6BM8 / ECL82 reverb driver?

[Henrik]

Hello everyone,

I'm toying with the ideas for a new build. I have gutted an earlier build, but will keep the transformers and a few other bits and pieces. Power amp will be 2xEL84 cathode biased.
I've been messing around with small pentodes in my last couple of projects and I really like them, both the obvious but problematic EF86 and the less obvious 5879. Therefore, I'm thinking my new build will have two channels, both with pentodes. I would also like to have reverb and I want it to work on both channels. I know I could just insert it between the preamps and the shared LTPPI, but that would mean feeding the reverb driver and tank with a distorted signal at higher gain settings. I think the reverb sounds better with a clean signal, so here's the general idea:

I would bleed off the signal directly from the input, through a cap into the grid of the triode section of an ECL82. This triode would then drive the grid of the pentode section through a cap and a volume pot. The pentode section would be transformer loaded through a standard Fender reverb transformer (I have one somewhere) to drive the tank. Recovery would be through a 12AX7 and mixed in just before the PI. The idea is that the ECL82 combo would be able to drive the tank well, even when the initial signal is at input level, and I can blend a clean reverb with an overdriven preamp. I know a schematic would be helpful, but I haven't gotten that far yet.

My reasons for posting are just a) Is this even going to work? and b) Will the reverb transformer cope with the pentode of the ECL82 and will the impedance matching be tolerable? The matched tank and transformer were designed for a paralleled 12AT7 driver.

Thanks!
Henrik

[Henrik]

I can see that the 6G15 and the Vibro King, which both used a power tube for the driver, actually have a different reverb transformer. A little bigger with a very different impedance ratio. I guess I'll have to get one of those if I'm going to try this.

[pdf64]

Those combined triode pentodes have become rather expensive of late, have you got a few handy?
I’d try using a tank with a high input impedance, and use cap coupling to the driver anode, eg as per Ampeg.
But I’m not a reverb fan, so won’t get around to tinkering with it.

[Pedro Vecino]

I think the reverb sounds better with a clean signal, so here's the general idea:
I would bleed off the signal directly from the input, through a cap into the grid of the triode section of an ECL82...

In my opinion, assigning the reverb from the amp input is not a good idea. It must have a tracking, not only with the distortion that the channels produce but with the equalization that they have.
This is something that happens in the Super Twin reverb: activating the added five-band EQ (this is not in series with the reverb) very strange effects appear on it with certain settings.

[Mick Bailey]

Those combined triode pentodes have become rather expensive of late...

Thanks for highlighting this. I have lots of NOS ECL82 in boxes and they used to be dirt cheap so I hadn't put any value on them - I don't track the market much these days. I was shocked to find that boxed Mullard OC44 transistors were on sale for £200 each and suddenly woke up to what was being charged for Ge transistors.

Using a standard reverb driver transformer may not give good results with a pentode, but is worth experimenting with. A 6G15 driver transformer may be better. You can get 'universal' 3.5w output transformers with multiple tappings that would give flexibility in arriving at the best match. Pretty much any signal output will provide some reverb, even if the transformer is badly matched, though I like to really spank the tray and the input transducer requires a fair signal to drive it to saturation to get a good 'surf' sound. I prefer capacitor coupling but you have to use a tray with a high-impedance input transducer otherwise a regular tray appears as a short to a pentode. To get round this you can use a MOSFET constant-current driver capacitor-coupled a regular tray and run it off a single triode and pick up the signal at the most appropriate point.

I've tried running reverb off an input signal and the results are poor to my ears, so I'm in agreement with Pedro Vecino on this. With any kind of preamp distortion it sounds like someone is playing along with a clean amp + reverb and the levels are always wrong.

[Henrik]

Thanks for chiming in. I'll probably try the pentode driver once I get a suitable transformer, but Ill take you advice and skip the whole "direct-from-input-and-join-up-at-PI"-thing.

[Mick Bailey]

I've had good results with this transformer - lots of impedance options. I don't know if they're available worldwide, though; https://uk.rs-online.com/web/p/audio...ormers/2106475

[Pedro Vecino]

The Super Twin's reverb driver is made with the 6CX8 pentode and maintains the typical 12AT7 transformer. Maybe it could be a clue comparing it to the ECL82's pentode.
There are also Hammond multiple impedance transformers in 3 and 5 watts: 125A and 125B.