Vibroking Stage 1 cathode follower - tone?

[Alex R]

A while ago a customer brought in a butchered b/f Bassman head and said could I strip out all the nonsense in there and build a Vibroking preamp, no vib or reverb, to replace one of the channels? So I did and it sounded wonderful, and raised a few questions I haven't since been able to answer.

I just thoughtlessly cloned this unusual circuit (with the minor filtering tweaks from the file on Ampage somewhere, as I had put these in a VK before and liked them). I included the first stage cap-coupled cathode follower despite the fact that that stage kind of looks to my non-designer's eye like it's mainly there to decouple the reverb... yet it's a bit complex for that function...??

For the other channel he said could I put in the standard b/f preamp. I did that too, and it sounded fine, but the VK channel was sweeter, crisper, rather more gainy, nicer breakup - superior all round we thought, I thought it sounded a bit like there was an EF86 in there.

I've been wondering about that first stage ever since, specifically whether it contributed to the lovely sweet gainy touch-sensitive nature of that VK preamp. No voltage gain but quite a bit of current gain, yes?

The schem's a bit confusing at first as V1a looks like it's stage 1, but of course it only drives the reverb. Minus reverb and vib here's how I think the structure goes, correct me if I'm wrong, do:

input coupled with a cap to
Stage 1 V2a - cathode follower
effects loop
Stage 2 v4a
some RC filtering a bit unusual to guitar amps
volume control
Stage 3 V4b
tone stack
PI

What would you expect Stage 1 to contribute to this preamp?

[Rick Erickson]

It's a buffer/driver for the effects loop. Voltage gain is slightly less than 1, but the output impedance is very low, compared to a guitar, and it can therefore drive long shielded cables without noticable signal loss. I would not put the effects loop in the front end of an amp, but that's a personal choice...

RE

[tubeswell]

Yep that's interesting - not only the FX loop, but the 'verb circuit is also at the start of the signal path. Why would you add verb (pardon the pun) and other FX and then want to EQ? I thought you'd more likely want do it the other way around??

[tubeswell]

Actually Can that (Dang - can't edit my new posts again!)- I have just had an afterthought that maybe 'verb works better if its added near the start of the signal path because you might get better amplification of the 'verb signal. But still I wonder why FX loop is added at the start?

[chipaudette]

My understanding of the history of the vibroking was to re-create the sound of an old fender with an fender stand-alone reverb unit plugged in the front. Those stand-alone units were pretty sweet and are the hallmark of surf music since the first time that Dick Dale plugged one into the front of his amp. So, to mimic this configuration in the vibroking, you've got to put the reverb at the start of the circuit.

Some people say that the combination of the stand-alone reverb unit in front of a dry fender amp just has a better sound than the typical Fender blackface configuration, which has the reverb built in AFTER the pre-amp. Given many people's positive reaction to the vibroking, maybe they're right.

Chip

[Alex R]

I'm making a bit more sense of it now. I see the purpose of the cathode follower driving the effects loop. But with no reverb and no effects loop would you expect Stage 1 to affect the sound and performance of the amp?