Which Fender circuit for Chimey Belltones?

[JazzGuitarGimp]

Hi All,

I am about to start my first build since I joined TAG. I have a friend whom I've done a few builds with in the last year, but this one is for me. It started out as a Weber kit for a 4x6V6 Deluxe Reverb. I have sourced trannies from Mercury Magnetics, and I've had all the parts sitting here for the better part of a year. During that time, I've been mulling around about what feature set I want, etc., and I think I've finally decided what this amp wants to be when it grows up.

The output section will be a quad of 6V6GT's in push pull, parallel configuration which will be pushed by a 12AX7 LTP PI followed by a 12AU7 CF buffer, direct coupled to the 6V6 grids. I am going to try and squeeze 60W out of it, running in class AB2. It will have defeatable GNFB, as well as a Lar/Mar MV between the PI and CF buffers.

By the way, this is about the fifth iteration of the design, and it literally has been all over the map. The last iteration had harmonic tremolo and spring reverb, which have been replaced by a buffered EFX Loop in this final iteration.

I got to thinking about two complete preamps. I should note that for the OD, I am roughly shooting for the early Robben Ford Dumble tone. So I am thinking about a two-triode preamp for the clean channel, and a four triode #102-inspired preamp for the OD channel. I'll use a few extra relays so that I can have A/B Y switching, giving me the option of a layered Clean and OD sound.

I am also going to build-in an active EFX Loop, patterned after the Dumblator.

I will be doing a build post here, with schematic and pictures.

Question:

I know that some of the Blackface era Fenders are capable of the squeaky-clean, chimey bell tone sound. Which circuit/model is best for this sound? Also, should I be looking at something Fender built before the Blackface era? Is there a Tweed, Brown, or Blond that does chimey bell tones better than any of the Blackface amps?

Many thanks, and thanks to all of you for making this forum 'the place to be',
Lou

[Cantplay]

When I think chime, the AC30 comes to mind more than any Fender.

John

[xtian]

a 12AX7 LTP PI followed by a 12AU7 CF buffer, direct coupled to the 6V6 grids

I've never seen a PI followed by another stage before the power section. Is that what you meant to say? Is there a precedent?

[JazzGuitarGimp]

When I think chime, the AC30 comes to mind more than any Fender.

John

Thanks, John.... But good grief, which one?! I just looked for the schematic, and found eleven variations of the AC30 - and that was on just ONE webpage!

Are you thinking of a top-boost, or a non-top-boost model? Any particular year?

Many thanks,
Lou

[JazzGuitarGimp]

a 12AX7 LTP PI followed by a 12AU7 CF buffer, direct coupled to the 6V6 grids

I've never seen a PI followed by another stage before the power section. Is that what you meant to say? Is there a precedent?

Hi Xtain,

Yes, that is what I meant to say. And yes, there are precedents of this circuit topology out there, though I cannot name an example (I don't know of one). But I believe this is used more frequently is bass amps as well as hifi amps. Direct-coupling a cathode follower yields a much lower-impedance connection to the power tube grids, making it easier to drive them into positive grid conduction.

I've attached a pdf of my preliminary power amp circuit. This should be considered a rough draft, and likely contains erroneous component values that will be determined during assembly. Also, note I am using a 50 / 50 bias strategy, where 10V of the bias voltage is derived from cathode-bias, and the other 10V of the bias voltage is derived from fixed-bias.

Regards,
Lou

[martin manning]

Ampeg's SVT has a common-cathode gain stage and a cathode follower on each of the PI's outputs, with the CF's being DC coupled to the power tube grids.

[Cantplay]

Maybe someone more technical can explain what causes chime.

I hear it much more on EL84 amps than any other tube.

I also hear it more in alnico speakers.

John

[JazzGuitarGimp]

Ah, okay. So it's more of an output tube thing than a preamp thing. Thanks, John...

[LeftyStrat]

Lou, I'm in the design stages of a multi-channel preamp. One thing to consider is that the Dumble classic preamp is identical to the Fender Blackface circuit except for the tone stack. So with some creative switching, you can get Fender BF, Dumble Clean, and Dumble ODS.

So:

- 12AX7, 100k plate, 1.5k cathode, bypass cap to taste.
- Relay into either
-- Dumble tone stack or
-- BF tone stack
- 12AX7, 100k plate, 1.5 cathode, bypass cap to taste
- Second half of relay.
- ODS

The Arduino is a nice and cheap microprocessor that can handle the switching duties. If you don't code I could whip up the programing in short order and share it with TAG.

