Has anyone used a 6X4 or EZ81 rectifier in a guitar amp? I'm working on a design and I can save about $15 on the transformer if I forgo the 5V tap. I'm leaning towards the EZ81 because the chassis I'm looking at already has socket holes for 9-pins and doesn't have any holes for 7-pin sockets.
The only thing I've seen about the EZ81 is that there's sometimes mechanical noise when it's warming up, but other than I don't know of any issues.
Just wanted to check before I go any further.
The only thing I've seen about the EZ81 is that there's sometimes mechanical noise when it's warming up, but other than I don't know of any issues.
Just wanted to check before I go any further.
I have a Hammond AO44 reverb amp that I rebuilt for guitar that uses an EZ81/6CA4 and have had no problems but it's a head, not a combo. So less vibration. I have heard that the 5 volt tubes are a little more robust, but others might have different opinions. I heard it form an old organ tech I've been in contact with and he seems to think most of the 6 volt rectifier tubes are a little more fragile than the 5 volt-5Y3, 5U4, etc.
This would be for a head, not a combo.
I just came across a reference to Marshall 18 Watt (I was Googling "6V rectifier tubes fragile" to follow up on your comment) which is a combo and apparently uses the EZ81. I hadn't ever looked at the schematics for those before. So cool, I guess an EZ81 is just fine for my needs.
I just came across a reference to Marshall 18 Watt (I was Googling "6V rectifier tubes fragile" to follow up on your comment) which is a combo and apparently uses the EZ81. I hadn't ever looked at the schematics for those before. So cool, I guess an EZ81 is just fine for my needs.
You have told us nothing of the circuit in which the rectifier would serve. Every rectifier tube has current ratings, and aside from fragility issues, you must determine if the 6v rectifier you propose can handle the circuit current requirements.
But an alternative thought: not only could you save the $15 for a 5v winding, you could also save the cost of the rectifier tube and socket by using a couple silicon diodes instead. They are cheap and reliable, and they suffer no fragility issues in a combo.
But an alternative thought: not only could you save the $15 for a 5v winding, you could also save the cost of the rectifier tube and socket by using a couple silicon diodes instead. They are cheap and reliable, and they suffer no fragility issues in a combo.
Enzo, I'm still toying with the design, but what I'm thinking of is a single ended, probably 6V6 (possibly a 6L6, but I doubt I'll want that much power) with 2 channels. Channel 1 is a 6SJ7 preamp and channel 2 is half of a 12AX7. Both of those would feed into the other half of the 12AX7 cathode follower to drive a fender-ish tone stack followed by a parallelled 12AU7 between that and the 6V6.
I'm pretty sure the EZ81 can handle that and the transformer has a 4A 6.3V tap, so that should handle the heater requirements.
As for silicon rectifiers, I thought of that. It's certainly easier and cheaper. But I've never played a tube amp with silicon rectification (I've played lots of solid state amps and lots of tube amps, but they've all been all one or the other) and I'm concerned I might not like the sound as much because of the loss of the sag and compression that comes with tube rectification. I tend to like a bluesy sounding amp and I'm under the impression tube rectification would be better for that. Feel free to correct me if I'm wrong.
I'm pretty sure the EZ81 can handle that and the transformer has a 4A 6.3V tap, so that should handle the heater requirements.
As for silicon rectifiers, I thought of that. It's certainly easier and cheaper. But I've never played a tube amp with silicon rectification (I've played lots of solid state amps and lots of tube amps, but they've all been all one or the other) and I'm concerned I might not like the sound as much because of the loss of the sag and compression that comes with tube rectification. I tend to like a bluesy sounding amp and I'm under the impression tube rectification would be better for that. Feel free to correct me if I'm wrong.
Hmmm. Actually, not sure that 2 channel idea will work as I have it. The cathode follower's grid is connected to the plate of the preceding stage... Like I said, still toying with the design.
Switching it up. How about this? I have a 6U8 I could use for channel 1 instead.
6U8 pentode as channel 1, 1/2 of 12AX7 for channel 2. They both feed into the other half of the 12AX7. The 12AX7 would then feed into the triode of the 6U8, which would be the cathode follower to drive the tone stack and then the paralleled 12AU7s after that... Would that work?
Switching it up. How about this? I have a 6U8 I could use for channel 1 instead.
