What a dumbo I am, I'm almost afraid to say what I just did. I fried the grids pins 4 and 6 of one of the 7591s by putting in the rectifier socket by mistake. I can't believe I did something so stupid. A nice GE 7591 that I just tested on a tube tester and both had no leaks now the grids are leaking and the tube is shot. I have no other 7591 and they are expensive.
Well, isn't always when you're doing a friend a favor for nothing you screw-up :BangHead:.
Keep with me, I'll just rewire the sockets for 6V6's I have a bunch of old stock USA, then go from there.
The pin-out is close for 6V6s just have to change the wire from pin 6 (G1) to pin 5 and the wire that was on pin 5 to pin 8 (K/G3) leaving pin 6 open. Sound correct?
What other changes do I have to make, grids to the bias off the 7591 were 120 ohm so that's Ok. I'll have to change the resistors in the bias, see schematic what do you recommend there? Schematic is posted above.
I'll also change the bias caps that was probably the cause of the high negative voltage. I guess 20uf at 50v is OK.
BTW, without the power tubes I still have -6.5 after the diode and of course nothing on the feed to the 220 or 120ohm since I blew up the 7591 :sad2:
stick with me, I 'll get this thing fixed, what a dumb A...
al
Can I wire it like the layout attached using one 25/50 cap with the 390K (instead of the 470ohm 1W) to the rectifiers tubes ac voltage?390K will likely be too big and cause the bias voltage to be too low. I'd start with a 100k first. Adjust the size up/down to give the bias voltage range you want.
Thanks for the info, I'll remember that if I ever convert but maybe the PT can't handle 6L6s.
The only thing is the PT doesn't have a low voltage bias tap so I figure I'll use the 390K like in the schematic.
Can I wire it like the layout attached using one 25/50 cap with the 390K (instead of the 470ohm 1W) to the rectifiers tubes ac voltage?
This circuit does not have tremolo so can run from the diode to bottom leg of the 10K-L bias pot and wire everything else like the layout?
So maybe I'm back to where I was before just that I have 6V6 now. Any changes to PI because of the 6V6's? Remember I'm using two 6Eu7s as in EA-65 schematic above.
Thanks all,
I just recheck my wiring again on the 6V6 conversion and check those 6EU7's pin outs too. Thanks Shrapnel for your advice.
I look at the amp's wiring and schematic for it (EA-65 above) and it's not wired like the schematic. Someone has made incorrect changes, no wonder it has strange voltage off the bias and no wonder it blew up in the first place. It's a true PTP rats nest but pretty much a simple amp. The voltages are way high as the choke is not even wired correctly.
I'll rewire the power supply correctly and see what happens, work the rail and bias voltage then put the tubes in and go from there.
I learned one thing today, no matter how simple a mistake can be, anyone can do it. Still can't believe I put a power tube in the rectifier socket. anyone else do that and turn the amp on...?
I'll get back with the results.
al
Your diode look good and your new caps value is OK.
Change the 51k for a higher value resistor in the bias circuit. This will help to have more bias negative voltage. Or you can also put a lower resitor in place of the 390 K, try a 220 K
You can also install an adjustable bias . Easy to do.
With a pot in serie with the 51 K. I'll use a 10 to 25 k pot in serie with the 51 K. Only the wiper of the pot must go to the two 220 K. You may have to try some resistor if bias is not enough negative for your tube.
Multi turn Cernet is the best and easy to use and cheap ( $2.50 )
is that electrical tape holding the two ceramic power resistors strapped in series together?
:laugh:
--DL
two: the EA-65 has some kind of photo cell LDR off the OT and volume Pot, see schematic, one leg broke and won't stay soldered to the photo cell (amp cuts out when the leg comes off) so I cut wire leading to it (green see pic)which came from the volume pod and that cap from the LDR to cut that cell from the circuit now where do I attach that sweeper off the volume (green wire) and what about the ceramic cap? See EA-65 schematic above look for the LDR.
EDIT I just figured out one answer on the volume and the cap, just attached the greed wire to the end of that ceramic cap..?
one: Some 6V6's I can't get the correct mV for biasing some tubes, the lowest I can get is 28 mV a little too hot (between a 1 ohm r on the cathodes}. I used Sluckey's recommended 100K off the acv to diode and a 10K L pod with a 15K on the leg to ground. Just like the Deluxe Reverb AB763 schematic above, except they had a 10K, didn't have one handy. How can I get more range using the 10K L pod and the 100K r? Change the 15K to what?Read my first post again. Decrease the 100K to decrease the 28mv reading across your 1 ohm cathode resistor.
