sirWheat
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For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 3, 2023 22:20:36 GMT -5
For over three weeks. And contrary to seemingly everyone else's experience, in my case there's been some kind of damage. This is a RedPlate Bluesmachine. It now produces a constant pretty loud hum no matter the setting; I can hear the guitar if I plug one in but it's garbled. I've swapped all tubes but for the reverb driver (don't have a spare at7) but this amp allows you to turn the reverb off so I assume that's not the issue. It's possible, via a particular setting and turning up particular knobs, to get a loud squeal and wavy noises as well. No overheating anywhere, including when I discovered that it'd been left on. I'm not expecting anyone to diagnose this, just hoping to find out about any usual suspects in such a case. Further, this amp is a custom build, modeled after the original owner's prized blackface Super so it has a tremolo circuit as well (which is also switchable and has no effect on the humming, etc.). Thanks for any future replies. I was already considering sending this back to RedPlate and having it turned into the latest version of Bluesmachine as this amp sounds decidedly different than others I've heard, particularly on the drive channel which gets ugly real quick. Maybe I should just do that... |
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Post by Lesterstrat on Dec 3, 2023 23:00:57 GMT -5
Sounds like a microphonic tube (the squealing). Try tapping on that at7 with a pencil to replicate the squeal.
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Post by reverendrob on Dec 4, 2023 2:12:22 GMT -5
One thing to keep in mind also is that the verb and trem circuits aren't "true bypass" in any real sense - so if the tube fails or goes microphonic, you can and will likely have issues even if off.
Willing to bet that Lesterstrat is probably right. If that doesn't fix the problem or point you in the right direction, contact the manufacturer.
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 4, 2023 8:01:04 GMT -5
Thanks! But I tried that. No reaction from any tube while tapping on it. Gonna pick up a new at7 anyway.
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pdf64
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Post by pdf64 on Dec 5, 2023 7:36:40 GMT -5
If you remove all the 12A_7 valves, does it still hum?
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 6, 2023 20:27:35 GMT -5
Yes, and more loudly. It is also the case that the 12at7 seems to be glowing rather brightly, if that means anything. Can't say whether that's different than normal.
Assuming that the at7 may be bad, can I put an ax7 in its place to prove it?
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Post by reverendrob on Dec 6, 2023 23:16:54 GMT -5
The AX7 will be noisier (about 40% more gain). Safety, probably so, but I usually go the other way (AT or AU into AX).
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DrKev
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It's just a guitar, it's not rocket science.
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Post by DrKev on Dec 7, 2023 4:46:11 GMT -5
If you remove all the 12A_7 valves, does it still hum? This again! Remove ALL preamp tubes. If you still have the noise it's a power amp issue. If the hum goes away, put the preamp tubes back in, one by one, starting from the phase inverter and working backwards to V1 until you get the noise again. That will help you narrow down where in the circuit the problem is. |
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pdf64
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Post by pdf64 on Dec 7, 2023 19:17:35 GMT -5
So with the valves in V1-6 sockets removed, the hum is worse? How about if the valve in phase splitter (V6) is then refitted? What’s the bias mV reading? The V# references are based on the descriptions in the link in post 1, which may not match your amp. The 12AT7 reverb driver will glow brighter, it’s a power output circuit with a much higher anode voltage than the 12AX7s. Due to the higher voltage and dissipation, I suggest not to put a 12AX7 in there. |
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 9, 2023 15:25:01 GMT -5
So I started anew; pulled all the pre-amp tubes, hum is the same as with them installed. Put V6 back in and no change. Not getting a real bias reading; starts out at around 10ish and descends to zero. Also, when putting the probe into the bias socket I was sometimes getting a small spark. ??
FWIW I also tried all of the above with a pair of 6v6 installed and got the same results.
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Post by funkykikuchiyo on Dec 9, 2023 15:44:14 GMT -5
I probably don't know enough about this stuff to add much to the conversation... but have you tried other inspections? Do you smell anything burning? If you look at the board, does anything look like it got toasted?
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 9, 2023 16:24:12 GMT -5
Thanks. I pulled the chassis just to have a look. Nah, nothing burnt-looking or smelling.
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pdf64
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Post by pdf64 on Dec 10, 2023 4:14:58 GMT -5
. ..Not getting a real bias reading; starts out at around 10ish and descends to zero. Also, when putting the probe into the bias socket I was sometimes getting a small spark. ?? . Hmm, that’s unusual. So was that with the meter set to the lowest DC voltage range? With the amp off, connect the meter probes as per the bias procedure, but with the meter set to measure resistance. The reading should be a very low resistance, 1 ohm, but a regular meter would read it a bit more, so under 2 ohms. It’s possible that a 6L6 has shorted for some reason, and popped that 1 ohm resistor. |
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 10, 2023 18:03:22 GMT -5
Yes, with the meter set to mV. Although, I realize now that I was trying to check the bias by putting the probe in the bias adjust socket (doh! I'm not as stupid as I sound, really) and that's where I was getting the occasional spark from.
Anyway, here's what I found doing it the correct way, expressed as the full range of the bias adjust pot;
3.8 - 50.8 mV
Resistance: 12.9 - 168.7
Thank you!
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pdf64
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Post by pdf64 on Dec 11, 2023 4:46:40 GMT -5
… Resistance: 12.9 - 168.7 The resistance reading needs to be made with the amp power off, from the results I’m not sure that was the case ? Whatever, I think the amp needs a bit of technical attention, my guess is that one of the output valves has shorted and damaged its screen grid resistor. If so, no valve can work in that socket until it’s fixed, hence the humming (due to the current imbalance) and lowish mV readings from the bias test point (shared between the 2 output valves). |
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sirWheat
Wholenote
For a better future, play Stevie Wonder for your children.
Posts: 319
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Post by sirWheat on Dec 11, 2023 17:58:30 GMT -5
Correct, I missed the part about powering off. Yes, it'll have to go see someone that has a clue. I can swap parts but diagnosis? Nah.
Thank you for having a go at it!
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Post by Lesterstrat on Dec 15, 2023 11:03:53 GMT -5
Please let us know what the issue ends up being. Thanks!
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