couple DYI 5f1 clone questions

[cubby01]

I have built a 5F1 clone to this design
http://www.dockeryamps.com/web/Docs/...amp-layout.pdf
http://www.dockeryamps.com/web/Docs/...-schematic.pdf
With the exception that I did not put in the 220K 1W bleeder resistor that is shown in parallel to the first filter cap. (didn't have one handy and thought it was optional?)

The power supply, tubes, and chassis were from an old magnavox amp chassis. The OT is a TF103-48 from
http://store.triodestore.com/tfchxfwi48oh.html.
Which has 4 ohm and 8 ohm taps. I'm using the 8 ohm taps.

All other parts I used parts I had on hand or ordered from an electronics supply house.

Upon completion I found it squealed, looking into this forum I saw recommendation to swap the OT secondary wires where they connect to the output jack. In other words, the "0" or ground secondary is now going to the jack "tip", the 8 ohm is now going to the jack sleeve. That stopped the squeal.

I get sound as I turn up gain doesn't act the way I expect. Gain starts picking up around 5 and up to that point is fairly clean, 5 to 9ish starts some ugly distortion that gets worse as volume increases up to nine. Almost a crackle when hitting low strings hard. Then it does something going from 9 to 10 the sound changes. And it's a very abrupt change, like something switched from one signal path to another. The ugly distortion is gone but so is a bit of the tone and it stays this way up to 12.

Questions... Regarding the 220K bleeder resistor. Do I need it? Would lacking it be causing this?

I'm using the 8 ohm tap on the OT. Should I have changed the resistor value in the feedback resistor. (something I read seemed to imply this but I donno).

When I toggle it off while hitting a big chord on the guitar the output volume drops very quickly. ie the caps seem to discharge in no time at all. This seems wierd. I've hit a big chord on other small amps and shut them down and it's a slow volume decline. Also I've opened it up several times to tinker and always attempt to short the caps to discharge them, but in each case no spark or anything. Does this sound right?

Thanks,
Cubby

[cubby01]

Well, I may have run across part of the problem. I noticed I had an extra ground wire running from the speaker jack to a ground lug. I removed it. Maybe that was creating some kind of ground loop, I donno. I also took the negative feedback resistor out of circuit for now. I may put this back in. To be honest I don't have a good grasp of the theory but I read where some recommend removing the NFB resistor in the Champ. I decided to try the OT 4 ohm secondary instead of the 8 ohm. I had a couple vintage 8 ohms 12 inch alnico speakers I threw into a baffle (no cabinet yet) and ran them in parallel to get 4 ohm. (The amp is a head unit, previously when I was running 8 ohm, I had been using either an 8 inch ceramic in a cabinet or just one of the above 12 inch in a cabinet)

After these changes sound is a lot better. Not yet perfect but it stomps all over the little Crate solid state practice amp I have.

[Paul P]

The 220k bleed resistor serves only to empty your filter capacitors when
you turn power off so you don't kill yourself next time you open your amp.

I'd install one next time you work on your amp (after you discharge your caps
using some other method than shorting them. A 220K resistor works well ).

It sounds like you have some sort of partial short to ground though.

Paul P

[cubby01]

The 220k bleed resistor serves only to empty your filter capacitors when
you turn power off so you don't kill yourself next time you open your amp.

I'd install one next time you work on your amp (after you discharge your caps
using some other method than shorting them. A 220K resistor works well ).

It sounds like you have some sort of partial short to ground though.

Paul P

Thanks, I'll re-trace the wiring against the layout again. I feel like I've done that several times though. Any tips on best way to locate a partial short to ground? Any "usual suspects" to look for?

Thanks,

[Paul P]

Do you have a multimeter ? Even a cheap one would be better than nothing.
You could measure the voltage across your filter caps after turning the amp
on and off, without playing anything, and see if they hold B+.

If one of the filter capacitors or the two 22nf capacitors is bad your B+ could
leek out through them.

With the caps drained (better yet disconnected) what's the resistance between
B+ and ground ?

