Reverb Circuit Hum

[earl2]

When I built this amp (no reverb) I definitely had to add a grid leak resistor on the triode before the phase inverter. Without it the amp made all kinds of howling noises. The original schematic doesn`t show one which always seemed strange to me. Don`t know if you added one. Mine also benefited a lot from a grid stopper on the phase inverter.

[Stevem]

Yes it sure does, sorry!

[mwaller]

have you tried a 100k reverb pot ala fender?

I have not, but I definitely will try this! I believe I have a 100K linear pot in my parts bin. Did Fender use a linear or audio taper?

[mwaller]

When I built this amp (no reverb) I definitely had to add a grid leak resistor on the triode before the phase inverter. Without it the amp made all kinds of howling noises. The original schematic doesn`t show one which always seemed strange to me. Don`t know if you added one. Mine also benefited a lot from a grid stopper on the phase inverter.

You're correct - OS/71 version of the schematic has an error in this regard. The 470K resistor (R29) is supposed to go between GND and the grid of V4b. That is how I built it. Likewise, the 100k resistor (R30) is supposed to go between GND and the grid of V3b.

There is a corrected version of the schematic labeled OS/026 which is included in the limited edition of the book Vox Amplifiers: The JMI Years
I don't own the book, but I've seen the schematic. It is much easier to interpret.

[mwaller]

have you tried a 100k reverb pot ala fender?

I tried this tonight... Unfortunately, no difference. Possibly even worse buzzing than before. I also shielded the grid wire to V4 gain stage, but that didn't seem to make a difference either.

I measured ac ripple in my power supply. The first node has 5.5v. The rest are more like 0.1 - 0.3. Is that typical?

[earl2]

There is a corrected version of the schematic labeled OS/026 which is included in the limited edition of the book Vox Amplifiers: The JMI Years
I don't own the book, but I've seen the schematic. It is much easier to interpret.

I didn`t know about that revised schematic. Had to make my own. In any case thanks for confirming what I worked out back then by a combination of logic and trial & error !
Looking at the Ampeg schematic it should have been obvious really. What threw me for a while were the photos of several originals which look for all the world as if they are indeed wired as shown in that original schematic.
Whats the primary impedance of that output transformer you`re using? I often wondered if Vox used the usual 8k or 6.3k as in the AC15.
Good luck with the hum. If I have any fresh ideas I`ll give you a shout.

[Stevem]

Well if 5.5 volts is true then your tube socket is bleeding heater voltage thru it, as that could be the only reason I can justify!

What I would do to confirm this is to disconnect all the wires from that socket but for the 2 heater wires and the test to see if you still read that 5.5 volts on any of the other socket terminals.

[sluckey]

I measured ac ripple in my power supply. The first node has 5.5v. The rest are more like 0.1 - 0.3. Is that typical?

It's very normal for the first node to have 5v or more ac ripple with a typical cap input filter.

[mwaller]

There is a corrected version of the schematic labeled OS/026 which is included in the limited edition of the book Vox Amplifiers: The JMI Years
I don't own the book, but I've seen the schematic. It is much easier to interpret.

I didn`t know about that revised schematic. Had to make my own. In any case thanks for confirming what I worked out back then by a combination of logic and trial & error !
Looking at the Ampeg schematic it should have been obvious really. What threw me for a while were the photos of several originals which look for all the world as if they are indeed wired as shown in that original schematic.
Whats the primary impedance of that output transformer you`re using? I often wondered if Vox used the usual 8k or 6.3k as in the AC15.
Good luck with the hum. If I have any fresh ideas I`ll give you a shout.

The OS/026 schematic shows an 8K-ohm output transformer. I'm using a Mercury Magnetics transformer that was designed for this amp. I assume it has an 8k primary, but I haven't verified.

[mwaller]

I measured ac ripple in my power supply. The first node has 5.5v. The rest are more like 0.1 - 0.3. Is that typical?

It's very normal for the first node to have 5v or more ac ripple with a typical cap input filter.

Could a leaky filter cap cause hum that isn't reflected in the RMS ripple measurement taken with my DMM?

The reverb circuit has always been a bit troublesome, but I seem to recall a time when the amp was otherwise quiet with reverb off. I even tested it with the higher efficiency speaker, and I don't recall hearing much hum.

Now I hear hum all the time, which suggests that something has changed. I've tried to put the circuit back to how it was originally before any updates, and I still have hum. I'm very suspicious of the ARS filter caps. I typically use F&T, but I decided to try the ARS because the Marshall guys seem to like them.
Perhaps I have a bad cap?!

[mwaller]

As I pulled the filter caps in anticipation of installing new ones tomorrow, I realized that my grounding scheme may be to blame for some of the hum. My original layout connected all the filter cap grounds to a lug near the inputs. When I install new caps, I believe I should connect the ground of the 32uF caps somewhere close to the power tubes. The 16uF caps should be grounded near the inputs as before. Does that sound right?

[Roe]

I recommend multistar grounding with one star grouding for each power supply node. Keep the distance between the different grounding points at least 2"

[earl2]

As I pulled the filter caps in anticipation of installing new ones tomorrow, I realized that my grounding scheme may be to blame for some of the hum. My original layout connected all the filter cap grounds to a lug near the inputs. When I install new caps, I believe I should connect the ground of the 32uF caps somewhere close to the power tubes. The 16uF caps should be grounded near the inputs as before. Does that sound right?

Yes I did it exactly like that and mine doesn`t hum. Not to say that`s your problem though.

[mwaller]

I recommend multistar grounding with one star grouding for each power supply node. Keep the distance between the different grounding points at least 2"

Would it be best to ground the power tube filter caps near the speaker jacks? Alternatively, I could simply ground it to the same location where the center taps are connected next to the power transformer. Thoughts?

[earl2]

I ground them where the center taps go next to the power transformer and everything preamp at the other end near the input jack. Never had any problems doing it like that.