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Vox AC15 EF86 with one power tube
Vox AC15 EF86 with one power tube
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Old 3rd July 2019, 01:03 PM   #1
jaidn is offline jaidn  Spain
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Default Vox AC15 EF86 with one power tube

Hello there!,

I love the Sluckey's 1960 Vox AC-15. I'd like to modify it to fit a single 6V6 power tube (I don't need 15W). What do I have to modify o transform the amp from PP to SE single tube?

I attach the Sluckey's 1960 Vox AC-15 pdf.

Cheers!
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File Type: pdf Vox_AC15.pdf (787.2 KB, 59 views)
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Old 3rd July 2019, 06:05 PM   #2
stephen_keller is offline stephen_keller  United States
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You'll have to redesign the phase inverter to be a mixing stage with suitable gain to drive a single-ended 6V6 power amp. The push-pull output transformer will need to be replaced by a suitable single-ended (gapped) transformer. The top-cut control will need to be replaced by something like a Garnet feedback tone control (assuming you want to retain that bit). There are likely to be other tweaks I'm overlooking. It probably won't sound much like the original circuit, though. The phase inverter contributes to the tone as does the push-pull EL84 output stage. That doesn't mean it won't sound good, just different.

You would get closer to AC-15 vibe, I think, by power-scaling existing EL84 power amp and adjusting the OT primary impedance to run the amp at 7-to-10 watts.

Stph
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Old 3rd July 2019, 08:48 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Why not turn the volume down ... ?
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Old 3rd July 2019, 09:45 PM   #4
Guerilla is offline Guerilla  Denmark
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Quote:
Originally Posted by JonSnell Electronic View Post
Why not turn the volume down ... ?
Isnt it common to like when an amp distort because turn it up higher than it can play clean? I thought that was the reason lowpowered guitaramps has become so popular
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Old 3rd July 2019, 11:27 PM   #5
leadbelly is offline leadbelly  Canada
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Do you really love it? If so, I would say the thing to do is build it as is (except cathode bias value) with 6K6 or 6W6 finals, as was done in the old days before power scaling took over.
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Old 4th July 2019, 01:26 AM   #6
JMFahey is offline JMFahey  Argentina
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Sell it and get/build an AC4
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Old 6th July 2019, 08:00 PM   #7
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by jaidn View Post
...Sluckey's Vox AC-15...I don't need 15W.
Going from push-pull to single-ended won't make the amp as much quieter as you'd expect, because of the way the human ear works. In fact, going from 15 W to 6 W is only about 4 dB SPL reduction, which is quite a slight change to the ear.

On top of that, SE output stages do sound different from PP. In my opinion, SE output stages produce better clean tones, while PP outputs produce better distorted / overdriven tones. (Of course "better" is entirely subjective, and you may or may not share my opinion.)

Quote:
Originally Posted by jaidn View Post
...modify or transform the amp from PP to SE...
You have to change almost everything in the power amp, including the OT. In effect, you would be designing a new amp, or bolting something a lot like a Fender Champ output stage onto your VOX EF86 preamp. Even if you do this, there's no telling if you'll like the result, so it will be an experiment.

So I would suggest you consider making a simple speaker attenuator instead. You only need two power resistors to make a simple fixed attenuator. You can quite easily cut the power from nominally 15 W to nominally 6 W, and see if that quietens it down enough for you (it may very well not.)

The attached schematic shows you how simple this is. You need two power resistors, which only cost a few bucks.

Consider the power ratings shown in the schematic a minimum. You can use a 50 W resistor for R1 and a 25 W resistor for R2 if you want a little more safety margin (cooler-running resistors.)

Remember, 6W can be very loud...this attenuator will do what you asked for (turn a 15W amp into a 6 W amp), but, depending on how you plan to use the amp, it may still be louder than you want.

It's not hard to change the values of R1 and R2 to lower the output power to less than 6 watts, and I've already made a spreadsheet of values you could use if it turns out that 6W is too loud for you.


-Gnobuddy
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Old 6th July 2019, 10:37 PM   #8
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
Why not turn the volume down ... ?
That's one of the problems with valve guitar amps, they only produce the timbre you want at one specific setting, which happens to generate the amount of nonlinear distortion you want from the output stage. Unfortunately, that setting may very well be too loud, depending on your intended use.

I did some calculations a few years ago that suggested that a few tens of milliwatts of power is all you need for apartment-friendly loudness levels from an overdriven guitar amp connected to a typical guitar speaker. As far as I know, there are very few commercial guitar amps that let you dial down the output that far.

Not long ago I breadboarded a single-ended guitar amp using a small-signal pentode as the output device, putting out a thundering 200 milliwatts or so at full chat. There's barely enough output power to warm up a quarter-watt 8 ohm resistor, but it was too loud to overdrive in my apartment at night!

(Clean tones were quite usable, though.)


-Gnobuddy
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Old 7th July 2019, 12:53 AM   #9
stephen_keller is offline stephen_keller  United States
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It occurs to me that one might build a PP power amp with a 7699 dual-pentode (see attached data sheet) or a similar wideband amplifier. These sorts of tubes were used in Tektronix oscilloscopes. The 7699 ought to be good for around a 1-2 watt power amp into a 10K plate-to-plate load. There were several industrial variants of this sort of thing used in scopes. Other tubes to consider for this sort of low-power PP amp, would AGC sync tubes such as the 6BU8, 6GS8 or 6KF8.

This sort of low-power PP amp would get much closer to the Vox AC15 vibe than a SE output stage. At 1/10 the power it would seem about half as loud (give or take).

Stph
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File Type: pdf 7699.pdf (262.6 KB, 11 views)
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Old 7th July 2019, 01:23 AM   #10
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by stephen_keller View Post
...one might build a PP power amp with a 7699 dual-pentode...or a similar wideband amplifier.
One might indeed, and it might be an interesting experimental project. But there is a chance it may not sound much like the Sluckey-simplified Vox the OP wants.

Along the lines of your proposal, a few years ago I built a small push-pull 2 watt amp using a pair of little 7-pin 6AK6 pentodes as the output valves. I paid $3 each for these. The 6AK6 was originally designed for audio output in radios, and has a 2.75-watt anode dissipation rating.

My more recent 200 mW single-ended experiment used a one-buck 6JW8, a 9-pin triode-pentode valve originally designed to be used in televisions. The pentode section is rated for only 1.2 watts maximum anode dissipation, the same as each of the two triodes in a 12AX7!

For my 6JW8 design, the OT primary needs to be about 20 - 25k, which I got from a little Hammond transformer, though a Fender reverb transformer would probably also work. There are only a few milliamps of DC anode current, and output power is miniscule, so my small PP output transformer seems to have no difficulty at all coping with SE usage.

The triode in the 6JW8 is used as the driver for the pentode, and I used a second one-buck valve (6AQ6) as the input gain stage. There is enough gain to insert a tweaked Fender two-knob tone control between 6AQ6 and 6JW8 triode, and go from clean tones to something in the classic rock era.

Quote:
Originally Posted by stephen_keller View Post
This sort of low-power PP amp would get much closer to the Vox AC15 vibe than a SE output stage. At 1/10 the power it would seem about half as loud (give or take).
I agree on both counts. And I'll add that half as loud as a 15-watt Vox is still very, very loud, depending on where you intend to use it!

For what it's worth, with a proper guitar speaker attached, my little 6AK6 is far too loud to overdrive in my apartment. Which is why I decided to try out a ten-times-less-powerful 200 mW amplifier design on the bread-board.


-Gnobuddy
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