vintage Vox power section Q AC10 vs AC15?

[MWaldorf]

Hi all.  I'm a tinkerer with a couple of builds behind me but not much in the way of electronics knowledge. 

I was looking at the schematics for the 60s Vox AC-10 and AC-15 and I noticed that the power sections look almost identical - the primary difference being 100 ohm resistors on the EL84 anodes in the AC-10 moved to grids in the AC-15.  The voltage on the anodes is 305V in the AC-10 and 310V in the AC-15.  So is the power difference between the two amps coming from the output transformer, or am I missing something fundamental that would be obvious to anyone who actually knows about this stuff?

Here's the power section of the AC10, and the AC15 below that.

[shooter]

no math so;
R41, R62/3 have an effect
my view;
bias a 10 hot or a 15 cold and you got an AC12.5 

[tubeswell]

The difference in B+ isn't going to account for much. The main difference is the 100R 'plate resistors' in the AC10, which eat up some of the voltage swing, which lowers power through the OT

[pdf64]

Are the OT primary impedances and HT regulation characteristics the same?

[66Strat]

Tubeswell has the answer.

[MWaldorf]

cool, thanks guys, that makes some sense.  So plate resistors in the AC10 limit output, and I assume the size/specs of the output transformer makes up the rest of the difference in output and headroom?

[shooter]

the OT might but guessing it's about efficiency.  If you have 97% versus 92% coupling..sorta thing.

with R41 in the 10, makes me think as a marketer, I can sell 2 "different" amps with lots of identical parts that I get cheaper by the dozen

[pdf64]

...bias a 10 hot or a 15 cold and you got an AC12.5 

But bias and power output are unrelated

[shooter]

 
bad habit 

the point, change the operational characteristics of a 15W amp and you have a 10w amp that looks strikingly similar 

[PRR]

> bias and power output are unrelated

Not much in class B. But these amps run near-A.

I figure "10" and "15" are tradenames, not technical promises. The Ford 289 "200HP" never had 200 horses. The Dodge 426 "425HP" had a lot more ponies than that. The Ford "500" didn't have 500 of anything.

[Williamblake]

If i remember correctly the name is explained somewhere in here:

Dont expect much.

[j_bruce]

Loved the video!

". . .anything worth doing is dangerous, and risky . . ." Brian May

[Jalmeida]

Hi all.  I'm a tinkerer with a couple of builds behind me but not much in the way of electronics knowledge. 

I was looking at the schematics for the 60s Vox AC-10 and AC-15 and I noticed that the power sections look almost identical - the primary difference being 100 ohm resistors on the EL84 anodes in the AC-10 moved to grids in the AC-15.  The voltage on the anodes is 305V in the AC-10 and 310V in the AC-15.  So is the power difference between the two amps coming from the output transformer, or am I missing something fundamental that would be obvious to anyone who actually knows about this stuff?

Here's the power section of the AC10, and the AC15 below that.

I know this is resurrecting a 3 year old post. And I not aure if the answer was found. I am not an expert, but wanted to share what I have found. But I have been researching this alot between a past AC10 and AC15 build. But neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. The AC15 had a higher B+ than the AC10 did. In fact the PT in the AC15 was higher than shown in the schematics and is often shown to be 300v same as shown on the AC10 schematic. The post 60 AC15 were closer to 315v unloaded. I found one AC10 that used a 275-0-275 radio spares transformer contrary to what the schematic shows. Same lamination format as the AC15. I am not entirely sure that these differences are equal to 5w delta between the two. Perhaps the schematics are showing different voltage numbers with one being loaded and other not? But the AC15 would load the PT a bit more with an extra tube for the Tremolo over the AC10.

[HotBluePlates]

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. ...

I will offer that after checking out some actual vintage amps, there is often some variance compared to what the schematic shows (especially with voltages).  We shouldn't assume schematic numbers are absolute-fact.

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. The AC15 had a higher B+ than the AC10 did. In fact the PT in the AC15 was higher than shown in the schematics and is often shown to be 300v same as shown on the AC10 schematic. The post 60 AC15 were closer to 315v unloaded. I found one AC10 that used a 275-0-275 radio spares transformer contrary to what the schematic shows. ...

I would suggest we consider what evidence shows the Radiospares transformer is a "factory part"?  Because if the factory (and Jennings contracted multiple companies to actually build their amps, maybe 3 or 4?) didn't use it originally, then its exact characteristics are not relevant to the wider "AC15" or "AC10" class of amps.

