Single ended EL34 amp thoughts

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nickfl
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Single ended EL34 amp thoughts

Post by nickfl »

This is my first post here, I've built four amps so far, starting with a 5e3 kit and moving slowly in a more independent direction, also repaired a couple of vintage craigslist finds. I usually post under the same name over on TDPRI, but as I move more away from Fender based builds I notice I get fewer responses, so I thought I'd post here in hopes of getting more feedback!

My latest interest is building some kind of single ended class A EL34 amp. I have done some reading about and listening to demos of different EL34 amps, both single ended and small push pulls and started to zero in on what I might want to do. I want to use an EF86 in the preamp both because it is something different and because the DR Z route 66 and the Matchless Clubman were among my favorites based on demos.

Here's a rough draft schematic I hacked together this evening:
Single Ended EL34 schematic.png
It uses much of the preamp of a Clubman, but with single and parallel inputs to v1 and a volume control added between 1v and v2, after the tone controls. The power section is based on an Allen class act, as that was the only single ended EL34 amp I've seen a schematic for. As a starting point, I've used a B+ similar to that of the Clubman, which seems to be in a pretty standard range for EL34s, though I am wondering if I should consider a lower voltage, esp for a single ended class A design.

Please chime in with any and all ideas, thoughts, and criticisms!
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dorrisant
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Re: Single ended EL34 amp thoughts

Post by dorrisant »

Welcome, Nick!

Looks like all but the bright cap over the Master pot on a Clubman preamp... I say go for it. Report back please.

What's up with the 18k plate resistor on V1? Would be 36k if the triodes were split... Seems way too small. It must work though.

You could also try fixed bias on the power tube... Different flavor.
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nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

dorrisant wrote: Tue Jul 25, 2017 2:12 am Welcome, Nick!

Looks like all but the bright cap over the Master pot on a Clubman preamp... I say go for it. Report back please.

What's up with the 18k plate resistor on V1? Would be 36k if the triodes were split... Seems way too small. It must work though.

You could also try fixed bias on the power tube... Different flavor.
Yeah, I thought that 18K seemed a little odd too, but its on the matchless schematic. Actually thats the value from the first clubman schematic I looked at, another I found has that resistor even lower at 10K!
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martin manning
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Re: Single ended EL34 amp thoughts

Post by martin manning »

See here for details of setting up a SE EL34 output stage: http://valvewizard.co.uk/se.html
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Re: Single ended EL34 amp thoughts

Post by didit »

A few thoughts: 2-3H choke, perhaps with additional capacitor too, added between rectifier and output transformer reduces hum. Read the last few paragraphs of the article Martin shared, and take to heart EL34 has a fragile screen.

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Re: Single ended EL34 amp thoughts

Post by Stevem »

I would go for more Around 300 volts on the plate and rework the power supply to a parallel configuration so you can still have you preamp tubes at a higher voltage.

The added plus of this will be you can do what didt posted .
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Re: Single ended EL34 amp thoughts

Post by tubeswell »

AX84 has/used to have an SE power amp schematic for EL34. (Page 3 of the attachment)

And what didit said about the fragile screen. The lower you can get the screen voltage the better. 20V below the plates would still be good - but bear in mind that the lower you go, the hotter your biasing needs to be to get tube current to flow.
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RJ Guitars
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Re: Single ended EL34 amp thoughts

Post by RJ Guitars »

Welcome, I'm along for the ride on this one... I'll also contribute the most recent schematic for the Eagle Supre. Using a pentode entry might be kinda cool to try so I'm watching with interest. I've taken a liking to 5879 pentode but I don't think it has the gain of the EF86 so it might not be enough without another gain stage to push the signal up a bit. rj
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nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

Stevem wrote: Tue Jul 25, 2017 6:16 pm I would go for more Around 300 volts on the plate and rework the power supply to a parallel configuration so you can still have you preamp tubes at a higher voltage.

The added plus of this will be you can do what didt posted .
Why would you go with 300v on the plate? I'm curious because choosing B+ voltage is my major hurdle right now, I more or less understand how to set things up, choose resistor sizes, transformer impedance, etc once I have a plate voltage in mind, but choosing a high vs low voltage is still a bit of a mystery to me. In the past I've looked at well known designs and gotten an idea of the range I wanted to be in from there, but there are so few single ended EL34 designs out there and I'm managing to find schematics for fewer still... I have the general understanding that more B+ tends to mean more headroom and somewhat more output, but I'm having trouble putting that into context with the rest of the amp design.
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Re: Single ended EL34 amp thoughts

Post by nickfl »

Tubeswell and RJ, thanks for the schematics! I've been having trouble finding single ended EL34 schematics to study, I'd only found one so far actually, and having two more is a huge help! RJ, what is the plate voltage on your schematic, based on the PT specs, I'm assuming its somewhere in the 375v range?

I had skimmed over that valve wizard article before, but I went ahead a reread it and it is very helpful for what I am after here. I also started reading the small pentode chapter in Designing Tube Preamps, as it definitely applies here too.
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Re: Single ended EL34 amp thoughts

Post by tubeswell »

nickfl wrote: Wed Jul 26, 2017 4:18 am ...choosing a high vs low voltage is still a bit of a mystery to me...
The most limiting factor in the equation of variables you potentially have at your disposal is going to be the reflected load of the OT that you obtain (as this is a pre-determined thing - unless you can wind your own output transformer). The plate voltage and plate power dissipation are related to the load, in as far as power requires a certain load to work into in order to stop the tube burning itself up.

