Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jewishjay on October 23, 2020, 10:48:46 pm

Title: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 23, 2020, 10:48:46 pm: Hot on the heels of my success with the Mojotone Vibro Champ, I scratch built a modified 5E3 type circuit. I took the Deluxe schematic and made it single input, and moved the higher gain 12ax7 to the first two stages and used a 6CN7 for the phase inverter. I started it up on the lightbulb current limiter and all the voltages look good, but I havent checked bias yet. The 6.3v is actually reading 7v...is that a problem? And theres a LOUD 60hz buzzzzzzz that does respond to the volume and tone knobs, and goes dead silent when i pull out the PI. Please look over this schematic, and tell me if theres something wrong with the design, because ive gone through it multiple times, and havent found any deviations yet.

I also have a question about the solid state rectification. When I buzzed out the circuit with a multimeter, each of the anode ends of the diodes beep for a moment, like continuity to ground, but only for a moment. Is that normal?
Title: Re: 60 Cycle HUMMMMMMM
Post by: tubeswell on October 24, 2020, 12:01:33 am: Got any pictures?
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 24, 2020, 05:06:22 am: 1- Q : Please look over this schematic, and tell me if theres something wrong with the design,
A : I won't built a amp with one side of heater wire at ground = more noise

2- Q :When I buzzed out the circuit with a multimeter, each of the anode ends of the diodes beep for a moment
A: I never use "bip" on meter alway ohms scale. Bip is not helpful to me

3- Finish amp with the right bias.

4- Good picture are a must.

5- Post schematic as built
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 24, 2020, 10:33:58 am: I can take close up pics of any spots youre concerned about, and the schematic I posted is exactly as built. Thanks for looking!
Title: Re: 60 Cycle HUMMMMMMM
Post by: shooter on October 24, 2020, 11:15:19 am: try putting a shorting jack on the input like switchcraft 12A
use shielded wire to connect it to the 1st tube
DON't run the wires under or near the filaments

that said, I suspect there's more but I quit there
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 24, 2020, 12:19:01 pm: Are rectifier diodes are ok ?
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 24, 2020, 01:08:15 pm: Quote from: Latole on October 24, 2020, 12:19:01 pm
Are rectifier diodes are ok ?

like...as a concept? lol
my new multi meter has a mode for that, but ive never used it before. i wonder if i can test them in circuit...
Title: Re: 60 Cycle HUMMMMMMM
Post by: HotBluePlates on October 24, 2020, 01:16:10 pm: Quote from: jewishjay on October 23, 2020, 10:48:46 pm
... theres a LOUD 60hz buzzzzzzz that does respond to the volume and tone knobs ...
Quote from: shooter on October 24, 2020, 11:15:19 am
try putting a shorting jack on the input like switchcraft 12A ...

100%

If you must keep this amp as-is, you'll need to plug a guitar into it, or rig a 1/4" shorting plug with the Tip connected to Sleeve.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 24, 2020, 02:24:16 pm: Quote from: jewishjay on October 24, 2020, 01:08:15 pm
Quote from: Latole on October 24, 2020, 12:19:01 pm
Are rectifier diodes are ok ?

like...as a concept? lol
my new multi meter has a mode for that, but ive never used it before. i wonder if i can test them in circuit...

Not concept ; are thy are working fine ? Power amp wires going to anodes ?
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 24, 2020, 05:19:32 pm: Quote from: Latole on October 24, 2020, 02:24:16 pm
Quote from: jewishjay on October 24, 2020, 01:08:15 pm
Quote from: Latole on October 24, 2020, 12:19:01 pm
Are rectifier diodes are ok ?

like...as a concept? lol
my new multi meter has a mode for that, but ive never used it before. i wonder if i can test them in circuit...

Not concept ; are thy are working fine ? Power amp wires going to anodes ?

AC is going in and DC is coming out, so I assume so. Could they be the source of the hum, and how could I check?
Title: Re: 60 Cycle HUMMMMMMM
Post by: AmberB on October 24, 2020, 07:34:33 pm: Since the heater winding on the PT puts out 6.3 volts between the 2 ends of the coil (give or take loading voltage drop), instead of having one end grounded, you really should build an artificial center tap for the heaters with a pair of 100 ohm to 220 ohm resistors. That may, or may not be the source of the hum, but it's just good design work...
Title: Re: 60 Cycle HUMMMMMMM
Post by: HotBluePlates on October 24, 2020, 09:14:42 pm: Quote from: jewishjay on October 24, 2020, 05:19:32 pm
... Could they be the source of the hum, and how could I check?

Your "hum source" is the un-grounded grid you have due to the non-shorting jack you used for the input. It's an antenna for noise in the air.

