18Watt.com

After successfully building a 6V6 'Superlite' with the Universal PCB I've decided to try an EL34 based '36 watt'.

I've finalised the schematic and a layout for the 'Super 36'. I should have the EF86 version layout done RSN [update] yes done now. (BTW - The '36 watt' amps are the 10th, 11th & 12th pages of the pdf)

I'm still pondering which one to do. I was originally thinking of doing the 'Superlite' version, but seeing as I just did a 'Superlite 6V6' I'm now thinking I should do the EF86 version. Hmmm...

[update] OK - I've decided. I'm going to do the EF86 Pentode pre-amp but with a silicon diode rectifier and the Antek Toroid TX.

[update 2]Actually I've done it with a Triode/Pentode (6U8-A) pre-amp tube up front, 5AR4 rectifier, AC-30 PT and an Edcor CXPP30 OT. It's the circuit called "Club Secret" on page 12 of the schematic PDF.

Pictures of progress to come.

I haven't really looked at the board, but can you do a 2 channel - channel 1 EF86, channel 2, Superlite TMB?

JMPGuitars wrote:...can you do a 2 channel - channel 1 EF86, channel 2, Superlite TMB?

Unfortunately not with the PCB tube compliment alone. It takes at most 2 noval 'pre-amp' tubes & two power tubes. There is provision to add additional external tube sockets via the PCB pads providing heater power, HT & signal path, but the straight PCB is intended to make 'single channel' amps only.
When we were designing the latest PCB we thought about adding another noval pre-amp socket, but we decided that it was too much of an outlier case to justify raising the base cost. We figured making it easy to add an external tube socket was the more flexible cost effective route.

Did you ever wind up building any of the 36Watt versions? I'm still deciding on a 36W with EL34s to build.

Yes absolutely !

It's wonderful. I thoroughly recommend it. It's convinced me completely on EL34s vs 6L6-GCs for this sort of power range.

Obviously you could do a quad of 84s but I honestly think a pair of 34s is the better choice.

Here are some photos of the build: -





The internal wiring is a little 'dodgy' because I converted it from silicon diode rectified to tube rectified and minimised my rewiring.

With this one I used a Triode/Pentode pre-amp tube up front which I really like. The topology is Triode -> Gain pot -> Pentode -> MOSFET Cathode Follower driven TMB -> Vol pot -> "18 watt" LTP PI -> EL34s @ around 390v 3.8K OT.

And here's an audio clip.

https://www.youtube.com/watch?v=DA--HUr ... e=youtu.be" onclick="window.open(this.href);return false;

Apologies for being so slack in getting around to posting this.

That's awesome. I'm probably going to build one soon, I love EL34 tubes. What transformers did you go with?

JMPGuitars wrote:... What transformers did you go with?

I used a W@b@r "AC30" style Power Transformer. It has 300vCT@250mA main secondary and a 6.3v & 5v set of filament secondaries. I used a 5AR4 for the rectifier to get a B+ of around 390v.
I used an Edcor CXPP30-MS-3.8K for the OT. The Edcor CXPP OTs are really, really good. Huge too. Way bigger than your average wimpy piece of iron These are full 20-20KHz @ continuous 30watt RMS load transformers. Highly recommended.
You just have to be prepared to wait for the 4-6 week lead time. They are made to order.
I'm sure Heyboer et al have suitable TXs, but they'll be more expensive.

I'll try and put together an 'as built' schematic in the next couple of days.

That would be great.

I'm familiar with the Edcor wait. I built a SE project with one of their OTs. Great quality stuff if you're patient enough for it.

jamesrr wrote: ↑

Thu 08/28/14 5:39 am

JMPGuitars wrote:... What transformers did you go with?

I used a W@b@r "AC30" style Power Transformer. It has 300vCT@250mA main secondary and a 6.3v & 5v set of filament secondaries. I used a 5AR4 for the rectifier to get a B+ of around 390v.
I used an Edcor CXPP30-MS-3.8K for the OT. The Edcor CXPP OTs are really, really good. Huge too. Way bigger than your average wimpy piece of iron These are full 20-20KHz @ continuous 30watt RMS load transformers. Highly recommended.
You just have to be prepared to wait for the 4-6 week lead time. They are made to order.
I'm sure Heyboer et al have suitable TXs, but they'll be more expensive.

I'll try and put together an 'as built' schematic in the next couple of days.

No Antek, just curious why you changed your mind? Toroids are unusual to see in guitar amps.

crgfrench wrote: ↑

Sat 05/04/19 11:50 am

No Antek, just curious why you changed your mind? Toroids are unusual to see in guitar amps.