6U8 pentode as channel 1, 1/2 of 12AX7 for channel 2. They both feed into the other half of the 12AX7. The 12AX7 would then feed into the triode of the 6U8, which would be the cathode follower to drive the tone stack and then the paralleled 12AU7s after that... Would that work?
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This is a first draft of what I have in mind.
http://s14.postimg.org/u33u9ojr5/Pete_1st_Draft.png
The output transformer isn't in the schematic, but the text represents where it would go and what I'm currently looking at.
I haven't started on the power supply design yet. I decided I'll probably go with the 6L6 after all (got greedy).
The pentode preamp and the paralleled 12AU7 section is largely ripped off from the AX84.com's blues pre-amp.
So I have some initial questions:
1: On the 6L6 I have 24V P-P as the goal for the input. Is that about right or did I get that wrong. I was kind of guesstimating. Any pointers on where to learn how to figure that out? I'm also assuming about 0.4V P-P on the guitar inputs as about what I'd expect from my Les Paul.
2: Related to #1, once I figure out what my overall gain needs to be (currently guesstimated at 60), how do you go about figuring out how to reduce the AC voltage between stages? Obviously I don't want 100 * 100 * 40 * 17 into the 6L6...
3: I wasn't sure at all how to pick the resistors for the cathode follower. Any pointers on how to do that would be great.
4: I wasn't sure how to calculate C7. Do I need a resistor to ground before U2b?
Is this just a complete train wreck? Should I just give up?
http://s14.postimg.org/u33u9ojr5/Pete_1st_Draft.png
The output transformer isn't in the schematic, but the text represents where it would go and what I'm currently looking at.
I haven't started on the power supply design yet. I decided I'll probably go with the 6L6 after all (got greedy).
The pentode preamp and the paralleled 12AU7 section is largely ripped off from the AX84.com's blues pre-amp.
So I have some initial questions:
1: On the 6L6 I have 24V P-P as the goal for the input. Is that about right or did I get that wrong. I was kind of guesstimating. Any pointers on where to learn how to figure that out? I'm also assuming about 0.4V P-P on the guitar inputs as about what I'd expect from my Les Paul.
2: Related to #1, once I figure out what my overall gain needs to be (currently guesstimated at 60), how do you go about figuring out how to reduce the AC voltage between stages? Obviously I don't want 100 * 100 * 40 * 17 into the 6L6...
3: I wasn't sure at all how to pick the resistors for the cathode follower. Any pointers on how to do that would be great.
4: I wasn't sure how to calculate C7. Do I need a resistor to ground before U2b?
Is this just a complete train wreck? Should I just give up?
Nevermind. Had a big epiphany last night as I was lying in bed unable to sleep and I think I answered all my own questions (or most of them and the others may fall into place)... Got a lot of recalculating to do...
I was just about to post that it seems that you want to re-visit many parts of the circuit. I would get rid of C1 and C2 and replace with 33K grid stoppers. Your plate load resistors R6 and R7 will give you much distortion and not much output voltage swing. They should be 100K or larger. With those cathode resistors, maybe even 150-180K.
The formula for bypass cap C7 is Freq.= 1/ (6.28 X R X C) where R is in ohms and C is in Farads, so 6.8uF would be entered as .0000068 or 6.8 X 10^-6.
Yes, you should have a resistor to ground before V2b. This could be a place to reduce gain. If you use 1Meg it will be greater gain than if you use 500K less.
As for the cathode follower, look at the 5F6-A Fender Bassman or several Marshall schematics for how to do this.
Also, there is usually 220K or so channel mixing resistors after C5 and C6.
The formula for bypass cap C7 is Freq.= 1/ (6.28 X R X C) where R is in ohms and C is in Farads, so 6.8uF would be entered as .0000068 or 6.8 X 10^-6.
Yes, you should have a resistor to ground before V2b. This could be a place to reduce gain. If you use 1Meg it will be greater gain than if you use 500K less.
As for the cathode follower, look at the 5F6-A Fender Bassman or several Marshall schematics for how to do this.
Also, there is usually 220K or so channel mixing resistors after C5 and C6.
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Awesome. Thanks for the info.
On the plate resistors, that was a big part of the "epiphany" I had last night.
I didn't know about the channel mixing resistors. Thanks. That makes a lot of sense, though.
