Preamp stage help please.

[Stratz]

Hi all. I've built or converted about 20 or so amps now and each one has basically had 2 or 2 1/2 gain stages. Mostly 2 gain stages with a blackface Fender style tone stack. I just started converting an old 1950's Bogen CHA-33 PA amp (schematic attached) to a guitar amp which has one 12AX7 (V1) one 6AV6 (V2) one 12AX7 (V3) with two 6L6's and two 5Y3 rectifiers.

So far I've stripped out the tone stack and installed a 1 meg Vol pot, 250k treble and bass pots ala Fender. I also bumped up the preamp voltage to approx 190-200 volts on the 12AX7 plates/140 on the 6AV6 plate and that's it.

I'd like to have 2 channels or at least 2 inputs and I'd like to put the tone stack either between the two triodes of V1 (12AX7) or after V1 and before the 6AV6 (V2) but I'd like to have one input run through 2 gain stages and the other input run through 2 1/2 or 3 gain stages (depending on what style phase inverter I use and if it takes 1/2 of a 12AXY or both triodes) but I dont know how to wire it up.

There are 2 schematics attached since one may be easier to read than the other. They're both the same but I imagine that they were drawn at different times.

I'd appreciate any help.
Thanks.
Rob.

[Leo_Gnardo]

Inquiring minds want to know: what do you mean by 1/2 a gain stage???

Either you pass signal thru A stage, or you don't. It's sort of ummm, quantum. One or zero. Half isn't a choice. Well as far as I know, maybe we'll learn something from your answer. Thanx in advance!

Sign me, puzzled.

[Justin Thomas]

Did you only want one tone stack but two channels?

So how bout one-half AX7 each per channel (voiced as you like), then mix the channels like the original Bogen, put the tone stack after the channels are summed. Then use the AV6 as a second shared gain stage, like the one in an AB165 Bassman, then the standard "booster" to push a Cathodyne PI driving the 6L6s? I tried something similar in an old Thor head, but wit one channel. It was Friggin Amazeballs as a rock machine. Might be a little over the top, though I only had a single Tone knob & Volume in stead of a more lossy Tone "Stack" a la Fender...

Basically each channel would have 3 gain stages, two of them shared... the main issue is, two gain stages & a Cathodyne without its driver may not be enough to push 6L6s, especially with a Fender stack in it... So figure either Cathodyne OR LTPI would effectively need two triodes to drive those big bottles, leaving 3 triode for two channels. The small Tweed Fenders are the most "triode-efficient" ones I know of.

Justin

Edit: you could also put the tone stack after the 6AV6, right before the PI "driver" stage...

[Stratz]

Thanks Justin. A single tone stack is fine but I'd like one channel/input to have more gain than the other. I can also add a switch for the cathode bypass caps/NFB. Something like that but I need help wiring it up.

How do I pass from one stage to the next but add another input at the same time? Thats where I start scratching my head.
Thank you.

Hi Leo. Some tubes have a single tube in the bottle and other tubes like a 12A*7 or 6EU7 have dual triodes which are basically 2 tubes in the same bottle so each half of the tube is considered 1/2 of a gain stage when placed in a preamp circuit.
Hope that helps you some.
Rob.

[Justin Thomas]

Hey Rob,

Each twin triode is capable of being 2 gain stages. Each triode section is A gain stage, potentially. So if you use a whole 12AX7 in one channel like a Fender Normal channel, that is two gain stages. The Reverb channel would have 3. You almost have to think of an AX7 as 2 completely separate tubes, as that is indeed what they are. You could wire up only one triode in the tube (like a Fender Bassman head) and it is still considered one gain stage, not half.

As far as how to wire the channels, see how the Bogen's Phono input bypasses the first tube & feeds the AV6 directly? You could do something like that... the main reason I was suggesting the Tweed way was because, for all intents and purposes, you have three gain stages to be used among two channels, after you eliminate one tube for the PI. Otherwise, you could use a single triode stage and bypass the tone stack, feeding it straight into the PI Driver... basically, move the wire from the 220K (R11) feeding into the AV6 and move it to feed V3A instead... that way, one channel has a straight Fender Normal T/B/Vkind of layout, and the other is like a 5F10 Harvard (I think) with just a Volume...

Just thinking out loud...

Justin

[Enzo]

yep, a stage is a stage, it doesn't matter how many of them are in a particular tube. A 6C10 has three triodes, compared to 12AX7 with two. It may sound trivial, but it matters when communicating that we all use the same terms. A two channel preamp might have two 12AX7s. I could wire them up as one 12AX7 serving two stages in one channel. Or I could wire them up as one side of each tube was one channel, and the remaining sides were the second channel.

I think most everyone knows what a gain stage is, though many people use it to refer to any triode stage. To me a gain stage has gain, a cathode follower stage doesn't really have gain, so I don't use the term gain stage for those. Or at least I try not to.

