AO-44 to AC10?

[MWaldorf]

A friend just gave me an AO-44 and an AO-50.  My plan is to use the AO-50 iron to make one of sluckey's AC-15s, but I'm wondering, does the AO-44 iron have enough guts to power an AC-10? 

The amp didn't come with tubes, so I'm not locked into ECL86s.  If I understand correctly an ECL86 is basically an EL84 with half a 12AX7 thrown in for good measure.  So that would cover the needs of the AC10 power section.  But will the AO-44 PT handle an additional EF86 and ECF82?

I'll be using a different chassis so I'm not concerned about fitting the circuit inside the AO-44 box.

Thanks everyone!

[sluckey]

But will the AO-44 PT handle an additional EF86 and ECF82?

I would say yes ***IF*** you stay with the 6GW8.  Might be a stretch to use EL84s. Maybe drop the tube rectifier and use SS.

[HotBluePlates]

... I'm not locked into ECL86s.  If I understand correctly an ECL86 is basically an EL84 with half a 12AX7 ...

... I would say yes ***IF*** you stay with the 6GW8.  Might be a stretch to use EL84s. ...

ECL86/6GW8 is a 9w tube (scroll to page 2), while EL84 is a 12w tube.  It's kinda like an EL84, but doesn't have the same grunt.  Heater current is also higher in the EL84 (though not by a lot).

I own a 1965 AC10.  The clean channel is useless to me as it's very clean & wimpy, using just the triode half of the ECF82/6U8 (having an Amplification Factor of 40 like a 12AY7) to drive the phase inverter.  The pentode section is used for the tremolo oscillator; I would look to build it with a MOSFET rather than a tube, and retain just the EF86 channel.  Then I'd use the existing triodes in the ECL86s for the long-tail phase inverter.

I suspect the whole thing could still fit in the AO-44 chassis, a la Sluckey's PeeWee.  That might depend on using compact caps for the trem oscillator.

[MWaldorf]

Thanks guys.  So if I stick to a single EF86 and keep the stock power section with the ECL86s I should be OK then.  If that's the case, would my best bet be to start with the AC15 normal channel as the preamp?  I can build a solid state tremolo for it, that's what I did with my current AO-44 amp (with a tweed Harvard pre-amp). 

[HotBluePlates]

...  So if I stick to a single EF86 and keep the stock power section with the ECL86s I should be OK then.  If that's the case, would my best bet be to start with the AC15 normal channel as the preamp?  ...

You can if that's the circuit you really want.  But the AC10 trem channel has a larger plate load resistor (possibly more gain).  And the real-hot ticket is when you turn the trem off with the footswitch, the Amplitude (Depth) control alters the gain of the EF86.

You can get some grindy tones at very low volume just by messing with that control.  I most-often play my AC10 at pretty low volume, trem off, and the Amplitude control full-up for max EF86-color.

[MWaldorf]

HBP - that's really interesting about the trem affecting the gain!  If the AO-44 iron can handle it, I'd prefer to have two channels because I usually use one for guitar and one for cheezy combo organ. 

Then, and forgive me if this is a stupid question, but if I stick to the ECL86s and the EZ81 can I run both an EF86 and ECF82?

[rayandkerry]

I suspect the whole thing could still fit in the AO-44 chassis, a la Sluckey's PeeWee.  That might depend on using compact caps for the trem oscillator.

If the AC10 doesn't work out, I can attest that the PeeWee sluckey designed is a nice little piece of work.  Easy to do and it sounds lovely (although not voxy).

[MWaldorf]

If the AC10 doesn't work out, I can attest that the PeeWee sluckey designed is a nice little piece of work.  Easy to do and it sounds lovely (although not voxy).

I believe it!  A friend built me an AO-44 with a 6AT6 tweed Harvard preamp which I then modified a bit, removing the conjunctive filter and negative feedback and adding a bass control, and that sounds pretty fantastic.  I'm looking to do something more Voxy than Fender and I want to try an EF86 preamp.

[HotBluePlates]

... if I stick to the ECL86s and the EZ81 can I run both an EF86 and ECF82?

I have no idea.  Maybe?

...  If the AO-44 iron can handle it, I'd prefer to have two channels because I usually use one for guitar and one for cheezy combo organ. ...

I've got blinders on a bit, because my preference would be to avoid a new chassis & metalwork, and simply add a 9-pin socket & rewire for the single-channel on the existing AO-44 chassis.  I suppose that would prevent getting a nice ready-made head shell for the amp.