Use a shorting plug, or clip a jumper from Hot to Ground at the jack, and most (all?) of your noise will go away.

(https://i.imgur.com/epsqUq8.jpg)
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 24, 2020, 10:48:28 pm: Quote from: AmberB on October 24, 2020, 07:34:33 pm
Since the heater winding on the PT puts out 6.3 volts between the 2 ends of the coil (give or take loading voltage drop), instead of having one end grounded, you really should build an artificial center tap for the heaters with a pair of 100 ohm to 220 ohm resistors. That may, or may not be the source of the hum, but it's just good design work...

forgive me, thats exactly what i did, i just failed to show it on the schematic. the virtual centertap is hidden under the pilot light, 100ohms each side to ground.

Quote from: HotBluePlates on October 24, 2020, 09:14:42 pm
Quote from: jewishjay on October 24, 2020, 05:19:32 pm
... Could they be the source of the hum, and how could I check?

Your "hum source" is the un-grounded grid you have due to the non-shorting jack you used for the input. It's an antenna for noise in the air.

Use a shorting plug, or clip a jumper from Hot to Ground at the jack, and most (all?) of your noise will go away.

(https://i.imgur.com/epsqUq8.jpg)

i jumpered hot to ground and that didnt change the hum at all, the search continues... thanks tho...
Title: Re: 60 Cycle HUMMMMMMM
Post by: HotBluePlates on October 25, 2020, 12:25:08 am: Quote from: jewishjay on October 24, 2020, 10:48:28 pm
... i jumpered hot to ground and that didnt change the hum at all, the search continues...

I'd move the 22µF feeding the preamp/phase inverter over to the preamp side of the chassis so that it grounds at the same spot where you have the rest of the preamp grounds. You can run a wire from the 10kΩ dropping resistor to the + of the cap, rather than running the long wire from the cap to the plate resistors.

If that doesn't nix it, unsolder one leg of the 0.022µF cap leading to the 6CN7 phase inverter tube. You're looking to break the circuit so you hear only the phase inverter/power section, so you can divide & conquer. There's a good chance of multiple hum-sources, boiling down to the construction method used and possibly heater-to-cathode leakage in tubes.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 25, 2020, 02:38:56 am: Quote from: HotBluePlates on October 24, 2020, 09:14:42 pm
Quote from: jewishjay on October 24, 2020, 05:19:32 pm
... Could they be the source of the hum, and how could I check?

Your "hum source" is the un-grounded grid you have due to the non-shorting jack you used for the input. It's an antenna for noise in the air.

Use a shorting plug, or clip a jumper from Hot to Ground at the jack, and most (all?) of your noise will go away.

(https://i.imgur.com/epsqUq8.jpg)

This kind of jack can make some noise, not the the one I understand by OP
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 25, 2020, 02:45:43 am: Quote from: jewishjay on October 24, 2020, 05:19:32 pm
Quote from: Latole on October 24, 2020, 02:24:16 pm
Quote from: jewishjay on October 24, 2020, 01:08:15 pm
Quote from: Latole on October 24, 2020, 12:19:01 pm
Are rectifier diodes are ok ?

like...as a concept? lol
my new multi meter has a mode for that, but ive never used it before. i wonder if i can test them in circuit...

Not concept ; are thy are working fine ? Power amp wires going to anodes ?

AC is going in and DC is coming out, so I assume so. Could they be the source of the hum, and how could I check?

I am not sure you understood me correctly;
1- Are the diodes well polarized?
2- Are the diodes in good condition?
To solder them you must use a heat sink otherwise the heat can damage them.
3- Disconnect one side of each of the diodes and check; the ohms
25/350 k in one direction, infinite / OL in the other direction.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 25, 2020, 02:54:31 am: What about jack, pots and filter caps grounds ?
Title: Re: 60 Cycle HUMMMMMMM
Post by: PRR on October 25, 2020, 12:14:55 pm: The current path from the PT HV CT seems to go into the chassis for several inches before it gets to the first filter cap "-" lead. This should be much shorter and more direct. Lift the PT CT lead from the blob and solder it to the - lead of the cap.

And this should be 120Hz hum. If it is really 60Hz, your rectifier is half-dead. It can be hard to know the difference because 60 and 120 are octaves of each other.
Title: Re: 60 Cycle HUMMMMMMM
Post by: AmberB on October 25, 2020, 08:20:01 pm: On one of my amp builds, I had to use an insulated input jack to solve the buzz problem. I grounded it to the negative side of the preamp filter cap.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 26, 2020, 03:30:17 am: Quote from: AmberB on October 25, 2020, 08:20:01 pm
On one of my amp builds, I had to use an insulated input jack to solve the buzz problem. I grounded it to the negative side of the preamp filter cap.

