Premier Spring Reverb Tone Tweak Suggestions?

[neddyboy]

Hello all. I have a '60s vintage tube spring reverb unit that I've been restoring. I'm competent enough to troubleshoot the basics: bad caps, poor lead routing, shielding, grounds, etc., but not good enough to answer this question: what part of the circuit makes these units sound so dark? I'd just like to tweak the overall tone to be a bit more bright. Can anyone tell me if there's a certain component I can replace to change the tone? Or is the dark tone a function of the overall circuit?

Thanks for any assistance.

[pompeiisneaks]

I'd guess that the C1 being only .01 means it passes a lot more bass frequency that may give it that darkness.  You could try replacing it with a .001 and see if that brightens it up a bit.

~Phil

[sluckey]

Does the unit sound dark if you disable the reverb with the footswitch or just turn the intensity pot all the way down?

[neddyboy]

Hi Sluckey, good question, will have to check tonight!

[neddyboy]

Sluckey, I checked the sounds with the footswitch off, and yes, it still sounds dark. What does that tell you? I've been thinking of making a 2-button footswitch, with one button for full bypass and the other being the current off/on. The preamp only option is nice to have, but I miss a full bypass.

[pompeiisneaks]

Makes me think my initial guess was right, C1 (and maybe C4) are in the flow even if the reverb is bypassed, so they could be allowing too much bass through?   Still a guess though.  Sluckey will likely know for sure.

~Phil

[neddyboy]

Hey Phil, I replaced those two caps with some I had in the box. .003 for C1 and .001 for C4. Definitely brighter.

[Willabe]

0.01uF is not a very large value for a coupling cap, .02uF to .1uF are very common in many classic guitar amps and the single 6EU7 preamp tube is not bypassed at the cathode (K) so no bass boost there.   

Plate dcv is pretty low, the 6EU7 has less than 100dcv on it's plate. Lower plate voltage can/will be less bright than higher plate dcv.

Black face Super Reverbs (BFSR's) had ~270dcv (?) on the 1st preamp tube plate, brighter and cleaner than a lot of other Fender amps.

Will that PT support a FWB rectifier? (I did that with my stand alone Fender reverb, working fine.)

[TIMBO]

WOW, that's freeky.
One has turned up on our local ebay.
Looks like there is a lot of interest....
http://www.ebay.com.au/itm/1960s-Multivox-Premier-Valve-Spring-Reverb-Unit/152119015006?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3D4a2efe65a0924f49b15d89f8ee4d5afb%26pid%3D100033%26rk%3D7%26rkt%3D8%26sd%3D191890548531

[ac427v]

I had one to play with for a summer. I loved the darkness of the reverb but it's definitely not for playing surf. I put in a bridge rectifier that boosted voltages and reduced hum without any problems. I did use new power supply caps with higher voltage ratings.

I did not notice a significant increase in brightness but then my ear is not very sophisticated (or perceptive any more). For your needs, I wonder if you would get enough bass cut by reducing the cathode bypass cap on the power tube. It is 100 uf now. Maybe try a 4.7 or even 2.2 uf?
--alt Craig B

[jojokeo]

1) Change J1 grid resistor R1 to 33K
2) I would not change the coupling caps to a smaller value to expect a brighter outcome at the expense of your lows and low mids. This is supposed to be a reverb unit not a treble boosted reverb unit. If anything you would think to make C3 and/or C4 larger.
3) A more reasonable solution would be to put bypassing caps one at a time to see how and what each does on R5 & R11, and treble bleeds on R8 and possibly R13.

[HotBluePlates]

0.01uF is not a very large value for a coupling cap, .02uF to .1uF are very common in many classic guitar amps ...

The output 6EU7 has a 0.01µF which sees 470kΩ plus some resistance from the pot before. 0.01µF and 470kΩ is -3dB at 33Hz.

2) I would not change the coupling caps to a smaller value to expect a brighter outcome at the expense of your lows and low mids. ...

This is a good point.

Try tacking a series resistor-cap across R12, maybe a 1kΩ in series with a 0.1µF. The goal here is to see if this causes a treble boost at all; values can be tuned later.

3) A more reasonable solution would be to put bypassing caps one at a time to see how and what each does on R5 & R11...

We can't do that, as it would kill the dry signal through the reverb. Look at the Normal control, then pretend the added bypass caps are a short to ground.

In my suggestion above, I'm uncertain if there is feedback as well as normal-signal feed-forward. So I honestly don't know if the paralleled cap will add treble or cut treble. It could be tried, and if it cuts treble (because of feedback action), then we'd know to use a cap-to-ground in this path between R12 & R13.

But in the end I suspect the savings of an extra 9-pin socket & circuitry of Fender's 6G15 forced a compromise which is giving this dark-verb result.

Of course, we could just take it for what it is (especially in a vintage unit), rather than ask it to be a 6G15.

[jojokeo]

3) A more reasonable solution would be to put bypassing caps one at a time to see how and what each does on R5 & R11...

We can't do that, as it would kill the dry signal through the reverb. Look at the Normal control, then pretend the added bypass caps are a short to ground.

In my suggestion above, I'm uncertain if there is feedback as well as normal-signal feed-forward. So I honestly don't know if the paralleled cap will add treble or cut treble. It could be tried, and if it cuts treble (because of feedback action), then we'd know to use a cap-to-ground in this path between R12 & R13.

But in the end I suspect the savings of an extra 9-pin socket & circuitry of Fender's 6G15 forced a compromise which is giving this dark-verb result.

Of course, we could just take it for what it is (especially in a vintage unit), rather than ask it to be a 6G15.

All good points HBP and part of why I said of "trying the caps one at a time to see how & what each does" in #3's comment (without knowing exactly until you try it).