Hi,
I remembered the JJ's when it was to late to edit the post. I figured someone or three would correct me 😉
Cheers!
Trout said:
I remembered the JJ's when it was to late to edit the post. I figured someone or three would correct me 😉
Cheers!
I am in the process of building one right now. As soon as I can I will post a schematic as it currently stands but a quick description is as follows.
Uses 3 12AX7s and two 6V6 (should be able to sub 6L6 or EL34 as well). Output tranny and PT are surplus from a 6L6 based prototype of someone elses.
A single input jack connects to a normal and bright channel that can be mixed in any proportion. A switch is being installed to allow the normal channel to be cascaded into the bright channel for additional gain. This feeds an additional gain stage with a cathode follower driving a Marshal style TMB stack.
The master volume after the tone stack feeds the LTP PI with a passive preout and power amp in with switching implemented in the jacks.
Power stage will be shared cathode bias of 250 ohm Rk bypassed by 1000uf. Screen resistors are 470 ohms.
PS will provide about 360V to the plates, 300 to the screens with progressively less to each AX7 ending at about 250 to 270V if all goes well.
I plan a full standby implementation with switched B+ and a diode switched into the heater circuit to idle them at about 1/2 heater power.
OT has 4,8,and 16 taps that are switched through a Hoffman impedance switch.
Planned speaker is an Eminence "Man-O-War" in the combo with a lower efficiency driver available in a separate cabinet. Both 12"
Will let you all know how it turns out. I am hoping to have it done by mid May.
mike
Uses 3 12AX7s and two 6V6 (should be able to sub 6L6 or EL34 as well). Output tranny and PT are surplus from a 6L6 based prototype of someone elses.
A single input jack connects to a normal and bright channel that can be mixed in any proportion. A switch is being installed to allow the normal channel to be cascaded into the bright channel for additional gain. This feeds an additional gain stage with a cathode follower driving a Marshal style TMB stack.
The master volume after the tone stack feeds the LTP PI with a passive preout and power amp in with switching implemented in the jacks.
Power stage will be shared cathode bias of 250 ohm Rk bypassed by 1000uf. Screen resistors are 470 ohms.
PS will provide about 360V to the plates, 300 to the screens with progressively less to each AX7 ending at about 250 to 270V if all goes well.
I plan a full standby implementation with switched B+ and a diode switched into the heater circuit to idle them at about 1/2 heater power.
OT has 4,8,and 16 taps that are switched through a Hoffman impedance switch.
Planned speaker is an Eminence "Man-O-War" in the combo with a lower efficiency driver available in a separate cabinet. Both 12"
Will let you all know how it turns out. I am hoping to have it done by mid May.
mike
sounds similar to what i had in mind.
I'm still tossing up whether i should try 2 or 4 6v6s in the power stage.
im definately going with jj's tho.
maybe 350 or so volts on the plates
about 290 volts will actually be usable making a swing of 580VpK on the output tranny of 8k - giving out about 21wattRMS...
it's going to power this cool vintage 12" speaker i have.
I'm still tossing up whether i should try 2 or 4 6v6s in the power stage.
im definately going with jj's tho.
maybe 350 or so volts on the plates
about 290 volts will actually be usable making a swing of 580VpK on the output tranny of 8k - giving out about 21wattRMS...
it's going to power this cool vintage 12" speaker i have.
mashaffer said:
Nice Job,
V1 is set up almost identically to my Marhall lead 100. I guess thats more a personal taste think, but I always felt at least in the Marshall that it tended to be a bit overwhelming on brightness.
I have a 73 PTP SL I restore last fall. Picked it up for $250.00 and other than some &^&* drilling an extra hole in the rear of the chassis, It had the wrong tubes in it. I got lucky and got the early 73 6550 version. The guy had put it EL34's and could not figure out why it sounded ratty.
One thing I would like to ask you about though. The Effects loop, I thought it was pretty much standard practice to allow some signal to pass through (bypass) the loop unaltered?
