I'm looking to drive this tank pulled from a 1990s Ampeg with tubes (originally driven by solid state). I don't know the input impedance, but the input DCR is 30 ohms. In the amp I'm modifying, I have a single 12AX7 section and a 6AU6 to play with. A little reading tells me driving the tank from the pentode plate may be a good way to go (then using the 12AX7 triode for gain recovery.) What I don't know is what current/voltage I can/should put into the tank, and if I have to be careful about burning out the input coil. Help appreciated!
Is there an Accutronics-style part number printed anywhere? Should be of the form {4,9}{A-F}{A-C}{1-3}{A-E}1{A-F} (example: 4CB2C1B).
This is a good article on spring reverb, including details about drive and recovery circuits.
This is a good article on spring reverb, including details about drive and recovery circuits.
Nothing visible beyond what's shown. The cardboard is glued on the bottom, so I don't know if there's anything printed inside.
I have read the Elliot Sound page, as well as the Valve Wizard page. I suppose I could rig up a 1 or 2 mA stage with a pot to attenuate the signal into the driving tube, and then adjust until it sounds good. The problem is, I have no idea how quickly I'll get to a point that may burn out the input coil.
I have read the Elliot Sound page, as well as the Valve Wizard page. I suppose I could rig up a 1 or 2 mA stage with a pot to attenuate the signal into the driving tube, and then adjust until it sounds good. The problem is, I have no idea how quickly I'll get to a point that may burn out the input coil.
R12R (built by SLM). I remember that the schematics they provide have no data on the tank. I'll see if I can find it anyway.
Based on your 30 ohm DCR measurement and Table 1 on the ESP page, the nominal input impedance should be around 150-250 ohms at 1kHz. So an RMS current of around 6mA with a peak of ~20mA.
The old Fender 6G15 reverb unit used a 6K6GT driver tube, which appears to be capable of more than 1W in that circuit—way more than the recommended peak input level.
The old Fender 6G15 reverb unit used a 6K6GT driver tube, which appears to be capable of more than 1W in that circuit—way more than the recommended peak input level.
It sounds like a 6AU6 may not be sufficient, then. This amp is a Silvertone 1482 I've converted into a head. I'm guessing the power transformer can't support much more than the original circuit draw. Maybe the better plan is to put in a follower to drive a standalone reverb unit.
I haven't tried running the numbers, but I think a 6AU6 might work OK with a 25K:200R transformer. Not sure where you might find such a thing though. The black panel Fender reverb amps (twin, deluxe, etc) used both sections of a 12AT7 in parallel into a 25K:8R transformer. If you were to get a different reverb tank with 8 ohm input impedance, one of those transformers might work.
I'm not sure I want to cram an extra transformer on this chassis. But a single ended radio output transformer might be close enough.
Just looking at the R12R schematic, the circuit looks like it would want the common 4EB type pan.
Nominal tank impedance is about 10X DCR so here around 300 ohm.
Way too low for any direct plate drive, way too high for classic Fender parallel 12AT7/transformer which expects some 8 ohm impedance drive coil (about 0.8 to 1 ohm DCR) your best bet here is, not surprisingly, to do exact same as Ampeg did: drive it with SS, I guess they used a boosted Op Amp.
But ... but .... I want a TUBE driven reverb!!!
Sorry, not with that tank.
Like the old Proverb says: "When life gives you lemons, you make lemonade" 🙂
Way too low for any direct plate drive, way too high for classic Fender parallel 12AT7/transformer which expects some 8 ohm impedance drive coil (about 0.8 to 1 ohm DCR) your best bet here is, not surprisingly, to do exact same as Ampeg did: drive it with SS, I guess they used a boosted Op Amp.
But ... but .... I want a TUBE driven reverb!!!
Sorry, not with that tank.
Like the old Proverb says: "When life gives you lemons, you make lemonade" 🙂
Well, I would prefer a tube reverb... but the real issue is, I don't want to add the power supply for SS. If I could get away with a source follower I would do that.
How about a cathode follower with some high transconductance valve that can be biased at 10 to 20 mA or so? For example, two ECC88 sections in parallel, or some cheaper RF valve.
That's more than I dare add to the power transformer load. I think my only good options may be a new tank, or an outboard unit with a simple follower driving it.
Then think about an OTL amp, capable of driving headphones, it´s a similar load and power level.
I´d start browsing the headphone Amp section for ideas.
I´d start browsing the headphone Amp section for ideas.