I'll have to deal with adding a grounded plug but the switch is a three way and in standby it supplies power to the screen grids o nthe power tubes, what's up with that?No it doesn't. Look at the schematic. It puts a short across the drive lines to the 6V6 control grid. This kills the signal going into the 6V6s which effectively puts the amp into standby mode WITHOUT actually turning off any power supply voltages. Sears did this on their twin twelve amp too.
That Photocell is part of a feedback circuit that I'm not 100% familiar with. About all I can tell is is the feedback off the output runs through this, odd to me, cap-diode & resistor circuit before going to the LDR. (Perhaps PRR or someone else can enlighten me on it.) The LDR then controls the varistor part of itself to adjust the resistance to ground, I presume attenuating the signal.
I'll have to deal with adding a grounded plug but the switch is a three way and in standby it supplies power to the screen grids o nthe power tubes, what's up with that?No it doesn't. Look at the schematic. It puts a short across the drive lines to the 6V6 control grid. This kills the signal going into the 6V6s which effectively puts the amp into standby mode WITHOUT actually turning off any power supply voltages. Sears did this on their twin twelve amp too.[/quote]
any clues on how to add a three prong power chord. Do I just run the ground like on the schematic and fuze the hot side eliminating the switch part that has the .022 cap?Just connect the black (hot) wire to the fuse. Connect the white (neutral) wire to the switch just like the existing power cord. Put a ring lug on the green wire and connect the lug securely to chassis using one of the power transformer nuts. Don't mess with the switch wiring. I'd leave the cap on the switch, but it won't hurt to clip it out completely.
Also the voltages are too high for the 6V6s, B+ 472, Plates 466, screens 461. Could I just use a 5Y3 or would a zener off the HV center tap be better? Or use the Zener before the choke? Would you use a bigger screen grid value then 1K5 1/2 watt.Putting 6V6s in that amp is a mistake IMO. Those 7591s were much beefier than a 6V6. Rather than use 6V6s and kludging the power supply to drop the B+, I'd use 6L6s and diddle the bias accordingly. Or spend $20 for a 7591.
Quoteany clues on how to add a three prong power chord. Do I just run the ground like on the schematic and fuze the hot side eliminating the switch part that has the .022 cap?Just connect the black (hot) wire to the fuse. Connect the white (neutral) wire to the switch just like the existing power cord. Put a ring lug on the green wire and connect the lug securely to chassis using one of the power transformer nuts. Don't mess with the switch wiring. I'd leave the cap on the switch, but it won't hurt to clip it out completely.QuoteAlso the voltages are too high for the 6V6s, B+ 472, Plates 466, screens 461. Could I just use a 5Y3 or would a zener off the HV center tap be better? Or use the Zener before the choke? Would you use a bigger screen grid value then 1K5 1/2 watt.Putting 6V6s in that amp is a mistake IMO. Those 7591s were much beefier than a 6V6. Rather than use 6V6s and kludging the power supply to drop the B+, I'd use 6L6s and diddle the bias accordingly. Or spend $20 for a 7591.
Yeah, makes sense, thanks again for all your help, sure wish I hadn't fried that 7591 but teaches me to be more careful, it could have been a nasty shock mistake. Makes me think...?
Anyway, what would your opinion be if I were to go with EL-34's, would the voltage be better suited for that tube? I believe the pin-out is basically the same as for 6L6s except grounding pin 1 to 8?
Thanks all again.
al
EL34: 1.5A each - 3.0A for two = + 1.4A
6L6: 0.9A each - 1.8A for two = + 0.2A
7591: 0.8A each - 1.6A for two = + 0.0A
I noticed there are no screen grid resistors in this amp, how about two 3 watt 470 ohms?good idea
first stage has just a 2.2Kto ground and second stage cathode doesn't go straight to ground so not sure where to use a cathode cap there.Put a cap on the first stage if you want to, but leave the second stage alone. It's the PI and if you put a bypass cap on it's cathode, you'll kill one of the drive signals to the PA.