Therefore a starting point is working out what voltage your PT will deliver on the High Tension supply rail. Then choose an off-the-shelf OT that gets you into ballpark plate voltage for load. The formula for optimum reflected load for centre-biased Class A operation is on that 'designing SE output stages' page of Merlin's. Take a plate voltage of 300V and the plate dissipation rating of 25W for example. (Incidentally Lord Valve reportedly recommends adopting a more conservative nominal maximum power dissipation rating of 20W for most modern-production EL34s (especially the ones from New Sensor)).

Assuming 25W

Zout = Va/(Pa/Va)

300/(25/300) = 3,600R. (3k6)

Assuming Lord Valves 20W recommendation

300/(20/300) = 4,500R (4k5)

So settling on an OT that is capable of reflecting 4k would be a good thing. YMMV
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So you asked for feedback?

Post by ChopSauce »

An example of SE EL34?

See : http://www.projetg5.com/Projets/G5/G5V3 ... cielle.pdf & http://www.projetg5.com/projets/g5

Note that I am no expert at all in this matter but, as indicated above, Merlin Blencowe's book abour preamps design is a recommended (not by me) book. It also deals with the EF86. It looks like you didn't read it, otherwise you wouldn't have designed this schematic - I guess. That doesn't mean you are wrong, though - just the reading of the book might give you the desired feedback (kinda.)

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Re: Single ended EL34 amp thoughts

Post by RJ Guitars »

nickfl wrote: Wed Jul 26, 2017 4:20 am...RJ, what is the plate voltage on your schematic, based on the PT specs, I'm assuming its somewhere in the 375v range?
Chopsauce has a good point that often what we get for voltages or our configuration isn't wrong, it's just what it is. Often it doesn't match the model we have or someone else's ideal specs but it sounds good and has withstood the test of time.

The voltages for the most recent iteration of an Eagle Supre was built by a guy that occasionally hangs out here called Angelo. I recently added a choke to that design so his voltage measurements are the first to come out of that updated design. These can be tweaked to suit your yourself once you get down to the fine tuning of things. My schematic was based on a Fender 5F1 Champ schematic and the use of an EL34 isn't really a stretch for it since it is a cathode biased output and if the transformers would handle the tube then the amp would probably work just fine. The major changes I made are the use of bigger transformers to handle the higher output. In my own previous builds I used a KT88 for the output tube and that was a great sounding amp.

I chose a custom wound Edcor power transformer with a secondary of 270-0-270 to power the amp. This was my shot at hitting the sweet spot between tone and headroom for a single ended EL34 based amp. Below are are the voltages that I was provided on the Eagle Supre with EL34. I'll reserve the right to edit these since Angelo is still tweaking on the amp a bit and these are really my understanding of his most recent measurements. Once these are confirmed I'll permanently add them to the schematic.

Rectifier out - 357v
V2 Plate - 350v
* V2 Screen - 348v
V1b plate - 164v
V1a plate - 163v
V2 Cathode - 19v
V1a cathode - 1.4v
V1b cathode - 1.3v

* Ange had to tweak on things a little to get the screen voltage to drop below the plate voltage. Initially they were exactly the same. This is a very cool little study all on it's own and don't be shocked if you see this in your build. It is possible and not necessarily "wrong" for the screen to even run a little bit higher than the plate in some cases.
Last edited by RJ Guitars on Wed Jul 26, 2017 1:15 pm, edited 2 times in total.
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nickfl
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Re: Single ended EL34 amp thoughts

Post by nickfl »

tubeswell wrote: Wed Jul 26, 2017 5:01 am
nickfl wrote: Wed Jul 26, 2017 4:18 am ...choosing a high vs low voltage is still a bit of a mystery to me...
The most limiting factor in the equation of variables you potentially have at your disposal is going to be the reflected load of the OT that you obtain (as this is a pre-determined thing - unless you can wind your own output transformer). The plate voltage and plate power dissipation are related to the load, in as far as power requires a certain load to work into in order to stop the tube burning itself up.

Therefore a starting point is working out what voltage your PT will deliver on the High Tension supply rail. Then choose an off-the-shelf OT that gets you into ballpark plate voltage for load. The formula for optimum reflected load for centre-biased Class A operation is on that 'designing SE output stages' page of Merlin's. Take a plate voltage of 300V and the plate dissipation rating of 25W for example. (Incidentally Lord Valve reportedly recommends adopting a more conservative nominal maximum power dissipation rating of 20W for most modern-production EL34s (especially the ones from New Sensor)).

Assuming 25W

Zout = Va/(Pa/Va)

300/(25/300) = 3,600R. (3k6)

Assuming Lord Valves 20W recommendation

300/(20/300) = 4,500R (4k5)

So settling on an OT that is capable of reflecting 4k would be a good thing. YMMV
I understand all of that, at least in a practical sense to the point that I can take any one starting variable and figure out what the others should be, but what I don't understand is how to choose the starting variable. I understand how to take a given power transformer and determine what the B+ should be and from there what primary impedance I need for the tube type, or what B+ I need for a given output transformer and tube type, but in this case I'm working from scratch with neither already selected, so how do I decide whether a plate voltage of say 300 or 375, or 425 is what I want? Really I don't see any practical reason why any of those wouldn't work, since I shouldn't have much trouble finding an appropriate OT for any of them. So, with no components already in hand, where should I start? Is it just a matter of lower B+ being dirtier and higher giving more headroom and output, or are there other ways this choice effects the way the amp will sound?
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Re: Single ended EL34 amp thoughts

Post by pompeiisneaks »

I think for that, you start with the datasheet on the tube and look at the operating conditions for a single valve in Class "A" and see the voltages. They specify on one I see either 250V or 300V. Then you can see the expected output wattage, and go from there. This sheet I"m seeing seems to show a single EL34 should only do 11W not 20 or 25 but maybe I missed somewhere earlier that you wanted parallel dual tubes? At any rate, as I understand it, that's where you decide what voltages you want is based on the datasheets.

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