As for the resistor before U2b and C7, I mentioned those in #4. I was pretty sure those were wrong.
I feel like I understand a lot of stuff today that I didn't yesterday. I'm looking forward to going home tonight and reworking some of this.
Thanks so much for the comments. I'll try to post an update tonight or tomorrow. I'm trying to sell a guitar to help pay for some of the parts I need (I have a chassis and speaker picked out). I'm going to hold off, of course, on transformers until I get this in a final draft.
I have an allowance for "toys" and it's not very much, so whatever else I spend on top of that has to come from stuff I can sell. I recently traded a hollowbody electric I didn't get much of a chance to play for a Les Paul (that I really wanted) and an acoustic I don't need. So I'm selling the acoustic and that should finance a good portion of this amp.
I've never done a ground-up build (or a complete design). My wife's uncle is a ham radio operator and a former radio shack owner. He helped me diagnose some issues with an old 60s amp I bought and he'll help me with the build. He's also the one that supplied me with my tubes (he still has boxes and boxes of them which I may further raid, but unfortunately, no iron, which is where the real expense is.)
On the plate resistors, that was a big part of the "epiphany" I had last night.
I didn't know about the channel mixing resistors. Thanks. That makes a lot of sense, though.
As for the resistor before U2b and C7, I mentioned those in #4. I was pretty sure those were wrong.
I feel like I understand a lot of stuff today that I didn't yesterday. I'm looking forward to going home tonight and reworking some of this.
Thanks so much for the comments. I'll try to post an update tonight or tomorrow. I'm trying to sell a guitar to help pay for some of the parts I need (I have a chassis and speaker picked out). I'm going to hold off, of course, on transformers until I get this in a final draft.
I have an allowance for "toys" and it's not very much, so whatever else I spend on top of that has to come from stuff I can sell. I recently traded a hollowbody electric I didn't get much of a chance to play for a Les Paul (that I really wanted) and an acoustic I don't need. So I'm selling the acoustic and that should finance a good portion of this amp.
I've never done a ground-up build (or a complete design). My wife's uncle is a ham radio operator and a former radio shack owner. He helped me diagnose some issues with an old 60s amp I bought and he'll help me with the build. He's also the one that supplied me with my tubes (he still has boxes and boxes of them which I may further raid, but unfortunately, no iron, which is where the real expense is.)
If you have limited funds, make sure you know how to calculate loads for power and output transformers as those are the most expensive items. Don't forget heater loads as heaters usually aren't on the schematic and could be easily forgotten.
I've been looking at Antek for power transformers (they have some $38 ones that seem like they may fit the bill).That was actually the motivation for my initial post here, regarding the EZ81 rectifiers, as they have 6.3 volt heaters and the Antek transformers don't have 5V taps (and the ones I was looking at had 4A 6.3V supplies which would handle all the tubes in my layout: 0.9A for 6L6GC, 1A for EZ81, 0.3 for the 12AX7, ~.5 for the 6U8)
The output transformer I'm thinking of is this one which also seemed really reasonably priced. Though obviously both are potentially subject to change...
The output transformer I'm thinking of is this one which also seemed really reasonably priced. Though obviously both are potentially subject to change...
Are you sure about that output tranny? This is for a 40 Watt push-pull output section, 2 6L6s. Your design was single ended, one 6L6.
No, apparently I'm not sure about it at all. I thought I was only looking at SE transformers... I was just kind of quickly going through looking for something relatively close to 3800 ohm, at least 30 watt (in case at some point I decided to use the extra tube slot in the chassis for a paralleled power tube) and not too expensive to stick on the schematic. I'm using LTSpice to draw the schematic (not doing any sims) and I couldn't find a transformer component so I just wanted something to put on the schematic as filler.
Obviously a push-pull OT won't work. Seemed awfully cheap for a 40 watt SE transformer.
Obviously a push-pull OT won't work. Seemed awfully cheap for a 40 watt SE transformer.
Edcor seems to have some inexpensive 25W OTs. That's probably plenty for 2 6L6s paralleled (doubt I'll ever do it, but just in case).
Get into the habit of looking up the tube data sheets.
This is a good site to find info:
Duncan's Amp Pages
For example the application data there will quickly tell you:
6X4: tpyical 70mA , 40uF max cap, 525 Ohms minimum Rseries for anodes
EZ81: typical 150mA, 50uF max cap, 230 Ohms minimum Rseries .
