Fender pro build 5E5/5E5A loud hum

[stephenhartley]

Hi - it's a while since I've been on here.

A few years ago, I built a Fender Pro copy that switched between cathode (5E5) and fixed (5E5A) bias. My thinking was to try and hear the difference between the two as a guitar player and to use the fixed bias setting to get a cleaner sound for recording bass guitar. I fitted it with a 15" bass speaker (JBL K140).

It sounded great until recently when it developed a loud hum, which didn't alter with the volume controls and got louder as the amp warmed up.

I noticed that the heater of one of the tubes (Russian military 6N3C = 6L6 - max plate voltage 400VDC, plate dissipation 19W) wasn't glowing.

I swapped them round and the non-glowing tube swapped too, so I replaced them with a fresh pair - no difference.

I swapped out the rectifier (GZ34) twice - no difference.

I swapped out the pre-amp and phase-inverter tubes one by one - no difference.

The PT is a Hammond 291 CEX (650 VAC).

Voltages read as follows:

VAC: 722

B+ 458

Plate to ground voltage outer power tube: 461 (far too high!)

Plate to ground voltage inner power tube: 461

Voltage 1st smoothing cap: 452

Voltage 2nd " 383

Voltage 3rd " 390

Voltage 4th " 330

The voltages on the last 3 smoothing caps starts to drop as the amp warms up and the hum gets louder.

There's a protector fuse from the rectifier and diodes on the pins too.

I feel like I'm missing a simple problem, but I'm not experienced enough to know where to go next. Could it be the PT?

I'd be most grateful for any suggestions.

Thanks

Stephen

[Helmholtz]

I swapped them round and the non-glowing tube swapped too, so I replaced them with a fresh pair - no difference.

Does that mean that even with new tubes one of them is not heating?

[The Dude]

^^^^^^ Yes that. If one of the heaters is not lighting up, check filament voltage at tube socket.

[pdf64]

Regarding the strangely high output valve anode voltages, probing them may be inducing oscillation.
Try repeating the measurement but with the valve in V3 socket removed (that should eliminate oscillation).

[stephenhartley]

Sorry - I should have been clearer - I meant no difference to the hum when I replaced the 6L6s. Yes the heaters on both the new tubes are lighting up. I'll check voltages without V3 in. V3 is the last AX7 before the 6L6s? Is 458 VDC acceptable B+ voltage for that transformer?

Thanks for the replies.

[pdf64]

722V AC seems a lot more than 650, but it’s only 10% higher and the latter is the voltage at full load, and we don’t yet know how much current the output valves are drawing at idle, or what the actual mains wall voltage being supplied to the amp is.

[stephenhartley]

Hi - I've measured all the voltages as advised, and now I'm beyond confused

Wall VAC: 246

I tried to use the output transformer method to determine output valve idle current.

OT resistance (pin 8 of rectifier to pin 3 of valve) inner valve: 91.2 ohms outer valve 99.1 ohms

Plate voltages: outer valve in cathode bias 498, in fixed bias 499. Inner valve cathode bias 498, fixed bias 499
taking out V3 didn't make any significant difference. I also checked with a different GZ34 and plate voltages the same. For some reason plate voltages have gone up since last night when they were ~460 VDC

Voltage drop across OT (pin 8 of rectifier to pin 3 of valve):
Outer valve in cathode bias: rises to 2.19, but then starts to drop as hum gets louder. That gives an idle current of only 24 mA and a dissipation wattage of 11.8 W. In fixed bias, 3.26 volts, giving 35mA and 17.8 W dissipation wattage
Inner valve in cathode bias: a whopping 12.5 volts, rising as hum gets louder and valve red plating - switched off - similar in fixed bias.

The amp has a progressive standby/on switch. I thought that might be the problem at the beginning, so I changed it and it made no difference

It seems that just the action of moving the multi-meter probe from pin 3 of the inner valve to pin 3 of the outer valve is associated with the valves red-plating?? I always switch off, unplug and discharge filter caps before moving probes?

[stephenhartley]

The cathode resister is 250R 10W

[stephenhartley]

Maybe the whopping plate voltages have fried the valves?

[stephenhartley]

Addendum: realised V3 not properly in its socket when I put it back - will re-measure OT voltage drop tomorrow.

[Helmholtz]

I don't think there's anything wrong with the PT and your DC voltages make sense.
Seems that the inner tube is running away, that will cause hum.

Check for bad tube, bad socket contact or leaky coupling cap.
Especially make sure the grid makes good contact.

Please post schematic.

[pdf64]

Probing output valve anodes can induce oscillation.

To prevent that, remove the valve in the phase splitter socket V3.

[stephenhartley]

Thanks.

I found a possible short between pins 6&7 of V3 socket which I replaced.

I replaced the output tubes (NOS Russian military 6n3c - supposedly matched pair) and the hum disappeared.

I tested all the voltages a number of times, including swapping round the output tubes, and there was initially variation, which has settled. The amp is now dead quiet.

In cathode bias mode:

Outer tube: OT voltage drop 6.4v. resistance 93.8R. idle current 68mA. Dissipation wattage 29.
Inner tube " 5.77v " 102R 56mA " 24

In fixed bias mode:

Outer tube: " 4.33v 46mA " 20
Inner tube " 3.56v 35mA " 15.6

So much for being a matched pair. Cathode resistor needs changing.

I don't actually have a schematic - I flipped between the 5E5 and 5E5A in Weber's book, then devised a switch to switch between the cathode bias of the 5E5 and the fixed of 5E5A.

I've previously added 470K 1W grid stopper resistors.

There's been an intermittent fault throughout - I'll get a decent matched pair of tubes and replace the coupling cap and double-check the sockets.

Thanks very much for the help.

Greatly appreciated

[stephenhartley]

Just wanted to say thanks again for the help in fixing my amp. Rehearsed with it on Fri with the bass through it and it sounded great. I never got round to drawing a schematic, but I drew a layout - one of attached pics. I'll be upgrading the output valves to 6L6GCs

[Helmholtz]

In cathode bias mode:

Outer tube: OT voltage drop 6.4v. resistance 93.8R. idle current 68mA. Dissipation wattage 29.
Inner tube " 5.77v " 102R 56mA " 24

In fixed bias mode:

Outer tube: " 4.33v 46mA " 20
Inner tube " 3.56v 35mA " 15.6

So much for being a matched pair. Cathode resistor needs changing.

A tube that has experienced severe redplating might no longer perform like new.