Thanks Simon I must have had my brain switched off when I read that post.
Carry on.
Thanks Simon I must have had my brain switched off when I read that post.
Carry on.
But C139 before that (and before some resistors giving a 1.1k overall resistance, and so C139 will be seeing a higher voltage than C8) is only rated at 100V, so it's more likely to be 60-80v. Still a lot better than some designs using maybe just 12v. It would have been jolly useful if they'd listed the voltages on the schematic!Originally Posted by DrNomis_44
Totally agree with you there, from a servicing point of view if a schematic also shows the typical voltages at various points on the schematic, that's definitely more of a help, I was a bit unsure what the +HT for the 12AX7 would typically be so I based my guess on the voltage-rating of C8, a good practice is to rate the cap higher than the expected circuit voltage so it doesn't explode, I would tend to err on the side of a bit too high, say for example the expected voltage is 100v, I would use a cap with a voltage-rating of 150V to 200V to ensure reliability.
I just had another look at the Carvin BX500 Rev. F schematic, and I found C139 which is the main +HT supply filter cap, and then there's those three 3k3 resistors in parallel (R247, 248, and 249) making a total of 1k1 as you said, and then right after is C8, so you're right.
So, what I can work out from the schematic is that, when the 12AX7 circuit is switched in Fet Q28 is switched off so it's not conducting from the drain to the source terminal, and Fet Q102 is switched-on so it's conducting from it's drain to the source terminal, when the 12AX7 circuit is bypassed, the opposite-state applies.
Basically, Q28 and Q102 are alternately switched on or off depending on whether the 12AX7 circuit is bypassed or not, and in the schematic, switch S2 A looks like it is responsible for alternately switching Q28 and Q102 on or off.
Last edited by DrNomis_44; 23-11-2020 at 06:58 PM.
Its a strange amp as the AC seems to be rectified to DC, then there must be a frequency inverter section to convert some power back to AC to pass through the transformer to get the higher DC voltage! There is a lower voltage DC output given as 40V, so you though they could have put the higher voltage on there, if only for safety reasons!
Yep, I noticed that and I agree, it's a bit unusual to go about generating the +HT supply in a bit of a roundabout way like that, perhaps it's a cost-cutting measure so that Carvin wouldn't have to put money into designing/manufacturing a power transformer with a separate HT secondary, I wonder if the high frequency signal from the inverter bleeds into the audio signal path of the amp, probably not if it's well filtered-out.
Not that I really know what is going on, but there is a small section near the bottom that says 'voltage doubling below 140v'. So I assume they are making a universal unit that works with the selection of 110v-130v and 220v-240v power supplies around the world without having to make region-specific variants. Also very useful for the musician if touring different countries and taking your equipment with you.
Most of this has gone over my head. I can confirm is that the back of the amp says it will handle 120 to 240 VAC. Sadly I have no world tours planned at the moment.
FWIW the F schematic should be fine. I have the G revision, but it only differs from F by the addition of one cap.
Other than that I understand "nur Bahnhoff" as the Germans say ;-) I had no idea how much EE talent there was on this board!
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The consensus we came up with is that the 12AX7 tube/valve doesn't really do much tonewise at low volumes due mainly to how it is configured in the circuit, it's not really providing much in the way of gain, it is really acting as more of a buffer.
That part I thought I understood. Glad to have it confirmed. I am not sure I have ever had the master turned up high enough to hear the difference. It would be sort of ironic if you could only hear the difference shortly before going deaf.
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