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ehedwr

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About ehedwr

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  1. If you stick with the rectifier tube just make sure you don't use that Standby switch. Leave it switched on, you don't need it. Your amp will thank you for it. My BC30 is running a pair of EL34's with a solid state rectifier. The voltage did go up so I replaced the 250 ohm 25 watt cathode resistor with a 330 ohm alternative to lower the plate current.
  2. There is no benefit in my opinion. Tube rectifiers aren't cheap, they don't last forever, can be unreliable and cause sag when the amp is pushed hard.
  3. Like others have done I snipped C3, C5 and C6. At first I liked it but later realised something was missing, you lose treble but you lose dimension too. I fitted a triple pole mini toggle switch and wired the capacitors through the switch so I can switch em back into circuit whenever I need. It's like a bright switch. I also replaced R3, R8, R15 and R21 with 1K/1 watt resistors. The GZ34 rectifier is now gone and replaced with a solid state recifier, this however has raised the plate voltage to 450 VDC. I wanted to fit EL34's so I replaced R45 and R46 with 1K/5 watt resistors, popped in a pair of EL34's and it worked fine however with 60mA of cathode current at zero signal I replaced the shared cathode resistor R44 with a 330 ohm 25 watt alternative which is now mounted if front of the fan for better cooling. Cathode current is now 50mA per tube and this amp is running like a dream. The fizz I had with the 6L6 tubes is gone, now I have creamy overdrives and gorgeous cleans. I use a THD hotplate attenuator so channel 1 cranked up gives the best overdrive tone of all. This amp is superb now. I might yet fit another input jack socket and maybe look into building a tremelo circuit and effects loop. But they are considerations for the future and not exactly easy mods to perform.
  4. If you want to rock at 5 watts then the Blackstar HT-5 head is about the best amp around. Next up from that I'd go for an Orange Tiny Terror @ 15 watts.
  5. The standby switch is not needed in this amp. Why? It has a tube rectifier which has to warm up itself before it will allow voltage through to the plates on the rest of the tubes. By the time that happens the rest of the valves are sufficiently warmed up and ready for action. If you have a solid state rectifier fitted like me, use the standby switch. When I do go back to a tube rectifier I'll be fitting some current limiting resistors to the plates to slow down the inrush.
  6. Many older amps didn't need a standby switch because the tube rectifier takes time to warm up and allow voltage through anyway. One thing I'm looking into at the moment is fitting limiting resistors to the rectifier plates which will reduce inrush. Check out the tube data for the GZ34. http://www.mif.pg.gda.pl/homepages/frank/sheets/035/5/5AR4.pdf Most amps don't have limiting resistors and manage fine. Something is very wrong with the design of this amp this amp. I'm wondering if it's the choke not holding back the current flow enough.
  7. I know what is causing the main fuse to blow. It's a design flaw. The rectifier tube pulls too much current on startup and arcs inside causing the main fuse to blow. I've had it happen enough times. Best start procedure from cold is to switch on the standby switch first, then the main on/off switch. The rectifier will allow voltage through more gradually this way and the other tubes wont get full voltage for at least thiry seconds anyway. Other than that replace the rectifier tube with a solid state rectifier designed to fit straight into the octal base. It will bump up the voltage slighlty but will work fine. My rectifier tubes are now very dodgy because of this, they work but the slightest current increase can make them arc and blow the fuse. If you want to stick with a GZ34 rectifier get a new one and it should be better.
  8. IMHO No. The preamp in your Vox is designed for high impedance, low voltage signals from guitar pickups. Amps in parallel sound fantastic but not in series. Why do that anyway?
