Vox AC10 and AC15 100R EL84 resistors

[Jalmeida]

First off, thanks for all the help with the help chasing down my reverb woes on the AC10 reverb build! After I resolved that I sent several days chasing ground loop issues. Got it 99% where I want it now.

Looking for comparisons and research I see that the AC10 and AC15 output sections are very similar except for the 100R resistors between the OT and the EL84s. The AC15 puts them on the Grids and the AC10 on the plates. Also it appears that the latter AC15 tied the EL84 differently as well. And through all my research I had done in the past about the AC10 I have found someone had mentioned in builds and even modifications on originals to omit the 100R plate resistors. however this would also effect the bias voltages on those tubes as well which should be adjusted on the cathode. I just am trying to wrap my head around the outputs of the AC10 and AC15 and understand what the design intent between the two differences. Perhaps Sluckey or someone else has some info to help educate me and these circuits?

[HotBluePlates]

... I see that the AC10 and AC15 output sections are very similar except for the 100R resistors between the OT and the EL84s. ... I just am trying to wrap my head around the outputs of the AC10 and AC15 and understand what the design intent ...

If you go far enough back, there was an AC15 but no AC10.  The AC15 had tremolo (only) on one channel that also had an EF86 preamp tube.

Later, the AC15 moved the EF86 to the other channel, and received an updated Vibrato circuit that could be crippled in a way to produce Tremolo.  So now the AC15 has a more complex Vib/Trem channel, and a no-effect EF86 channel.

Enter the AC10.  This amp is exactly what the AC15 used to be, except it has one or two 10" speakers.  The AC10 has the old AC15 circuit with tremolo on the EF86 channel, plus a clean/dry channel.  I surmise that Vox needed to differentiate the AC10 & AC15 more than just with the Vibrato & speaker-size, so they mildly knocked-back power output on the AC10 with the 100Ω resistors between plate & OT.

... I have found someone had mentioned in builds and even modifications on originals to omit the 100R plate resistors. however this would also effect the bias voltages on those tubes ...

Don't worry about it.

Screen voltage & Cathode Resistor matter the most for idle bias of that cathode-biased amp.  And you'll find that it also matters whether you have tubes that will even idle in the proper range with your cathode resistor.  That's because cathode bias lets the tubes "find their own idle point" and even reduces variations between tubes somewhat.

Owning a 1965 AC10 Twin, I'd say your biggest consideration oughta be making sure your B+ voltage is not too high.  Shoot for under-300v just to be sure you're not running too hot all the time.

[Jalmeida]

... I see that the AC10 and AC15 output sections are very similar except for the 100R resistors between the OT and the EL84s. ... I just am trying to wrap my head around the outputs of the AC10 and AC15 and understand what the design intent ...

If you go far enough back, there was an AC15 but no AC10.  The AC15 had tremolo (only) on one channel that also had an EF86 preamp tube.

Later, the AC15 moved the EF86 to the other channel, and received an updated Vibrato circuit that could be crippled in a way to produce Tremolo.  So now the AC15 has a more complex Vib/Trem channel, and a no-effect EF86 channel.

Enter the AC10.  This amp is exactly what the AC15 used to be, except it has one or two 10" speakers.  The AC10 has the old AC15 circuit with tremolo on the EF86 channel, plus a clean/dry channel.  I surmise that Vox needed to differentiate the AC10 & AC15 more than just with the Vibrato & speaker-size, so they mildly knocked-back power output on the AC10 with the 100Ω resistors between plate & OT.

... I have found someone had mentioned in builds and even modifications on originals to omit the 100R plate resistors. however this would also effect the bias voltages on those tubes ...

Don't worry about it.

Screen voltage & Cathode Resistor matter the most for idle bias of that cathode-biased amp.  And you'll find that it also matters whether you have tubes that will even idle in the proper range with your cathode resistor.  That's because cathode bias lets the tubes "find their own idle point" and even reduces variations between tubes somewhat.