Also, why is the cap after the master such a large value? .68?
I would have though perhaps .1 or ,05.
Just curious.
I have been thinking about grabbing one of these to try out, looks very well done and the add in kit is very reasonable.
http://mysite.verizon.net/resqaaya/products.html
Trout
Trout said:
I've had horrible luck with the JJ 7591s in my Fisher amps. Three different quads I've ordered have had at least one tube that failed within an hour of installing it. I've switched to the EH 7591A and haven't looked back since.
Doc Trout;
The .68 was actually just a guess I haven't run any equations at this point. The "FX loop" is actually going to be used most often as a preamp out into a PA in situations where his band leader demands it. Given the low Zin often encountered with SS gear I want to be sure that I don't get excessive roll off.
As to the pass through I had not initially planned on a preout but found out that it was in fact a requirement. I also was given a short timeline so went for the simplest implementation I could come up with. Since the amp is for my Grandson I have the luxury of making changes at a later date if needed. 🙂 Also I think that many pedals have a bypass built in do they not?
When you say V1 are you referring to the input tube (U8/U7 in my drawing) or the output tube (U1)? Some have suggested that the 330uf bypass cap on the normal channel is much too large. What is your opinion?
Thanks, mike
Love the Avarar BTW.
The .68 was actually just a guess I haven't run any equations at this point. The "FX loop" is actually going to be used most often as a preamp out into a PA in situations where his band leader demands it. Given the low Zin often encountered with SS gear I want to be sure that I don't get excessive roll off.
As to the pass through I had not initially planned on a preout but found out that it was in fact a requirement. I also was given a short timeline so went for the simplest implementation I could come up with. Since the amp is for my Grandson I have the luxury of making changes at a later date if needed. 🙂 Also I think that many pedals have a bypass built in do they not?
When you say V1 are you referring to the input tube (U8/U7 in my drawing) or the output tube (U1)? Some have suggested that the 330uf bypass cap on the normal channel is much too large. What is your opinion?
Thanks, mike
Love the Avarar BTW.
BTW, I found the 2.7K resistors that I thought that I had forgotten to order so I now have changed the OT grid resistors to 2.7k. I also am adding a 220K resistor in parallel with the first PS filter cap as a discharge. This figures to 1.6 mA draw which should not hurt anything and will provide a safety bleed down.
mike
Oh, BTW I have all the sockets, jacks and switches wired except where they connect to the turret board and where parts are still not in (e.g. diode for standby switch). I started to populate the turret board last night so it is getting much closer to D day.
mike
Oh, BTW I have all the sockets, jacks and switches wired except where they connect to the turret board and where parts are still not in (e.g. diode for standby switch). I started to populate the turret board last night so it is getting much closer to D day.
hellomashaffer said:
me too looking for some guitar amplifer
maybe will buy ;; maybe can i build?
mashaffer, can i ask you
1. this schema: http://www.diyaudio.com/forums/attachment.php?s=&postid=1484262&stamp=1208217851
can i change tube? what is another 12AX7
i have at home 6922, only
2. 6v6 is expensive? alternative?
3. where can buy output transformatore in europe?
Is another guitar amplifer better for me?
i want easy to build, not difficult
tank u very much all help
Jack
Jack I am kind of new at this, in fact this is my first but I will give you what help I can.
1) The 6922 is a great tube for hifi but it is not really suitable for this application because of the same things that make it a great hifi tube. In a guitar amp you need a high mu triode or possibly a pentode. There are other high mu (70+) triodes out there but I don't have a list. Some have used 6SL7 (octal base) and 12AT7 (9 pin like the AX7). There are also probably suitable choices in the NOS 7 pin tubes out there.
2) The circuit could be massaged for use with EL84 (9 pin). In Octals 6L6 and EL34 are possibilities as well.
3) I have no clue.
