Peavey DB 210 mods

[Teleman55]

I am about to do some mods to this amp that I have been reading at "Blue Guitar".I read in a previous post here about a cathode bias mod but can't find it.I would like to give it a try and could use some help.

[Enzo]

Go back to Blue Guitar and look up Peavey Classic 30 mods. Except for the added tremolo, the DB and C30 are the same.

[Teleman55]

Thanks Enzo-I found the article,I need to give it a study to make sure I understand everything.

[Teleman55]

It seems I have no 100 ohm 5W resistors the closest I have is 110 Ohm.Can I use this with the 100 mf bypass cap? By the way Enzo why did you remove the flyback diodes?

[Enzo]

100 ohm, 110 ohm, wouldn't make a lot of difference, so yes, of course it would work. A 10% 100 ohm resistor could measure 110 ohms anyway.

I don't recall the flyback diodes, but I would assume I did it because of all the trace cutting I had to do. In the stock amp, the cathodes are grounded. There is a main ground trace running across the board for this. The cathodes ground to it, but so do the flybacks and a couple other things. When I isolated the cathodes so they could be switched, that left the flybacks without a solid ground. SO it was a matter of rerouting their grounds or just eliminating them.

But I am trying to recreate some thinking I did a few years back, not remembering specifically.

And would the flybacks still do their job of connected to the cathodes? Probably, but there may have been other mitigating factors. Look over the layout of the traces for what was cut and see if any components wound up orphaned.

[Teleman55]

Thanks Enzo,Now I get it.I can give the flybacks their own ground connection if I want to keep them.I had the amp up and running after doing most of Steve's mods and it sound great up until it blew a fuse.I gotta go over what I did and find the problem,I thought it seemed to be running real hot so I used an IR thermometer and the power tube were at 319 degrees.I don't know if that is in normal range or not.

[Enzo]

So measure the voltage across them, measure the actual resistance of your 110 ohm resistor and then the voltage across it while under power. Then calculate the current through the tube from that. Current times voltage gives the dissipation of the tubes. That will tell you what is happening, I have no idea what the temperature should be, though 300 SOUNDS high. But are you meassuring the temp of the glas bottle with your IR or are you really measuring the temp of the innards through the glass. The glass ought not be that hot, but the innards sure would be.

[Teleman55]

I did some checking of my work,made a list of recommended mods,a list of original values and a list of what I installed.I found that when I added the 110/5W cathode resistor and 100mf/100v bypass cap. I neglected to cut the jumper that connected the cathodes to ground.Big Whoops!!Working too late and not using a checklist.I am most unorganised when experimenting on my own gear.I did not blow a fuse.I hope I didn't ruin the Output trans., the primaries test at 73 and 95 ohms.I'm gonna be away until saturday further testing will have to wait until then.I will post more if your still interested.

[Enzo]

Due to the way they are wound, that odd resistance mis-match on the OT is normal. That is not damage.

Was it as simple as cutting a jumper? I seem to recall having to cut traces AND install new wires for the other things that had been grounded to the same trace as the cathodes. On the other hand I was also adding the switch so it could be set back to fixed bias when desired.

We always like to know how projects turn out.

[Teleman55]

Yeah, I had overlooked that .I will post this weekend when I get back to it.Tonite I'm off to a John McQuen concert.

[Teleman55]

I did some testing and found the power trans is dead.I hope Peavey will sell a replacement unless you can recommend a substitute.I disconnected the PT from its terminal strip and got no resistance reading across the primaries or from each primary to ground,connected it to power and on the secondaries measured millivolts.There are 3 caps on the PT terminal strip one is a round black unit what is it?

[seizethedave]

Teleman,

I am getting ready to do the same mods on my DB. Do you have any suggestions or advice before I tinker too much? I don't believe that I'm going to do the bias cathode mod, though.

Did you do the Master Volume mod?

Dave

[Enzo]

CALL Peavey and ask for the parts department. they will sell you any parts, and their prices are reasonable. I have not found those power transformers to be a trouble spot, so sorry yours failed. They will also send you the schematic upon request.

The black thing would be an inrush limiter - a thermistor. If it is open it is bad, if it measures some low resistance, it is OK.

[Teleman55]

Thanks Enzo-the thermistor reads open.When I first fired up the amp I used a current limiter(100 watt bulb)and initially everything seemed fine, after about 5 mins. I heard a click and the bulb lit up indicating a short.I checked everything overand passed the current limiter test.Fired up the amp at full voltage and it died as described,when it died I again heard a click.I wonder how the cathode ground mistake shorted the PT without blowing any of the 3 fuses? More may be revealed when I reply to Dave.
Dave,Welcome to the discussion.I am glad to share what I can.Check out "The Blue Guitar".The FTP area ,there are Peavey files with many mods,browse through them and read carefully as some are Tweed Bassman mods.At the bottom of the FTP index there are more mod note,read them.As for what works it depends on your goals.I found my DB21O lacked openness and clarity.I recommend that you do what I eventually did to locate my problem,down an original schematic,it lists part numbers (C1,C2-R1,R2 and such),download another schematic that lists part values and a layout for ease in locating parts on the board.Then make a list with 4 columns.Column 1 is a list of part numbers to be changed,column 2 is original part values,Column3 is recommended changes,column4 is actual values used.I some parts that were close so I noted that and checked off work done.When removing the boards I found that getting the tube socket board out first made it possible to tip the whole assembly and weasle the pots out of the chassis.Be careful ther is limited room between the boards and the chassis and some parts are a real tight fit,the tube sockets have indents that let you see how moch room is available.Hope this helps,don't do what I did and have fun.-Kevin

[Teleman55]

I have installed my new PT-Peavey was a pleasure to deal with-after fixing the cathode ground mistake and testing for shorts I powered up and with in minutes saw smoke,eventually discovered that the epxoy I had used to secure cathode resistor lead to the board was cooking.Apparently the cathode to resistor connection gets quite hot.I know from guitar repair that epxoy melts at 180 degrees,this was burned.I also found that the boost seems to reduce gain when engaged.I believe Steve A. suggests a 760pf cap I had a 690pf available for C11.All in all the amp is much improved.