[sluckey]

I would just put an EF86 preamp in front of the original power amp inside the original chassis. This is the easy way to see just how voxy it will sound. My gut says it would sound good. I've attached a schematic of my idea.

However, if I had to buy a pair of 6GW8s, I'd just shelf the AO-44 or consider converting to use 6BM8s. A pair of new 6GW8s will cost $120 and a pair of new 6BM8s will cost $44. It's a lot of work to convert to 6BM8s inside that chassis.

The AO-39 is well suited for your AC-10 idea.

[bmccowan]

It's a lot of work to convert to 6BM8s inside that chassis.

Sluckey, did you maybe mean that its not a lot of work? I was thinking of doing the same to a 44 chassis I have, since I really like the sound of 6BM8s in a HoSo Lite that Tubenit helped me with, and it does not seem like a lot of socket pin changes from the 6GW8?

[MWaldorf]

Fate has been kind and I was able to find a pair of ECL86s for $45, just have to wait till they get here from France!

Sluckey, thank you so much for the revised PeeWee!  I'll definitely drop the negative feedback and conjunctive filter.  Am I correct that I can drop in a Vox style tone cut control?  While the component values are different in the AO-44 power amp, the topography looks very similar to an AC10. 

[sluckey]

Sluckey, did you maybe mean that its not a lot of work?

Pins 2, 3, 7, and 8 have to be shuffled around. Looks simple enough on paper, but when you get inside the P2P chassis it's gonna be a different story. Gonna be a bigger job than you would think. It's doable but definitely not just a 2 beer job! 

If you do this, please post some pics.

[sluckey]

Fate has been kind and I was able to find a pair of ECL86s for $45, just have to wait till they get here from France!

Am I correct that I can drop in a Vox style tone cut control?

Great! You may want to look at my Pee Wee to get some ideas how to place the extra tube in the chassis. The AO-44 uses a cathodyne PI and that cut control acts funny with the cathodyne, but it's easy enough to just stick it in and see if you like it. It would probably be better to rewire the triodes to the AC-10 LTP PI. Then the cut would work fine.

[bmccowan]

Gonna be a bigger job than you would think.

Thanks for the warning. This is also true of pretty much every project I've ever done; and not just amps! Fixing cars & motorcycles, building a cottage, even cooking dinner. The list is long. I do recall struggling with rewiring AO-39s and there is a bit more room in there. Fat arthritic fingers don't help the cause. There are no triode/pentode octal tubes, are there?

[bmccowan]

I just pulled mine out of its hiding place. I had forgotten how crowded things are in there! So maybe I have some ECL86s around here somewhere?

[HotBluePlates]

... A pair of new 6GW8s will cost $120 ...

Wow, they've gone way up!  $60 each?

I've got 12 bought from eBay back in 2015 that are a mix of Telefunken, Amperex and Philips, plus a couple Sylvanias. Net price was just under $18/each shipped.

There's also a group of hams near me that at last ask were selling them for $6/each if they test like-new.  However, I mighta bought the last one they had...

[sluckey]

HBP, did you ever do anything with your AO-44?

[HotBluePlates]

HBP, did you ever do anything with your AO-44?

I let it be what it was: a clip-on Reverb amplifier for amps with no reverb.  Kinda like the Gibson/Matesro GA-1RT (except no trem).

I still have grand delusions about reworking the resistor/bulb attenuator to be able to adjust the dwell on it.

[sluckey]

I played with mine as a separate reverb amp for a week before I converted it. I really liked the sound of the separate reverb.

[rayandkerry]

I believe it!  A friend built me an AO-44 with a 6AT6 tweed Harvard preamp which I then modified a bit, removing the conjunctive filter and negative feedback and adding a bass control, and that sounds pretty fantastic.  I'm looking to do something more Voxy than Fender and I want to try an EF86 preamp.

Cool!  And ditto on the ef86.  I'm building an AC30 right now.  Been working up to it for years.  It'll have both the ef86 and the top boost 12ax7 preamps.  I can't wait! 

Although I'm going to have to due to normal life taking up too much hobby time, LOL.  The chassis is cut with the tube sockets installed and maybe 40 - 50 percent of the wiring done.  Good luck with your project!

