please help - building a tube driven reverb

[Godwin]

let me start by saying I know nothing, so any help or guidance would be great. sorry about the length of this, I got a bit carried away.

oh, and sorry if this is in the wrong place.

I want to build a stand alone valve reverb unit, as an introduction to this stuff, and I have about 3 billion questions, which I can't seem to work out from the internet.

first off, I've got the tank, Accutronics 8BB2C1B, whos website states input and output impedances of 190, and 2575 ohms respectively though both rca plugs read at about 200 ohms, is it likely to be broken or am i missing something? will the output impedance change to the proper value when signal is put through the tank?

next, valves. I've got an ecc83, ecc85, ef80, ech81, and ebf89. all mullard shield yellow print, pinched from an early 60's am/fm tuner. they all look to be in good condition, especially the 85, 81, and 88.

since I'm on a really tight budget, I would like to use these, and replace them when i know the project is a good'un. specifically the ecc85.

so with that in mind, the ecc85 is is a dual triode, which means i could use one half as the driver, and the other half as the receiver

power supply.
I know I need 6.3v from the transformer for the filament, at 435ma, and then i need a higher DC voltage for the plate. assuming I use a cathode resistor to ground, do i need any other taps from the transformer? is there any benefit to rectifying the filament supply? if there is a benefit, do i need to look for a transformer with a 9.1 tap to compensate for the voltage drop across the diodes?

input stage.
I want to be able to run this unit either in an FXloop, or directly from a guitar, but if i have to choose, I'll choose the loop. do i need an input coupling capacitor? some schematics I've seen don't use them. can I just go for a high input impedance, and let that suffice for both guitar and FXsend?

I've read that a 1meg grid resistor is usual, will this translate to 1meg input impedance, and be suitable for what i want?

the datasheets seem to suggest that the plate resistor should be 1.8kohms, for the most linear operation which i think is what i want. this is where i start to get lost...

do i need to match the output of the first stage to the input of the tank (190ohms)? how can i do that when the 1.8kohm plate resistor gives the best linearity, and the output impedance is controlled by the plate resistor and internal plate resistance?

-------------------

I think i should probably stop there, because I've already asked to many questions, and I could write a thesis on "the things I do not know about what I'm trying to do"

as I said, any help/guidance, is really appreciated, thanks for taking the time to read this.

Harry

[Enzo]

Are you sure that 1.8k resistor isn't a cathode resistor?

As to the pan, resistance is not impedancd. If there is continuity through each end, it is OK.

You don't need parts, you need a circuit. Starting with a tube is in my humble opinion like the people in that TV commercial who ask the architect to design a house around a faucet.

For example look up the circuit for the old Fender tube reverb. it is a stand alone spring reverb unit.

[Rob Mercure]

Right off the bat your reverb pan has the wrong impedance for a typical tube type transformer driven reverb - 8 ohms would be correct. Instead yours is made for being driven by a solid state source. So you've got the option of using an IC or a cathode follower or seeing how well it would plate drive through a capacitor. The last is my favorite method due to better tone and elimination of the transformer (cost) but the input impedance for this method is typically around 2,000 ohm - but I have driven 8 ohm tanks with this method successfully and since yours is almost 200 ohms it would probably work. Your preferred valve crosses to the US 6AQ8 which has sufficient plate power but the mu is only 60 which may not provide sufficient gain for reverb return especially if you're driving the reverb input from a transformer or capacitor tube source due to the impedance mismatch. But you could always try.

If you're determined to limit your valve types to those listed (and you could sell the Mullard ECC83 for enough money to purchase a tube type reverb pan, matching tranny, several JJ ECC83s and ECC81s, power tranny, chassis, filter caps, etc. and still have enough for a pint of bitters or two) I'd use the ECC85 with the two sections tied together can cap drive the reverb (or tranny with the mismatch) and use either the ECC83 (if you don't sell it) or one of the pentode sections of the EF80 or EBF89 as the recovery tube. With the reverb tank and tube types you've got it wouldn't be that hard for an experienced tinkerer to come up with a useable reverb but for a newby you're adding difficult, but not insurmountable, challenges.