Relays would have to be 5 volts, and you'd need something between 9 and 5 volts for the Arduino.

As far as chime, certainly output tubes play a role. But I think lowered filtering and sag also plays a role. I think of chime as a glassy animated treble. IMHO the Vox AC10 is way up there, so you might want to look at that circuit.

Here's a vid from a TAG member (asatplayer), playing an original AC10:

http://www.youtube.com/watch?v=Xy8wffW-m-A

[Cliff Schecht]

When I think of clean Fender chimy tones, I think of the 10" speaker, 6L6 powered brownface era amps. The magic in these is a combination of the interesting tapped treble pot, the PI setup, the plate voltages throughout the amp and the speakers.

If you really want to open up the high end on any amp, reduce/remove the negative feedback around the output stage. This will allow for more power amp gain but also tends to give a bump to the overall bandwidth as well. What you are looking for is high end harmonic content and this will give it to you easily. That's what makes the AC30/Rocket style circuits stand out so much, they lack NFB which gives them high end sparkle.

Not to be rude, but if you have gone through five iterations of design without building anything then you are sort of shooting in the dark. The post PI cathode followers sound sort of ridiculous, any non AX7 driver tube will be able to handle the grid current drawn by the 6V6's and so you aren't really gaining much there other than complexity. The only time I consider doing this is when I have an AX7 driving a quad of KT88 or larger output tubes. As far as the other features in your amp, well that's all up to you, but I think a post-PI CF is superfluous.[/list]

[10thTx]

There is an interesting thread on the Hoffman forum about the post PI cathode follower:

http://www.el34world.com/Forum/index.php?topic=15679.0

Check out reply #10 from HotBluePlates.

With respect, 10thtx

[martin manning]

There is an interesting thread on the Hoffman forum about the post PI cathode follower: http://www.el34world.com/Forum/index.php?topic=15679.0
Check out reply #10 from HotBluePlates.

Good discussion on the gain and voltage, but as important is the low-impedance source driving the output tubes, particularly in the case of large bass amps (like the SVT).

[chief mushroom cloud]

When I think of clean Fender chimy tones, I think of the 10" speaker, 6L6 powered brownface era amps. The magic in these is a combination of the interesting tapped treble pot, the PI setup, the plate voltages throughout the amp and the speakers.

If you really want to open up the high end on any amp, reduce/remove the negative feedback around the output stage. This will allow for more power amp gain but also tends to give a bump to the overall bandwidth as well. What you are looking for is high end harmonic content and this will give it to you easily. That's what makes the AC30/Rocket style circuits stand out so much, they lack NFB which gives them high end sparkle.

+1
BF Vibrolux w/ NFB removed thru Alnicos......

[JazzGuitarGimp]

There is an interesting thread on the Hoffman forum about the post PI cathode follower:

http://www.el34world.com/Forum/index.php?topic=15679.0

Check out reply #10 from HotBluePlates.

With respect, 10thtx

Response #12 is of interest too. I will admit that this is as much a learning experience for me as it is anything else.

As I understand it, the usual LTP PI doesn't have the ability to push the power tube grids higher than the power tube cathode voltage. It runs out of steam because when the grid is pressed beyond the cathode, it draws excessive current. Here's my thinking; and I speak theoretically for now - once I am done with this build, I hope to know the ins and outs of class AB2 but for now it's all conjecture: Let's say I have a quad of 6V6GC's, pushed by the usual LPT PI with 380V HT. And let's say that at full power, the 6V6 grids swing down to 60V.

Let's also say, for sake of discussion, the 6V6 has an amplification factor of 10. If I can push the grids to 3V more positive than the cathode voltage without distortion by using the cathode followers, I'll get 30V more swing on the output. Now the plates are pulling down to 30V, rather than 60V. Let's say the OT Zpri is 4K2. Here are the calculations for clean output power with and without the cathode follower:

Without the follower:

[(380 - 60)^2] * 2 / 4,200 = 48.76 Watts

With the follower:

[(380-30)^2] * 2 / 4,200 = 58.33 Watts

At this point, I have no idea how much current the grids will pull when pushed to +3V with respect to the cathodes, so I don't know if I will be able to achieve my goal. I know the 12AU7 has a max Ip of 22mA, and I am hoping that will be enough to push two 6V6 grids to +3V. I am looking forward to having a better understanding of this approach when I get through to the other side.

Cheers,
Lou

[JazzGuitarGimp]

And by the way, I am enjoying this discussion and I thank every one of you for your participation. This is going to be a fun build.