Best bet is to pop over to...
http://www.18watt.com (Discussion of Marshall 18W based designs only) My design is based roughly on the 18W output stage which originally used the EL84s.
and http://ax84.com/
The guys there can help you better than I ever could.
mike
1) The 6922 is a great tube for hifi but it is not really suitable for this application because of the same things that make it a great hifi tube. In a guitar amp you need a high mu triode or possibly a pentode. There are other high mu (70+) triodes out there but I don't have a list. Some have used 6SL7 (octal base) and 12AT7 (9 pin like the AX7). There are also probably suitable choices in the NOS 7 pin tubes out there.
2) The circuit could be massaged for use with EL84 (9 pin). In Octals 6L6 and EL34 are possibilities as well.
3) I have no clue.
Best bet is to pop over to...
http://www.18watt.com (Discussion of Marshall 18W based designs only) My design is based roughly on the 18W output stage which originally used the EL84s.
and http://ax84.com/
The guys there can help you better than I ever could.
mike
BTW. http://www.ppwatt.com/ is a sister board of 18Watt where a broader range of discussion is welcomed. That could be a very good resource as well.
mike
mike
6V6 should be cheap. They were used in many old valve radios. I have a box full of old, but good, used ones.
Opinion on 12AT7 for last gain and CF
BTW, I would like the experts opinion on changing the last gain stage and CF (driving tone stack and preamp out) to a 12AT7 instead of the 12AX7 to provide more drive when connected to a solid state PA amp via longish cables. Would you think this advisable?
Off hand I would say that I would want to reduce the load resistors by about 50% or 60% right? Might possibly need to change the 820 ohm cathode resistor but maybe not as I think the AT likes larger bias voltage anyway. Will take a look at the curves at lunch today.
mike
BTW, I would like the experts opinion on changing the last gain stage and CF (driving tone stack and preamp out) to a 12AT7 instead of the 12AX7 to provide more drive when connected to a solid state PA amp via longish cables. Would you think this advisable?
Off hand I would say that I would want to reduce the load resistors by about 50% or 60% right? Might possibly need to change the 820 ohm cathode resistor but maybe not as I think the AT likes larger bias voltage anyway. Will take a look at the curves at lunch today.
mike
6922 works great at the input of a guitar amp if you use it as a cascode. After having converted from EF86 to this tube I am really satisfied.
If you are temped by the 7591, as someone else also mentioned stay away from JJ. EH works splendid though. And I like 7591 far better than 6V6.
Brgds
If you are temped by the 7591, as someone else also mentioned stay away from JJ. EH works splendid though. And I like 7591 far better than 6V6.
Brgds
My first guitar amp... under construction is a hybrid...
I changed my tube order to 12AX7EH made by Electro Harmonix ... after I read the EI ones are no go in guitar amps due to microphonics. just a shame OPTs are so expensive +-$150 here...
But the next one will be a Mesa Boogie clone of the Mark 1... i actualy want to build the dual/tripple rectifier, but allthe switches have me too confused at this stage... allthoug it might be reverse engineerable form looking at diffirent schematics of their models...
http://www.schematicheaven.com/boogieamps/boogie_mki_reissue.pdf
They make the nicest sounding amps in my books...
http://www.schematicheaven.com/mesaboogie.htm
I changed my tube order to 12AX7EH made by Electro Harmonix ... after I read the EI ones are no go in guitar amps due to microphonics. just a shame OPTs are so expensive +-$150 here...
But the next one will be a Mesa Boogie clone of the Mark 1... i actualy want to build the dual/tripple rectifier, but allthe switches have me too confused at this stage... allthoug it might be reverse engineerable form looking at diffirent schematics of their models...
http://www.schematicheaven.com/boogieamps/boogie_mki_reissue.pdf
They make the nicest sounding amps in my books...
http://www.schematicheaven.com/mesaboogie.htm
Hey Guitar Joe, did you ever decide what to do? I finished mine, in fact Wes is jammin' on it right now. That thing gets loud! I changed a few values and decided to skip the diode in the heater standby supply.
mike
mike
Re: Opinion on 12AT7 for last gain and CF
I wouldn't bother- the ECC81/12AT7 draws a LOT of grid current, which makes it generally poor sounding in overdriven guitar amps, except sometimes as a long tailed pair. I would go for an ECC82/12AU7 instead, if you don't mind the drop in gain. They're much more bluesy, and you can get your lower Zout that way (as you say, by reducing the cathode load resistor).