So, to echo Enzo's advice: start with a circuit - then sell the ECC83 to buy the parts <grin>

Rob

[Godwin]

thanks Enzo, I've seen that your a very knowledgeable guy and contribute a lot to this forum. I really appreciate it your time and thoughts.

fender reverb schematic

I've got the fender 6G15 schematic in front of me, its somewhat more complex than i was hoping for. am i right in thinking that the 7025 on the right is the same valve as an ecc83?

I suppose the sensible route is just to buy all that stuff and put it together, though i was hoping to design my own unit, however badly, in an attempt to understand how the different variables in a circuit are chosen.

is it feasible to adapt that circuit to accept the valves I have? or should i just go ahead and copy that schematic?

also, I've read, perhaps on this board, that the dwell control is not very good. is it an easy mod to improve it?

thanks, Harry

[Godwin]

thanks rob, how much would the ecc83 sell for? I've got 2 more from the matching Hifi amp. also 4 ecl86's.

unless its silly money, I'll probably keep them. though they do make some lovely real ales round here.

since I'm a long way from an experienced tinkerer, I think I'll start from scratch, like you both suggested.

right. Im off to get a part time job.

and to think i thought this would be as simple as 2 common cathode gain stages with the right resistors & caps.

[Rob Mercure]

Godwin,

I'd suggest that you check e-bay to determine prices although I think they're only "slightly" inflated <grin>. But I recently sold 5 used ones - from a Tektronics oscilloscope - to someone who wanted them for a Rivera amp ("mellow man") for $200 US and he was so eager to get them that I'm sure that I undersold. While I love my tube amps for stage performance even I don't bother to use my Mullards and Telefunkens and instead choose to sell 'em - I do choose to use RCA black plates as I've got a few more and my Fenders were designed around them (my Bogen used 6EU7s - a 12AX7/ECC83 variant and I also have it in black plate). So unless you've got your own tube amps your using and feel that the sound is too "harsh" you might want to unload the Mullard ECC83s and let the "HiFi nuts" - with their "oxygen free cerebrums <grin>" pay big money for them. Anyhoo, any decent tech should be able to tweak their equipment's performance to match the tubes they have available.

If you're totally new at this you might want to copy the Fender as the entire circuit including the power supply is there. If you've got enough experience to cobble up a PS the Ampeg capacitor coupled circuit is simpler and, IMHO, better sounding than the Fender. If you do copy the Fender, or craft your own, I really don't see any gain in using a tube rectifier which should save you a few steps/parts. No matter which, as I noted before, you really need a different reverb pan but new ones are only about $20 US. Duncan's web site has a power supply designer program if you roll your own.

Oh, the 7025 is a low hum, low noise variant of the 12AX7/ECC83 - European designation schemes moved the characters around so the 7025 equivalent would be something like an E83CC or EC83C or such.

Rob

[Godwin]

thanks rob.

actually i was wondering about the PS on the reverb circuit, it seems to be 3 series diodes and a choke, with a couple of smoothing caps either side. why not add a fourth diode and wire it into a bridge?

I've built a bridge rectified 5vdc regulated power supply, which works great except for a dodgy diode which makes it cut intermittently. I probably damaged it whilst ham fisted soldering using an iron which resembles a large screwdriver, and is put to better use as one too. never mind, got a better iron now. I'll fix it, soon.

I might take the mullard valves down to my local tech. he must have some picky customers who require the originals. just seen a pair of the same ecc83's are at 36 pounds, with 12hours to go on ebay. ('bout $70? maybe) thats more than a couple of pints of bitter! I'll be too drunk to build anything.


could you point me in the right direction for the ampeg schematic? I can't find anything on google except a few vague memories of an "echo satelite" or an "echo jet"

thanks again, Harry