Something was wrong with your amp. I built fews amps from scratch and never need a insulating jack.

jewishjay,

Repairing, over the Internet, a home-made amplifier that has never worked well is often a very difficult job.

Not having the amp in front of you prevents us from seeing details and the problem here probably comes from details.That the photo does not allow us to see or you to mention us unintentionally.
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on October 26, 2020, 11:25:24 am: Quote from: Latole on October 26, 2020, 03:30:17 am

Repairing, over the Internet, a home-made amplifier that has never worked well is often a very difficult job.

Not having the amp in front of you prevents us from seeing details and the problem here probably comes from details.That the photo does not allow us to see or you to mention us unintentionally.

Absolutely, I understand that, and I appreciate everyones suggestions. Let me make some of the suggested adjustments, redraw that schematic to be more accurate and then I'll post again. Thanks everybody!
Title: Re: 60 Cycle HUMMMMMMM
Post by: 1blueheron on October 26, 2020, 02:12:10 pm: I have no clue but based on observation of amps I have worked on and books/tutorials I have read, general best practice is considered to be keeping your heater wires as low and close to the chassis, preferably in corners as possible and then run your signal path wires as far away from them as possible to avoid inductive currents.

I have seen manufacturers use different amounts of twist and vary in their degree of keeping twist, but seldom have I seen heater wires that aren't kept low and as close as possible to the chassis. If they were, some form of isolation was provided for the signal path.

In your case you seem to have accomplished the exact opposite with your heater wires flying high above and signal wires running under them near the chassis.

Was this wiring method recommended by the kit provider?
Title: Re: 60 Cycle HUMMMMMMM
Post by: sluckey on October 26, 2020, 02:30:49 pm: Many of the old Fender amps had the filament wires running high above the sockets. Like this '64 DR...

(http://sluckeyamps.com/64DR/01.jpg)
Title: Re: 60 Cycle HUMMMMMMM
Post by: 1blueheron on October 26, 2020, 02:58:46 pm: Quote from: sluckey on October 26, 2020, 02:30:49 pm
Many of the old Fender amps had the filament wires running high above the sockets. Like this '64 DR...

(http://sluckeyamps.com/64DR/01.jpg)
Looks like they made a conscious effort to keep all signal path wires crossing filament wires at close to right angles to avoid inductive current/noise. I would guess that counts as some form of isolation although it is a deviation from keeping 'em low:worthy1: IIRC Soldano get's away with intentionally not twisting wires as well...
Title: Re: 60 Cycle HUMMMMMMM
Post by: vampwizzard on October 26, 2020, 05:51:36 pm: Quote from: sluckey on October 26, 2020, 02:30:49 pm
Many of the old Fender amps had the filament wires running high above the sockets. Like this '64 DR...

(http://sluckeyamps.com/64DR/01.jpg)

This makes me feel things. Maybe been shut in the house too long! Thats pretty!
Title: Re: 60 Cycle HUMMMMMMM
Post by: sluckey on October 26, 2020, 06:10:15 pm: I have 12 more pics of that amp. It's not mine. I thought they were pretty too.

http://sluckeyamps.com/64DR/index.htm
Title: Re: 60 Cycle HUMMMMMMM
Post by: shooter on October 26, 2020, 06:27:24 pm: really clean amp, the shoe though is way to clean :icon_biggrin:
Title: Re: 60 Cycle HUMMMMMMM
Post by: Latole on October 27, 2020, 03:24:06 am: Quote from: sluckey on October 26, 2020, 02:30:49 pm

Many of the old Fender amps had the filament wires running high above the sockets. Like this '64 DR...

Many ?? All of Leo Fender desing amps are built like that.

With a clean and "special " wire dress as 1Blueheron wrote ;

"Looks like they made a conscious effort to keep all signal path wires crossing filament wires at close to right angles to avoid inductive current/noise.
Title: Re: 60 Cycle HUMMMMMMM
Post by: sluckey on October 27, 2020, 07:11:16 am: Quote from: Latole on October 27, 2020, 03:24:06 am
Quote from: sluckey on October 26, 2020, 02:30:49 pm
Many of the old Fender amps had the filament wires running high above the sockets. Like this '64 DR...

Many ?? All of Leo Fender desing amps are built like that.
Not so! Maybe you should look at some 5XX tweed amps. Most all the tweed amps have the filament wires laying down against the chassis.

