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Advice on 807 Amp build

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nzoomed

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Joined 2015
#1
Im looking at building a class A push pull 807 amplifier.
Im not sure how many watts per channel I would get in this configuration, but am not expecting much more than 25W.

Any advice on tube selection?
Im thinking I would like to make use of some octal tubes, mainly for looks more than anything.
I have in mind to use an EF37A or 6J7G for the driver stage and a 6SN7 for the phase inverter.

Any other tubes i could consider, particularly to replace the 6SN7?

Any information that could assist me in designing such a build would be good, this is going to be the first amp im designing.

TIA
 
#4
I'm probably in the minority, but for whatever reason(s), over the years I've come to really respect the combination of 4 output tubes per channel in a strictly class-A push-pull configuration. Dunno… the use of 4 finals, 2 per phase, in parallel just seems to deliver the “oomph” that (in my mind) an amplifier ought to have across the whole frequency range.

Going strictly class A tho' also burns a lot of quiescent juice. If I were using 807's, I'd definitely push each valve to about 20 W quiescent (no-signal) burn rate. 80 W for the 4 of them. I'd also run them at 550 V or so. High enough voltage to get the benefits of the 807's pentode linearity, low enough to also push up the average current. And not require a hard-to-handle 750 volt power supply.

35 mA quiescent per tube, 70 per phase, 140 for a quad. Since — again for whatever reason — I build monoblocks, not dual (or more) channel all-in-one-box amplifiers, then I'd probably design each amp to have a ⁶SL₇ front end gain stage, driving the classic 12AT7 phase splitter, then a ⁶SN₇ cathode follower for each phase to drive the 807s. I'd use a MOSFET series regulated power supply, CLC prefiltering to cut buzz, silicon rectification, a 4 stage slow-start voltage reference, and modest running reservoirs. 100 µF only (E = ½CV² = ½ 100×10⁻⁶ 550² → 15 joules)

If doing this today, based on the enthusiasm by certain DIYaudio members, I'd very likely outfit each block with a commercial heavy-duty power toroid … as an output transformer. 416 primary (industrial transformer), 24 volt secondary. 250 VA… Delivers over 80 VA at 20 Hz.

Anyway, probably TMI.
Happy 4th of July all.

GoatGuy
 
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#5
Look up the STC (Australian) application report for the 807, it has all the information you need.

GoatGuy - as much as I like using power toroidals as output transformers, I don't think they would be applicable in this case due to the relatively high voltages and primary impedance. Unless operation at low frequencies is not required and saturation can be avoided.
 
#6
tikiroo said:
GoatGuy - as much as I like using power toroidals as output transformers, I don't think they would be applicable in this case due to the relatively high voltages and primary impedance. Unless operation at low frequencies is not required and saturation can be avoided.
Click to expand...

This is why I derated the 250 VA "power rating" of the hypothetical toroidal to ¼ its nominal 60 Hz value.

250 VA × 20 Hz ÷ 60 Hz • (1 - 20%) → 67 VA​

I'm certainly not expecting the quad of 807's to cough forth much more than that at 20 Hz, regardless of what source I provide. Most — as it always is — of what I play "passing for music" is great big high dynamics symphonic stuff and delightful 24 bit 96 kHz sampled live jazz with huge dynamics. Live dynamics.

I think over-spec'ing the toroid by the % above (well, derating it so…) would likely do the trick.

416 V : 24 V is 17:1 … which in impedance is 300:1. If driving 8 Ω, that's 300 × 8 = 2400 Ω primary. While this may be a bit low, it still is a reasonable match for 4 807's in PPP (parallel push pull).

I personally have never worried overly much about how perfect the impedance match/mismatch for the output final. ±30%? Its OK. Maybe less perfect coupling, but the robustness of valves — themselves (by design) not driven deeply into their 'do not go here' region — keeps perfect impedance matching a fool's errand.

Just saying.
GoatGuy
 
#7
GoatGuy said:
...as an output transformer. 416 primary (industrial transformer), 24 volt secondary. 250 VA… Delivers over 80 VA at 20 Hz.
Click to expand...

I have have never used the 807 but assuming the curves are the same as the 6L6 or very similar that doesn't sound like an efficient choice for class A operation if you run the single tube at just 35 mA and 550V plate voltage. Each tube will see 2.4K load impedance in class A mode with your transformer choice into 8R and cannot swing more than 34mA peak in pure class A or 34mA rms in practical class A (i.e. constant or nearly constant average overall current draw for the PSU). That means about 5.5W and 11W, respectively. With 80W plate dissipation it is not really efficient by any standard (less than 7% and 14%, respectively). In other words that sounds more like class AB operation.
I think for that load you need to give about 70mA current per tube. Assuming a knee around 50V and 2.4K load per tube (1/2 of plate-to-plate and again x2 because of the parallel mode). An anode voltage of 290V should be enough (with 175V on G2) for pure class A. Output power will be 24W for a total plate dissipation of about 81W (290Vx70mA = 20.3W per tube). 30% efficiency for pure class A in pentode connection sounds right to me. Don't you think so?
 
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#8
perhaps i should be going with class AB instead?
Thats probably where i got the 25-30W figure in mind.
So are we only looking around 5-7W per channel in class A?
I was going with push pull, not single ended. 5W would seem what I would expect in Single ended class A.
 
#10
H713 said:
Nothing wrong with Class AB. I'm in the design process of a PP 807 amp as well (OPT hunting...). Here are two threads that have a fair bit of discussion about my 807 project. You might find them useful. Don't be put off by the thread names, I got off topic within five posts.

Question about building an amp without a preamp stage

AA-100 output transformer power rating
Click to expand...

Yeah I think class AB will be fine.
I had been looking at this schematic for ideas.
I think the use of a magic eye as a graphical display is cool too.

Its unclear to me if this magic eye is missing the right channel on the schematic or not though.
An externally hosted image should be here but it was not working when we last tested it.
 
#11
nzoomed said:
Yeah I think class AB will be fine.
I had been looking at this schematic for ideas.
I think the use of a magic eye as a graphical display is cool too.

Its unclear to me if this magic eye is missing the right channel on the schematic or not though.
An externally hosted image should be here but it was not working when we last tested it.
Click to expand...

IMHO, that circuit is not suited for true Class AB because the LM317 bias circuit should work like a CCS. It's for class A operation. Anyway 12W from a single pair of 807's in pentode mode is not bad at all and you won't hear much difference in comparison to 25W in terms of SPL. You will likely notice a difference in sound between class A and AB though...
 
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#16
nzoomed said:
I found the schematic on this forum here:
Antique Radio Forums • View topic - Williams 807 Amp EDIT: Made Decision
They claim they are getting 20 watts(per channel?) out of it. Yes its based on the oddwatt design.
Click to expand...

That's possible considering each tube can handle 30W plate dissipation. My previous figure was based on other numbers (mainly a specific transformer) but not optimised for squeezing out max power.
Anyway 12W or 20W will make little practical difference unless you use very inefficient speakers...maybe.
 
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