AC30 cathode follower Q

[Chuck H]

The design I'm working on is going to have an AC30 type preamp as a switch option. I've never owned one of these or had one in my hands to test. Looking at the schem I see that the cathode follower uses a 56k resistor with no external bias supply. The plate of the supporting triode (and therefor the grid bias for the cathode follwer triode) uses a 100k plate load. In a "Marshall" type amp (a circuit I'm much more familiar with) the supporting triode plate and the cathode follower cathode R are the same and this arrangement does achieve an acceptible bias. The "Vox" cathode follower doesn't. The lower Rk on the cathode follower holds the cathode at a lower relative potential to it's grid (from the supporting triodes 100k plate R). My Q is... Is there some special mojo, unique distortion, compression, etc. that the Vox circuit may be imparting that I need to be sure to capture if I want my amp to sound "right". Notice on the schem that the cathode and grid are indicated at the same voltage!?! So, no bias for the Vox cathode follower??? I ask because my circuit will probably be self biased to make it switchable.

[exclamationmark]

Usually with 100k plate/100k cathode circuits you get a little bit of a compression. However, with a 56k cathode resistor you would get significant compression. This bias point is also set a little bit cooler than most marshalls though, which increases the quiescent grid current draw! Also, those are some looooong resistors in the schematic!

The vox designers were probably lazy and decided the output of the cathode follower would be "close enough" to the input voltage of 180, and in reality this is probably true, with only 1-3 volts difference. However, in real life, there would probably be a difference of something like 10 or more volts, due to the grid current flowing through cathode follower and increasing the voltage across the cathode resistor. Or maybe they were just confused about what was happening and decided to omit the proper voltage...

All direct coupled cathode followers self-bias. In fact the reason for their compressive qualities is the fact that they self bias. I suggest you read The Valve Wizard

[Chuck H]

Yes... I read Merlin's site on cathode followers. But it doesn't cover the typical not self contained bias circuit like the direct coupled Marshall and Vox circuits. In the amp I'm working on I needed a way to drive or recover the reverb with one triode. I employed a 12au7 triode as the reverb driver and thought to use the other half as the AC30 preamp type cathode follower. I'm still working out the particulars of the power supply. But I'm trying to figure out if using a triode other than that of the same tube (like a typical CF in guitar amps) will require special consideration to get similar performance. For better or worse, as it were.

[clyde1]

Hoffman has a circuit that uses a 12AX7 for reverb drive and recovery. You won't channel Dick Dale but I've used it several times with excellent results.

[Tage]

Hey chuck in Merlins book he goes over this, the lower cathode resister on the dc coupled cathode follower does increase compression. There is some stuff about it stealing current from the gain stage and that causing the compression, the jist of it though is that the lower cathode resistor causes a heavily asymmetrical waveform, generating more comression and sustain.

The oscilloscope clips on his website are the same as in the book, although in the book, these refer to the Marshall/Vox style DC couple circuit. I have done a few marshall style amps and used the 56k cathode resistor on the cathode follower and always liked the results although I never A/B against a 100k.

JM2C, Tage

[Chuck H]

I'm also wondering if I may lose anything by not direct coupling. As in the gain theft/compression mentioned above. Since it'll be a 12ax7 triode with a 100k plate and a 12au7 triode with a, what, 22k cathode? roughly? I can't imagine I'll be able to direct couple for bias. I've looked into using a 12dw7 with the AU triode as the reverb driver and the AX triode as the cathode follower. But JJ is the only current MFG of 12dw7's. And appearently JJ preamp tubes like to fail when used as cathode followers. Part of my criteria is tube availability.

Thank you for all the replies so far. I do need to get this out of the design phase and start building very soon.

So... Any experience, theories, opinions on what I may lose by not direct coupling???

[Tage]

Well if it's the AC30 sound your going for then I think some of it's sound will be lost without the DC coupled follower. As far as I can tell you would only lose the compression added by the follower, and an AC30 style output stage will be plenty squishy as regardless. But I'm sure the amp would sound excellent with the AU triode as the follower, it may even sound better? but we will never know if you don't build it.

[Chuck H]

Well I'll be using a pair of big bottles for the power amp. This amp will be used for clean-ish tones. But my guy does like the AC30 jangle. He also wants a BF type amp option. o he challenged me to make an amp that could morph from Fender to Vox on a switch... Add trem and reverb!!! He also want's (needs actually) a simple control panel. I'm enjoying the project so far. I've tried to pay attention to certain criteria that are characteristic of each type of amp. My personal criteria also come into play. I don't like the idea of building two complete preamps to switch between, add reverb and trem and end up with seven freaking preamp tubes with one triode unused for what amounts to a single channel amp that doesn't even have an effects loop. I don't know why, but an usused triode bugs me. So does having most of a preamp sitting around unused when the other "channel" is in use. So I do have the preamp triodes serving double duty other than the cathode follower. I have this design down to five preamp tubes. Which I think has to be the minimum for this amp with an all tube design. But my next move might be to add a tube, run the reverb on two triodes and dedicate a tube for the direct coupled cathode follower and do away with the 12au7. But I REALLY like the design I have now and I also like that it can be done with five 12xx7 tubes instead of six. For what amounts to a simple amp (for the user) the lower tube count has a certain aesthetic.

