Randall RG50TC 'frying bacon' noise

[Stratfordade]

I'm fixing up a very nice Randall RG50TC (2xEL34 power tubes) whose owner reported reduced volume. Narrowed this down to a PA problem and subsequently discovered one half of the OT primary was shorted! Replaced the OT with a Hammond 1750N that required only minor modifications to the fixings to mount onto the chassis, and was rewarded with lovely high volume on all channels.

But fixing the volume problem exposed a new issue — there's a 'frying bacon' noise which is only there on the 'boost' channel. The Randall has a clean channel with 'normal' and 'boost' switch enabling some crunch on the clean channel, and an overdrive channel with two gain levels. All the channel switching logic works fine but I get this frying bacon noise only on the 'boost' channel. One clue may be that the clean channel level control (which affects both the 'normal' and 'boost' level of the clean channel) doesn't affect the volume of the noise, but acts more like a treble control. When the gain of the clean channel is maxed the frying noise is much more trebly!

The noise isn't there on power up, but starts after amp has been on about ten minutes with no signal applied but with a jack in input socket. The noise (all noise actually) disappears when I pull V2.

I've cleaned all pots and valve sockets especially pre-amp where I'm sure the problem lies. I've also changed each of the pre-amp tubes (3 12AX7 and a 12AT7 splitter) for a known good 12AX7 (I used it to replace the 12AT7 too as didn't have a spare 12AT7) with no change.

I'm thinking there could be problem with a plate resistor but looking at the schematic I'm having trouble identifying which tube is identified with the Boost channel, and I can't even see a switch for the Boost channel on the schematic apart from the connector for an external foot switch. If I can please have some help with decoding the schematic then I'm sure I can pin down what could be causing the problem on Boost — the amp is so nearly ready. Which components should I be looking at that are uniquely associated with the Boost setting?

Thanks for any help. Meantime I'll keep staring at the schematic!

[Stratfordade]

Very curious. I waited till the amp was making that noise again, and swapped over V1 and V2 both 12AX7. Noise stopped and didn't restart with tubes swapped warmed up. Swapped them back to confirm and now I can't persuade the noise to start again! Perhaps it was just bad socket but I'd cleaned them by spraying another 12AX7 and pushing it in and out of each socket a few times, spraying with cleaner in between moving between sockets. Anyway the amp sounds fine now!

[g1]

Just for your info, the OD channel basically puts and extra triode before the first V2 stage.
For the clean channel, the 'boost' essentially changes R207 from 82K to around 1M. So you have R207 attenuating the signal when boost is off (Q1 shorts out R18).

[Stratfordade]

Just for your info, the OD channel basically puts and extra triode before the first V2 stage.
For the clean channel, the 'boost' essentially changes R207 from 82K to around 1M. So you have R207 attenuating the signal when boost is off (Q1 shorts out R18).

Thanks very much! I could see the extra triode for Overdrive but hadn't understood how the boost worked but it's obvious now you explain it, and cunning how 'boost' is actually removing attenuation which I was not looking for!

Is it possible that the crackling noise was caused by a leaky C12 in that boost circuitry? I've buttoned the amp up now though — soak tested it for hours on boost and no crackles since I pulled and reinserted V2.

[The Dude]

It's not uncommon for a preamp tube to work fine in one circuit, but present problems in another.

[Stratfordade]

Thanks. That seems to be what’s happening in this case. Good experience for future issues!

[Stratfordade]

Naturally once I'd put the amp all back together again and tested it the crackly/frying bacon sound was back after using the Boost channel for a few minutes, so I took the chassis out again.

Even removed a board to change components which involved a lot of nuts, LEDs, and interconnects. g1 was good enough to explain how the Boost works (turning off FET so it no longer shorts out R18 resulting in less attenuation). I replaced all components where the signal may flow when boost on — R18, Q1, D3 — but made no difference! This was after I'd used freeze spray on each of those original components while it was crackling but that made no difference either. Maybe it's something intermittent in the push/latch button SW3 to switch from Clean to Boost but wiggling the button and the switch body didn't provoke crackling. It's something time/temp dependent but I've had to put the amp back together now so will have to do without the Boost channel till I can have it back to look at, and just rely on Clean and the 2 OD gain levels. You'd think it would be simple with so few components involved in Boost mode so it's maddening. Really the only extra component involved with signal flow when Boost is selected (turning off thew FET) should be that R18 1M so shouldn't be such an elusive fix.

If anyone has other suggestions for things to check when I have the amp back I'd be grateful!

[g1]

Have you repeated the tube swapping that seemed to get rid of the problem earlier? It's possible there is some intermittent or bad solder connection (R18 or R207 ?) that is getting flexed when the socket is pressured.
Otherwise, it's rare but a jfet could make crackling like that. De-soldering the source or drain leg (or removing fet) will put it in full time boost mode for testing.

[Stratfordade]

I removed R18 and soldered in a new 1M so think I’ve covered the possibility of bad joint there. I didn’t change R207 as it works fine in Clean mode when there’s more current through it.

Pulling FET altogether would be a great test! Should have thought of that but I did replace it with another one albeit J112 not the original J108. I thought any n-channel would be ok as small differences in d-s resistance irrelevant in series with 82K.

All solder joints looked excellent though, but I didn’t check for track cracks visually. But I have prodded board vigorously and couldn’t provoke crackling/rustling. R18 and R207 sit on a separate board (input sockets/controls/switches) to the one mounting the tubes so socket flexing shouldn’t be an issue. Originally the noise did disappear after swapping tubes but may have been coincidence as it’s now back!

I swapped V1 and V2 with no change. I tried a new tube in both positions and V3 but no change. The noise did get a lot louder when I pulled V1 though.

I need to hand amp back with this fault and advise to use Clean and OD only which is disappointing as personally I like the Boost sound best (the original fault was shorted OT now replaced)! Will do more testing on it when it’s not needed for a spell.

[Stratfordade]

Also there are some interesting switching arrangements going on to generate the boost switching signal so maybe I’ll find something in that area - even if no audio signal may be dodgy switching?

[g1]

That business with those 4 transistor bases tied together does not make any sense to me.

[Stratfordade]

That business with those 4 transistor bases tied together does not make any sense to me.

That puzzled me too!