Paul P

[cubby01]

Thanks Paul. Yup, gots several meters around, older analog and a cheap DMM. Although I had not yet checked the DC voltage while turning the amp on and then off, I had earlier checked resistance between ground and B+ side of each of the 3 big caps and shows no continuity. I did this with caps drained but still connected.

I will check the voltages but may not have an opportunity until tomorrow. I've not done this check before. I assume I do this with all tubes in. I just clip the meter into the ground on one lead and clip other into B+ for a filter cap then toggle amp on, wait till it warms up then toggle off watching the meter for values? Will that do it? Same scenario for each of the three filter caps?

[Paul P]

Do it just once for the filter caps (they're in parallel) and watch what happens
to your voltage once you turn your amp off (without playing anything). It
shouldn't drop to zero. If it does there's something connecting B+ to ground
that leaks.

Paul P

[cubby01]

Ok, I checked voltages. When amp is turned off it drops to near zero in about 2 seconds.

While I was there I checked voltages at each filter cap. It's +375 out of the 5Y3GT at the first cap. +315 at the second, and +269 at the third. These seem higher than what I had seen on a 5F1 schematic somewhere. I believe the PT is 325-0-325 and it's showing about 327 VAC going to the 5Y3 rectifier.

I noticed when I first switched on the B+ voltage runs up (for example on the first cap) to about +460 then backs down to +375. As it backs down the audible hum goes up as well. Is this a clue?

I have another set of filter caps but would like to narrow down the problem first if possible before swaping these out.

EDIT - I just recalled that the first time I powered up the amp I believe it did sustain sound out of the speaker for longer when I tuned off the amp. Since the caps are rated at 450v and it looks like it's kicking out a bit more than that on start up I'm wondering if that was enough to cause the caps to fail?

[Paul P]

Your filter caps shouldn't be dropping to zero in two seconds.

You've got 6ma running through the 10k power supply resistor and 2ma
through the 22k, which don't seem terribly wrong.

I think your PT secondaries should be higher than 327 since the PT is
rated at 320 for full current draw. So it looks like a lot of current is
flowing somewhere, but not at the preamp end of things (I'm presuming
your AC mains voltage is somewhere near 120vac). There're only two
things between B+ and ground before the preamp/PI, besides the filter
caps, and they are the output transformer and the power tube.

I don't know enough about tubes to know if one can partially short internally
and still operate to some extent. Do you have another one you can try ?

The output transformer could have some part of its winding shorted to
the frame. Did you at any time run the amp without a speaker attached ?

You're going to have to pull parts out until your caps remain full when you
power off (if they're good). Then the last part you pulled should be the
culprit. Or you could have two solder joints partially touching somewhere
I suppose.

I'm no expert at this so I'm only thowing out ideas for you to think about.

Do you know if the original 5f1 Champ had a 320vac power transformer ?
It seems kind of high to me for an old Champ.

Paul P

[cubby01]

Thanks. You've been a big help Paul. I've got another 6V6 I can swap. And take your advise to pull parts work backwards along the chain.

I had a problem right up front where I was getting squeel, and (based on something I had read in this forum where someone else was using the same OT) swapped the secondaries on the OT. I don't recall ever running it without a speaker attached. I understand that to be a no-no.

I saw where some other parts suppliers sold similar rated PTs for 5F1 clone kits. I will need to go back an check those again. Maybe they were wired differently. Seems I am close though. It is a working amplifier if not yet fully debugged.

[cubby01]

Well, I think I'm going to start from scratch on this one. To tell the truth the wiring is pretty sloppy anyway. I was trying to make due with an existing chassis and ended up with a birds nest. I'm going to fabricate a chassis better suited to the layout.

I did not locate the leak but I feel there must be a short I'm not finding. I removed the 6v6 tube and the 12AX7, and even disconnected the OT and it still looked like it was discharging, albeit slower. I fiddled withthe volume pot as well. Then after I put everything back together and I get no sound. The caps are now at a slow discharge but its like a ghost. I check B+ and it shows zero one minute then next minute it shows +70 or something. I discharge it by shorting out the filter caps and check a few other things and somehow comes back. At one point I checked B+ side of all caps and they showed zero or +1 and later while I was poking around the wiring on the 5Y3 I got a nasty little jolt. So I put the meter back on the caps and they show like +34. I'm sure the thing is haunted now.