[jimboyogi]

I guess that there are numerous differences between a Fender and Vox TB amp, but for the sake of debate I'll reduce these to 2 key differences.

1 - extra asymmetric compression and increased 2nd harmonic distortion in the Vox CF, and

2 - no global NFB in the Vox

If you used the same channel for both, but for 'fender' used standard (1.5K Rk) biasing on V1b for lower 2nd HD, but for 'Vox' switched in a higher value Rk here for colder biasing and more 2nd HD, as well as removing the NFB. If you juggled around the values of Rk and the NFB ratio, you might be able to equal the total gains so that the volume stayed the same when switching?

[Chuck H]

Thanks. I actually burned some oil last night and I'm pretty happy with the new schem. In looking at the BF type reverb (bench mark?) there is considerable loading and voltage division. I can actually use the other half of the UA7 as the mixer and still get 90% of the AC reverb at the PI input as the BF type circuit. So now I can dedicate an AX7 triode for the Vox mode cathode follower and direct couple.

FWIW the things I'm switching between the BF and TB modes are the tonestack location, tonestack values, cathode follower, nfb and the high cut circuit. The high cut circuit will actually be fixed rather than variable and I'll tune it by ear.

[Wombaticus]

Thanks. I actually burned some oil last night and I'm pretty happy with the new schem. In looking at the BF type reverb (bench mark?) there is considerable loading and voltage division. I can actually use the other half of the UA7 as the mixer and still get 90% of the AC reverb at the PI input as the BF type circuit. So now I can dedicate an AX7 triode for the Vox mode cathode follower and direct couple.

FWIW the things I'm switching between the BF and TB modes are the tonestack location, tonestack values, cathode follower, nfb and the high cut circuit. The high cut circuit will actually be fixed rather than variable and I'll tune it by ear.

There's actually no particular problem direct coupling a 12AU7 cathode follower to a 12AX7 gain stage, I've done it and it works just fine. Just use a smallish Rk for the CF (22k isn't far off the mark). If you want it to draw grid current you'll probably find however that you'll need to bias the preceding stage pretty cold.
On the other hand, I've also used the JJ ECC832 quite a bit, and would not hesitate in the slightest to use the 12AX7 half in a CF arrangement - especially if you're elevating the heater voltage.

[soundmasterg]

I did an AC100 build, where I also added my own higher gain preamp channel. I used JJ 12DW7's in quite a few places in the amp, though I don't recall at the moment if I used any AU7 halves for the CF. Actually I'm pretty sure I used the AX7 half for the CF. In my own channel I used an EH 12AY7 for the CF in that channel. All of them work just fine, though I did elevate the AC filaments to around 60v too.

I stayed with the 56k cathode resistor ala Vox and it works fine. In my high gain channel, I used a 75k cathode resistor on the CF and that works fine too. The Vox circuit with EL34's sounds better IMHO than the Vox circuit with EL84's. The floating paraphase PI sounds a bit different than the LTP used in the AC30, but it still sounds good. YMMV.

Greg

Greg

[yunger]

i wonder(out of pure speculation) if could you use the 12au7 triode as an anode follower/3rd stage behind a typical fender 2 stage? then use some fancy switching to activate the au7 and move the tone stack behind it.
i only say this because i recently built a little amp with a 12au7 as a se output tube and it really seems to compress the more i drive it. the sound is really good. it should drive a tone stack better than a plate driven 12ax7 triode too.
sorry if this is way off. i'm not a pro. just speaking in the spirit of fun.

[Chuck H]

It's a great idea... For an original design. Build it and see!

The amp I'm working on is to be used for predominantly clean to clean-ish tones and should replicate a BF or TB type amp. I chose a pair of el34's at high-ish voltage (worked for Dumble for "better than BF) and works for getting a "Brittish" tone when no NFB is employed. And should produce about sixty clean watts. Which, if there is occasional need for some grit, is about the right amount of power for the medium size stages my friend commonly plays. I would say friend/customer. But this guy IS a friend and I've already built a couple of amps for him on contract. This one is going to be a "thank you" parts cost only project. But the idea and the eventual design is very gratifying. It should be able to go from Dick Dale to "Jingle Bell Rock" to Beatles or Tom Petty (Mike Campbell) with the flip of a switch! I tease this guy that he has better amps than I do, and I built them all.!. I'm definitely building one of these for myself since I have most of the spare parts. My amp will be using some SS supporting circuitry though, to save on space and power supply requirements.

[Gingertube]

Chuck,
I would not expect the same sonic results from using a 12AU7 for the cathode follower.
In the part schematic you posted you can see that the 12AX7 cathode follower is running with zero bias (180V on grid and cathode). 180V across 56K means that the tube is running 3.2mA, that is a hell of a lot for a 12AX7 and to get that current the bias is down to zero. I expect it is drawing a fair amount of grid current from the previous stage and I also expect it to simply not respond at all to positive signal swings on the grid, that is it will pretty much only reproduce the negative going half cycle as it is already saturated. I can see that I'm going to have to dig out the Wiz'es book and have a look a what he says. When ever I see a 12Ax7 running at 3mA I immediately think that its not good design and there should be a better way to achieve the same thing - sort of like a friend of mine with his classic car restorations, when he reaches for the hammer I always say "hang on , there has to be a better way" - 90% of the time I'm right.
Cheers,
Ian