If you still want high gain, a 12BZ7 or even an ECC832/12DW7 is an option.
mashaffer said:
I wouldn't bother- the ECC81/12AT7 draws a LOT of grid current, which makes it generally poor sounding in overdriven guitar amps, except sometimes as a long tailed pair. I would go for an ECC82/12AU7 instead, if you don't mind the drop in gain. They're much more bluesy, and you can get your lower Zout that way (as you say, by reducing the cathode load resistor).
If you still want high gain, a 12BZ7 or even an ECC832/12DW7 is an option.
Thanks for the tip. I have delivered the finished product already though. I ended up leaving the 12AX7 and it is doing just fine. The pre out is driving a DI box which drives a mixing board and it worked great. The only problem was bleed through into the PI when the pre out was connected. A shorting plug on the power amp in "fixed" the problem fo now until we can figure out where it is being picked up and generated.
I did switch to a 12AY7 for the input tube due to excessive microphonics with the AX7. The AY works great in that position. Still plenty of gain with undetectable microphonics and very quiet considering the amount of gain involved. When the amp is turned up to ear bleeding levels with the bright channel in use you can detect a little hiss. His pedals are noisier.
mike
I did switch to a 12AY7 for the input tube due to excessive microphonics with the AX7. The AY works great in that position. Still plenty of gain with undetectable microphonics and very quiet considering the amount of gain involved. When the amp is turned up to ear bleeding levels with the bright channel in use you can detect a little hiss. His pedals are noisier.
mike
Parallel Push Pull 6V6 are just stunning.
I've recently completed a London Power "Standard" (from Kevin's TUT5). I changed the power tranny to a Hammond 370FX and the Output Tranny to a 50W Marshall unit and then drove it with 4 6V6G (the old coke bottle shape),
It is a full power scaling amp and each output tube pair can be hot switched between pentode and triode mode AND between cathode bias and fixed bias. You may not be surprised to find that 90% of the time I run the amp with both output tube pairs in triode mode with cathode biasing. The Cathode biasing is via a 470 Ohm 5W for each pair (shared between the 2 tubes of the pair) and NO bypass capacitor. B+ is 350 Volt and the tubes idle at 25mA.
BTW I am seriously impressed with the "standard" 2 12AX7 preamp and the reverb driven by push pull 12AT7 is lovely too. I experimented with various Reverb Tanks, 2 spring and 3 spring in Medium Delay and Long Delay versions. The 3 spring Long Delay is the way to go.
Cheers,
Ian
I've recently completed a London Power "Standard" (from Kevin's TUT5). I changed the power tranny to a Hammond 370FX and the Output Tranny to a 50W Marshall unit and then drove it with 4 6V6G (the old coke bottle shape),
It is a full power scaling amp and each output tube pair can be hot switched between pentode and triode mode AND between cathode bias and fixed bias. You may not be surprised to find that 90% of the time I run the amp with both output tube pairs in triode mode with cathode biasing. The Cathode biasing is via a 470 Ohm 5W for each pair (shared between the 2 tubes of the pair) and NO bypass capacitor. B+ is 350 Volt and the tubes idle at 25mA.
BTW I am seriously impressed with the "standard" 2 12AX7 preamp and the reverb driven by push pull 12AT7 is lovely too. I experimented with various Reverb Tanks, 2 spring and 3 spring in Medium Delay and Long Delay versions. The 3 spring Long Delay is the way to go.
Cheers,
Ian
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