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About Guitarnut

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  1. Since posting this, I've found out the following. What happens is that when in Pentode mode, the screens are connected to the "B" node of the PS. This gives a steady voltage to the screen placing it in it's intended role witin the Pentode. With the switch in the Triode mode, the screens are connected to the plates and act as secondary anodes making the tube act more like a triode. Fascinating... Peace, Mark
  2. This is a portion of the Epi BC30 schematic that shows the output stage. The amp has a switch that switches from triode to pentode... though the tube appears to be tetrode. Maybe they're showing the screen grid (pin 4) and not the suppressor grid since it takes it's power from the cathode supply and isn't connected directly to a pin? My first question is about the switch. In triode position, each tube's screen grid is connected to the "B" tap of the power supply. In pentode, they're connected to the plates. How does this alter the power output? Second question: All the other tube stages have load (anode) resistors but the plates of the 5881s don't. Why not? Is the output Tx the load in this case? And I guess since the Tx won't pass the DC component of the power stage, no coupling cap is needed to block it? Peace, Mark
  3. Thansk for the replies and info. Much appreciated. I think I have my head around this now. I'm a visual learner so I did a quick animation to map it out and see it "in action". Hope I have it correct. If not, feel free to corrct me and I'll update the animation.
  4. I'm trying to get my head around tube amp design and operation. I was making good progerss in my studies this morning until I started looking at the PS. The rectifier in particular. I understand the concept of a diode tube, and how electrons only flow from cathode to anode. That the AC supply off either end of the center tapped seconday PT is out of phase with the other end and the rectifier only conducts on the positive side of the sign wave. Filter caps fill in the gaps but for a bit of ripple that is handled by the choke. My big question is what exactly is going on in the tube. From this schematic, I see that 5 VAC is going to the heater and the HT is connected to each of the plates. But I aslo notice that the resulting DC output is connected to the 5 VAC line. How is it possible that these 2 are on the same "buss"? Is it because the output of the tube is only the positive side of an AC voltage and is only considered DC when in a circuit that connects it to ground? And, if electrons only flow from neg to pos, or cathode to anode, how is a higher voltage created on the cathode? Is the cathode pos in this case? Peace, Mark
  5. Modifying amps is more than prevalent, it's epidemic. What can be modded depends on what you want to accomplish. On the BC30, the non-polorized caps and all of the resistors are top mounted on the PCB. The mods I have planned are lifting a couple of bright caps and adding a Master Volume which can be done from the top side. The electrolytic caps are radial packages and both of the leads go thru the board...the can sits on top of them. So, the PCB has to be removed to replace or meter them directly. Peace, Mark
  6. Not directly because their terminals are under the PCB. And they wouldn't hold a voltage very long anyway since the balance/bleeder resistors, R50 and R51 mentioned above, give them a direct path to ground. Peace, Mark
  7. I have been trying to do some mods on my BC30. Understanding the importance of draining the caps first, I uplugged the amp, removed the chasis, set my DVM to 3000VDC and checked for voltage at the anode resistor (R4) of V1...nothing. Checked every resistor coming off of the power tubes (R40 - R46) ...nothing. I can't get to the pins of the caps without removing the PCB. Anyone who's done the C5, C6 mod, MV mod care to share how they drained the caps? Is it possible the circuit has bleed resistors that drain them off before I can get the tubes cooled and the chasis removed? Did anyone ever actually meter any voltage in the circuit? I've tested the DVM on several known DC sources...9V battery, 12VDC/24VDC power supply and it appears to be fine. I've never had any issues with it in the past. Jefrs has been a huge help in explaining the mods but he's 6 hours ahead of me and probably sound asleep as I'm posting this. I was hoping to get this buttoned up tonight. Short of metering the chasis when it's hot, I'm at a loss. BC30 Schematic Peace, Mark
  8. Thanks. I saw your MV mod before pics in the Mods thread but I didn't see any of the finished mod. Did you wire it up the same as Dannyboyee82 did here? Could you share a drawing of how you did it? Peace, Mark
  9. How are you guys draining the filter caps? I plan to short the anode/plate resistor (R4) on V1 pin 1 to ground. I've posted this question in several forums and not gotten any answers. I would sure appreciate knowing how you guys are doing it. Peace, Mark
  10. I know it's been along time since this post but I'm in need of some clarification on this. I want to discharge my BC30 in order to do the MV mod but I need to be sure I'm discharging properly. So the R that Andyroid is referring to would be R4 coming off of pin 1? ...see attachment. Once the above step has been taken, how can I verify that the caps are indeed drained? Meter from the + side of the cap to ground? Thanks, Mark
  11. Thanks Cheeks. That's a great idea. I'll give it a try. I'm thinking that a standard 2 switch pedal is all I need. Peace, Mark
  12. I'm in the US. Indiana, specifically. I've seen the switch listed in the UK but figured I could find a substitute here in the States cheaper...maybe even build a custom switch myself. I have the schematic but I'm not sure how to interpret it. Peace, Mark
  13. I'm looking for a footswitch for my BC30. I've seen that the switch for the SoCal also works on the BC30 but damned if I can find either in stock anywhere. I'm to the point I could buy a third party switch or wire up my own. Anyone know the switch and wiring config? Is it just as simple as 2 on/off switches and a TRS plug? What 3rd party or other brand switches would work for me? Peace, Mark
  14. Well, it's out. My plan to use the Weldon 16 cement worked like a charm. I cut two small pieces of .060" plastic binding and glued them together on their sides. I then cut a notch in the end that was deep enough to reach the bottom of the socket and narrow enough that it didn't touch the sides of the socket. With the amp upside down, I then applied a small dab of cement to the end of the binding and eased it into the socket. It centered itself due to the curvature of the remaining plastic pin in the socket. I held it in place for about a minute until the cement started to take hold, then let it cure over night. This morning I was able to slowly pull it from the bottom of the socket. It eased up to the top and then popped out...breaking into smaller pieces as it cleared the edge. All clean and clear. Peace, Mark
  15. Thanks. I thought of that but the sockets are soldered to the PCB in the BC30...no holes in the center. You can see the two 6L6 sockets at the bottom center and right in this pic. I wish it were that easy though. Peace, Mark
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