There seem to be some differences in the connections around pin 6 (the anode) of the ‘vol.’ EF86:
The resistor from pin 6 to the power supply cap should be 220k (not 2.2M).
The 7.5k resistor connecting to the wiper of the vol. pot seems very strange.
The resistor from pin 6 to the power supply cap should be 220k (not 2.2M).
The 7.5k resistor connecting to the wiper of the vol. pot seems very strange.
Thanks Malcolm,
I'll double check the diag and circuit and replace as necessary.
I'll let you know how I get on.
I'll double check the diag and circuit and replace as necessary.
I'll let you know how I get on.
+1
Yes the gain will be a bit "cough" high..😀
Unless your trying to play 78 records with crystal pickup..😀
Regards
M. Gregg
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Sorry for delay in responding but the week's been taken up with that work thing.
My bad the 2.2M is actually a 220k
and the 7.5k is actually a 1.5k and is as the original circuit.
Re: the "now 1.5k" connection through a capacitor to pin 6 seeming odd, I snipped the capacitor from pin 6 and the amp came to life. Still not quite as loud as the original, but pretty much 90% of it and with the extra distortion/ preamp I built into the amplifier it should certainly do. There's a big hum coming through but I have wires and connections all over the place that need putting back before I raise this as an issue. Need to get it tested with the good valves and speaker too. Will report back as to how it goes.
For the meantime though, thanks to you Malcolm and to M for your help in rescuing me from the swamp.
My bad the 2.2M is actually a 220k
and the 7.5k is actually a 1.5k and is as the original circuit.
Re: the "now 1.5k" connection through a capacitor to pin 6 seeming odd, I snipped the capacitor from pin 6 and the amp came to life. Still not quite as loud as the original, but pretty much 90% of it and with the extra distortion/ preamp I built into the amplifier it should certainly do. There's a big hum coming through but I have wires and connections all over the place that need putting back before I raise this as an issue. Need to get it tested with the good valves and speaker too. Will report back as to how it goes.
For the meantime though, thanks to you Malcolm and to M for your help in rescuing me from the swamp.
Looking,
at the schematic Malcolm posted and comparing your diagram.
You show a resistor of what looks like 1.5K wrong..if you look at the bottom of the tone controls on Malcolms schematic you will see a 15K (under the Bass pot), but its in series with the pot etc so its closer to 300K in total.
This is what I think you should have the 15K in the red box I believe should be 300K. I have also added a 1 meg on the input (in blue).
its just a scratch drawing. I have hacked the schematic to remove the tone controls.
NB there is a 10M across the first tube but you don't have it on your drawing so I assume your other amp doesn't have it!
I also think the input resistor shown as 2.2K is way to small..I would use something like 22K, on the schematic its shown as 220K.
If the input resistor is to small the input pot will go noisy very quickly.
Regards
M. Gregg
at the schematic Malcolm posted and comparing your diagram.
You show a resistor of what looks like 1.5K wrong..if you look at the bottom of the tone controls on Malcolms schematic you will see a 15K (under the Bass pot), but its in series with the pot etc so its closer to 300K in total.
This is what I think you should have the 15K in the red box I believe should be 300K. I have also added a 1 meg on the input (in blue).
its just a scratch drawing. I have hacked the schematic to remove the tone controls.
NB there is a 10M across the first tube but you don't have it on your drawing so I assume your other amp doesn't have it!
I also think the input resistor shown as 2.2K is way to small..I would use something like 22K, on the schematic its shown as 220K.
If the input resistor is to small the input pot will go noisy very quickly.
Regards
M. Gregg
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I agree. The connections you show around that 1.5K don't seem right. In your diagram you show the 1.5k connecting the wiper of the vol. pot to the output side of the 0.1uF coupling cap on pin 6 (the anode). If it is really like that, I think it would put a massive negative feedback around the first (vol.) EF86, killing the gain completely.
Wire it as in post 26# and let us know..I would use the 300K in place of the 15K<<you show what looks like 1.5K but that's not what should be there if you remove the tone controls. As Malcolm says the 1.5K is wired wrong on your drawing anyway.
Compare to the circuit posted By Malcolm:
http://www.chambonino.com/carlsbro/conc30a.html
Regards
M. Gregg
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Sorry for delay in responding, work takes the upper hand.