The EZ81 (6CA4) is clearly the higher rated tube.
I built an ORANGE Tiny Terror Clone with push pull ECL86 (6GW8) + a 12AX7, approx. 14 Watts max., I used a EZ81 for that, 47uF 1st cap and the minimum Rseries was met (by secondary winding resistance + reflected primary wiring resistance) without needing to add any physical resistors to the anodes. Slightly less power than a Tiny Terror so I called it the "Anklebiter". The local guitar god bought it off me on the spot, the first time he tried it out.
Cheers,
Ian
This is a good site to find info:
Duncan's Amp Pages
For example the application data there will quickly tell you:
6X4: tpyical 70mA , 40uF max cap, 525 Ohms minimum Rseries for anodes
EZ81: typical 150mA, 50uF max cap, 230 Ohms minimum Rseries .
The EZ81 (6CA4) is clearly the higher rated tube.
I built an ORANGE Tiny Terror Clone with push pull ECL86 (6GW8) + a 12AX7, approx. 14 Watts max., I used a EZ81 for that, 47uF 1st cap and the minimum Rseries was met (by secondary winding resistance + reflected primary wiring resistance) without needing to add any physical resistors to the anodes. Slightly less power than a Tiny Terror so I called it the "Anklebiter". The local guitar god bought it off me on the spot, the first time he tried it out.
Cheers,
Ian
Yeah, I was looking at the EZ81 instead of the 6X4. The 6X4 wouldn't handle enough current for the 6L6.
As for datasheets, I Google the tube. Usually there's at least one source of datasheets. I'll usually look at a few if there are multiple ones. Especially when looking at pentode average characteristics since you can sometimes find graphs for different screen grid voltages across different datasheets.
My current amp has a 6GW8 (and 6AV6 and 12AX7) , but it's single ended. It's an old 1960s Ace Tone Elite. Probably about 3 watts. 1/2 of the 12AX7 is the stage 1 pre-amp and then the 6AV6 is the second stage. The other half of the 12AX7 drives the reverb and the 6GW8 triode drives the tremolo. I think I paid $100 for it. It's a nice little amp.
As for datasheets, I Google the tube. Usually there's at least one source of datasheets. I'll usually look at a few if there are multiple ones. Especially when looking at pentode average characteristics since you can sometimes find graphs for different screen grid voltages across different datasheets.
My current amp has a 6GW8 (and 6AV6 and 12AX7) , but it's single ended. It's an old 1960s Ace Tone Elite. Probably about 3 watts. 1/2 of the 12AX7 is the stage 1 pre-amp and then the 6AV6 is the second stage. The other half of the 12AX7 drives the reverb and the 6GW8 triode drives the tremolo. I think I paid $100 for it. It's a nice little amp.
Remember, if you add a second tube later, you will need to lower your primary impedance by 1/2. I don't know how this will work for picking which tranny to buy. Doubling your power only increases loudness by 3dB, so it might not be worth it. But yes, 25 watts should be good for 2 6L6s paralleled. You might want to decide ahead of time which power level you want to optimise your pri. impedance of your OT.
I hadn't thought about that. I'm just going to go with the 1 6L6 in that case and I'll drop down to a 15W transformer. I'd have to upgrade the rectifier as well (I was reminded of that when gingertube posted the ratings). I hadn't thought about the fact that the EZ81 was only going to handle 150 ma. The vast majority of that will be used up by the current tube set. The 6L6GC alone is going to be more than half of it.
PD, there is no wrong choice between tube and solid state rectification, just a matter of taste.
But I am surprised to hear you say you have never played a tube amp with silicon rectifiers. here is a short list of common amps that have diodes instead of tubes in the power supply. You have never played these?
Fender
Hot Rod DeVille
Bassman heads
Twin Reverb
AMpeg
V4
SVT
Peavey
Classic 30
Classic 50
5150/6505
Marshall
Oh, too many to list
But I am surprised to hear you say you have never played a tube amp with silicon rectifiers. here is a short list of common amps that have diodes instead of tubes in the power supply. You have never played these?
Fender
Hot Rod DeVille
Bassman heads
Twin Reverb
AMpeg
V4
SVT
Peavey
Classic 30
Classic 50
5150/6505
Marshall
Oh, too many to list
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