I've used Toroidal PTs in quite a few guitar amps now. They've worked really well for me. The Antek ones are very cost effective. You just need to know how to wire them up.
They have 2 6.3v heater supplies along with the HT taps, so you can use a few different tube rectifiers including the EZ81, which is common here (in 18 watt land).
I used the AC30 PT in this particular build as it was convenient as I had one sitting on my shelf, and it seemed like a good choice for a 36watt.
Worked really well.

Incidentally, I've logged on to 18watt after a break for some time, as I'm just in the process of designing a "complete" 36 watt, i.e. A full 18watt with both the normal & trem channels using the U-PCB and with an EL34 power stage. So I should have a new 36watt thread starting soon

p.s. The schematic I'm currently working with is viewable in the link on the 1st post from this thread...

I was just thinking about that now that I have a UPCB in hand, looking forward to seeing the EL34 design!

I think the Anteks are the best value out there for PTs. I drove over to Kearny NJ about a month ago and John sold me an AS1T300 and an AS1T275 for $20 apiece. I'm using the 275 for my current 18W build.

James,

How did you like the EF86-PI-2xEL34 topology compared to the standard 12AX7 front end? I am rather interested in this. Did you have a preference for one preamp over another?

yeah i'll be interested in this one as well, notice the el34 schematics are using 18W el84 values for the phase inverter and power section except for the wattage of the bias resistor? i thought you had to reduce the value of the phase tail resistor for el34's.
i've been patiently waiting for Josh to post his superlite el34 schematic/layout but i think i'll end up looking like this before he do'es!

colossal wrote: ↑

Fri 07/19/19 6:55 pm

How did you like the EF86-PI-2xEL34 topology compared to the standard 12AX7 front end?

I really like it. Sounds great. I'm not sure if I "prefer" one to the other, but I sure want at least one amp with an EF86 channel
I typically use moderate biasing values to get high gain, but not stupid high... I've avoided the problems with them that are commonly reported using this approach.
I also use the Russian 6J32P valves which are readily available and typically much more reasonably priced.

geoff 1965 wrote: ↑

Fri 07/19/19 7:58 pm

... notice the el34 schematics are using 18W el84 values for the phase inverter and power section except for the wattage of the bias resistor? i thought you had to reduce the value of the phase tail resistor for el34's.

Well I haven't seen the need to change them myself. The datasheets I have for the EL84 and EL34 show them with very similar sensitivity characteristics.
For example, a reference setup for EL84s shows a 300Va, 270R cathode resistor per valve - (equiv 135R shared), 8k OT load producing 17watts with a 20v r.m.s. grid to grid input signal.
A reference setup for EL34s shows 375Va, 130R common cathode, 3.4k OT producing 35watts with a 21v r.m.s. input signal.
You can play around with the PI values to see what you like, but I see no need to change them as a starting point.

that's interesting and good to know! i think my next build will be el34.
typically looking at different schematics when using the 18W phase inverter the tail resistor is dropped to 22K for 6V6's and i'm sure i read somewhere zaphod phil recommending 10K for el34's.maybe that's for a plexi type power section?

jamesrr wrote: ↑

Sat 07/20/19 5:31 pm

I also use the Russian 6J32P valves which are readily available and typically much more reasonably priced.

Interesting! I just ordered some to try them out. They certainly cost less than new EF86 tubes. If they perform better, as some people claim on a couple forums I read, then even better.

geoff 1965 wrote: ↑

Sat 07/20/19 6:13 pm

... i think my next build will be el34.
typically looking at different schematics when using the 18W phase inverter the tail resistor is dropped to 22K for 6V6's and i'm sure i read somewhere zaphod phil recommending 10K for el34's.maybe that's for a plexi type power section?

There's hours of entertaining discussions to be had arguing the 'right' values for the LTP component values
Just to be clear, the EL34 datasheet I was talking about shows 20v RMS at the individual grid, ie. 40v grid to grid, so about twice the absolute voltage of the EL84s.
Now IMHO this is no great drama at all, as the circuit with the '18 watt' LTP values & preceding gain stage with very low loss tone stack means getting 20+v signal swing to the power tubes is no issue whatsoever, even with wimpy single coils...
It's useful to look at the following gain calculator for the LTP PI: -
https://www.ampbooks.com/mobile/amplifi ... iled-pair/
If you set the values to 18watt values you get gain of 28+ on both the inverted and non-inverted signals, i.e. you need less than 1v into the LTP to get our EL34s to full power.
You'll also notice that changing the tail resistor to 22K or even 10K doesn't increase that gain factor much at all. Indeed, what it does most is lessen the balance. This is why you'll see 82K and occasionally 91K used on the inverting anode load resistor.
What has far more impact is changing the power tube grid resistors.
If you drop them from the 18watt 470K to 220K with a 10K tail (The "Plexi" values) you drop the gain from around 28 to 25.
Ergo... IMHO (and in my experience) you don't need or want to change the tail resistor here...