Tried a few little change here and there and then decided best idea would be dismantle it and as you suggest M, and start the pre amp again but from the schematic of the original rather than my bodged one. I'll let you know how I get on. But as I say, work take the upper hand, so it won't be tomorrow 😉
Thanks.
Tried a few little change here and there and then decided best idea would be dismantle it and as you suggest M, and start the pre amp again but from the schematic of the original rather than my bodged one. I'll let you know how I get on. But as I say, work take the upper hand, so it won't be tomorrow 😉
Thanks.
Hiya,
eventually got some time to spend some time on the amp.
Have rebuilt the front end to the schematic and it works, a few buzzes and stuff here and there, but need to reroute some cables before I chase them on this forum. Also, I wired up the tone capacitors the wrong way round, so still got to change them round.
So thanks for all your help - lesson learned - don't run before there's a bird in the hand - or something.
One little question:
One of the buzzes (with nothing plugged in) was like the classic guitar lead plugged in but no guitar on the end. The input jack is the 'switched' kind, so I shorted the +ve jack to ground when the lead is unplugged, which seemed to stop the noise. Lead plugged in breaks this short. Shorting it with a resistor didn't stop the buzz, only a direct short. So just wondering if this is symptomatic of something mis-wired in the circuit (have checked the earths) or if it would overload the circuit in some way, or if it should be ok, as it appears to be?
Any thoughts would be much appreciated.
John
eventually got some time to spend some time on the amp.
Have rebuilt the front end to the schematic and it works, a few buzzes and stuff here and there, but need to reroute some cables before I chase them on this forum. Also, I wired up the tone capacitors the wrong way round, so still got to change them round.
So thanks for all your help - lesson learned - don't run before there's a bird in the hand - or something.
One little question:
One of the buzzes (with nothing plugged in) was like the classic guitar lead plugged in but no guitar on the end. The input jack is the 'switched' kind, so I shorted the +ve jack to ground when the lead is unplugged, which seemed to stop the noise. Lead plugged in breaks this short. Shorting it with a resistor didn't stop the buzz, only a direct short. So just wondering if this is symptomatic of something mis-wired in the circuit (have checked the earths) or if it would overload the circuit in some way, or if it should be ok, as it appears to be?
Any thoughts would be much appreciated.
John
I had a problem with a Switchcraft shorting jack in a recent project. Even though the 'switched' arm was contacting the hot arm, it wasn't making a good contact. I ended up cleaning it with acetone (wet a business card and pulled it through the contacts) and a piece of very fine (1200 grit) abrasive, which seemed to cure the problem.
I don't know if there was some oxidation or mill oil on the contact, but it definitely wasn't shorting properly at first, even though I had it wired to short to ground when no plug was inserted.
Loud and quiet
Update: The Amp is now Loud and Quiet, yes loud and quiet.
Cleaned the input jack as suggested, it gave an improvement in the amount of hum, also found a bit of a dry solder joint on the earth to the input too, that improved it too.
The cables from the input jack, to volly pot, to first EF86, to tone controls, I replaced with screened cable.
I installed a bias control on each EL34 and balanced them up.
Also rerouted a couple of heater wires, and tightened all the nuts and bolts.
I sourced some Tropical Fish caps for the tone controls,
soldered them in place,
turned it on
and hey presto!! loud and quiet.
On full volly and full tone controls there is a slight hum, but well within acceptable bounds. Don't think I've ever come across an amp (esp a valve amp) that was totally quiet anyhow.
So a big thanks to you all for helping me reverse out of my cul de sac and back onto the main road - I'm a happy camper once again.
Donation to the site on it's way.
🙂
Update: The Amp is now Loud and Quiet, yes loud and quiet.
Cleaned the input jack as suggested, it gave an improvement in the amount of hum, also found a bit of a dry solder joint on the earth to the input too, that improved it too.
The cables from the input jack, to volly pot, to first EF86, to tone controls, I replaced with screened cable.
I installed a bias control on each EL34 and balanced them up.
Also rerouted a couple of heater wires, and tightened all the nuts and bolts.
I sourced some Tropical Fish caps for the tone controls,
soldered them in place,
turned it on
and hey presto!! loud and quiet.
On full volly and full tone controls there is a slight hum, but well within acceptable bounds. Don't think I've ever come across an amp (esp a valve amp) that was totally quiet anyhow.
So a big thanks to you all for helping me reverse out of my cul de sac and back onto the main road - I'm a happy camper once again.
Donation to the site on it's way.
🙂
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