So I have been given free reign to gut a Blues Jr and do whatever I want with it

[Dave H]

I'm playing around with Duncan's tone stack calculator. How do I know what my Zsrc and load are?

Zsrc is about 1k for a cathode follower, 35k for 12AX7 gain stage with 100k plate load and bypassed cathode resistor and about 200k for your EF86 stage.

[mort]

ok so this is what I'm getting on the Duncan simulator.


there are lines for all controls at 1(not zero), 5 and 10.

I see the -20db loss with controls at 5, but I don't see the "x10" (?). Does this look like a healthy configuration?

[dchang0]

For reference, this is the best layout I found for the widely-shared one-tube reverb circuit.

I wish I could remember who made the layout so I could give them proper credit and thanks, but I can't find the bookmark I had for the original thread. It is designed to fit on a "daughter-card" that is easy to splice into any amp.

Anyway, I followed this exact layout for the reverb upgrade for my customer, and he loved/loves it. Very warm and fat (as opposed to my "cold/dry room" one-tube 6BM8 reverb driver design based on Eric Barbour's example of a 6BM8 reverb recovery).

[Dave H]

I see the -20db loss with controls at 5, but I don't see the "x10" (?). Does this look like a healthy configuration?

Sorry about that, it was poor terminology. By "x10" I meant that the signal would be attenuated by a factor of 10 i.e. -20dB.

I see that the mid notch of your tone control has shifted down to 100Hz. Is that OK? It's usually between 500Hz and 1kHz

[mort]

So the reverb tank in this thing is 800 ohm input impedance and all the reverb transformers I'm seeing have 8 ohm secondaries. Is there a simple way to manipulate this to make them work together or do I just need to get an 8 ohm tank? Or since I still don't have a reverb transformer, is there one that's 800 ohm secondary?

[dchang0]

Ah yes, I forgot to mention that I ended up buying a new tank for the tube reverb conversion I did. It was only $15 or $19 for a MOD tank at the time.

[mort]

Yeah, no big deal to buy one. Just didn't want to get one unnecessarily.

[dchang0]

Before you build the reverb circuit, you could always go with a capacitor-coupled reverb driver. This would save you having to buy a reverb transformer and a new reverb tank as well as save space inside the amp.

[mort]

Before you build the reverb circuit, you could always go with a capacitor-coupled reverb driver. This would save you having to buy a reverb transformer and a new reverb tank as well as save space inside the amp.

I'm all ears! Got a link to a drawing?

[mort]

I found this one but it uses a different tube type.

Channel Road Amplification: Transformerless Reverb Driver

[dchang0]

Here's a pretty good analysis:

Doug Circuits

It shows several examples from real-world amp designs.

IIRC, Ampeg and Marshall had capacitor-coupled reverb drivers (Ampeg Gemini).

I also designed and built a 6BM8 one-tube capacitor-coupled reverb. It's pretty powerful and can overdrive the tank. Here is the thread--look at post 55 for the schematic:

http://music-electronics-forum.com/t22458-2/

You should probably change the 470nF coupling cap to something like a 0.022uF. I kinda overkilled it with the 470nF.

[Leo_Gnardo]

I'm all ears! Got a link to a drawing?

Have a look at a schemo for mid60's Ampeg, like a Gemini I or Reverberocket. Instead of driving a transformer, they have a sturdy 5W 10K resistor between paralleled tube plates and hi voltage supply, a 12AU7 or 6CG7/6FQ7, pick the signal off thru a film cap, 0.47 uF in most cases but you can use as little as 0.1 uF. I'd go 600V on that cap. Send that signal to your tank input. Not the most efficient circuit in the world, but it does work and people love the sound of those old Ampeg reverbs. One refinement I'd suggest, put a load resistor say 100K to 470K on the reverb side of that cap so that it doesn't charge up. That will save you 2 things, 1) getting a shock if you touch the plug intended to drive the tank, and B) wrecking the tank transducer if the cap is charged, then you plug it into the tank and it discharges thru the transducer. Ampeg left this out because they assumed the tank would always be plugged in (duh) also it saved a couple pennies during construction, always good, for them.

[mort]

YYou guys think that a single 12ax7 triode will do the work of the whole 6cg7 in this schematic?

[Leo_Gnardo]

YYou guys think that a single 12ax7 triode will do the work of the whole 6cg7 in this schematic?

Like will a Porsche 911 do the job of a Ford 350 pickup truck? Hm lemme think about that...... . . . . . . .

NO.

Ampeg is using one triode for a little gain, the second as a power delivery system. Power, you'll need it. Please consider, if not 6CG7, how about a 12AU7? If you simply sub in a 12AX7 here, you may find yourself subbing that 12AX7 a lot, as it will be way overstressed in this application. Yes I know, Mesa Boogie use an AX to drive their reverb, 400V on the plate OW! - good for them - I don't approve, that's my position & I'm gonna stick to it.

FWIW I have a similar project to yours to complete in March. I'm considering using a JJ ECC832, equiv.to 12DW7/7247. Dual triode, usual footprint. You find them used in Ampeg SVT and some other amps. One triode is like a 12AX7, hi gain low power, the other 12AU7 high power low gain. You may find you do not need the extra gain stage as shown in your Ampeg schemo. Instead of a whole extra gain stage, how about a bypass cap across the cathode resistor in the drive amp? That'll give us some cheap gain, almost for free, bypass cap might cost a couple of dimes . I'm planning to use the hi power triode to drive the tank, and hi gain to recover the signal.

[Justin Thomas]

Not sure of the 70s, but the Reverberocket 2, GU12, Rocket 2, all used 6U10 triple triodes...

Justin