And one final nit. V numbers for tubes - V1, V2, V3, etc - refer to the schematic, not the chassis. In many cases, even most cases, the V numbers on the schematic are the same as the tube order across the chassis. But not always. Myself, I'd organize my drawings so the numbers agreed with tube position. SO if the schematic calls a tube V4, do not assume it is the fourth tube over until you determine if that is true. Look at the PV 5150/6505 amps as examples.

I'll yak about my other thing - misuse of "comprise" - somewhere else.

[Stratz]

As far as how to wire the channels, see how the Bogen's Phono input bypasses the first tube & feeds the AV6 directly? You could do something like that... the main reason I was suggesting the Tweed way was because, for all intents and purposes, you have three gain stages to be used among two channels, after you eliminate one tube for the PI.
Justin

Actually I'll have 4 gain stages total if I us a a split load/cathodyne PI. I can run input 2 just like the phono input was wired on the schematic and it still goes through the tone stack. So, I'd have 2 gain stages on one channel and up to 4 gain stages on the other input. 4 would be a bit much but 3 would probably work ok with a blackface Vol/Treble/Bass tone stack.
Do you see any issues with Justin?

Thanks!

[Stratz]

And one final nit. V numbers for tubes - V1, V2, V3, etc - refer to the schematic, not the chassis.

The Bogen CHA33 I'm working on right now is a perfect example Enzo. The 6AV6 is first valve in line then come the two 12AX7's.

It can get confusing when all of the valves are the same. Thats when I have to get the eraser (solder sucker) out

[Stratz]

Another question if you guys dont mind. I cant tell from the drawing if the plate of the first triode is going to the grid of the second triode of the PI or not.
Is it a Paraphase or chathodyne PI????
Thanks

[Justin Thomas]

In your channel with only 2 gain stages, a circuit of Stage 1 => Tone Stack&/or Volume => Stage 2 => Cathodyne => 6L6s May not be enough to drive the output tubes to your satisfaction... if you were driving EL84s or 6V6s, maybe. It's n or that it wouldn't "work," I just wonder if the results would be disappointing in that channel. You really lose a lot in that tone stack, compared to if you used a single Tone control like the Harvard. I may be entirely wrong - I'm basing my experience on my one big-bottle build...

Justin

PS it's not a Cathodyne...

[Justin Thomas]

@ Enzo:

A 12AX7 comprises 2 separate triodes?

Justin

I had to look it up. Though I didn't see where it was used in the thread?

[Enzo]

Yes, thank you for understanding. I even have seen "comprised of" in such publications as Scientific American and National Geographic, though not often.

[Chuck H]

Justin has a good idea, but there's a potential rub (no offense brother). The block diagram below outlines something like the idea. As you'll see there is no good way to incorporate fully independent volume control. In fact there's only one way and that would be to place the volume control for the lower gain channel between the input jack and that channels first gain stage. This has historically proven to be tonally undesirable. It also has the drawback of needing a large-ish value "mix" resistor after the higher gain channel volume to avoid control interaction and that would attenuate the higher gain channel considerably. The idea also has the drawback of floating the higher gain channel input stage when the lower gain channel is selected. Since you'll need to keep the second gain stage in use for the higher gain channel you can't use a grounding switch scheme. That will mean some extra high gain hiss riding on the lower gain channel when it's selected. That said...

If independent volume control isn't critical then you could put a second volume control at any of the other locations and it should be fine. Just a little noisier than necessary and without independent channel control for the higher gain channel. Mesa actually did some similar designs and people liked them just fine.

I came up with an idea some time ago and this seems like a good application for it. It also doesn't give you independent control, but it offers less noise, simpler switching and one less jack to wire. See the schematic below.

You would use all three available triodes (the concertina PI isn't a gain stage) for both the low and high gain "channels". Using this circuit on the first gain stage will provide more gain through the overall circuit. Using it on the second stage should still provide plenty of gain and would allow you to fully bypass the first gain stage. Which IME makes for lower hum noise. The 10k fixed cathode resistance should keep things pretty clean for the lower gain channel. The circuit provides a 17dB boost down to the knee of the bypass cap. Because this is a boost you probably wouldn't want to fully bypass the switched cathode or the clipped tone could have too much LF. I indicated probable useful values to try in the schem. The 25k pot would be on the control panel as a boost level control with a range from maybe 3dB up to the full 17dB. The impedance of the cathode circuit should be low enough to avoid any trouble with noise or capacitance in the footswitch cable. Sort of like channel switching without the logic devices or relays

[Justin Thomas]

Chuck,

No offense taken here! Ain't the first time I learnt from you, prolly won't be the last!

Justin

[Chuck H]

Chuck,

No offense taken here! Ain't the first time I learnt from you, prolly won't be the last!

Justin

I've learned plenty from everyone here. Including you. And anyway, Mesa was selling amps with designs like your idea when we still had long hair