M Fowler wrote:I see your using the 280v taps which are yellow to the diodes. I blew your pictures up and went over everything and I did not see anything wrong. I see that you ran your OT primary CT blue to the 1 K 25w resistor rather than to the Standby switch but is okay. I am assuming that there is a wire under the board from C9 the 0.1uf OD to R10 1m which goes to V3 pin 7.
Other than switching the OT primaries it looks good to me. If the amp howls on start up it means the OT primary is out of phase so by reversing the wires takes cares of the phase on primary side.
Mark
Thanks for combing over it Mark, I just swapped the primary wires, ran it for a solid 20 min, no more howling!!! So that (hopefully) takes care of that!
C9 Is jumped under the board to R10, it just looked cleaner that way. On all of the wires i fed under the board, I stripped enough wire so they stuck up through the top of the turret, that way I can solder each turret only once.
Now on to the scratchy pots... I measured 2.5V DC on the Pot side of C2 (500pF between pin 6 on V1 and treble pot) and I measured 12.5V DC on lug 3 (or is it 1) on the presence pot, so may be .1uF C9 is leaking DC from B+4?
The presence pot will always have DC on it because it's connected to the tail of the LTP and there is DC there. As to the 500pF, you probably should disconnect the cold side of each pot in the tone stack and test them individually. If one of the .02s is leaking, that could show up on the treble pot (of course the mid or bass pot would be scratchy too.)
First post on the forum but... shouldn't the pins 1 & 8 be tied together on the EL34 sockets? I know that the voltage drop across the 1 ohm resistor is less than 1 V but any voltage differential between those 2 pins may not be the best idea?
Firestorm wrote:The presence pot will always have DC on it because it's connected to the tail of the LTP and there is DC there. As to the 500pF, you probably should disconnect the cold side of each pot in the tone stack and test them individually. If one of the .02s is leaking, that could show up on the treble pot (of course the mid or bass pot would be scratchy too.)
Thanks Firestorm, I was thinking about the PI the wrong way, thinking that the .1uF after B+4 should block it, but I see now why it's there. I appreciate the tips, it makes sense to disconnect the cold sides of the pots and isolate each connection, after that it should be obvious where the dc is coming in...
meddler wrote:First post on the forum but... shouldn't the pins 1 & 8 be tied together on the EL34 sockets? I know that the voltage drop across the 1 ohm resistor is less than 1 V but any voltage differential between those 2 pins may not be the best idea?
You can actually actually do it either way, but I did read before (and can't remember the explanation why) that it's better to tie 1&8 together and then to the 1ohm resistor in terms of getting an accurate reading.
meddler wrote:First post on the forum but... shouldn't the pins 1 & 8 be tied together on the EL34 sockets? I know that the voltage drop across the 1 ohm resistor is less than 1 V but any voltage differential between those 2 pins may not be the best idea?
Yea, pins 1 and 8 were tied together with a jumper, I removed the jumper to just be able to check cathode current across that one ohm, after the bias is set I will put the jumper back so both pin 1 and 8 are tied together again, then directly to ground. I've seen it done both ways, seems a lot of people just leave the resistor in between ground and 1 and 8 tied together, I didn't want to move the flyback diodes, and if I left them before the 1 ohm, if something did happen that resistor wouldn't last long. So I just un-solder/solder the jumper when I want to bias the amp, seemed like the best way to do it. I read a snippet from one of O'Connors TOT books (or it may just have been an article he wrote) that either method is acceptable, either ...
Pin1 - Pin8 - 1ohm - Gnd
or
Pin8 - 1ohm - Pin1 - Gnd
meddler wrote:First post on the forum but... shouldn't the pins 1 & 8 be tied together on the EL34 sockets? I know that the voltage drop across the 1 ohm resistor is less than 1 V but any voltage differential between those 2 pins may not be the best idea?
You can actually actually do it either way, but I did read before (and can't remember the explanation why) that it's better to tie 1&8 together and then to the 1ohm resistor in terms of getting an accurate reading.
Man, everyone's quick with the replies today, by the time I get a reply in there's another one before me I've heard that by just reading the cathode current without it connected to suppressor, it will give you a little colder bias than you actually are reading, I also don't completely grasp that yet, but I'm gonna go out in the man cave and try it both ways and see if there is any difference in current, then check those number against the transformer shunt method and see what the difference is... I'll report back later with my findings
Got it all worked out, one of the .02uF caps (C4 on the schematic) had almost 14V on the opposite side of B+... Replaced it, 0V now, no more scratchy pots!!! No more squealing!!! My initial B+ is still a little low, sitting at around 380V with the amp running, but everything is proportional after that, so I'm not gonna sweat it for now... So all is well with my new Express, Thank you to everyone who helped me work out the bugs, couldn't have done it without T.A.G.!!!
Was that a brand new orange drop that was leaking?
Is C4 the mid cap?
Shazam, I got a scratchy volume pot on my D'Lite that may be from that same thing.
Also I have some hash on my signal, I wonder if it is something similar to what you have?
How do you check it?
Un-solder the end towards the pot and measure for DC there to ground?
Structo wrote:I'm not a Train Wrecker so excuse me.
Was that a brand new orange drop that was leaking?
Is C4 the mid cap?
Shazam, I got a scratchy volume pot on my D'Lite that may be from that same thing.
Also I have some hash on my signal, I wonder if it is something similar to what you have?
How do you check it?
Un-solder the end towards the pot and measure for DC there to ground?
Tom,
That was a brand new Cornell Dublier brand polyester film cap that was bad. I measured the capacitance of it before I built the board, didn't think about checking it for shorts. C4 is a coupling cap with (if memory serves me correctly) around 160VDC on the non Pot side, I was seeing about 10% of that coming through, after changing the cap it was zero... In the end, I just un-soldered each wire running from the pre-amp board to pots and checked each one individually for DCV from pot side to ground. If you don't remove them all at the same time, one may influence another and you'll have more than one questionable cap. I don't know if there is a better way to do it, thats how I did it and if worked for me...
I've heard that by just reading the cathode current without it connected to suppressor, it will give you a little colder bias than you actually are reading, I also don't completely grasp that yet, but I'm gonna go out in the man cave and try it both ways and see if there is any difference in current, then check those number against the transformer shunt method and see what the difference is... I'll report back later with my findings 
