wire. Oh, and put the 22K resistor back on the board.

[king_me]

Thanks Tubeswell for your feedback


Sluckey wired up as mention by your comments. Now i have a better swing -68 max -30min. set about -48.5.. Looks like i may have taken out the tubes since i have no sound..Now this is my first crack at it so didn't have to begin with since the 470 problem.  double checking wiring just eliminate any solder joints or misplaced connections before another set goes in. For another question and it should be the simplest for you guys of the whole build and my hardest to picture. The speaker output. Im reusing the jack (3 lug only). I have the green 8ohm to tip connected to FB 820 ohm of the board. sleeve and shunt to ground. Black from output transformer to ground. grn/yellow and white not connected to anything. sorry i do not have a good visual to show. Is that correct?

[sluckey]

Sounds correct. Show us some hi-rez pics of your build. Resolution needs to be high enough to read component values.

[king_me]

Gentlemen Sluckey and Tubewell thank you for the communication on my first build!. Sluckey I have sound and now for my embarrassing find... I forgot to connect B+. Oh gosh did not want to share that lol.


Again thank you guys!!!

[sluckey]

now for my embarrassing find... I forgot to connect B+. Oh gosh did not want to share that lol.

Don't think you are special! We've all had those kinds of embarrassing moments. Best part... this was a non destructive moment.