Seymour Duncan Convertable 60W combo Wierdness

[guitarbill55]

[quote author="Kid Squid"]Changed the 1k 5w's , still got the funky waveforms, on the o/p and B+.

out of curio, one ended the diodes, joined together , and i got the classic full wave , with no filtering., as soon as i connect the parallel cr filter, the waveform is DC with a 100hz ripple of 25 V.A.C., B+is 450v DC.
is this a lot of ripple from a paralell CR filter, considering, that the caps are 165uf (2x330uf series)?
this is the ripple that you can see on the B+ waveforms, on the previous page.
the two 330uf caps are new replacements also.
checked the resistors, equal volts dropped across each, so they seem o.k.
any ideas ?

Could it be those 330uf caps ?
the other voltage tap-offs , step down o.k ( (A) 2.6v ripple 440vdc, (C) 70-80mv ripple 404vdc,)
but i'm thinking, for such a big value of capacitance, in the parallel filter, there should be hardly any ripple, let alone the 25v i'm getting....

oh, and across the 8 ohm o/p, (50w resistor)i'm getting 150mv AC with no i/p.....
I've had a couple of tins, :guinness: and 450vDC is not to be dissed...

so i'm going to have a puff, then hit the sack. my daughter wore me out today !! :grin:

Nos Da,

Steve :thumb:[/quote]
hiya kid, I had to go cross country to take care of me mum; been off line and quite busied out lately. It sounds like you have someting goofed with those 330 uf caps or the circuit board the caps are mounted on. Check all your grounds at the device in question. Don't assume anything -- especially with the board out of the case. Those old boards get hairline cracks over time that are very hard to see with naked eye. Use your ohmeter with the unit powered down to find them. Check that the correct terminal goes to ground ( see below ). Don't forget to let the caps discharge or do it with a 10k 2 watter.

Another thing you should definitely check is: Make absolutely sure that all the diodes in the PS are installed correctly. If some bone head got in there before you (which is very likely BTW ) he may have gone to lunch after removing them for testing and put them back in wrong. Make sure those caps are also oriented on the board correctly. Like I've said so many times before: Never assume anything.

Now for another tin! :sam:
gb

 

[Kid Squid]

Hi Bill,
Haven't had much time at the minute , been busy as hell with work n stuff,
Anyway, Had a check over it again briefly,

Ignore previous posts ***

O/T checked ok.
Valves checked o.k.

Ripple is 100Hz - so I am getting F/W Rect, what was I thinking !
Ripple will be about 24v a.c. at the B+ tap off, it's before any hard filtering !, relying on OPT to 'cancel out' (like a humbucker) the ripple.
The PSU checks out o.k, all diodes, caps etc,

Problem is around the driver stage.
i'm getting amplification to the grid of the first section of the driver, but the o/p going to the EL34's, looks like its being ' modulated'?, the EL34's are amplifying this modulation, and this is what is giving me the funny output.

With the driver tube removed, I'm not getting any hum
i'm getting a negative bias voltage, from the -ve bias supply,
but inbetween the supply, and the cathode, there is a diode, and with the driver tube in, the supply side of the diode (cathode), i'm getting -ve, but at the other end of the diode, i'm getting a positive voltage .
what is the purpose of this diode - it is a current regulating diode - CIL1304, i've never come across any of these before ??,
Also,
the feedback winding off the OPT , is connected to the grid of the other section of the driver tube, could it be something to do with this ?, I've checked the winding, and it seems to be o.k.
any thoughts ?

Schematics are here

http://www.diyguitarist.com/GuitarAmps/Convert60.htm
at bottom of page.

Steve :thumb:

 

[Walrus]

The CR1304 is acting as a constant current source for the longtailed pair which is the driver valve. Same idea as the input circuit for the 2520 DOA except that is transistors.
I once had fault with a feedback transformer on an old Shure power amp (transistor not valve) that was causing a hum on the output. I compared resistance readings to a new one and they appeared identical, but substitution cured my fault. Not sure if you have a simlilar problem....

 

[Kid Squid]

Cheers Walrus,

Regarding the LTP, would it be possible, to remove the feedback winding off the 2nd grid, and connect it more like a standard marshall / fender type ( i haven't checked any drawings yet tho - to be certain),
Or another way of testing the Driver stage, without the feedback connected ?
Should i be getting negative voltage at both ends of the bias 'diode' ? , currently, i'm getting between anode of diode, and grid - about -1.5 v, but between anode of diode and earth, i'm getting about 16v ( other side of the diode is -60 odd Volts.),
I don't fully understand whats happening with the Constant current diode thingy !

havent had much time for it today - my only day off the last fortnight !

Steve :thumb:

 

[Walrus]

I'm not up on design so I can't comment on changing the feedback wiring.

What is the voltage at E, and is it smooth DC. A fault there could inject hum into the driver tube cathodes and hence into the output stage.
If the CR1304 is working correctly, then there should be about 8-9 Volts across R39.