Owning a 1965 AC10 Twin, I'd say your biggest consideration oughta be making sure your B+ voltage is not too high.  Shoot for under-300v just to be sure you're not running too hot all the time.

Thanks for the info. Yeah, researching I did note the stark resemblance between the earlier 1959 AC15 circuits and the AC10. It seemed the early amps were very much more of a scaled derivative than latter models.

What are the differences and effects of changing the OT primary connection to the Plate versus the Grid?

As far as bias, I omitted the 100R resistor and have 7vdc bias across the cathode resistor. I have some more resistor values coming so I can get the bias to the schematic spec of 12VDC.
And it seems the AC15 also calls for -12vdc Cathode bias voltage.

[sluckey]

As far as bias, I omitted the 100R resistor and have -7vdc bias across the cathode resistor. I have some more resistor values coming so I can get the bias to the schematic spec of -12VDC.
And it seems the AC15 also calls for -12vdc Cathode bias voltage.

Where do you see any negative voltages on either of those schematics?

[Jalmeida]

As far as bias, I omitted the 100R resistor and have -7vdc bias across the cathode resistor. I have some more resistor values coming so I can get the bias to the schematic spec of -12VDC.
And it seems the AC15 also calls for -12vdc Cathode bias voltage.

Where do you see any negative voltages on either of those schematics?

You are correct, nowhere does it say negative voltage. I am ringing across the cathode resistor ringing the drop across it to ground.

[tubeswell]

As far as bias, I omitted the 100R resistor and have -7vdc bias across the cathode resistor. I have some more resistor values coming so I can get the bias to the schematic spec of -12VDC.
And it seems the AC15 also calls for -12vdc Cathode bias voltage.

Where do you see any negative voltages on either of those schematics?

You are correct, nowhere does it say negative voltage. I am ringing across the cathode resistor ringing the drop across it to ground.

Swap your V-meter probes around

[Jalmeida]

As far as bias, I omitted the 100R resistor and have -7vdc bias across the cathode resistor. I have some more resistor values coming so I can get the bias to the schematic spec of -12VDC.
And it seems the AC15 also calls for -12vdc Cathode bias voltage.

Where do you see any negative voltages on either of those schematics?

You are correct, nowhere does it say negative voltage. I am ringing across the cathode resistor ringing the drop across it to ground.

Swap your V-meter probes around

It is definitely +7vdc drop across the 130R resistor. I was incorrect stating it as negative.

[HotBluePlates]

It is definitely +7vdc drop across the 130R resistor.

If you have measured the resistance of that resistor (with the amp off) and get 130Ω (more or less), then the tubes likely just idle cold and/or are very responsive to voltage-input.

I might leave it as-is, or try a different pair of EL84s.

[Jalmeida]

Mystery solved. Shortly after I went and ordered an assortment of resistors to correct, my meter battery died completely. I replaced my battery and rechecked and now I have 10.5v across that resistor🙄🤦🏻‍♂️🤷🏻‍♂️

But in the midst of reading about this, I spoke with an expert who explained that the early AC15 actually had 350V-0-350v CT transformers. It is thought that these were transformers they had laying around from an earlier organ and used up until about 1961. And despite the fact that the schematics for the AC15 show 300V-0-300v, that they were not necessarily 300V. Some were 315V-0-315. So the 100R resistors are to have been to protect the EL84 seeing they were way over on B+ voltage. And the AC10 may have used the same higher voltage transformers.

Here is a good explanation on the AC15

https://www.voxac30.org.uk/vox_ac15_first_circuit.html

[HotBluePlates]

... I spoke with an expert who explained that the early AC15 actually had 350V-0-350v CT transformers. ... And despite the fact that the schematics for the AC15 show 300V-0-300v, that they were not necessarily 300V. Some were 315V-0-315. ...

There are Vox variations not documented in a schematic it seems.

Take for instance the early AC30 amps that used a pair of EL34s rather than four EL84s.  I think I've only seen that in Jim Elyea's book, if you get the version with the Schematics Folio.

Perhaps that early AC15 is the same way.