That said, I measured my own ~1965 AC10 Twin and got:
   -  Almost exactly 300-0-300v at the PT high voltage secondary.
   -  334vdc at the rectifier output (rather than 320vdc)
   -  310v at the 2nd filter cap (that the AC10 schematic calls "305v source").
   -  There was 300v at the OT primary ends, and 296v at the EL84 plates (on the other side of the 100Ω resistors).

I'd consider all that "very close agreement" to the AC10 schematic.

I do not own a vintage AC15, and haven't had access to one to measure myself.  I can't say what is or isn't inside those.  But do notice the AC10 adds a 200Ω resistor between rectifier and 1st filter cap, which the AC15 does not have.  That will lower supply voltage somewhat in the AC10 if the same transformers are used in both.

[astronomicum]

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. ...

I will offer that check out some actual vintage amps, there is often some variance compared to what the schematic shows (especially with voltages).  We shouldn't assume schematic numbers are absolute-fact.

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. The AC15 had a higher B+ than the AC10 did. In fact the PT in the AC15 was higher than shown in the schematics and is often shown to be 300v same as shown on the AC10 schematic. The post 60 AC15 were closer to 315v unloaded. I found one AC10 that used a 275-0-275 radio spares transformer contrary to what the schematic shows. ...

I would suggest we consider what evidence shows the Radiospares transformer is a "factory part"?  Because if the factory (and Jennings contracted multiple companies to actually build their amps, maybe 3 or 4?) didn't use it originally, then its exact characteristics are not relevant to the wider "AC15" or "AC10" class of amps.

That said, I measured my own ~1965 AC10 Twin and got:
   -  Almost exactly 300-0-300v at the PT high voltage secondary.
   -  334vdc at the rectifier output (rather than 320vdc)
   -  310v at the 2nd filter cap (that the AC10 schematic calls "305v source").
   -  There was 300v at the OT primary ends, and 296v at the EL84 plates (on the other side of the 100Ω resistors).

I'd consider all that "very close agreement" to the AC10 schematic.

I would also conclude that the AC-10 schematic is accurate based on voltage measurements from this example (attached).

[Jalmeida]

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. ...

I will offer that check out some actual vintage amps, there is often some variance compared to what the schematic shows (especially with voltages).  We shouldn't assume schematic numbers are absolute-fact.

... neither the AC15 nor the AC10 schematics are accurate in regards to the power transformers. The AC15 had a higher B+ than the AC10 did. In fact the PT in the AC15 was higher than shown in the schematics and is often shown to be 300v same as shown on the AC10 schematic. The post 60 AC15 were closer to 315v unloaded. I found one AC10 that used a 275-0-275 radio spares transformer contrary to what the schematic shows. ...

I would suggest we consider what evidence shows the Radiospares transformer is a "factory part"?  Because if the factory (and Jennings contracted multiple companies to actually build their amps, maybe 3 or 4?) didn't use it originally, then its exact characteristics are not relevant to the wider "AC15" or "AC10" class of amps.

That said, I measured my own ~1965 AC10 Twin and got:
   -  Almost exactly 300-0-300v at the PT high voltage secondary.
   -  334vdc at the rectifier output (rather than 320vdc)
   -  310v at the 2nd filter cap (that the AC10 schematic calls "305v source").
   -  There was 300v at the OT primary ends, and 296v at the EL84 plates (on the other side of the 100Ω resistors).

I'd consider all that "very close agreement" to the AC10 schematic.

I do not own a vintage AC15, and haven't had access to one to measure myself.  I can't say what is or isn't inside those.  But do notice the AC10 adds a 200Ω resistor between rectifier and 1st filter cap, which the AC15 does not have.  That will lower supply voltage someone the AC10 if the same transformers are used in both.

I don’t doubt your voltage readings. Were all the tubes in? And the AC10 schematic may be correct. But all things similar between the AC10 and the AC15 schematic, voltages post rectifier doesn’t jive between them. Is the transformer in yours original?

I don’t entirely disagree about the Radio Spares transformer. However Vox’s contractors did also use Radio Spares transformers on some amp so, I just don’t know. Markings are likely under the bell cover. If the above picture is a factory original AC10 factory PT then 300v may come down to the originals being above rated spec. Or maybe schematic
Voltages between the two models being measured differently.

Those 100R resistors are odd. I am skeptical that these alone could cause a 5w difference. Also note the AC15 has 100R resistors between pins 9 and the B+ of the EL84 while the AC10 has them between pin 7 and the OT. And the AC30/4 has the 100R resistors on pins 9 as well. I heard a theory that goes back to the very early AC15 that had 350-0-350 PTs. And that the100r resistors on pin 7 were intended for OT protection because they were pushing the EL84s hard with nearly 400v B+. And the AC10 and very early AC15 being of the same lineage being carried over to the AC10.