  9. Well a fine gentleman from another forum informed me that the filter cap maximum for a GZ34/5AR4 rectifier tube is 64uF. C33 and C34 in series work out as 50uF. I had replaced mine with with a pair of 220uF/400V caps which gave me 100uF. This was drawing too much current through the rectifier on startup. I put the stock caps back in and then modified the way the standby switch operates. At present when you switch on your main on/off switch power goes straight to the heater filaments for all tubes including the rectifier. When you switch your standby switch on current rushes through your warmed up rectifier to fill up your filter caps and provide power to the rest of your amp circuit. I believe this current spike and the arcing in the rectifier is what's causing the main fuse to blow for alot of people. Options? Fit a solid state rectifier which can take more current or alter the position of the standby switch in the circuit. I'm still awaiting my solid state rectifier so I dropped the standby switch out of it's current position in the circuit, instead the rectifier is now connected directly to the transformer. I have used one of the standby contacts and wired it in series with the fuse F8 on the HT line. It now switches the 420VDC output from the rectifier to the choke T4. Now when I switch the amp on the rectifier comes online with the main switch and starts to trickle charge the filter caps as it warms up. A little more gently than before. On switching the standby switch the filter caps are already full which aids the rectifier in charging the rest of the circuit. Well thats my understanding of it and it's working at the moment! Only thing is I'm not entirely happy with switching 420 volts with the cheaply made switch that comes with the amp. I'll keep you posted of any developments.
  10. I've ordered a Sovtek Solid State rectifier from Hotroxuk.com which plugs straight into the rectifier octal base. I'm expecting the voltages to be a little higher so I'll be watching out for that but at least it'll take more current than a tube. I will also fit a second temporary standby switch at a different point in the circuit to control the amount of inrush current on startup. I'll probably put the original filter caps back in too. If that works then fine but if not I'll be testing the transformer with a megger to see if it's breaking down. I'll keep you posted. Finding a replacement TX could be difficult because of the unusual 12V winding.
  11. Ok. I just tried this again to see what current readings I would get with the EL34's fitted and the good rectifier tube installed. At first all was well. The plate voltage for some reason had risen from 420 VDC (with 6L6's) to 430 VDC. I turned off to measure mains current, started it back up and POP! Another fuse gone, the power TX farted and a flash from the rectifier tube. What next? I know the heater filaments draw more current with EL34's but the 6.3V winding is capable of 5 amps and only feeds the output tube heaters. Maybe it's time to look for a new power transformer and fit a solid state rectifier. Oh and finally I must add that I uprated the filter caps from a pair of 100mF/400V electrolytics to a pair of 220mF/400V electrolytics. I did this to reduce sag. Do you think the larger caps are sucking too much current on startup? --------------------------------------------------------------------------------
  12. I've recently done a few mods to my BC30 like fitting a bright switch and converting to EL34 output tubes. Whilst playing the other day the mains fuse (F9) blew on me. Thinking it was because I had changed tubes I put the 6L6 tubes back in. I tried a few more fuses and they kept on blowing when I switched from standby to play. I then pulled all the internal fuses and tried it by putting em back in different stages to try and isolate the fault. Guess what? From then on it worked fine. Today I had the chassis out taking notes of different voltages at various points. Everything was hunky dorey until I plugged in the guitar and started giving it a bashing. Pop! The fuse blew again! I replaced the fuse and connected my multimeter inline with the power supply to see what was happening. Don't compare my readings here in the UK to yours elsewhere. US currents would double what I have. On switching on the power switch the current quickly settled at about 300 mA. When I switched on the standby switch it rose to 500mA. The original fuse was rated at 1.6 amps. I tried this a few times when I noticed the rectifier tube would sometimes arc whilst switching on the standby switch. The reading on my meter spiked and a crack could heard coming from my mains transformer. I had a Harma GZ34 fitted and only a few weeks old too! I put the original Chinese rectifier tube back in. Now there is no spiking, arcing or cracking and with Channel 2 cranked right up, full volume, full gain, full sag, it's drawing 650 mA. I'm hoping that this is a just a case of an iffy valve and not some other problem causing the rectifier to go bad. And I hope I caught it in time to save my power supply TX from blowing. As for the EL34's? They'll be going back in soon as soon as I'm absolutely positive they didn't cause this. Though I don't see how they could have. Anybody have any insights on this?
  13. I have replaced the Sovtek 6L6 tubes with Harma 6L6GC STR. A definate improvement. As for the preamp it's mix and match. In v1 I fitted a Sovtek 12AX7LPS and in v5 I have a Mullard 12AT7 to calm down the reverb. I have snipped C3,C5 and C6 like others have done but I'm about to wire them back in via a mini toggle switch which will act as a bright switch. I'm still considering fitting a presence control. I have just taken delivery of a pair of EH EL34's which I'm gonna fit after some alterations. I'll let you know how that one goes. One thing I will say is this. Watch out for that schematic that Epiphone has issued, quite alot of component values are different on my amp to what the schematic says.
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