(http://sluckeyamps.com/harvard/h03.jpg)

(https://cdn.shopify.com/s/files/1/0230/9164/2448/files/1958-Fender-Bassman-034_ba1b6568-7750-440f-86a6-5d5408a1268d.jpg?v=1585191498)
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on November 02, 2020, 10:46:52 am: SOLVED! After applying some of your suggestions (shunted jack and shielded wire) I found a bad solder joint in the heater circuit. Flowing a little more solder made the amp come to life and it works great! Maybe a little too well....there's so much gain, its super loud at volume 1 and nicely distorted at volume 3. I might change the 12AX7 to a 12AT7 and or swap the 68k input to 100k. I'm open to further input and can post a video in the coming days to show you where its at. The plate voltage is 320 and the plate dissipation is 10.8

THANKS SO MUCH for your support and advice.
Title: Re: 60 Cycle HUMMMMMMM
Post by: shooter on November 02, 2020, 11:05:57 am: Quote
works great! Maybe a little too well

start easy
lift the 25uF cap off the 1st gain stage
IF that got you close
decrease the 25uF cap on the 2nd gain stage to 10uF
play for a couple hours, then decide if it's better/worse, or just different
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on November 02, 2020, 01:39:25 pm: Ok thanks, I'm also considering RobRob's NFB mod to "civilize" this notoriously hot circuit.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Willabe on November 03, 2020, 10:49:34 am: Quote from: jewishjay on November 02, 2020, 01:39:25 pm
Ok thanks, I'm also considering RobRob's NFB mod to "civilize" this notoriously hot circuit.

Before you try the RobRob mod, put a 12AY7 (gain of 40) in the 1st stage, 12AT7 is still kind of strong/hot, (12AT7 = gain of 60, and 12AU7 = gain of 17 to 20, depending on which spec sheet you look at, 12AX7 = gain of 100.) No NFB retains harmonics/chime, doesn't kill them off like NFB does. That's a big part of the 5E3 sound.

And, I don't know if you know this, but the volume controls are interactive on a 5E3. Try plugging into either channel then turn up the volume on the other channel. Try 12:00 on the channel your plugged into and full up on the other channel. That setting gets the cleanest sound. Now leave the settings as they are and plug into the other channel, that's max mid and distortion. The tone control is on 1 channels volume pot, so try reversing what I suggested, it will be a little different. Play with the volume settings to taste. You can use an A/B box to switch between channels.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Willabe on November 03, 2020, 11:09:59 am: Quote from: jewishjay on October 23, 2020, 10:48:46 pm
..... I scratch built a modified 5E3 type circuit. I took the Deluxe schematic and made it single input, ...

Just saw this.

IMO, a big part of the charm of a 5E3 is the interactive channels. Gives more options/tones.

Neil Young has played/recorded with a 5E3 for decades using the interactive controls. He had a box built with servo motors that are midi controlled to change settings on the fly/live. It sits on top of the 5E3. It has a memory for the servo motors that change the amps control settings, that are foot switch activated.

The 1st amp I built was/is a 5E3, still have it.
Title: Re: 60 Cycle HUMMMMMMM
Post by: Willabe on November 03, 2020, 11:27:44 am: When you went with a single channel, you lost 1 of the 1M volume pots.

After trying a 12AY7 for the 1st preamp tube, if still too hot, I'd try putting a 1M R from the 2nd gain stages grid to ground, that will cut your gain a little. If still too hot, you could try 500K R there.

I'd try those things before trying NFB loop.
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on November 06, 2020, 10:26:12 am: So, this was never intended to be a 5e3 clone. The Deluxe was only a jumping off point for a small PP 6v6. Here is the schematic as it exist right now, and I like the sound. It's not a 5e3, that was never the intent....but it is a good little amp. It's very touch sensitive, and the volume and tone controls are very interactive, so many tones are possible. I have just a couple questions in closing.

1. the heater circuit reads 7v is that a long term problem that needs to be addressed?

2. with so many changes, it's not much like a 5e3....but it's closer to that than anything else I can think of. seems like lot of the Fenders beyond this are grid biased. Is there another PP 6v6 cathode biased amp that I should be aware of?
Title: Re: 60 Cycle HUMMMMMMM
Post by: sluckey on November 06, 2020, 10:44:05 am: I would not be concerned with the heater voltage. There are probably more cathode biased 6V6 circuits than fixed biased 6V6 circuits.
Title: Re: 60 Cycle HUMMMMMMM
Post by: jewishjay on November 06, 2020, 10:52:52 am: Quote from: sluckey on November 06, 2020, 10:44:05 am
There are probably more cathode biased 6V6 circuits than fixed biased 6V6 circuits.

single ended for sure. but when i look through tweed schematics, all the push pulls i see are fixed.
Title: Re: 60 Cycle HUMMMMMMM
Post by: sluckey on November 06, 2020, 11:29:11 am: I'm talking push/pull cat biased 6V6s. Fender only has a couple. The rest of the world has thousands. Browse through Hoffman's schematic library. Link at bottom of this page. Look under Ampeg, Gibson, Silvertone, Valco, Supro for starters.