[MWaldorf]

Just did a quick test on the cap can in my AO-44.  It's supposed to be 30/30/30/20, and currently reads 28/22/24/19.  I assume I should replace it?  If so, I see that CE manufacturing makes a 30/30/30/10 and a 30/30/30/30 cap can.  Which would be a better choice - to over or under filter the last stage?

Thanks, Mel

[sluckey]

Based on that info, I'd let it be.

[HotBluePlates]

Just did a quick test on the cap can in my AO-44.  It's supposed to be 30/30/30/20, and currently reads 28/22/24/19.  I assume I should replace it?  ...

That cap can probably didn't measure very-different when it was new.  Meaning, its tolerances are likely wider than the values you measured.

[MWaldorf]

Thanks Sluckey and HBP.  I'll keep that in place.  Just got some tube sockets in the mail so now I can start working on the preamp.

Actually, another stupid question - am I correct that I install a Vox style cut control?  The AO-44 phase inverter is different than a Vox type (cathodyne vs Long Tail Pair, I think?), but the input to the power pentodes looks very similar between them.  Regardless of the phase inverter type, the signals going to the pentodes are out of phase with each other, correct? 

Thanks as always, Mel

[sluckey]

The Vox cut control works very well with a LTP PI but it acts very wonky with the cathodyne. There was a long discussion about this earlier this year. Don't remember which thread. I think it was 2deaf or pdf64 that finally gave the answer for why it doesn't work.
Had to do with the high plate impedance versus the low cathode impedance of the cathodyne.

[DummyLoad]

... I'm not locked into ECL86s.  If I understand correctly an ECL86 is basically an EL84 with half a 12AX7 ...

... I would say yes ***IF*** you stay with the 6GW8.  Might be a stretch to use EL84s. ...

I own a 1965 AC10.  The clean channel is useless to me as it's very clean & wimpy, using just the triode half of the ECF82/6U8 (having an Amplification Factor of 40 like a 12AY7) to drive the phase inverter.  The pentode section is used for the tremolo oscillator; I would look to build it with a MOSFET rather than a tube, and retain just the EF86 channel.  Then I'd use the existing triodes in the ECL86s for the long-tail phase inverter.

have you ever tried to sub in a 6JW8 for the ECF82/6U8? pin compatible with 6U8/ECF82, pentodes should work in LFO application, but triode has 2x mu over 6U8/ECF82.

--pete

[HotBluePlates]

... Vox style cut control?  The AO-44 phase inverter is different ... (cathodyne vs Long Tail Pair, I think?) ...

The Vox cut control works very well with a LTP PI but it acts very wonky with the cathodyne. There was a long discussion about this earlier this year. Don't remember which thread. I think it was 2deaf or pdf64 that finally gave the answer for why it doesn't work. ...

FWIW, I've been using a Vox-style Cut control on my 25L15-style amp since, what, 2012?  That amp has a cathodyne inverter & it's been sounding good since the build (though it initially took a bit of tweaking to dial it in).

A caveat is that amp simply doesn't break up on the output tubes, even with the volume full-up (it might if I also max the tone controls).

I'll have to go look for the thread; I'm thinking it may be more a caution about weirdness once the output tubes start drawing grid-current.

... have you ever tried to sub in a 6JW8 for the ECF82/6U8? pin compatible with 6U8/ECF82, pentodes should work in LFO application, but triode has 2x mu over 6U8/ECF82. ...

No I haven't!  Thanks for the tip!

I'll look to see if I have any 6JW8s, but there's a good chance any I had were in that big batch of tubes I sent you some time back.  And I wound up trashing something like 400-500 additional oddball types some time ago once I accepted I'd likely never have a use for them.  I'll have to look at what survived the cut...

[MWaldorf]

thanks again guys.  For the moment I'll get it up and running with the cathodyne PI and then maybe change it to LTP if it seems to need the cut control.

[DummyLoad]

... Vox style cut control?  The AO-44 phase inverter is different ... (cathodyne vs Long Tail Pair, I think?) ...

... have you ever tried to sub in a 6JW8 for the ECF82/6U8? pin compatible with 6U8/ECF82, pentodes should work in LFO application, but triode has 2x mu over 6U8/ECF82. ...

No I haven't!  Thanks for the tip!