[dwinstonwood]

...they were pushing the EL84s hard with nearly 400v B+...

Off topic, but Fender tried that for a short period, too:
https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9.pdf

[Jalmeida]

...they were pushing the EL84s hard with nearly 400v B+...

Off topic, but Fender tried that for a short period, too:
https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9.pdf

I’m sure the older EL84s took the high B+ in stride.

[HotBluePlates]

... your voltage readings. Were all the tubes in? ...

Yes, all my readings were taken with all tubes installed.

With no tubes (especially the EL84s), the voltage reading is meaningless because the unloaded supply will rise to the peak of the power transformer's AC voltage.

... Is the transformer in yours original? ...

Yes.  Other than replaced filter caps & an added mains fuse, my AC10 Twin is completely original.  The speakers in the photo were removed to re-install the original silver Elac 10s.  The transformers are very likely Parmeko all around (the choke certainly is, being marked as such).

(open in a new tab for full size)


 

I don’t entirely disagree about the Radio Spares transformer. However Vox’s contractors did also use Radio Spares transformers on some amp so, I just don’t know. Markings are likely under the bell cover. If the above picture is a factory original AC10 factory PT then ...

Jim Elyea's Vox Book (all 682 pages) is probably the most complete & authoritative reference.  He mentions Radio Spares being used for short runs, prototypes, and "oops!" moments.  But Elyea's information suggests they were seen in very early 1960 AC30s, and mostly not seen again.

Instead, the companies that do get mentioned for transformers are Albion, Drake, Haddon, JD, Lemark, Morley, Partridge, Parmeko, and Woden.  Albion, Haddon and Woden would be the most-common, with Parmeko coming in 4th.  Elyea makes special mention that Parmeko transformers were common for AC10s.

... Those 100R resistors are odd. I am skeptical that these alone could cause a 5w difference. Also note the AC15 has 100R resistors between [Screen] and the B+ of the EL84 while the AC10 has them between [Plate] and the OT. ... I heard a theory that goes back to the very early AC15 that had 350-0-350 PTs. And that the100r resistors on pin 7 were intended for OT protection because they were pushing the EL84s hard with nearly 400v B+. ...

Anyone have a schematic or an actual vintage AC15 to substantiate the theory?

I found it was easy to convince myself of all kinds of things when discussing theory on a forum.  But then you get the actual vintage amp in-hand & find out the theories were wrong.  I've heard about a bunch of strange, unexpected stuff from folks who work with only vintage guitars & amps all day for decades.

FWIW, the "G1/10" amplifier model already existed as a Jennings product before Dick Denney showed up with "his amp" and was a copy of a Univox circuit.  The Denney amp was the early "15w amp" that was first released as the "G1/15" and at first didn't use what we might call the "AC15 circuit."  Denney eventually changed the AC15 to have an EF86 and tremolo on one channel, and eventually the "G1/10" was renamed to "AC10" and given essentially the same circuit.  The G1/15 (and AC15) had a 12" speaker, while the G1/10 (and AC10) used a 10" speaker.

The AC10 seems to have been intentionally hobbled to differentiate it from the AC15, by adding that 200Ω resistor before the 1st filter cap, and perhaps with those 100Ω plate resistors as well.  While the ad copy says "10 watts" for the AC10 and "15 watts" for the AC15, I never saw a successful salesman that let the truth get in the way of a compelling pitch.

Eventually, the AC15 further differentiated itself by moving to have an actual Vibrato circuit, and a Brilliance switch.  The aC10 seemed to stay with the slightly-hobbled AC15 circuit.

[sluckey]

... Those 100R resistors are odd. I am skeptical that these alone could cause a 5w difference. Also note the AC15 has 100R resistors between [Screen] and the B+ of the EL84 while the AC10 has them between [Plate] and the OT. ... I heard a theory that goes back to the very early AC15 that had 350-0-350 PTs. And that the100r resistors on pin 7 were intended for OT protection because they were pushing the EL84s hard with nearly 400v B+. ...

Anyone have a schematic or an actual vintage AC15 to substantiate the theory?

This is the schematic I used to build my AC-15...

     https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf

And here it is redrawn in my style...

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

[Jalmeida]

... your voltage readings. Were all the tubes in? ...

 

I don’t entirely disagree about the Radio Spares transformer. However Vox’s contractors did also use Radio Spares transformers on some amp so, I just don’t know. Markings are likely under the bell cover. If the above picture is a factory original AC10 factory PT then ...