Maybe part of my problem is I started down a path using one layout (the old drawn one you see with the death cap) then went a Webber layout and then changed to a Dockery layout because it looked like it addressed a lot of hum issues. What is the best, easily available, known to work well, layout for this amp?

Thanks all,

[tubeswell]

What is the best, easily available, known to work well, layout for this amp?

Thanks all,

IMHO the Fender layout is tried and true. You could improve it with a CT-grounded heater that is either grounded to one of the PT bolts along with the AC mains ground and the high voltage CT, or you can connect the heater ground to the output tube cathode (to lift the bias of the heater circuit above the ground in the rest of the amp - to avoid 6VAC hum getting back into the signal path.). Heaters wired this way are wired in parallel with each end of the PT winding going to opposite sides of the heater filaments in the tubes. If your heater winding hasn't got a CT, you can simulate a ground by running a 100r resistor from each side of the heater winding to ground (and grounding those to the output tube cathode).

I recently built a 5F2A (close relative of the 5F1) more of less following the Fender layout, but I used a split ground system, where the pre-amp tube cathode resistors, vol & tone grounds, and pre-amp filter cap ground were all connected to one of the input socket grounds via thick multi stranded copper wire, and the other grounds incl output tube cathode resistor, other filter cap grounds, heater CT, high voltage CT, PT shield and AC mains ground) were all grounded at one of the PT bolts at the other end of the chassis. It works pretty damn well.

The pictures show more or less how I did it

If I can get an m4a file to load up (without having too many problems), the clip shows what it sounds like with the NFB switch lifted, but before I added the extra 20uF filter cap to the reservoir cap (which lifted the B+ about 8VDC to 368VDC). (The lack of good timing somewhere in the middle of the clip is simply because I couldn't hear the backing track properly when I was doing the widdly bit - I just used Garageband on my eMac, with the built in mic on the mac, and dubbed the lead back over, whilst trying to listen to the playback of the backing track through the feeble mac built-in speakers. Ahh... excuses, excuses ;-)

[cubby01]

Well I havn't had much time in past few days to mess with the amp but a quick update. have not moved to different chassis yet. I did replace the 25-10-10 mF electrolytics with better quality caps. And I changed the grounding point the HV CT from the PT. A bit better sounding but still showing the caps drain right away after powering down. Still had an ugly distortion at volume above 6 or so. Then I yanked the artificial filiment center tap. That made a big improvement in the sound. Got more hum but better sound. Ugly breakup is gone.

Caps still drain quickly. Within about 5 seconds of turning off B+ goes from about +315 to +2 and will set there. As I do this guitar and speaker are connected but not playing. Where the volume control is set seems to have no bearing on the rate of fall.

[Paul P]

Then I yanked the artificial filiment center tap. That made a big improvement in the sound. Got more hum but better sound. Ugly breakup is gone.

How are your heaters wired ? Fender grounded one side of the loop and you
couldn't use a center tap with that method.

All I can say about your caps discharging is "curiouser and curiouser".

It would be nice to have a picture of the inside of your amp.

Paul P

[cubby01]

How are your heaters wired ? Fender grounded one side of the loop and you
couldn't use a center tap with that method.

Was wired to this layout
http://www.dockeryamps.com/web/Docs/...amp-layout.pdf
http://www.dockeryamps.com/web/Docs/...-schematic.pdf
With the exception that I did not put in the 220K 1W bleeder resistor.

By removing the artifical center tap all I did was remove the two 100ohm resisters between pins 7-8 and 8-2 on the 6V6. In looking at this now I see that the layout doesn't match the schematic in this regard. And that layout and schemo are about a year apart. hmmm.

I will change heater wiring back to the original Fender wiring and see what I get. Will go back and check other wiring against original Fender layout as well.

Thanks again.