Q: AC15 w trem/vib on EF86 channel?

[MWaldorf]

Gang,

I'm looking to build an AC15, starting with iron from a Hammond AO-50.  I've just finished building Sluckey's Voxy PeeWee AO-44 with EF86 preamp and I love it.  I'd really like to have that EF86 tone with the AC15 trem/vibrato circuit that uses both sides of a 12AX7.

I've looked at the AC10 schematic, which has tremolo on the EF86 channel, and tried to compare it to the more complex trem/vib circuit in the AC15, but having no electronic expertise beyond minor mods, I'm at a loss as to have the swap could be done.

Is it possible to attach the trem/vib circuit to an EF86?  If so, would anyone with electronics knowledge be willing to write up a schematic?

Thanks as always,
Mel

[HotBluePlates]

... I'm looking to build an AC15, starting with iron from a Hammond AO-50. ...  I'd really like to have that EF86 tone with the AC15 trem/vibrato circuit that uses both sides of a 12AX7.

I've looked at the AC10 schematic, which has tremolo on the EF86 channel, and tried to compare it to the more complex trem/vib circuit in the AC15 ...

Is it possible to attach the trem/vib circuit to an EF86?  ...

Yes & No.

No, you can't do it as you've imagined.  The AC15/AC30 Vib/Trem circuit uses one & a half 12AX7s, and both sides of a 12AU7 just for the Vib/Trem.  So looking at a readable AC15 schematic, that's all of V4 and V6, and the 2nd half of V1.  The 1st half of V1 (pins 1, 2, 3) are the single preamp triode for that channel.

If you want to have the EF86 and the full Vib/Trem, the EF86 will take the place of the 1st half of V1.  Then copy everything that follows, all the way out to the multiple 0.005µF/1MΩ high-pass sections and the 500kΩ Volume control (R30).

The AC10 does things more simply, but you get tremolo only.  On the up-side, turn off the trem with the footswitch and the Amplitude control allows you to drop the screen voltage on the EF86 for more grind.

I haven't read the thread, but it seems someone else has already done this exact conversion before.

I've got a '65 AC10 Twin and a couple early-60s AC30s.  It's been a little while since I've played the AC10, but from memory the AC30's trem seemed a bit deeper.  If you're not building a clone, I'd recommend a pot instead of the switch/resistors for the Speed control.

[MWaldorf]

Thanks HBP!

It'll show my level of ignorance that I thought both sides of V1 where part of the preamp.  I'm pretty keen on getting the full AC15 vib/trem circuit as I've really liked both the vibrato and tremolo on the ones I've tried. 

As for the AO50, all I'm using is the chassis and the iron, it's gutted otherwise.  The factory tube complement is 2xEF86, 2x12AX7, 2x7247, 2xEL84, 1xEZ81.  So, if I understand it correctly, I should be able to run all the AC15 tubes (1xEF86, 1x12AU7, 3x12AX7, 2xEL84, 1xEZ81) and even power an additional EF86 with the AO50 PT.  If that's the case, I should be able change the circuit to have the extra EF86 in place of the first triode of V1?  That would leave a spare triode for something.

Looking at the AC15 schematic, it looks like I would copy the normal channel EF86 in place of V1a, and pin 6 of the EF86 would connect to C4 then to pin 7 of V1b?

I really appreciate the help!

[sluckey]

Looking at the AC15 schematic, it looks like I would copy the normal channel EF86 in place of V1a, and pin 6 of the EF86 would connect to C4 then to pin 7 of V1b?

I suggest to include C12, C17, and SW3, then to V1 pin 7. C4 not needed in this case.

Here's a better schematic (IMO) that shows the AC-15 circuit in a more logical and straight forward manner. Look at pages 2 and 3...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

[MWaldorf]

Thanks, Sluckey!  That actually makes it even easier for me.  I'll be using your six position rotary in place of C17 and SW3, I've used that in two AO-44 conversions based on your VoxyPeeWee and it's a great addition.  I'll be using your AC15 layout and circuit upgrades as a starting point with a few minor changes - two inputs on each channel, and a pentode/triode morph control on the updated vib/trem channel. 

[sluckey]

Will this be a single channel amp? If so, you need to ground the unused LTP PI input. For example, you will not use the top input. Then change C27 to a .1µF and connect the left side of the cap to ground. Right side still connects to V5-2.

[MWaldorf]

I'm keeping it two channels.  Generally I use one channel for guitar and one channel for combo organ.  Vib/trem channel will be for guitar.  Since I rarely use more than two fingers at a time when playing organ, the standard normal channel can stay nice and simple.  ;)

[sluckey]

I'll be interested to hear how this turns out. I've always preferred the EF86 channel on my AC-15. The vibrato is OK, but it doesn't even come close to the sound of my Magnatones.

[MWaldorf]

will do.  Once I get started I'll make a build thread.

BTW, here's a demo I made of my VoxyPeeWee.  Note that I'm a putz and forgot to mention your creation of the circuit in the video; at least I got you in the description.

[sluckey]

I've just finished building Sluckey's Voxy PeeWee AO-44 with EF86 preamp and I love it.

Just a bit of clarification. My PeeWee AO-44 was never anything Voxy. It had a Fender AB763 preamp in it for a while. Then it became a donor for my L'il Maggie, a Magnatone M2 amp. I suspect you may be thinking about my AO-39 conversion which was an AC-15 Lite that later became a donor for my Dual Lite?

[MWaldorf]

Actually, in another thread a month or so ago, I was looking into turning an AO-44 into an AC10, and you drafted a schematic with the AC15 EF86 normal channel as the preamp, with an additional change to the last resistor in the power supply to get the right voltage for the EF86. 

Here's the schematic with some additional changes I made - pentode/triode switch and cathode bypass cap switch.  Afterwards I replaced the pentode/triode switch with switchable morph controls, as seen in the video.  I can update the schematic if you think it would be useful to have for future reference.

[tubeswell]

FWIW, the AC10 and AC15 power amp sections and phase inverters are almost identical circuits, except that the AC10 has 100R plate resistors in series with the EL84 plates and the OT primary that eats up the output power a little bit (and the AC10 has a 200R resistor in series with the rectifier tube that eats up the B+ voltage a little bit)

[sluckey]

Actually, in another thread a month or so ago, I was looking into turning an AO-44 into an AC10, and you drafted a schematic with the AC15 EF86 normal channel as the preamp, with an additional change to the last resistor in the power supply to get the right voltage for the EF86.

Haha. Memory is a beeatch when you get old. 

[MWaldorf]

Haha. Memory is a beeatch when you get old. 

To be fair, we do have more memories to forget as we age!

[MWaldorf]

How does this look for the revised vib/trem channel with EF86?  Note I've included the relay switching for dual morph/blend controls on the EF86, which I also used on the VoxyPeeWee.