I'll look to see if I have any 6JW8s, but there's a good chance any I had were in that big batch of tubes I sent you some time back.  And I wound up trashing something like 400-500 additional oddball types some time ago once I accepted I'd likely never have a use for them.  I'll have to look at what survived the cut...

as it turns out the 6LX8 is same as 6JW8 - one of the 6JW8 tubes in the pile you sent me had a hand-written note on the box. so i scrounged for 6LX8 - found 3! so i have on-hand 2x 6JW8 and 3x 6LX8 - PM snail-mail address and i'll send you a pair.

--pete

[DummyLoad]

HBP - if you have a 6CM8 in your stash, give that one a spin as well: pin compatible with 6U8 but has a 12AX7-ish triode (mu 100, rp 50k, gm 2k) and sharp cou-off pent., however, the pent. has very high rp (0.6MΩ).

we are still chatting about a AC10. 

--pete

[HotBluePlates]

HBP - if you have a 6CM8 in your stash, give that one a spin as well: pin compatible with 6U8 but has a 12AX7-ish triode (mu 100, rp 50k, gm 2k) and sharp cou-off pent., however, the pent. has very high rp (0.6MΩ). ...

I'll take a look!  I'm a little pessimistic about finding any, though.  After I sent you those tubes one time, I went through my tube stash and threw away ~400 tubes that seemed like oddball types I'd never get around to trying.  I still have some lesser-used tubes but decided to stick to only keeping largely typical guitar-amp tubes just to stave off the hoarding tendency.

[MWaldorf]

It's alive!!!  All wired up to Sluckey's VoxyPeeWee schematic (with OT R/C filter and NFB removed) and it's raw and raunchy.  Pretty great just running out of a very tired Oxford 12L6-4.

Immediate observation - this makes a lot of treble!  I'm thinking of changing the values of the Brilliant switch, dropping the 360pf at the thin side and adding a .022uF at the low end side.  Or should I go even to .033uF or .047uF?  I suppose I can do that testing.

The other thing I'd like to try is a cut control.  I know I have to switch over to a LTP phase inverter.  I've tried to modify Sluckey's Voxy PeeWee with an AC-10 power section.  I know the pentode section of the ECL86 isn't the same as an EL84, so I assume some of the component values need to change? 

I appreciate all the advice here, and thanks for your continued assistance!

Best, Mel

[PRR]

> a very tired Oxford 12L6-4.............- this makes a lot of treble!

Speaker sitting open, no baffle at all, is gut-less (so you only hear treble).

Don't start voicing until you try several properly-used speaker systems.

[HotBluePlates]

... The other thing I'd like to try is a cut control.  I know I have to switch over to a LTP phase inverter.   ...

I made an amp with a split-load/cathodyne inverter with 22kΩ plate & cathode load resistors.  I put a Cut control on it using a 250kΩ audio pot (wired as a rheostat, like every other Cut control), with 0.047µF caps on both ends to block d.c.

In my amp it works fine.

I think I found the thread Sluckey referenced: 5E3 Deluxe Cut Control

They mention the split-load inverter can drive a lot of capacitance without treble loss.  But the guy trying it was using 0.0047µF, which is 1/4 the total capacitance I used (Cut control all the way off puts the caps in series, resulting in an effective  "0.0235µF cap").

I found I guessed a value for the required caps, didn't hear an effect and then had to keep adjusting until I heard the treble-reduction I was looking to get.  Not hard really, as long as you have a selection of caps handy.

[MWaldorf]

Thanks guys. 

PRR - I tried it through the biggest, baddest speaker cabinet I had - a Fender Showman 1x15" tone ring cabinet.  More bass, almost passable on the most open of the brilliant switch selections.

HBP - thanks, I'll give that a try, seems like less effort / modification than a full switchover to the LTP.

[MWaldorf]

OK gang, a few updates:

HBP, I tried your cut circuit, and while it did something, it didn't do much.  I wound up tacking a modified tweed Harvard tone control after the volume pot, which got me the top end control I wanted without taking any of the tonal character away.  Next I added a switchable 1uF cathode bypass capacitor on V1A which gives a nice volume boost.

Finally, I also added a triode/pentode switch on the EF86, based on the circuit in the AC15H1TV.  I get a pretty pronounced popping when switching, which I didn't hear in demos of the Vox amp.  From what I've read, it seems this popping is the nature of the beast.  I tried high value resistors in various configurations around the switch with no difference (between outer legs, between both outer legs and the wiper, between one or the other leg and the wiper)  Anyone have any ideas?

The final schematic is below.  I've updated Sluckey's schematic to note what I changed.

Once again, thank you everyone for your help!