Jim Elyea's Vox Book (all 682 pages) is probably the most complete & authoritative reference.  He mentions Radio Spares being used for short runs, prototypes, and "oops!" moments.  But Elyea's information suggests they were seen in very early 1960 AC30s, and mostly not seen again.

Instead, the companies that do get mentioned for transformers are Albion, Drake, Haddon, JD, Lemark, Morley, Partridge, Parmeko, and Woden.  Albion, Haddon and Woden would be the most-common, with Parmeko coming in 4th.  Elyea makes special mention that Parmeko transformers were common for AC10s.

... Those 100R resistors are odd. I am skeptical that these alone could cause a 5w difference. Also note the AC15 has 100R resistors between [Screen] and the B+ of the EL84 while the AC10 has them between [Plate] and the OT. ... I heard a theory that goes back to the very early AC15 that had 350-0-350 PTs. And that the100r resistors on pin 7 were intended for OT protection because they were pushing the EL84s hard with nearly 400v B+. ...

Anyone have a schematic or an actual vintage AC15 to substantiate the theory?


The AC10 seems to have been intentionally hobbled to differentiate it from the AC15, by adding that 200Ω resistor before the 1st filter cap, and perhaps with those 100Ω plate resistors as well.  While the ad copy says "10 watts" for the AC10 and "15 watts" for the AC15, I never saw a successful salesman that let the truth get in the way of a compelling pitch.

Eventually, the AC15 further differentiated itself by moving to have an actual Vibrato circuit, and a Brilliance switch.  The aC10 seemed to stay with the slightly-hobbled AC15 circuit.

You very well may be 100% correct in the 200R resistor before the filter caps and those 100R resistors at the plates. Then the question is would the 100R resistors at the plates on the AC10(going to the OT Center tap)have the same effects on wattage as the 100R resistors on the Screen on the AC15(going to the OT primaries)Although the 200R resistors seem to not be consistently there either. Who knows why. But these of these two schematics don’t even agree on the 200R resistor. And even a google search shows some have the resistor some don’t. Of course anyone could remove it too. But all I know if guys I know who know way more than I do know that the AC15 power into the rectifier reads closer to 310-320v range. No clue why. I want to know though. 10-20v into the rectifier makes a bit more post rectifier. And I agree, hands on an original AC15 is the only way to know. Perhaps that is how I convince my wife to let me finally buy a Vintage JMI AC15 Although >$6000 is an expensive 10-20v theory


Now let me butter up my princess for $7000 to buy a JMI era AC15. If I don’t respond in the next 3 days, it’s because she killed me and is digging a shallow grave. 

[PRR]

The AC10 seems to have been intentionally hobbled to differentiate it from the AC15, ...... While the ad copy says "10 watts" for the AC10 and "15 watts" for the AC15, I never saw a successful salesman that let the truth get in the way of a compelling pitch.

My parents bought a Galaxie and a Mustang. With identical 289 2bbl engines. (Later I learned way too much about the 289.) The Galaxie was rated 195HP, the Mustang 200HP. Said so right on the air cleaner lids. (Both numbers were fictional, "Gross HP".)

They just make this stuff up. 

[astronomicum]

But these two schematics don’t even agree on the 200R resistor.

They also don't agree on the chokes with a higher inductance on the schematic missing the resistor which may explain why it was removed.

[Jalmeida]

But these two schematics don’t even agree on the 200R resistor.

They also don't agree on the chokes with a higher inductance on the schematic missing the resistor which may explain why it was removed.

Good point. Not sure.

[Jalmeida]

So, a bit more digging and research into this as I am putting together a layout diagram for the JMI period AC10. I have a acquaintance who dogs rather deep into these Vintage Vox amps and shared some good info that I wanted to share.

The AC10 OT is basically a budget version of the AC15 OT. 1” lamination stack versus the 1.5” on the AC15.

The AC10 PT initially until somewhere in the mid sixties had the 300-0-300 transformer like the AC15, but again with a shorter lamination stack by ~1/2”. Somewhere in the mid 60s the PT dropped to 285-0-285.

Also, the chokes varied somewhat from 5hy and beyond. But were 100Ω DCR.

So no real earth shaking epiphanies or data.

But with the smaller transformer lamination it is quite possible that the clean headroom between the two would be different. Maybe not too though. Or maybe indistinguishable. Also, the smaller somewhat fragile speakers and the 220k input resistors along with the resistors on the power supply and the EL84 very well does appear they tried to throttle back the AC10 to give way for the bigger brother.

So what has been said here about the marketing terminology I believe to be part true.

What would be cool would be to get both together and run through similar speaker cabs to compare. That said, once I get the layout together I will share here for any interested.